malloc
malloc
malloc
malloc
malloc-Related Functions
printf
getopt
argp_parse Function
argp_parse
argp_help Function
argp_help Function
sysconf
pathconf
The C language provides no built-in facilities for performing such common operations as input/output, memory management, string manipulation, and the like. Instead, these facilities are defined in a standard library, which you compile and link with your programs.
The GNU C library, described in this document, defines all of the library functions that are specified by the ISO C standard, as well as additional features specific to POSIX and other derivatives of the Unix operating system, and extensions specific to the GNU system.
The purpose of this manual is to tell you how to use the facilities of the GNU library. We have mentioned which features belong to which standards to help you identify things that are potentially non-portable to other systems. But the emphasis in this manual is not on strict portability.
This manual is written with the assumption that you are at least somewhat familiar with the C programming language and basic programming concepts. Specifically, familiarity with ISO standard C (see section ISO C), rather than "traditional" pre-ISO C dialects, is assumed.
The GNU C library includes several header files, each of which provides definitions and declarations for a group of related facilities; this information is used by the C compiler when processing your program. For example, the header file `stdio.h' declares facilities for performing input and output, and the header file `string.h' declares string processing utilities. The organization of this manual generally follows the same division as the header files.
If you are reading this manual for the first time, you should read all of the introductory material and skim the remaining chapters. There are a lot of functions in the GNU C library and it's not realistic to expect that you will be able to remember exactly how to use each and every one of them. It's more important to become generally familiar with the kinds of facilities that the library provides, so that when you are writing your programs you can recognize when to make use of library functions, and where in this manual you can find more specific information about them.
This section discusses the various standards and other sources that the GNU C library is based upon. These sources include the ISO C and POSIX standards, and the System V and Berkeley Unix implementations.
The primary focus of this manual is to tell you how to make effective use of the GNU library facilities. But if you are concerned about making your programs compatible with these standards, or portable to operating systems other than GNU, this can affect how you use the library. This section gives you an overview of these standards, so that you will know what they are when they are mentioned in other parts of the manual.
See section Summary of Library Facilities, for an alphabetical list of the functions and other symbols provided by the library. This list also states which standards each function or symbol comes from.
The GNU C library is compatible with the C standard adopted by the American National Standards Institute (ANSI): American National Standard X3.159-1989---"ANSI C" and later by the International Standardization Organization (ISO): ISO/IEC 9899:1990, "Programming languages--C". We here refer to the standard as ISO C since this is the more general standard in respect of ratification. The header files and library facilities that make up the GNU library are a superset of those specified by the ISO C standard.
If you are concerned about strict adherence to the ISO C standard, you should use the `-ansi' option when you compile your programs with the GNU C compiler. This tells the compiler to define only ISO standard features from the library header files, unless you explicitly ask for additional features. See section Feature Test Macros, for information on how to do this.
Being able to restrict the library to include only ISO C features is important because ISO C puts limitations on what names can be defined by the library implementation, and the GNU extensions don't fit these limitations. See section Reserved Names, for more information about these restrictions.
This manual does not attempt to give you complete details on the differences between ISO C and older dialects. It gives advice on how to write programs to work portably under multiple C dialects, but does not aim for completeness.
The GNU library is also compatible with the ISO POSIX family of standards, known more formally as the Portable Operating System Interface for Computer Environments (ISO/IEC 9945). They were also published as ANSI/IEEE Std 1003. POSIX is derived mostly from various versions of the Unix operating system.
The library facilities specified by the POSIX standards are a superset of those required by ISO C; POSIX specifies additional features for ISO C functions, as well as specifying new additional functions. In general, the additional requirements and functionality defined by the POSIX standards are aimed at providing lower-level support for a particular kind of operating system environment, rather than general programming language support which can run in many diverse operating system environments.
The GNU C library implements all of the functions specified in ISO/IEC 9945-1:1996, the POSIX System Application Program Interface, commonly referred to as POSIX.1. The primary extensions to the ISO C facilities specified by this standard include file system interface primitives (see section File System Interface), device-specific terminal control functions (see section Low-Level Terminal Interface), and process control functions (see section Processes).
Some facilities from ISO/IEC 9945-2:1993, the POSIX Shell and Utilities standard (POSIX.2) are also implemented in the GNU library. These include utilities for dealing with regular expressions and other pattern matching facilities (see section Pattern Matching).
The GNU C library defines facilities from some versions of Unix which are not formally standardized, specifically from the 4.2 BSD, 4.3 BSD, and 4.4 BSD Unix systems (also known as Berkeley Unix) and from SunOS (a popular 4.2 BSD derivative that includes some Unix System V functionality). These systems support most of the ISO C and POSIX facilities, and 4.4 BSD and newer releases of SunOS in fact support them all.
The BSD facilities include symbolic links (see section Symbolic Links), the
select function (see section Waiting for Input or Output), the BSD signal
functions (see section BSD Signal Handling), and sockets (see section Sockets).
The System V Interface Description (SVID) is a document describing the AT&T Unix System V operating system. It is to some extent a superset of the POSIX standard (see section POSIX (The Portable Operating System Interface)).
The GNU C library defines most of the facilities required by the SVID that are not also required by the ISO C or POSIX standards, for compatibility with System V Unix and other Unix systems (such as SunOS) which include these facilities. However, many of the more obscure and less generally useful facilities required by the SVID are not included. (In fact, Unix System V itself does not provide them all.)
The supported facilities from System V include the methods for
inter-process communication and shared memory, the hsearch and
drand48 families of functions, fmtmsg and several of the
mathematical functions.
The X/Open Portability Guide, published by the X/Open Company, Ltd., is a more general standard than POSIX. X/Open owns the Unix copyright and the XPG specifies the requirements for systems which are intended to be a Unix system.
The GNU C library complies to the X/Open Portability Guide, Issue 4.2, with all extensions common to XSI (X/Open System Interface) compliant systems and also all X/Open UNIX extensions.
The additions on top of POSIX are mainly derived from functionality available in System V and BSD systems. Some of the really bad mistakes in System V systems were corrected, though. Since fulfilling the XPG standard with the Unix extensions is a precondition for getting the Unix brand chances are good that the functionality is available on commercial systems.
This section describes some of the practical issues involved in using the GNU C library.
Libraries for use by C programs really consist of two parts: header files that define types and macros and declare variables and functions; and the actual library or archive that contains the definitions of the variables and functions.
(Recall that in C, a declaration merely provides information that a function or variable exists and gives its type. For a function declaration, information about the types of its arguments might be provided as well. The purpose of declarations is to allow the compiler to correctly process references to the declared variables and functions. A definition, on the other hand, actually allocates storage for a variable or says what a function does.)
In order to use the facilities in the GNU C library, you should be sure that your program source files include the appropriate header files. This is so that the compiler has declarations of these facilities available and can correctly process references to them. Once your program has been compiled, the linker resolves these references to the actual definitions provided in the archive file.
Header files are included into a program source file by the `#include' preprocessor directive. The C language supports two forms of this directive; the first,
#include "header"
is typically used to include a header file header that you write yourself; this would contain definitions and declarations describing the interfaces between the different parts of your particular application. By contrast,
#include <file.h>
is typically used to include a header file `file.h' that contains definitions and declarations for a standard library. This file would normally be installed in a standard place by your system administrator. You should use this second form for the C library header files.
Typically, `#include' directives are placed at the top of the C source file, before any other code. If you begin your source files with some comments explaining what the code in the file does (a good idea), put the `#include' directives immediately afterwards, following the feature test macro definition (see section Feature Test Macros).
For more information about the use of header files and `#include' directives, see section `Header Files' in The GNU C Preprocessor Manual.
The GNU C library provides several header files, each of which contains the type and macro definitions and variable and function declarations for a group of related facilities. This means that your programs may need to include several header files, depending on exactly which facilities you are using.
Some library header files include other library header files automatically. However, as a matter of programming style, you should not rely on this; it is better to explicitly include all the header files required for the library facilities you are using. The GNU C library header files have been written in such a way that it doesn't matter if a header file is accidentally included more than once; including a header file a second time has no effect. Likewise, if your program needs to include multiple header files, the order in which they are included doesn't matter.
Compatibility Note: Inclusion of standard header files in any order and any number of times works in any ISO C implementation. However, this has traditionally not been the case in many older C implementations.
Strictly speaking, you don't have to include a header file to use a function it declares; you could declare the function explicitly yourself, according to the specifications in this manual. But it is usually better to include the header file because it may define types and macros that are not otherwise available and because it may define more efficient macro replacements for some functions. It is also a sure way to have the correct declaration.
If we describe something as a function in this manual, it may have a macro definition as well. This normally has no effect on how your program runs--the macro definition does the same thing as the function would. In particular, macro equivalents for library functions evaluate arguments exactly once, in the same way that a function call would. The main reason for these macro definitions is that sometimes they can produce an inline expansion that is considerably faster than an actual function call.
Taking the address of a library function works even if it is also defined as a macro. This is because, in this context, the name of the function isn't followed by the left parenthesis that is syntactically necessary to recognize a macro call.
You might occasionally want to avoid using the macro definition of a function--perhaps to make your program easier to debug. There are two ways you can do this:
For example, suppose the header file `stdlib.h' declares a function
named abs with
extern int abs (int);
and also provides a macro definition for abs. Then, in:
#include <stdlib.h>
int f (int *i) { return abs (++*i); }
the reference to abs might refer to either a macro or a function.
On the other hand, in each of the following examples the reference is
to a function and not a macro.
#include <stdlib.h>
int g (int *i) { return (abs) (++*i); }
#undef abs
int h (int *i) { return abs (++*i); }
Since macro definitions that double for a function behave in exactly the same way as the actual function version, there is usually no need for any of these methods. In fact, removing macro definitions usually just makes your program slower.
The names of all library types, macros, variables and functions that come from the ISO C standard are reserved unconditionally; your program may not redefine these names. All other library names are reserved if your program explicitly includes the header file that defines or declares them. There are several reasons for these restrictions:
exit to do something completely different from
what the standard exit function does, for example. Preventing
this situation helps to make your programs easier to understand and
contributes to modularity and maintainability.
In addition to the names documented in this manual, reserved names include all external identifiers (global functions and variables) that begin with an underscore (`_') and all identifiers regardless of use that begin with either two underscores or an underscore followed by a capital letter are reserved names. This is so that the library and header files can define functions, variables, and macros for internal purposes without risk of conflict with names in user programs.
Some additional classes of identifier names are reserved for future extensions to the C language or the POSIX.1 environment. While using these names for your own purposes right now might not cause a problem, they do raise the possibility of conflict with future versions of the C or POSIX standards, so you should avoid these names.
float and long double arguments,
respectively.
In addition, some individual header files reserve names beyond those that they actually define. You only need to worry about these restrictions if your program includes that particular header file.
The exact set of features available when you compile a source file is controlled by which feature test macros you define.
If you compile your programs using `gcc -ansi', you get only the ISO C library features, unless you explicitly request additional features by defining one or more of the feature macros. See section `GNU CC Command Options' in The GNU CC Manual, for more information about GCC options.
You should define these macros by using `#define' preprocessor
directives at the top of your source code files. These directives
must come before any #include of a system header file. It
is best to make them the very first thing in the file, preceded only by
comments. You could also use the `-D' option to GCC, but it's
better if you make the source files indicate their own meaning in a
self-contained way.
This system exists to allow the library to conform to multiple standards.
Although the different standards are often described as supersets of each
other, they are usually incompatible because larger standards require
functions with names that smaller ones reserve to the user program. This
is not mere pedantry -- it has been a problem in practice. For instance,
some non-GNU programs define functions named getline that have
nothing to do with this library's getline. They would not be
compilable if all features were enabled indiscriminately.
This should not be used to verify that a program conforms to a limited standard. It is insufficient for this purpose, as it will not protect you from including header files outside the standard, or relying on semantics undefined within the standard.
The state of _POSIX_SOURCE is irrelevant if you define the
macro _POSIX_C_SOURCE to a positive integer.
If you define this macro to a value greater than or equal to 1,
then the functionality from the 1990 edition of the POSIX.1 standard
(IEEE Standard 1003.1-1990) is made available.
If you define this macro to a value greater than or equal to 2,
then the functionality from the 1992 edition of the POSIX.2 standard
(IEEE Standard 1003.2-1992) is made available.
If you define this macro to a value greater than or equal to 199309L,
then the functionality from the 1993 edition of the POSIX.1b standard
(IEEE Standard 1003.1b-1993) is made available.
Greater values for _POSIX_C_SOURCE will enable future extensions.
The POSIX standards process will define these values as necessary, and
the GNU C Library should support them some time after they become standardized.
The 1996 edition of POSIX.1 (ISO/IEC 9945-1: 1996) states that
if you define _POSIX_C_SOURCE to a value greater than
or equal to 199506L, then the functionality from the 1996
edition is made available.
Some of the features derived from 4.3 BSD Unix conflict with the corresponding features specified by the POSIX.1 standard. If this macro is defined, the 4.3 BSD definitions take precedence over the POSIX definitions.
Due to the nature of some of the conflicts between 4.3 BSD and POSIX.1,
you need to use a special BSD compatibility library when linking
programs compiled for BSD compatibility. This is because some functions
must be defined in two different ways, one of them in the normal C
library, and one of them in the compatibility library. If your program
defines _BSD_SOURCE, you must give the option `-lbsd-compat'
to the compiler or linker when linking the program, to tell it to find
functions in this special compatibility library before looking for them in
the normal C library.
_POSIX_SOURCE and
_POSIX_C_SOURCE are automatically defined.
As the unification of all Unices, functionality only available in BSD and SVID is also included.
If the macro _XOPEN_SOURCE_EXTENDED is also defined, even more
functionality is available. The extra functions will make all functions
available which are necessary for the X/Open Unix brand.
If the macro _XOPEN_SOURCE has the value @math{500} this includes
all functionality described so far plus some new definitions from the
Single Unix Specification, version 2.
fseeko and ftello are available. Without
these functions the difference between the ISO C interface
(fseek, ftell) and the low-level POSIX interface
(lseek) would lead to problems.
This macro was introduced as part of the Large File Support extension (LFS).
The new functionality is made available by a new set of types and
functions which replace the existing ones. The names of these new objects
contain 64 to indicate the intention, e.g., off_t
vs. off64_t and fseeko vs. fseeko64.
This macro was introduced as part of the Large File Support extension
(LFS). It is a transition interface for the period when 64 bit
offsets are not generally used (see _FILE_OFFSET_BITS).
_LARGEFILE64_SOURCE makes the 64
bit interface available as an additional interface,
_FILE_OFFSET_BITS allows the 64 bit interface to
replace the old interface.
If _FILE_OFFSET_BITS is undefined, or if it is defined to the
value 32, nothing changes. The 32 bit interface is used and
types like off_t have a size of 32 bits on 32 bit
systems.
If the macro is defined to the value 64, the large file interface
replaces the old interface. I.e., the functions are not made available
under different names (as they are with _LARGEFILE64_SOURCE).
Instead the old function names now reference the new functions, e.g., a
call to fseeko now indeed calls fseeko64.
This macro should only be selected if the system provides mechanisms for
handling large files. On 64 bit systems this macro has no effect
since the *64 functions are identical to the normal functions.
This macro was introduced as part of the Large File Support extension (LFS).
_ISOC99_SOURCE should be defined.
If you want to get the full effect of _GNU_SOURCE but make the
BSD definitions take precedence over the POSIX definitions, use this
sequence of definitions:
#define _GNU_SOURCE #define _BSD_SOURCE #define _SVID_SOURCE
Note that if you do this, you must link your program with the BSD compatibility library by passing the `-lbsd-compat' option to the compiler or linker. Note: If you forget to do this, you may get very strange errors at run time.
Unlike on some other systems, no special version of the C library must be used for linking. There is only one version but while compiling this it must have been specified to compile as thread safe.
We recommend you use _GNU_SOURCE in new programs. If you don't
specify the `-ansi' option to GCC and don't define any of these
macros explicitly, the effect is the same as defining
_POSIX_C_SOURCE to 2 and _POSIX_SOURCE,
_SVID_SOURCE, and _BSD_SOURCE to 1.
When you define a feature test macro to request a larger class of features,
it is harmless to define in addition a feature test macro for a subset of
those features. For example, if you define _POSIX_C_SOURCE, then
defining _POSIX_SOURCE as well has no effect. Likewise, if you
define _GNU_SOURCE, then defining either _POSIX_SOURCE or
_POSIX_C_SOURCE or _SVID_SOURCE as well has no effect.
Note, however, that the features of _BSD_SOURCE are not a subset of
any of the other feature test macros supported. This is because it defines
BSD features that take precedence over the POSIX features that are
requested by the other macros. For this reason, defining
_BSD_SOURCE in addition to the other feature test macros does have
an effect: it causes the BSD features to take priority over the conflicting
POSIX features.
Here is an overview of the contents of the remaining chapters of this manual.
sizeof
operator and the symbolic constant NULL, how to write functions
accepting variable numbers of arguments, and constants describing the
ranges and other properties of the numerical types. There is also a simple
debugging mechanism which allows you to put assertions in your code, and
have diagnostic messages printed if the tests fail.
isspace) and functions for
performing case conversion.
FILE * objects). These are the normal C library functions
from `stdio.h'.
char data type.
setjmp and
longjmp functions. These functions provide a facility for
goto-like jumps which can jump from one function to another.
If you already know the name of the facility you are interested in, you can look it up in section Summary of Library Facilities. This gives you a summary of its syntax and a pointer to where you can find a more detailed description. This appendix is particularly useful if you just want to verify the order and type of arguments to a function, for example. It also tells you what standard or system each function, variable, or macro is derived from.
Many functions in the GNU C library detect and report error conditions, and sometimes your programs need to check for these error conditions. For example, when you open an input file, you should verify that the file was actually opened correctly, and print an error message or take other appropriate action if the call to the library function failed.
This chapter describes how the error reporting facility works. Your program should include the header file `errno.h' to use this facility.
Most library functions return a special value to indicate that they have
failed. The special value is typically -1, a null pointer, or a
constant such as EOF that is defined for that purpose. But this
return value tells you only that an error has occurred. To find out
what kind of error it was, you need to look at the error code stored in the
variable errno. This variable is declared in the header file
`errno.h'.
errno contains the system error number. You can
change the value of errno.
Since errno is declared volatile, it might be changed
asynchronously by a signal handler; see section Defining Signal Handlers.
However, a properly written signal handler saves and restores the value
of errno, so you generally do not need to worry about this
possibility except when writing signal handlers.
The initial value of errno at program startup is zero. Many
library functions are guaranteed to set it to certain nonzero values
when they encounter certain kinds of errors. These error conditions are
listed for each function. These functions do not change errno
when they succeed; thus, the value of errno after a successful
call is not necessarily zero, and you should not use errno to
determine whether a call failed. The proper way to do that is
documented for each function. If the call failed, you can
examine errno.
Many library functions can set errno to a nonzero value as a
result of calling other library functions which might fail. You should
assume that any library function might alter errno when the
function returns an error.
Portability Note: ISO C specifies errno as a
"modifiable lvalue" rather than as a variable, permitting it to be
implemented as a macro. For example, its expansion might involve a
function call, like *_errno (). In fact, that is what it is
on the GNU system itself. The GNU library, on non-GNU systems, does
whatever is right for the particular system.
There are a few library functions, like sqrt and atan,
that return a perfectly legitimate value in case of an error, but also
set errno. For these functions, if you want to check to see
whether an error occurred, the recommended method is to set errno
to zero before calling the function, and then check its value afterward.
All the error codes have symbolic names; they are macros defined in `errno.h'. The names start with `E' and an upper-case letter or digit; you should consider names of this form to be reserved names. See section Reserved Names.
The error code values are all positive integers and are all distinct,
with one exception: EWOULDBLOCK and EAGAIN are the same.
Since the values are distinct, you can use them as labels in a
switch statement; just don't use both EWOULDBLOCK and
EAGAIN. Your program should not make any other assumptions about
the specific values of these symbolic constants.
The value of errno doesn't necessarily have to correspond to any
of these macros, since some library functions might return other error
codes of their own for other situations. The only values that are
guaranteed to be meaningful for a particular library function are the
ones that this manual lists for that function.
On non-GNU systems, almost any system call can return EFAULT if
it is given an invalid pointer as an argument. Since this could only
happen as a result of a bug in your program, and since it will not
happen on the GNU system, we have saved space by not mentioning
EFAULT in the descriptions of individual functions.
In some Unix systems, many system calls can also return EFAULT if
given as an argument a pointer into the stack, and the kernel for some
obscure reason fails in its attempt to extend the stack. If this ever
happens, you should probably try using statically or dynamically
allocated memory instead of stack memory on that system.
The error code macros are defined in the header file `errno.h'. All of them expand into integer constant values. Some of these error codes can't occur on the GNU system, but they can occur using the GNU library on other systems.
You can choose to have functions resume after a signal that is handled,
rather than failing with EINTR; see section Primitives Interrupted by Signals.
exec functions (see section Executing a File) occupy too much memory space. This condition never arises in the
GNU system.
exec functions; see section Executing a File.
link (see section Hard Links) but
also when you rename a file with rename (see section Renaming Files).
In BSD and GNU, the number of open files is controlled by a resource
limit that can usually be increased. If you get this error, you might
want to increase the RLIMIT_NOFILE limit or make it unlimited;
see section Limiting Resource Usage.
rename can cause this error if the file being renamed already has
as many links as it can take (see section Renaming Files).
SIGPIPE signal; this signal terminates the program if not handled
or blocked. Thus, your program will never actually see EPIPE
unless it has handled or blocked SIGPIPE.
EWOULDBLOCK is another name for EAGAIN;
they are always the same in the GNU C library.
This error can happen in a few different situations:
select to find out
when the operation will be possible; see section Waiting for Input or Output.
Portability Note: In many older Unix systems, this condition
was indicated by EWOULDBLOCK, which was a distinct error code
different from EAGAIN. To make your program portable, you should
check for both codes and treat them the same.
fork
can return this error. It indicates that the shortage is expected to
pass, so your program can try the call again later and it may succeed.
It is probably a good idea to delay for a few seconds before trying it
again, to allow time for other processes to release scarce resources.
Such shortages are usually fairly serious and affect the whole system,
so usually an interactive program should report the error to the user
and return to its command loop.
EAGAIN (above).
The values are always the same, on every operating system.
C libraries in many older Unix systems have EWOULDBLOCK as a
separate error code.
connect; see section Making a Connection) never return
EAGAIN. Instead, they return EINPROGRESS to indicate that
the operation has begun and will take some time. Attempts to manipulate
the object before the call completes return EALREADY. You can
use the select function to find out when the pending operation
has completed; see section Waiting for Input or Output.
ENOMEM; you may get one or the
other from network operations.
EDESTADDRREQ instead.
connect.
PATH_MAX; see section Limits on File System Capacity) or host name too long (in gethostname or
sethostname; see section Host Identification).
fork. See section Limiting Resource Usage, for details on
the RLIMIT_NPROC limit.
On some systems chmod returns this error if you try to set the
sticky bit on a non-directory file; see section Assigning File Permissions.
ENOSYS unless you
install a new version of the C library or the operating system.
If the entire function is not available at all in the implementation,
it returns ENOSYS instead.
term protocol return
this error for certain operations when the caller is not in the
foreground process group of the terminal. Users do not usually see this
error because functions such as read and write translate
it into a SIGTTIN or SIGTTOU signal. See section Job Control,
for information on process groups and these signals.
The following error codes are defined by the Linux/i386 kernel. They are not yet documented.
The library has functions and variables designed to make it easy for
your program to report informative error messages in the customary
format about the failure of a library call. The functions
strerror and perror give you the standard error message
for a given error code; the variable
program_invocation_short_name gives you convenient access to the
name of the program that encountered the error.
strerror function maps the error code (see section Checking for Errors) specified by the errnum argument to a descriptive error
message string. The return value is a pointer to this string.
The value errnum normally comes from the variable errno.
You should not modify the string returned by strerror. Also, if
you make subsequent calls to strerror, the string might be
overwritten. (But it's guaranteed that no library function ever calls
strerror behind your back.)
The function strerror is declared in `string.h'.
strerror_r function works like strerror but instead of
returning the error message in a statically allocated buffer shared by
all threads in the process, it returns a private copy for the
thread. This might be either some permanent global data or a message
string in the user supplied buffer starting at buf with the
length of n bytes.
At most n characters are written (including the NUL byte) so it is up to the user to select the buffer large enough.
This function should always be used in multi-threaded programs since
there is no way to guarantee the string returned by strerror
really belongs to the last call of the current thread.
This function strerror_r is a GNU extension and it is declared in
`string.h'.
stderr;
see section Standard Streams.
If you call perror with a message that is either a null
pointer or an empty string, perror just prints the error message
corresponding to errno, adding a trailing newline.
If you supply a non-null message argument, then perror
prefixes its output with this string. It adds a colon and a space
character to separate the message from the error string corresponding
to errno.
The function perror is declared in `stdio.h'.
strerror and perror produce the exact same message for any
given error code; the precise text varies from system to system. On the
GNU system, the messages are fairly short; there are no multi-line
messages or embedded newlines. Each error message begins with a capital
letter and does not include any terminating punctuation.
Compatibility Note: The strerror function is a new
feature of ISO C. Many older C systems do not support this function
yet.
Many programs that don't read input from the terminal are designed to
exit if any system call fails. By convention, the error message from
such a program should start with the program's name, sans directories.
You can find that name in the variable
program_invocation_short_name; the full file name is stored the
variable program_invocation_name.
argv[0]. Note
that this is not necessarily a useful file name; often it contains no
directory names. See section Program Arguments.
program_invocation_name minus
everything up to the last slash, if any.)
The library initialization code sets up both of these variables before
calling main.
Portability Note: These two variables are GNU extensions. If
you want your program to work with non-GNU libraries, you must save the
value of argv[0] in main, and then strip off the directory
names yourself. We added these extensions to make it possible to write
self-contained error-reporting subroutines that require no explicit
cooperation from main.
Here is an example showing how to handle failure to open a file
correctly. The function open_sesame tries to open the named file
for reading and returns a stream if successful. The fopen
library function returns a null pointer if it couldn't open the file for
some reason. In that situation, open_sesame constructs an
appropriate error message using the strerror function, and
terminates the program. If we were going to make some other library
calls before passing the error code to strerror, we'd have to
save it in a local variable instead, because those other library
functions might overwrite errno in the meantime.
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
FILE *
open_sesame (char *name)
{
FILE *stream;
errno = 0;
stream = fopen (name, "r");
if (stream == NULL)
{
fprintf (stderr, "%s: Couldn't open file %s; %s\n",
program_invocation_short_name, name, strerror (errno));
exit (EXIT_FAILURE);
}
else
return stream;
}
This chapter describes how processes manage and use memory in a system that uses the GNU C library.
The GNU C Library has several functions for dynamically allocating virtual memory in various ways. They vary in generality and in efficiency. The library also provides functions for controlling paging and allocation of real memory.
Memory mapped I/O is not discussed in this chapter. See section Memory-mapped I/O.
One of the most basic resources a process has available to it is memory. There are a lot of different ways systems organize memory, but in a typical one, each process has one linear virtual address space, with addresses running from zero to some huge maximum. It need not be contiguous; i.e. not all of these addresses actually can be used to store data.
The virtual memory is divided into pages (4 kilobytes is typical). Backing each page of virtual memory is a page of real memory (called a frame) or some secondary storage, usually disk space. The disk space might be swap space or just some ordinary disk file. Actually, a page of all zeroes sometimes has nothing at all backing it -- there's just a flag saying it is all zeroes.
The same frame of real memory or backing store can back multiple virtual
pages belonging to multiple processes. This is normally the case, for
example, with virtual memory occupied by GNU C library code. The same
real memory frame containing the printf function backs a virtual
memory page in each of the existing processes that has a printf
call in its program.
In order for a program to access any part of a virtual page, the page must at that moment be backed by ("connected to") a real frame. But because there is usually a lot more virtual memory than real memory, the pages must move back and forth between real memory and backing store regularly, coming into real memory when a process needs to access them and then retreating to backing store when not needed anymore. This movement is called paging.
When a program attempts to access a page which is not at that moment backed by real memory, this is known as a page fault. When a page fault occurs, the kernel suspends the process, places the page into a real page frame (this is called "paging in" or "faulting in"), then resumes the process so that from the process' point of view, the page was in real memory all along. In fact, to the process, all pages always seem to be in real memory. Except for one thing: the elapsed execution time of an instruction that would normally be a few nanoseconds is suddenly much, much, longer (because the kernel normally has to do I/O to complete the page-in). For programs sensitive to that, the functions described in section Locking Pages can control it.
Within each virtual address space, a process has to keep track of what is at which addresses, and that process is called memory allocation. Allocation usually brings to mind meting out scarce resources, but in the case of virtual memory, that's not a major goal, because there is generally much more of it than anyone needs. Memory allocation within a process is mainly just a matter of making sure that the same byte of memory isn't used to store two different things.
Processes allocate memory in two major ways: by exec and programmatically. Actually, forking is a third way, but it's not very interesting. See section Creating a Process.
Exec is the operation of creating a virtual address space for a process,
loading its basic program into it, and executing the program. It is
done by the "exec" family of functions (e.g. execl). The
operation takes a program file (an executable), it allocates space to
load all the data in the executable, loads it, and transfers control to
it. That data is most notably the instructions of the program (the
text), but also literals and constants in the program and even
some variables: C variables with the static storage class (see section Memory Allocation in C Programs).
Once that program begins to execute, it uses programmatic allocation to gain additional memory. In a C program with the GNU C library, there are two kinds of programmatic allocation: automatic and dynamic. See section Memory Allocation in C Programs.
Memory-mapped I/O is another form of dynamic virtual memory allocation. Mapping memory to a file means declaring that the contents of certain range of a process' addresses shall be identical to the contents of a specified regular file. The system makes the virtual memory initially contain the contents of the file, and if you modify the memory, the system writes the same modification to the file. Note that due to the magic of virtual memory and page faults, there is no reason for the system to do I/O to read the file, or allocate real memory for its contents, until the program accesses the virtual memory. See section Memory-mapped I/O.
Just as it programmatically allocates memory, the program can programmatically deallocate (free) it. You can't free the memory that was allocated by exec. When the program exits or execs, you might say that all its memory gets freed, but since in both cases the address space ceases to exist, the point is really moot. See section Program Termination.
A process' virtual address space is divided into segments. A segment is a contiguous range of virtual addresses. Three important segments are:
This section covers how ordinary programs manage storage for their data,
including the famous malloc function and some fancier facilities
special the GNU C library and GNU Compiler.
The C language supports two kinds of memory allocation through the variables in C programs:
A third important kind of memory allocation, dynamic allocation, is not supported by C variables but is available via GNU C library functions.
Dynamic memory allocation is a technique in which programs determine as they are running where to store some information. You need dynamic allocation when the amount of memory you need, or how long you continue to need it, depends on factors that are not known before the program runs.
For example, you may need a block to store a line read from an input file; since there is no limit to how long a line can be, you must allocate the memory dynamically and make it dynamically larger as you read more of the line.
Or, you may need a block for each record or each definition in the input data; since you can't know in advance how many there will be, you must allocate a new block for each record or definition as you read it.
When you use dynamic allocation, the allocation of a block of memory is an action that the program requests explicitly. You call a function or macro when you want to allocate space, and specify the size with an argument. If you want to free the space, you do so by calling another function or macro. You can do these things whenever you want, as often as you want.
Dynamic allocation is not supported by C variables; there is no storage class "dynamic", and there can never be a C variable whose value is stored in dynamically allocated space. The only way to get dynamically allocated memory is via a system call (which is generally via a GNU C library function call), and the only way to refer to dynamically allocated space is through a pointer. Because it is less convenient, and because the actual process of dynamic allocation requires more computation time, programmers generally use dynamic allocation only when neither static nor automatic allocation will serve.
For example, if you want to allocate dynamically some space to hold a
struct foobar, you cannot declare a variable of type struct
foobar whose contents are the dynamically allocated space. But you can
declare a variable of pointer type struct foobar * and assign it the
address of the space. Then you can use the operators `*' and
`->' on this pointer variable to refer to the contents of the space:
{
struct foobar *ptr
= (struct foobar *) malloc (sizeof (struct foobar));
ptr->name = x;
ptr->next = current_foobar;
current_foobar = ptr;
}
The most general dynamic allocation facility is malloc. It
allows you to allocate blocks of memory of any size at any time, make
them bigger or smaller at any time, and free the blocks individually at
any time (or never).
To allocate a block of memory, call malloc. The prototype for
this function is in `stdlib.h'.
The contents of the block are undefined; you must initialize it yourself
(or use calloc instead; see section Allocating Cleared Space).
Normally you would cast the value as a pointer to the kind of object
that you want to store in the block. Here we show an example of doing
so, and of initializing the space with zeros using the library function
memset (see section Copying and Concatenation):
struct foo *ptr; ... ptr = (struct foo *) malloc (sizeof (struct foo)); if (ptr == 0) abort (); memset (ptr, 0, sizeof (struct foo));
You can store the result of malloc into any pointer variable
without a cast, because ISO C automatically converts the type
void * to another type of pointer when necessary. But the cast
is necessary in contexts other than assignment operators or if you might
want your code to run in traditional C.
Remember that when allocating space for a string, the argument to
malloc must be one plus the length of the string. This is
because a string is terminated with a null character that doesn't count
in the "length" of the string but does need space. For example:
char *ptr; ... ptr = (char *) malloc (length + 1);
See section Representation of Strings, for more information about this.
malloc
If no more space is available, malloc returns a null pointer.
You should check the value of every call to malloc. It is
useful to write a subroutine that calls malloc and reports an
error if the value is a null pointer, returning only if the value is
nonzero. This function is conventionally called xmalloc. Here
it is:
void *
xmalloc (size_t size)
{
register void *value = malloc (size);
if (value == 0)
fatal ("virtual memory exhausted");
return value;
}
Here is a real example of using malloc (by way of xmalloc).
The function savestring will copy a sequence of characters into
a newly allocated null-terminated string:
char *
savestring (const char *ptr, size_t len)
{
register char *value = (char *) xmalloc (len + 1);
value[len] = '\0';
return (char *) memcpy (value, ptr, len);
}
The block that malloc gives you is guaranteed to be aligned so
that it can hold any type of data. In the GNU system, the address is
always a multiple of eight on most systems, and a multiple of 16 on
64-bit systems. Only rarely is any higher boundary (such as a page
boundary) necessary; for those cases, use memalign,
posix_memalign or valloc (see section Allocating Aligned Memory Blocks).
Note that the memory located after the end of the block is likely to be
in use for something else; perhaps a block already allocated by another
call to malloc. If you attempt to treat the block as longer than
you asked for it to be, you are liable to destroy the data that
malloc uses to keep track of its blocks, or you may destroy the
contents of another block. If you have already allocated a block and
discover you want it to be bigger, use realloc (see section Changing the Size of a Block).
malloc
When you no longer need a block that you got with malloc, use the
function free to make the block available to be allocated again.
The prototype for this function is in `stdlib.h'.
free function deallocates the block of memory pointed at
by ptr.
free. It's provided for
backward compatibility with SunOS; you should use free instead.
Freeing a block alters the contents of the block. Do not expect to find any data (such as a pointer to the next block in a chain of blocks) in the block after freeing it. Copy whatever you need out of the block before freeing it! Here is an example of the proper way to free all the blocks in a chain, and the strings that they point to:
struct chain
{
struct chain *next;
char *name;
}
void
free_chain (struct chain *chain)
{
while (chain != 0)
{
struct chain *next = chain->next;
free (chain->name);
free (chain);
chain = next;
}
}
Occasionally, free can actually return memory to the operating
system and make the process smaller. Usually, all it can do is allow a
later call to malloc to reuse the space. In the meantime, the
space remains in your program as part of a free-list used internally by
malloc.
There is no point in freeing blocks at the end of a program, because all of the program's space is given back to the system when the process terminates.
Often you do not know for certain how big a block you will ultimately need at the time you must begin to use the block. For example, the block might be a buffer that you use to hold a line being read from a file; no matter how long you make the buffer initially, you may encounter a line that is longer.
You can make the block longer by calling realloc. This function
is declared in `stdlib.h'.
realloc function changes the size of the block whose address is
ptr to be newsize.
Since the space after the end of the block may be in use, realloc
may find it necessary to copy the block to a new address where more free
space is available. The value of realloc is the new address of the
block. If the block needs to be moved, realloc copies the old
contents.
If you pass a null pointer for ptr, realloc behaves just
like `malloc (newsize)'. This can be convenient, but beware
that older implementations (before ISO C) may not support this
behavior, and will probably crash when realloc is passed a null
pointer.
Like malloc, realloc may return a null pointer if no
memory space is available to make the block bigger. When this happens,
the original block is untouched; it has not been modified or relocated.
In most cases it makes no difference what happens to the original block
when realloc fails, because the application program cannot continue
when it is out of memory, and the only thing to do is to give a fatal error
message. Often it is convenient to write and use a subroutine,
conventionally called xrealloc, that takes care of the error message
as xmalloc does for malloc:
void *
xrealloc (void *ptr, size_t size)
{
register void *value = realloc (ptr, size);
if (value == 0)
fatal ("Virtual memory exhausted");
return value;
}
You can also use realloc to make a block smaller. The reason you
would do this is to avoid tying up a lot of memory space when only a little
is needed.
In several allocation implementations, making a block smaller sometimes
necessitates copying it, so it can fail if no other space is available.
If the new size you specify is the same as the old size, realloc
is guaranteed to change nothing and return the same address that you gave.
The function calloc allocates memory and clears it to zero. It
is declared in `stdlib.h'.
calloc returns.
You could define calloc as follows:
void *
calloc (size_t count, size_t eltsize)
{
size_t size = count * eltsize;
void *value = malloc (size);
if (value != 0)
memset (value, 0, size);
return value;
}
But in general, it is not guaranteed that calloc calls
malloc internally. Therefore, if an application provides its own
malloc/realloc/free outside the C library, it
should always define calloc, too.
malloc
As opposed to other versions, the malloc in the GNU C Library
does not round up block sizes to powers of two, neither for large nor
for small sizes. Neighboring chunks can be coalesced on a free
no matter what their size is. This makes the implementation suitable
for all kinds of allocation patterns without generally incurring high
memory waste through fragmentation.
Very large blocks (much larger than a page) are allocated with
mmap (anonymous or via /dev/zero) by this implementation.
This has the great advantage that these chunks are returned to the
system immediately when they are freed. Therefore, it cannot happen
that a large chunk becomes "locked" in between smaller ones and even
after calling free wastes memory. The size threshold for
mmap to be used can be adjusted with mallopt. The use of
mmap can also be disabled completely.
The address of a block returned by malloc or realloc in
the GNU system is always a multiple of eight (or sixteen on 64-bit
systems). If you need a block whose address is a multiple of a higher
power of two than that, use memalign, posix_memalign, or
valloc. These functions are declared in `stdlib.h'.
With the GNU library, you can use free to free the blocks that
memalign, posix_memalign, and valloc return. That
does not work in BSD, however--BSD does not provide any way to free
such blocks.
memalign function allocates a block of size bytes whose
address is a multiple of boundary. The boundary must be a
power of two! The function memalign works by allocating a
somewhat larger block, and then returning an address within the block
that is on the specified boundary.
posix_memalign function is similar to the memalign
function in that it returns a buffer of size bytes aligned to a
multiple of alignment. But it adds one requirement to the
parameter alignment: the value must be a power of two multiple of
sizeof (void *).
If the function succeeds in allocation memory a pointer to the allocated
memory is returned in *memptr and the return value is zero.
Otherwise the function returns an error value indicating the problem.
This function was introduced in POSIX 1003.1d.
valloc is like using memalign and passing the page size
as the value of the second argument. It is implemented like this:
void *
valloc (size_t size)
{
return memalign (getpagesize (), size);
}
section How to get information about the memory subsystem? for more information about the memory subsystem.
You can adjust some parameters for dynamic memory allocation with the
mallopt function. This function is the general SVID/XPG
interface, defined in `malloc.h'.
mallopt, the param argument specifies the
parameter to be set, and value the new value to be set. Possible
choices for param, as defined in `malloc.h', are:
M_TRIM_THRESHOLD
sbrk to be called with a negative argument in
order to return memory to the system.
M_TOP_PAD
sbrk is required. It also specifies the
number of bytes to retain when shrinking the heap by calling sbrk
with a negative argument. This provides the necessary hysteresis in
heap size such that excessive amounts of system calls can be avoided.
M_MMAP_THRESHOLD
mmap system call. This way it is guaranteed
that the memory for these chunks can be returned to the system on
free.
M_MMAP_MAX
mmap. Setting this
to zero disables all use of mmap.
You can ask malloc to check the consistency of dynamic memory by
using the mcheck function. This function is a GNU extension,
declared in `mcheck.h'.
mcheck tells malloc to perform occasional
consistency checks. These will catch things such as writing
past the end of a block that was allocated with malloc.
The abortfn argument is the function to call when an inconsistency
is found. If you supply a null pointer, then mcheck uses a
default function which prints a message and calls abort
(see section Aborting a Program). The function you supply is called with
one argument, which says what sort of inconsistency was detected; its
type is described below.
It is too late to begin allocation checking once you have allocated
anything with malloc. So mcheck does nothing in that
case. The function returns -1 if you call it too late, and
0 otherwise (when it is successful).
The easiest way to arrange to call mcheck early enough is to use
the option `-lmcheck' when you link your program; then you don't
need to modify your program source at all. Alternatively you might use
a debugger to insert a call to mcheck whenever the program is
started, for example these gdb commands will automatically call mcheck
whenever the program starts:
(gdb) break main Breakpoint 1, main (argc=2, argv=0xbffff964) at whatever.c:10 (gdb) command 1 Type commands for when breakpoint 1 is hit, one per line. End with a line saying just "end". >call mcheck(0) >continue >end (gdb) ...
This will however only work if no initialization function of any object
involved calls any of the malloc functions since mcheck
must be called before the first such function.
mprobe function lets you explicitly check for inconsistencies
in a particular allocated block. You must have already called
mcheck at the beginning of the program, to do its occasional
checks; calling mprobe requests an additional consistency check
to be done at the time of the call.
The argument pointer must be a pointer returned by malloc
or realloc. mprobe returns a value that says what
inconsistency, if any, was found. The values are described below.
MCHECK_DISABLED
mcheck was not called before the first allocation.
No consistency checking can be done.
MCHECK_OK
MCHECK_HEAD
MCHECK_TAIL
MCHECK_FREE
Another possibility to check for and guard against bugs in the use of
malloc, realloc and free is to set the environment
variable MALLOC_CHECK_. When MALLOC_CHECK_ is set, a
special (less efficient) implementation is used which is designed to be
tolerant against simple errors, such as double calls of free with
the same argument, or overruns of a single byte (off-by-one bugs). Not
all such errors can be protected against, however, and memory leaks can
result. If MALLOC_CHECK_ is set to 0, any detected heap
corruption is silently ignored; if set to 1, a diagnostic is
printed on stderr; if set to 2, abort is called
immediately. This can be useful because otherwise a crash may happen
much later, and the true cause for the problem is then very hard to
track down.
There is one problem with MALLOC_CHECK_: in SUID or SGID binaries
it could possibly be exploited since diverging from the normal programs
behaviour it now writes something to the standard error desriptor.
Therefore the use of MALLOC_CHECK_ is disabled by default for
SUID and SGID binaries. It can be enabled again by the system
administrator by adding a file `/etc/suid-debug' (the content is
not important it could be empty).
So, what's the difference between using MALLOC_CHECK_ and linking
with `-lmcheck'? MALLOC_CHECK_ is orthogonal with respect to
`-lmcheck'. `-lmcheck' has been added for backward
compatibility. Both MALLOC_CHECK_ and `-lmcheck' should
uncover the same bugs - but using MALLOC_CHECK_ you don't need to
recompile your application.
The GNU C library lets you modify the behavior of malloc,
realloc, and free by specifying appropriate hook
functions. You can use these hooks to help you debug programs that use
dynamic memory allocation, for example.
The hook variables are declared in `malloc.h'.
malloc uses whenever it is called. You should define this
function to look like malloc; that is, like:
void *function (size_t size, const void *caller)
The value of caller is the return address found on the stack when
the malloc function was called. This value allows you to trace
the memory consumption of the program.
realloc
uses whenever it is called. You should define this function to look
like realloc; that is, like:
void *function (void *ptr, size_t size, const void *caller)
The value of caller is the return address found on the stack when
the realloc function was called. This value allows you to trace the
memory consumption of the program.
free
uses whenever it is called. You should define this function to look
like free; that is, like:
void function (void *ptr, const void *caller)
The value of caller is the return address found on the stack when
the free function was called. This value allows you to trace the
memory consumption of the program.
memalign
uses whenever it is called. You should define this function to look
like memalign; that is, like:
void *function (size_t size, size_t alignment, const void *caller)
The value of caller is the return address found on the stack when
the memalign function was called. This value allows you to trace the
memory consumption of the program.
You must make sure that the function you install as a hook for one of these functions does not call that function recursively without restoring the old value of the hook first! Otherwise, your program will get stuck in an infinite recursion. Before calling the function recursively, one should make sure to restore all the hooks to their previous value. When coming back from the recursive call, all the hooks should be resaved since a hook might modify itself.
void (*__malloc_initialize_hook) (void) = my_init_hook;
An issue to look out for is the time at which the malloc hook functions
can be safely installed. If the hook functions call the malloc-related
functions recursively, it is necessary that malloc has already properly
initialized itself at the time when __malloc_hook etc. is
assigned to. On the other hand, if the hook functions provide a
complete malloc implementation of their own, it is vital that the hooks
are assigned to before the very first malloc call has
completed, because otherwise a chunk obtained from the ordinary,
un-hooked malloc may later be handed to __free_hook, for example.
In both cases, the problem can be solved by setting up the hooks from
within a user-defined function pointed to by
__malloc_initialize_hook---then the hooks will be set up safely
at the right time.
Here is an example showing how to use __malloc_hook and
__free_hook properly. It installs a function that prints out
information every time malloc or free is called. We just
assume here that realloc and memalign are not used in our
program.
/* Prototypes for __malloc_hook, __free_hook */
#include <malloc.h>
/* Prototypes for our hooks. */
static void *my_init_hook (void);
static void *my_malloc_hook (size_t, const void *);
static void my_free_hook (void*, const void *);
/* Override initializing hook from the C library. */
void (*__malloc_initialize_hook) (void) = my_init_hook;
static void
my_init_hook (void)
{
old_malloc_hook = __malloc_hook;
old_free_hook = __free_hook;
__malloc_hook = my_malloc_hook;
__free_hook = my_free_hook;
}
static void *
my_malloc_hook (size_t size, const void *caller)
{
void *result;
/* Restore all old hooks */
__malloc_hook = old_malloc_hook;
__free_hook = old_free_hook;
/* Call recursively */
result = malloc (size);
/* Save underlaying hooks */
old_malloc_hook = __malloc_hook;
old_free_hook = __free_hook;
/* printf might call malloc, so protect it too. */
printf ("malloc (%u) returns %p\n", (unsigned int) size, result);
/* Restore our own hooks */
__malloc_hook = my_malloc_hook;
__free_hook = my_free_hook;
return result;
}
static void *
my_free_hook (void *ptr, const void *caller)
{
/* Restore all old hooks */
__malloc_hook = old_malloc_hook;
__free_hook = old_free_hook;
/* Call recursively */
free (ptr);
/* Save underlaying hooks */
old_malloc_hook = __malloc_hook;
old_free_hook = __free_hook;
/* printf might call free, so protect it too. */
printf ("freed pointer %p\n", ptr);
/* Restore our own hooks */
__malloc_hook = my_malloc_hook;
__free_hook = my_free_hook;
}
main ()
{
...
}
The mcheck function (see section Heap Consistency Checking) works by
installing such hooks.
malloc
You can get information about dynamic memory allocation by calling the
mallinfo function. This function and its associated data type
are declared in `malloc.h'; they are an extension of the standard
SVID/XPG version.
int arena
sbrk by
malloc, in bytes.
int ordblks
malloc requests; see
section Efficiency Considerations for malloc.)
int smblks
int hblks
mmap.
int hblkhd
mmap, in bytes.
int usmblks
int fsmblks
int uordblks
malloc.
int fordblks
int keepcost
struct mallinfo.
malloc-Related Functions
Here is a summary of the functions that work with malloc:
void *malloc (size_t size)
void free (void *addr)
malloc. See section Freeing Memory Allocated with malloc.
void *realloc (void *addr, size_t size)
malloc larger or smaller,
possibly by copying it to a new location. See section Changing the Size of a Block.
void *calloc (size_t count, size_t eltsize)
malloc, and set its contents to zero. See section Allocating Cleared Space.
void *valloc (size_t size)
void *memalign (size_t size, size_t boundary)
int mallopt (int param, int value)
int mcheck (void (*abortfn) (void))
malloc to perform occasional consistency checks on
dynamically allocated memory, and to call abortfn when an
inconsistency is found. See section Heap Consistency Checking.
void *(*__malloc_hook) (size_t size, const void *caller)
malloc uses whenever it is called.
void *(*__realloc_hook) (void *ptr, size_t size, const void *caller)
realloc uses whenever it is called.
void (*__free_hook) (void *ptr, const void *caller)
free uses whenever it is called.
void (*__memalign_hook) (size_t size, size_t alignment, const void *caller)
memalign uses whenever it is called.
struct mallinfo mallinfo (void)
malloc.
A complicated task when programming with languages which do not use garbage collected dynamic memory allocation is to find memory leaks. Long running programs must assure that dynamically allocated objects are freed at the end of their lifetime. If this does not happen the system runs out of memory, sooner or later.
The malloc implementation in the GNU C library provides some
simple means to detect such leaks and obtain some information to find
the location. To do this the application must be started in a special
mode which is enabled by an environment variable. There are no speed
penalties for the program if the debugging mode is not enabled.
mtrace function is called it looks for an environment
variable named MALLOC_TRACE. This variable is supposed to
contain a valid file name. The user must have write access. If the
file already exists it is truncated. If the environment variable is not
set or it does not name a valid file which can be opened for writing
nothing is done. The behaviour of malloc etc. is not changed.
For obvious reasons this also happens if the application is installed
with the SUID or SGID bit set.
If the named file is successfully opened, mtrace installs special
handlers for the functions malloc, realloc, and
free (see section Memory Allocation Hooks). From then on, all uses of these
functions are traced and protocolled into the file. There is now of
course a speed penalty for all calls to the traced functions so tracing
should not be enabled during normal use.
This function is a GNU extension and generally not available on other systems. The prototype can be found in `mcheck.h'.
muntrace function can be called after mtrace was used
to enable tracing the malloc calls. If no (succesful) call of
mtrace was made muntrace does nothing.
Otherwise it deinstalls the handlers for malloc, realloc,
and free and then closes the protocol file. No calls are
protocolled anymore and the program runs again at full speed.
This function is a GNU extension and generally not available on other systems. The prototype can be found in `mcheck.h'.
Even though the tracing functionality does not influence the runtime
behaviour of the program it is not a good idea to call mtrace in
all programs. Just imagine that you debug a program using mtrace
and all other programs used in the debugging session also trace their
malloc calls. The output file would be the same for all programs
and thus is unusable. Therefore one should call mtrace only if
compiled for debugging. A program could therefore start like this:
#include <mcheck.h>
int
main (int argc, char *argv[])
{
#ifdef DEBUGGING
mtrace ();
#endif
...
}
This is all what is needed if you want to trace the calls during the
whole runtime of the program. Alternatively you can stop the tracing at
any time with a call to muntrace. It is even possible to restart
the tracing again with a new call to mtrace. But this can cause
unreliable results since there may be calls of the functions which are
not called. Please note that not only the application uses the traced
functions, also libraries (including the C library itself) use these
functions.
This last point is also why it is no good idea to call muntrace
before the program terminated. The libraries are informed about the
termination of the program only after the program returns from
main or calls exit and so cannot free the memory they use
before this time.
So the best thing one can do is to call mtrace as the very first
function in the program and never call muntrace. So the program
traces almost all uses of the malloc functions (except those
calls which are executed by constructors of the program or used
libraries).
You know the situation. The program is prepared for debugging and in all debugging sessions it runs well. But once it is started without debugging the error shows up. A typical example is a memory leak that becomes visible only when we turn off the debugging. If you foresee such situations you can still win. Simply use something equivalent to the following little program:
#include <mcheck.h>
#include <signal.h>
static void
enable (int sig)
{
mtrace ();
signal (SIGUSR1, enable);
}
static void
disable (int sig)
{
muntrace ();
signal (SIGUSR2, disable);
}
int
main (int argc, char *argv[])
{
...
signal (SIGUSR1, enable);
signal (SIGUSR2, disable);
...
}
I.e., the user can start the memory debugger any time s/he wants if the
program was started with MALLOC_TRACE set in the environment.
The output will of course not show the allocations which happened before
the first signal but if there is a memory leak this will show up
nevertheless.
If you take a look at the output it will look similar to this:
= Start [0x8048209] - 0x8064cc8 [0x8048209] - 0x8064ce0 [0x8048209] - 0x8064cf8 [0x80481eb] + 0x8064c48 0x14 [0x80481eb] + 0x8064c60 0x14 [0x80481eb] + 0x8064c78 0x14 [0x80481eb] + 0x8064c90 0x14 = End
What this all means is not really important since the trace file is not
meant to be read by a human. Therefore no attention is given to
readability. Instead there is a program which comes with the GNU C
library which interprets the traces and outputs a summary in an
user-friendly way. The program is called mtrace (it is in fact a
Perl script) and it takes one or two arguments. In any case the name of
the file with the trace output must be specified. If an optional
argument precedes the name of the trace file this must be the name of
the program which generated the trace.
drepper$ mtrace tst-mtrace log No memory leaks.
In this case the program tst-mtrace was run and it produced a
trace file `log'. The message printed by mtrace shows there
are no problems with the code, all allocated memory was freed
afterwards.
If we call mtrace on the example trace given above we would get a
different outout:
drepper$ mtrace errlog - 0x08064cc8 Free 2 was never alloc'd 0x8048209 - 0x08064ce0 Free 3 was never alloc'd 0x8048209 - 0x08064cf8 Free 4 was never alloc'd 0x8048209 Memory not freed: ----------------- Address Size Caller 0x08064c48 0x14 at 0x80481eb 0x08064c60 0x14 at 0x80481eb 0x08064c78 0x14 at 0x80481eb 0x08064c90 0x14 at 0x80481eb
We have called mtrace with only one argument and so the script
has no chance to find out what is meant with the addresses given in the
trace. We can do better:
drepper$ mtrace tst errlog - 0x08064cc8 Free 2 was never alloc'd /home/drepper/tst.c:39 - 0x08064ce0 Free 3 was never alloc'd /home/drepper/tst.c:39 - 0x08064cf8 Free 4 was never alloc'd /home/drepper/tst.c:39 Memory not freed: ----------------- Address Size Caller 0x08064c48 0x14 at /home/drepper/tst.c:33 0x08064c60 0x14 at /home/drepper/tst.c:33 0x08064c78 0x14 at /home/drepper/tst.c:33 0x08064c90 0x14 at /home/drepper/tst.c:33
Suddenly the output makes much more sense and the user can see immediately where the function calls causing the trouble can be found.
Interpreting this output is not complicated. There are at most two
different situations being detected. First, free was called for
pointers which were never returned by one of the allocation functions.
This is usually a very bad problem and what this looks like is shown in
the first three lines of the output. Situations like this are quite
rare and if they appear they show up very drastically: the program
normally crashes.
The other situation which is much harder to detect are memory leaks. As
you can see in the output the mtrace function collects all this
information and so can say that the program calls an allocation function
from line 33 in the source file `/home/drepper/tst-mtrace.c' four
times without freeing this memory before the program terminates.
Whether this is a real problem remains to be investigated.
An obstack is a pool of memory containing a stack of objects. You can create any number of separate obstacks, and then allocate objects in specified obstacks. Within each obstack, the last object allocated must always be the first one freed, but distinct obstacks are independent of each other.
Aside from this one constraint of order of freeing, obstacks are totally general: an obstack can contain any number of objects of any size. They are implemented with macros, so allocation is usually very fast as long as the objects are usually small. And the only space overhead per object is the padding needed to start each object on a suitable boundary.
The utilities for manipulating obstacks are declared in the header file `obstack.h'.
struct
obstack. This structure has a small fixed size; it records the status
of the obstack and how to find the space in which objects are allocated.
It does not contain any of the objects themselves. You should not try
to access the contents of the structure directly; use only the functions
described in this chapter.
You can declare variables of type struct obstack and use them as
obstacks, or you can allocate obstacks dynamically like any other kind
of object. Dynamic allocation of obstacks allows your program to have a
variable number of different stacks. (You can even allocate an
obstack structure in another obstack, but this is rarely useful.)
All the functions that work with obstacks require you to specify which
obstack to use. You do this with a pointer of type struct obstack
*. In the following, we often say "an obstack" when strictly
speaking the object at hand is such a pointer.
The objects in the obstack are packed into large blocks called
chunks. The struct obstack structure points to a chain of
the chunks currently in use.
The obstack library obtains a new chunk whenever you allocate an object
that won't fit in the previous chunk. Since the obstack library manages
chunks automatically, you don't need to pay much attention to them, but
you do need to supply a function which the obstack library should use to
get a chunk. Usually you supply a function which uses malloc
directly or indirectly. You must also supply a function to free a chunk.
These matters are described in the following section.
Each source file in which you plan to use the obstack functions must include the header file `obstack.h', like this:
#include <obstack.h>
Also, if the source file uses the macro obstack_init, it must
declare or define two functions or macros that will be called by the
obstack library. One, obstack_chunk_alloc, is used to allocate
the chunks of memory into which objects are packed. The other,
obstack_chunk_free, is used to return chunks when the objects in
them are freed. These macros should appear before any use of obstacks
in the source file.
Usually these are defined to use malloc via the intermediary
xmalloc (see section Unconstrained Allocation). This is done with
the following pair of macro definitions:
#define obstack_chunk_alloc xmalloc #define obstack_chunk_free free
Though the memory you get using obstacks really comes from malloc,
using obstacks is faster because malloc is called less often, for
larger blocks of memory. See section Obstack Chunks, for full details.
At run time, before the program can use a struct obstack object
as an obstack, it must initialize the obstack by calling
obstack_init.
obstack_chunk_alloc function. If
allocation of memory fails, the function pointed to by
obstack_alloc_failed_handler is called. The obstack_init
function always returns 1 (Compatibility notice: Former versions of
obstack returned 0 if allocation failed).
Here are two examples of how to allocate the space for an obstack and initialize it. First, an obstack that is a static variable:
static struct obstack myobstack; ... obstack_init (&myobstack);
Second, an obstack that is itself dynamically allocated:
struct obstack *myobstack_ptr = (struct obstack *) xmalloc (sizeof (struct obstack)); obstack_init (myobstack_ptr);
obstack uses when obstack_chunk_alloc fails to allocate
memory. The default action is to print a message and abort.
You should supply a function that either calls exit
(see section Program Termination) or longjmp (see section Non-Local Exits) and doesn't return.
void my_obstack_alloc_failed (void) ... obstack_alloc_failed_handler = &my_obstack_alloc_failed;
The most direct way to allocate an object in an obstack is with
obstack_alloc, which is invoked almost like malloc.
struct obstack
object which represents the obstack. Each obstack function or macro
requires you to specify an obstack-ptr as the first argument.
This function calls the obstack's obstack_chunk_alloc function if
it needs to allocate a new chunk of memory; it calls
obstack_alloc_failed_handler if allocation of memory by
obstack_chunk_alloc failed.
For example, here is a function that allocates a copy of a string str
in a specific obstack, which is in the variable string_obstack:
struct obstack string_obstack;
char *
copystring (char *string)
{
size_t len = strlen (string) + 1;
char *s = (char *) obstack_alloc (&string_obstack, len);
memcpy (s, string, len);
return s;
}
To allocate a block with specified contents, use the function
obstack_copy, declared like this:
obstack_alloc_failed_handler if allocation of memory by
obstack_chunk_alloc failed.
obstack_copy, but appends an extra byte containing a null
character. This extra byte is not counted in the argument size.
The obstack_copy0 function is convenient for copying a sequence
of characters into an obstack as a null-terminated string. Here is an
example of its use:
char *
obstack_savestring (char *addr, int size)
{
return obstack_copy0 (&myobstack, addr, size);
}
Contrast this with the previous example of savestring using
malloc (see section Basic Memory Allocation).
To free an object allocated in an obstack, use the function
obstack_free. Since the obstack is a stack of objects, freeing
one object automatically frees all other objects allocated more recently
in the same obstack.
Note that if object is a null pointer, the result is an
uninitialized obstack. To free all memory in an obstack but leave it
valid for further allocation, call obstack_free with the address
of the first object allocated on the obstack:
obstack_free (obstack_ptr, first_object_allocated_ptr);
Recall that the objects in an obstack are grouped into chunks. When all the objects in a chunk become free, the obstack library automatically frees the chunk (see section Preparing for Using Obstacks). Then other obstacks, or non-obstack allocation, can reuse the space of the chunk.
The interfaces for using obstacks may be defined either as functions or as macros, depending on the compiler. The obstack facility works with all C compilers, including both ISO C and traditional C, but there are precautions you must take if you plan to use compilers other than GNU C.
If you are using an old-fashioned non-ISO C compiler, all the obstack "functions" are actually defined only as macros. You can call these macros like functions, but you cannot use them in any other way (for example, you cannot take their address).
Calling the macros requires a special precaution: namely, the first operand (the obstack pointer) may not contain any side effects, because it may be computed more than once. For example, if you write this:
obstack_alloc (get_obstack (), 4);
you will find that get_obstack may be called several times.
If you use *obstack_list_ptr++ as the obstack pointer argument,
you will get very strange results since the incrementation may occur
several times.
In ISO C, each function has both a macro definition and a function definition. The function definition is used if you take the address of the function without calling it. An ordinary call uses the macro definition by default, but you can request the function definition instead by writing the function name in parentheses, as shown here:
char *x; void *(*funcp) (); /* Use the macro. */ x = (char *) obstack_alloc (obptr, size); /* Call the function. */ x = (char *) (obstack_alloc) (obptr, size); /* Take the address of the function. */ funcp = obstack_alloc;
This is the same situation that exists in ISO C for the standard library functions. See section Macro Definitions of Functions.
Warning: When you do use the macros, you must observe the precaution of avoiding side effects in the first operand, even in ISO C.
If you use the GNU C compiler, this precaution is not necessary, because various language extensions in GNU C permit defining the macros so as to compute each argument only once.
Because memory in obstack chunks is used sequentially, it is possible to build up an object step by step, adding one or more bytes at a time to the end of the object. With this technique, you do not need to know how much data you will put in the object until you come to the end of it. We call this the technique of growing objects. The special functions for adding data to the growing object are described in this section.
You don't need to do anything special when you start to grow an object.
Using one of the functions to add data to the object automatically
starts it. However, it is necessary to say explicitly when the object is
finished. This is done with the function obstack_finish.
The actual address of the object thus built up is not known until the object is finished. Until then, it always remains possible that you will add so much data that the object must be copied into a new chunk.
While the obstack is in use for a growing object, you cannot use it for ordinary allocation of another object. If you try to do so, the space already added to the growing object will become part of the other object.
obstack_blank, which adds space without initializing it.
obstack_grow, which is
the growing-object analogue of obstack_copy. It adds size
bytes of data to the growing object, copying the contents from
data.
obstack_copy0. It adds
size bytes copied from data, followed by an additional null
character.
obstack_1grow.
It adds a single byte containing c to the growing object.
obstack_ptr_grow. It adds sizeof (void *) bytes
containing the value of data.
int can be added by using the
obstack_int_grow function. It adds sizeof (int) bytes to
the growing object and initializes them with the value of data.
obstack_finish to close it off and return its final address.
Once you have finished the object, the obstack is available for ordinary allocation or for growing another object.
This function can return a null pointer under the same conditions as
obstack_alloc (see section Allocation in an Obstack).
When you build an object by growing it, you will probably need to know
afterward how long it became. You need not keep track of this as you grow
the object, because you can find out the length from the obstack just
before finishing the object with the function obstack_object_size,
declared as follows:
obstack_object_size will return zero.
If you have started growing an object and wish to cancel it, you should finish it and then free it, like this:
obstack_free (obstack_ptr, obstack_finish (obstack_ptr));
This has no effect if no object was growing.
You can use obstack_blank with a negative size argument to make
the current object smaller. Just don't try to shrink it beyond zero
length--there's no telling what will happen if you do that.
The usual functions for growing objects incur overhead for checking whether there is room for the new growth in the current chunk. If you are frequently constructing objects in small steps of growth, this overhead can be significant.
You can reduce the overhead by using special "fast growth" functions that grow the object without checking. In order to have a robust program, you must do the checking yourself. If you do this checking in the simplest way each time you are about to add data to the object, you have not saved anything, because that is what the ordinary growth functions do. But if you can arrange to check less often, or check more efficiently, then you make the program faster.
The function obstack_room returns the amount of room available
in the current chunk. It is declared as follows:
While you know there is room, you can use these fast growth functions for adding data to a growing object:
obstack_1grow_fast adds one byte containing the
character c to the growing object in obstack obstack-ptr.
obstack_ptr_grow_fast adds sizeof (void *)
bytes containing the value of data to the growing object in
obstack obstack-ptr.
obstack_int_grow_fast adds sizeof (int) bytes
containing the value of data to the growing object in obstack
obstack-ptr.
obstack_blank_fast adds size bytes to the
growing object in obstack obstack-ptr without initializing them.
When you check for space using obstack_room and there is not
enough room for what you want to add, the fast growth functions
are not safe. In this case, simply use the corresponding ordinary
growth function instead. Very soon this will copy the object to a
new chunk; then there will be lots of room available again.
So, each time you use an ordinary growth function, check afterward for
sufficient space using obstack_room. Once the object is copied
to a new chunk, there will be plenty of space again, so the program will
start using the fast growth functions again.
Here is an example:
void
add_string (struct obstack *obstack, const char *ptr, int len)
{
while (len > 0)
{
int room = obstack_room (obstack);
if (room == 0)
{
/* Not enough room. Add one character slowly,
which may copy to a new chunk and make room. */
obstack_1grow (obstack, *ptr++);
len--;
}
else
{
if (room > len)
room = len;
/* Add fast as much as we have room for. */
len -= room;
while (room-- > 0)
obstack_1grow_fast (obstack, *ptr++);
}
}
}
Here are functions that provide information on the current status of allocation in an obstack. You can use them to learn about an object while still growing it.
If no object is growing, this value says where the next object you allocate will start (once again assuming it fits in the current chunk).
obstack_next_free
returns the same value as obstack_base.
obstack_next_free (obstack-ptr) - obstack_base (obstack-ptr)
Each obstack has an alignment boundary; each object allocated in the obstack automatically starts on an address that is a multiple of the specified boundary. By default, this boundary is 4 bytes.
To access an obstack's alignment boundary, use the macro
obstack_alignment_mask, whose function prototype looks like
this:
The expansion of the macro obstack_alignment_mask is an lvalue,
so you can alter the mask by assignment. For example, this statement:
obstack_alignment_mask (obstack_ptr) = 0;
has the effect of turning off alignment processing in the specified obstack.
Note that a change in alignment mask does not take effect until
after the next time an object is allocated or finished in the
obstack. If you are not growing an object, you can make the new
alignment mask take effect immediately by calling obstack_finish.
This will finish a zero-length object and then do proper alignment for
the next object.
Obstacks work by allocating space for themselves in large chunks, and then parceling out space in the chunks to satisfy your requests. Chunks are normally 4096 bytes long unless you specify a different chunk size. The chunk size includes 8 bytes of overhead that are not actually used for storing objects. Regardless of the specified size, longer chunks will be allocated when necessary for long objects.
The obstack library allocates chunks by calling the function
obstack_chunk_alloc, which you must define. When a chunk is no
longer needed because you have freed all the objects in it, the obstack
library frees the chunk by calling obstack_chunk_free, which you
must also define.
These two must be defined (as macros) or declared (as functions) in each
source file that uses obstack_init (see section Creating Obstacks).
Most often they are defined as macros like this:
#define obstack_chunk_alloc malloc #define obstack_chunk_free free
Note that these are simple macros (no arguments). Macro definitions with
arguments will not work! It is necessary that obstack_chunk_alloc
or obstack_chunk_free, alone, expand into a function name if it is
not itself a function name.
If you allocate chunks with malloc, the chunk size should be a
power of 2. The default chunk size, 4096, was chosen because it is long
enough to satisfy many typical requests on the obstack yet short enough
not to waste too much memory in the portion of the last chunk not yet used.
Since this macro expands to an lvalue, you can specify a new chunk size by assigning it a new value. Doing so does not affect the chunks already allocated, but will change the size of chunks allocated for that particular obstack in the future. It is unlikely to be useful to make the chunk size smaller, but making it larger might improve efficiency if you are allocating many objects whose size is comparable to the chunk size. Here is how to do so cleanly:
if (obstack_chunk_size (obstack_ptr) < new-chunk-size) obstack_chunk_size (obstack_ptr) = new-chunk-size;
Here is a summary of all the functions associated with obstacks. Each
takes the address of an obstack (struct obstack *) as its first
argument.
void obstack_init (struct obstack *obstack-ptr)
void *obstack_alloc (struct obstack *obstack-ptr, int size)
void *obstack_copy (struct obstack *obstack-ptr, void *address, int size)
void *obstack_copy0 (struct obstack *obstack-ptr, void *address, int size)
void obstack_free (struct obstack *obstack-ptr, void *object)
void obstack_blank (struct obstack *obstack-ptr, int size)
void obstack_grow (struct obstack *obstack-ptr, void *address, int size)
void obstack_grow0 (struct obstack *obstack-ptr, void *address, int size)
void obstack_1grow (struct obstack *obstack-ptr, char data-char)
void *obstack_finish (struct obstack *obstack-ptr)
int obstack_object_size (struct obstack *obstack-ptr)
void obstack_blank_fast (struct obstack *obstack-ptr, int size)
void obstack_1grow_fast (struct obstack *obstack-ptr, char data-char)
int obstack_room (struct obstack *obstack-ptr)
int obstack_alignment_mask (struct obstack *obstack-ptr)
int obstack_chunk_size (struct obstack *obstack-ptr)
void *obstack_base (struct obstack *obstack-ptr)
void *obstack_next_free (struct obstack *obstack-ptr)
The function alloca supports a kind of half-dynamic allocation in
which blocks are allocated dynamically but freed automatically.
Allocating a block with alloca is an explicit action; you can
allocate as many blocks as you wish, and compute the size at run time. But
all the blocks are freed when you exit the function that alloca was
called from, just as if they were automatic variables declared in that
function. There is no way to free the space explicitly.
The prototype for alloca is in `stdlib.h'. This function is
a BSD extension.
alloca is the address of a block of size
bytes of memory, allocated in the stack frame of the calling function.
Do not use alloca inside the arguments of a function call--you
will get unpredictable results, because the stack space for the
alloca would appear on the stack in the middle of the space for
the function arguments. An example of what to avoid is foo (x,
alloca (4), y).
alloca Example
As an example of the use of alloca, here is a function that opens
a file name made from concatenating two argument strings, and returns a
file descriptor or minus one signifying failure:
int
open2 (char *str1, char *str2, int flags, int mode)
{
char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
stpcpy (stpcpy (name, str1), str2);
return open (name, flags, mode);
}
Here is how you would get the same results with malloc and
free:
int
open2 (char *str1, char *str2, int flags, int mode)
{
char *name = (char *) malloc (strlen (str1) + strlen (str2) + 1);
int desc;
if (name == 0)
fatal ("virtual memory exceeded");
stpcpy (stpcpy (name, str1), str2);
desc = open (name, flags, mode);
free (name);
return desc;
}
As you can see, it is simpler with alloca. But alloca has
other, more important advantages, and some disadvantages.
alloca
Here are the reasons why alloca may be preferable to malloc:
alloca wastes very little space and is very fast. (It is
open-coded by the GNU C compiler.)
alloca does not have separate pools for different sizes of
block, space used for any size block can be reused for any other size.
alloca does not cause memory fragmentation.
longjmp (see section Non-Local Exits)
automatically free the space allocated with alloca when they exit
through the function that called alloca. This is the most
important reason to use alloca.
To illustrate this, suppose you have a function
open_or_report_error which returns a descriptor, like
open, if it succeeds, but does not return to its caller if it
fails. If the file cannot be opened, it prints an error message and
jumps out to the command level of your program using longjmp.
Let's change open2 (see section alloca Example) to use this
subroutine:
int
open2 (char *str1, char *str2, int flags, int mode)
{
char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
stpcpy (stpcpy (name, str1), str2);
return open_or_report_error (name, flags, mode);
}
Because of the way alloca works, the memory it allocates is
freed even when an error occurs, with no special effort required.
By contrast, the previous definition of open2 (which uses
malloc and free) would develop a memory leak if it were
changed in this way. Even if you are willing to make more changes to
fix it, there is no easy way to do so.
alloca
These are the disadvantages of alloca in comparison with
malloc:
alloca, so it is less
portable. However, a slower emulation of alloca written in C
is available for use on systems with this deficiency.
In GNU C, you can replace most uses of alloca with an array of
variable size. Here is how open2 would look then:
int open2 (char *str1, char *str2, int flags, int mode)
{
char name[strlen (str1) + strlen (str2) + 1];
stpcpy (stpcpy (name, str1), str2);
return open (name, flags, mode);
}
But alloca is not always equivalent to a variable-sized array, for
several reasons:
alloca
remains until the end of the function.
alloca within a loop, allocating an
additional block on each iteration. This is impossible with
variable-sized arrays.
Note: If you mix use of alloca and variable-sized arrays
within one function, exiting a scope in which a variable-sized array was
declared frees all blocks allocated with alloca during the
execution of that scope.
The symbols in this section are declared in `unistd.h'.
You will not normally use the functions in this section, because the functions described in section Allocating Storage For Program Data are easier to use. Those are interfaces to a GNU C Library memory allocator that uses the functions below itself. The functions below are simple interfaces to system calls.
brk sets the high end of the calling process' data segment to
addr.
The address of the end of a segment is defined to be the address of the last byte in the segment plus 1.
The function has no effect if addr is lower than the low end of the data segment. (This is considered success, by the way).
The function fails if it would cause the data segment to overlap another segment or exceed the process' data storage limit (see section Limiting Resource Usage).
The function is named for a common historical case where data storage and the stack are in the same segment. Data storage allocation grows upward from the bottom of the segment while the stack grows downward toward it from the top of the segment and the curtain between them is called the break.
The return value is zero on success. On failure, the return value is
-1 and errno is set accordingly. The following errno
values are specific to this function:
ENOMEM
brk except that you specify the new
end of the data segment as an offset delta from the current end
and on success the return value is the address of the resulting end of
the data segment instead of zero.
This means you can use `sbrk(0)' to find out what the current end of the data segment is.
You can tell the system to associate a particular virtual memory page with a real page frame and keep it that way -- i.e. cause the page to be paged in if it isn't already and mark it so it will never be paged out and consequently will never cause a page fault. This is called locking a page.
The functions in this chapter lock and unlock the calling process' pages.
Because page faults cause paged out pages to be paged in transparently, a process rarely needs to be concerned about locking pages. However, there are two reasons people sometimes are:
Be aware that when you lock a page, that's one fewer page frame that can be used to back other virtual memory (by the same or other processes), which can mean more page faults, which means the system runs more slowly. In fact, if you lock enough memory, some programs may not be able to run at all for lack of real memory.
A memory lock is associated with a virtual page, not a real frame. The paging rule is: If a frame backs at least one locked page, don't page it out.
Memory locks do not stack. I.e. you can't lock a particular page twice so that it has to be unlocked twice before it is truly unlocked. It is either locked or it isn't.
A memory lock persists until the process that owns the memory explicitly unlocks it. (But process termination and exec cause the virtual memory to cease to exist, which you might say means it isn't locked any more).
Memory locks are not inherited by child processes. (But note that on a modern Unix system, immediately after a fork, the parent's and the child's virtual address space are backed by the same real page frames, so the child enjoys the parent's locks). See section Creating a Process.
Because of its ability to impact other processes, only the superuser can lock a page. Any process can unlock its own page.
The system sets limits on the amount of memory a process can have locked and the amount of real memory it can have dedicated to it. See section Limiting Resource Usage.
In Linux, locked pages aren't as locked as you might think. Two virtual pages that are not shared memory can nonetheless be backed by the same real frame. The kernel does this in the name of efficiency when it knows both virtual pages contain identical data, and does it even if one or both of the virtual pages are locked.
But when a process modifies one of those pages, the kernel must get it a separate frame and fill it with the page's data. This is known as a copy-on-write page fault. It takes a small amount of time and in a pathological case, getting that frame may require I/O.
To make sure this doesn't happen to your program, don't just lock the pages. Write to them as well, unless you know you won't write to them ever. And to make sure you have pre-allocated frames for your stack, enter a scope that declares a C automatic variable larger than the maximum stack size you will need, set it to something, then return from its scope.
The symbols in this section are declared in `sys/mman.h'. These functions are defined by POSIX.1b, but their availability depends on your kernel. If your kernel doesn't allow these functions, they exist but always fail. They are available with a Linux kernel.
Portability Note: POSIX.1b requires that when the mlock
and munlock functions are available, the file `unistd.h'
define the macro _POSIX_MEMLOCK_RANGE and the file
limits.h define the macro PAGESIZE to be the size of a
memory page in bytes. It requires that when the mlockall and
munlockall functions are available, the `unistd.h' file
define the macro _POSIX_MEMLOCK. The GNU C library conforms to
this requirement.
mlock locks a range of the calling process' virtual pages.
The range of memory starts at address addr and is len bytes long. Actually, since you must lock whole pages, it is the range of pages that include any part of the specified range.
When the function returns successfully, each of those pages is backed by (connected to) a real frame (is resident) and is marked to stay that way. This means the function may cause page-ins and have to wait for them.
When the function fails, it does not affect the lock status of any pages.
The return value is zero if the function succeeds. Otherwise, it is
-1 and errno is set accordingly. errno values
specific to this function are:
ENOMEM
EPERM
EINVAL
ENOSYS
mlock capability.
You can lock all a process' memory with mlockall. You
unlock memory with munlock or munlockall.
To avoid all page faults in a C program, you have to use
mlockall, because some of the memory a program uses is hidden
from the C code, e.g. the stack and automatic variables, and you
wouldn't know what address to tell mlock.
mlock unlocks a range of the calling process' virtual pages.
munlock is the inverse of mlock and functions completely
analogously to mlock, except that there is no EPERM
failure.
mlockall locks all the pages in a process' virtual memory address
space, and/or any that are added to it in the future. This includes the
pages of the code, data and stack segment, as well as shared libraries,
user space kernel data, shared memory, and memory mapped files.
flags is a string of single bit flags represented by the following
macros. They tell mlockall which of its functions you want. All
other bits must be zero.
MCL_CURRENT
MCL_FUTURE
MCL_FUTURE.
See section Executing a File.
When the function returns successfully, and you specified
MCL_CURRENT, all of the process' pages are backed by (connected
to) real frames (they are resident) and are marked to stay that way.
This means the function may cause page-ins and have to wait for them.
When the process is in MCL_FUTURE mode because it successfully
executed this function and specified MCL_CURRENT, any system call
by the process that requires space be added to its virtual address space
fails with errno = ENOMEM if locking the additional space
would cause the process to exceed its locked page limit. In the case
that the address space addition that can't be accomodated is stack
expansion, the stack expansion fails and the kernel sends a
SIGSEGV signal to the process.
When the function fails, it does not affect the lock status of any pages or the future locking mode.
The return value is zero if the function succeeds. Otherwise, it is
-1 and errno is set accordingly. errno values
specific to this function are:
ENOMEM
EPERM
EINVAL
ENOSYS
mlockall capability.
You can lock just specific pages with mlock. You unlock pages
with munlockall and munlock.
munlockall unlocks every page in the calling process' virtual
address space and turn off MCL_FUTURE future locking mode.
The return value is zero if the function succeeds. Otherwise, it is
-1 and errno is set accordingly. The only way this
function can fail is for generic reasons that all functions and system
calls can fail, so there are no specific errno values.
Programs that work with characters and strings often need to classify a character--is it alphabetic, is it a digit, is it whitespace, and so on--and perform case conversion operations on characters. The functions in the header file `ctype.h' are provided for this purpose.
Since the choice of locale and character set can alter the
classifications of particular character codes, all of these functions
are affected by the current locale. (More precisely, they are affected
by the locale currently selected for character classification--the
LC_CTYPE category; see section Categories of Activities that Locales Affect.)
The ISO C standard specifies two different sets of functions. The
one set works on char type characters, the other one on
wchar_t wide characters (see section Introduction to Extended Characters).
This section explains the library functions for classifying characters.
For example, isalpha is the function to test for an alphabetic
character. It takes one argument, the character to test, and returns a
nonzero integer if the character is alphabetic, and zero otherwise. You
would use it like this:
if (isalpha (c))
printf ("The character `%c' is alphabetic.\n", c);
Each of the functions in this section tests for membership in a
particular class of characters; each has a name starting with `is'.
Each of them takes one argument, which is a character to test, and
returns an int which is treated as a boolean value. The
character argument is passed as an int, and it may be the
constant value EOF instead of a real character.
The attributes of any given character can vary between locales. See section Locales and Internationalization, for more information on locales.
These functions are declared in the header file `ctype.h'.
islower or isupper is true of a character, then
isalpha is also true.
In some locales, there may be additional characters for which
isalpha is true--letters which are neither upper case nor lower
case. But in the standard "C" locale, there are no such
additional characters.
isalpha or isdigit is
true of a character, then isalnum is also true.
"C" locale, isspace returns true for only the standard
whitespace characters:
' '
'\f'
'\n'
'\r'
'\t'
'\v'
unsigned char value that fits
into the US/UK ASCII character set. This function is a BSD extension
and is also an SVID extension.
This section explains the library functions for performing conversions
such as case mappings on characters. For example, toupper
converts any character to upper case if possible. If the character
can't be converted, toupper returns it unchanged.
These functions take one argument of type int, which is the
character to convert, and return the converted character as an
int. If the conversion is not applicable to the argument given,
the argument is returned unchanged.
Compatibility Note: In pre-ISO C dialects, instead of
returning the argument unchanged, these functions may fail when the
argument is not suitable for the conversion. Thus for portability, you
may need to write islower(c) ? toupper(c) : c rather than just
toupper(c).
These functions are declared in the header file `ctype.h'.
tolower returns the corresponding
lower-case letter. If c is not an upper-case letter,
c is returned unchanged.
toupper returns the corresponding
upper-case letter. Otherwise c is returned unchanged.
unsigned char value
that fits into the US/UK ASCII character set, by clearing the high-order
bits. This function is a BSD extension and is also an SVID extension.
tolower, and is provided for compatibility
with the SVID. See section SVID (The System V Interface Description).
toupper, and is provided for compatibility
with the SVID.
Amendment 1 to ISO C90 defines functions to classify wide
characters. Although the original ISO C90 standard already defined
the type wchar_t, no functions operating on them were defined.
The general design of the classification functions for wide characters
is more general. It allows extensions to the set of available
classifications, beyond those which are always available. The POSIX
standard specifies how extensions can be made, and this is already
implemented in the GNU C library implementation of the localedef
program.
The character class functions are normally implemented with bitsets, with a bitset per character. For a given character, the appropriate bitset is read from a table and a test is performed as to whether a certain bit is set. Which bit is tested for is determined by the class.
For the wide character classification functions this is made visible.
There is a type classification type defined, a function to retrieve this
value for a given class, and a function to test whether a given
character is in this class, using the classification value. On top of
this the normal character classification functions as used for
char objects can be defined.
wctype_t can hold a value which represents a character class.
The only defined way to generate such a value is by using the
wctype function.
wctype returns a value representing a class of wide
characters which is identified by the string property. Beside
some standard properties each locale can define its own ones. In case
no property with the given name is known for the current locale
selected for the LC_CTYPE category, the function returns zero.
The properties known in every locale are:
@multitable @columnfractions .25 .25 .25 .25
"alnum" @tab "alpha" @tab "cntrl" @tab "digit"
"graph" @tab "lower" @tab "print" @tab "punct"
"space" @tab "upper" @tab "xdigit"
This function is declared in `wctype.h'.
wctype.
This function is declared in `wctype.h'.
wctype if the property string is one of the known character
classes. In some situations it is desirable to construct the property
strings, and then it is important that wctype can also handle the
standard classes.
iswalpha
or iswdigit is true of a character, then iswalnum is also
true.
This function can be implemented using
iswctype (wc, wctype ("alnum"))
It is declared in `wctype.h'.
iswlower or iswupper is true of a character, then
iswalpha is also true.
In some locales, there may be additional characters for which
iswalpha is true--letters which are neither upper case nor lower
case. But in the standard "C" locale, there are no such
additional characters.
This function can be implemented using
iswctype (wc, wctype ("alpha"))
It is declared in `wctype.h'.
iswctype (wc, wctype ("cntrl"))
It is declared in `wctype.h'.
n = 0;
while (iswdigit (*wc))
{
n *= 10;
n += *wc++ - L'0';
}
This function can be implemented using
iswctype (wc, wctype ("digit"))
It is declared in `wctype.h'.
iswctype (wc, wctype ("graph"))
It is declared in `wctype.h'.
iswctype (wc, wctype ("lower"))
It is declared in `wctype.h'.
iswctype (wc, wctype ("print"))
It is declared in `wctype.h'.
iswctype (wc, wctype ("punct"))
It is declared in `wctype.h'.
"C" locale, iswspace returns true for only the standard
whitespace characters:
L' '
L'\f'
L'\n'
L'\r'
L'\t'
L'\v'
iswctype (wc, wctype ("space"))
It is declared in `wctype.h'.
iswctype (wc, wctype ("upper"))
It is declared in `wctype.h'.
iswctype (wc, wctype ("xdigit"))
It is declared in `wctype.h'.
The first note is probably not astonishing but still occasionally a
cause of problems. The iswXXX functions can be implemented
using macros and in fact, the GNU C library does this. They are still
available as real functions but when the `wctype.h' header is
included the macros will be used. This is the same as the
char type versions of these functions.
The second note covers something new. It can be best illustrated by a (real-world) example. The first piece of code is an excerpt from the original code. It is truncated a bit but the intention should be clear.
int
is_in_class (int c, const char *class)
{
if (strcmp (class, "alnum") == 0)
return isalnum (c);
if (strcmp (class, "alpha") == 0)
return isalpha (c);
if (strcmp (class, "cntrl") == 0)
return iscntrl (c);
...
return 0;
}
Now, with the wctype and iswctype you can avoid the
if cascades, but rewriting the code as follows is wrong:
int
is_in_class (int c, const char *class)
{
wctype_t desc = wctype (class);
return desc ? iswctype ((wint_t) c, desc) : 0;
}
The problem is that it is not guaranteed that the wide character representation of a single-byte character can be found using casting. In fact, usually this fails miserably. The correct solution to this problem is to write the code as follows:
int
is_in_class (int c, const char *class)
{
wctype_t desc = wctype (class);
return desc ? iswctype (btowc (c), desc) : 0;
}
See section Converting Single Characters, for more information on btowc.
Note that this change probably does not improve the performance
of the program a lot since the wctype function still has to make
the string comparisons. It gets really interesting if the
is_in_class function is called more than once for the
same class name. In this case the variable desc could be computed
once and reused for all the calls. Therefore the above form of the
function is probably not the final one.
The classification functions are also generalized by the ISO C
standard. Instead of just allowing the two standard mappings, a
locale can contain others. Again, the localedef program
already supports generating such locale data files.
wctrans function.
wctrans function has to be used to find out whether a named
mapping is defined in the current locale selected for the
LC_CTYPE category. If the returned value is non-zero, you can use
it afterwards in calls to towctrans. If the return value is
zero no such mapping is known in the current locale.
Beside locale-specific mappings there are two mappings which are guaranteed to be available in every locale:
@multitable @columnfractions .5 .5
"tolower" @tab "toupper"
These functions are declared in `wctype.h'.
towctrans maps the input character wc
according to the rules of the mapping for which desc is a
descriptor, and returns the value it finds. desc must be
obtained by a successful call to wctrans.
This function is declared in `wctype.h'.
wctrans
for them.
towlower returns the corresponding
lower-case letter. If wc is not an upper-case letter,
wc is returned unchanged.
towlower can be implemented using
towctrans (wc, wctrans ("tolower"))
This function is declared in `wctype.h'.
towupper returns the corresponding
upper-case letter. Otherwise wc is returned unchanged.
towupper can be implemented using
towctrans (wc, wctrans ("toupper"))
This function is declared in `wctype.h'.
char type value to a wint_t and use it as an
argument to towctrans calls.
Operations on strings (or arrays of characters) are an important part of
many programs. The GNU C library provides an extensive set of string
utility functions, including functions for copying, concatenating,
comparing, and searching strings. Many of these functions can also
operate on arbitrary regions of storage; for example, the memcpy
function can be used to copy the contents of any kind of array.
It's fairly common for beginning C programmers to "reinvent the wheel" by duplicating this functionality in their own code, but it pays to become familiar with the library functions and to make use of them, since this offers benefits in maintenance, efficiency, and portability.
For instance, you could easily compare one string to another in two
lines of C code, but if you use the built-in strcmp function,
you're less likely to make a mistake. And, since these library
functions are typically highly optimized, your program may run faster
too.
This section is a quick summary of string concepts for beginning C programmers. It describes how character strings are represented in C and some common pitfalls. If you are already familiar with this material, you can skip this section.
A string is an array of char objects. But string-valued
variables are usually declared to be pointers of type char *.
Such variables do not include space for the text of a string; that has
to be stored somewhere else--in an array variable, a string constant,
or dynamically allocated memory (see section Allocating Storage For Program Data). It's up to
you to store the address of the chosen memory space into the pointer
variable. Alternatively you can store a null pointer in the
pointer variable. The null pointer does not point anywhere, so
attempting to reference the string it points to gets an error.
"string" normally refers to multibyte character strings as opposed to
wide character strings. Wide character strings are arrays of type
wchar_t and as for multibyte character strings usually pointers
of type wchar_t * are used.
By convention, a null character, '\0', marks the end of a
multibyte character string and the null wide character,
L'\0', marks the end of a wide character string. For example, in
testing to see whether the char * variable p points to a
null character marking the end of a string, you can write
!*p or *p == '\0'.
A null character is quite different conceptually from a null pointer,
although both are represented by the integer 0.
String literals appear in C program source as strings of
characters between double-quote characters (`"') where the initial
double-quote character is immediately preceded by a capital `L'
(ell) character (as in L"foo"). In ISO C, string literals
can also be formed by string concatenation: "a" "b" is the
same as "ab". For wide character strings one can either use
L"a" L"b" or L"a" "b". Modification of string literals is
not allowed by the GNU C compiler, because literals are placed in
read-only storage.
Character arrays that are declared const cannot be modified
either. It's generally good style to declare non-modifiable string
pointers to be of type const char *, since this often allows the
C compiler to detect accidental modifications as well as providing some
amount of documentation about what your program intends to do with the
string.
The amount of memory allocated for the character array may extend past the null character that normally marks the end of the string. In this document, the term allocated size is always used to refer to the total amount of memory allocated for the string, while the term length refers to the number of characters up to (but not including) the terminating null character.
A notorious source of program bugs is trying to put more characters in a string than fit in its allocated size. When writing code that extends strings or moves characters into a pre-allocated array, you should be very careful to keep track of the length of the text and make explicit checks for overflowing the array. Many of the library functions do not do this for you! Remember also that you need to allocate an extra byte to hold the null character that marks the end of the string.
Originally strings were sequences of bytes where each byte represents a single character. This is still true today if the strings are encoded using a single-byte character encoding. Things are different if the strings are encoded using a multibyte encoding (for more information on encodings see section Introduction to Extended Characters). There is no difference in the programming interface for these two kind of strings; the programmer has to be aware of this and interpret the byte sequences accordingly.
But since there is no separate interface taking care of these
differences the byte-based string functions are sometimes hard to use.
Since the count parameters of these functions specify bytes a call to
strncpy could cut a multibyte character in the middle and put an
incomplete (and therefore unusable) byte sequence in the target buffer.
To avoid these problems later versions of the ISO C standard introduce a second set of functions which are operating on wide characters (see section Introduction to Extended Characters). These functions don't have the problems the single-byte versions have since every wide character is a legal, interpretable value. This does not mean that cutting wide character strings at arbitrary points is without problems. It normally is for alphabet-based languages (except for non-normalized text) but languages based on syllables still have the problem that more than one wide character is necessary to complete a logical unit. This is a higher level problem which the C library functions are not designed to solve. But it is at least good that no invalid byte sequences can be created. Also, the higher level functions can also much easier operate on wide character than on multibyte characters so that a general advise is to use wide characters internally whenever text is more than simply copied.
The remaining of this chapter will discuss the functions for handling wide character strings in parallel with the discussion of the multibyte character strings since there is almost always an exact equivalent available.
This chapter describes both functions that work on arbitrary arrays or blocks of memory, and functions that are specific to null-terminated arrays of characters and wide characters.
Functions that operate on arbitrary blocks of memory have names
beginning with `mem' and `wmem' (such as memcpy and
wmemcpy) and invariably take an argument which specifies the size
(in bytes and wide characters respectively) of the block of memory to
operate on. The array arguments and return values for these functions
have type void * or wchar_t. As a matter of style, the
elements of the arrays used with the `mem' functions are referred
to as "bytes". You can pass any kind of pointer to these functions,
and the sizeof operator is useful in computing the value for the
size argument. Parameters to the `wmem' functions must be of type
wchar_t *. These functions are not really usable with anything
but arrays of this type.
In contrast, functions that operate specifically on strings and wide
character strings have names beginning with `str' and `wcs'
respectively (such as strcpy and wcscpy) and look for a
null character to terminate the string instead of requiring an explicit
size argument to be passed. (Some of these functions accept a specified
maximum length, but they also check for premature termination with a
null character.) The array arguments and return values for these
functions have type char * and wchar_t * respectively, and
the array elements are referred to as "characters" and "wide
characters".
In many cases, there are both `mem' and `str'/`wcs' versions of a function. The one that is more appropriate to use depends on the exact situation. When your program is manipulating arbitrary arrays or blocks of storage, then you should always use the `mem' functions. On the other hand, when you are manipulating null-terminated strings it is usually more convenient to use the `str'/`wcs' functions, unless you already know the length of the string in advance. The `wmem' functions should be used for wide character arrays with known size.
Some of the memory and string functions take single characters as
arguments. Since a value of type char is automatically promoted
into an value of type int when used as a parameter, the functions
are declared with int as the type of the parameter in question.
In case of the wide character function the situation is similarly: the
parameter type for a single wide character is wint_t and not
wchar_t. This would for many implementations not be necessary
since the wchar_t is large enough to not be automatically
promoted, but since the ISO C standard does not require such a
choice of types the wint_t type is used.
You can get the length of a string using the strlen function.
This function is declared in the header file `string.h'.
strlen function returns the length of the null-terminated
string s in bytes. (In other words, it returns the offset of the
terminating null character within the array.)
For example,
strlen ("hello, world")
=> 12
When applied to a character array, the strlen function returns
the length of the string stored there, not its allocated size. You can
get the allocated size of the character array that holds a string using
the sizeof operator:
char string[32] = "hello, world";
sizeof (string)
=> 32
strlen (string)
=> 12
But beware, this will not work unless string is the character array itself, not a pointer to it. For example:
char string[32] = "hello, world";
char *ptr = string;
sizeof (string)
=> 32
sizeof (ptr)
=> 4 /* (on a machine with 4 byte pointers) */
This is an easy mistake to make when you are working with functions that take string arguments; those arguments are always pointers, not arrays.
It must also be noted that for multibyte encoded strings the return
value does not have to correspond to the number of characters in the
string. To get this value the string can be converted to wide
characters and wcslen can be used or something like the following
code can be used:
/* The input is instring. The length is expected inn. */ { mbstate_t t; char *scopy = string; /* In initial state. */ memset (&t, '\0', sizeof (t)); /* Determine number of characters. */ n = mbsrtowcs (NULL, &scopy, strlen (scopy), &t); }
This is cumbersome to do so if the number of characters (as opposed to bytes) is needed often it is better to work with wide characters.
The wide character equivalent is declared in `wchar.h'.
wcslen function is the wide character equivalent to
strlen. The return value is the number of wide characters in the
wide character string pointed to by ws (this is also the offset of
the terminating null wide character of ws).
Since there are no multi wide character sequences making up one character the return value is not only the offset in the array, it is also the number of wide characters.
This function was introduced in Amendment 1 to ISO C90.
strnlen function returns the length of the string s in
bytes if this length is smaller than maxlen bytes. Otherwise it
returns maxlen. Therefore this function is equivalent to
(strlen (s) < n ? strlen (s) : maxlen) but it
is more efficient and works even if the string s is not
null-terminated.
char string[32] = "hello, world";
strnlen (string, 32)
=> 12
strnlen (string, 5)
=> 5
This function is a GNU extension and is declared in `string.h'.
wcsnlen is the wide character equivalent to strnlen. The
maxlen parameter specifies the maximum number of wide characters.
This function is a GNU extension and is declared in `wchar.h'.
You can use the functions described in this section to copy the contents of strings and arrays, or to append the contents of one string to another. The `str' and `mem' functions are declared in the header file `string.h' while the `wstr' and `wmem' functions are declared in the file `wchar.h'.
A helpful way to remember the ordering of the arguments to the functions in this section is that it corresponds to an assignment expression, with the destination array specified to the left of the source array. All of these functions return the address of the destination array.
Most of these functions do not work properly if the source and destination arrays overlap. For example, if the beginning of the destination array overlaps the end of the source array, the original contents of that part of the source array may get overwritten before it is copied. Even worse, in the case of the string functions, the null character marking the end of the string may be lost, and the copy function might get stuck in a loop trashing all the memory allocated to your program.
All functions that have problems copying between overlapping arrays are
explicitly identified in this manual. In addition to functions in this
section, there are a few others like sprintf (see section Formatted Output Functions) and scanf (see section Formatted Input Functions).
memcpy function copies size bytes from the object
beginning at from into the object beginning at to. The
behavior of this function is undefined if the two arrays to and
from overlap; use memmove instead if overlapping is possible.
The value returned by memcpy is the value of to.
Here is an example of how you might use memcpy to copy the
contents of an array:
struct foo *oldarray, *newarray; int arraysize; ... memcpy (new, old, arraysize * sizeof (struct foo));
wmemcpy function copies size wide characters from the object
beginning at wfrom into the object beginning at wto. The
behavior of this function is undefined if the two arrays wto and
wfrom overlap; use wmemmove instead if overlapping is possible.
The following is a possible implementation of wmemcpy but there
are more optimizations possible.
wchar_t *
wmemcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
size_t size)
{
return (wchar_t *) memcpy (wto, wfrom, size * sizeof (wchar_t));
}
The value returned by wmemcpy is the value of wto.
This function was introduced in Amendment 1 to ISO C90.
mempcpy function is nearly identical to the memcpy
function. It copies size bytes from the object beginning at
from into the object pointed to by to. But instead of
returning the value of to it returns a pointer to the byte
following the last written byte in the object beginning at to.
I.e., the value is ((void *) ((char *) to + size)).
This function is useful in situations where a number of objects shall be copied to consecutive memory positions.
void *
combine (void *o1, size_t s1, void *o2, size_t s2)
{
void *result = malloc (s1 + s2);
if (result != NULL)
mempcpy (mempcpy (result, o1, s1), o2, s2);
return result;
}
This function is a GNU extension.
wmempcpy function is nearly identical to the wmemcpy
function. It copies size wide characters from the object
beginning at wfrom into the object pointed to by wto. But
instead of returning the value of wto it returns a pointer to the
wide character following the last written wide character in the object
beginning at wto. I.e., the value is wto + size.
This function is useful in situations where a number of objects shall be copied to consecutive memory positions.
The following is a possible implementation of wmemcpy but there
are more optimizations possible.
wchar_t *
wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
size_t size)
{
return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t));
}
This function is a GNU extension.
memmove copies the size bytes at from into the
size bytes at to, even if those two blocks of space
overlap. In the case of overlap, memmove is careful to copy the
original values of the bytes in the block at from, including those
bytes which also belong to the block at to.
The value returned by memmove is the value of to.
wmemmove copies the size wide characters at wfrom
into the size wide characters at wto, even if those two
blocks of space overlap. In the case of overlap, memmove is
careful to copy the original values of the wide characters in the block
at wfrom, including those wide characters which also belong to the
block at wto.
The following is a possible implementation of wmemcpy but there
are more optimizations possible.
wchar_t *
wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
size_t size)
{
return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t));
}
The value returned by wmemmove is the value of wto.
This function is a GNU extension.
unsigned char) into each of the first size bytes of the
object beginning at block. It returns the value of block.
memcpy, this function has undefined results if the strings
overlap. The return value is the value of to.
wmemcpy, this function has undefined results if
the strings overlap. The return value is the value of wto.
strcpy but always copies exactly
size characters into to.
If the length of from is more than size, then strncpy
copies just the first size characters. Note that in this case
there is no null terminator written into to.
If the length of from is less than size, then strncpy
copies all of from, followed by enough null characters to add up
to size characters in all. This behavior is rarely useful, but it
is specified by the ISO C standard.
The behavior of strncpy is undefined if the strings overlap.
Using strncpy as opposed to strcpy is a way to avoid bugs
relating to writing past the end of the allocated space for to.
However, it can also make your program much slower in one common case:
copying a string which is probably small into a potentially large buffer.
In this case, size may be large, and when it is, strncpy will
waste a considerable amount of time copying null characters.
wcscpy but always copies exactly
size wide characters into wto.
If the length of wfrom is more than size, then
wcsncpy copies just the first size wide characters. Note
that in this case there is no null terminator written into wto.
If the length of wfrom is less than size, then
wcsncpy copies all of wfrom, followed by enough null wide
characters to add up to size wide characters in all. This
behavior is rarely useful, but it is specified by the ISO C
standard.
The behavior of wcsncpy is undefined if the strings overlap.
Using wcsncpy as opposed to wcscpy is a way to avoid bugs
relating to writing past the end of the allocated space for wto.
However, it can also make your program much slower in one common case:
copying a string which is probably small into a potentially large buffer.
In this case, size may be large, and when it is, wcsncpy will
waste a considerable amount of time copying null wide characters.
malloc; see
section Unconstrained Allocation. If malloc cannot allocate space
for the new string, strdup returns a null pointer. Otherwise it
returns a pointer to the new string.
malloc; see section Unconstrained Allocation. If malloc
cannot allocate space for the new string, wcsdup returns a null
pointer. Otherwise it returns a pointer to the new wide character
string.
This function is a GNU extension.
strdup but always copies at most
size characters into the newly allocated string.
If the length of s is more than size, then strndup
copies just the first size characters and adds a closing null
terminator. Otherwise all characters are copied and the string is
terminated.
This function is different to strncpy in that it always
terminates the destination string.
strndup is a GNU extension.
strcpy, except that it returns a pointer to
the end of the string to (that is, the address of the terminating
null character to + strlen (from)) rather than the beginning.
For example, this program uses stpcpy to concatenate `foo'
and `bar' to produce `foobar', which it then prints.
#include <string.h>
#include <stdio.h>
int
main (void)
{
char buffer[10];
char *to = buffer;
to = stpcpy (to, "foo");
to = stpcpy (to, "bar");
puts (buffer);
return 0;
}
This function is not part of the ISO or POSIX standards, and is not customary on Unix systems, but we did not invent it either. Perhaps it comes from MS-DOG.
Its behavior is undefined if the strings overlap. The function is declared in `string.h'.
wcscpy, except that it returns a pointer to
the end of the string wto (that is, the address of the terminating
null character wto + strlen (wfrom)) rather than the beginning.
This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself.
The behavior of wcpcpy is undefined if the strings overlap.
wcpcpy is a GNU extension and is declared in `wchar.h'.
stpcpy but copies always exactly
size characters into to.
If the length of from is more then size, then stpncpy
copies just the first size characters and returns a pointer to the
character directly following the one which was copied last. Note that in
this case there is no null terminator written into to.
If the length of from is less than size, then stpncpy
copies all of from, followed by enough null characters to add up
to size characters in all. This behaviour is rarely useful, but it
is implemented to be useful in contexts where this behaviour of the
strncpy is used. stpncpy returns a pointer to the
first written null character.
This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself.
Its behaviour is undefined if the strings overlap. The function is declared in `string.h'.
wcpcpy but copies always exactly
wsize characters into wto.
If the length of wfrom is more then size, then
wcpncpy copies just the first size wide characters and
returns a pointer to the wide character directly following the one which
was copied last. Note that in this case there is no null terminator
written into wto.
If the length of wfrom is less than size, then wcpncpy
copies all of wfrom, followed by enough null characters to add up
to size characters in all. This behaviour is rarely useful, but it
is implemented to be useful in contexts where this behaviour of the
wcsncpy is used. wcpncpy returns a pointer to the
first written null character.
This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself.
Its behaviour is undefined if the strings overlap.
wcpncpy is a GNU extension and is declared in `wchar.h'.
strdup but allocates the new string
using alloca instead of malloc (see section Automatic Storage with Variable Size). This means of course the returned string has the same
limitations as any block of memory allocated using alloca.
For obvious reasons strdupa is implemented only as a macro;
you cannot get the address of this function. Despite this limitation
it is a useful function. The following code shows a situation where
using malloc would be a lot more expensive.
#include <paths.h>
#include <string.h>
#include <stdio.h>
const char path[] = _PATH_STDPATH;
int
main (void)
{
char *wr_path = strdupa (path);
char *cp = strtok (wr_path, ":");
while (cp != NULL)
{
puts (cp);
cp = strtok (NULL, ":");
}
return 0;
}
Please note that calling strtok using path directly is
invalid. It is also not allowed to call strdupa in the argument
list of strtok since strdupa uses alloca
(see section Automatic Storage with Variable Size) can interfere with the parameter
passing.
This function is only available if GNU CC is used.
strndup but like strdupa it
allocates the new string using alloca
see section Automatic Storage with Variable Size. The same advantages and limitations
of strdupa are valid for strndupa, too.
This function is implemented only as a macro, just like strdupa.
Just as strdupa this macro also must not be used inside the
parameter list in a function call.
strndupa is only available if GNU CC is used.
strcat function is similar to strcpy, except that the
characters from from are concatenated or appended to the end of
to, instead of overwriting it. That is, the first character from
from overwrites the null character marking the end of to.
An equivalent definition for strcat would be:
char *
strcat (char *restrict to, const char *restrict from)
{
strcpy (to + strlen (to), from);
return to;
}
This function has undefined results if the strings overlap.
wcscat function is similar to wcscpy, except that the
characters from wfrom are concatenated or appended to the end of
wto, instead of overwriting it. That is, the first character from
wfrom overwrites the null character marking the end of wto.
An equivalent definition for wcscat would be:
wchar_t *
wcscat (wchar_t *wto, const wchar_t *wfrom)
{
wcscpy (wto + wcslen (wto), wfrom);
return wto;
}
This function has undefined results if the strings overlap.
Programmers using the strcat or wcscat function (or the
following strncat or wcsncar functions for that matter)
can easily be recognized as lazy and reckless. In almost all situations
the lengths of the participating strings are known (it better should be
since how can one otherwise ensure the allocated size of the buffer is
sufficient?) Or at least, one could know them if one keeps track of the
results of the various function calls. But then it is very inefficient
to use strcat/wcscat. A lot of time is wasted finding the
end of the destination string so that the actual copying can start.
This is a common example:
/* This function concatenates arbitrarily many strings. The last parameter must beNULL. */ char * concat (const char *str, ...) { va_list ap, ap2; size_t total = 1; const char *s; char *result; va_start (ap, str); /* Actuallyva_copy, but this is the name more gcc versions understand. */ __va_copy (ap2, ap); /* Determine how much space we need. */ for (s = str; s != NULL; s = va_arg (ap, const char *)) total += strlen (s); va_end (ap); result = (char *) malloc (total); if (result != NULL) { result[0] = '\0'; /* Copy the strings. */ for (s = str; s != NULL; s = va_arg (ap2, const char *)) strcat (result, s); } va_end (ap2); return result; }
This looks quite simple, especially the second loop where the strings are actually copied. But these innocent lines hide a major performance penalty. Just imagine that ten strings of 100 bytes each have to be concatenated. For the second string we search the already stored 100 bytes for the end of the string so that we can append the next string. For all strings in total the comparisons necessary to find the end of the intermediate results sums up to 5500! If we combine the copying with the search for the allocation we can write this function more efficient:
char *
concat (const char *str, ...)
{
va_list ap;
size_t allocated = 100;
char *result = (char *) malloc (allocated);
char *wp;
if (allocated != NULL)
{
char *newp;
va_start (ap, atr);
wp = result;
for (s = str; s != NULL; s = va_arg (ap, const char *))
{
size_t len = strlen (s);
/* Resize the allocated memory if necessary. */
if (wp + len + 1 > result + allocated)
{
allocated = (allocated + len) * 2;
newp = (char *) realloc (result, allocated);
if (newp == NULL)
{
free (result);
return NULL;
}
wp = newp + (wp - result);
result = newp;
}
wp = mempcpy (wp, s, len);
}
/* Terminate the result string. */
*wp++ = '\0';
/* Resize memory to the optimal size. */
newp = realloc (result, wp - result);
if (newp != NULL)
result = newp;
va_end (ap);
}
return result;
}
With a bit more knowledge about the input strings one could fine-tune
the memory allocation. The difference we are pointing to here is that
we don't use strcat anymore. We always keep track of the length
of the current intermediate result so we can safe us the search for the
end of the string and use mempcpy. Please note that we also
don't use stpcpy which might seem more natural since we handle
with strings. But this is not necessary since we already know the
length of the string and therefore can use the faster memory copying
function. The example would work for wide characters the same way.
Whenever a programmer feels the need to use strcat she or he
should think twice and look through the program whether the code cannot
be rewritten to take advantage of already calculated results. Again: it
is almost always unnecessary to use strcat.
strcat except that not more than size
characters from from are appended to the end of to. A
single null character is also always appended to to, so the total
allocated size of to must be at least size + 1 bytes
longer than its initial length.
The strncat function could be implemented like this:
char *
strncat (char *to, const char *from, size_t size)
{
to[strlen (to) + size] = '\0';
strncpy (to + strlen (to), from, size);
return to;
}
The behavior of strncat is undefined if the strings overlap.
wcscat except that not more than size
characters from from are appended to the end of to. A
single null character is also always appended to to, so the total
allocated size of to must be at least size + 1 bytes
longer than its initial length.
The wcsncat function could be implemented like this:
wchar_t *
wcsncat (wchar_t *restrict wto, const wchar_t *restrict wfrom,
size_t size)
{
wto[wcslen (to) + size] = L'\0';
wcsncpy (wto + wcslen (wto), wfrom, size);
return wto;
}
The behavior of wcsncat is undefined if the strings overlap.
Here is an example showing the use of strncpy and strncat
(the wide character version is equivalent). Notice how, in the call to
strncat, the size parameter is computed to avoid
overflowing the character array buffer.
#include <string.h>
#include <stdio.h>
#define SIZE 10
static char buffer[SIZE];
main ()
{
strncpy (buffer, "hello", SIZE);
puts (buffer);
strncat (buffer, ", world", SIZE - strlen (buffer) - 1);
puts (buffer);
}
The output produced by this program looks like:
hello hello, wo
memmove, derived from
BSD. Note that it is not quite equivalent to memmove, because the
arguments are not in the same order and there is no return value.
memset, derived from
BSD. Note that it is not as general as memset, because the only
value it can store is zero.
You can use the functions in this section to perform comparisons on the contents of strings and arrays. As well as checking for equality, these functions can also be used as the ordering functions for sorting operations. See section Searching and Sorting, for an example of this.
Unlike most comparison operations in C, the string comparison functions return a nonzero value if the strings are not equivalent rather than if they are. The sign of the value indicates the relative ordering of the first characters in the strings that are not equivalent: a negative value indicates that the first string is "less" than the second, while a positive value indicates that the first string is "greater".
The most common use of these functions is to check only for equality. This is canonically done with an expression like `! strcmp (s1, s2)'.
All of these functions are declared in the header file `string.h'.
memcmp compares the size bytes of memory
beginning at a1 against the size bytes of memory beginning
at a2. The value returned has the same sign as the difference
between the first differing pair of bytes (interpreted as unsigned
char objects, then promoted to int).
If the contents of the two blocks are equal, memcmp returns
0.
wmemcmp compares the size wide characters
beginning at a1 against the size wide characters beginning
at a2. The value returned is smaller than or larger than zero
depending on whether the first differing wide character is a1 is
smaller or larger than the corresponding character in a2.
If the contents of the two blocks are equal, wmemcmp returns
0.
On arbitrary arrays, the memcmp function is mostly useful for
testing equality. It usually isn't meaningful to do byte-wise ordering
comparisons on arrays of things other than bytes. For example, a
byte-wise comparison on the bytes that make up floating-point numbers
isn't likely to tell you anything about the relationship between the
values of the floating-point numbers.
wmemcmp is really only useful to compare arrays of type
wchar_t since the function looks at sizeof (wchar_t) bytes
at a time and this number of bytes is system dependent.
You should also be careful about using memcmp to compare objects
that can contain "holes", such as the padding inserted into structure
objects to enforce alignment requirements, extra space at the end of
unions, and extra characters at the ends of strings whose length is less
than their allocated size. The contents of these "holes" are
indeterminate and may cause strange behavior when performing byte-wise
comparisons. For more predictable results, perform an explicit
component-wise comparison.
For example, given a structure type definition like:
struct foo
{
unsigned char tag;
union
{
double f;
long i;
char *p;
} value;
};
you are better off writing a specialized comparison function to compare
struct foo objects instead of comparing them with memcmp.
strcmp function compares the string s1 against
s2, returning a value that has the same sign as the difference
between the first differing pair of characters (interpreted as
unsigned char objects, then promoted to int).
If the two strings are equal, strcmp returns 0.
A consequence of the ordering used by strcmp is that if s1
is an initial substring of s2, then s1 is considered to be
"less than" s2.
strcmp does not take sorting conventions of the language the
strings are written in into account. To get that one has to use
strcoll.
The wcscmp function compares the wide character string ws1
against ws2. The value returned is smaller than or larger than zero
depending on whether the first differing wide character is ws1 is
smaller or larger than the corresponding character in ws2.
If the two strings are equal, wcscmp returns 0.
A consequence of the ordering used by wcscmp is that if ws1
is an initial substring of ws2, then ws1 is considered to be
"less than" ws2.
wcscmp does not take sorting conventions of the language the
strings are written in into account. To get that one has to use
wcscoll.
strcmp, except that differences in case are
ignored. How uppercase and lowercase characters are related is
determined by the currently selected locale. In the standard "C"
locale the characters @"A and @"a do not match but in a locale which
regards these characters as parts of the alphabet they do match.
strcasecmp is derived from BSD.
wcscmp, except that differences in case are
ignored. How uppercase and lowercase characters are related is
determined by the currently selected locale. In the standard "C"
locale the characters @"A and @"a do not match but in a locale which
regards these characters as parts of the alphabet they do match.
wcscasecmp is a GNU extension.
strcmp, except that no more than
size wide characters are compared. In other words, if the two
strings are the same in their first size wide characters, the
return value is zero.
wcscmp, except that no more than
size wide characters are compared. In other words, if the two
strings are the same in their first size wide characters, the
return value is zero.
strncmp, except that differences in case
are ignored. Like strcasecmp, it is locale dependent how
uppercase and lowercase characters are related.
strncasecmp is a GNU extension.
wcsncmp, except that differences in case
are ignored. Like wcscasecmp, it is locale dependent how
uppercase and lowercase characters are related.
wcsncasecmp is a GNU extension.
Here are some examples showing the use of strcmp and
strncmp (equivalent examples can be constructed for the wide
character functions). These examples assume the use of the ASCII
character set. (If some other character set--say, EBCDIC--is used
instead, then the glyphs are associated with different numeric codes,
and the return values and ordering may differ.)
strcmp ("hello", "hello")
=> 0 /* These two strings are the same. */
strcmp ("hello", "Hello")
=> 32 /* Comparisons are case-sensitive. */
strcmp ("hello", "world")
=> -15 /* The character 'h' comes before 'w'. */
strcmp ("hello", "hello, world")
=> -44 /* Comparing a null character against a comma. */
strncmp ("hello", "hello, world", 5)
=> 0 /* The initial 5 characters are the same. */
strncmp ("hello, world", "hello, stupid world!!!", 5)
=> 0 /* The initial 5 characters are the same. */
strverscmp function compares the string s1 against
s2, considering them as holding indices/version numbers. Return
value follows the same conventions as found in the strverscmp
function. In fact, if s1 and s2 contain no digits,
strverscmp behaves like strcmp.
Basically, we compare strings normally (character by character), until we find a digit in each string - then we enter a special comparison mode, where each sequence of digits is taken as a whole. If we reach the end of these two parts without noticing a difference, we return to the standard comparison mode. There are two types of numeric parts: "integral" and "fractional" (those begin with a '0'). The types of the numeric parts affect the way we sort them:
strverscmp ("no digit", "no digit")
=> 0 /* same behaviour as strcmp. */
strverscmp ("item#99", "item#100")
=> <0 /* same prefix, but 99 < 100. */
strverscmp ("alpha1", "alpha001")
=> >0 /* fractional part inferior to integral one. */
strverscmp ("part1_f012", "part1_f01")
=> >0 /* two fractional parts. */
strverscmp ("foo.009", "foo.0")
=> <0 /* idem, but with leading zeroes only. */
This function is especially useful when dealing with filename sorting, because filenames frequently hold indices/version numbers.
strverscmp is a GNU extension.
memcmp, derived from BSD.
In some locales, the conventions for lexicographic ordering differ from the strict numeric ordering of character codes. For example, in Spanish most glyphs with diacritical marks such as accents are not considered distinct letters for the purposes of collation. On the other hand, the two-character sequence `ll' is treated as a single letter that is collated immediately after `l'.
You can use the functions strcoll and strxfrm (declared in
the headers file `string.h') and wcscoll and wcsxfrm
(declared in the headers file `wchar') to compare strings using a
collation ordering appropriate for the current locale. The locale used
by these functions in particular can be specified by setting the locale
for the LC_COLLATE category; see section Locales and Internationalization.
In the standard C locale, the collation sequence for strcoll is
the same as that for strcmp. Similarly, wcscoll and
wcscmp are the same in this situation.
Effectively, the way these functions work is by applying a mapping to transform the characters in a string to a byte sequence that represents the string's position in the collating sequence of the current locale. Comparing two such byte sequences in a simple fashion is equivalent to comparing the strings with the locale's collating sequence.
The functions strcoll and wcscoll perform this translation
implicitly, in order to do one comparison. By contrast, strxfrm
and wcsxfrm perform the mapping explicitly. If you are making
multiple comparisons using the same string or set of strings, it is
likely to be more efficient to use strxfrm or wcsxfrm to
transform all the strings just once, and subsequently compare the
transformed strings with strcmp or wcscmp.
strcoll function is similar to strcmp but uses the
collating sequence of the current locale for collation (the
LC_COLLATE locale).
wcscoll function is similar to wcscmp but uses the
collating sequence of the current locale for collation (the
LC_COLLATE locale).
Here is an example of sorting an array of strings, using strcoll
to compare them. The actual sort algorithm is not written here; it
comes from qsort (see section Array Sort Function). The job of the
code shown here is to say how to compare the strings while sorting them.
(Later on in this section, we will show a way to do this more
efficiently using strxfrm.)
/* This is the comparison function used withqsort. */ int compare_elements (char **p1, char **p2) { return strcoll (*p1, *p2); } /* This is the entry point--the function to sort strings using the locale's collating sequence. */ void sort_strings (char **array, int nstrings) { /* Sorttemp_arrayby comparing the strings. */ qsort (array, nstrings, sizeof (char *), compare_elements); }
strxfrm transforms the string from using the
collation transformation determined by the locale currently selected for
collation, and stores the transformed string in the array to. Up
to size characters (including a terminating null character) are
stored.
The behavior is undefined if the strings to and from overlap; see section Copying and Concatenation.
The return value is the length of the entire transformed string. This
value is not affected by the value of size, but if it is greater
or equal than size, it means that the transformed string did not
entirely fit in the array to. In this case, only as much of the
string as actually fits was stored. To get the whole transformed
string, call strxfrm again with a bigger output array.
The transformed string may be longer than the original string, and it may also be shorter.
If size is zero, no characters are stored in to. In this
case, strxfrm simply returns the number of characters that would
be the length of the transformed string. This is useful for determining
what size the allocated array should be. It does not matter what
to is if size is zero; to may even be a null pointer.
wcsxfrm transforms wide character string wfrom
using the collation transformation determined by the locale currently
selected for collation, and stores the transformed string in the array
wto. Up to size wide characters (including a terminating null
character) are stored.
The behavior is undefined if the strings wto and wfrom overlap; see section Copying and Concatenation.
The return value is the length of the entire transformed wide character
string. This value is not affected by the value of size, but if
it is greater or equal than size, it means that the transformed
wide character string did not entirely fit in the array wto. In
this case, only as much of the wide character string as actually fits
was stored. To get the whole transformed wide character string, call
wcsxfrm again with a bigger output array.
The transformed wide character string may be longer than the original wide character string, and it may also be shorter.
If size is zero, no characters are stored in to. In this
case, wcsxfrm simply returns the number of wide characters that
would be the length of the transformed wide character string. This is
useful for determining what size the allocated array should be (remember
to multiply with sizeof (wchar_t)). It does not matter what
wto is if size is zero; wto may even be a null pointer.
Here is an example of how you can use strxfrm when
you plan to do many comparisons. It does the same thing as the previous
example, but much faster, because it has to transform each string only
once, no matter how many times it is compared with other strings. Even
the time needed to allocate and free storage is much less than the time
we save, when there are many strings.
struct sorter { char *input; char *transformed; };
/* This is the comparison function used with qsort
to sort an array of struct sorter. */
int
compare_elements (struct sorter *p1, struct sorter *p2)
{
return strcmp (p1->transformed, p2->transformed);
}
/* This is the entry point--the function to sort
strings using the locale's collating sequence. */
void
sort_strings_fast (char **array, int nstrings)
{
struct sorter temp_array[nstrings];
int i;
/* Set up temp_array. Each element contains
one input string and its transformed string. */
for (i = 0; i < nstrings; i++)
{
size_t length = strlen (array[i]) * 2;
char *transformed;
size_t transformed_length;
temp_array[i].input = array[i];
/* First try a buffer perhaps big enough. */
transformed = (char *) xmalloc (length);
/* Transform array[i]. */
transformed_length = strxfrm (transformed, array[i], length);
/* If the buffer was not large enough, resize it
and try again. */
if (transformed_length >= length)
{
/* Allocate the needed space. +1 for terminating
NUL character. */
transformed = (char *) xrealloc (transformed,
transformed_length + 1);
/* The return value is not interesting because we know
how long the transformed string is. */
(void) strxfrm (transformed, array[i],
transformed_length + 1);
}
temp_array[i].transformed = transformed;
}
/* Sort temp_array by comparing transformed strings. */
qsort (temp_array, sizeof (struct sorter),
nstrings, compare_elements);
/* Put the elements back in the permanent array
in their sorted order. */
for (i = 0; i < nstrings; i++)
array[i] = temp_array[i].input;
/* Free the strings we allocated. */
for (i = 0; i < nstrings; i++)
free (temp_array[i].transformed);
}
The interesting part of this code for the wide character version would look like this:
void
sort_strings_fast (wchar_t **array, int nstrings)
{
...
/* Transform array[i]. */
transformed_length = wcsxfrm (transformed, array[i], length);
/* If the buffer was not large enough, resize it
and try again. */
if (transformed_length >= length)
{
/* Allocate the needed space. +1 for terminating
NUL character. */
transformed = (wchar_t *) xrealloc (transformed,
(transformed_length + 1)
* sizeof (wchar_t));
/* The return value is not interesting because we know
how long the transformed string is. */
(void) wcsxfrm (transformed, array[i],
transformed_length + 1);
}
...
Note the additional multiplication with sizeof (wchar_t) in the
realloc call.
Compatibility Note: The string collation functions are a new feature of ISO C90. Older C dialects have no equivalent feature. The wide character versions were introduced in Amendment 1 to ISO C90.
This section describes library functions which perform various kinds of searching operations on strings and arrays. These functions are declared in the header file `string.h'.
unsigned char) in the initial size bytes of the
object beginning at block. The return value is a pointer to the
located byte, or a null pointer if no match was found.
memchr function is used with the knowledge that the
byte c is available in the memory block specified by the
parameters. But this means that the size parameter is not really
needed and that the tests performed with it at runtime (to check whether
the end of the block is reached) are not needed.
The rawmemchr function exists for just this situation which is
surprisingly frequent. The interface is similar to memchr except
that the size parameter is missing. The function will look beyond
the end of the block pointed to by block in case the programmer
made an error in assuming that the byte c is present in the block.
In this case the result is unspecified. Otherwise the return value is a
pointer to the located byte.
This function is of special interest when looking for the end of a string. Since all strings are terminated by a null byte a call like
rawmemchr (str, '\0')
will never go beyond the end of the string.
This function is a GNU extension.
memrchr is like memchr, except that it searches
backwards from the end of the block defined by block and size
(instead of forwards from the front).
strchr function finds the first occurrence of the character
c (converted to a char) in the null-terminated string
beginning at string. The return value is a pointer to the located
character, or a null pointer if no match was found.
For example,
strchr ("hello, world", 'l')
=> "llo, world"
strchr ("hello, world", '?')
=> NULL
The terminating null character is considered to be part of the string,
so you can use this function get a pointer to the end of a string by
specifying a null character as the value of the c argument. It
would be better (but less portable) to use strchrnul in this
case, though.
wcschr function finds the first occurrence of the wide
character wc in the null-terminated wide character string
beginning at wstring. The return value is a pointer to the
located wide character, or a null pointer if no match was found.
The terminating null character is considered to be part of the wide
character string, so you can use this function get a pointer to the end
of a wide character string by specifying a null wude character as the
value of the wc argument. It would be better (but less portable)
to use wcschrnul in this case, though.
strchrnul is the same as strchr except that if it does
not find the character, it returns a pointer to string's terminating
null character rather than a null pointer.
This function is a GNU extension.
wcschrnul is the same as wcschr except that if it does not
find the wide character, it returns a pointer to wide character string's
terminating null wide character rather than a null pointer.
This function is a GNU extension.
One useful, but unusual, use of the strchr
function is when one wants to have a pointer pointing to the NUL byte
terminating a string. This is often written in this way:
s += strlen (s);
This is almost optimal but the addition operation duplicated a bit of
the work already done in the strlen function. A better solution
is this:
s = strchr (s, '\0');
There is no restriction on the second parameter of strchr so it
could very well also be the NUL character. Those readers thinking very
hard about this might now point out that the strchr function is
more expensive than the strlen function since we have two abort
criteria. This is right. But in the GNU C library the implementation of
strchr is optimized in a special way so that strchr
actually is faster.
strrchr is like strchr, except that it searches
backwards from the end of the string string (instead of forwards
from the front).
For example,
strrchr ("hello, world", 'l')
=> "ld"
wcsrchr is like wcschr, except that it searches
backwards from the end of the string wstring (instead of forwards
from the front).
strchr, except that it searches haystack for a
substring needle rather than just a single character. It
returns a pointer into the string haystack that is the first
character of the substring, or a null pointer if no match was found. If
needle is an empty string, the function returns haystack.
For example,
strstr ("hello, world", "l")
=> "llo, world"
strstr ("hello, world", "wo")
=> "world"
wcschr, except that it searches haystack for a
substring needle rather than just a single wide character. It
returns a pointer into the string haystack that is the first wide
character of the substring, or a null pointer if no match was found. If
needle is an empty string, the function returns haystack.
wcsstr is an depricated alias for wcsstr. This is the
name originally used in the X/Open Portability Guide before the
Amendment 1 to ISO C90 was published.
strstr, except that it ignores case in searching for
the substring. Like strcasecmp, it is locale dependent how
uppercase and lowercase characters are related.
For example,
strstr ("hello, world", "L")
=> "llo, world"
strstr ("hello, World", "wo")
=> "World"
strstr, but needle and haystack are byte
arrays rather than null-terminated strings. needle-len is the
length of needle and haystack-len is the length of
haystack.
This function is a GNU extension.
strspn ("string span") function returns the length of the
initial substring of string that consists entirely of characters that
are members of the set specified by the string skipset. The order
of the characters in skipset is not important.
For example,
strspn ("hello, world", "abcdefghijklmnopqrstuvwxyz")
=> 5
Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.
wcsspn ("wide character string span") function returns the
length of the initial substring of wstring that consists entirely
of wide characters that are members of the set specified by the string
skipset. The order of the wide characters in skipset is not
important.
strcspn ("string complement span") function returns the length
of the initial substring of string that consists entirely of characters
that are not members of the set specified by the string stopset.
(In other words, it returns the offset of the first character in string
that is a member of the set stopset.)
For example,
strcspn ("hello, world", " \t\n,.;!?")
=> 5
Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.
wcscspn ("wide character string complement span") function
returns the length of the initial substring of wstring that
consists entirely of wide characters that are not members of the
set specified by the string stopset. (In other words, it returns
the offset of the first character in string that is a member of
the set stopset.)
strpbrk ("string pointer break") function is related to
strcspn, except that it returns a pointer to the first character
in string that is a member of the set stopset instead of the
length of the initial substring. It returns a null pointer if no such
character from stopset is found.
For example,
strpbrk ("hello, world", " \t\n,.;!?")
=> ", world"
Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.
wcspbrk ("wide character string pointer break") function is
related to wcscspn, except that it returns a pointer to the first
wide character in wstring that is a member of the set
stopset instead of the length of the initial substring. It
returns a null pointer if no such character from stopset is found.
index is another name for strchr; they are exactly the same.
New code should always use strchr since this name is defined in
ISO C while index is a BSD invention which never was available
on System V derived systems.
rindex is another name for strrchr; they are exactly the same.
New code should always use strrchr since this name is defined in
ISO C while rindex is a BSD invention which never was available
on System V derived systems.
It's fairly common for programs to have a need to do some simple kinds
of lexical analysis and parsing, such as splitting a command string up
into tokens. You can do this with the strtok function, declared
in the header file `string.h'.
strtok.
The string to be split up is passed as the newstring argument on
the first call only. The strtok function uses this to set up
some internal state information. Subsequent calls to get additional
tokens from the same string are indicated by passing a null pointer as
the newstring argument. Calling strtok with another
non-null newstring argument reinitializes the state information.
It is guaranteed that no other library function ever calls strtok
behind your back (which would mess up this internal state information).
The delimiters argument is a string that specifies a set of delimiters that may surround the token being extracted. All the initial characters that are members of this set are discarded. The first character that is not a member of this set of delimiters marks the beginning of the next token. The end of the token is found by looking for the next character that is a member of the delimiter set. This character in the original string newstring is overwritten by a null character, and the pointer to the beginning of the token in newstring is returned.
On the next call to strtok, the searching begins at the next
character beyond the one that marked the end of the previous token.
Note that the set of delimiters delimiters do not have to be the
same on every call in a series of calls to strtok.
If the end of the string newstring is reached, or if the remainder of
string consists only of delimiter characters, strtok returns
a null pointer.
Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.
Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.
wcstok.
The string to be split up is passed as the newstring argument on
the first call only. The wcstok function uses this to set up
some internal state information. Subsequent calls to get additional
tokens from the same wide character string are indicated by passing a
null pointer as the newstring argument. Calling wcstok
with another non-null newstring argument reinitializes the state
information. It is guaranteed that no other library function ever calls
wcstok behind your back (which would mess up this internal state
information).
The delimiters argument is a wide character string that specifies a set of delimiters that may surround the token being extracted. All the initial wide characters that are members of this set are discarded. The first wide character that is not a member of this set of delimiters marks the beginning of the next token. The end of the token is found by looking for the next wide character that is a member of the delimiter set. This wide character in the original wide character string newstring is overwritten by a null wide character, and the pointer to the beginning of the token in newstring is returned.
On the next call to wcstok, the searching begins at the next
wide character beyond the one that marked the end of the previous token.
Note that the set of delimiters delimiters do not have to be the
same on every call in a series of calls to wcstok.
If the end of the wide character string newstring is reached, or
if the remainder of string consists only of delimiter wide characters,
wcstok returns a null pointer.
Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.
Warning: Since strtok and wcstok alter the string
they is parsing, you should always copy the string to a temporary buffer
before parsing it with strtok/wcstok (see section Copying and Concatenation). If you allow strtok or wcstok to modify
a string that came from another part of your program, you are asking for
trouble; that string might be used for other purposes after
strtok or wcstok has modified it, and it would not have
the expected value.
The string that you are operating on might even be a constant. Then
when strtok or wcstok tries to modify it, your program
will get a fatal signal for writing in read-only memory. See section Program Error Signals. Even if the operation of strtok or wcstok
would not require a modification of the string (e.g., if there is
exactly one token) the string can (and in the GNU libc case will) be
modified.
This is a special case of a general principle: if a part of a program does not have as its purpose the modification of a certain data structure, then it is error-prone to modify the data structure temporarily.
The functions strtok and wcstok are not reentrant.
See section Signal Handling and Nonreentrant Functions, for a discussion of where and why reentrancy is
important.
Here is a simple example showing the use of strtok.
#include <string.h> #include <stddef.h> ... const char string[] = "words separated by spaces -- and, punctuation!"; const char delimiters[] = " .,;:!-"; char *token, *cp; ... cp = strdupa (string); /* Make writable copy. */ token = strtok (cp, delimiters); /* token => "words" */ token = strtok (NULL, delimiters); /* token => "separated" */ token = strtok (NULL, delimiters); /* token => "by" */ token = strtok (NULL, delimiters); /* token => "spaces" */ token = strtok (NULL, delimiters); /* token => "and" */ token = strtok (NULL, delimiters); /* token => "punctuation" */ token = strtok (NULL, delimiters); /* token => NULL */
The GNU C library contains two more functions for tokenizing a string which overcome the limitation of non-reentrancy. They are only available for multibyte character strings.
strtok, this function splits the string into several
tokens which can be accessed by successive calls to strtok_r.
The difference is that the information about the next token is stored in
the space pointed to by the third argument, save_ptr, which is a
pointer to a string pointer. Calling strtok_r with a null
pointer for newstring and leaving save_ptr between the calls
unchanged does the job without hindering reentrancy.
This function is defined in POSIX.1 and can be found on many systems which support multi-threading.
strtok_r with the
newstring argument replaced by the save_ptr argument. The
initialization of the moving pointer has to be done by the user.
Successive calls to strsep move the pointer along the tokens
separated by delimiter, returning the address of the next token
and updating string_ptr to point to the beginning of the next
token.
One difference between strsep and strtok_r is that if the
input string contains more than one character from delimiter in a
row strsep returns an empty string for each pair of characters
from delimiter. This means that a program normally should test
for strsep returning an empty string before processing it.
This function was introduced in 4.3BSD and therefore is widely available.
Here is how the above example looks like when strsep is used.
#include <string.h> #include <stddef.h> ... const char string[] = "words separated by spaces -- and, punctuation!"; const char delimiters[] = " .,;:!-"; char *running; char *token; ... running = strdupa (string); token = strsep (&running, delimiters); /* token => "words" */ token = strsep (&running, delimiters); /* token => "separated" */ token = strsep (&running, delimiters); /* token => "by" */ token = strsep (&running, delimiters); /* token => "spaces" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "and" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "punctuation" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => NULL */
basename function returns the last
component of the path in filename. This function is the prefered
usage, since it does not modify the argument, filename, and
respects trailing slashes. The prototype for basename can be
found in `string.h'. Note, this function is overriden by the XPG
version, if `libgen.h' is included.
Example of using GNU basename:
#include <string.h>
int
main (int argc, char *argv[])
{
char *prog = basename (argv[0]);
if (argc < 2)
{
fprintf (stderr, "Usage %s <arg>\n", prog);
exit (1);
}
...
}
Portability Note: This function may produce different results on different systems.
basename. It is similar in
spirit to the GNU version, but may modify the path by removing
trailing '/' characters. If the path is made up entirely of '/'
characters, then "/" will be returned. Also, if path is
NULL or an empty string, then "." is returned. The prototype for
the XPG version can be found in `libgen.h'.
Example of using XPG basename:
#include <libgen.h>
int
main (int argc, char *argv[])
{
char *prog;
char *path = strdupa (argv[0]);
prog = basename (path);
if (argc < 2)
{
fprintf (stderr, "Usage %s <arg>\n", prog);
exit (1);
}
...
}
dirname function is the compliment to the XPG version of
basename. It returns the parent directory of the file specified
by path. If path is NULL, an empty string, or
contains no '/' characters, then "." is returned. The prototype for this
function can be found in `libgen.h'.
The function below addresses the perennial programming quandary: "How do
I take good data in string form and painlessly turn it into garbage?"
This is actually a fairly simple task for C programmers who do not use
the GNU C library string functions, but for programs based on the GNU C
library, the strfry function is the preferred method for
destroying string data.
The prototype for this function is in `string.h'.
strfry creates a pseudorandom anagram of a string, replacing the
input with the anagram in place. For each position in the string,
strfry swaps it with a position in the string selected at random
(from a uniform distribution). The two positions may be the same.
The return value of strfry is always string.
Portability Note: This function is unique to the GNU C library.
The memfrob function converts an array of data to something
unrecognizable and back again. It is not encryption in its usual sense
since it is easy for someone to convert the encrypted data back to clear
text. The transformation is analogous to Usenet's "Rot13" encryption
method for obscuring offensive jokes from sensitive eyes and such.
Unlike Rot13, memfrob works on arbitrary binary data, not just
text.
For true encryption, See section DES Encryption and Password Handling.
This function is declared in `string.h'.
memfrob transforms (frobnicates) each byte of the data structure
at mem, which is length bytes long, by bitwise exclusive
oring it with binary 00101010. It does the transformation in place and
its return value is always mem.
Note that memfrob a second time on the same data structure
returns it to its original state.
This is a good function for hiding information from someone who doesn't want to see it or doesn't want to see it very much. To really prevent people from retrieving the information, use stronger encryption such as that described in See section DES Encryption and Password Handling.
Portability Note: This function is unique to the GNU C library.
To store or transfer binary data in environments which only support text one has to encode the binary data by mapping the input bytes to characters in the range allowed for storing or transfering. SVID systems (and nowadays XPG compliant systems) provide minimal support for this task.
l64a is undefined if n is negative. In the GNU
implementation, l64a treats its argument as unsigned, so it will
return a sensible encoding for any nonzero n; however, portable
programs should not rely on this.
To encode a large buffer l64a must be called in a loop, once for
each 32-bit word of the buffer. For example, one could do something
like this:
char *
encode (const void *buf, size_t len)
{
/* We know in advance how long the buffer has to be. */
unsigned char *in = (unsigned char *) buf;
char *out = malloc (6 + ((len + 3) / 4) * 6 + 1);
char *cp = out;
/* Encode the length. */
/* Using `htonl' is necessary so that the data can be
decoded even on machines with different byte order. */
cp = mempcpy (cp, l64a (htonl (len)), 6);
while (len > 3)
{
unsigned long int n = *in++;
n = (n << 8) | *in++;
n = (n << 8) | *in++;
n = (n << 8) | *in++;
len -= 4;
if (n)
cp = mempcpy (cp, l64a (htonl (n)), 6);
else
/* `l64a' returns the empty string for n==0, so we
must generate its encoding ("......") by hand. */
cp = stpcpy (cp, "......");
}
if (len > 0)
{
unsigned long int n = *in++;
if (--len > 0)
{
n = (n << 8) | *in++;
if (--len > 0)
n = (n << 8) | *in;
}
memcpy (cp, l64a (htonl (n)), 6);
cp += 6;
}
*cp = '\0';
return out;
}
It is strange that the library does not provide the complete functionality needed but so be it.
To decode data produced with l64a the following function should be
used.
l64a. The function processes at least 6 characters of
this string, and decodes the characters it finds according to the table
below. It stops decoding when it finds a character not in the table,
rather like atoi; if you have a buffer which has been broken into
lines, you must be careful to skip over the end-of-line characters.
The decoded number is returned as a long int value.
The l64a and a64l functions use a base 64 encoding, in
which each character of an encoded string represents six bits of an
input word. These symbols are used for the base 64 digits:
@multitable {xxxxx} {xxx} {xxx} {xxx} {xxx} {xxx} {xxx} {xxx} {xxx}
. @tab / @tab 0 @tab 1
@tab 2 @tab 3 @tab 4 @tab 5
6 @tab 7 @tab 8 @tab 9
@tab A @tab B @tab C @tab D
E @tab F @tab G @tab H
@tab I @tab J @tab K @tab L
M @tab N @tab O @tab P
@tab Q @tab R @tab S @tab T
U @tab V @tab W @tab X
@tab Y @tab Z @tab a @tab b
c @tab d @tab e @tab f
@tab g @tab h @tab i @tab j
k @tab l @tab m @tab n
@tab o @tab p @tab q @tab r
s @tab t @tab u @tab v
@tab w @tab x @tab y @tab z
This encoding scheme is not standard. There are some other encoding
methods which are much more widely used (UU encoding, MIME encoding).
Generally, it is better to use one of these encodings.
argz vectors are vectors of strings in a contiguous block of
memory, each element separated from its neighbors by null-characters
('\0').
Envz vectors are an extension of argz vectors where each element is a
name-value pair, separated by a '=' character (as in a Unix
environment).
Each argz vector is represented by a pointer to the first element, of
type char *, and a size, of type size_t, both of which can
be initialized to 0 to represent an empty argz vector. All argz
functions accept either a pointer and a size argument, or pointers to
them, if they will be modified.
The argz functions use malloc/realloc to allocate/grow
argz vectors, and so any argz vector creating using these functions may
be freed by using free; conversely, any argz function that may
grow a string expects that string to have been allocated using
malloc (those argz functions that only examine their arguments or
modify them in place will work on any sort of memory).
See section Unconstrained Allocation.
All argz functions that do memory allocation have a return type of
error_t, and return 0 for success, and ENOMEM if an
allocation error occurs.
These functions are declared in the standard include file `argz.h'.
argz_create function converts the Unix-style argument vector
argv (a vector of pointers to normal C strings, terminated by
(char *)0; see section Program Arguments) into an argz vector with
the same elements, which is returned in argz and argz_len.
argz_create_sep function converts the null-terminated string
string into an argz vector (returned in argz and
argz_len) by splitting it into elements at every occurrence of the
character sep.
argz_extract function converts the argz vector argz and
argz_len into a Unix-style argument vector stored in argv,
by putting pointers to every element in argz into successive
positions in argv, followed by a terminator of 0.
Argv must be pre-allocated with enough space to hold all the
elements in argz plus the terminating (char *)0
((argz_count (argz, argz_len) + 1) * sizeof (char *)
bytes should be enough). Note that the string pointers stored into
argv point into argz---they are not copies--and so
argz must be copied if it will be changed while argv is
still active. This function is useful for passing the elements in
argz to an exec function (see section Executing a File).
argz_stringify converts argz into a normal string with
the elements separated by the character sep, by replacing each
'\0' inside argz (except the last one, which terminates the
string) with sep. This is handy for printing argz in a
readable manner.
argz_add function adds the string str to the end of the
argz vector *argz, and updates *argz and
*argz_len accordingly.
argz_add_sep function is similar to argz_add, but
str is split into separate elements in the result at occurrences of
the character delim. This is useful, for instance, for
adding the components of a Unix search path to an argz vector, by using
a value of ':' for delim.
argz_append function appends buf_len bytes starting at
buf to the argz vector *argz, reallocating
*argz to accommodate it, and adding buf_len to
*argz_len.
*argz, the argz_delete function will
remove this entry and reallocate *argz, modifying
*argz and *argz_len accordingly. Note that as
destructive argz functions usually reallocate their argz argument,
pointers into argz vectors such as entry will then become invalid.
argz_insert function inserts the string entry into the
argz vector *argz at a point just before the existing
element pointed to by before, reallocating *argz and
updating *argz and *argz_len. If before
is 0, entry is added to the end instead (as if by
argz_add). Since the first element is in fact the same as
*argz, passing in *argz as the value of
before will result in entry being inserted at the beginning.
argz_next function provides a convenient way of iterating
over the elements in the argz vector argz. It returns a pointer
to the next element in argz after the element entry, or
0 if there are no elements following entry. If entry
is 0, the first element of argz is returned.
This behavior suggests two styles of iteration:
char *entry = 0;
while ((entry = argz_next (argz, argz_len, entry)))
action;
(the double parentheses are necessary to make some C compilers shut up
about what they consider a questionable while-test) and:
char *entry;
for (entry = argz;
entry;
entry = argz_next (argz, argz_len, entry))
action;
Note that the latter depends on argz having a value of 0 if
it is empty (rather than a pointer to an empty block of memory); this
invariant is maintained for argz vectors created by the functions here.
*replace_count will be
incremented by number of replacements performed.
Envz vectors are just argz vectors with additional constraints on the form of each element; as such, argz functions can also be used on them, where it makes sense.
Each element in an envz vector is a name-value pair, separated by a '='
character; if multiple '=' characters are present in an element, those
after the first are considered part of the value, and treated like all other
non-'\0' characters.
If no '=' characters are present in an element, that element is
considered the name of a "null" entry, as distinct from an entry with an
empty value: envz_get will return 0 if given the name of null
entry, whereas an entry with an empty value would result in a value of
""; envz_entry will still find such entries, however. Null
entries can be removed with envz_strip function.
As with argz functions, envz functions that may allocate memory (and thus
fail) have a return type of error_t, and return either 0 or
ENOMEM.
These functions are declared in the standard include file `envz.h'.
envz_entry function finds the entry in envz with the name
name, and returns a pointer to the whole entry--that is, the argz
element which begins with name followed by a '=' character. If
there is no entry with that name, 0 is returned.
envz_get function finds the entry in envz with the name
name (like envz_entry), and returns a pointer to the value
portion of that entry (following the '='). If there is no entry with
that name (or only a null entry), 0 is returned.
envz_add function adds an entry to *envz
(updating *envz and *envz_len) with the name
name, and value value. If an entry with the same name
already exists in envz, it is removed first. If value is
0, then the new entry will the special null type of entry
(mentioned above).
envz_merge function adds each entry in envz2 to envz,
as if with envz_add, updating *envz and
*envz_len. If override is true, then values in envz2
will supersede those with the same name in envz, otherwise not.
Null entries are treated just like other entries in this respect, so a null entry in envz can prevent an entry of the same name in envz2 from being added to envz, if override is false.
envz_strip function removes any null entries from envz,
updating *envz and *envz_len.
@ifnottex @macro cal{text} \text\
Character sets used in the early days of computing had only six, seven, or eight bits for each character: there was never a case where more than eight bits (one byte) were used to represent a single character. The limitations of this approach became more apparent as more people grappled with non-Roman character sets, where not all the characters that make up a language's character set can be represented by @math{2^8} choices. This chapter shows the functionality which was added to the C library to support multiple character sets.
A variety of solutions to overcome the differences between character sets with a 1:1 relation between bytes and characters and character sets with ratios of 2:1 or 4:1 exist. The remainder of this section gives a few examples to help understand the design decisions made while developing the functionality of the C library.
A distinction we have to make right away is between internal and external representation. Internal representation means the representation used by a program while keeping the text in memory. External representations are used when text is stored or transmitted through whatever communication channel. Examples of external representations include files lying in a directory that are going to be read and parsed.
Traditionally there has been no difference between the two representations. It was equally comfortable and useful to use the same single-byte representation internally and externally. This changes with more and larger character sets.
One of the problems to overcome with the internal representation is handling text that is externally encoded using different character sets. Assume a program which reads two texts and compares them using some metric. The comparison can be usefully done only if the texts are internally kept in a common format.
For such a common format (@math{=} character set) eight bits are certainly no longer enough. So the smallest entity will have to grow: wide characters will now be used. Instead of one byte, two or four will be used instead. (Three are not good to address in memory and more than four bytes seem not to be necessary).
As shown in some other part of this manual,
there exists a completely new family of functions which can handle texts
of this kind in memory. The most commonly used character sets for such
internal wide character representations are Unicode and ISO 10646
(also known as UCS for Universal Character Set). Unicode was originally
planned as a 16-bit character set, whereas ISO 10646 was designed to
be a 31-bit large code space. The two standards are practically identical.
They have the same character repertoire and code table, but Unicode specifies
added semantics. At the moment, only characters in the first 0x10000
code positions (the so-called Basic Multilingual Plane, BMP) have been
assigned, but the assignment of more specialized characters outside this
16-bit space is already in progress. A number of encodings have been
defined for Unicode and ISO 10646 characters:
UCS-2 is a 16-bit word that can only represent characters
from the BMP, UCS-4 is a 32-bit word than can represent any Unicode
and ISO 10646 character, UTF-8 is an ASCII compatible encoding where
ASCII characters are represented by ASCII bytes and non-ASCII characters
by sequences of 2-6 non-ASCII bytes, and finally UTF-16 is an extension
of UCS-2 in which pairs of certain UCS-2 words can be used to encode
non-BMP characters up to 0x10ffff.
To represent wide characters the char type is not suitable. For
this reason the ISO C standard introduces a new type which is
designed to keep one character of a wide character string. To maintain
the similarity there is also a type corresponding to int for
those functions which take a single wide character.
char[]
for multibyte character strings. The type is defined in `stddef.h'.
The ISO C90 standard, where this type was introduced, does not say
anything specific about the representation. It only requires that this
type is capable of storing all elements of the basic character set.
Therefore it would be legitimate to define wchar_t as
char. This might make sense for embedded systems.
But for GNU systems this type is always 32 bits wide. It is therefore
capable of representing all UCS-4 values and therefore covering all of
ISO 10646. Some Unix systems define wchar_t as a 16-bit type and
thereby follow Unicode very strictly. This is perfectly fine with the
standard but it also means that to represent all characters from Unicode
and ISO 10646 one has to use UTF-16 surrogate characters which is in
fact a multi-wide-character encoding. But this contradicts the purpose
of the wchar_t type.
wint_t is a data type used for parameters and variables which
contain a single wide character. As the name already suggests it is the
equivalent to int when using the normal char strings. The
types wchar_t and wint_t have often the same
representation if their size if 32 bits wide but if wchar_t is
defined as char the type wint_t must be defined as
int due to the parameter promotion.
This type is defined in `wchar.h' and got introduced in Amendment 1 to ISO C90.
As there are for the char data type there also exist macros
specifying the minimum and maximum value representable in an object of
type wchar_t.
WCHAR_MIN evaluates to the minimum value representable
by an object of type wint_t.
This macro got introduced in Amendment 1 to ISO C90.
WCHAR_MAX evaluates to the maximum value representable
by an object of type wint_t.
This macro got introduced in Amendment 1 to ISO C90.
Another special wide character value is the equivalent to EOF.
WEOF evaluates to a constant expression of type
wint_t whose value is different from any member of the extended
character set.
WEOF need not be the same value as EOF and unlike
EOF it also need not be negative. I.e., sloppy code like
{
int c;
...
while ((c = getc (fp)) < 0)
...
}
has to be rewritten to explicitly use WEOF when wide characters
are used.
{
wint_t c;
...
while ((c = wgetc (fp)) != WEOF)
...
}
This macro was introduced in Amendment 1 to ISO C90 and is defined in `wchar.h'.
These internal representations present problems when it comes to storing and transmittal, since a single wide character consists of more than one byte they are effected by byte-ordering. I.e., machines with different endianesses would see different value accessing the same data. This also applies for communication protocols which are all byte-based and therefore the sender has to decide about splitting the wide character in bytes. A last (but not least important) point is that wide characters often require more storage space than an customized byte oriented character set.
For all the above reasons, an external encoding which is different
from the internal encoding is often used if the latter is UCS-2 or UCS-4.
The external encoding is byte-based and can be chosen appropriately for
the environment and for the texts to be handled. There exist a variety
of different character sets which can be used for this external
encoding. Information which will not be exhaustively presented
here--instead, a description of the major groups will suffice. All of
the ASCII-based character sets [_bkoz_: do you mean Roman character
sets? If not, what do you mean here?] fulfill one requirement: they are
"filesystem safe". This means that the character '/' is used in
the encoding only to represent itself. Things are a bit
different for character sets like EBCDIC (Extended Binary Coded Decimal
Interchange Code, a character set family used by IBM) but if the
operation system does not understand EBCDIC directly the parameters to
system calls have to be converted first anyhow.
0xc2 0x61 (non-spacing
acute accent, following by lower-case `a') to get the "small a with
acute" character. To get the acute accent character on its own, one has
to write 0xc2 0x20 (the non-spacing acute followed by a space).
This type of character set is used in some embedded systems such as
teletex.
The question remaining is: how to select the character set or encoding to use. The answer: you cannot decide about it yourself, it is decided by the developers of the system or the majority of the users. Since the goal is interoperability one has to use whatever the other people one works with use. If there are no constraints the selection is based on the requirements the expected circle of users will have. I.e., if a project is expected to only be used in, say, Russia it is fine to use KOI8-R or a similar character set. But if at the same time people from, say, Greece are participating one should use a character set which allows all people to collaborate.
The most widely useful solution seems to be: go with the most general character set, namely ISO 10646. Use UTF-8 as the external encoding and problems about users not being able to use their own language adequately are a thing of the past.
One final comment about the choice of the wide character representation
is necessary at this point. We have said above that the natural choice
is using Unicode or ISO 10646. This is not required, but at least
encouraged, by the ISO C standard. The standard defines at least a
macro __STDC_ISO_10646__ that is only defined on systems where
the wchar_t type encodes ISO 10646 characters. If this
symbol is not defined one should as much as possible avoid making
assumption about the wide character representation. If the programmer
uses only the functions provided by the C library to handle wide
character strings there should not be any compatibility problems with
other systems.
A Unix C library contains three different sets of functions in two families to handle character set conversion. The one function family is specified in the ISO C standard and therefore is portable even beyond the Unix world.
The most commonly known set of functions, coming from the ISO C90 standard, is unfortunately the least useful one. In fact, these functions should be avoided whenever possible, especially when developing libraries (as opposed to applications).
The second family of functions got introduced in the early Unix standards (XPG2) and is still part of the latest and greatest Unix standard: Unix 98. It is also the most powerful and useful set of functions. But we will start with the functions defined in Amendment 1 to ISO C90.
The ISO C standard defines functions to convert strings from a multibyte representation to wide character strings. There are a number of peculiarities:
LC_CTYPE category of the current locale is used; see
section Categories of Activities that Locales Affect.
Despite these limitations the ISO C functions can very well be used
in many contexts. In graphical user interfaces, for instance, it is not
uncommon to have functions which require text to be displayed in a wide
character string if it is not simple ASCII. The text itself might come
from a file with translations and the user should decide about the
current locale which determines the translation and therefore also the
external encoding used. In such a situation (and many others) the
functions described here are perfect. If more freedom while performing
the conversion is necessary take a look at the iconv functions
(see section Generic Charset Conversion).
We already said above that the currently selected locale for the
LC_CTYPE category decides about the conversion which is performed
by the functions we are about to describe. Each locale uses its own
character set (given as an argument to localedef) and this is the
one assumed as the external multibyte encoding. The wide character
character set always is UCS-4, at least on GNU systems.
A characteristic of each multibyte character set is the maximum number of bytes which can be necessary to represent one character. This information is quite important when writing code which uses the conversion functions. In the examples below we will see some examples. The ISO C standard defines two macros which provide this information.
MB_CUR_MAX expands into a positive integer expression that is the
maximum number of bytes in a multibyte character in the current locale.
The value is never greater than MB_LEN_MAX. Unlike
MB_LEN_MAX this macro need not be a compile-time constant and in
fact, in the GNU C library it is not.
Two different macros are necessary since strictly ISO C90 compilers do not allow variable length array definitions but still it is desirable to avoid dynamic allocation. This incomplete piece of code shows the problem:
{
char buf[MB_LEN_MAX];
ssize_t len = 0;
while (! feof (fp))
{
fread (&buf[len], 1, MB_CUR_MAX - len, fp);
/* ... process buf */
len -= used;
}
}
The code in the inner loop is expected to have always enough bytes in
the array buf to convert one multibyte character. The array
buf has to be sized statically since many compilers do not allow a
variable size. The fread call makes sure that always
MB_CUR_MAX bytes are available in buf. Note that it isn't
a problem if MB_CUR_MAX is not a compile-time constant.
In the introduction of this chapter it was said that certain character sets use a stateful encoding. I.e., the encoded values depend in some way on the previous bytes in the text.
Since the conversion functions allow converting a text in more than one step we must have a way to pass this information from one call of the functions to another.
mbstate_t can contain all the information
about the shift state needed from one call to a conversion
function to another.
This type is defined in `wchar.h'. It got introduced in Amendment 1 to ISO C90.
To use objects of this type the programmer has to define such objects (normally as local variables on the stack) and pass a pointer to the object to the conversion functions. This way the conversion function can update the object if the current multibyte character set is stateful.
There is no specific function or initializer to put the state object in any specific state. The rules are that the object should always represent the initial state before the first use and this is achieved by clearing the whole variable with code such as follows:
{
mbstate_t state;
memset (&state, '\0', sizeof (state));
/* from now on state can be used. */
...
}
When using the conversion functions to generate output it is often necessary to test whether the current state corresponds to the initial state. This is necessary, for example, to decide whether or not to emit escape sequences to set the state to the initial state at certain sequence points. Communication protocols often require this.
This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'.
Code using this function often looks similar to this:
{
mbstate_t state;
memset (&state, '\0', sizeof (state));
/* Use state. */
...
if (! mbsinit (&state))
{
/* Emit code to return to initial state. */
const wchar_t empty[] = L"";
const wchar_t *srcp = empty;
wcsrtombs (outbuf, &srcp, outbuflen, &state);
}
...
}
The code to emit the escape sequence to get back to the initial state is
interesting. The wcsrtombs function can be used to determine the
necessary output code (see section Converting Multibyte and Wide Character Strings). Please note that on
GNU systems it is not necessary to perform this extra action for the
conversion from multibyte text to wide character text since the wide
character encoding is not stateful. But there is nothing mentioned in
any standard which prohibits making wchar_t using a stateful
encoding.
The most fundamental of the conversion functions are those dealing with single characters. Please note that this does not always mean single bytes. But since there is very often a subset of the multibyte character set which consists of single byte sequences there are functions to help with converting bytes. One very important and often applicable scenario is where ASCII is a subpart of the multibyte character set. I.e., all ASCII characters stand for itself and all other characters have at least a first byte which is beyond the range @math{0} to @math{127}.
btowc function ("byte to wide character") converts a valid
single byte character c in the initial shift state into the wide
character equivalent using the conversion rules from the currently
selected locale of the LC_CTYPE category.
If (unsigned char) c is no valid single byte multibyte
character or if c is EOF the function returns WEOF.
Please note the restriction of c being tested for validity only in
the initial shift state. There is no mbstate_t object used from
which the state information is taken and the function also does not use
any static state.
This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'.
Despite the limitation that the single byte value always is interpreted in the initial state this function is actually useful most of the time. Most characters are either entirely single-byte character sets or they are extension to ASCII. But then it is possible to write code like this (not that this specific example is very useful):
wchar_t *
itow (unsigned long int val)
{
static wchar_t buf[30];
wchar_t *wcp = &buf[29];
*wcp = L'\0';
while (val != 0)
{
*--wcp = btowc ('0' + val % 10);
val /= 10;
}
if (wcp == &buf[29])
*--wcp = L'0';
return wcp;
}
Why is it necessary to use such a complicated implementation and not
simply cast '0' + val % 10 to a wide character? The answer is
that there is no guarantee that one can perform this kind of arithmetic
on the character of the character set used for wchar_t
representation. In other situations the bytes are not constant at
compile time and so the compiler cannot do the work. In situations like
this it is necessary btowc.
There also is a function for the conversion in the other direction.
wctob function ("wide character to byte") takes as the
parameter a valid wide character. If the multibyte representation for
this character in the initial state is exactly one byte long the return
value of this function is this character. Otherwise the return value is
EOF.
This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'.
There are more general functions to convert single character from multibyte representation to wide characters and vice versa. These functions pose no limit on the length of the multibyte representation and they also do not require it to be in the initial state.
mbrtowc function ("multibyte restartable to wide
character") converts the next multibyte character in the string pointed
to by s into a wide character and stores it in the wide character
string pointed to by pwc. The conversion is performed according
to the locale currently selected for the LC_CTYPE category. If
the conversion for the character set used in the locale requires a state
the multibyte string is interpreted in the state represented by the
object pointed to by ps. If ps is a null pointer, a static,
internal state variable used only by the mbrtowc function is
used.
If the next multibyte character corresponds to the NUL wide character
the return value of the function is @math{0} and the state object is
afterwards in the initial state. If the next n or fewer bytes
form a correct multibyte character the return value is the number of
bytes starting from s which form the multibyte character. The
conversion state is updated according to the bytes consumed in the
conversion. In both cases the wide character (either the L'\0'
or the one found in the conversion) is stored in the string pointer to
by pwc iff pwc is not null.
If the first n bytes of the multibyte string possibly form a valid
multibyte character but there are more than n bytes needed to
complete it the return value of the function is (size_t) -2 and
no value is stored. Please note that this can happen even if n
has a value greater or equal to MB_CUR_MAX since the input might
contain redundant shift sequences.
If the first n bytes of the multibyte string cannot possibly form
a valid multibyte character also no value is stored, the global variable
errno is set to the value EILSEQ and the function returns
(size_t) -1. The conversion state is afterwards undefined.
This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'.
Using this function is straight forward. A function which copies a multibyte string into a wide character string while at the same time converting all lowercase character into uppercase could look like this (this is not the final version, just an example; it has no error checking, and leaks sometimes memory):
wchar_t *
mbstouwcs (const char *s)
{
size_t len = strlen (s);
wchar_t *result = malloc ((len + 1) * sizeof (wchar_t));
wchar_t *wcp = result;
wchar_t tmp[1];
mbstate_t state;
size_t nbytes;
memset (&state, '\0', sizeof (state));
while ((nbytes = mbrtowc (tmp, s, len, &state)) > 0)
{
if (nbytes >= (size_t) -2)
/* Invalid input string. */
return NULL;
*result++ = towupper (tmp[0]);
len -= nbytes;
s += nbytes;
}
return result;
}
The use of mbrtowc should be clear. A single wide character is
stored in tmp[0] and the number of consumed bytes is stored
in the variable nbytes. In case the the conversion was successful
the uppercase variant of the wide character is stored in the
result array and the pointer to the input string and the number of
available bytes is adjusted.
The only non-obvious thing about the function might be the way memory is allocated for the result. The above code uses the fact that there can never be more wide characters in the converted results than there are bytes in the multibyte input string. This method yields to a pessimistic guess about the size of the result and if many wide character strings have to be constructed this way or the strings are long, the extra memory required allocated because the input string contains multibyte characters might be significant. It would be possible to resize the allocated memory block to the correct size before returning it. A better solution might be to allocate just the right amount of space for the result right away. Unfortunately there is no function to compute the length of the wide character string directly from the multibyte string. But there is a function which does part of the work.
mbrlen function ("multibyte restartable length") computes
the number of at most n bytes starting at s which form the
next valid and complete multibyte character.
If the next multibyte character corresponds to the NUL wide character the return value is @math{0}. If the next n bytes form a valid multibyte character the number of bytes belonging to this multibyte character byte sequence is returned.
If the the first n bytes possibly form a valid multibyte
character but it is incomplete the return value is (size_t) -2.
Otherwise the multibyte character sequence is invalid and the return
value is (size_t) -1.
The multibyte sequence is interpreted in the state represented by the
object pointed to by ps. If ps is a null pointer, a state
object local to mbrlen is used.
This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'.
The tentative reader now will of course note that mbrlen can be
implemented as
mbrtowc (NULL, s, n, ps != NULL ? ps : &internal)
This is true and in fact is mentioned in the official specification.
Now, how can this function be used to determine the length of the wide
character string created from a multibyte character string? It is not
directly usable but we can define a function mbslen using it:
size_t
mbslen (const char *s)
{
mbstate_t state;
size_t result = 0;
size_t nbytes;
memset (&state, '\0', sizeof (state));
while ((nbytes = mbrlen (s, MB_LEN_MAX, &state)) > 0)
{
if (nbytes >= (size_t) -2)
/* Something is wrong. */
return (size_t) -1;
s += nbytes;
++result;
}
return result;
}
This function simply calls mbrlen for each multibyte character
in the string and counts the number of function calls. Please note that
we here use MB_LEN_MAX as the size argument in the mbrlen
call. This is OK since a) this value is larger then the length of the
longest multibyte character sequence and b) because we know that the
string s ends with a NUL byte which cannot be part of any other
multibyte character sequence but the one representing the NUL wide
character. Therefore the mbrlen function will never read invalid
memory.
Now that this function is available (just to make this clear, this function is not part of the GNU C library) we can compute the number of wide character required to store the converted multibyte character string s using
wcs_bytes = (mbslen (s) + 1) * sizeof (wchar_t);
Please note that the mbslen function is quite inefficient. The
implementation of mbstouwcs implemented using mbslen would
have to perform the conversion of the multibyte character input string
twice and this conversion might be quite expensive. So it is necessary
to think about the consequences of using the easier but imprecise method
before doing the work twice.
wcrtomb function ("wide character restartable to
multibyte") converts a single wide character into a multibyte string
corresponding to that wide character.
If s is a null pointer the function resets the the state stored in
the objects pointer to by ps (or the internal mbstate_t
object) to the initial state. This can also be achieved by a call like
this:
wcrtombs (temp_buf, L'\0', ps)
since if s is a null pointer wcrtomb performs as if it
writes into an internal buffer which is guaranteed to be large enough.
If wc is the NUL wide character wcrtomb emits, if
necessary, a shift sequence to get the state ps into the initial
state followed by a single NUL byte is stored in the string s.
Otherwise a byte sequence (possibly including shift sequences) is
written into the string s. This of only happens if wc is a
valid wide character, i.e., it has a multibyte representation in the
character set selected by locale of the LC_CTYPE category. If
wc is no valid wide character nothing is stored in the strings
s, errno is set to EILSEQ, the conversion state in
ps is undefined and the return value is (size_t) -1.
If no error occurred the function returns the number of bytes stored in the string s. This includes all byte representing shift sequences.
One word about the interface of the function: there is no parameter
specifying the length of the array s. Instead the function
assumes that there are at least MB_CUR_MAX bytes available since
this is the maximum length of any byte sequence representing a single
character. So the caller has to make sure that there is enough space
available, otherwise buffer overruns can occur.
This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'.
Using this function is as easy as using mbrtowc. The following
example appends a wide character string to a multibyte character string.
Again, the code is not really useful (and correct), it is simply here to
demonstrate the use and some problems.
char *
mbscatwcs (char *s, size_t len, const wchar_t *ws)
{
mbstate_t state;
/* Find the end of the existing string. */
char *wp = strchr (s, '\0');
len -= wp - s;
memset (&state, '\0', sizeof (state));
do
{
size_t nbytes;
if (len < MB_CUR_LEN)
{
/* We cannot guarantee that the next
character fits into the buffer, so
return an error. */
errno = E2BIG;
return NULL;
}
nbytes = wcrtomb (wp, *ws, &state);
if (nbytes == (size_t) -1)
/* Error in the conversion. */
return NULL;
len -= nbytes;
wp += nbytes;
}
while (*ws++ != L'\0');
return s;
}
First the function has to find the end of the string currently in the
array s. The strchr call does this very efficiently since a
requirement for multibyte character representations is that the NUL byte
never is used except to represent itself (and in this context, the end
of the string).
After initializing the state object the loop is entered where the first
task is to make sure there is enough room in the array s. We
abort if there are not at least MB_CUR_LEN bytes available. This
is not always optimal but we have no other choice. We might have less
than MB_CUR_LEN bytes available but the next multibyte character
might also be only one byte long. At the time the wcrtomb call
returns it is too late to decide whether the buffer was large enough or
not. If this solution is really unsuitable there is a very slow but
more accurate solution.
...
if (len < MB_CUR_LEN)
{
mbstate_t temp_state;
memcpy (&temp_state, &state, sizeof (state));
if (wcrtomb (NULL, *ws, &temp_state) > len)
{
/* We cannot guarantee that the next
character fits into the buffer, so
return an error. */
errno = E2BIG;
return NULL;
}
}
...
Here we do perform the conversion which might overflow the buffer so
that we are afterwards in the position to make an exact decision about
the buffer size. Please note the NULL argument for the
destination buffer in the new wcrtomb call; since we are not
interested in the converted text at this point this is a nice way to
express this. The most unusual thing about this piece of code certainly
is the duplication of the conversion state object. But think about
this: if a change of the state is necessary to emit the next multibyte
character we want to have the same shift state change performed in the
real conversion. Therefore we have to preserve the initial shift state
information.
There are certainly many more and even better solutions to this problem. This example is only meant for educational purposes.
The functions described in the previous section only convert a single character at a time. Most operations to be performed in real-world programs include strings and therefore the ISO C standard also defines conversions on entire strings. However, the defined set of functions is quite limited, thus the GNU C library contains a few extensions which can help in some important situations.
mbsrtowcs function ("multibyte string restartable to wide
character string") converts an NUL terminated multibyte character
string at *src into an equivalent wide character string,
including the NUL wide character at the end. The conversion is started
using the state information from the object pointed to by ps or
from an internal object of mbsrtowcs if ps is a null
pointer. Before returning the state object to match the state after the
last converted character. The state is the initial state if the
terminating NUL byte is reached and converted.
If dst is not a null pointer the result is stored in the array pointed to by dst, otherwise the conversion result is not available since it is stored in an internal buffer.
If len wide characters are stored in the array dst before reaching the end of the input string the conversion stops and len is returned. If dst is a null pointer len is never checked.
Another reason for a premature return from the function call is if the
input string contains an invalid multibyte sequence. In this case the
global variable errno is set to EILSEQ and the function
returns (size_t) -1.
In all other cases the function returns the number of wide characters
converted during this call. If dst is not null mbsrtowcs
stores in the pointer pointed to by src a null pointer (if the NUL
byte in the input string was reached) or the address of the byte
following the last converted multibyte character.
This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'.
The definition of this function has one limitation which has to be
understood. The requirement that dst has to be a NUL terminated
string provides problems if one wants to convert buffers with text. A
buffer is normally no collection of NUL terminated strings but instead a
continuous collection of lines, separated by newline characters. Now
assume a function to convert one line from a buffer is needed. Since
the line is not NUL terminated the source pointer cannot directly point
into the unmodified text buffer. This means, either one inserts the NUL
byte at the appropriate place for the time of the mbsrtowcs
function call (which is not doable for a read-only buffer or in a
multi-threaded application) or one copies the line in an extra buffer
where it can be terminated by a NUL byte. Note that it is not in
general possible to limit the number of characters to convert by setting
the parameter len to any specific value. Since it is not known
how many bytes each multibyte character sequence is in length one always
could do only a guess.
There is still a problem with the method of NUL-terminating a line right after the newline character which could lead to very strange results. As said in the description of the mbsrtowcs function above the conversion state is guaranteed to be in the initial shift state after processing the NUL byte at the end of the input string. But this NUL byte is not really part of the text. I.e., the conversion state after the newline in the original text could be something different than the initial shift state and therefore the first character of the next line is encoded using this state. But the state in question is never accessible to the user since the conversion stops after the NUL byte (which resets the state). Most stateful character sets in use today require that the shift state after a newline is the initial state--but this is not a strict guarantee. Therefore simply NUL terminating a piece of a running text is not always an adequate solution and therefore never should be used in generally used code.
The generic conversion interface (see section Generic Charset Conversion)
does not have this limitation (it simply works on buffers, not
strings), and the GNU C library contains a set of functions which take
additional parameters specifying the maximal number of bytes which are
consumed from the input string. This way the problem of
mbsrtowcs's example above could be solved by determining the line
length and passing this length to the function.
wcsrtombs function ("wide character string restartable to
multibyte string") converts the NUL terminated wide character string at
*src into an equivalent multibyte character string and
stores the result in the array pointed to by dst. The NUL wide
character is also converted. The conversion starts in the state
described in the object pointed to by ps or by a state object
locally to wcsrtombs in case ps is a null pointer. If
dst is a null pointer the conversion is performed as usual but the
result is not available. If all characters of the input string were
successfully converted and if dst is not a null pointer the
pointer pointed to by src gets assigned a null pointer.
If one of the wide characters in the input string has no valid multibyte
character equivalent the conversion stops early, sets the global
variable errno to EILSEQ, and returns (size_t) -1.
Another reason for a premature stop is if dst is not a null pointer and the next converted character would require more than len bytes in total to the array dst. In this case (and if dest is not a null pointer) the pointer pointed to by src is assigned a value pointing to the wide character right after the last one successfully converted.
Except in the case of an encoding error the return value of the function is the number of bytes in all the multibyte character sequences stored in dst. Before returning the state in the object pointed to by ps (or the internal object in case ps is a null pointer) is updated to reflect the state after the last conversion. The state is the initial shift state in case the terminating NUL wide character was converted.
This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'.
The restriction mentions above for the mbsrtowcs function applies
also here. There is no possibility to directly control the number of
input characters. One has to place the NUL wide character at the
correct place or control the consumed input indirectly via the available
output array size (the len parameter).
mbsnrtowcs function is very similar to the mbsrtowcs
function. All the parameters are the same except for nmc which is
new. The return value is the same as for mbsrtowcs.
This new parameter specifies how many bytes at most can be used from the
multibyte character string. I.e., the multibyte character string
*src need not be NUL terminated. But if a NUL byte is
found within the nmc first bytes of the string the conversion
stops here.
This function is a GNU extensions. It is meant to work around the problems mentioned above. Now it is possible to convert buffer with multibyte character text piece for piece without having to care about inserting NUL bytes and the effect of NUL bytes on the conversion state.
A function to convert a multibyte string into a wide character string and display it could be written like this (this is not a really useful example):
void
showmbs (const char *src, FILE *fp)
{
mbstate_t state;
int cnt = 0;
memset (&state, '\0', sizeof (state));
while (1)
{
wchar_t linebuf[100];
const char *endp = strchr (src, '\n');
size_t n;
/* Exit if there is no more line. */
if (endp == NULL)
break;
n = mbsnrtowcs (linebuf, &src, endp - src, 99, &state);
linebuf[n] = L'\0';
fprintf (fp, "line %d: \"%S\"\n", linebuf);
}
}
There is no problem with the state after a call to mbsnrtowcs.
Since we don't insert characters in the strings which were not in there
right from the beginning and we use state only for the conversion
of the given buffer there is no problem with altering the state.
wcsnrtombs function implements the conversion from wide
character strings to multibyte character strings. It is similar to
wcsrtombs but it takes, just like mbsnrtowcs, an extra
parameter which specifies the length of the input string.
No more than nwc wide characters from the input string
*src are converted. If the input string contains a NUL
wide character in the first nwc character to conversion stops at
this place.
This function is a GNU extension and just like mbsnrtowcs is
helps in situations where no NUL terminated input strings are available.
The example programs given in the last sections are only brief and do
not contain all the error checking etc. Presented here is a complete
and documented example. It features the mbrtowc function but it
should be easy to derive versions using the other functions.
int
file_mbsrtowcs (int input, int output)
{
/* Note the use of MB_LEN_MAX.
MB_CUR_MAX cannot portably be used here. */
char buffer[BUFSIZ + MB_LEN_MAX];
mbstate_t state;
int filled = 0;
int eof = 0;
/* Initialize the state. */
memset (&state, '\0', sizeof (state));
while (!eof)
{
ssize_t nread;
ssize_t nwrite;
char *inp = buffer;
wchar_t outbuf[BUFSIZ];
wchar_t *outp = outbuf;
/* Fill up the buffer from the input file. */
nread = read (input, buffer + filled, BUFSIZ);
if (nread < 0)
{
perror ("read");
return 0;
}
/* If we reach end of file, make a note to read no more. */
if (nread == 0)
eof = 1;
/* filled is now the number of bytes in buffer. */
filled += nread;
/* Convert those bytes to wide characters--as many as we can. */
while (1)
{
size_t thislen = mbrtowc (outp, inp, filled, &state);
/* Stop converting at invalid character;
this can mean we have read just the first part
of a valid character. */
if (thislen == (size_t) -1)
break;
/* We want to handle embedded NUL bytes
but the return value is 0. Correct this. */
if (thislen == 0)
thislen = 1;
/* Advance past this character. */
inp += thislen;
filled -= thislen;
++outp;
}
/* Write the wide characters we just made. */
nwrite = write (output, outbuf,
(outp - outbuf) * sizeof (wchar_t));
if (nwrite < 0)
{
perror ("write");
return 0;
}
/* See if we have a real invalid character. */
if ((eof && filled > 0) || filled >= MB_CUR_MAX)
{
error (0, 0, "invalid multibyte character");
return 0;
}
/* If any characters must be carried forward,
put them at the beginning of buffer. */
if (filled > 0)
memmove (inp, buffer, filled);
}
return 1;
}
The functions described in the last chapter are defined in Amendment 1 to ISO C90. But the original ISO C90 standard also contained functions for character set conversion. The reason that they are not described in the first place is that they are almost entirely useless.
The problem is that all the functions for conversion defined in ISO C90 use a local state. This implies that multiple conversions at the same time (not only when using threads) cannot be done, and that you cannot first convert single characters and then strings since you cannot tell the conversion functions which state to use.
These functions are therefore usable only in a very limited set of situations. One must complete converting the entire string before starting a new one and each string/text must be converted with the same function (there is no problem with the library itself; it is guaranteed that no library function changes the state of any of these functions). For the above reasons it is highly requested that the functions from the last section are used in place of non-reentrant conversion functions.
mbtowc ("multibyte to wide character") function when called
with non-null string converts the first multibyte character
beginning at string to its corresponding wide character code. It
stores the result in *result.
mbtowc never examines more than size bytes. (The idea is
to supply for size the number of bytes of data you have in hand.)
mbtowc with non-null string distinguishes three
possibilities: the first size bytes at string start with
valid multibyte character, they start with an invalid byte sequence or
just part of a character, or string points to an empty string (a
null character).
For a valid multibyte character, mbtowc converts it to a wide
character and stores that in *result, and returns the
number of bytes in that character (always at least @math{1}, and never
more than size).
For an invalid byte sequence, mbtowc returns @math{-1}. For an
empty string, it returns @math{0}, also storing '\0' in
*result.
If the multibyte character code uses shift characters, then
mbtowc maintains and updates a shift state as it scans. If you
call mbtowc with a null pointer for string, that
initializes the shift state to its standard initial value. It also
returns nonzero if the multibyte character code in use actually has a
shift state. See section States in Non-reentrant Functions.
wctomb ("wide character to multibyte") function converts
the wide character code wchar to its corresponding multibyte
character sequence, and stores the result in bytes starting at
string. At most MB_CUR_MAX characters are stored.
wctomb with non-null string distinguishes three
possibilities for wchar: a valid wide character code (one that can
be translated to a multibyte character), an invalid code, and L'\0'.
Given a valid code, wctomb converts it to a multibyte character,
storing the bytes starting at string. Then it returns the number
of bytes in that character (always at least @math{1}, and never more
than MB_CUR_MAX).
If wchar is an invalid wide character code, wctomb returns
@math{-1}. If wchar is L'\0', it returns 0, also
storing '\0' in *string.
If the multibyte character code uses shift characters, then
wctomb maintains and updates a shift state as it scans. If you
call wctomb with a null pointer for string, that
initializes the shift state to its standard initial value. It also
returns nonzero if the multibyte character code in use actually has a
shift state. See section States in Non-reentrant Functions.
Calling this function with a wchar argument of zero when
string is not null has the side-effect of reinitializing the
stored shift state as well as storing the multibyte character
'\0' and returning @math{0}.
Similar to mbrlen there is also a non-reentrant function which
computes the length of a multibyte character. It can be defined in
terms of mbtowc.
mblen function with a non-null string argument returns
the number of bytes that make up the multibyte character beginning at
string, never examining more than size bytes. (The idea is
to supply for size the number of bytes of data you have in hand.)
The return value of mblen distinguishes three possibilities: the
first size bytes at string start with valid multibyte
character, they start with an invalid byte sequence or just part of a
character, or string points to an empty string (a null character).
For a valid multibyte character, mblen returns the number of
bytes in that character (always at least 1, and never more than
size). For an invalid byte sequence, mblen returns
@math{-1}. For an empty string, it returns @math{0}.
If the multibyte character code uses shift characters, then mblen
maintains and updates a shift state as it scans. If you call
mblen with a null pointer for string, that initializes the
shift state to its standard initial value. It also returns a nonzero
value if the multibyte character code in use actually has a shift state.
See section States in Non-reentrant Functions.
For convenience reasons the ISO C90 standard defines also functions to convert entire strings instead of single characters. These functions suffer from the same problems as their reentrant counterparts from Amendment 1 to ISO C90; see section Converting Multibyte and Wide Character Strings.
mbstowcs ("multibyte string to wide character string")
function converts the null-terminated string of multibyte characters
string to an array of wide character codes, storing not more than
size wide characters into the array beginning at wstring.
The terminating null character counts towards the size, so if size
is less than the actual number of wide characters resulting from
string, no terminating null character is stored.
The conversion of characters from string begins in the initial shift state.
If an invalid multibyte character sequence is found, this function returns a value of @math{-1}. Otherwise, it returns the number of wide characters stored in the array wstring. This number does not include the terminating null character, which is present if the number is less than size.
Here is an example showing how to convert a string of multibyte characters, allocating enough space for the result.
wchar_t *
mbstowcs_alloc (const char *string)
{
size_t size = strlen (string) + 1;
wchar_t *buf = xmalloc (size * sizeof (wchar_t));
size = mbstowcs (buf, string, size);
if (size == (size_t) -1)
return NULL;
buf = xrealloc (buf, (size + 1) * sizeof (wchar_t));
return buf;
}
wcstombs ("wide character string to multibyte string")
function converts the null-terminated wide character array wstring
into a string containing multibyte characters, storing not more than
size bytes starting at string, followed by a terminating
null character if there is room. The conversion of characters begins in
the initial shift state.
The terminating null character counts towards the size, so if size is less than or equal to the number of bytes needed in wstring, no terminating null character is stored.
If a code that does not correspond to a valid multibyte character is found, this function returns a value of @math{-1}. Otherwise, the return value is the number of bytes stored in the array string. This number does not include the terminating null character, which is present if the number is less than size.
In some multibyte character codes, the meaning of any particular byte sequence is not fixed; it depends on what other sequences have come earlier in the same string. Typically there are just a few sequences that can change the meaning of other sequences; these few are called shift sequences and we say that they set the shift state for other sequences that follow.
To illustrate shift state and shift sequences, suppose we decide that
the sequence 0200 (just one byte) enters Japanese mode, in which
pairs of bytes in the range from 0240 to 0377 are single
characters, while 0201 enters Latin-1 mode, in which single bytes
in the range from 0240 to 0377 are characters, and
interpreted according to the ISO Latin-1 character set. This is a
multibyte code which has two alternative shift states ("Japanese mode"
and "Latin-1 mode"), and two shift sequences that specify particular
shift states.
When the multibyte character code in use has shift states, then
mblen, mbtowc and wctomb must maintain and update
the current shift state as they scan the string. To make this work
properly, you must follow these rules:
mblen (NULL,
0). This initializes the shift state to its standard initial value.
Here is an example of using mblen following these rules:
void
scan_string (char *s)
{
int length = strlen (s);
/* Initialize shift state. */
mblen (NULL, 0);
while (1)
{
int thischar = mblen (s, length);
/* Deal with end of string and invalid characters. */
if (thischar == 0)
break;
if (thischar == -1)
{
error ("invalid multibyte character");
break;
}
/* Advance past this character. */
s += thischar;
length -= thischar;
}
}
The functions mblen, mbtowc and wctomb are not
reentrant when using a multibyte code that uses a shift state. However,
no other library functions call these functions, so you don't have to
worry that the shift state will be changed mysteriously.
The conversion functions mentioned so far in this chapter all had in
common that they operate on character sets which are not directly
specified by the functions. The multibyte encoding used is specified by
the currently selected locale for the LC_CTYPE category. The
wide character set is fixed by the implementation (in the case of GNU C
library it always is UCS-4 encoded ISO 10646.
This has of course several problems when it comes to general character conversion:
LC_CTYPE category,
one has to change the LC_CTYPE locale using setlocale.
This introduces major problems for the rest of the programs since
several more functions (e.g., the character classification functions,
see section Classification of Characters) use the LC_CTYPE category.
LC_CTYPE selection is global and shared by all
threads.
wchar_t representation there is at least a two-step
process necessary to convert a text using the functions above. One
would have to select the source character set as the multibyte encoding,
convert the text into a wchar_t text, select the destination
character set as the multibyte encoding and convert the wide character
text to the multibyte (@math{=} destination) character set.
Even if this is possible (which is not guaranteed) it is a very tiring
work. Plus it suffers from the other two raised points even more due to
the steady changing of the locale.
The XPG2 standard defines a completely new set of functions which has none of these limitations. They are not at all coupled to the selected locales and they but no constraints on the character sets selected for source and destination. Only the set of available conversions is limiting them. The standard does not specify that any conversion at all must be available. It is a measure of the quality of the implementation.
In the following text first the interface to iconv, the
conversion function, will be described. Comparisons with other
implementations will show what pitfalls lie on the way of portable
applications. At last, the implementation is described as far as
interesting to the advanced user who wants to extend the conversion
capabilities.
This set of functions follows the traditional cycle of using a resource: open--use--close. The interface consists of three functions, each of which implement one step.
Before the interfaces are described it is necessary to introduce a datatype. Just like other open--use--close interface the functions introduced here work using a handles and the `iconv.h' header defines a special type for the handles used.
Objects of this type can get assigned handles for the conversions using
the iconv functions. The objects themselves need not be freed but
the conversions for which the handles stand for have to.
The first step is the function to create a handle.
iconv_open function has to be used before starting a
conversion. The two parameters this function takes determine the
source and destination character set for the conversion and if the
implementation has the possibility to perform such a conversion the
function returns a handle.
If the wanted conversion is not available the function returns
(iconv_t) -1. In this case the global variable errno can
have the following values:
EMFILE
OPEN_MAX file descriptors open.
ENFILE
ENOMEM
EINVAL
It is not possible to use the same descriptor in different threads to perform independent conversions. Within the data structures associated with the descriptor there is information about the conversion state. This must not be messed up by using it in different conversions.
An iconv descriptor is like a file descriptor as for every use a
new descriptor must be created. The descriptor does not stand for all
of the conversions from fromset to toset.
The GNU C library implementation of iconv_open has one
significant extension to other implementations. To ease the extension
of the set of available conversions the implementation allows storing
the necessary files with data and code in arbitrarily many directories.
How this extension has to be written will be explained below
(see section The iconv Implementation in the GNU C library). Here it is only important to say
that all directories mentioned in the GCONV_PATH environment
variable are considered if they contain a file `gconv-modules'.
These directories need not necessarily be created by the system
administrator. In fact, this extension is introduced to help users
writing and using their own, new conversions. Of course this does not work
for security reasons in SUID binaries; in this case only the system
directory is considered and this normally is
`prefix/lib/gconv'. The GCONV_PATH environment
variable is examined exactly once at the first call of the
iconv_open function. Later modifications of the variable have no
effect.
This function got introduced early in the X/Open Portability Guide, version 2. It is supported by all commercial Unices as it is required for the Unix branding. However, the quality and completeness of the implementation varies widely. The function is declared in `iconv.h'.
The iconv implementation can associate large data structure with
the handle returned by iconv_open. Therefore it is crucial to
free all the resources once all conversions are carried out and the
conversion is not needed anymore.
iconv_close function frees all resources associated with the
handle cd which must have been returned by a successful call to
the iconv_open function.
If the function call was successful the return value is @math{0}.
Otherwise it is @math{-1} and errno is set appropriately.
Defined error are:
EBADF
This function was introduced together with the rest of the iconv
functions in XPG2 and it is declared in `iconv.h'.
The standard defines only one actual conversion function. This has therefore the most general interface: it allows conversion from one buffer to another. Conversion from a file to a buffer, vice versa, or even file to file can be implemented on top of it.
iconv function converts the text in the input buffer
according to the rules associated with the descriptor cd and
stores the result in the output buffer. It is possible to call the
function for the same text several times in a row since for stateful
character sets the necessary state information is kept in the data
structures associated with the descriptor.
The input buffer is specified by *inbuf and it contains
*inbytesleft bytes. The extra indirection is necessary for
communicating the used input back to the caller (see below). It is
important to note that the buffer pointer is of type char and the
length is measured in bytes even if the input text is encoded in wide
characters.
The output buffer is specified in a similar way. *outbuf
points to the beginning of the buffer with at least
*outbytesleft bytes room for the result. The buffer
pointer again is of type char and the length is measured in
bytes. If outbuf or *outbuf is a null pointer the
conversion is performed but no output is available.
If inbuf is a null pointer the iconv function performs the
necessary action to put the state of the conversion into the initial
state. This is obviously a no-op for non-stateful encodings, but if the
encoding has a state such a function call might put some byte sequences
in the output buffer which perform the necessary state changes. The
next call with inbuf not being a null pointer then simply goes on
from the initial state. It is important that the programmer never makes
any assumption on whether the conversion has to deal with states or not.
Even if the input and output character sets are not stateful the
implementation might still have to keep states. This is due to the
implementation chosen for the GNU C library as it is described below.
Therefore an iconv call to reset the state should always be
performed if some protocol requires this for the output text.
The conversion stops for three reasons. The first is that all characters from the input buffer are converted. This actually can mean two things: really all bytes from the input buffer are consumed or there are some bytes at the end of the buffer which possibly can form a complete character but the input is incomplete. The second reason for a stop is when the output buffer is full. And the third reason is that the input contains invalid characters.
In all these cases the buffer pointers after the last successful conversion, for input and output buffer, are stored in inbuf and outbuf and the available room in each buffer is stored in inbytesleft and outbytesleft.
Since the character sets selected in the iconv_open call can be
almost arbitrary there can be situations where the input buffer contains
valid characters which have no identical representation in the output
character set. The behavior in this situation is undefined. The
current behavior of the GNU C library in this situation is to
return with an error immediately. This certainly is not the most
desirable solution. Therefore future versions will provide better ones
but they are not yet finished.
If all input from the input buffer is successfully converted and stored
in the output buffer the function returns the number of non-reversible
conversions performed. In all other cases the return value is
(size_t) -1 and errno is set appropriately. In this case
the value pointed to by inbytesleft is nonzero.
EILSEQ
*inbuf points at the first byte of the
invalid byte sequence.
E2BIG
EINVAL
EBADF
This function was introduced in the XPG2 standard and is declared in the `iconv.h' header.
The definition of the iconv function is quite good overall. It
provides quite flexible functionality. The only problems lie in the
boundary cases which are incomplete byte sequences at the end of the
input buffer and invalid input. A third problem, which is not really
a design problem, is the way conversions are selected. The standard
does not say anything about the legitimate names, a minimal set of
available conversions. We will see how this negatively impacts other
implementations, as is demonstrated below.
iconv example
The example below features a solution for a common problem. Given that
one knows the internal encoding used by the system for wchar_t
strings one often is in the position to read text from a file and store
it in wide character buffers. One can do this using mbsrtowcs
but then we run into the problems discussed above.
int
file2wcs (int fd, const char *charset, wchar_t *outbuf, size_t avail)
{
char inbuf[BUFSIZ];
size_t insize = 0;
char *wrptr = (char *) outbuf;
int result = 0;
iconv_t cd;
cd = iconv_open ("WCHAR_T", charset);
if (cd == (iconv_t) -1)
{
/* Something went wrong. */
if (errno == EINVAL)
error (0, 0, "conversion from '%s' to wchar_t not available",
charset);
else
perror ("iconv_open");
/* Terminate the output string. */
*outbuf = L'\0';
return -1;
}
while (avail > 0)
{
size_t nread;
size_t nconv;
char *inptr = inbuf;
/* Read more input. */
nread = read (fd, inbuf + insize, sizeof (inbuf) - insize);
if (nread == 0)
{
/* When we come here the file is completely read.
This still could mean there are some unused
characters in the inbuf. Put them back. */
if (lseek (fd, -insize, SEEK_CUR) == -1)
result = -1;
/* Now write out the byte sequence to get into the
initial state if this is necessary. */
iconv (cd, NULL, NULL, &wrptr, &avail);
break;
}
insize += nread;
/* Do the conversion. */
nconv = iconv (cd, &inptr, &insize, &wrptr, &avail);
if (nconv == (size_t) -1)
{
/* Not everything went right. It might only be
an unfinished byte sequence at the end of the
buffer. Or it is a real problem. */
if (errno == EINVAL)
/* This is harmless. Simply move the unused
bytes to the beginning of the buffer so that
they can be used in the next round. */
memmove (inbuf, inptr, insize);
else
{
/* It is a real problem. Maybe we ran out of
space in the output buffer or we have invalid
input. In any case back the file pointer to
the position of the last processed byte. */
lseek (fd, -insize, SEEK_CUR);
result = -1;
break;
}
}
}
/* Terminate the output string. */
if (avail >= sizeof (wchar_t))
*((wchar_t *) wrptr) = L'\0';
if (iconv_close (cd) != 0)
perror ("iconv_close");
return (wchar_t *) wrptr - outbuf;
}
This example shows the most important aspects of using the iconv
functions. It shows how successive calls to iconv can be used to
convert large amounts of text. The user does not have to care about
stateful encodings as the functions take care of everything.
An interesting point is the case where iconv return an error and
errno is set to EINVAL. This is not really an error in
the transformation. It can happen whenever the input character set
contains byte sequences of more than one byte for some character and
texts are not processed in one piece. In this case there is a chance
that a multibyte sequence is cut. The caller than can simply read the
remainder of the takes and feed the offending bytes together with new
character from the input to iconv and continue the work. The
internal state kept in the descriptor is not unspecified after
such an event as it is the case with the conversion functions from the
ISO C standard.
The example also shows the problem of using wide character strings with
iconv. As explained in the description of the iconv
function above the function always takes a pointer to a char
array and the available space is measured in bytes. In the example the
output buffer is a wide character buffer. Therefore we use a local
variable wrptr of type char * which is used in the
iconv calls.
This looks rather innocent but can lead to problems on platforms which
have tight restriction on alignment. Therefore the caller of
iconv has to make sure that the pointers passed are suitable for
access of characters from the appropriate character set. Since in the
above case the input parameter to the function is a wchar_t
pointer this is the case (unless the user violates alignment when
computing the parameter). But in other situations, especially when
writing generic functions where one does not know what type of character
set one uses and therefore treats text as a sequence of bytes, it might
become tricky.
iconv Implementations
This is not really the place to discuss the iconv implementation
of other systems but it is necessary to know a bit about them to write
portable programs. The above mentioned problems with the specification
of the iconv functions can lead to portability issues.
The first thing to notice is that due to the large number of character sets in use it is certainly not practical to encode the conversions directly in the C library. Therefore the conversion information must come from files outside the C library. This is usually done in one or both of the following ways:
Some implementations in commercial Unices implement a mixture of these these possibilities, the majority only the second solution. Using loadable modules moves the code out of the library itself and keeps the door open for extensions and improvements. But this design is also limiting on some platforms since not many platforms support dynamic loading in statically linked programs. On platforms without his capability it is therefore not possible to use this interface in statically linked programs. The GNU C library has on ELF platforms no problems with dynamic loading in in these situations and therefore this point is moot. The danger is that one gets acquainted with this and forgets about the restrictions on other systems.
A second thing to know about other iconv implementations is that
the number of available conversions is often very limited. Some
implementations provide in the standard release (not special
international or developer releases) at most 100 to 200 conversion
possibilities. This does not mean 200 different character sets are
supported. E.g., conversions from one character set to a set of, say,
10 others counts as 10 conversion. Together with the other direction
this makes already 20. One can imagine the thin coverage these platform
provide. Some Unix vendors even provide only a handful of conversions
which renders them useless for almost all uses.
This directly leads to a third and probably the most problematic point.
The way the iconv conversion functions are implemented on all
known Unix system and the availability of the conversion functions from
character set @math{@cal{A}} to @math{@cal{B}} and the conversion from
@math{@cal{B}} to @math{@cal{C}} does not imply that the
conversion from @math{@cal{A}} to @math{@cal{C}} is available.
This might not seem unreasonable and problematic at first but it is a quite big problem as one will notice shortly after hitting it. To show the problem we assume to write a program which has to convert from @math{@cal{A}} to @math{@cal{C}}. A call like
cd = iconv_open ("@math{@cal{C}}", "@math{@cal{A}}");
does fail according to the assumption above. But what does the program do now? The conversion is really necessary and therefore simply giving up is no possibility.
This is a nuisance. The iconv function should take care of this.
But how should the program proceed from here on? If it would try to
convert to character set @math{@cal{B}} first the two iconv_open
calls
cd1 = iconv_open ("@math{@cal{B}}", "@math{@cal{A}}");
and
cd2 = iconv_open ("@math{@cal{C}}", "@math{@cal{B}}");
will succeed but how to find @math{@cal{B}}?
Unfortunately, the answer is: there is no general solution. On some systems guessing might help. On those systems most character sets can convert to and from UTF-8 encoded ISO 10646 or Unicode text. Beside this only some very system-specific methods can help. Since the conversion functions come from loadable modules and these modules must be stored somewhere in the filesystem, one could try to find them and determine from the available file which conversions are available and whether there is an indirect route from @math{@cal{A}} to @math{@cal{C}}.
This shows one of the design errors of iconv mentioned above. It
should at least be possible to determine the list of available
conversion programmatically so that if iconv_open says there is
no such conversion, one could make sure this also is true for indirect
routes.
iconv Implementation in the GNU C library
After reading about the problems of iconv implementations in the
last section it is certainly good to note that the implementation in
the GNU C library has none of the problems mentioned above. What
follows is a step-by-step analysis of the points raised above. The
evaluation is based on the current state of the development (as of
January 1999). The development of the iconv functions is not
complete, but basic functionality has solidified.
The GNU C library's iconv implementation uses shared loadable
modules to implement the conversions. A very small number of
conversions are built into the library itself but these are only rather
trivial conversions.
All the benefits of loadable modules are available in the GNU C library
implementation. This is especially appealing since the interface is
well documented (see below) and it therefore is easy to write new
conversion modules. The drawback of using loadable objects is not a
problem in the GNU C library, at least on ELF systems. Since the
library is able to load shared objects even in statically linked
binaries this means that static linking needs not to be forbidden in
case one wants to use iconv.
The second mentioned problem is the number of supported conversions. Currently, the GNU C library supports more than 150 character sets. The way the implementation is designed the number of supported conversions is greater than 22350 (@math{150} times @math{149}). If any conversion from or to a character set is missing it can easily be added.
Particularly impressive as it may be, this high number is due to the
fact that the GNU C library implementation of iconv does not have
the third problem mentioned above. I.e., whenever there is a conversion
from a character set @math{@cal{A}} to @math{@cal{B}} and from
@math{@cal{B}} to @math{@cal{C}} it is always possible to convert from
@math{@cal{A}} to @math{@cal{C}} directly. If the iconv_open
returns an error and sets errno to EINVAL this really
means there is no known way, directly or indirectly, to perform the
wanted conversion.
This is achieved by providing for each character set a conversion from and to UCS-4 encoded ISO 10646. Using ISO 10646 as an intermediate representation it is possible to triangulate, i.e., converting with an intermediate representation.
There is no inherent requirement to provide a conversion to ISO 10646 for a new character set and it is also possible to provide other conversions where neither source nor destination character set is ISO 10646. The currently existing set of conversions is simply meant to cover all conversions which might be of interest.
All currently available conversions use the triangulation method above, making conversion run unnecessarily slow. If, e.g., somebody often needs the conversion from ISO-2022-JP to EUC-JP, a quicker solution would involve direct conversion between the two character sets, skipping the input to ISO 10646 first. The two character sets of interest are much more similar to each other than to ISO 10646.
In such a situation one can easy write a new conversion and provide it
as a better alternative. The GNU C library iconv implementation
would automatically use the module implementing the conversion if it is
specified to be more efficient.
All information about the available conversions comes from a file named
`gconv-modules' which can be found in any of the directories along
the GCONV_PATH. The `gconv-modules' files are line-oriented
text files, where each of the lines has one of the following formats:
alias define an alias name for a character
set. There are two more words expected on the line. The first one
defines the alias name and the second defines the original name of the
character set. The effect is that it is possible to use the alias name
in the fromset or toset parameters of iconv_open and
achieve the same result as when using the real character set name.
This is quite important as a character set has often many different
names. There is normally always an official name but this need not
correspond to the most popular name. Beside this many character sets
have special names which are somehow constructed. E.g., all character
sets specified by the ISO have an alias of the form
ISO-IR-nnn where nnn is the registration number.
This allows programs which know about the registration number to
construct character set names and use them in iconv_open calls.
More on the available names and aliases follows below.
module introduce an available conversion
module. These lines must contain three or four more words.
The first word specifies the source character set, the second word the
destination character set of conversion implemented in this module. The
third word is the name of the loadable module. The filename is
constructed by appending the usual shared object suffix (normally
`.so') and this file is then supposed to be found in the same
directory the `gconv-modules' file is in. The last word on the
line, which is optional, is a numeric value representing the cost of the
conversion. If this word is missing a cost of @math{1} is assumed. The
numeric value itself does not matter that much; what counts are the
relative values of the sums of costs for all possible conversion paths.
Below is a more precise description of the use of the cost value.
Returning to the example above where one has written a module to directly convert from ISO-2022-JP to EUC-JP and back. All what has to be done is to put the new module, be its name ISO2022JP-EUCJP.so, in a directory and add a file `gconv-modules' with the following content in the same directory:
module ISO-2022-JP// EUC-JP// ISO2022JP-EUCJP 1 module EUC-JP// ISO-2022-JP// ISO2022JP-EUCJP 1
To see why this is sufficient, it is necessary to understand how the
conversion used by iconv (and described in the descriptor) is
selected. The approach to this problem is quite simple.
At the first call of the iconv_open function the program reads
all available `gconv-modules' files and builds up two tables: one
containing all the known aliases and another which contains the
information about the conversions and which shared object implements
them.
iconv
The set of available conversions form a directed graph with weighted
edges. The weights on the edges are the costs specified in the
`gconv-modules' files. The iconv_open function uses an
algorithm suitable for search for the best path in such a graph and so
constructs a list of conversions which must be performed in succession
to get the transformation from the source to the destination character
set.
Explaining why the above `gconv-modules' files allows the
iconv implementation to resolve the specific ISO-2022-JP to
EUC-JP conversion module instead of the conversion coming with the
library itself is straightforward. Since the latter conversion takes two
steps (from ISO-2022-JP to ISO 10646 and then from ISO 10646 to
EUC-JP) the cost is @math{1+1 = 2}. But the above `gconv-modules'
file specifies that the new conversion modules can perform this
conversion with only the cost of @math{1}.
A mysterious piece about the `gconv-modules' file above (and also
the file coming with the GNU C library) are the names of the character
sets specified in the module lines. Why do almost all the names
end in //? And this is not all: the names can actually be
regular expressions. At this point of time this mystery should not be
revealed, unless you have the relevant spell-casting materials: ashes
from an original DOS 6.2 boot disk burnt in effigy, a crucifix
blessed by St. Emacs, assorted herbal roots from Central America, sand
from Cebu, etc. Sorry! The part of the implementation where
this is used is not yet finished. For now please simply follow the
existing examples. It'll become clearer once it is. --drepper
A last remark about the `gconv-modules' is about the names not
ending with //. There often is a character set named
INTERNAL mentioned. From the discussion above and the chosen
name it should have become clear that this is the name for the
representation used in the intermediate step of the triangulation. We
have said that this is UCS-4 but actually it is not quite right. The
UCS-4 specification also includes the specification of the byte ordering
used. Since a UCS-4 value consists of four bytes a stored value is
effected by byte ordering. The internal representation is not
the same as UCS-4 in case the byte ordering of the processor (or at least
the running process) is not the same as the one required for UCS-4. This
is done for performance reasons as one does not want to perform
unnecessary byte-swapping operations if one is not interested in actually
seeing the result in UCS-4. To avoid trouble with endianess the internal
representation consistently is named INTERNAL even on big-endian
systems where the representations are identical.
iconv module data structuresSo far this section described how modules are located and considered to be used. What remains to be described is the interface of the modules so that one can write new ones. This section describes the interface as it is in use in January 1999. The interface will change in future a bit but hopefully only in an upward compatible way.
The definitions necessary to write new modules are publicly available in the non-standard header `gconv.h'. The following text will therefore describe the definitions from this header file. But first it is necessary to get an overview.
From the perspective of the user of iconv the interface is quite
simple: the iconv_open function returns a handle which can be
used in calls to iconv and finally the handle is freed with a call
to iconv_close. The problem is: the handle has to be able to
represent the possibly long sequences of conversion steps and also the
state of each conversion since the handle is all which is passed to the
iconv function. Therefore the data structures are really the
elements to understanding the implementation.
We need two different kinds of data structures. The first describes the conversion and the second describes the state etc. There are really two type definitions like this in `gconv.h'.
struct __gconv_loaded_object *__shlib_handle
const char *__modname
int __counter
const char *__from_name
const char *__to_name
__from_name and __to_name contain the names of the source and
destination character sets. They can be used to identify the actual
conversion to be carried out since one module might implement
conversions for more than one character set and/or direction.
gconv_fct __fct
gconv_init_fct __init_fct
gconv_end_fct __end_fct
int __min_needed_from
int __max_needed_from
int __min_needed_to
int __max_needed_to;
__min_needed_from value specifies how many bytes a character of
the source character set at least needs. The __max_needed_from
specifies the maximum value which also includes possible shift
sequences.
The __min_needed_to and __max_needed_to values serve the
same purpose but this time for the destination character set.
It is crucial that these values are accurate since otherwise the
conversion functions will have problems or not work at all.
int __stateful
void *__data
__data
element must not contain data specific to one specific use of the
conversion function.
char *__outbuf
char *__outbufend
__outbuf element points to the beginning of the buffer and
__outbufend points to the byte following the last byte in the
buffer. The conversion function must not assume anything about the size
of the buffer but it can be safely assumed the there is room for at
least one complete character in the output buffer.
Once the conversion is finished and the conversion is the last step the
__outbuf element must be modified to point after last last byte
written into the buffer to signal how much output is available. If this
conversion step is not the last one the element must not be modified.
The __outbufend element must not be modified.
int __is_last
int __invocation_counter
int __internal_use
mbsrtowcs et.al. I.e., the
function is not used directly through the iconv interface.
This sometimes makes a difference as it is expected that the
iconv functions are used to translate entire texts while the
mbsrtowcs functions are normally only used to convert single
strings and might be used multiple times to convert entire texts.
But in this situation we would have problem complying with some rules of
the character set specification. Some character sets require a prolog
which must appear exactly once for an entire text. If a number of
mbsrtowcs calls are used to convert the text only the first call
must add the prolog. But since there is no communication between the
different calls of mbsrtowcs the conversion functions have no
possibility to find this out. The situation is different for sequences
of iconv calls since the handle allows access to the needed
information.
This element is mostly used together with __invocation_counter in
a way like this:
if (!data->__internal_use
&& data->__invocation_counter == 0)
/* Emit prolog. */
...
This element must never be modified.
mbstate_t *__statep
__statep element points to an object of type mbstate_t
(see section Representing the state of the conversion). The conversion of an stateful character
set must use the object pointed to by this element to store information
about the conversion state. The __statep element itself must
never be modified.
mbstate_t __state
iconv module interfacesWith the knowledge about the data structures we now can describe the conversion functions itself. To understand the interface a bit of knowledge about the functionality in the C library which loads the objects with the conversions is necessary.
It is often the case that one conversion is used more than once. I.e.,
there are several iconv_open calls for the same set of character
sets during one program run. The mbsrtowcs et.al. functions in
the GNU C library also use the iconv functionality which
increases the number of uses of the same functions even more.
For this reason the modules do not get loaded exclusively for one
conversion. Instead a module once loaded can be used by arbitrarily many
iconv or mbsrtowcs calls at the same time. The splitting
of the information between conversion function specific information and
conversion data makes this possible. The last section showed the two
data structures used to do this.
This is of course also reflected in the interface and semantics of the functions the modules must provide. There are three functions which must have the following names:
gconv_init
gconv_init function initializes the conversion function
specific data structure. This very same object is shared by all
conversion which use this conversion and therefore no state information
about the conversion itself must be stored in here. If a module
implements more than one conversion the gconv_init function will be
called multiple times.
gconv_end
gconv_end function is responsible to free all resources
allocated by the gconv_init function. If there is nothing to do
this function can be missing. Special care must be taken if the module
implements more than one conversion and the gconv_init function
does not allocate the same resources for all conversions.
gconv
gconv_init and the conversion data, specific to
this use of the conversion functions.
There are three data types defined for the three module interface function and these define the interface.
As explained int the description of the struct __gconv_step data
structure above the initialization function has to initialize parts of
it.
__min_needed_from
__max_needed_from
__min_needed_to
__max_needed_to
__stateful
If the initialization function needs to communication some information
to the conversion function this can happen using the __data
element of the __gconv_step structure. But since this data is
shared by all the conversion is must not be modified by the conversion
function. How this can be used is shown in the example below.
#define MIN_NEEDED_FROM 1
#define MAX_NEEDED_FROM 4
#define MIN_NEEDED_TO 4
#define MAX_NEEDED_TO 4
int
gconv_init (struct __gconv_step *step)
{
/* Determine which direction. */
struct iso2022jp_data *new_data;
enum direction dir = illegal_dir;
enum variant var = illegal_var;
int result;
if (__strcasecmp (step->__from_name, "ISO-2022-JP//") == 0)
{
dir = from_iso2022jp;
var = iso2022jp;
}
else if (__strcasecmp (step->__to_name, "ISO-2022-JP//") == 0)
{
dir = to_iso2022jp;
var = iso2022jp;
}
else if (__strcasecmp (step->__from_name, "ISO-2022-JP-2//") == 0)
{
dir = from_iso2022jp;
var = iso2022jp2;
}
else if (__strcasecmp (step->__to_name, "ISO-2022-JP-2//") == 0)
{
dir = to_iso2022jp;
var = iso2022jp2;
}
result = __GCONV_NOCONV;
if (dir != illegal_dir)
{
new_data = (struct iso2022jp_data *)
malloc (sizeof (struct iso2022jp_data));
result = __GCONV_NOMEM;
if (new_data != NULL)
{
new_data->dir = dir;
new_data->var = var;
step->__data = new_data;
if (dir == from_iso2022jp)
{
step->__min_needed_from = MIN_NEEDED_FROM;
step->__max_needed_from = MAX_NEEDED_FROM;
step->__min_needed_to = MIN_NEEDED_TO;
step->__max_needed_to = MAX_NEEDED_TO;
}
else
{
step->__min_needed_from = MIN_NEEDED_TO;
step->__max_needed_from = MAX_NEEDED_TO;
step->__min_needed_to = MIN_NEEDED_FROM;
step->__max_needed_to = MAX_NEEDED_FROM + 2;
}
/* Yes, this is a stateful encoding. */
step->__stateful = 1;
result = __GCONV_OK;
}
}
return result;
}
The function first checks which conversion is wanted. The module from which this function is taken implements four different conversion and which one is selected can be determined by comparing the names. The comparison should always be done without paying attention to the case.
Then a data structure is allocated which contains the necessary
information about which conversion is selected. The data structure
struct iso2022jp_data is locally defined since outside the module
this data is not used at all. Please note that if all four conversions
this modules supports are requested there are four data blocks.
One interesting thing is the initialization of the __min_ and
__max_ elements of the step data object. A single ISO-2022-JP
character can consist of one to four bytes. Therefore the
MIN_NEEDED_FROM and MAX_NEEDED_FROM macros are defined
this way. The output is always the INTERNAL character set (aka
UCS-4) and therefore each character consists of exactly four bytes. For
the conversion from INTERNAL to ISO-2022-JP we have to take into
account that escape sequences might be necessary to switch the character
sets. Therefore the __max_needed_to element for this direction
gets assigned MAX_NEEDED_FROM + 2. This takes into account the
two bytes needed for the escape sequences to single the switching. The
asymmetry in the maximum values for the two directions can be explained
easily: when reading ISO-2022-JP text escape sequences can be handled
alone. I.e., it is not necessary to process a real character since the
effect of the escape sequence can be recorded in the state information.
The situation is different for the other direction. Since it is in
general not known which character comes next one cannot emit escape
sequences to change the state in advance. This means the escape
sequences which have to be emitted together with the next character.
Therefore one needs more room then only for the character itself.
The possible return values of the initialization function are:
__GCONV_OK
__GCONV_NOCONV
__GCONV_NOMEM
The functions called before the module is unloaded is significantly easier. It often has nothing at all to do in which case it can be left out completely.
__data element of
the object pointed to by the argument is of interest. Continuing the
example from the initialization function, the finalization function
looks like this:
void
gconv_end (struct __gconv_step *data)
{
free (data->__data);
}
The most important function is the conversion function itself. It can get quite complicated for complex character sets. But since this is not of interest here we will only describe a possible skeleton for the conversion function.
iconv
function it can be seen why the flushing mode is necessary. What mode
is selected is determined by the sixth argument, an integer. If it is
nonzero it means that flushing is selected.
Common to both mode is where the output buffer can be found. The
information about this buffer is stored in the conversion step data. A
pointer to this is passed as the second argument to this function. The
description of the struct __gconv_step_data structure has more
information on this.
What has to be done for flushing depends on the source character set.
If it is not stateful nothing has to be done. Otherwise the function
has to emit a byte sequence to bring the state object in the initial
state. Once this all happened the other conversion modules in the chain
of conversions have to get the same chance. Whether another step
follows can be determined from the __is_last element of the step
data structure to which the first parameter points.
The more interesting mode is when actually text has to be converted. The first step in this case is to convert as much text as possible from the input buffer and store the result in the output buffer. The start of the input buffer is determined by the third argument which is a pointer to a pointer variable referencing the beginning of the buffer. The fourth argument is a pointer to the byte right after the last byte in the buffer.
The conversion has to be performed according to the current state if the
character set is stateful. The state is stored in an object pointed to
by the __statep element of the step data (second argument). Once
either the input buffer is empty or the output buffer is full the
conversion stops. At this point the pointer variable referenced by the
third parameter must point to the byte following the last processed
byte. I.e., if all of the input is consumed this pointer and the fourth
parameter have the same value.
What now happens depends on whether this step is the last one or not.
If it is the last step the only thing which has to be done is to update
the __outbuf element of the step data structure to point after the
last written byte. This gives the caller the information on how much
text is available in the output buffer. Beside this the variable
pointed to by the fifth parameter, which is of type size_t, must
be incremented by the number of characters (not bytes) which were
converted in a non-reversible way. Then the function can return.
In case the step is not the last one the later conversion functions have to get a chance to do their work. Therefore the appropriate conversion function has to be called. The information about the functions is stored in the conversion data structures, passed as the first parameter. This information and the step data are stored in arrays so the next element in both cases can be found by simple pointer arithmetic:
int
gconv (struct __gconv_step *step, struct __gconv_step_data *data,
const char **inbuf, const char *inbufend, size_t *written,
int do_flush)
{
struct __gconv_step *next_step = step + 1;
struct __gconv_step_data *next_data = data + 1;
...
The next_step pointer references the next step information and
next_data the next data record. The call of the next function
therefore will look similar to this:
next_step->__fct (next_step, next_data, &outerr, outbuf,
written, 0)
But this is not yet all. Once the function call returns the conversion
function might have some more to do. If the return value of the
function is __GCONV_EMPTY_INPUT this means there is more room in
the output buffer. Unless the input buffer is empty the conversion
functions start all over again and processes the rest of the input
buffer. If the return value is not __GCONV_EMPTY_INPUT something
went wrong and we have to recover from this.
A requirement for the conversion function is that the input buffer pointer (the third argument) always points to the last character which was put in the converted form in the output buffer. This is trivially true after the conversion performed in the current step. But if the conversion functions deeper down the stream stop prematurely not all characters from the output buffer are consumed and therefore the input buffer pointers must be backed of to the right position.
This is easy to do if the input and output character sets have a fixed width for all characters. In this situation we can compute how many characters are left in the output buffer and therefore can correct the input buffer pointer appropriate with a similar computation. Things are getting tricky if either character set has character represented with variable length byte sequences and it gets even more complicated if the conversion has to take care of the state. In these cases the conversion has to be performed once again, from the known state before the initial conversion. I.e., if necessary the state of the conversion has to be reset and the conversion loop has to be executed again. The difference now is that it is known how much input must be created and the conversion can stop before converting the first unused character. Once this is done the input buffer pointers must be updated again and the function can return.
One final thing should be mentioned. If it is necessary for the
conversion to know whether it is the first invocation (in case a prolog
has to be emitted) the conversion function should just before returning
to the caller increment the __invocation_counter element of the
step data structure. See the description of the struct
__gconv_step_data structure above for more information on how this can
be used.
The return value must be one of the following values:
__GCONV_EMPTY_INPUT
__GCONV_FULL_OUTPUT
__GCONV_INCOMPLETE_INPUT
The following example provides a framework for a conversion function. In case a new conversion has to be written the holes in this implementation have to be filled and that is it.
int
gconv (struct __gconv_step *step, struct __gconv_step_data *data,
const char **inbuf, const char *inbufend, size_t *written,
int do_flush)
{
struct __gconv_step *next_step = step + 1;
struct __gconv_step_data *next_data = data + 1;
gconv_fct fct = next_step->__fct;
int status;
/* If the function is called with no input this means we have
to reset to the initial state. The possibly partly
converted input is dropped. */
if (do_flush)
{
status = __GCONV_OK;
/* Possible emit a byte sequence which put the state object
into the initial state. */
/* Call the steps down the chain if there are any but only
if we successfully emitted the escape sequence. */
if (status == __GCONV_OK && ! data->__is_last)
status = fct (next_step, next_data, NULL, NULL,
written, 1);
}
else
{
/* We preserve the initial values of the pointer variables. */
const char *inptr = *inbuf;
char *outbuf = data->__outbuf;
char *outend = data->__outbufend;
char *outptr;
do
{
/* Remember the start value for this round. */
inptr = *inbuf;
/* The outbuf buffer is empty. */
outptr = outbuf;
/* For stateful encodings the state must be safe here. */
/* Run the conversion loop. status is set
appropriately afterwards. */
/* If this is the last step leave the loop, there is
nothing we can do. */
if (data->__is_last)
{
/* Store information about how many bytes are
available. */
data->__outbuf = outbuf;
/* If any non-reversible conversions were performed,
add the number to *written. */
break;
}
/* Write out all output which was produced. */
if (outbuf > outptr)
{
const char *outerr = data->__outbuf;
int result;
result = fct (next_step, next_data, &outerr,
outbuf, written, 0);
if (result != __GCONV_EMPTY_INPUT)
{
if (outerr != outbuf)
{
/* Reset the input buffer pointer. We
document here the complex case. */
size_t nstatus;
/* Reload the pointers. */
*inbuf = inptr;
outbuf = outptr;
/* Possibly reset the state. */
/* Redo the conversion, but this time
the end of the output buffer is at
outerr. */
}
/* Change the status. */
status = result;
}
else
/* All the output is consumed, we can make
another run if everything was ok. */
if (status == __GCONV_FULL_OUTPUT)
status = __GCONV_OK;
}
}
while (status == __GCONV_OK);
/* We finished one use of this step. */
++data->__invocation_counter;
}
return status;
}
This information should be sufficient to write new modules. Anybody doing so should also take a look at the available source code in the GNU C library sources. It contains many examples of working and optimized modules.
Different countries and cultures have varying conventions for how to communicate. These conventions range from very simple ones, such as the format for representing dates and times, to very complex ones, such as the language spoken.
Internationalization of software means programming it to be able to adapt to the user's favorite conventions. In ISO C, internationalization works by means of locales. Each locale specifies a collection of conventions, one convention for each purpose. The user chooses a set of conventions by specifying a locale (via environment variables).
All programs inherit the chosen locale as part of their environment. Provided the programs are written to obey the choice of locale, they will follow the conventions preferred by the user.
Each locale specifies conventions for several purposes, including the following:
Some aspects of adapting to the specified locale are handled
automatically by the library subroutines. For example, all your program
needs to do in order to use the collating sequence of the chosen locale
is to use strcoll or strxfrm to compare strings.
Other aspects of locales are beyond the comprehension of the library. For example, the library can't automatically translate your program's output messages into other languages. The only way you can support output in the user's favorite language is to program this more or less by hand. The C library provides functions to handle translations for multiple languages easily.
This chapter discusses the mechanism by which you can modify the current locale. The effects of the current locale on specific library functions are discussed in more detail in the descriptions of those functions.
The simplest way for the user to choose a locale is to set the
environment variable LANG. This specifies a single locale to use
for all purposes. For example, a user could specify a hypothetical
locale named `espana-castellano' to use the standard conventions of
most of Spain.
The set of locales supported depends on the operating system you are using, and so do their names. We can't make any promises about what locales will exist, except for one standard locale called `C' or `POSIX'. Later we will describe how to construct locales.
A user also has the option of specifying different locales for different purposes--in effect, choosing a mixture of multiple locales.
For example, the user might specify the locale `espana-castellano' for most purposes, but specify the locale `usa-english' for currency formatting. This might make sense if the user is a Spanish-speaking American, working in Spanish, but representing monetary amounts in US dollars.
Note that both locales `espana-castellano' and `usa-english', like all locales, would include conventions for all of the purposes to which locales apply. However, the user can choose to use each locale for a particular subset of those purposes.
The purposes that locales serve are grouped into categories, so
that a user or a program can choose the locale for each category
independently. Here is a table of categories; each name is both an
environment variable that a user can set, and a macro name that you can
use as an argument to setlocale.
LC_COLLATE
strcoll
and strxfrm); see section Collation Functions.
LC_CTYPE
LC_MONETARY
LC_NUMERIC
LC_TIME
LC_MESSAGES
LC_ALL
setlocale to set a single locale for all purposes. Setting
this environment variable overwrites all selections by the other
LC_* variables or LANG.
LANG
When developing the message translation functions it was felt that the
functionality provided by the variables above is not sufficient. For
example, it should be possible to specify more than one locale name.
Take a Swedish user who better speaks German than English, and a program
whose messages are output in English by default. It should be possible
to specify that the first choice of language is Swedish, the second
German, and if this also fails to use English. This is
possible with the variable LANGUAGE. For further description of
this GNU extension see section User influence on gettext.
A C program inherits its locale environment variables when it starts up.
This happens automatically. However, these variables do not
automatically control the locale used by the library functions, because
ISO C says that all programs start by default in the standard `C'
locale. To use the locales specified by the environment, you must call
setlocale. Call it as follows:
setlocale (LC_ALL, "");
to select a locale based on the user choice of the appropriate environment variables.
You can also use setlocale to specify a particular locale, for
general use or for a specific category.
The symbols in this section are defined in the header file `locale.h'.
setlocale sets the current locale for category
category to locale. A list of all the locales the system
provides can be created by running
locale -a
If category is LC_ALL, this specifies the locale for all
purposes. The other possible values of category specify an
single purpose (see section Categories of Activities that Locales Affect).
You can also use this function to find out the current locale by passing
a null pointer as the locale argument. In this case,
setlocale returns a string that is the name of the locale
currently selected for category category.
The string returned by setlocale can be overwritten by subsequent
calls, so you should make a copy of the string (see section Copying and Concatenation) if you want to save it past any further calls to
setlocale. (The standard library is guaranteed never to call
setlocale itself.)
You should not modify the string returned by setlocale. It might
be the same string that was passed as an argument in a previous call to
setlocale. One requirement is that the category must be
the same in the call the string was returned and the one when the string
is passed in as locale parameter.
When you read the current locale for category LC_ALL, the value
encodes the entire combination of selected locales for all categories.
In this case, the value is not just a single locale name. In fact, we
don't make any promises about what it looks like. But if you specify
the same "locale name" with LC_ALL in a subsequent call to
setlocale, it restores the same combination of locale selections.
To be sure you can use the returned string encoding the currently selected locale at a later time, you must make a copy of the string. It is not guaranteed that the returned pointer remains valid over time.
When the locale argument is not a null pointer, the string returned
by setlocale reflects the newly-modified locale.
If you specify an empty string for locale, this means to read the appropriate environment variable and use its value to select the locale for category.
If a nonempty string is given for locale, then the locale of that name is used if possible.
If you specify an invalid locale name, setlocale returns a null
pointer and leaves the current locale unchanged.
Here is an example showing how you might use setlocale to
temporarily switch to a new locale.
#include <stddef.h>
#include <locale.h>
#include <stdlib.h>
#include <string.h>
void
with_other_locale (char *new_locale,
void (*subroutine) (int),
int argument)
{
char *old_locale, *saved_locale;
/* Get the name of the current locale. */
old_locale = setlocale (LC_ALL, NULL);
/* Copy the name so it won't be clobbered by setlocale. */
saved_locale = strdup (old_locale);
if (saved_locale == NULL)
fatal ("Out of memory");
/* Now change the locale and do some stuff with it. */
setlocale (LC_ALL, new_locale);
(*subroutine) (argument);
/* Restore the original locale. */
setlocale (LC_ALL, saved_locale);
free (saved_locale);
}
Portability Note: Some ISO C systems may define additional locale categories, and future versions of the library will do so. For portability, assume that any symbol beginning with `LC_' might be defined in `locale.h'.
The only locale names you can count on finding on all operating systems are these three standard ones:
"C"
"POSIX"
""
Defining and installing named locales is normally a responsibility of the system administrator at your site (or the person who installed the GNU C library). It is also possible for the user to create private locales. All this will be discussed later when describing the tool to do so.
If your program needs to use something other than the `C' locale, it will be more portable if you use whatever locale the user specifies with the environment, rather than trying to specify some non-standard locale explicitly by name. Remember, different machines might have different sets of locales installed.
There are several ways to access locale information. The simplest way is to let the C library itself do the work. Several of the functions in this library implicitly access the locale data, and use what information is provided by the currently selected locale. This is how the locale model is meant to work normally.
As an example take the strftime function, which is meant to nicely
format date and time information (see section Formatting Calendar Time).
Part of the standard information contained in the LC_TIME
category is the names of the months. Instead of requiring the
programmer to take care of providing the translations the
strftime function does this all by itself. %A
in the format string is replaced by the appropriate weekday
name of the locale currently selected by LC_TIME. This is an
easy example, and wherever possible functions do things automatically
in this way.
But there are quite often situations when there is simply no function
to perform the task, or it is simply not possible to do the work
automatically. For these cases it is necessary to access the
information in the locale directly. To do this the C library provides
two functions: localeconv and nl_langinfo. The former is
part of ISO C and therefore portable, but has a brain-damaged
interface. The second is part of the Unix interface and is portable in
as far as the system follows the Unix standards.
localeconv: It is portable but ...
Together with the setlocale function the ISO C people
invented the localeconv function. It is a masterpiece of poor
design. It is expensive to use, not extendable, and not generally
usable as it provides access to only LC_MONETARY and
LC_NUMERIC related information. Nevertheless, if it is
applicable to a given situation it should be used since it is very
portable. The function strfmon formats monetary amounts
according to the selected locale using this information.
localeconv function returns a pointer to a structure whose
components contain information about how numeric and monetary values
should be formatted in the current locale.
You should not modify the structure or its contents. The structure might
be overwritten by subsequent calls to localeconv, or by calls to
setlocale, but no other function in the library overwrites this
value.
localeconv's return value is of this data type. Its elements are
described in the following subsections.
If a member of the structure struct lconv has type char,
and the value is CHAR_MAX, it means that the current locale has
no value for that parameter.
These are the standard members of struct lconv; there may be
others.
char *decimal_point
char *mon_decimal_point
decimal_point is ".", and the value of
mon_decimal_point is "".
char *thousands_sep
char *mon_thousands_sep
"" (the empty string).
char *grouping
char *mon_grouping
grouping applies to non-monetary quantities
and mon_grouping applies to monetary quantities. Use either
thousands_sep or mon_thousands_sep to separate the digit
groups.
Each member of these strings is to be interpreted as an integer value of
type char. Successive numbers (from left to right) give the
sizes of successive groups (from right to left, starting at the decimal
point.) The last member is either 0, in which case the previous
member is used over and over again for all the remaining groups, or
CHAR_MAX, in which case there is no more grouping--or, put
another way, any remaining digits form one large group without
separators.
For example, if grouping is "\04\03\02", the correct
grouping for the number 123456787654321 is `12', `34',
`56', `78', `765', `4321'. This uses a group of 4
digits at the end, preceded by a group of 3 digits, preceded by groups
of 2 digits (as many as needed). With a separator of `,', the
number would be printed as `12,34,56,78,765,4321'.
A value of "\03" indicates repeated groups of three digits, as
normally used in the U.S.
In the standard `C' locale, both grouping and
mon_grouping have a value of "". This value specifies no
grouping at all.
char int_frac_digits
char frac_digits
CHAR_MAX, meaning "unspecified". The ISO standard doesn't say
what to do when you find this value; we recommend printing no
fractional digits. (This locale also specifies the empty string for
mon_decimal_point, so printing any fractional digits would be
confusing!)
These members of the struct lconv structure specify how to print
the symbol to identify a monetary value--the international analog of
`$' for US dollars.
Each country has two standard currency symbols. The local currency symbol is used commonly within the country, while the international currency symbol is used internationally to refer to that country's currency when it is necessary to indicate the country unambiguously.
For example, many countries use the dollar as their monetary unit, and when dealing with international currencies it's important to specify that one is dealing with (say) Canadian dollars instead of U.S. dollars or Australian dollars. But when the context is known to be Canada, there is no need to make this explicit--dollar amounts are implicitly assumed to be in Canadian dollars.
char *currency_symbol
""
(the empty string), meaning "unspecified". The ISO standard doesn't
say what to do when you find this value; we recommend you simply print
the empty string as you would print any other string pointed to by this
variable.
char *int_curr_symbol
int_curr_symbol should normally consist of a
three-letter abbreviation determined by the international standard
ISO 4217 Codes for the Representation of Currency and Funds,
followed by a one-character separator (often a space).
In the standard `C' locale, this member has a value of ""
(the empty string), meaning "unspecified". We recommend you simply print
the empty string as you would print any other string pointed to by this
variable.
char p_cs_precedes
char n_cs_precedes
char int_p_cs_precedes
char int_n_cs_precedes
1 if the currency_symbol or
int_curr_symbol strings should precede the value of a monetary
amount, or 0 if the strings should follow the value. The
p_cs_precedes and int_p_cs_precedes members apply to
positive amounts (or zero), and the n_cs_precedes and
int_n_cs_precedes members apply to negative amounts.
In the standard `C' locale, all of these members have a value of
CHAR_MAX, meaning "unspecified". The ISO standard doesn't say
what to do when you find this value. We recommend printing the
currency symbol before the amount, which is right for most countries.
In other words, treat all nonzero values alike in these members.
The members with the int_ prefix apply to the
int_curr_symbol while the other two apply to
currency_symbol.
char p_sep_by_space
char n_sep_by_space
char int_p_sep_by_space
char int_n_sep_by_space
1 if a space should appear between the
currency_symbol or int_curr_symbol strings and the
amount, or 0 if no space should appear. The
p_sep_by_space and int_p_sep_by_space members apply to
positive amounts (or zero), and the n_sep_by_space and
int_n_sep_by_space members apply to negative amounts.
In the standard `C' locale, all of these members have a value of
CHAR_MAX, meaning "unspecified". The ISO standard doesn't say
what you should do when you find this value; we suggest you treat it as
1 (print a space). In other words, treat all nonzero values alike in
these members.
The members with the int_ prefix apply to the
int_curr_symbol while the other two apply to
currency_symbol. There is one specialty with the
int_curr_symbol, though. Since all legal values contain a space
at the end the string one either printf this space (if the currency
symbol must appear in front and must be separated) or one has to avoid
printing this character at all (especially when at the end of the
string).
These members of the struct lconv structure specify how to print
the sign (if any) of a monetary value.
char *positive_sign
char *negative_sign
"" (the empty string), meaning "unspecified".
The ISO standard doesn't say what to do when you find this value; we
recommend printing positive_sign as you find it, even if it is
empty. For a negative value, print negative_sign as you find it
unless both it and positive_sign are empty, in which case print
`-' instead. (Failing to indicate the sign at all seems rather
unreasonable.)
char p_sign_posn
char n_sign_posn
char int_p_sign_posn
char int_n_sign_posn
positive_sign or negative_sign.) The possible values are
as follows:
0
1
2
3
4
CHAR_MAX
CHAR_MAX. We recommend you print the sign after the currency
symbol.
The members with the int_ prefix apply to the
int_curr_symbol while the other two apply to
currency_symbol.
When writing the X/Open Portability Guide the authors realized that the
localeconv function is not enough to provide reasonable access to
locale information. The information which was meant to be available
in the locale (as later specified in the POSIX.1 standard) requires more
ways to access it. Therefore the nl_langinfo function
was introduced.
nl_langinfo function can be used to access individual
elements of the locale categories. Unlike the localeconv
function, which returns all the information, nl_langinfo
lets the caller select what information it requires. This is very
fast and it is not a problem to call this function multiple times.
A second advantage is that in addition to the numeric and monetary
formatting information, information from the
LC_TIME and LC_MESSAGES categories is available.
The type nl_type is defined in `nl_types.h'. The argument
item is a numeric value defined in the header `langinfo.h'.
The X/Open standard defines the following values:
CODESET
nl_langinfo returns a string with the name of the coded character
set used in the selected locale.
ABDAY_1
ABDAY_2
ABDAY_3
ABDAY_4
ABDAY_5
ABDAY_6
ABDAY_7
nl_langinfo returns the abbreviated weekday name. ABDAY_1
corresponds to Sunday.
DAY_1
DAY_2
DAY_3
DAY_4
DAY_5
DAY_6
DAY_7
ABDAY_1 etc., but here the return value is the
unabbreviated weekday name.
ABMON_1
ABMON_2
ABMON_3
ABMON_4
ABMON_5
ABMON_6
ABMON_7
ABMON_8
ABMON_9
ABMON_10
ABMON_11
ABMON_12
ABMON_1
corresponds to January.
MON_1
MON_2
MON_3
MON_4
MON_5
MON_6
MON_7
MON_8
MON_9
MON_10
MON_11
MON_12
ABMON_1 etc., but here the month names are not abbreviated.
Here the first value MON_1 also corresponds to January.
AM_STR
PM_STR
D_T_FMT
strftime to
represent time and date in a locale-specific way.
D_FMT
strftime to
represent a date in a locale-specific way.
T_FMT
strftime to
represent time in a locale-specific way.
T_FMT_AMPM
strftime to
represent time in the am/pm format.
Note that if the am/pm format does not make any sense for the
selected locale, the return value might be the same as the one for
T_FMT.
ERA
E modifier in their format strings causes the
strftime functions to use this information. The format of the
returned string is not specified, and therefore you should not assume
knowledge of it on different systems.
ERA_YEAR
ERA it should not be necessary to use this value directly.
ERA_D_T_FMT
strftime to
represent dates and times in a locale-specific era-based way.
ERA_D_FMT
strftime to
represent a date in a locale-specific era-based way.
ERA_T_FMT
strftime to
represent time in a locale-specific era-based way.
ALT_DIGITS
ERA this
value is not intended to be used directly, but instead indirectly
through the strftime function. When the modifier O is
used in a format which would otherwise use numerals to represent hours,
minutes, seconds, weekdays, months, or weeks, the appropriate value for
the locale is used instead.
INT_CURR_SYMBOL
localeconv in the
int_curr_symbol element of the struct lconv.
CURRENCY_SYMBOL
CRNCYSTR
localeconv in the
currency_symbol element of the struct lconv.
CRNCYSTR is a deprecated alias still required by Unix98.
MON_DECIMAL_POINT
localeconv in the
mon_decimal_point element of the struct lconv.
MON_THOUSANDS_SEP
localeconv in the
mon_thousands_sep element of the struct lconv.
MON_GROUPING
localeconv in the
mon_grouping element of the struct lconv.
POSITIVE_SIGN
localeconv in the
positive_sign element of the struct lconv.
NEGATIVE_SIGN
localeconv in the
negative_sign element of the struct lconv.
INT_FRAC_DIGITS
localeconv in the
int_frac_digits element of the struct lconv.
FRAC_DIGITS
localeconv in the
frac_digits element of the struct lconv.
P_CS_PRECEDES
localeconv in the
p_cs_precedes element of the struct lconv.
P_SEP_BY_SPACE
localeconv in the
p_sep_by_space element of the struct lconv.
N_CS_PRECEDES
localeconv in the
n_cs_precedes element of the struct lconv.
N_SEP_BY_SPACE
localeconv in the
n_sep_by_space element of the struct lconv.
P_SIGN_POSN
localeconv in the
p_sign_posn element of the struct lconv.
N_SIGN_POSN
localeconv in the
n_sign_posn element of the struct lconv.
INT_P_CS_PRECEDES
localeconv in the
int_p_cs_precedes element of the struct lconv.
INT_P_SEP_BY_SPACE
localeconv in the
int_p_sep_by_space element of the struct lconv.
INT_N_CS_PRECEDES
localeconv in the
int_n_cs_precedes element of the struct lconv.
INT_N_SEP_BY_SPACE
localeconv in the
int_n_sep_by_space element of the struct lconv.
INT_P_SIGN_POSN
localeconv in the
int_p_sign_posn element of the struct lconv.
INT_N_SIGN_POSN
localeconv in the
int_n_sign_posn element of the struct lconv.
DECIMAL_POINT
RADIXCHAR
localeconv in the
decimal_point element of the struct lconv.
The name RADIXCHAR is a deprecated alias still used in Unix98.
THOUSANDS_SEP
THOUSEP
localeconv in the
thousands_sep element of the struct lconv.
The name THOUSEP is a deprecated alias still used in Unix98.
GROUPING
localeconv in the
grouping element of the struct lconv.
YESEXPR
regex function to recognize a positive response to a yes/no
question.
NOEXPR
regex function to recognize a negative response to a yes/no
question.
YESSTR
NOSTR
YESSTR is also true here.
The use of this symbol is deprecated. Instead message translation
should be used.
The file `langinfo.h' defines a lot more symbols but none of them is official. Using them is not portable, and the format of the return values might change. Therefore we recommended you not use them.
Note that the return value for any valid argument can be used for
in all situations (with the possible exception of the am/pm time formatting
codes). If the user has not selected any locale for the
appropriate category, nl_langinfo returns the information from the
"C" locale. It is therefore possible to use this function as
shown in the example below.
If the argument item is not valid, a pointer to an empty string is returned.
An example of nl_langinfo usage is a function which has to
print a given date and time in a locale-specific way. At first one
might think that, since strftime internally uses the locale
information, writing something like the following is enough:
size_t
i18n_time_n_data (char *s, size_t len, const struct tm *tp)
{
return strftime (s, len, "%X %D", tp);
}
The format contains no weekday or month names and therefore is
internationally usable. Wrong! The output produced is something like
"hh:mm:ss MM/DD/YY". This format is only recognizable in the
USA. Other countries use different formats. Therefore the function
should be rewritten like this:
size_t
i18n_time_n_data (char *s, size_t len, const struct tm *tp)
{
return strftime (s, len, nl_langinfo (D_T_FMT), tp);
}
Now it uses the date and time format of the locale selected when the program runs. If the user selects the locale correctly there should never be a misunderstanding over the time and date format.
We have seen that the structure returned by localeconv as well as
the values given to nl_langinfo allow you to retrieve the various
pieces of locale-specific information to format numbers and monetary
amounts. We have also seen that the underlying rules are quite complex.
Therefore the X/Open standards introduce a function which uses such locale information, making it easier for the user to format numbers according to these rules.
strfmon function is similar to the strftime function
in that it takes a buffer, its size, a format string,
and values to write into the buffer as text in a form specified
by the format string. Like strftime, the function
also returns the number of bytes written into the buffer.
There are two differences: strfmon can take more than one
argument, and, of course, the format specification is different. Like
strftime, the format string consists of normal text, which is
output as is, and format specifiers, which are indicated by a `%'.
Immediately after the `%', you can optionally specify various flags
and formatting information before the main formatting character, in a
similar way to printf:
LC_MONETARY
category of the locale selected at program runtime.
The next part of a specification is an optional field width. If no width is specified @math{0} is taken. During output, the function first determines how much space is required. If it requires at least as many characters as given by the field width, it is output using as much space as necessary. Otherwise, it is extended to use the full width by filling with the space character. The presence or absence of the `-' flag determines the side at which such padding occurs. If present, the spaces are added at the right making the output left-justified, and vice versa.
So far the format looks familiar, being similar to the printf and
strftime formats. However, the next two optional fields
introduce something new. The first one is a `#' character followed
by a decimal digit string. The value of the digit string specifies the
number of digit positions to the left of the decimal point (or
equivalent). This does not include the grouping character when
the `^' flag is not given. If the space needed to print the number
does not fill the whole width, the field is padded at the left side with
the fill character, which can be selected using the `=' flag and by
default is a space. For example, if the field width is selected as 6
and the number is @math{123}, the fill character is `*' the result
will be `***123'.
The second optional field starts with a `.' (period) and consists
of another decimal digit string. Its value describes the number of
characters printed after the decimal point. The default is selected
from the current locale (frac_digits, int_frac_digits, see
see section Generic Numeric Formatting Parameters). If the exact representation needs more digits
than given by the field width, the displayed value is rounded. If the
number of fractional digits is selected to be zero, no decimal point is
printed.
As a GNU extension, the strfmon implementation in the GNU libc
allows an optional `L' next as a format modifier. If this modifier
is given, the argument is expected to be a long double instead of
a double value.
Finally, the last component is a format specifier. There are three specifiers defined:
As for printf, the function reads the format string
from left to right and uses the values passed to the function following
the format string. The values are expected to be either of type
double or long double, depending on the presence of the
modifier `L'. The result is stored in the buffer pointed to by
s. At most maxsize characters are stored.
The return value of the function is the number of characters stored in
s, including the terminating NULL byte. If the number of
characters stored would exceed maxsize, the function returns
@math{-1} and the content of the buffer s is unspecified. In this
case errno is set to E2BIG.
A few examples should make clear how the function works. It is
assumed that all the following pieces of code are executed in a program
which uses the USA locale (en_US). The simplest
form of the format is this:
strfmon (buf, 100, "@%[email protected]%[email protected]%[email protected]", 123.45, -567.89, 12345.678);
The output produced is
"@[email protected][email protected]$12,[email protected]"
We can notice several things here. First, the widths of the output
numbers are different. We have not specified a width in the format
string, and so this is no wonder. Second, the third number is printed
using thousands separators. The thousands separator for the
en_US locale is a comma. The number is also rounded.
@math{.678} is rounded to @math{.68} since the format does not specify a
precision and the default value in the locale is @math{2}. Finally,
note that the national currency symbol is printed since `%n' was
used, not `i'. The next example shows how we can align the output.
strfmon (buf, 100, "@%=*[email protected]%=*[email protected]%=*[email protected]", 123.45, -567.89, 12345.678);
The output this time is:
"@ [email protected] [email protected] $12,[email protected]"
Two things stand out. Firstly, all fields have the same width (eleven characters) since this is the width given in the format and since no number required more characters to be printed. The second important point is that the fill character is not used. This is correct since the white space was not used to achieve a precision given by a `#' modifier, but instead to fill to the given width. The difference becomes obvious if we now add a width specification.
strfmon (buf, 100, "@%=*11#[email protected]%=*11#[email protected]%=*11#[email protected]", 123.45, -567.89, 12345.678);
The output is
"@ $***[email protected]$***[email protected] $12,[email protected]"
Here we can see that all the currency symbols are now aligned, and that the space between the currency sign and the number is filled with the selected fill character. Note that although the width is selected to be @math{5} and @math{123.45} has three digits left of the decimal point, the space is filled with three asterisks. This is correct since, as explained above, the width does not include the positions used to store thousands separators. One last example should explain the remaining functionality.
strfmon (buf, 100, "@%=0(16#[email protected]%=0(16#[email protected]%=0(16#[email protected]", 123.45, -567.89, 12345.678);
This rather complex format string produces the following output:
"@ USD 000123,450 @(USD 000567.890)@ USD 12,345.678 @"
The most noticeable change is the alternative way of representing
negative numbers. In financial circles this is often done using
parentheses, and this is what the `(' flag selected. The fill
character is now `0'. Note that this `0' character is not
regarded as a numeric zero, and therefore the first and second numbers
are not printed using a thousands separator. Since we used the format
specifier `i' instead of `n', the international form of the
currency symbol is used. This is a four letter string, in this case
"USD ". The last point is that since the precision right of the
decimal point is selected to be three, the first and second numbers are
printed with an extra zero at the end and the third number is printed
without rounding.
The program's interface with the human should be designed in a way to ease the human the task. One of the possibilities is to use messages in whatever language the user prefers.
Printing messages in different languages can be implemented in different ways. One could add all the different languages in the source code and add among the variants every time a message has to be printed. This is certainly no good solution since extending the set of languages is difficult (the code must be changed) and the code itself can become really big with dozens of message sets.
A better solution is to keep the message sets for each language are kept in separate files which are loaded at runtime depending on the language selection of the user.
The GNU C Library provides two different sets of functions to support
message translation. The problem is that neither of the interfaces is
officially defined by the POSIX standard. The catgets family of
functions is defined in the X/Open standard but this is derived from
industry decisions and therefore not necessarily based on reasonable
decisions.
As mentioned above the message catalog handling provides easy extendibility by using external data files which contain the message translations. I.e., these files contain for each of the messages used in the program a translation for the appropriate language. So the tasks of the message handling functions are
The two approaches mainly differ in the implementation of this last step. The design decisions made for this influences the whole rest.
The catgets functions are based on the simple scheme:
Associate every message to translate in the source code with a unique identifier. To retrieve a message from a catalog file solely the identifier is used.
This means for the author of the program that s/he will have to make sure the meaning of the identifier in the program code and in the message catalogs are always the same.
Before a message can be translated the catalog file must be located. The user of the program must be able to guide the responsible function to find whatever catalog the user wants. This is separated from what the programmer had in mind.
All the types, constants and functions for the catgets functions
are defined/declared in the `nl_types.h' header file.
catgets function family
catgets function tries to locate the message data file names
cat_name and loads it when found. The return value is of an
opaque type and can be used in calls to the other functions to refer to
this loaded catalog.
The return value is (nl_catd) -1 in case the function failed and
no catalog was loaded. The global variable errno contains a code
for the error causing the failure. But even if the function call
succeeded this does not mean that all messages can be translated.
Locating the catalog file must happen in a way which lets the user of the program influence the decision. It is up to the user to decide about the language to use and sometimes it is useful to use alternate catalog files. All this can be specified by the user by setting some environment variables.
The first problem is to find out where all the message catalogs are stored. Every program could have its own place to keep all the different files but usually the catalog files are grouped by languages and the catalogs for all programs are kept in the same place.
To tell the catopen function where the catalog for the program
can be found the user can set the environment variable NLSPATH to
a value which describes her/his choice. Since this value must be usable
for different languages and locales it cannot be a simple string.
Instead it is a format string (similar to printf's). An example
is
/usr/share/locale/%L/%N:/usr/share/locale/%L/LC_MESSAGES/%N
First one can see that more than one directory can be specified (with
the usual syntax of separating them by colons). The next things to
observe are the format string, %L and %N in this case.
The catopen function knows about several of them and the
replacement for all of them is of course different.
%N
catgets.
%L
%l
lang[_terr[.codeset]] and this format uses the
first part lang.
%t
%c
%%
% is used in a meta character there must be a way to
express the % character in the result itself. Using %%
does this just like it works for printf.
Using NLSPATH allows arbitrary directories to be searched for
message catalogs while still allowing different languages to be used.
If the NLSPATH environment variable is not set, the default value
is
prefix/share/locale/%L/%N:prefix/share/locale/%L/LC_MESSAGES/%N
where prefix is given to configure while installing the GNU
C Library (this value is in many cases /usr or the empty string).
The remaining problem is to decide which must be used. The value
decides about the substitution of the format elements mentioned above.
First of all the user can specify a path in the message catalog name
(i.e., the name contains a slash character). In this situation the
NLSPATH environment variable is not used. The catalog must exist
as specified in the program, perhaps relative to the current working
directory. This situation in not desirable and catalogs names never
should be written this way. Beside this, this behavior is not portable
to all other platforms providing the catgets interface.
Otherwise the values of environment variables from the standard
environment are examined (see section Standard Environment Variables). Which
variables are examined is decided by the flag parameter of
catopen. If the value is NL_CAT_LOCALE (which is defined
in `nl_types.h') then the catopen function use the name of
the locale currently selected for the LC_MESSAGES category.
If flag is zero the LANG environment variable is examined.
This is a left-over from the early days where the concept of the locales
had not even reached the level of POSIX locales.
The environment variable and the locale name should have a value of the
form lang[_terr[.codeset]] as explained above.
If no environment variable is set the "C" locale is used which
prevents any translation.
The return value of the function is in any case a valid string. Either it is a translation from a message catalog or it is the same as the string parameter. So a piece of code to decide whether a translation actually happened must look like this:
{
char *trans = catgets (desc, set, msg, input_string);
if (trans == input_string)
{
/* Something went wrong. */
}
}
When an error occurred the global variable errno is set to
While it sometimes can be useful to test for errors programs normally will avoid any test. If the translation is not available it is no big problem if the original, untranslated message is printed. Either the user understands this as well or s/he will look for the reason why the messages are not translated.
Please note that the currently selected locale does not depend on a call
to the setlocale function. It is not necessary that the locale
data files for this locale exist and calling setlocale succeeds.
The catopen function directly reads the values of the environment
variables.
catgets has to be used to access the massage catalog
previously opened using the catopen function. The
catalog_desc parameter must be a value previously returned by
catopen.
The next two parameters, set and message, reflect the internal organization of the message catalog files. This will be explained in detail below. For now it is interesting to know that a catalog can consists of several set and the messages in each thread are individually numbered using numbers. Neither the set number nor the message number must be consecutive. They can be arbitrarily chosen. But each message (unless equal to another one) must have its own unique pair of set and message number.
Since it is not guaranteed that the message catalog for the language selected by the user exists the last parameter string helps to handle this case gracefully. If no matching string can be found string is returned. This means for the programmer that
It is somewhat uncomfortable to write a program using the catgets
functions if no supporting functionality is available. Since each
set/message number tuple must be unique the programmer must keep lists
of the messages at the same time the code is written. And the work
between several people working on the same project must be coordinated.
We will see some how these problems can be relaxed a bit (see section How to use the catgets interface).
catclose function can be used to free the resources
associated with a message catalog which previously was opened by a call
to catopen. If the resources can be successfully freed the
function returns 0. Otherwise it return -1 and the
global variable errno is set. Errors can occur if the catalog
descriptor catalog_desc is not valid in which case errno is
set to EBADF.
The only reasonable way the translate all the messages of a function and
store the result in a message catalog file which can be read by the
catopen function is to write all the message text to the
translator and let her/him translate them all. I.e., we must have a
file with entries which associate the set/message tuple with a specific
translation. This file format is specified in the X/Open standard and
is as follows:
$
followed by a whitespace character are comment and are also ignored.
$set followed by a whitespace character an additional argument
is required to follow. This argument can either be:
catgets interface.
It is an error if a symbol name appears more than once. All following
messages are placed in a set with this number.
$delset followed by a whitespace character an additional argument
is required to follow. This argument can either be:
$set command again messages could be added and these
messages will appear in the output.
$quote, the quoting character used for this input file is
changed to the first non-whitespace character following the
$quote. If no non-whitespace character is present before the
line ends quoting is disable.
By default no quoting character is used. In this mode strings are
terminated with the first unescaped line break. If there is a
$quote sequence present newline need not be escaped. Instead a
string is terminated with the first unescaped appearance of the quote
character.
A common usage of this feature would be to set the quote character to
". Then any appearance of the " in the strings must
be escaped using the backslash (i.e., \" must be written).
catgets interface). There is
one limitation with the identifier: it must not be Set. The
reason will be explained below.
The text of the messages can contain escape characters. The usual bunch
of characters known from the ISO C language are recognized
(\n, \t, \v, \b, \r, \f,
\\, and \nnn, where nnn is the octal coding of
a character code).
Important: The handling of identifiers instead of numbers for the set and messages is a GNU extension. Systems strictly following the X/Open specification do not have this feature. An example for a message catalog file is this:
$ This is a leading comment. $quote " $set SetOne 1 Message with ID 1. two " Message with ID \"two\", which gets the value 2 assigned" $set SetTwo $ Since the last set got the number 1 assigned this set has number 2. 4000 "The numbers can be arbitrary, they need not start at one."
This small example shows various aspects:
$ followed by
a whitespace.
". Otherwise the quotes in the
message definition would have to be left away and in this case the
message with the identifier two would loose its leading whitespace.
While this file format is pretty easy it is not the best possible for
use in a running program. The catopen function would have to
parser the file and handle syntactic errors gracefully. This is not so
easy and the whole process is pretty slow. Therefore the catgets
functions expect the data in another more compact and ready-to-use file
format. There is a special program gencat which is explained in
detail in the next section.
Files in this other format are not human readable. To be easy to use by programs it is a binary file. But the format is byte order independent so translation files can be shared by systems of arbitrary architecture (as long as they use the GNU C Library).
Details about the binary file format are not important to know since
these files are always created by the gencat program. The
sources of the GNU C Library also provide the sources for the
gencat program and so the interested reader can look through
these source files to learn about the file format.
The gencat program is specified in the X/Open standard and the
GNU implementation follows this specification and so processes
all correctly formed input files. Additionally some extension are
implemented which help to work in a more reasonable way with the
catgets functions.
The gencat program can be invoked in two ways:
`gencat [Option]... [Output-File [Input-File]...]`
This is the interface defined in the X/Open standard. If no Input-File parameter is given input will be read from standard input. Multiple input files will be read as if they are concatenated. If Output-File is also missing, the output will be written to standard output. To provide the interface one is used to from other programs a second interface is provided.
`gencat [Option]... -o Output-File [Input-File]...`
The option `-o' is used to specify the output file and all file arguments are used as input files.
Beside this one can use `-' or `/dev/stdin' for Input-File to denote the standard input. Corresponding one can use `-' and `/dev/stdout' for Output-File to denote standard output. Using `-' as a file name is allowed in X/Open while using the device names is a GNU extension.
The gencat program works by concatenating all input files and
then merge the resulting collection of message sets with a
possibly existing output file. This is done by removing all messages
with set/message number tuples matching any of the generated messages
from the output file and then adding all the new messages. To
regenerate a catalog file while ignoring the old contents therefore
requires to remove the output file if it exists. If the output is
written to standard output no merging takes place.
The following table shows the options understood by the gencat
program. The X/Open standard does not specify any option for the
program so all of these are GNU extensions.
#defines to associate a name with a
number.
Please note that the generated file only contains the symbols from the
input files. If the output is merged with the previous content of the
output file the possibly existing symbols from the file(s) which
generated the old output files are not in the generated header file.
catgets interface
The catgets functions can be used in two different ways. By
following slavishly the X/Open specs and not relying on the extension
and by using the GNU extensions. We will take a look at the former
method first to understand the benefits of extensions.
Since the X/Open format of the message catalog files does not allow symbol names we have to work with numbers all the time. When we start writing a program we have to replace all appearances of translatable strings with something like
catgets (catdesc, set, msg, "string")
catgets is retrieved from a call to catopen which is
normally done once at the program start. The "string" is the
string we want to translate. The problems start with the set and
message numbers.
In a bigger program several programmers usually work at the same time on the program and so coordinating the number allocation is crucial. Though no two different strings must be indexed by the same tuple of numbers it is highly desirable to reuse the numbers for equal strings with equal translations (please note that there might be strings which are equal in one language but have different translations due to difference contexts).
The allocation process can be relaxed a bit by different set numbers for
different parts of the program. So the number of developers who have to
coordinate the allocation can be reduced. But still lists must be keep
track of the allocation and errors can easily happen. These errors
cannot be discovered by the compiler or the catgets functions.
Only the user of the program might see wrong messages printed. In the
worst cases the messages are so irritating that they cannot be
recognized as wrong. Think about the translations for "true" and
"false" being exchanged. This could result in a disaster.
The problems mentioned in the last section derive from the fact that:
By constantly using symbolic names and by providing a method which maps the string content to a symbolic name (however this will happen) one can prevent both problems above. The cost of this is that the programmer has to write a complete message catalog file while s/he is writing the program itself.
This is necessary since the symbolic names must be mapped to numbers
before the program sources can be compiled. In the last section it was
described how to generate a header containing the mapping of the names.
E.g., for the example message file given in the last section we could
call the gencat program as follow (assume `ex.msg' contains
the sources).
gencat -H ex.h -o ex.cat ex.msg
This generates a header file with the following content:
#define SetTwoSet 0x2 /* ex.msg:8 */ #define SetOneSet 0x1 /* ex.msg:4 */ #define SetOnetwo 0x2 /* ex.msg:6 */
As can be seen the various symbols given in the source file are mangled
to generate unique identifiers and these identifiers get numbers
assigned. Reading the source file and knowing about the rules will
allow to predict the content of the header file (it is deterministic)
but this is not necessary. The gencat program can take care for
everything. All the programmer has to do is to put the generated header
file in the dependency list of the source files of her/his project and
to add a rules to regenerate the header of any of the input files
change.
One word about the symbol mangling. Every symbol consists of two parts:
the name of the message set plus the name of the message or the special
string Set. So SetOnetwo means this macro can be used to
access the translation with identifier two in the message set
SetOne.
The other names denote the names of the message sets. The special
string Set is used in the place of the message identifier.
If in the code the second string of the set SetOne is used the C
code should look like this:
catgets (catdesc, SetOneSet, SetOnetwo,
" Message with ID \"two\", which gets the value 2 assigned")
Writing the function this way will allow to change the message number and even the set number without requiring any change in the C source code. (The text of the string is normally not the same; this is only for this example.)
To illustrate the usual way to work with the symbolic version numbers here is a little example. Assume we want to write the very complex and famous greeting program. We start by writing the code as usual:
#include <stdio.h>
int
main (void)
{
printf ("Hello, world!\n");
return 0;
}
Now we want to internationalize the message and therefore replace the message with whatever the user wants.
#include <nl_types.h>
#include <stdio.h>
#include "msgnrs.h"
int
main (void)
{
nl_catd catdesc = catopen ("hello.cat", NL_CAT_LOCALE);
printf (catgets (catdesc, SetMainSet, SetMainHello,
"Hello, world!\n"));
catclose (catdesc);
return 0;
}
We see how the catalog object is opened and the returned descriptor used in the other function calls. It is not really necessary to check for failure of any of the functions since even in these situations the functions will behave reasonable. They simply will be return a translation.
What remains unspecified here are the constants SetMainSet and
SetMainHello. These are the symbolic names describing the
message. To get the actual definitions which match the information in
the catalog file we have to create the message catalog source file and
process it using the gencat program.
$ Messages for the famous greeting program. $quote " $set Main Hello "Hallo, Welt!\n"
Now we can start building the program (assume the message catalog source file is named `hello.msg' and the program source file `hello.c'):
% gencat -H msgnrs.h -o hello.cat hello.msg % cat msgnrs.h #define MainSet 0x1 /* hello.msg:4 */ #define MainHello 0x1 /* hello.msg:5 */ % gcc -o hello hello.c -I. % cp hello.cat /usr/share/locale/de/LC_MESSAGES % echo $LC_ALL de % ./hello Hallo, Welt! %
The call of the gencat program creates the missing header file
`msgnrs.h' as well as the message catalog binary. The former is
used in the compilation of `hello.c' while the later is placed in a
directory in which the catopen function will try to locate it.
Please check the LC_ALL environment variable and the default path
for catopen presented in the description above.
Sun Microsystems tried to standardize a different approach to message translation in the Uniforum group. There never was a real standard defined but still the interface was used in Sun's operation systems. Since this approach fits better in the development process of free software it is also used throughout the GNU project and the GNU `gettext' package provides support for this outside the GNU C Library.
The code of the `libintl' from GNU `gettext' is the same as the code in the GNU C Library. So the documentation in the GNU `gettext' manual is also valid for the functionality here. The following text will describe the library functions in detail. But the numerous helper programs are not described in this manual. Instead people should read the GNU `gettext' manual (see section `GNU gettext utilities' in Native Language Support Library and Tools). We will only give a short overview.
Though the catgets functions are available by default on more
systems the gettext interface is at least as portable as the
former. The GNU `gettext' package can be used wherever the
functions are not available.
gettext family of functions
The paradigms underlying the gettext approach to message
translations is different from that of the catgets functions the
basic functionally is equivalent. There are functions of the following
categories:
The gettext functions have a very simple interface. The most
basic function just takes the string which shall be translated as the
argument and it returns the translation. This is fundamentally
different from the catgets approach where an extra key is
necessary and the original string is only used for the error case.
If the string which has to be translated is the only argument this of
course means the string itself is the key. I.e., the translation will
be selected based on the original string. The message catalogs must
therefore contain the original strings plus one translation for any such
string. The task of the gettext function is it to compare the
argument string with the available strings in the catalog and return the
appropriate translation. Of course this process is optimized so that
this process is not more expensive than an access using an atomic key
like in catgets.
The gettext approach has some advantages but also some
disadvantages. Please see the GNU `gettext' manual for a detailed
discussion of the pros and cons.
All the definitions and declarations for gettext can be found in
the `libintl.h' header file. On systems where these functions are
not part of the C library they can be found in a separate library named
`libintl.a' (or accordingly different for shared libraries).
gettext function searches the currently selected message
catalogs for a string which is equal to msgid. If there is such a
string available it is returned. Otherwise the argument string
msgid is returned.
Please note that all though the return value is char * the
returned string must not be changed. This broken type results from the
history of the function and does not reflect the way the function should
be used.
Please note that above we wrote "message catalogs" (plural). This is a specialty of the GNU implementation of these functions and we will say more about this when we talk about the ways message catalogs are selected (see section How to determine which catalog to be used).
The gettext function does not modify the value of the global
errno variable. This is necessary to make it possible to write
something like
printf (gettext ("Operation failed: %m\n"));
Here the errno value is used in the printf function while
processing the %m format element and if the gettext
function would change this value (it is called before printf is
called) we would get a wrong message.
So there is no easy way to detect a missing message catalog beside comparing the argument string with the result. But it is normally the task of the user to react on missing catalogs. The program cannot guess when a message catalog is really necessary since for a user who speaks the language the program was developed in does not need any translation.
The remaining two functions to access the message catalog add some
functionality to select a message catalog which is not the default one.
This is important if parts of the program are developed independently.
Every part can have its own message catalog and all of them can be used
at the same time. The C library itself is an example: internally it
uses the gettext functions but since it must not depend on a
currently selected default message catalog it must specify all ambiguous
information.
dgettext functions acts just like the gettext
function. It only takes an additional first argument domainname
which guides the selection of the message catalogs which are searched
for the translation. If the domainname parameter is the null
pointer the dgettext function is exactly equivalent to
gettext since the default value for the domain name is used.
As for gettext the return value type is char * which is an
anachronism. The returned string must never be modified.
dcgettext adds another argument to those which
dgettext takes. This argument category specifies the last
piece of information needed to localize the message catalog. I.e., the
domain name and the locale category exactly specify which message
catalog has to be used (relative to a given directory, see below).
The dgettext function can be expressed in terms of
dcgettext by using
dcgettext (domain, string, LC_MESSAGES)
instead of
dgettext (domain, string)
This also shows which values are expected for the third parameter. One
has to use the available selectors for the categories available in
`locale.h'. Normally the available values are LC_CTYPE,
LC_COLLATE, LC_MESSAGES, LC_MONETARY,
LC_NUMERIC, and LC_TIME. Please note that LC_ALL
must not be used and even though the names might suggest this, there is
no relation to the environments variables of this name.
The dcgettext function is only implemented for compatibility with
other systems which have gettext functions. There is not really
any situation where it is necessary (or useful) to use a different value
but LC_MESSAGES in for the category parameter. We are
dealing with messages here and any other choice can only be irritating.
As for gettext the return value type is char * which is an
anachronism. The returned string must never be modified.
When using the three functions above in a program it is a frequent case
that the msgid argument is a constant string. So it is worth to
optimize this case. Thinking shortly about this one will realize that
as long as no new message catalog is loaded the translation of a message
will not change. This optimization is actually implemented by the
gettext, dgettext and dcgettext functions.
The functions to retrieve the translations for a given message have a remarkable simple interface. But to provide the user of the program still the opportunity to select exactly the translation s/he wants and also to provide the programmer the possibility to influence the way to locate the search for catalogs files there is a quite complicated underlying mechanism which controls all this. The code is complicated the use is easy.
Basically we have two different tasks to perform which can also be
performed by the catgets functions:
This is the functionality required by the specifications for
gettext and this is also what the catgets functions are
able to do. But there are some problems unresolved:
de, german, or
deutsch and the program should always react the same.
de_DE.ISO-8859-1 which means German, spoken in Germany,
coded using the ISO 8859-1 character set there is the possibility
that a message catalog matching this exactly is not available. But
there could be a catalog matching de and if the character set
used on the machine is always ISO 8859-1 there is no reason why this
later message catalog should not be used. (We call this message
inheritance.)
We can divide the configuration actions in two parts: the one is performed by the programmer, the other by the user. We will start with the functions the programmer can use since the user configuration will be based on this.
As the functions described in the last sections already mention separate
sets of messages can be selected by a domain name. This is a
simple string which should be unique for each program part with uses a
separate domain. It is possible to use in one program arbitrary many
domains at the same time. E.g., the GNU C Library itself uses a domain
named libc while the program using the C Library could use a
domain named foo. The important point is that at any time
exactly one domain is active. This is controlled with the following
function.
textdomain function sets the default domain, which is used in
all future gettext calls, to domainname. Please note that
dgettext and dcgettext calls are not influenced if the
domainname parameter of these functions is not the null pointer.
Before the first call to textdomain the default domain is
messages. This is the name specified in the specification of
the gettext API. This name is as good as any other name. No
program should ever really use a domain with this name since this can
only lead to problems.
The function returns the value which is from now on taken as the default
domain. If the system went out of memory the returned value is
NULL and the global variable errno is set to ENOMEM.
Despite the return value type being char * the return string must
not be changed. It is allocated internally by the textdomain
function.
If the domainname parameter is the null pointer no new default domain is set. Instead the currently selected default domain is returned.
If the domainname parameter is the empty string the default domain
is reset to its initial value, the domain with the name messages.
This possibility is questionable to use since the domain messages
really never should be used.
bindtextdomain function can be used to specify the directory
which contains the message catalogs for domain domainname for the
different languages. To be correct, this is the directory where the
hierarchy of directories is expected. Details are explained below.
For the programmer it is important to note that the translations which
come with the program have be placed in a directory hierarchy starting
at, say, `/foo/bar'. Then the program should make a
bindtextdomain call to bind the domain for the current program to
this directory. So it is made sure the catalogs are found. A correctly
running program does not depend on the user setting an environment
variable.
The bindtextdomain function can be used several times and if the
domainname argument is different the previously bound domains
will not be overwritten.
If the program which wish to use bindtextdomain at some point of
time use the chdir function to change the current working
directory it is important that the dirname strings ought to be an
absolute pathname. Otherwise the addressed directory might vary with
the time.
If the dirname parameter is the null pointer bindtextdomain
returns the currently selected directory for the domain with the name
domainname.
The bindtextdomain function returns a pointer to a string
containing the name of the selected directory name. The string is
allocated internally in the function and must not be changed by the
user. If the system went out of core during the execution of
bindtextdomain the return value is NULL and the global
variable errno is set accordingly.
The functions of the gettext family described so far (and all the
catgets functions as well) have one problem in the real world
which have been neglected completely in all existing approaches. What
is meant here is the handling of plural forms.
Looking through Unix source code before the time anybody thought about internationalization (and, sadly, even afterwards) one can often find code similar to the following:
printf ("%d file%s deleted", n, n == 1 ? "" : "s");
After the first complaints from people internationalizing the code people
either completely avoided formulations like this or used strings like
"file(s)". Both look unnatural and should be avoided. First
tries to solve the problem correctly looked like this:
if (n == 1)
printf ("%d file deleted", n);
else
printf ("%d files deleted", n);
But this does not solve the problem. It helps languages where the plural form of a noun is not simply constructed by adding an `s' but that is all. Once again people fell into the trap of believing the rules their language is using are universal. But the handling of plural forms differs widely between the language families. There are two things we can differ between (and even inside language families);
The consequence of this is that application writers should not try to
solve the problem in their code. This would be localization since it is
only usable for certain, hardcoded language environments. Instead the
extended gettext interface should be used.
These extra functions are taking instead of the one key string two
strings and an numerical argument. The idea behind this is that using
the numerical argument and the first string as a key, the implementation
can select using rules specified by the translator the right plural
form. The two string arguments then will be used to provide a return
value in case no message catalog is found (similar to the normal
gettext behavior). In this case the rules for Germanic language
is used and it is assumed that the first string argument is the singular
form, the second the plural form.
This has the consequence that programs without language catalogs can
display the correct strings only if the program itself is written using
a Germanic language. This is a limitation but since the GNU C library
(as well as the GNU gettext package) are written as part of the
GNU package and the coding standards for the GNU project require program
being written in English, this solution nevertheless fulfills its
purpose.
ngettext function is similar to the gettext function
as it finds the message catalogs in the same way. But it takes two
extra arguments. The msgid1 parameter must contain the singular
form of the string to be converted. It is also used as the key for the
search in the catalog. The msgid2 parameter is the plural form.
The parameter n is used to determine the plural form. If no
message catalog is found msgid1 is returned if n == 1,
otherwise msgid2.
An example for the us of this function is:
printf (ngettext ("%d file removed", "%d files removed", n), n);
Please note that the numeric value n has to be passed to the
printf function as well. It is not sufficient to pass it only to
ngettext.
dngettext is similar to the dgettext function in the
way the message catalog is selected. The difference is that it takes
two extra parameter to provide the correct plural form. These two
parameters are handled in the same way ngettext handles them.
dcngettext is similar to the dcgettext function in the
way the message catalog is selected. The difference is that it takes
two extra parameter to provide the correct plural form. These two
parameters are handled in the same way ngettext handles them.
A description of the problem can be found at the beginning of the last section. Now there is the question how to solve it. Without the input of linguists (which was not available) it was not possible to determine whether there are only a few different forms in which plural forms are formed or whether the number can increase with every new supported language.
Therefore the solution implemented is to allow the translator to specify
the rules of how to select the plural form. Since the formula varies
with every language this is the only viable solution except for
hardcoding the information in the code (which still would require the
possibility of extensions to not prevent the use of new languages). The
details are explained in the GNU gettext manual. Here only a a
bit of information is provided.
The information about the plural form selection has to be stored in the
header entry (the one with the empty (msgid string). It looks
like this:
Plural-Forms: nplurals=2; plural=n == 1 ? 0 : 1;
The nplurals value must be a decimal number which specifies how
many different plural forms exist for this language. The string
following plural is an expression which is using the C language
syntax. Exceptions are that no negative number are allowed, numbers
must be decimal, and the only variable allowed is n. This
expression will be evaluated whenever one of the functions
ngettext, dngettext, or dcngettext is called. The
numeric value passed to these functions is then substituted for all uses
of the variable n in the expression. The resulting value then
must be greater or equal to zero and smaller than the value given as the
value of nplurals.
The following rules are known at this point. The language with families are listed. But this does not necessarily mean the information can be generalized for the whole family (as can be easily seen in the table below).(1).}
Plural-Forms: nplurals=1; plural=0;Languages with this property include:
Plural-Forms: nplurals=2; plural=n != 1;(Note: this uses the feature of C expressions that boolean expressions have to value zero or one.) Languages with this property include:
Plural-Forms: nplurals=2; plural=n>1;Languages with this property include:
Plural-Forms: nplurals=3; plural=n==1 ? 0 : n==2 ? 1 : 2;Languages with this property include:
Plural-Forms: nplurals=3; \
plural=n%100/10==1 ? 2 : n%10==1 ? 0 : (n+9)%10>3 ? 2 : 1;
Languages with this property include:
Plural-Forms: nplurals=3; \
plural=n==1 ? 0 : \
n%10>=2 && n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2;
(Continuation in the next line is possible.)
Languages with this property include:
Plural-Forms: nplurals=4; \
plural=n==1 ? 0 : n%10==2 ? 1 : n%10==3 || n%10==4 ? 2 : 3;
Languages with this property include:
gettext uses
gettext not only looks up a translation in a message catalog. It
also converts the translation on the fly to the desired output character
set. This is useful if the user is working in a different character set
than the translator who created the message catalog, because it avoids
distributing variants of message catalogs which differ only in the
character set.
The output character set is, by default, the value of nl_langinfo
(CODESET), which depends on the LC_CTYPE part of the current
locale. But programs which store strings in a locale independent way
(e.g. UTF-8) can request that gettext and related functions
return the translations in that encoding, by use of the
bind_textdomain_codeset function.
Note that the msgid argument to gettext is not subject to
character set conversion. Also, when gettext does not find a
translation for msgid, it returns msgid unchanged --
independently of the current output character set. It is therefore
recommended that all msgids be US-ASCII strings.
bind_textdomain_codeset function can be used to specify the
output character set for message catalogs for domain domainname.
The codeset argument must be a valid codeset name which can be used
for the iconv_open function, or a null pointer.
If the codeset parameter is the null pointer,
bind_textdomain_codeset returns the currently selected codeset
for the domain with the name domainname. It returns NULL if
no codeset has yet been selected.
The bind_textdomain_codeset function can be used several times.
If used multiple times with the same domainname argument, the
later call overrides the settings made by the earlier one.
The bind_textdomain_codeset function returns a pointer to a
string containing the name of the selected codeset. The string is
allocated internally in the function and must not be changed by the
user. If the system went out of core during the execution of
bind_textdomain_codeset, the return value is NULL and the
global variable errno is set accordingly. @end deftypefun
gettext in GUI programs
One place where the gettext functions, if used normally, have big
problems is within programs with graphical user interfaces (GUIs). The
problem is that many of the strings which have to be translated are very
short. They have to appear in pull-down menus which restricts the
length. But strings which are not containing entire sentences or at
least large fragments of a sentence may appear in more than one
situation in the program but might have different translations. This is
especially true for the one-word strings which are frequently used in
GUI programs.
As a consequence many people say that the gettext approach is
wrong and instead catgets should be used which indeed does not
have this problem. But there is a very simple and powerful method to
handle these kind of problems with the gettext functions.
As as example consider the following fictional situation. A GUI program has a menu bar with the following entries:
+------------+------------+--------------------------------------+
| File | Printer | |
+------------+------------+--------------------------------------+
| Open | | Select |
| New | | Open |
+----------+ | Connect |
+----------+
To have the strings File, Printer, Open,
New, Select, and Connect translated there has to be
at some point in the code a call to a function of the gettext
family. But in two places the string passed into the function would be
Open. The translations might not be the same and therefore we
are in the dilemma described above.
One solution to this problem is to artificially enlengthen the strings to make them unambiguous. But what would the program do if no translation is available? The enlengthened string is not what should be printed. So we should use a little bit modified version of the functions.
To enlengthen the strings a uniform method should be used. E.g., in the example above the strings could be chosen as
Menu|File Menu|Printer Menu|File|Open Menu|File|New Menu|Printer|Select Menu|Printer|Open Menu|Printer|Connect
Now all the strings are different and if now instead of gettext
the following little wrapper function is used, everything works just
fine:
char *
sgettext (const char *msgid)
{
char *msgval = gettext (msgid);
if (msgval == msgid)
msgval = strrchr (msgid, '|') + 1;
return msgval;
}
What this little function does is to recognize the case when no
translation is available. This can be done very efficiently by a
pointer comparison since the return value is the input value. If there
is no translation we know that the input string is in the format we used
for the Menu entries and therefore contains a | character. We
simply search for the last occurrence of this character and return a
pointer to the character following it. That's it!
If one now consistently uses the enlengthened string form and replaces
the gettext calls with calls to sgettext (this is normally
limited to very few places in the GUI implementation) then it is
possible to produce a program which can be internationalized.
With advanced compilers (such as GNU C) one can write the
sgettext functions as an inline function or as a macro like this:
#define sgettext(msgid) \
({ const char *__msgid = (msgid); \
char *__msgstr = gettext (__msgid); \
if (__msgval == __msgid) \
__msgval = strrchr (__msgid, '|') + 1; \
__msgval; })
The other gettext functions (dgettext, dcgettext
and the ngettext equivalents) can and should have corresponding
functions as well which look almost identical, except for the parameters
and the call to the underlying function.
Now there is of course the question why such functions do not exist in the GNU C library? There are two parts of the answer to this question.
| which is a quite good choice because it
resembles a notation frequently used in this context and it also is a
character not often used in message strings.
But what if the character is used in message strings. Or if the chose
character is not available in the character set on the machine one
compiles (e.g., | is not required to exist for ISO C; this is
why the `iso646.h' file exists in ISO C programming environments).
There is only one more comment to make left. The wrapper function above require that the translations strings are not enlengthened themselves. This is only logical. There is no need to disambiguate the strings (since they are never used as keys for a search) and one also saves quite some memory and disk space by doing this.
gettextThe last sections described what the programmer can do to internationalize the messages of the program. But it is finally up to the user to select the message s/he wants to see. S/He must understand them.
The POSIX locale model uses the environment variables LC_COLLATE,
LC_CTYPE, LC_MESSAGES, LC_MONETARY, NUMERIC,
and LC_TIME to select the locale which is to be used. This way
the user can influence lots of functions. As we mentioned above the
gettext functions also take advantage of this.
To understand how this happens it is necessary to take a look at the various components of the filename which gets computed to locate a message catalog. It is composed as follows:
dir_name/locale/LC_category/domain_name.mo
The default value for dir_name is system specific. It is computed from the value given as the prefix while configuring the C library. This value normally is `/usr' or `/'. For the former the complete dir_name is:
/usr/share/locale
We can use `/usr/share' since the `.mo' files containing the
message catalogs are system independent, so all systems can use the same
files. If the program executed the bindtextdomain function for
the message domain that is currently handled, the dir_name
component is exactly the value which was given to the function as
the second parameter. I.e., bindtextdomain allows overwriting
the only system dependent and fixed value to make it possible to
address files anywhere in the filesystem.
The category is the name of the locale category which was selected
in the program code. For gettext and dgettext this is
always LC_MESSAGES, for dcgettext this is selected by the
value of the third parameter. As said above it should be avoided to
ever use a category other than LC_MESSAGES.
The locale component is computed based on the category used. Just
like for the setlocale function here comes the user selection
into the play. Some environment variables are examined in a fixed order
and the first environment variable set determines the return value of
the lookup process. In detail, for the category LC_xxx the
following variables in this order are examined:
LANGUAGE
LC_ALL
LC_xxx
LANG
This looks very familiar. With the exception of the LANGUAGE
environment variable this is exactly the lookup order the
setlocale function uses. But why introducing the LANGUAGE
variable?
The reason is that the syntax of the values these variables can have is
different to what is expected by the setlocale function. If we
would set LC_ALL to a value following the extended syntax that
would mean the setlocale function will never be able to use the
value of this variable as well. An additional variable removes this
problem plus we can select the language independently of the locale
setting which sometimes is useful.
While for the LC_xxx variables the value should consist of
exactly one specification of a locale the LANGUAGE variable's
value can consist of a colon separated list of locale names. The
attentive reader will realize that this is the way we manage to
implement one of our additional demands above: we want to be able to
specify an ordered list of language.
Back to the constructed filename we have only one component missing.
The domain_name part is the name which was either registered using
the textdomain function or which was given to dgettext or
dcgettext as the first parameter. Now it becomes obvious that a
good choice for the domain name in the program code is a string which is
closely related to the program/package name. E.g., for the GNU C
Library the domain name is libc.
A limit piece of example code should show how the programmer is supposed to work:
{
setlocale (LC_ALL, "");
textdomain ("test-package");
bindtextdomain ("test-package", "/usr/local/share/locale");
puts (gettext ("Hello, world!"));
}
At the program start the default domain is messages, and the
default locale is "C". The setlocale call sets the locale
according to the user's environment variables; remember that correct
functioning of gettext relies on the correct setting of the
LC_MESSAGES locale (for looking up the message catalog) and
of the LC_CTYPE locale (for the character set conversion).
The textdomain call changes the default domain to
test-package. The bindtextdomain call specifies that
the message catalogs for the domain test-package can be found
below the directory `/usr/local/share/locale'.
If now the user set in her/his environment the variable LANGUAGE
to de the gettext function will try to use the
translations from the file
/usr/local/share/locale/de/LC_MESSAGES/test-package.mo
From the above descriptions it should be clear which component of this filename is determined by which source.
In the above example we assumed that the LANGUAGE environment
variable to de. This might be an appropriate selection but what
happens if the user wants to use LC_ALL because of the wider
usability and here the required value is de_DE.ISO-8859-1? We
already mentioned above that a situation like this is not infrequent.
E.g., a person might prefer reading a dialect and if this is not
available fall back on the standard language.
The gettext functions know about situations like this and can
handle them gracefully. The functions recognize the format of the value
of the environment variable. It can split the value is different pieces
and by leaving out the only or the other part it can construct new
values. This happens of course in a predictable way. To understand
this one must know the format of the environment variable value. There
are two more or less standardized forms:
language[_territory[.codeset]][@modifier]
language[_territory][+audience][+special][,[sponsor][_revision]]
The functions will automatically recognize which format is used. Less specific locale names will be stripped of in the order of the following list:
revision
sponsor
special
codeset
normalized codeset
territory
audience/modifier
From the last entry one can see that the meaning of the modifier
field in the X/Open format and the audience format have the same
meaning. Beside one can see that the language field for obvious
reasons never will be dropped.
The only new thing is the normalized codeset entry. This is
another goodie which is introduced to help reducing the chaos which
derives from the inability of the people to standardize the names of
character sets. Instead of ISO-8859-1 one can often see 8859-1,
88591, iso8859-1, or iso_8859-1. The normalized
codeset value is generated from the user-provided character set name by
applying the following rules:
"iso".
So all of the above name will be normalized to iso88591. This
allows the program user much more freely choosing the locale name.
Even this extended functionality still does not help to solve the
problem that completely different names can be used to denote the same
locale (e.g., de and german). To be of help in this
situation the locale implementation and also the gettext
functions know about aliases.
The file `/usr/share/locale/locale.alias' (replace `/usr' with whatever prefix you used for configuring the C library) contains a mapping of alternative names to more regular names. The system manager is free to add new entries to fill her/his own needs. The selected locale from the environment is compared with the entries in the first column of this file ignoring the case. If they match the value of the second column is used instead for the further handling.
In the description of the format of the environment variables we already mentioned the character set as a factor in the selection of the message catalog. In fact, only catalogs which contain text written using the character set of the system/program can be used (directly; there will come a solution for this some day). This means for the user that s/he will always have to take care for this. If in the collection of the message catalogs there are files for the same language but coded using different character sets the user has to be careful.
gettext
The GNU C Library does not contain the source code for the programs to
handle message catalogs for the gettext functions. As part of
the GNU project the GNU gettext package contains everything the
developer needs. The functionality provided by the tools in this
package by far exceeds the abilities of the gencat program
described above for the catgets functions.
There is a program msgfmt which is the equivalent program to the
gencat program. It generates from the human-readable and
-editable form of the message catalog a binary file which can be used by
the gettext functions. But there are several more programs
available.
The xgettext program can be used to automatically extract the
translatable messages from a source file. I.e., the programmer need not
take care for the translations and the list of messages which have to be
translated. S/He will simply wrap the translatable string in calls to
gettext et.al and the rest will be done by xgettext. This
program has a lot of option which help to customize the output or do
help to understand the input better.
Other programs help to manage development cycle when new messages appear in the source files or when a new translation of the messages appear. here it should only be noted that using all the tools in GNU gettext it is possible to completely automize the handling of message catalog. Beside marking the translatable string in the source code and generating the translations the developers do not have anything to do themselves.
This chapter describes functions for searching and sorting arrays of arbitrary objects. You pass the appropriate comparison function to be applied as an argument, along with the size of the objects in the array and the total number of elements.
In order to use the sorted array library functions, you have to describe how to compare the elements of the array.
To do this, you supply a comparison function to compare two elements of
the array. The library will call this function, passing as arguments
pointers to two array elements to be compared. Your comparison function
should return a value the way strcmp (see section String/Array Comparison) does: negative if the first argument is "less" than the
second, zero if they are "equal", and positive if the first argument
is "greater".
Here is an example of a comparison function which works with an array of
numbers of type double:
int
compare_doubles (const void *a, const void *b)
{
const double *da = (const double *) a;
const double *db = (const double *) b;
return (*da > *db) - (*da < *db);
}
The header file `stdlib.h' defines a name for the data type of comparison functions. This type is a GNU extension.
int comparison_fn_t (const void *, const void *);
Generally searching for a specific element in an array means that potentially all elements must be checked. The GNU C library contains functions to perform linear search. The prototypes for the following two functions can be found in `search.h'.
lfind function searches in the array with *nmemb
elements of size bytes pointed to by base for an element
which matches the one pointed to by key. The function pointed to
by compar is used decide whether two elements match.
The return value is a pointer to the matching element in the array
starting at base if it is found. If no matching element is
available NULL is returned.
The mean runtime of this function is *nmemb/2. This
function should only be used elements often get added to or deleted from
the array in which case it might not be useful to sort the array before
searching.
lsearch function is similar to the lfind function. It
searches the given array for an element and returns it if found. The
difference is that if no matching element is found the lsearch
function adds the object pointed to by key (with a size of
size bytes) at the end of the array and it increments the value of
*nmemb to reflect this addition.
This means for the caller that if it is not sure that the array contains
the element one is searching for the memory allocated for the array
starting at base must have room for at least size more
bytes. If one is sure the element is in the array it is better to use
lfind so having more room in the array is always necessary when
calling lsearch.
To search a sorted array for an element matching the key, use the
bsearch function. The prototype for this function is in
the header file `stdlib.h'.
bsearch function searches the sorted array array for an object
that is equivalent to key. The array contains count elements,
each of which is of size size bytes.
The compare function is used to perform the comparison. This function is called with two pointer arguments and should return an integer less than, equal to, or greater than zero corresponding to whether its first argument is considered less than, equal to, or greater than its second argument. The elements of the array must already be sorted in ascending order according to this comparison function.
The return value is a pointer to the matching array element, or a null pointer if no match is found. If the array contains more than one element that matches, the one that is returned is unspecified.
This function derives its name from the fact that it is implemented using the binary search algorithm.
To sort an array using an arbitrary comparison function, use the
qsort function. The prototype for this function is in
`stdlib.h'.
The compare function is used to perform the comparison on the array elements. This function is called with two pointer arguments and should return an integer less than, equal to, or greater than zero corresponding to whether its first argument is considered less than, equal to, or greater than its second argument.
Warning: If two objects compare as equal, their order after sorting is unpredictable. That is to say, the sorting is not stable. This can make a difference when the comparison considers only part of the elements. Two elements with the same sort key may differ in other respects.
If you want the effect of a stable sort, you can get this result by writing the comparison function so that, lacking other reason distinguish between two elements, it compares them by their addresses. Note that doing this may make the sorting algorithm less efficient, so do it only if necessary.
Here is a simple example of sorting an array of doubles in numerical order, using the comparison function defined above (see section Defining the Comparison Function):
{
double *array;
int size;
...
qsort (array, size, sizeof (double), compare_doubles);
}
The qsort function derives its name from the fact that it was
originally implemented using the "quick sort" algorithm.
The implementation of qsort in this library might not be an
in-place sort and might thereby use an extra amount of memory to store
the array.
Here is an example showing the use of qsort and bsearch
with an array of structures. The objects in the array are sorted
by comparing their name fields with the strcmp function.
Then, we can look up individual objects based on their names.
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
/* Define an array of critters to sort. */
struct critter
{
const char *name;
const char *species;
};
struct critter muppets[] =
{
{"Kermit", "frog"},
{"Piggy", "pig"},
{"Gonzo", "whatever"},
{"Fozzie", "bear"},
{"Sam", "eagle"},
{"Robin", "frog"},
{"Animal", "animal"},
{"Camilla", "chicken"},
{"Sweetums", "monster"},
{"Dr. Strangepork", "pig"},
{"Link Hogthrob", "pig"},
{"Zoot", "human"},
{"Dr. Bunsen Honeydew", "human"},
{"Beaker", "human"},
{"Swedish Chef", "human"}
};
int count = sizeof (muppets) / sizeof (struct critter);
/* This is the comparison function used for sorting and searching. */
int
critter_cmp (const struct critter *c1, const struct critter *c2)
{
return strcmp (c1->name, c2->name);
}
/* Print information about a critter. */
void
print_critter (const struct critter *c)
{
printf ("%s, the %s\n", c->name, c->species);
}
/* Do the lookup into the sorted array. */
void
find_critter (const char *name)
{
struct critter target, *result;
target.name = name;
result = bsearch (&target, muppets, count, sizeof (struct critter),
critter_cmp);
if (result)
print_critter (result);
else
printf ("Couldn't find %s.\n", name);
}
/* Main program. */
int
main (void)
{
int i;
for (i = 0; i < count; i++)
print_critter (&muppets[i]);
printf ("\n");
qsort (muppets, count, sizeof (struct critter), critter_cmp);
for (i = 0; i < count; i++)
print_critter (&muppets[i]);
printf ("\n");
find_critter ("Kermit");
find_critter ("Gonzo");
find_critter ("Janice");
return 0;
}
The output from this program looks like:
Kermit, the frog Piggy, the pig Gonzo, the whatever Fozzie, the bear Sam, the eagle Robin, the frog Animal, the animal Camilla, the chicken Sweetums, the monster Dr. Strangepork, the pig Link Hogthrob, the pig Zoot, the human Dr. Bunsen Honeydew, the human Beaker, the human Swedish Chef, the human Animal, the animal Beaker, the human Camilla, the chicken Dr. Bunsen Honeydew, the human Dr. Strangepork, the pig Fozzie, the bear Gonzo, the whatever Kermit, the frog Link Hogthrob, the pig Piggy, the pig Robin, the frog Sam, the eagle Swedish Chef, the human Sweetums, the monster Zoot, the human Kermit, the frog Gonzo, the whatever Couldn't find Janice.
hsearch function.The functions mentioned so far in this chapter are searching in a sorted or unsorted array. There are other methods to organize information which later should be searched. The costs of insert, delete and search differ. One possible implementation is using hashing tables.
hcreate function creates a hashing table which can contain at
least nel elements. There is no possibility to grow this table so
it is necessary to choose the value for nel wisely. The used
methods to implement this function might make it necessary to make the
number of elements in the hashing table larger than the expected maximal
number of elements. Hashing tables usually work inefficient if they are
filled 80% or more. The constant access time guaranteed by hashing can
only be achieved if few collisions exist. See Knuth's "The Art of
Computer Programming, Part 3: Searching and Sorting" for more
information.
The weakest aspect of this function is that there can be at most one hashing table used through the whole program. The table is allocated in local memory out of control of the programmer. As an extension the GNU C library provides an additional set of functions with an reentrant interface which provide a similar interface but which allow to keep arbitrarily many hashing tables.
It is possible to use more than one hashing table in the program run if
the former table is first destroyed by a call to hdestroy.
The function returns a non-zero value if successful. If it return zero something went wrong. This could either mean there is already a hashing table in use or the program runs out of memory.
hdestroy function can be used to free all the resources
allocated in a previous call of hcreate. After a call to this
function it is again possible to call hcreate and allocate a new
table with possibly different size.
It is important to remember that the elements contained in the hashing
table at the time hdestroy is called are not freed by this
function. It is the responsibility of the program code to free those
strings (if necessary at all). Freeing all the element memory is not
possible without extra, separately kept information since there is no
function to iterate through all available elements in the hashing table.
If it is really necessary to free a table and all elements the
programmer has to keep a list of all table elements and before calling
hdestroy s/he has to free all element's data using this list.
This is a very unpleasant mechanism and it also shows that this kind of
hashing tables is mainly meant for tables which are created once and
used until the end of the program run.
Entries of the hashing table and keys for the search are defined using this type:
hsearch functions. They can only be used for data sets which use
the NUL character always and solely to terminate the records. It is not
possible to handle general binary data.
char *key
char *data
hcreate the
hsearch function must be used. This function can perform simple
search for an element (if action has the FIND) or it can
alternatively insert the key element into the hashing table, possibly
replacing a previous value (if action is ENTER).
The key is denoted by a pointer to an object of type ENTRY. For
locating the corresponding position in the hashing table only the
key element of the structure is used.
The return value depends on the action parameter value. If it is
FIND the value is a pointer to the matching element in the
hashing table or NULL if no matching element exists. If
action is ENTER the return value is only NULL if the
programs runs out of memory while adding the new element to the table.
Otherwise the return value is a pointer to the element in the hashing
table which contains newly added element based on the data in key.
As mentioned before the hashing table used by the functions described so
far is global and there can be at any time at most one hashing table in
the program. A solution is to use the following functions which are a
GNU extension. All have in common that they operate on a hashing table
which is described by the content of an object of the type struct
hsearch_data. This type should be treated as opaque, none of its
members should be changed directly.
hcreate_r function initializes the object pointed to by
htab to contain a hashing table with at least nel elements.
So this function is equivalent to the hcreate function except
that the initialized data structure is controlled by the user.
This allows having more than one hashing table at one time. The
memory necessary for the struct hsearch_data object can be
allocated dynamically.
The return value is non-zero if the operation were successful. if the return value is zero something went wrong which probably means the programs runs out of memory.
hdestroy_r function frees all resources allocated by the
hcreate_r function for this very same object htab. As for
hdestroy it is the programs responsibility to free the strings
for the elements of the table.
hsearch_r function is equivalent to hsearch. The
meaning of the first two arguments is identical. But instead of
operating on a single global hashing table the function works on the
table described by the object pointed to by htab (which is
initialized by a call to hcreate_r).
Another difference to hcreate is that the pointer to the found
entry in the table is not the return value of the functions. It is
returned by storing it in a pointer variables pointed to by the
retval parameter. The return value of the function is an integer
value indicating success if it is non-zero and failure if it is zero.
In the latter case the global variable errno signals the reason for
the failure.
ENOMEM
hsearch_r was called with an so far
unknown key and action set to ENTER.
ESRCH
FIND and no corresponding element
is found in the table.
tsearch function.
Another common form to organize data for efficient search is to use
trees. The tsearch function family provides a nice interface to
functions to organize possibly large amounts of data by providing a mean
access time proportional to the logarithm of the number of elements.
The GNU C library implementation even guarantees that this bound is
never exceeded even for input data which cause problems for simple
binary tree implementations.
The functions described in the chapter are all described in the System V and X/Open specifications and are therefore quite portable.
In contrast to the hsearch functions the tsearch functions
can be used with arbitrary data and not only zero-terminated strings.
The tsearch functions have the advantage that no function to
initialize data structures is necessary. A simple pointer of type
void * initialized to NULL is a valid tree and can be
extended or searched.
tsearch function searches in the tree pointed to by
*rootp for an element matching key. The function
pointed to by compar is used to determine whether two elements
match. See section Defining the Comparison Function, for a specification of the functions
which can be used for the compar parameter.
If the tree does not contain a matching entry the key value will
be added to the tree. tsearch does not make a copy of the object
pointed to by key (how could it since the size is unknown).
Instead it adds a reference to this object which means the object must
be available as long as the tree data structure is used.
The tree is represented by a pointer to a pointer since it is sometimes
necessary to change the root node of the tree. So it must not be
assumed that the variable pointed to by rootp has the same value
after the call. This also shows that it is not safe to call the
tsearch function more than once at the same time using the same
tree. It is no problem to run it more than once at a time on different
trees.
The return value is a pointer to the matching element in the tree. If a
new element was created the pointer points to the new data (which is in
fact key). If an entry had to be created and the program ran out
of space NULL is returned.
tfind function is similar to the tsearch function. It
locates an element matching the one pointed to by key and returns
a pointer to this element. But if no matching element is available no
new element is entered (note that the rootp parameter points to a
constant pointer). Instead the function returns NULL.
Another advantage of the tsearch function in contrast to the
hsearch functions is that there is an easy way to remove
elements.
tdelete can be used. It locates the matching element using the
same method as tfind. The corresponding element is then removed
and a pointer to the parent of the deleted node is returned by the
function. If there is no matching entry in the tree nothing can be
deleted and the function returns NULL. If the root of the tree
is deleted tdelete returns some unspecified value not equal to
NULL.
tdestroy. It frees all resources allocated by the tsearch
function to generate the tree pointed to by vroot.
For the data in each tree node the function freefct is called. The pointer to the data is passed as the argument to the function. If no such work is necessary freefct must point to a function doing nothing. It is called in any case.
This function is a GNU extension and not covered by the System V or X/Open specifications.
In addition to the function to create and destroy the tree data structure, there is another function which allows you to apply a function to all elements of the tree. The function must have this type:
void __action_fn_t (const void *nodep, VISIT value, int level);
The nodep is the data value of the current node (once given as the
key argument to tsearch). level is a numeric value
which corresponds to the depth of the current node in the tree. The
root node has the depth @math{0} and its children have a depth of
@math{1} and so on. The VISIT type is an enumeration type.
VISIT value indicates the status of the current node in the
tree and how the function is called. The status of a node is either
`leaf' or `internal node'. For each leaf node the function is called
exactly once, for each internal node it is called three times: before
the first child is processed, after the first child is processed and
after both children are processed. This makes it possible to handle all
three methods of tree traversal (or even a combination of them).
preorder
postorder
endorder
leaf
twalk function calls the function provided by the parameter
action. For leaf nodes the function is called exactly once with
value set to leaf. For internal nodes the function is
called three times, setting the value parameter or action to
the appropriate value. The level argument for the action
function is computed while descending the tree with increasing the value
by one for the descend to a child, starting with the value @math{0} for
the root node.
Since the functions used for the action parameter to twalk
must not modify the tree data, it is safe to run twalk in more
than one thread at the same time, working on the same tree. It is also
safe to call tfind in parallel. Functions which modify the tree
must not be used, otherwise the behaviour is undefined.
The GNU C Library provides pattern matching facilities for two kinds of patterns: regular expressions and file-name wildcards. The library also provides a facility for expanding variable and command references and parsing text into words in the way the shell does.
This section describes how to match a wildcard pattern against a particular string. The result is a yes or no answer: does the string fit the pattern or not. The symbols described here are all declared in `fnmatch.h'.
0 if they do match; otherwise, it
returns the nonzero value FNM_NOMATCH. The arguments
pattern and string are both strings.
The argument flags is a combination of flag bits that alter the details of matching. See below for a list of the defined flags.
In the GNU C Library, fnmatch cannot experience an "error"---it
always returns an answer for whether the match succeeds. However, other
implementations of fnmatch might sometimes report "errors".
They would do so by returning nonzero values that are not equal to
FNM_NOMATCH.
These are the available flags for the flags argument:
FNM_FILE_NAME
FNM_PATHNAME
FNM_FILE_NAME; it comes from POSIX.2. We
don't recommend this name because we don't use the term "pathname" for
file names.
FNM_PERIOD
FNM_PERIOD and FNM_FILE_NAME, then the
special treatment applies to `.' following `/' as well as to
`.' at the beginning of string. (The shell uses the
FNM_PERIOD and FNM_FILE_NAME flags together for matching
file names.)
FNM_NOESCAPE
FNM_NOESCAPE, then `\' is an ordinary character.
FNM_LEADING_DIR
FNM_CASEFOLD
FNM_EXTMATCH
|
separated list of patterns.
?(pattern-list)
*(pattern-list)
+(pattern-list)
@(pattern-list)
!(pattern-list)
The archetypal use of wildcards is for matching against the files in a directory, and making a list of all the matches. This is called globbing.
You could do this using fnmatch, by reading the directory entries
one by one and testing each one with fnmatch. But that would be
slow (and complex, since you would have to handle subdirectories by
hand).
The library provides a function glob to make this particular use
of wildcards convenient. glob and the other symbols in this
section are declared in `glob.h'.
glob
The result of globbing is a vector of file names (strings). To return
this vector, glob uses a special data type, glob_t, which
is a structure. You pass glob the address of the structure, and
it fills in the structure's fields to tell you about the results.
gl_pathc
gl_pathv
char **.
gl_offs
gl_pathv field. Unlike the other fields, this
is always an input to glob, rather than an output from it.
If you use a nonzero offset, then that many elements at the beginning of
the vector are left empty. (The glob function fills them with
null pointers.)
The gl_offs field is meaningful only if you use the
GLOB_DOOFFS flag. Otherwise, the offset is always zero
regardless of what is in this field, and the first real element comes at
the beginning of the vector.
gl_closedir
closedir
function. It is used if the GLOB_ALTDIRFUNC bit is set in
the flag parameter. The type of this field is
void (*) (void *).
This is a GNU extension.
gl_readdir
readdir
function used to read the contents of a directory. It is used if the
GLOB_ALTDIRFUNC bit is set in the flag parameter. The type of
this field is struct dirent *(*) (void *).
This is a GNU extension.
gl_opendir
opendir
function. It is used if the GLOB_ALTDIRFUNC bit is set in
the flag parameter. The type of this field is
void *(*) (const char *).
This is a GNU extension.
gl_stat
stat function
to get information about an object in the filesystem. It is used if the
GLOB_ALTDIRFUNC bit is set in the flag parameter. The type of
this field is int (*) (const char *, struct stat *).
This is a GNU extension.
gl_lstat
lstat
function to get information about an object in the filesystems, not
following symbolic links. It is used if the GLOB_ALTDIRFUNC bit
is set in the flag parameter. The type of this field is int
(*) (const char *, struct stat *).
This is a GNU extension.
For use in the glob64 function `glob.h' contains another
definition for a very similar type. glob64_t differs from
glob_t only in the types of the members gl_readdir,
gl_stat, and gl_lstat.
gl_pathc
gl_pathv
char **.
gl_offs
gl_pathv field. Unlike the other fields, this
is always an input to glob, rather than an output from it.
If you use a nonzero offset, then that many elements at the beginning of
the vector are left empty. (The glob function fills them with
null pointers.)
The gl_offs field is meaningful only if you use the
GLOB_DOOFFS flag. Otherwise, the offset is always zero
regardless of what is in this field, and the first real element comes at
the beginning of the vector.
gl_closedir
closedir
function. It is used if the GLOB_ALTDIRFUNC bit is set in
the flag parameter. The type of this field is
void (*) (void *).
This is a GNU extension.
gl_readdir
readdir64
function used to read the contents of a directory. It is used if the
GLOB_ALTDIRFUNC bit is set in the flag parameter. The type of
this field is struct dirent64 *(*) (void *).
This is a GNU extension.
gl_opendir
opendir
function. It is used if the GLOB_ALTDIRFUNC bit is set in
the flag parameter. The type of this field is
void *(*) (const char *).
This is a GNU extension.
gl_stat
stat64 function
to get information about an object in the filesystem. It is used if the
GLOB_ALTDIRFUNC bit is set in the flag parameter. The type of
this field is int (*) (const char *, struct stat64 *).
This is a GNU extension.
gl_lstat
lstat64
function to get information about an object in the filesystems, not
following symbolic links. It is used if the GLOB_ALTDIRFUNC bit
is set in the flag parameter. The type of this field is int
(*) (const char *, struct stat64 *).
This is a GNU extension.
glob does globbing using the pattern pattern
in the current directory. It puts the result in a newly allocated
vector, and stores the size and address of this vector into
*vector-ptr. The argument flags is a combination of
bit flags; see section Flags for Globbing, for details of the flags.
The result of globbing is a sequence of file names. The function
glob allocates a string for each resulting word, then
allocates a vector of type char ** to store the addresses of
these strings. The last element of the vector is a null pointer.
This vector is called the word vector.
To return this vector, glob stores both its address and its
length (number of elements, not counting the terminating null pointer)
into *vector-ptr.
Normally, glob sorts the file names alphabetically before
returning them. You can turn this off with the flag GLOB_NOSORT
if you want to get the information as fast as possible. Usually it's
a good idea to let glob sort them--if you process the files in
alphabetical order, the users will have a feel for the rate of progress
that your application is making.
If glob succeeds, it returns 0. Otherwise, it returns one
of these error codes:
GLOB_ABORTED
GLOB_ERR or your specified errfunc returned a nonzero
value.
See below
for an explanation of the GLOB_ERR flag and errfunc.
GLOB_NOMATCH
GLOB_NOCHECK flag, then you never get this error code, because
that flag tells glob to pretend that the pattern matched
at least one file.
GLOB_NOSPACE
In the event of an error, glob stores information in
*vector-ptr about all the matches it has found so far.
It is important to notive that the glob function will not fail if
it encounters directories or files which cannot be handled without the
LFS interfaces. The implementation of glob is supposed to use
these functions internally. This at least is the assumptions made by
the Unix standard. The GNU extension of allowing the user to provide
own directory handling and stat functions complicates things a
bit. If these callback functions are used and a large file or directory
is encountered glob can fail.
glob64 function was added as part of the Large File Summit
extensions but is not part of the original LFS proposal. The reason for
this is simple: it is not necessary. The necessity for a glob64
function is added by the extensions of the GNU glob
implementation which allows the user to provide own directory handling
and stat functions. The readdir and stat functions
do depend on the choice of _FILE_OFFSET_BITS since the definition
of the types struct dirent and struct stat will change
depending on the choice.
Beside this difference the glob64 works just like glob in
all aspects.
This function is a GNU extension.
This section describes the flags that you can specify in the
flags argument to glob. Choose the flags you want,
and combine them with the C bitwise OR operator |.
GLOB_APPEND
glob. This way you can effectively expand
several words as if they were concatenated with spaces between them.
In order for appending to work, you must not modify the contents of the
word vector structure between calls to glob. And, if you set
GLOB_DOOFFS in the first call to glob, you must also
set it when you append to the results.
Note that the pointer stored in gl_pathv may no longer be valid
after you call glob the second time, because glob might
have relocated the vector. So always fetch gl_pathv from the
glob_t structure after each glob call; never save
the pointer across calls.
GLOB_DOOFFS
gl_offs field says how many slots to leave.
The blank slots contain null pointers.
GLOB_ERR
glob tries its best to keep
on going despite any errors, reading whatever directories it can.
You can exercise even more control than this by specifying an
error-handler function errfunc when you call glob. If
errfunc is not a null pointer, then glob doesn't give up
right away when it can't read a directory; instead, it calls
errfunc with two arguments, like this:
(*errfunc) (filename, error-code)The argument filename is the name of the directory that
glob couldn't open or couldn't read, and error-code is the
errno value that was reported to glob.
If the error handler function returns nonzero, then glob gives up
right away. Otherwise, it continues.
GLOB_MARK
GLOB_NOCHECK
glob returns that there were no
matches.)
GLOB_NOSORT
GLOB_NOESCAPE
GLOB_NOESCAPE, then `\' is an ordinary character.
glob does its work by calling the function fnmatch
repeatedly. It handles the flag GLOB_NOESCAPE by turning on the
FNM_NOESCAPE flag in calls to fnmatch.
Beside the flags described in the last section, the GNU implementation of
glob allows a few more flags which are also defined in the
`glob.h' file. Some of the extensions implement functionality
which is available in modern shell implementations.
GLOB_PERIOD
. character (period) is treated special. It cannot be
matched by wildcards. See section Wildcard Matching, FNM_PERIOD.
GLOB_MAGCHAR
GLOB_MAGCHAR value is not to be given to glob in the
flags parameter. Instead, glob sets this bit in the
gl_flags element of the glob_t structure provided as the
result if the pattern used for matching contains any wildcard character.
GLOB_ALTDIRFUNC
glob implementation uses the user-supplied
functions specified in the structure pointed to by pglob
parameter. For more information about the functions refer to the
sections about directory handling see section Accessing Directories, and
section Reading the Attributes of a File.
GLOB_BRACE
, (comma) characters. The commas
themself are discarded. Please note what we said above about recursive
brace expressions. The commas used to separate the subexpressions must
be at the same level. Commas in brace subexpressions are not matched.
They are used during expansion of the brace expression of the deeper
level. The example below shows this
glob ("{foo/{,bar,biz},baz}", GLOB_BRACE, NULL, &result)
is equivalent to the sequence
glob ("foo/", GLOB_BRACE, NULL, &result)
glob ("foo/bar", GLOB_BRACE|GLOB_APPEND, NULL, &result)
glob ("foo/biz", GLOB_BRACE|GLOB_APPEND, NULL, &result)
glob ("baz", GLOB_BRACE|GLOB_APPEND, NULL, &result)
if we leave aside error handling.
GLOB_NOMAGIC
GLOB_TILDE
~ (tilde) is handled special
if it appears at the beginning of the pattern. Instead of being taken
verbatim it is used to represent the home directory of a known user.
If ~ is the only character in pattern or it is followed by a
/ (slash), the home directory of the process owner is
substituted. Using getlogin and getpwnam the information
is read from the system databases. As an example take user bart
with his home directory at `/home/bart'. For him a call like
glob ("~/bin/*", GLOB_TILDE, NULL, &result)
would return the contents of the directory `/home/bart/bin'.
Instead of referring to the own home directory it is also possible to
name the home directory of other users. To do so one has to append the
user name after the tilde character. So the contents of user
homer's `bin' directory can be retrieved by
glob ("~homer/bin/*", GLOB_TILDE, NULL, &result)
If the user name is not valid or the home directory cannot be determined
for some reason the pattern is left untouched and itself used as the
result. I.e., if in the last example home is not available the
tilde expansion yields to "~homer/bin/*" and glob is not
looking for a directory named ~homer.
This functionality is equivalent to what is available in C-shells if the
nonomatch flag is set.
GLOB_TILDE_CHECK
glob behaves like as if GLOB_TILDE is
given. The only difference is that if the user name is not available or
the home directory cannot be determined for other reasons this leads to
an error. glob will return GLOB_NOMATCH instead of using
the pattern itself as the name.
This functionality is equivalent to what is available in C-shells if
nonomatch flag is not set.
GLOB_ONLYDIR
glob
implementation. It is mainly used internally to increase the
performance but might be useful for a user as well and therefore is
documented here.
Calling glob will in most cases allocate resources which are used
to represent the result of the function call. If the same object of
type glob_t is used in multiple call to glob the resources
are freed or reused so that no leaks appear. But this does not include
the time when all glob calls are done.
globfree function frees all resources allocated by previous
calls to glob associated with the object pointed to by
pglob. This function should be called whenever the currently used
glob_t typed object isn't used anymore.
globfree but it frees records of
type glob64_t which were allocated by glob64.
The GNU C library supports two interfaces for matching regular expressions. One is the standard POSIX.2 interface, and the other is what the GNU system has had for many years.
Both interfaces are declared in the header file `regex.h'.
If you define _POSIX_C_SOURCE, then only the POSIX.2
functions, structures, and constants are declared.
Before you can actually match a regular expression, you must compile it. This is not true compilation--it produces a special data structure, not machine instructions. But it is like ordinary compilation in that its purpose is to enable you to "execute" the pattern fast. (See section Matching a Compiled POSIX Regular Expression, for how to use the compiled regular expression for matching.)
There is a special data type for compiled regular expressions:
re_nsub
There are several other fields, but we don't describe them here, because only the functions in the library should use them.
After you create a regex_t object, you can compile a regular
expression into it by calling regcomp.
regcomp "compiles" a regular expression into a
data structure that you can use with regexec to match against a
string. The compiled regular expression format is designed for
efficient matching. regcomp stores it into *compiled.
It's up to you to allocate an object of type regex_t and pass its
address to regcomp.
The argument cflags lets you specify various options that control the syntax and semantics of regular expressions. See section Flags for POSIX Regular Expressions.
If you use the flag REG_NOSUB, then regcomp omits from
the compiled regular expression the information necessary to record
how subexpressions actually match. In this case, you might as well
pass 0 for the matchptr and nmatch arguments when
you call regexec.
If you don't use REG_NOSUB, then the compiled regular expression
does have the capacity to record how subexpressions match. Also,
regcomp tells you how many subexpressions pattern has, by
storing the number in compiled->re_nsub. You can use that
value to decide how long an array to allocate to hold information about
subexpression matches.
regcomp returns 0 if it succeeds in compiling the regular
expression; otherwise, it returns a nonzero error code (see the table
below). You can use regerror to produce an error message string
describing the reason for a nonzero value; see section POSIX Regexp Matching Cleanup.
Here are the possible nonzero values that regcomp can return:
REG_BADBR
REG_BADPAT
REG_BADRPT
REG_ECOLLATE
REG_ECTYPE
REG_EESCAPE
REG_ESUBREG
REG_EBRACK
REG_EPAREN
REG_EBRACE
REG_ERANGE
REG_ESPACE
regcomp ran out of memory.
These are the bit flags that you can use in the cflags operand when
compiling a regular expression with regcomp.
REG_EXTENDED
REG_ICASE
REG_NOSUB
REG_NEWLINE
Once you have compiled a regular expression, as described in section POSIX Regular Expression Compilation, you can match it against strings using
regexec. A match anywhere inside the string counts as success,
unless the regular expression contains anchor characters (`^' or
`$').
*compiled against string.
regexec returns 0 if the regular expression matches;
otherwise, it returns a nonzero value. See the table below for
what nonzero values mean. You can use regerror to produce an
error message string describing the reason for a nonzero value;
see section POSIX Regexp Matching Cleanup.
The argument eflags is a word of bit flags that enable various options.
If you want to get information about what part of string actually
matched the regular expression or its subexpressions, use the arguments
matchptr and nmatch. Otherwise, pass 0 for
nmatch, and NULL for matchptr. See section Match Results with Subexpressions.
You must match the regular expression with the same set of current locales that were in effect when you compiled the regular expression.
The function regexec accepts the following flags in the
eflags argument:
REG_NOTBOL
REG_NOTEOL
Here are the possible nonzero values that regexec can return:
REG_NOMATCH
REG_ESPACE
regexec ran out of memory.
When regexec matches parenthetical subexpressions of
pattern, it records which parts of string they match. It
returns that information by storing the offsets into an array whose
elements are structures of type regmatch_t. The first element of
the array (index 0) records the part of the string that matched
the entire regular expression. Each other element of the array records
the beginning and end of the part that matched a single parenthetical
subexpression.
regexec. It contains two structure fields, as follows:
rm_so
rm_eo
regoff_t is an alias for another signed integer type.
The fields of regmatch_t have type regoff_t.
The regmatch_t elements correspond to subexpressions
positionally; the first element (index 1) records where the first
subexpression matched, the second element records the second
subexpression, and so on. The order of the subexpressions is the order
in which they begin.
When you call regexec, you specify how long the matchptr
array is, with the nmatch argument. This tells regexec how
many elements to store. If the actual regular expression has more than
nmatch subexpressions, then you won't get offset information about
the rest of them. But this doesn't alter whether the pattern matches a
particular string or not.
If you don't want regexec to return any information about where
the subexpressions matched, you can either supply 0 for
nmatch, or use the flag REG_NOSUB when you compile the
pattern with regcomp.
Sometimes a subexpression matches a substring of no characters. This
happens when `f\(o*\)' matches the string `fum'. (It really
matches just the `f'.) In this case, both of the offsets identify
the point in the string where the null substring was found. In this
example, the offsets are both 1.
Sometimes the entire regular expression can match without using some of
its subexpressions at all--for example, when `ba\(na\)*' matches the
string `ba', the parenthetical subexpression is not used. When
this happens, regexec stores -1 in both fields of the
element for that subexpression.
Sometimes matching the entire regular expression can match a particular
subexpression more than once--for example, when `ba\(na\)*'
matches the string `bananana', the parenthetical subexpression
matches three times. When this happens, regexec usually stores
the offsets of the last part of the string that matched the
subexpression. In the case of `bananana', these offsets are
6 and 8.
But the last match is not always the one that is chosen. It's more
accurate to say that the last opportunity to match is the one
that takes precedence. What this means is that when one subexpression
appears within another, then the results reported for the inner
subexpression reflect whatever happened on the last match of the outer
subexpression. For an example, consider `\(ba\(na\)*s \)*' matching
the string `bananas bas '. The last time the inner expression
actually matches is near the end of the first word. But it is
considered again in the second word, and fails to match there.
regexec reports nonuse of the "na" subexpression.
Another place where this rule applies is when the regular expression
\(ba\(na\)*s \|nefer\(ti\)* \)*
matches `bananas nefertiti'. The "na" subexpression does match
in the first word, but it doesn't match in the second word because the
other alternative is used there. Once again, the second repetition of
the outer subexpression overrides the first, and within that second
repetition, the "na" subexpression is not used. So regexec
reports nonuse of the "na" subexpression.
When you are finished using a compiled regular expression, you can
free the storage it uses by calling regfree.
regfree frees all the storage that *compiled
points to. This includes various internal fields of the regex_t
structure that aren't documented in this manual.
regfree does not free the object *compiled itself.
You should always free the space in a regex_t structure with
regfree before using the structure to compile another regular
expression.
When regcomp or regexec reports an error, you can use
the function regerror to turn it into an error message string.
regcomp or
regexec was working with when it got the error. Alternatively,
you can supply NULL for compiled; you will still get a
meaningful error message, but it might not be as detailed.
If the error message can't fit in length bytes (including a
terminating null character), then regerror truncates it.
The string that regerror stores is always null-terminated
even if it has been truncated.
The return value of regerror is the minimum length needed to
store the entire error message. If this is less than length, then
the error message was not truncated, and you can use it. Otherwise, you
should call regerror again with a larger buffer.
Here is a function which uses regerror, but always dynamically
allocates a buffer for the error message:
char *get_regerror (int errcode, regex_t *compiled)
{
size_t length = regerror (errcode, compiled, NULL, 0);
char *buffer = xmalloc (length);
(void) regerror (errcode, compiled, buffer, length);
return buffer;
}
Word expansion means the process of splitting a string into words and substituting for variables, commands, and wildcards just as the shell does.
For example, when you write `ls -l foo.c', this string is split into three separate words---`ls', `-l' and `foo.c'. This is the most basic function of word expansion.
When you write `ls *.c', this can become many words, because the word `*.c' can be replaced with any number of file names. This is called wildcard expansion, and it is also a part of word expansion.
When you use `echo $PATH' to print your path, you are taking advantage of variable substitution, which is also part of word expansion.
Ordinary programs can perform word expansion just like the shell by
calling the library function wordexp.
When word expansion is applied to a sequence of words, it performs the following transformations in the order shown here:
For the details of these transformations, and how to write the constructs that use them, see The BASH Manual (to appear).
wordexpAll the functions, constants and data types for word expansion are declared in the header file `wordexp.h'.
Word expansion produces a vector of words (strings). To return this
vector, wordexp uses a special data type, wordexp_t, which
is a structure. You pass wordexp the address of the structure,
and it fills in the structure's fields to tell you about the results.
we_wordc
we_wordv
char **.
we_offs
we_wordv field. Unlike the other fields, this
is always an input to wordexp, rather than an output from it.
If you use a nonzero offset, then that many elements at the beginning of
the vector are left empty. (The wordexp function fills them with
null pointers.)
The we_offs field is meaningful only if you use the
WRDE_DOOFFS flag. Otherwise, the offset is always zero
regardless of what is in this field, and the first real element comes at
the beginning of the vector.
*word-vector-ptr. The argument flags is a
combination of bit flags; see section Flags for Word Expansion, for details of
the flags.
You shouldn't use any of the characters `|&;<>' in the string
words unless they are quoted; likewise for newline. If you use
these characters unquoted, you will get the WRDE_BADCHAR error
code. Don't use parentheses or braces unless they are quoted or part of
a word expansion construct. If you use quotation characters `'"`',
they should come in pairs that balance.
The results of word expansion are a sequence of words. The function
wordexp allocates a string for each resulting word, then
allocates a vector of type char ** to store the addresses of
these strings. The last element of the vector is a null pointer.
This vector is called the word vector.
To return this vector, wordexp stores both its address and its
length (number of elements, not counting the terminating null pointer)
into *word-vector-ptr.
If wordexp succeeds, it returns 0. Otherwise, it returns one
of these error codes:
WRDE_BADCHAR
WRDE_BADVAL
WRDE_UNDEF to forbid such references.
WRDE_CMDSUB
WRDE_NOCMD to forbid command substitution.
WRDE_NOSPACE
wordexp can store part of the results--as much as it could
allocate room for.
WRDE_SYNTAX
*word-vector-ptr points to. This does not free the
structure *word-vector-ptr itself--only the other
data it points to.
This section describes the flags that you can specify in the
flags argument to wordexp. Choose the flags you want,
and combine them with the C operator |.
WRDE_APPEND
wordexp. This way you can effectively expand
several words as if they were concatenated with spaces between them.
In order for appending to work, you must not modify the contents of the
word vector structure between calls to wordexp. And, if you set
WRDE_DOOFFS in the first call to wordexp, you must also
set it when you append to the results.
WRDE_DOOFFS
we_offs field says how many slots to leave.
The blank slots contain null pointers.
WRDE_NOCMD
WRDE_REUSE
wordexp.
Instead of allocating a new vector of words, this call to wordexp
will use the vector that already exists (making it larger if necessary).
Note that the vector may move, so it is not safe to save an old pointer
and use it again after calling wordexp. You must fetch
we_pathv anew after each call.
WRDE_SHOWERR
wordexp gives these
commands a standard error stream that discards all output.
WRDE_UNDEF
wordexp Example
Here is an example of using wordexp to expand several strings
and use the results to run a shell command. It also shows the use of
WRDE_APPEND to concatenate the expansions and of wordfree
to free the space allocated by wordexp.
int
expand_and_execute (const char *program, const char *options)
{
wordexp_t result;
pid_t pid
int status, i;
/* Expand the string for the program to run. */
switch (wordexp (program, &result, 0))
{
case 0: /* Successful. */
break;
case WRDE_NOSPACE:
/* If the error was WRDE_NOSPACE,
then perhaps part of the result was allocated. */
wordfree (&result);
default: /* Some other error. */
return -1;
}
/* Expand the strings specified for the arguments. */
for (i = 0; args[i]; i++)
{
if (wordexp (options, &result, WRDE_APPEND))
{
wordfree (&result);
return -1;
}
}
pid = fork ();
if (pid == 0)
{
/* This is the child process. Execute the command. */
execv (result.we_wordv[0], result.we_wordv);
exit (EXIT_FAILURE);
}
else if (pid < 0)
/* The fork failed. Report failure. */
status = -1;
else
/* This is the parent process. Wait for the child to complete. */
if (waitpid (pid, &status, 0) != pid)
status = -1;
wordfree (&result);
return status;
}
It's a standard part of shell syntax that you can use `~' at the beginning of a file name to stand for your own home directory. You can use `~user' to stand for user's home directory.
Tilde expansion is the process of converting these abbreviations to the directory names that they stand for.
Tilde expansion applies to the `~' plus all following characters up to whitespace or a slash. It takes place only at the beginning of a word, and only if none of the characters to be transformed is quoted in any way.
Plain `~' uses the value of the environment variable HOME
as the proper home directory name. `~' followed by a user name
uses getpwname to look up that user in the user database, and
uses whatever directory is recorded there. Thus, `~' followed
by your own name can give different results from plain `~', if
the value of HOME is not really your home directory.
Part of ordinary shell syntax is the use of `$variable' to substitute the value of a shell variable into a command. This is called variable substitution, and it is one part of doing word expansion.
There are two basic ways you can write a variable reference for substitution:
${variable}
$variable
foo and expands
into `tractor-bar'.
When you use braces, you can also use various constructs to modify the value that is substituted, or test it in various ways.
${variable:-default}
${variable:=default}
${variable:?message}
${variable:+replacement}
${#variable}
These variants of variable substitution let you remove part of the variable's value before substituting it. The prefix and suffix are not mere strings; they are wildcard patterns, just like the patterns that you use to match multiple file names. But in this context, they match against parts of the variable value rather than against file names.
${variable%%suffix}
${variable%suffix}
${variable##prefix}
${variable#prefix}
Most programs need to do either input (reading data) or output (writing data), or most frequently both, in order to do anything useful. The GNU C library provides such a large selection of input and output functions that the hardest part is often deciding which function is most appropriate!
This chapter introduces concepts and terminology relating to input and output. Other chapters relating to the GNU I/O facilities are:
Before you can read or write the contents of a file, you must establish a connection or communications channel to the file. This process is called opening the file. You can open a file for reading, writing, or both.
The connection to an open file is represented either as a stream or as a file descriptor. You pass this as an argument to the functions that do the actual read or write operations, to tell them which file to operate on. Certain functions expect streams, and others are designed to operate on file descriptors.
When you have finished reading to or writing from the file, you can terminate the connection by closing the file. Once you have closed a stream or file descriptor, you cannot do any more input or output operations on it.
When you want to do input or output to a file, you have a choice of two
basic mechanisms for representing the connection between your program
and the file: file descriptors and streams. File descriptors are
represented as objects of type int, while streams are represented
as FILE * objects.
File descriptors provide a primitive, low-level interface to input and output operations. Both file descriptors and streams can represent a connection to a device (such as a terminal), or a pipe or socket for communicating with another process, as well as a normal file. But, if you want to do control operations that are specific to a particular kind of device, you must use a file descriptor; there are no facilities to use streams in this way. You must also use file descriptors if your program needs to do input or output in special modes, such as nonblocking (or polled) input (see section File Status Flags).
Streams provide a higher-level interface, layered on top of the primitive file descriptor facilities. The stream interface treats all kinds of files pretty much alike--the sole exception being the three styles of buffering that you can choose (see section Stream Buffering).
The main advantage of using the stream interface is that the set of
functions for performing actual input and output operations (as opposed
to control operations) on streams is much richer and more powerful than
the corresponding facilities for file descriptors. The file descriptor
interface provides only simple functions for transferring blocks of
characters, but the stream interface also provides powerful formatted
input and output functions (printf and scanf) as well as
functions for character- and line-oriented input and output.
Since streams are implemented in terms of file descriptors, you can extract the file descriptor from a stream and perform low-level operations directly on the file descriptor. You can also initially open a connection as a file descriptor and then make a stream associated with that file descriptor.
In general, you should stick with using streams rather than file descriptors, unless there is some specific operation you want to do that can only be done on a file descriptor. If you are a beginning programmer and aren't sure what functions to use, we suggest that you concentrate on the formatted input functions (see section Formatted Input) and formatted output functions (see section Formatted Output).
If you are concerned about portability of your programs to systems other than GNU, you should also be aware that file descriptors are not as portable as streams. You can expect any system running ISO C to support streams, but non-GNU systems may not support file descriptors at all, or may only implement a subset of the GNU functions that operate on file descriptors. Most of the file descriptor functions in the GNU library are included in the POSIX.1 standard, however.
One of the attributes of an open file is its file position that keeps track of where in the file the next character is to be read or written. In the GNU system, and all POSIX.1 systems, the file position is simply an integer representing the number of bytes from the beginning of the file.
The file position is normally set to the beginning of the file when it is opened, and each time a character is read or written, the file position is incremented. In other words, access to the file is normally sequential.
Ordinary files permit read or write operations at any position within
the file. Some other kinds of files may also permit this. Files which
do permit this are sometimes referred to as random-access files.
You can change the file position using the fseek function on a
stream (see section File Positioning) or the lseek function on a file
descriptor (see section Input and Output Primitives). If you try to change the file
position on a file that doesn't support random access, you get the
ESPIPE error.
Streams and descriptors that are opened for append access are treated specially for output: output to such files is always appended sequentially to the end of the file, regardless of the file position. However, the file position is still used to control where in the file reading is done.
If you think about it, you'll realize that several programs can read a given file at the same time. In order for each program to be able to read the file at its own pace, each program must have its own file pointer, which is not affected by anything the other programs do.
In fact, each opening of a file creates a separate file position. Thus, if you open a file twice even in the same program, you get two streams or descriptors with independent file positions.
By contrast, if you open a descriptor and then duplicate it to get another descriptor, these two descriptors share the same file position: changing the file position of one descriptor will affect the other.
In order to open a connection to a file, or to perform other operations such as deleting a file, you need some way to refer to the file. Nearly all files have names that are strings--even files which are actually devices such as tape drives or terminals. These strings are called file names. You specify the file name to say which file you want to open or operate on.
This section describes the conventions for file names and how the operating system works with them.
In order to understand the syntax of file names, you need to understand how the file system is organized into a hierarchy of directories.
A directory is a file that contains information to associate other files with names; these associations are called links or directory entries. Sometimes, people speak of "files in a directory", but in reality, a directory only contains pointers to files, not the files themselves.
The name of a file contained in a directory entry is called a file name component. In general, a file name consists of a sequence of one or more such components, separated by the slash character (`/'). A file name which is just one component names a file with respect to its directory. A file name with multiple components names a directory, and then a file in that directory, and so on.
Some other documents, such as the POSIX standard, use the term
pathname for what we call a file name, and either filename
or pathname component for what this manual calls a file name
component. We don't use this terminology because a "path" is
something completely different (a list of directories to search), and we
think that "pathname" used for something else will confuse users. We
always use "file name" and "file name component" (or sometimes just
"component", where the context is obvious) in GNU documentation. Some
macros use the POSIX terminology in their names, such as
PATH_MAX. These macros are defined by the POSIX standard, so we
cannot change their names.
You can find more detailed information about operations on directories in section File System Interface.
A file name consists of file name components separated by slash (`/') characters. On the systems that the GNU C library supports, multiple successive `/' characters are equivalent to a single `/' character.
The process of determining what file a file name refers to is called file name resolution. This is performed by examining the components that make up a file name in left-to-right order, and locating each successive component in the directory named by the previous component. Of course, each of the files that are referenced as directories must actually exist, be directories instead of regular files, and have the appropriate permissions to be accessible by the process; otherwise the file name resolution fails.
If a file name begins with a `/', the first component in the file name is located in the root directory of the process (usually all processes on the system have the same root directory). Such a file name is called an absolute file name.
Otherwise, the first component in the file name is located in the current working directory (see section Working Directory). This kind of file name is called a relative file name.
The file name components `.' ("dot") and `..' ("dot-dot") have special meanings. Every directory has entries for these file name components. The file name component `.' refers to the directory itself, while the file name component `..' refers to its parent directory (the directory that contains the link for the directory in question). As a special case, `..' in the root directory refers to the root directory itself, since it has no parent; thus `/..' is the same as `/'.
Here are some examples of file names:
A file name that names a directory may optionally end in a `/'. You can specify a file name of `/' to refer to the root directory, but the empty string is not a meaningful file name. If you want to refer to the current working directory, use a file name of `.' or `./'.
Unlike some other operating systems, the GNU system doesn't have any built-in support for file types (or extensions) or file versions as part of its file name syntax. Many programs and utilities use conventions for file names--for example, files containing C source code usually have names suffixed with `.c'---but there is nothing in the file system itself that enforces this kind of convention.
Functions that accept file name arguments usually detect these
errno error conditions relating to the file name syntax or
trouble finding the named file. These errors are referred to throughout
this manual as the usual file name errors.
EACCES
ENAMETOOLONG
PATH_MAX, or when an individual file name component
has a length greater than NAME_MAX. See section Limits on File System Capacity.
In the GNU system, there is no imposed limit on overall file name
length, but some file systems may place limits on the length of a
component.
ENOENT
ENOTDIR
ELOOP
The rules for the syntax of file names discussed in section File Names, are the rules normally used by the GNU system and by other POSIX systems. However, other operating systems may use other conventions.
There are two reasons why it can be important for you to be aware of file name portability issues:
The ISO C standard says very little about file name syntax, only that file names are strings. In addition to varying restrictions on the length of file names and what characters can validly appear in a file name, different operating systems use different conventions and syntax for concepts such as structured directories and file types or extensions. Some concepts such as file versions might be supported in some operating systems and not by others.
The POSIX.1 standard allows implementations to put additional restrictions on file name syntax, concerning what characters are permitted in file names and on the length of file name and file name component strings. However, in the GNU system, you do not need to worry about these restrictions; any character except the null character is permitted in a file name string, and there are no limits on the length of file name strings.
This chapter describes the functions for creating streams and performing input and output operations on them. As discussed in section Input/Output Overview, a stream is a fairly abstract, high-level concept representing a communications channel to a file, device, or process.
For historical reasons, the type of the C data structure that represents
a stream is called FILE rather than "stream". Since most of
the library functions deal with objects of type FILE *, sometimes
the term file pointer is also used to mean "stream". This leads
to unfortunate confusion over terminology in many books on C. This
manual, however, is careful to use the terms "file" and "stream"
only in the technical sense.
The FILE type is declared in the header file `stdio.h'.
FILE
object holds all of the internal state information about the connection
to the associated file, including such things as the file position
indicator and buffering information. Each stream also has error and
end-of-file status indicators that can be tested with the ferror
and feof functions; see section End-Of-File and Errors.
FILE objects are allocated and managed internally by the
input/output library functions. Don't try to create your own objects of
type FILE; let the library do it. Your programs should
deal only with pointers to these objects (that is, FILE * values)
rather than the objects themselves.
When the main function of your program is invoked, it already has
three predefined streams open and available for use. These represent
the "standard" input and output channels that have been established
for the process.
These streams are declared in the header file `stdio.h'.
In the GNU system, you can specify what files or processes correspond to these streams using the pipe and redirection facilities provided by the shell. (The primitives shells use to implement these facilities are described in section File System Interface.) Most other operating systems provide similar mechanisms, but the details of how to use them can vary.
In the GNU C library, stdin, stdout, and stderr are
normal variables which you can set just like any others. For example,
to redirect the standard output to a file, you could do:
fclose (stdout);
stdout = fopen ("standard-output-file", "w");
Note however, that in other systems stdin, stdout, and
stderr are macros that you cannot assign to in the normal way.
But you can use freopen to get the effect of closing one and
reopening it. See section Opening Streams.
The three streams stdin, stdout, and stderr are not
unoriented at program start (see section Streams in Internationalized Applications).
Opening a file with the fopen function creates a new stream and
establishes a connection between the stream and a file. This may
involve creating a new file.
Everything described in this section is declared in the header file `stdio.h'.
fopen function opens a stream for I/O to the file
filename, and returns a pointer to the stream.
The opentype argument is a string that controls how the file is opened and specifies attributes of the resulting stream. It must begin with one of the following sequences of characters:
As you can see, `+' requests a stream that can do both input and
output. The ISO standard says that when using such a stream, you must
call fflush (see section Stream Buffering) or a file positioning
function such as fseek (see section File Positioning) when switching
from reading to writing or vice versa. Otherwise, internal buffers
might not be emptied properly. The GNU C library does not have this
limitation; you can do arbitrary reading and writing operations on a
stream in whatever order.
Additional characters may appear after these to specify flags for the call. Always put the mode (`r', `w+', etc.) first; that is the only part you are guaranteed will be understood by all systems.
The GNU C library defines one additional character for use in
opentype: the character `x' insists on creating a new
file--if a file filename already exists, fopen fails
rather than opening it. If you use `x' you are guaranteed that
you will not clobber an existing file. This is equivalent to the
O_EXCL option to the open function (see section Opening and Closing Files).
The character `b' in opentype has a standard meaning; it requests a binary stream rather than a text stream. But this makes no difference in POSIX systems (including the GNU system). If both `+' and `b' are specified, they can appear in either order. See section Text and Binary Streams.
If the opentype string contains the sequence
,ccs=STRING then STRING is taken as the name of a
coded character set and fopen will mark the stream as
wide-oriented which appropriate conversion functions in place to convert
from and to the character set STRING is place. Any other stream
is opened initially unoriented and the orientation is decided with the
first file operation. If the first operation is a wide character
operation, the stream is not only marked as wide-oriented, also the
conversion functions to convert to the coded character set used for the
current locale are loaded. This will not change anymore from this point
on even if the locale selected for the LC_CTYPE category is
changed.
Any other characters in opentype are simply ignored. They may be meaningful in other systems.
If the open fails, fopen returns a null pointer.
When the sources are compiling with _FILE_OFFSET_BITS == 64 on a
32 bit machine this function is in fact fopen64 since the LFS
interface replaces transparently the old interface.
You can have multiple streams (or file descriptors) pointing to the same file open at the same time. If you do only input, this works straightforwardly, but you must be careful if any output streams are included. See section Dangers of Mixing Streams and Descriptors. This is equally true whether the streams are in one program (not usual) or in several programs (which can easily happen). It may be advantageous to use the file locking facilities to avoid simultaneous access. See section File Locks.
fopen but the stream it returns a
pointer for is opened using open64. Therefore this stream can be
used even on files larger then @math{2^31} bytes on 32 bit machines.
Please note that the return type is still FILE *. There is no
special FILE type for the LFS interface.
If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32
bits machine this function is available under the name fopen
and so transparently replaces the old interface.
stdin, stdout, and stderr. In POSIX.1 systems this
value is determined by the OPEN_MAX parameter; see section General Capacity Limits. In BSD and GNU, it is controlled by the RLIMIT_NOFILE
resource limit; see section Limiting Resource Usage.
fclose and fopen.
It first closes the stream referred to by stream, ignoring any
errors that are detected in the process. (Because errors are ignored,
you should not use freopen on an output stream if you have
actually done any output using the stream.) Then the file named by
filename is opened with mode opentype as for fopen,
and associated with the same stream object stream.
If the operation fails, a null pointer is returned; otherwise,
freopen returns stream.
freopen has traditionally been used to connect a standard stream
such as stdin with a file of your own choice. This is useful in
programs in which use of a standard stream for certain purposes is
hard-coded. In the GNU C library, you can simply close the standard
streams and open new ones with fopen. But other systems lack
this ability, so using freopen is more portable.
When the sources are compiling with _FILE_OFFSET_BITS == 64 on a
32 bit machine this function is in fact freopen64 since the LFS
interface replaces transparently the old interface.
freopen. The only difference is that
on 32 bit machine the stream returned is able to read beyond the
@math{2^31} bytes limits imposed by the normal interface. It should be
noted that the stream pointed to by stream need not be opened
using fopen64 or freopen64 since its mode is not important
for this function.
If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32
bits machine this function is available under the name freopen
and so transparently replaces the old interface.
In some situations it is useful to know whether a given stream is available for reading or writing. This information is normally not available and would have to be remembered separately. Solaris introduced a few functions to get this information from the stream descriptor and these functions are also available in the GNU C library.
__freadable function determines whether the stream
stream was opened to allow reading. In this case the return value
is nonzero. For write-only streams the function returns zero.
This function is declared in `stdio_ext.h'.
__fwritable function determines whether the stream
stream was opened to allow writing. In this case the return value
is nonzero. For read-only streams the function returns zero.
This function is declared in `stdio_ext.h'.
For slightly different kind of problems there are two more functions. They provide even finer-grained information.
__freading function determines whether the stream
stream was last read from or whether it is opened read-only. In
this case the return value is nonzero, otherwise it is zero.
Determining whether a stream opened for reading and writing was last
used for writing allows to draw conclusions about the content about the
buffer, among other things.
This function is declared in `stdio_ext.h'.
__fwriting function determines whether the stream
stream was last written to or whether it is opened write-only. In
this case the return value is nonzero, otherwise it is zero.
This function is declared in `stdio_ext.h'.
When a stream is closed with fclose, the connection between the
stream and the file is cancelled. After you have closed a stream, you
cannot perform any additional operations on it.
fclose function returns
a value of 0 if the file was closed successfully, and EOF
if an error was detected.
It is important to check for errors when you call fclose to close
an output stream, because real, everyday errors can be detected at this
time. For example, when fclose writes the remaining buffered
output, it might get an error because the disk is full. Even if you
know the buffer is empty, errors can still occur when closing a file if
you are using NFS.
The function fclose is declared in `stdio.h'.
To close all streams currently available the GNU C Library provides another function.
fcloseall
function returns a value of 0 if all the files were closed
successfully, and EOF if an error was detected.
This function should be used only in special situations, e.g., when an error occurred and the program must be aborted. Normally each single stream should be closed separately so that problems with individual streams can be identified. It is also problematic since the standard streams (see section Standard Streams) will also be closed.
The function fcloseall is declared in `stdio.h'.
If the main function to your program returns, or if you call the
exit function (see section Normal Termination), all open streams are
automatically closed properly. If your program terminates in any other
manner, such as by calling the abort function (see section Aborting a Program) or from a fatal signal (see section Signal Handling), open streams
might not be closed properly. Buffered output might not be flushed and
files may be incomplete. For more information on buffering of streams,
see section Stream Buffering.
Streams can be used in multi-threaded applications in the same way they are used in single-threaded applications. But the programmer must be aware of a the possible complications. It is important to know about these also if the program one writes never use threads since the design and implementation of many stream functions is heavily influenced by the requirements added by multi-threaded programming.
The POSIX standard requires that by default the stream operations are atomic. I.e., issueing two stream operations for the same stream in two threads at the same time will cause the operations to be executed as if they were issued sequentially. The buffer operations performed while reading or writing are protected from other uses of the same stream. To do this each stream has an internal lock object which has to be (implicitly) acquired before any work can be done.
But there are situations where this is not enough and there are also situations where this is not wanted. The implicit locking is not enough if the program requires more than one stream function call to happen atomically. One example would be if an output line a program wants to generate is created by several function calls. The functions by themselves would ensure only atomicity of their own operation, but not atomicity over all the function calls. For this it is necessary to perform the stream locking in the application code.
flockfile function acquires the internal locking object
associated with the stream stream. This ensures that no other
thread can explicitly through flockfile/ftrylockfile or
implicit through a call of a stream function lock the stream. The
thread will block until the lock is acquired. An explicit call to
funlockfile has to be used to release the lock.
ftrylockfile function tries to acquire the internal locking
object associated with the stream stream just like
flockfile. But unlike flockfile this function does not
block if the lock is not available. ftrylockfile returns zero if
the lock was successfully acquired. Otherwise the stream is locked by
another thread.
funlockfile function releases the internal locking object of
the stream stream. The stream must have been locked before by a
call to flockfile or a successful call of ftrylockfile.
The implicit locking performed by the stream operations do not count.
The funlockfile function does not return an error status and the
behavior of a call for a stream which is not locked by the current
thread is undefined.
The following example shows how the functions above can be used to
generate an output line atomically even in multi-threaded applications
(yes, the same job could be done with one fprintf call but it is
sometimes not possible):
FILE *fp;
{
...
flockfile (fp);
fputs ("This is test number ", fp);
fprintf (fp, "%d\n", test);
funlockfile (fp)
}
Without the explicit locking it would be possible for another thread to
use the stream fp after the fputs call return and before
fprintf was called with the result that the number does not
follow the word `number'.
From this description it might already be clear that the locking objects
in streams are no simple mutexes. Since locking the same stream twice
in the same thread is allowed the locking objects must be equivalent to
recursive mutexes. These mutexes keep track of the owner and the number
of times the lock is acquired. The same number of funlockfile
calls by the same threads is necessary to unlock the stream completely.
For instance:
void
foo (FILE *fp)
{
ftrylockfile (fp);
fputs ("in foo\n", fp);
/* This is very wrong!!! */
funlockfile (fp);
}
It is important here that the funlockfile function is only called
if the ftrylockfile function succeeded in locking the stream. It
is therefore always wrong to ignore the result of ftrylockfile.
And it makes no sense since otherwise one would use flockfile.
The result of code like that above is that either funlockfile
tries to free a stream that hasn't been locked by the current thread or it
frees the stream prematurely. The code should look like this:
void
foo (FILE *fp)
{
if (ftrylockfile (fp) == 0)
{
fputs ("in foo\n", fp);
funlockfile (fp);
}
}
Now that we covered why it is necessary to have these locking it is necessary to talk about situations when locking is unwanted and what can be done. The locking operations (explicit or implicit) don't come for free. Even if a lock is not taken the cost is not zero. The operations which have to be performed require memory operations which are save in multi-processor environments. With the many local caches involved in such systems this is quite costly. So it is best to avoid the locking completely if it is known that the code using the stream is never used in a context where more than one thread can use the stream at one time. This can be determined most of the time for application code; for library code which can be used in many contexts one should default to be conservative and use locking.
There are two basic mechanisms to avoid locking. The first is to use
the _unlocked variants of the stream operations. The POSIX
standard defines quite a few of those and the GNU library adds a few
more. These variants of the functions behave just like the functions
with the name without the suffix except that they are not locking the
stream. Using these functions is very desirable since they are
potentially much faster. This is not only because the locking
operation itself is avoided. More importantly, functions like
putc and getc are very simple and tradionally (before the
introduction of threads) were implemented as macros which are very fast
if the buffer is not empty. With locking required these functions are
now no macros anymore (the code generated would be too much). But these
macros are still available with the same functionality under the new
names putc_unlocked and getc_unlocked. This possibly huge
difference of speed also suggests the use of the _unlocked
functions even if locking is required. The difference is that the
locking then has to be performed in the program:
void
foo (FILE *fp, char *buf)
{
flockfile (fp);
while (*buf != '/')
putc_unlocked (*buf++, fp);
funlockfile (fp);
}
If in this example the putc function would be used and the
explicit locking would be missing the putc function would have to
acquire the lock in every call, potentially many times depending on when
the loop terminates. Writing it the way illustrated above allows the
putc_unlocked macro to be used which means no locking and direct
manipulation of the buffer of the stream.
A second way to avoid locking is by using a non-standard function which was introduced in Solaris and is available in the GNU C library as well.
The __fsetlocking function can be used to select whether the
stream operations will implicitly acquire the locking object of the
stream stream. By default this is done but it can be disabled and
reinstated using this function. There are three values defined for the
type parameter.
FSETLOCKING_INTERNAL
stream will from now on use the default internal
locking. Every stream operation with exception of the _unlocked
variants will implicitly lock the stream.
FSETLOCKING_BYCALLER
__fsetlocking function returns the user is responsible
for locking the stream. None of the stream operations will implicitly
do this anymore until the state is set back to
FSETLOCKING_INTERNAL.
FSETLOCKING_QUERY
__fsetlocking only queries the current locking state of the
stream. The return value will be FSETLOCKING_INTERNAL or
FSETLOCKING_BYCALLER depending on the state.
The return value of __fsetlocking is either
FSETLOCKING_INTERNAL or FSETLOCKING_BYCALLER depending on
the state of the stream before the call.
This function and the values for the type parameter are declared in `stdio_ext.h'.
This function is especially useful when program code has to be used
which is written without knowledge about the _unlocked functions
(or if the programmer was to lazy to use them).
ISO C90 introduced the new type wchar_t to allow handling
larger character sets. What was missing was a possibility to output
strings of wchar_t directly. One had to convert them into
multibyte strings using mbstowcs (there was no mbsrtowcs
yet) and then use the normal stream functions. While this is doable it
is very cumbersome since performing the conversions is not trivial and
greatly increases program complexity and size.
The Unix standard early on (I think in XPG4.2) introduced two additional
format specifiers for the printf and scanf families of
functions. Printing and reading of single wide characters was made
possible using the %C specifier and wide character strings can be
handled with %S. These modifiers behave just like %c and
%s only that they expect the corresponding argument to have the
wide character type and that the wide character and string are
transformed into/from multibyte strings before being used.
This was a beginning but it is still not good enough. Not always is it
desirable to use printf and scanf. The other, smaller and
faster functions cannot handle wide characters. Second, it is not
possible to have a format string for printf and scanf
consisting of wide characters. The result is that format strings would
have to be generated if they have to contain non-basic characters.
In the Amendment 1 to ISO C90 a whole new set of functions was
added to solve the problem. Most of the stream functions got a
counterpart which take a wide character or wide character string instead
of a character or string respectively. The new functions operate on the
same streams (like stdout). This is different from the model of
the C++ runtime library where separate streams for wide and normal I/O
are used.
Being able to use the same stream for wide and normal operations comes
with a restriction: a stream can be used either for wide operations or
for normal operations. Once it is decided there is no way back. Only a
call to freopen or freopen64 can reset the
orientation. The orientation can be decided in three ways:
fread and fwrite functions) the stream is marked as not
wide oriented.
fwide function can be used to set the orientation either way.
It is important to never mix the use of wide and not wide operations on
a stream. There are no diagnostics issued. The application behavior
will simply be strange or the application will simply crash. The
fwide function can help avoiding this.
The fwide function can be used to set and query the state of the
orientation of the stream stream. If the mode parameter has
a positive value the streams get wide oriented, for negative values
narrow oriented. It is not possible to overwrite previous orientations
with fwide. I.e., if the stream stream was already
oriented before the call nothing is done.
If mode is zero the current orientation state is queried and nothing is changed.
The fwide function returns a negative value, zero, or a positive
value if the stream is narrow, not at all, or wide oriented
respectively.
This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'.
It is generally a good idea to orient a stream as early as possible.
This can prevent surprise especially for the standard streams
stdin, stdout, and stderr. If some library
function in some situations uses one of these streams and this use
orients the stream in a different way the rest of the application
expects it one might end up with hard to reproduce errors. Remember
that no errors are signal if the streams are used incorrectly. Leaving
a stream unoriented after creation is normally only necessary for
library functions which create streams which can be used in different
contexts.
When writing code which uses streams and which can be used in different contexts it is important to query the orientation of the stream before using it (unless the rules of the library interface demand a specific orientation). The following little, silly function illustrates this.
void
print_f (FILE *fp)
{
if (fwide (fp, 0) > 0)
/* Positive return value means wide orientation. */
fputwc (L'f', fp);
else
fputc ('f', fp);
}
Note that in this case the function print_f decides about the
orientation of the stream if it was unoriented before (will not happen
if the advise above is followed).
The encoding used for the wchar_t values is unspecified and the
user must not make any assumptions about it. For I/O of wchar_t
values this means that it is impossible to write these values directly
to the stream. This is not what follows from the ISO C locale model
either. What happens instead is that the bytes read from or written to
the underlying media are first converted into the internal encoding
chosen by the implementation for wchar_t. The external encoding
is determined by the LC_CTYPE category of the current locale or
by the `ccs' part of the mode specification given to fopen,
fopen64, freopen, or freopen64. How and when the
conversion happens is unspecified and it happens invisible to the user.
Since a stream is created in the unoriented state it has at that point
no conversion associated with it. The conversion which will be used is
determined by the LC_CTYPE category selected at the time the
stream is oriented. If the locales are changed at the runtime this
might produce surprising results unless one pays attention. This is
just another good reason to orient the stream explicitly as soon as
possible, perhaps with a call to fwide.
This section describes functions for performing character- and line-oriented output.
These narrow streams functions are declared in the header file `stdio.h' and the wide stream functions in `wchar.h'.
fputc function converts the character c to type
unsigned char, and writes it to the stream stream.
EOF is returned if a write error occurs; otherwise the
character c is returned.
fputwc function writes the wide character wc to the
stream stream. WEOF is returned if a write error occurs;
otherwise the character wc is returned.
fputc_unlocked function is equivalent to the fputc
function except that it does not implicitly lock the stream.
fputwc_unlocked function is equivalent to the fputwc
function except that it does not implicitly lock the stream.
This function is a GNU extension.
fputc, except that most systems implement it as
a macro, making it faster. One consequence is that it may evaluate the
stream argument more than once, which is an exception to the
general rule for macros. putc is usually the best function to
use for writing a single character.
fputwc, except that it can be implement as
a macro, making it faster. One consequence is that it may evaluate the
stream argument more than once, which is an exception to the
general rule for macros. putwc is usually the best function to
use for writing a single wide character.
putc_unlocked function is equivalent to the putc
function except that it does not implicitly lock the stream.
putwc_unlocked function is equivalent to the putwc
function except that it does not implicitly lock the stream.
This function is a GNU extension.
putchar function is equivalent to putc with
stdout as the value of the stream argument.
putwchar function is equivalent to putwc with
stdout as the value of the stream argument.
putchar_unlocked function is equivalent to the putchar
function except that it does not implicitly lock the stream.
putwchar_unlocked function is equivalent to the putwchar
function except that it does not implicitly lock the stream.
This function is a GNU extension.
fputs writes the string s to the stream
stream. The terminating null character is not written.
This function does not add a newline character, either.
It outputs only the characters in the string.
This function returns EOF if a write error occurs, and otherwise
a non-negative value.
For example:
fputs ("Are ", stdout);
fputs ("you ", stdout);
fputs ("hungry?\n", stdout);
outputs the text `Are you hungry?' followed by a newline.
fputws writes the wide character string ws to
the stream stream. The terminating null character is not written.
This function does not add a newline character, either. It
outputs only the characters in the string.
This function returns WEOF if a write error occurs, and otherwise
a non-negative value.
fputs_unlocked function is equivalent to the fputs
function except that it does not implicitly lock the stream.
This function is a GNU extension.
fputws_unlocked function is equivalent to the fputws
function except that it does not implicitly lock the stream.
This function is a GNU extension.
puts function writes the string s to the stream
stdout followed by a newline. The terminating null character of
the string is not written. (Note that fputs does not
write a newline as this function does.)
puts is the most convenient function for printing simple
messages. For example:
puts ("This is a message.");
outputs the text `This is a message.' followed by a newline.
int) to
stream. It is provided for compatibility with SVID, but we
recommend you use fwrite instead (see section Block Input/Output).
This section describes functions for performing character-oriented input. These narrow streams functions are declared in the header file `stdio.h' and the wide character functions are declared in `wchar.h'.
These functions return an int or wint_t value (for narrow
and wide stream functions respectively) that is either a character of
input, or the special value EOF/WEOF (usually -1). For
the narrow stream functions it is important to store the result of these
functions in a variable of type int instead of char, even
when you plan to use it only as a character. Storing EOF in a
char variable truncates its value to the size of a character, so
that it is no longer distinguishable from the valid character
`(char) -1'. So always use an int for the result of
getc and friends, and check for EOF after the call; once
you've verified that the result is not EOF, you can be sure that
it will fit in a `char' variable without loss of information.
unsigned char from
the stream stream and returns its value, converted to an
int. If an end-of-file condition or read error occurs,
EOF is returned instead.
WEOF is returned instead.
fgetc_unlocked function is equivalent to the fgetc
function except that it does not implicitly lock the stream.
fgetwc_unlocked function is equivalent to the fgetwc
function except that it does not implicitly lock the stream.
This function is a GNU extension.
fgetc, except that it is permissible (and
typical) for it to be implemented as a macro that evaluates the
stream argument more than once. getc is often highly
optimized, so it is usually the best function to use to read a single
character.
fgetwc, except that it is permissible for it to
be implemented as a macro that evaluates the stream argument more
than once. getwc can be highly optimized, so it is usually the
best function to use to read a single wide character.
getc_unlocked function is equivalent to the getc
function except that it does not implicitly lock the stream.
getwc_unlocked function is equivalent to the getwc
function except that it does not implicitly lock the stream.
This function is a GNU extension.
getchar function is equivalent to getc with stdin
as the value of the stream argument.
getwchar function is equivalent to getwc with stdin
as the value of the stream argument.
getchar_unlocked function is equivalent to the getchar
function except that it does not implicitly lock the stream.
getwchar_unlocked function is equivalent to the getwchar
function except that it does not implicitly lock the stream.
This function is a GNU extension.
Here is an example of a function that does input using fgetc. It
would work just as well using getc instead, or using
getchar () instead of fgetc (stdin). The code would
also work the same for the wide character stream functions.
int
y_or_n_p (const char *question)
{
fputs (question, stdout);
while (1)
{
int c, answer;
/* Write a space to separate answer from question. */
fputc (' ', stdout);
/* Read the first character of the line.
This should be the answer character, but might not be. */
c = tolower (fgetc (stdin));
answer = c;
/* Discard rest of input line. */
while (c != '\n' && c != EOF)
c = fgetc (stdin);
/* Obey the answer if it was valid. */
if (answer == 'y')
return 1;
if (answer == 'n')
return 0;
/* Answer was invalid: ask for valid answer. */
fputs ("Please answer y or n:", stdout);
}
}
int) from stream.
It's provided for compatibility with SVID. We recommend you use
fread instead (see section Block Input/Output). Unlike getc,
any int value could be a valid result. getw returns
EOF when it encounters end-of-file or an error, but there is no
way to distinguish this from an input word with value -1.
Since many programs interpret input on the basis of lines, it is convenient to have functions to read a line of text from a stream.
Standard C has functions to do this, but they aren't very safe: null
characters and even (for gets) long lines can confuse them. So
the GNU library provides the nonstandard getline function that
makes it easy to read lines reliably.
Another GNU extension, getdelim, generalizes getline. It
reads a delimited record, defined as everything through the next
occurrence of a specified delimiter character.
All these functions are declared in `stdio.h'.
*lineptr.
Before calling getline, you should place in *lineptr
the address of a buffer *n bytes long, allocated with
malloc. If this buffer is long enough to hold the line,
getline stores the line in this buffer. Otherwise,
getline makes the buffer bigger using realloc, storing the
new buffer address back in *lineptr and the increased size
back in *n.
See section Unconstrained Allocation.
If you set *lineptr to a null pointer, and *n
to zero, before the call, then getline allocates the initial
buffer for you by calling malloc.
In either case, when getline returns, *lineptr is
a char * which points to the text of the line.
When getline is successful, it returns the number of characters
read (including the newline, but not including the terminating null).
This value enables you to distinguish null characters that are part of
the line from the null character inserted as a terminator.
This function is a GNU extension, but it is the recommended way to read lines from a stream. The alternative standard functions are unreliable.
If an error occurs or end of file is reached without any bytes read,
getline returns -1.
getline except that the character which
tells it to stop reading is not necessarily newline. The argument
delimiter specifies the delimiter character; getdelim keeps
reading until it sees that character (or end of file).
The text is stored in lineptr, including the delimiter character
and a terminating null. Like getline, getdelim makes
lineptr bigger if it isn't big enough.
getline is in fact implemented in terms of getdelim, just
like this:
ssize_t
getline (char **lineptr, size_t *n, FILE *stream)
{
return getdelim (lineptr, n, '\n', stream);
}
fgets function reads characters from the stream stream
up to and including a newline character and stores them in the string
s, adding a null character to mark the end of the string. You
must supply count characters worth of space in s, but the
number of characters read is at most count - 1. The extra
character space is used to hold the null character at the end of the
string.
If the system is already at end of file when you call fgets, then
the contents of the array s are unchanged and a null pointer is
returned. A null pointer is also returned if a read error occurs.
Otherwise, the return value is the pointer s.
Warning: If the input data has a null character, you can't tell.
So don't use fgets unless you know the data cannot contain a null.
Don't use it to read files edited by the user because, if the user inserts
a null character, you should either handle it properly or print a clear
error message. We recommend using getline instead of fgets.
fgetws function reads wide characters from the stream
stream up to and including a newline character and stores them in
the string ws, adding a null wide character to mark the end of the
string. You must supply count wide characters worth of space in
ws, but the number of characters read is at most count
- 1. The extra character space is used to hold the null wide
character at the end of the string.
If the system is already at end of file when you call fgetws, then
the contents of the array ws are unchanged and a null pointer is
returned. A null pointer is also returned if a read error occurs.
Otherwise, the return value is the pointer ws.
Warning: If the input data has a null wide character (which are
null bytes in the input stream), you can't tell. So don't use
fgetws unless you know the data cannot contain a null. Don't use
it to read files edited by the user because, if the user inserts a null
character, you should either handle it properly or print a clear error
message.
fgets_unlocked function is equivalent to the fgets
function except that it does not implicitly lock the stream.
This function is a GNU extension.
fgetws_unlocked function is equivalent to the fgetws
function except that it does not implicitly lock the stream.
This function is a GNU extension.
gets reads characters from the stream stdin
up to the next newline character, and stores them in the string s.
The newline character is discarded (note that this differs from the
behavior of fgets, which copies the newline character into the
string). If gets encounters a read error or end-of-file, it
returns a null pointer; otherwise it returns s.
Warning: The gets function is very dangerous
because it provides no protection against overflowing the string
s. The GNU library includes it for compatibility only. You
should always use fgets or getline instead. To
remind you of this, the linker (if using GNU ld) will issue a
warning whenever you use gets.
In parser programs it is often useful to examine the next character in the input stream without removing it from the stream. This is called "peeking ahead" at the input because your program gets a glimpse of the input it will read next.
Using stream I/O, you can peek ahead at input by first reading it and
then unreading it (also called pushing it back on the stream).
Unreading a character makes it available to be input again from the stream,
by the next call to fgetc or other input function on that stream.
Here is a pictorial explanation of unreading. Suppose you have a stream reading a file that contains just six characters, the letters `foobar'. Suppose you have read three characters so far. The situation looks like this:
f o o b a r
^
so the next input character will be `b'.
If instead of reading `b' you unread the letter `o', you get a situation like this:
f o o b a r
|
o--
^
so that the next input characters will be `o' and `b'.
If you unread `9' instead of `o', you get this situation:
f o o b a r
|
9--
^
so that the next input characters will be `9' and `b'.
ungetc To Do Unreading
The function to unread a character is called ungetc, because it
reverses the action of getc.
ungetc function pushes back the character c onto the
input stream stream. So the next input from stream will
read c before anything else.
If c is EOF, ungetc does nothing and just returns
EOF. This lets you call ungetc with the return value of
getc without needing to check for an error from getc.
The character that you push back doesn't have to be the same as the last
character that was actually read from the stream. In fact, it isn't
necessary to actually read any characters from the stream before
unreading them with ungetc! But that is a strange way to write
a program; usually ungetc is used only to unread a character
that was just read from the same stream.
The GNU C library only supports one character of pushback--in other
words, it does not work to call ungetc twice without doing input
in between. Other systems might let you push back multiple characters;
then reading from the stream retrieves the characters in the reverse
order that they were pushed.
Pushing back characters doesn't alter the file; only the internal
buffering for the stream is affected. If a file positioning function
(such as fseek, fseeko or rewind; see section File Positioning) is called, any pending pushed-back characters are
discarded.
Unreading a character on a stream that is at end of file clears the end-of-file indicator for the stream, because it makes the character of input available. After you read that character, trying to read again will encounter end of file.
ungetwc function behaves just like ungetc just that it
pushes back a wide character.
Here is an example showing the use of getc and ungetc to
skip over whitespace characters. When this function reaches a
non-whitespace character, it unreads that character to be seen again on
the next read operation on the stream.
#include <stdio.h>
#include <ctype.h>
void
skip_whitespace (FILE *stream)
{
int c;
do
/* No need to check for EOF because it is not
isspace, and ungetc ignores EOF. */
c = getc (stream);
while (isspace (c));
ungetc (c, stream);
}
This section describes how to do input and output operations on blocks of data. You can use these functions to read and write binary data, as well as to read and write text in fixed-size blocks instead of by characters or lines.
Binary files are typically used to read and write blocks of data in the same format as is used to represent the data in a running program. In other words, arbitrary blocks of memory--not just character or string objects--can be written to a binary file, and meaningfully read in again by the same program.
Storing data in binary form is often considerably more efficient than using the formatted I/O functions. Also, for floating-point numbers, the binary form avoids possible loss of precision in the conversion process. On the other hand, binary files can't be examined or modified easily using many standard file utilities (such as text editors), and are not portable between different implementations of the language, or different kinds of computers.
These functions are declared in `stdio.h'.
If fread encounters end of file in the middle of an object, it
returns the number of complete objects read, and discards the partial
object. Therefore, the stream remains at the actual end of the file.
fread_unlocked function is equivalent to the fread
function except that it does not implicitly lock the stream.
This function is a GNU extension.
fwrite_unlocked function is equivalent to the fwrite
function except that it does not implicitly lock the stream.
This function is a GNU extension.
The functions described in this section (printf and related
functions) provide a convenient way to perform formatted output. You
call printf with a format string or template string
that specifies how to format the values of the remaining arguments.
Unless your program is a filter that specifically performs line- or
character-oriented processing, using printf or one of the other
related functions described in this section is usually the easiest and
most concise way to perform output. These functions are especially
useful for printing error messages, tables of data, and the like.
The printf function can be used to print any number of arguments.
The template string argument you supply in a call provides
information not only about the number of additional arguments, but also
about their types and what style should be used for printing them.
Ordinary characters in the template string are simply written to the output stream as-is, while conversion specifications introduced by a `%' character in the template cause subsequent arguments to be formatted and written to the output stream. For example,
int pct = 37;
char filename[] = "foo.txt";
printf ("Processing of `%s' is %d%% finished.\nPlease be patient.\n",
filename, pct);
produces output like
Processing of `foo.txt' is 37% finished. Please be patient.
This example shows the use of the `%d' conversion to specify that
an int argument should be printed in decimal notation, the
`%s' conversion to specify printing of a string argument, and
the `%%' conversion to print a literal `%' character.
There are also conversions for printing an integer argument as an unsigned value in octal, decimal, or hexadecimal radix (`%o', `%u', or `%x', respectively); or as a character value (`%c').
Floating-point numbers can be printed in normal, fixed-point notation using the `%f' conversion or in exponential notation using the `%e' conversion. The `%g' conversion uses either `%e' or `%f' format, depending on what is more appropriate for the magnitude of the particular number.
You can control formatting more precisely by writing modifiers between the `%' and the character that indicates which conversion to apply. These slightly alter the ordinary behavior of the conversion. For example, most conversion specifications permit you to specify a minimum field width and a flag indicating whether you want the result left- or right-justified within the field.
The specific flags and modifiers that are permitted and their interpretation vary depending on the particular conversion. They're all described in more detail in the following sections. Don't worry if this all seems excessively complicated at first; you can almost always get reasonable free-format output without using any of the modifiers at all. The modifiers are mostly used to make the output look "prettier" in tables.
This section provides details about the precise syntax of conversion
specifications that can appear in a printf template
string.
Characters in the template string that are not part of a conversion specification are printed as-is to the output stream. Multibyte character sequences (see section Character Set Handling) are permitted in a template string.
The conversion specifications in a printf template string have
the general form:
% [ param-no $] flags width [ . precision ] type conversion
For example, in the conversion specifier `%-10.8ld', the `-'
is a flag, `10' specifies the field width, the precision is
`8', the letter `l' is a type modifier, and `d' specifies
the conversion style. (This particular type specifier says to
print a long int argument in decimal notation, with a minimum of
8 digits left-justified in a field at least 10 characters wide.)
In more detail, output conversion specifications consist of an initial `%' character followed in sequence by:
printf function are assigned to the
formats in the order of appearance in the format string. But in some
situations (such as message translation) this is not desirable and this
extension allows an explicit parameter to be specified.
The param-no part of the format must be an integer in the range of
1 to the maximum number of arguments present to the function call. Some
implementations limit this number to a certainly upper bound. The exact
limit can be retrieved by the following constant.
ARGMAX is the maximum value allowed for the
specification of an positional parameter in a printf call. The
actual value in effect at runtime can be retrieved by using
sysconf using the _SC_NL_ARGMAX parameter see section Definition of sysconf.
Some system have a quite low limit such as @math{9} for System V
systems. The GNU C library has no real limit.
int.
If the value is negative, this means to set the `-' flag (see
below) and to use the absolute value as the field width.
int, and is ignored
if it is negative. If you specify `*' for both the field width and
precision, the field width argument precedes the precision argument.
Other C library versions may not recognize this syntax.
int,
but you can specify `h', `l', or `L' for other integer
types.)
The exact options that are permitted and how they are interpreted vary between the different conversion specifiers. See the descriptions of the individual conversions for information about the particular options that they use.
With the `-Wformat' option, the GNU C compiler checks calls to
printf and related functions. It examines the format string and
verifies that the correct number and types of arguments are supplied.
There is also a GNU C syntax to tell the compiler that a function you
write uses a printf-style format string.
See section `Declaring Attributes of Functions' in Using GNU CC, for more information.
Here is a table summarizing what all the different conversions do:
scanf for input
(see section Table of Input Conversions).
errno.
(This is a GNU extension.)
See section Other Output Conversions.
If the syntax of a conversion specification is invalid, unpredictable things will happen, so don't do this. If there aren't enough function arguments provided to supply values for all the conversion specifications in the template string, or if the arguments are not of the correct types, the results are unpredictable. If you supply more arguments than conversion specifications, the extra argument values are simply ignored; this is sometimes useful.
This section describes the options for the `%d', `%i', `%o', `%u', `%x', and `%X' conversion specifications. These conversions print integers in various formats.
The `%d' and `%i' conversion specifications both print an
int argument as a signed decimal number; while `%o',
`%u', and `%x' print the argument as an unsigned octal,
decimal, or hexadecimal number (respectively). The `%X' conversion
specification is just like `%x' except that it uses the characters
`ABCDEF' as digits instead of `abcdef'.
The following flags are meaningful:
strtoul function (see section Parsing of Integers) and scanf with the `%i' conversion
(see section Numeric Input Conversions).
LC_NUMERIC category; see section Generic Numeric Formatting Parameters. This flag is a
GNU extension.
If a precision is supplied, it specifies the minimum number of digits to appear; leading zeros are produced if necessary. If you don't specify a precision, the number is printed with as many digits as it needs. If you convert a value of zero with an explicit precision of zero, then no characters at all are produced.
Without a type modifier, the corresponding argument is treated as an
int (for the signed conversions `%i' and `%d') or
unsigned int (for the unsigned conversions `%o', `%u',
`%x', and `%X'). Recall that since printf and friends
are variadic, any char and short arguments are
automatically converted to int by the default argument
promotions. For arguments of other integer types, you can use these
modifiers:
signed char or unsigned
char, as appropriate. A char argument is converted to an
int or unsigned int by the default argument promotions
anyway, but the `h' modifier says to convert it back to a
char again.
This modifier was introduced in ISO C99.
short int or unsigned
short int, as appropriate. A short argument is converted to an
int or unsigned int by the default argument promotions
anyway, but the `h' modifier says to convert it back to a
short again.
intmax_t or uintmax_t, as
appropriate.
This modifier was introduced in ISO C99.
long int or unsigned long
int, as appropriate. Two `l' characters is like the `L'
modifier, below.
If used with `%c' or `%s' the corresponding parameter is
considered as a wide character or wide character string respectively.
This use of `l' was introduced in Amendment 1 to ISO C90.
long long int. (This type is
an extension supported by the GNU C compiler. On systems that don't
support extra-long integers, this is the same as long int.)
The `q' modifier is another name for the same thing, which comes
from 4.4 BSD; a long long int is sometimes called a "quad"
int.
ptrdiff_t.
This modifier was introduced in ISO C99.
size_t.
`z' was introduced in ISO C99. `Z' is a GNU extension
predating this addition and should not be used in new code.
Here is an example. Using the template string:
"|%5d|%-5d|%+5d|%+-5d|% 5d|%05d|%5.0d|%5.2d|%d|\n"
to print numbers using the different options for the `%d' conversion gives results like:
| 0|0 | +0|+0 | 0|00000| | 00|0| | 1|1 | +1|+1 | 1|00001| 1| 01|1| | -1|-1 | -1|-1 | -1|-0001| -1| -01|-1| |100000|100000|+100000| 100000|100000|100000|100000|100000|
In particular, notice what happens in the last case where the number is too large to fit in the minimum field width specified.
Here are some more examples showing how unsigned integers print under various format options, using the template string:
"|%5u|%5o|%5x|%5X|%#5o|%#5x|%#5X|%#10.8x|\n"
| 0| 0| 0| 0| 0| 0x0| 0X0|0x00000000| | 1| 1| 1| 1| 01| 0x1| 0X1|0x00000001| |100000|303240|186a0|186A0|0303240|0x186a0|0X186A0|0x000186a0|
This section discusses the conversion specifications for floating-point numbers: the `%f', `%e', `%E', `%g', and `%G' conversions.
The `%f' conversion prints its argument in fixed-point notation,
producing output of the form
[-]ddd.ddd,
where the number of digits following the decimal point is controlled
by the precision you specify.
The `%e' conversion prints its argument in exponential notation,
producing output of the form
[-]d.ddde[+|-]dd.
Again, the number of digits following the decimal point is controlled by
the precision. The exponent always contains at least two digits. The
`%E' conversion is similar but the exponent is marked with the letter
`E' instead of `e'.
The `%g' and `%G' conversions print the argument in the style of `%e' or `%E' (respectively) if the exponent would be less than -4 or greater than or equal to the precision; otherwise they use the `%f' style. Trailing zeros are removed from the fractional portion of the result and a decimal-point character appears only if it is followed by a digit.
The `%a' and `%A' conversions are meant for representing
floating-point numbers exactly in textual form so that they can be
exchanged as texts between different programs and/or machines. The
numbers are represented is the form
[-]0xh.hhhp[+|-]dd.
At the left of the decimal-point character exactly one digit is print.
This character is only 0 if the number is denormalized.
Otherwise the value is unspecified; it is implementation dependent how many
bits are used. The number of hexadecimal digits on the right side of
the decimal-point character is equal to the precision. If the precision
is zero it is determined to be large enough to provide an exact
representation of the number (or it is large enough to distinguish two
adjacent values if the FLT_RADIX is not a power of 2,
see section Floating Point Parameters). For the `%a' conversion
lower-case characters are used to represent the hexadecimal number and
the prefix and exponent sign are printed as 0x and p
respectively. Otherwise upper-case characters are used and 0X
and P are used for the representation of prefix and exponent
string. The exponent to the base of two is printed as a decimal number
using at least one digit but at most as many digits as necessary to
represent the value exactly.
If the value to be printed represents infinity or a NaN, the output is
[-]inf or nan respectively if the conversion
specifier is `%a', `%e', `%f', or `%g' and it is
[-]INF or NAN respectively if the conversion is
`%A', `%E', or `%G'.
The following flags can be used to modify the behavior:
LC_NUMERIC category;
see section Generic Numeric Formatting Parameters. This flag is a GNU extension.
The precision specifies how many digits follow the decimal-point
character for the `%f', `%e', and `%E' conversions. For
these conversions, the default precision is 6. If the precision
is explicitly 0, this suppresses the decimal point character
entirely. For the `%g' and `%G' conversions, the precision
specifies how many significant digits to print. Significant digits are
the first digit before the decimal point, and all the digits after it.
If the precision is 0 or not specified for `%g' or `%G',
it is treated like a value of 1. If the value being printed
cannot be expressed accurately in the specified number of digits, the
value is rounded to the nearest number that fits.
Without a type modifier, the floating-point conversions use an argument
of type double. (By the default argument promotions, any
float arguments are automatically converted to double.)
The following type modifier is supported:
long
double.
Here are some examples showing how numbers print using the various floating-point conversions. All of the numbers were printed using this template string:
"|%13.4a|%13.4f|%13.4e|%13.4g|\n"
Here is the output:
| 0x0.0000p+0| 0.0000| 0.0000e+00| 0| | 0x1.0000p-1| 0.5000| 5.0000e-01| 0.5| | 0x1.0000p+0| 1.0000| 1.0000e+00| 1| | -0x1.0000p+0| -1.0000| -1.0000e+00| -1| | 0x1.9000p+6| 100.0000| 1.0000e+02| 100| | 0x1.f400p+9| 1000.0000| 1.0000e+03| 1000| | 0x1.3880p+13| 10000.0000| 1.0000e+04| 1e+04| | 0x1.81c8p+13| 12345.0000| 1.2345e+04| 1.234e+04| | 0x1.86a0p+16| 100000.0000| 1.0000e+05| 1e+05| | 0x1.e240p+16| 123456.0000| 1.2346e+05| 1.235e+05|
Notice how the `%g' conversion drops trailing zeros.
This section describes miscellaneous conversions for printf.
The `%c' conversion prints a single character. In case there is no
`l' modifier the int argument is first converted to an
unsigned char. Then, if used in a wide stream function, the
character is converted into the corresponding wide character. The
`-' flag can be used to specify left-justification in the field,
but no other flags are defined, and no precision or type modifier can be
given. For example:
printf ("%c%c%c%c%c", 'h', 'e', 'l', 'l', 'o');
prints `hello'.
If there is a `l' modifier present the argument is expected to be
of type wint_t. If used in a multibyte function the wide
character is converted into a multibyte character before being added to
the output. In this case more than one output byte can be produced.
The `%s' conversion prints a string. If no `l' modifier is
present the corresponding argument must be of type char * (or
const char *). If used in a wide stream function the string is
first converted in a wide character string. A precision can be
specified to indicate the maximum number of characters to write;
otherwise characters in the string up to but not including the
terminating null character are written to the output stream. The
`-' flag can be used to specify left-justification in the field,
but no other flags or type modifiers are defined for this conversion.
For example:
printf ("%3s%-6s", "no", "where");
prints ` nowhere '.
If there is a `l' modifier present the argument is expected to be of type wchar_t (or const wchar_t *).
If you accidentally pass a null pointer as the argument for a `%s' conversion, the GNU library prints it as `(null)'. We think this is more useful than crashing. But it's not good practice to pass a null argument intentionally.
The `%m' conversion prints the string corresponding to the error
code in errno. See section Error Messages. Thus:
fprintf (stderr, "can't open `%s': %m\n", filename);
is equivalent to:
fprintf (stderr, "can't open `%s': %s\n", filename, strerror (errno));
The `%m' conversion is a GNU C library extension.
The `%p' conversion prints a pointer value. The corresponding
argument must be of type void *. In practice, you can use any
type of pointer.
In the GNU system, non-null pointers are printed as unsigned integers, as if a `%#x' conversion were used. Null pointers print as `(nil)'. (Pointers might print differently in other systems.)
For example:
printf ("%p", "testing");
prints `0x' followed by a hexadecimal number--the address of the
string constant "testing". It does not print the word
`testing'.
You can supply the `-' flag with the `%p' conversion to specify left-justification, but no other flags, precision, or type modifiers are defined.
The `%n' conversion is unlike any of the other output conversions.
It uses an argument which must be a pointer to an int, but
instead of printing anything it stores the number of characters printed
so far by this call at that location. The `h' and `l' type
modifiers are permitted to specify that the argument is of type
short int * or long int * instead of int *, but no
flags, field width, or precision are permitted.
For example,
int nchar;
printf ("%d %s%n\n", 3, "bears", &nchar);
prints:
3 bears
and sets nchar to 7, because `3 bears' is seven
characters.
The `%%' conversion prints a literal `%' character. This conversion doesn't use an argument, and no flags, field width, precision, or type modifiers are permitted.
This section describes how to call printf and related functions.
Prototypes for these functions are in the header file `stdio.h'.
Because these functions take a variable number of arguments, you
must declare prototypes for them before using them. Of course,
the easiest way to make sure you have all the right prototypes is to
just include `stdio.h'.
printf function prints the optional arguments under the
control of the template string template to the stream
stdout. It returns the number of characters printed, or a
negative value if there was an output error.
wprintf function prints the optional arguments under the
control of the wide template string template to the stream
stdout. It returns the number of wide characters printed, or a
negative value if there was an output error.
printf, except that the output is
written to the stream stream instead of stdout.
wprintf, except that the output is
written to the stream stream instead of stdout.
printf, except that the output is stored in the character
array s instead of written to a stream. A null character is written
to mark the end of the string.
The sprintf function returns the number of characters stored in
the array s, not including the terminating null character.
The behavior of this function is undefined if copying takes place between objects that overlap--for example, if s is also given as an argument to be printed under control of the `%s' conversion. See section Copying and Concatenation.
Warning: The sprintf function can be dangerous
because it can potentially output more characters than can fit in the
allocation size of the string s. Remember that the field width
given in a conversion specification is only a minimum value.
To avoid this problem, you can use snprintf or asprintf,
described below.
wprintf, except that the output is stored in the
wide character array ws instead of written to a stream. A null
wide character is written to mark the end of the string. The size
argument specifies the maximum number of characters to produce. The
trailing null character is counted towards this limit, so you should
allocate at least size wide characters for the string ws.
The return value is the number of characters which would be generated for the given input, excluding the trailing null. If this value is greater or equal to size, not all characters from the result have been stored in ws. You should try again with a bigger output string.
Note that the corresponding narrow stream function takes fewer
parameters. swprintf in fact corresponds to the snprintf
function. Since the sprintf function can be dangerous and should
be avoided the ISO C committee refused to make the same mistake
again and decided to not define an function exactly corresponding to
sprintf.
snprintf function is similar to sprintf, except that
the size argument specifies the maximum number of characters to
produce. The trailing null character is counted towards this limit, so
you should allocate at least size characters for the string s.
The return value is the number of characters which would be generated for the given input, excluding the trailing null. If this value is greater or equal to size, not all characters from the result have been stored in s. You should try again with a bigger output string. Here is an example of doing this:
/* Construct a message describing the value of a variable
whose name is name and whose value is value. */
char *
make_message (char *name, char *value)
{
/* Guess we need no more than 100 chars of space. */
int size = 100;
char *buffer = (char *) xmalloc (size);
int nchars;
if (buffer == NULL)
return NULL;
/* Try to print in the allocated space. */
nchars = snprintf (buffer, size, "value of %s is %s",
name, value);
if (nchars >= size)
{
/* Reallocate buffer now that we know
how much space is needed. */
buffer = (char *) xrealloc (buffer, nchars + 1);
if (buffer != NULL)
/* Try again. */
snprintf (buffer, size, "value of %s is %s",
name, value);
}
/* The last call worked, return the string. */
return buffer;
}
In practice, it is often easier just to use asprintf, below.
Attention: In versions of the GNU C library prior to 2.1 the
return value is the number of characters stored, not including the
terminating null; unless there was not enough space in s to
store the result in which case -1 is returned. This was
changed in order to comply with the ISO C99 standard.
The functions in this section do formatted output and place the results in dynamically allocated memory.
sprintf, except that it dynamically
allocates a string (as with malloc; see section Unconstrained Allocation) to hold the output, instead of putting the output in a
buffer you allocate in advance. The ptr argument should be the
address of a char * object, and asprintf stores a pointer
to the newly allocated string at that location.
The return value is the number of characters allocated for the buffer, or less than zero if an error occured. Usually this means that the buffer could not be allocated.
Here is how to use asprintf to get the same result as the
snprintf example, but more easily:
/* Construct a message describing the value of a variable
whose name is name and whose value is value. */
char *
make_message (char *name, char *value)
{
char *result;
if (asprintf (&result, "value of %s is %s", name, value) < 0)
return NULL;
return result;
}
asprintf, except that it uses the
obstack obstack to allocate the space. See section Obstacks.
The characters are written onto the end of the current object.
To get at them, you must finish the object with obstack_finish
(see section Growing Objects).
The functions vprintf and friends are provided so that you can
define your own variadic printf-like functions that make use of
the same internals as the built-in formatted output functions.
The most natural way to define such functions would be to use a language
construct to say, "Call printf and pass this template plus all
of my arguments after the first five." But there is no way to do this
in C, and it would be hard to provide a way, since at the C language
level there is no way to tell how many arguments your function received.
Since that method is impossible, we provide alternative functions, the
vprintf series, which lets you pass a va_list to describe
"all of my arguments after the first five."
When it is sufficient to define a macro rather than a real function, the GNU C compiler provides a way to do this much more easily with macros. For example:
#define myprintf(a, b, c, d, e, rest...) \
printf (mytemplate , ## rest...)
See section `Macros with Variable Numbers of Arguments' in Using GNU CC, for details. But this is limited to macros, and does not apply to real functions at all.
Before calling vprintf or the other functions listed in this
section, you must call va_start (see section Variadic Functions) to initialize a pointer to the variable arguments. Then you
can call va_arg to fetch the arguments that you want to handle
yourself. This advances the pointer past those arguments.
Once your va_list pointer is pointing at the argument of your
choice, you are ready to call vprintf. That argument and all
subsequent arguments that were passed to your function are used by
vprintf along with the template that you specified separately.
In some other systems, the va_list pointer may become invalid
after the call to vprintf, so you must not use va_arg
after you call vprintf. Instead, you should call va_end
to retire the pointer from service. However, you can safely call
va_start on another pointer variable and begin fetching the
arguments again through that pointer. Calling vprintf does not
destroy the argument list of your function, merely the particular
pointer that you passed to it.
GNU C does not have such restrictions. You can safely continue to fetch
arguments from a va_list pointer after passing it to
vprintf, and va_end is a no-op. (Note, however, that
subsequent va_arg calls will fetch the same arguments which
vprintf previously used.)
Prototypes for these functions are declared in `stdio.h'.
printf except that, instead of taking
a variable number of arguments directly, it takes an argument list
pointer ap.
wprintf except that, instead of taking
a variable number of arguments directly, it takes an argument list
pointer ap.
fprintf with the variable argument list
specified directly as for vprintf.
fwprintf with the variable argument list
specified directly as for vwprintf.
sprintf with the variable argument list
specified directly as for vprintf.
swprintf with the variable argument list
specified directly as for vwprintf.
snprintf with the variable argument list
specified directly as for vprintf.
vasprintf function is the equivalent of asprintf with the
variable argument list specified directly as for vprintf.
obstack_vprintf function is the equivalent of
obstack_printf with the variable argument list specified directly
as for vprintf.
Here's an example showing how you might use vfprintf. This is a
function that prints error messages to the stream stderr, along
with a prefix indicating the name of the program
(see section Error Messages, for a description of
program_invocation_short_name).
#include <stdio.h>
#include <stdarg.h>
void
eprintf (const char *template, ...)
{
va_list ap;
extern char *program_invocation_short_name;
fprintf (stderr, "%s: ", program_invocation_short_name);
va_start (ap, template);
vfprintf (stderr, template, ap);
va_end (ap);
}
You could call eprintf like this:
eprintf ("file `%s' does not exist\n", filename);
In GNU C, there is a special construct you can use to let the compiler
know that a function uses a printf-style format string. Then it
can check the number and types of arguments in each call to the
function, and warn you when they do not match the format string.
For example, take this declaration of eprintf:
void eprintf (const char *template, ...)
__attribute__ ((format (printf, 1, 2)));
This tells the compiler that eprintf uses a format string like
printf (as opposed to scanf; see section Formatted Input);
the format string appears as the first argument;
and the arguments to satisfy the format begin with the second.
See section `Declaring Attributes of Functions' in Using GNU CC, for more information.
You can use the function parse_printf_format to obtain
information about the number and types of arguments that are expected by
a given template string. This function permits interpreters that
provide interfaces to printf to avoid passing along invalid
arguments from the user's program, which could cause a crash.
All the symbols described in this section are declared in the header file `printf.h'.
printf template string template.
The information is stored in the array argtypes; each element of
this array describes one argument. This information is encoded using
the various `PA_' macros, listed below.
The argument n specifies the number of elements in the array
argtypes. This is the maximum number of elements that
parse_printf_format will try to write.
parse_printf_format returns the total number of arguments required
by template. If this number is greater than n, then the
information returned describes only the first n arguments. If you
want information about additional arguments, allocate a bigger
array and call parse_printf_format again.
The argument types are encoded as a combination of a basic type and modifier flag bits.
(argtypes[i] & PA_FLAG_MASK) to extract just the
flag bits for an argument, or (argtypes[i] & ~PA_FLAG_MASK) to
extract just the basic type code.
Here are symbolic constants that represent the basic types; they stand for integer values.
PA_INT
int.
PA_CHAR
int, cast to char.
PA_STRING
char *, a null-terminated string.
PA_POINTER
void *, an arbitrary pointer.
PA_FLOAT
float.
PA_DOUBLE
double.
PA_LAST
PA_LAST. For example, if you have data types `foo'
and `bar' with their own specialized printf conversions,
you could define encodings for these types as:
#define PA_FOO PA_LAST #define PA_BAR (PA_LAST + 1)
Here are the flag bits that modify a basic type. They are combined with the code for the basic type using inclusive-or.
PA_FLAG_PTR
PA_FLAG_SHORT
short. (This corresponds to the `h' type modifier.)
PA_FLAG_LONG
long. (This corresponds to the `l' type modifier.)
PA_FLAG_LONG_LONG
long long. (This corresponds to the `L' type modifier.)
PA_FLAG_LONG_DOUBLE
PA_FLAG_LONG_LONG, used by convention with
a base type of PA_DOUBLE to indicate a type of long double.
Here is an example of decoding argument types for a format string. We
assume this is part of an interpreter which contains arguments of type
NUMBER, CHAR, STRING and STRUCTURE (and
perhaps others which are not valid here).
/* Test whether the nargs specified objects
in the vector args are valid
for the format string format:
if so, return 1.
If not, return 0 after printing an error message. */
int
validate_args (char *format, int nargs, OBJECT *args)
{
int *argtypes;
int nwanted;
/* Get the information about the arguments.
Each conversion specification must be at least two characters
long, so there cannot be more specifications than half the
length of the string. */
argtypes = (int *) alloca (strlen (format) / 2 * sizeof (int));
nwanted = parse_printf_format (string, nelts, argtypes);
/* Check the number of arguments. */
if (nwanted > nargs)
{
error ("too few arguments (at least %d required)", nwanted);
return 0;
}
/* Check the C type wanted for each argument
and see if the object given is suitable. */
for (i = 0; i < nwanted; i++)
{
int wanted;
if (argtypes[i] & PA_FLAG_PTR)
wanted = STRUCTURE;
else
switch (argtypes[i] & ~PA_FLAG_MASK)
{
case PA_INT:
case PA_FLOAT:
case PA_DOUBLE:
wanted = NUMBER;
break;
case PA_CHAR:
wanted = CHAR;
break;
case PA_STRING:
wanted = STRING;
break;
case PA_POINTER:
wanted = STRUCTURE;
break;
}
if (TYPE (args[i]) != wanted)
{
error ("type mismatch for arg number %d", i);
return 0;
}
}
return 1;
}
printf
The GNU C library lets you define your own custom conversion specifiers
for printf template strings, to teach printf clever ways
to print the important data structures of your program.
The way you do this is by registering the conversion with the function
register_printf_function; see section Registering New Conversions.
One of the arguments you pass to this function is a pointer to a handler
function that produces the actual output; see section Defining the Output Handler, for information on how to write this function.
You can also install a function that just returns information about the number and type of arguments expected by the conversion specifier. See section Parsing a Template String, for information about this.
The facilities of this section are declared in the header file `printf.h'.
Portability Note: The ability to extend the syntax of
printf template strings is a GNU extension. ISO standard C has
nothing similar.
The function to register a new output conversion is
register_printf_function, declared in `printf.h'.
'Y', it defines the conversion `%Y'.
You can redefine the built-in conversions like `%s', but flag
characters like `#' and type modifiers like `l' can never be
used as conversions; calling register_printf_function for those
characters has no effect. It is advisable not to use lowercase letters,
since the ISO C standard warns that additional lowercase letters may be
standardized in future editions of the standard.
The handler-function is the function called by printf and
friends when this conversion appears in a template string.
See section Defining the Output Handler, for information about how to define
a function to pass as this argument. If you specify a null pointer, any
existing handler function for spec is removed.
The arginfo-function is the function called by
parse_printf_format when this conversion appears in a
template string. See section Parsing a Template String, for information
about this.
Attention: In the GNU C library versions before 2.0 the
arginfo-function function did not need to be installed unless
the user used the parse_printf_format function. This has changed.
Now a call to any of the printf functions will call this
function when this format specifier appears in the format string.
The return value is 0 on success, and -1 on failure
(which occurs if spec is out of range).
You can redefine the standard output conversions, but this is probably not a good idea because of the potential for confusion. Library routines written by other people could break if you do this.
If you define a meaning for `%A', what if the template contains `%+23A' or `%-#A'? To implement a sensible meaning for these, the handler when called needs to be able to get the options specified in the template.
Both the handler-function and arginfo-function accept an
argument that points to a struct printf_info, which contains
information about the options appearing in an instance of the conversion
specifier. This data type is declared in the header file
`printf.h'.
printf template
string to the handler and arginfo functions for that specifier. It
contains the following members:
int prec
-1 if no precision
was specified. If the precision was given as `*', the
printf_info structure passed to the handler function contains the
actual value retrieved from the argument list. But the structure passed
to the arginfo function contains a value of INT_MIN, since the
actual value is not known.
int width
0 if no
width was specified. If the field width was given as `*', the
printf_info structure passed to the handler function contains the
actual value retrieved from the argument list. But the structure passed
to the arginfo function contains a value of INT_MIN, since the
actual value is not known.
wchar_t spec
unsigned int is_long_double
long long int, as opposed to long double for floating
point conversions.
unsigned int is_char
unsigned int is_short
unsigned int is_long
unsigned int alt
unsigned int space
unsigned int left
unsigned int showsign
unsigned int group
unsigned int extra
printf function this variable always contains the value
0.
unsigned int wide
wchar_t pad
'0' if the `0' flag was specified, and
' ' otherwise.
Now let's look at how to define the handler and arginfo functions
which are passed as arguments to register_printf_function.
Compatibility Note: The interface changed in GNU libc
version 2.0. Previously the third argument was of type
va_list *.
You should define your handler functions with a prototype like:
int function (FILE *stream, const struct printf_info *info,
const void *const *args)
The stream argument passed to the handler function is the stream to which it should write output.
The info argument is a pointer to a structure that contains information about the various options that were included with the conversion in the template string. You should not modify this structure inside your handler function. See section Conversion Specifier Options, for a description of this data structure.
The args is a vector of pointers to the arguments data. The number of arguments was determined by calling the argument information function provided by the user.
Your handler function should return a value just like printf
does: it should return the number of characters it has written, or a
negative value to indicate an error.
If you are going to use parse_printf_format in your
application, you must also define a function to pass as the
arginfo-function argument for each new conversion you install with
register_printf_function.
You have to define these functions with a prototype like:
int function (const struct printf_info *info,
size_t n, int *argtypes)
The return value from the function should be the number of arguments the
conversion expects. The function should also fill in no more than
n elements of the argtypes array with information about the
types of each of these arguments. This information is encoded using the
various `PA_' macros. (You will notice that this is the same
calling convention parse_printf_format itself uses.)
printf Extension Example
Here is an example showing how to define a printf handler function.
This program defines a data structure called a Widget and
defines the `%W' conversion to print information about Widget *
arguments, including the pointer value and the name stored in the data
structure. The `%W' conversion supports the minimum field width and
left-justification options, but ignores everything else.
#include <stdio.h>
#include <stdlib.h>
#include <printf.h>
typedef struct
{
char *name;
}
Widget;
int
print_widget (FILE *stream,
const struct printf_info *info,
const void *const *args)
{
const Widget *w;
char *buffer;
int len;
/* Format the output into a string. */
w = *((const Widget **) (args[0]));
len = asprintf (&buffer, "<Widget %p: %s>", w, w->name);
if (len == -1)
return -1;
/* Pad to the minimum field width and print to the stream. */
len = fprintf (stream, "%*s",
(info->left ? -info->width : info->width),
buffer);
/* Clean up and return. */
free (buffer);
return len;
}
int
print_widget_arginfo (const struct printf_info *info, size_t n,
int *argtypes)
{
/* We always take exactly one argument and this is a pointer to the
structure.. */
if (n > 0)
argtypes[0] = PA_POINTER;
return 1;
}
int
main (void)
{
/* Make a widget to print. */
Widget mywidget;
mywidget.name = "mywidget";
/* Register the print function for widgets. */
register_printf_function ('W', print_widget, print_widget_arginfo);
/* Now print the widget. */
printf ("|%W|\n", &mywidget);
printf ("|%35W|\n", &mywidget);
printf ("|%-35W|\n", &mywidget);
return 0;
}
The output produced by this program looks like:
|<Widget 0xffeffb7c: mywidget>| | <Widget 0xffeffb7c: mywidget>| |<Widget 0xffeffb7c: mywidget> |
printf Handlers
The GNU libc also contains a concrete and useful application of the
printf handler extension. There are two functions available
which implement a special way to print floating-point numbers.
%f except
that there is a postfix character indicating the divisor for the
number to make this less than 1000. There are two possible divisors:
powers of 1024 or powers of 1000. Which one is used depends on the
format character specified while registered this handler. If the
character is of lower case, 1024 is used. For upper case characters,
1000 is used.
The postfix tag corresponds to bytes, kilobytes, megabytes, gigabytes, etc. The full table is:
The default precision is 3, i.e., 1024 is printed with a lower-case
format character as if it were %.3fk and will yield 1.000k.
Due to the requirements of register_printf_function we must also
provide the function which returns information about the arguments.
vfprintf implementation expects
it. The format always takes one argument.
To use these functions both functions must be registered with a call like
register_printf_function ('B', printf_size, printf_size_info);
Here we register the functions to print numbers as powers of 1000 since
the format character 'B' is an upper-case character. If we
would additionally use 'b' in a line like
register_printf_function ('b', printf_size, printf_size_info);
we could also print using a power of 1024. Please note that all that is
different in these two lines is the format specifier. The
printf_size function knows about the difference between lower and upper
case format specifiers.
The use of 'B' and 'b' is no coincidence. Rather it is
the preferred way to use this functionality since it is available on
some other systems which also use format specifiers.
The functions described in this section (scanf and related
functions) provide facilities for formatted input analogous to the
formatted output facilities. These functions provide a mechanism for
reading arbitrary values under the control of a format string or
template string.
Calls to scanf are superficially similar to calls to
printf in that arbitrary arguments are read under the control of
a template string. While the syntax of the conversion specifications in
the template is very similar to that for printf, the
interpretation of the template is oriented more towards free-format
input and simple pattern matching, rather than fixed-field formatting.
For example, most scanf conversions skip over any amount of
"white space" (including spaces, tabs, and newlines) in the input
file, and there is no concept of precision for the numeric input
conversions as there is for the corresponding output conversions.
Ordinarily, non-whitespace characters in the template are expected to
match characters in the input stream exactly, but a matching failure is
distinct from an input error on the stream.
Another area of difference between scanf and printf is
that you must remember to supply pointers rather than immediate values
as the optional arguments to scanf; the values that are read are
stored in the objects that the pointers point to. Even experienced
programmers tend to forget this occasionally, so if your program is
getting strange errors that seem to be related to scanf, you
might want to double-check this.
When a matching failure occurs, scanf returns immediately,
leaving the first non-matching character as the next character to be
read from the stream. The normal return value from scanf is the
number of values that were assigned, so you can use this to determine if
a matching error happened before all the expected values were read.
The scanf function is typically used for things like reading in
the contents of tables. For example, here is a function that uses
scanf to initialize an array of double:
void
readarray (double *array, int n)
{
int i;
for (i=0; i<n; i++)
if (scanf (" %lf", &(array[i])) != 1)
invalid_input_error ();
}
The formatted input functions are not used as frequently as the formatted output functions. Partly, this is because it takes some care to use them properly. Another reason is that it is difficult to recover from a matching error.
If you are trying to read input that doesn't match a single, fixed
pattern, you may be better off using a tool such as Flex to generate a
lexical scanner, or Bison to generate a parser, rather than using
scanf. For more information about these tools, see section `' in Flex: The Lexical Scanner Generator, and section `' in The Bison Reference Manual.
A scanf template string is a string that contains ordinary
multibyte characters interspersed with conversion specifications that
start with `%'.
Any whitespace character (as defined by the isspace function;
see section Classification of Characters) in the template causes any number
of whitespace characters in the input stream to be read and discarded.
The whitespace characters that are matched need not be exactly the same
whitespace characters that appear in the template string. For example,
write ` , ' in the template to recognize a comma with optional
whitespace before and after.
Other characters in the template string that are not part of conversion specifications must match characters in the input stream exactly; if this is not the case, a matching failure occurs.
The conversion specifications in a scanf template string
have the general form:
% flags width type conversion
In more detail, an input conversion specification consists of an initial `%' character followed in sequence by:
scanf finds a conversion
specification that uses this flag, it reads input as directed by the
rest of the conversion specification, but it discards this input, does
not use a pointer argument, and does not increment the count of
successful assignments.
long int
rather than a pointer to an int.
The exact options that are permitted and how they are interpreted vary between the different conversion specifiers. See the descriptions of the individual conversions for information about the particular options that they allow.
With the `-Wformat' option, the GNU C compiler checks calls to
scanf and related functions. It examines the format string and
verifies that the correct number and types of arguments are supplied.
There is also a GNU C syntax to tell the compiler that a function you
write uses a scanf-style format string.
See section `Declaring Attributes of Functions' in Using GNU CC, for more information.
Here is a table that summarizes the various conversion specifications:
wcrtomb into a
multibyte string. This means that the buffer must provide room for
MB_CUR_MAX bytes for each wide character read. In case
`%ls' is used in a multibyte function the result is converted into
wide characters as with multiple calls of mbrtowc before being
stored in the user provided buffer.
wcrtomb into a
multibyte string. This means that the buffer must provide room for
MB_CUR_MAX bytes for each wide character read. In case
`%l[' is used in a multibyte function the result is converted into
wide characters as with multiple calls of mbrtowc before being
stored in the user provided buffer.
MB_CUR_MAX bytes for each character. If `%lc' is used in
a multibyte function the input is treated as a multibyte sequence (and
not bytes) and the result is converted as with calls to mbrtowc.
printf. See section Other Input Conversions.
If the syntax of a conversion specification is invalid, the behavior is undefined. If there aren't enough function arguments provided to supply addresses for all the conversion specifications in the template strings that perform assignments, or if the arguments are not of the correct types, the behavior is also undefined. On the other hand, extra arguments are simply ignored.
This section describes the scanf conversions for reading numeric
values.
The `%d' conversion matches an optionally signed integer in decimal
radix. The syntax that is recognized is the same as that for the
strtol function (see section Parsing of Integers) with the value
10 for the base argument.
The `%i' conversion matches an optionally signed integer in any of
the formats that the C language defines for specifying an integer
constant. The syntax that is recognized is the same as that for the
strtol function (see section Parsing of Integers) with the value
0 for the base argument. (You can print integers in this
syntax with printf by using the `#' flag character with the
`%x', `%o', or `%d' conversion. See section Integer Conversions.)
For example, any of the strings `10', `0xa', or `012'
could be read in as integers under the `%i' conversion. Each of
these specifies a number with decimal value 10.
The `%o', `%u', and `%x' conversions match unsigned
integers in octal, decimal, and hexadecimal radices, respectively. The
syntax that is recognized is the same as that for the strtoul
function (see section Parsing of Integers) with the appropriate value
(8, 10, or 16) for the base argument.
The `%X' conversion is identical to the `%x' conversion. They both permit either uppercase or lowercase letters to be used as digits.
The default type of the corresponding argument for the %d and
%i conversions is int *, and unsigned int * for the
other integer conversions. You can use the following type modifiers to
specify other sizes of integer:
signed char * or unsigned
char *.
This modifier was introduced in ISO C99.
short int * or unsigned
short int *.
intmax_t * or uintmax_t *.
This modifier was introduced in ISO C99.
long int * or unsigned
long int *. Two `l' characters is like the `L' modifier, below.
If used with `%c' or `%s' the corresponding parameter is
considered as a pointer to a wide character or wide character string
respectively. This use of `l' was introduced in Amendment 1 to
ISO C90.
long long int * or unsigned long long int *. (The long long type is an extension supported by the
GNU C compiler. For systems that don't provide extra-long integers, this
is the same as long int.)
The `q' modifier is another name for the same thing, which comes
from 4.4 BSD; a long long int is sometimes called a "quad"
int.
ptrdiff_t *.
This modifier was introduced in ISO C99.
size_t *.
This modifier was introduced in ISO C99.
All of the `%e', `%f', `%g', `%E', and `%G'
input conversions are interchangeable. They all match an optionally
signed floating point number, in the same syntax as for the
strtod function (see section Parsing of Floats).
For the floating-point input conversions, the default argument type is
float *. (This is different from the corresponding output
conversions, where the default type is double; remember that
float arguments to printf are converted to double
by the default argument promotions, but float * arguments are
not promoted to double *.) You can specify other sizes of float
using these type modifiers:
double *.
long double *.
For all the above number parsing formats there is an additional optional
flag `''. When this flag is given the scanf function
expects the number represented in the input string to be formatted
according to the grouping rules of the currently selected locale
(see section Generic Numeric Formatting Parameters).
If the "C" or "POSIX" locale is selected there is no
difference. But for a locale which specifies values for the appropriate
fields in the locale the input must have the correct form in the input.
Otherwise the longest prefix with a correct form is processed.
This section describes the scanf input conversions for reading
string and character values: `%s', `%S', `%[', `%c',
and `%C'.
You have two options for how to receive the input from these conversions:
char * or wchar_t * (the
latter of the `l' modifier is present).
Warning: To make a robust program, you must make sure that the
input (plus its terminating null) cannot possibly exceed the size of the
buffer you provide. In general, the only way to do this is to specify a
maximum field width one less than the buffer size. If you
provide the buffer, always specify a maximum field width to prevent
overflow.
scanf to allocate a big enough buffer, by specifying the
`a' flag character. This is a GNU extension. You should provide
an argument of type char ** for the buffer address to be stored
in. See section Dynamically Allocating String Conversions.
The `%c' conversion is the simplest: it matches a fixed number of characters, always. The maximum field width says how many characters to read; if you don't specify the maximum, the default is 1. This conversion doesn't append a null character to the end of the text it reads. It also does not skip over initial whitespace characters. It reads precisely the next n characters, and fails if it cannot get that many. Since there is always a maximum field width with `%c' (whether specified, or 1 by default), you can always prevent overflow by making the buffer long enough.
If the format is `%lc' or `%C' the function stores wide
characters which are converted using the conversion determined at the
time the stream was opened from the external byte stream. The number of
bytes read from the medium is limited by MB_CUR_LEN * n but
at most n wide character get stored in the output string.
The `%s' conversion matches a string of non-whitespace characters. It skips and discards initial whitespace, but stops when it encounters more whitespace after having read something. It stores a null character at the end of the text that it reads.
For example, reading the input:
hello, world
with the conversion `%10c' produces " hello, wo", but
reading the same input with the conversion `%10s' produces
"hello,".
Warning: If you do not specify a field width for `%s', then the number of characters read is limited only by where the next whitespace character appears. This almost certainly means that invalid input can make your program crash--which is a bug.
The `%ls' and `%S' format are handled just like `%s'
except that the external byte sequence is converted using the conversion
associated with the stream to wide characters with their own encoding.
A width or precision specified with the format do not directly determine
how many bytes are read from the stream since they measure wide
characters. But an upper limit can be computed by multiplying the value
of the width or precision by MB_CUR_MAX.
To read in characters that belong to an arbitrary set of your choice, use the `%[' conversion. You specify the set between the `[' character and a following `]' character, using the same syntax used in regular expressions. As special cases:
The `%[' conversion does not skip over initial whitespace characters.
Here are some examples of `%[' conversions and what they mean:
As for `%c' and `%s' the `%[' format is also modified to produce wide characters if the `l' modifier is present. All what is said about `%ls' above is true for `%l['.
One more reminder: the `%s' and `%[' conversions are dangerous if you don't specify a maximum width or use the `a' flag, because input too long would overflow whatever buffer you have provided for it. No matter how long your buffer is, a user could supply input that is longer. A well-written program reports invalid input with a comprehensible error message, not with a crash.
A GNU extension to formatted input lets you safely read a string with no
maximum size. Using this feature, you don't supply a buffer; instead,
scanf allocates a buffer big enough to hold the data and gives
you its address. To use this feature, write `a' as a flag
character, as in `%as' or `%a[0-9a-z]'.
The pointer argument you supply for where to store the input should have
type char **. The scanf function allocates a buffer and
stores its address in the word that the argument points to. You should
free the buffer with free when you no longer need it.
Here is an example of using the `a' flag with the `%[...]' conversion specification to read a "variable assignment" of the form `variable = value'.
{
char *variable, *value;
if (2 > scanf ("%a[a-zA-Z0-9] = %a[^\n]\n",
&variable, &value))
{
invalid_input_error ();
return 0;
}
...
}
This section describes the miscellaneous input conversions.
The `%p' conversion is used to read a pointer value. It recognizes
the same syntax used by the `%p' output conversion for
printf (see section Other Output Conversions); that is, a hexadecimal
number just as the `%x' conversion accepts. The corresponding
argument should be of type void **; that is, the address of a
place to store a pointer.
The resulting pointer value is not guaranteed to be valid if it was not originally written during the same program execution that reads it in.
The `%n' conversion produces the number of characters read so far
by this call. The corresponding argument should be of type int *.
This conversion works in the same way as the `%n' conversion for
printf; see section Other Output Conversions, for an example.
The `%n' conversion is the only mechanism for determining the
success of literal matches or conversions with suppressed assignments.
If the `%n' follows the locus of a matching failure, then no value
is stored for it since scanf returns before processing the
`%n'. If you store -1 in that argument slot before calling
scanf, the presence of -1 after scanf indicates an
error occurred before the `%n' was reached.
Finally, the `%%' conversion matches a literal `%' character in the input stream, without using an argument. This conversion does not permit any flags, field width, or type modifier to be specified.
Here are the descriptions of the functions for performing formatted input. Prototypes for these functions are in the header file `stdio.h'.
scanf function reads formatted input from the stream
stdin under the control of the template string template.
The optional arguments are pointers to the places which receive the
resulting values.
The return value is normally the number of successful assignments. If
an end-of-file condition is detected before any matches are performed,
including matches against whitespace and literal characters in the
template, then EOF is returned.
wscanf function reads formatted input from the stream
stdin under the control of the template string template.
The optional arguments are pointers to the places which receive the
resulting values.
The return value is normally the number of successful assignments. If
an end-of-file condition is detected before any matches are performed,
including matches against whitespace and literal characters in the
template, then WEOF is returned.
scanf, except that the input is read
from the stream stream instead of stdin.
wscanf, except that the input is read
from the stream stream instead of stdin.
scanf, except that the characters are taken from the
null-terminated string s instead of from a stream. Reaching the
end of the string is treated as an end-of-file condition.
The behavior of this function is undefined if copying takes place between objects that overlap--for example, if s is also given as an argument to receive a string read under control of the `%s', `%S', or `%[' conversion.
wscanf, except that the characters are taken from the
null-terminated string ws instead of from a stream. Reaching the
end of the string is treated as an end-of-file condition.
The behavior of this function is undefined if copying takes place between objects that overlap--for example, if ws is also given as an argument to receive a string read under control of the `%s', `%S', or `%[' conversion.
The functions vscanf and friends are provided so that you can
define your own variadic scanf-like functions that make use of
the same internals as the built-in formatted output functions.
These functions are analogous to the vprintf series of output
functions. See section Variable Arguments Output Functions, for important
information on how to use them.
Portability Note: The functions listed in this section were introduced in ISO C99 and were before available as GNU extensions.
scanf, but instead of taking
a variable number of arguments directly, it takes an argument list
pointer ap of type va_list (see section Variadic Functions).
wscanf, but instead of taking
a variable number of arguments directly, it takes an argument list
pointer ap of type va_list (see section Variadic Functions).
fscanf with the variable argument list
specified directly as for vscanf.
fwscanf with the variable argument list
specified directly as for vwscanf.
sscanf with the variable argument list
specified directly as for vscanf.
swscanf with the variable argument list
specified directly as for vwscanf.
In GNU C, there is a special construct you can use to let the compiler
know that a function uses a scanf-style format string. Then it
can check the number and types of arguments in each call to the
function, and warn you when they do not match the format string.
For details, See section `Declaring Attributes of Functions' in Using GNU CC.
Many of the functions described in this chapter return the value of the
macro EOF to indicate unsuccessful completion of the operation.
Since EOF is used to report both end of file and random errors,
it's often better to use the feof function to check explicitly
for end of file and ferror to check for errors. These functions
check indicators that are part of the internal state of the stream
object, indicators set if the appropriate condition was detected by a
previous I/O operation on that stream.
EOF is -1. In
other libraries, its value may be some other negative number.
This symbol is declared in `stdio.h'.
WEOF is -1. In
other libraries, its value may be some other negative number.
This symbol is declared in `wchar.h'.
feof function returns nonzero if and only if the end-of-file
indicator for the stream stream is set.
This symbol is declared in `stdio.h'.
feof_unlocked function is equivalent to the feof
function except that it does not implicitly lock the stream.
This function is a GNU extension.
This symbol is declared in `stdio.h'.
ferror function returns nonzero if and only if the error
indicator for the stream stream is set, indicating that an error
has occurred on a previous operation on the stream.
This symbol is declared in `stdio.h'.
ferror_unlocked function is equivalent to the ferror
function except that it does not implicitly lock the stream.
This function is a GNU extension.
This symbol is declared in `stdio.h'.
In addition to setting the error indicator associated with the stream,
the functions that operate on streams also set errno in the same
way as the corresponding low-level functions that operate on file
descriptors. For example, all of the functions that perform output to a
stream--such as fputc, printf, and fflush---are
implemented in terms of write, and all of the errno error
conditions defined for write are meaningful for these functions.
For more information about the descriptor-level I/O functions, see
section Low-Level Input/Output.
You may explicitly clear the error and EOF flags with the clearerr
function.
The file positioning functions (see section File Positioning) also clear the end-of-file indicator for the stream.
clearerr_unlocked function is equivalent to the clearerr
function except that it does not implicitly lock the stream.
This function is a GNU extension.
Note that it is not correct to just clear the error flag and retry a failed stream operation. After a failed write, any number of characters since the last buffer flush may have been committed to the file, while some buffered data may have been discarded. Merely retrying can thus cause lost or repeated data.
A failed read may leave the file pointer in an inappropriate position for a second try. In both cases, you should seek to a known position before retrying.
Most errors that can happen are not recoverable -- a second try will always fail again in the same way. So usually it is best to give up and report the error to the user, rather than install complicated recovery logic.
One important exception is EINTR (see section Primitives Interrupted by Signals).
Many stream I/O implementations will treat it as an ordinary error, which
can be quite inconvenient. You can avoid this hassle by installing all
signals with the SA_RESTART flag.
For similar reasons, setting nonblocking I/O on a stream's file descriptor is not usually advisable.
The GNU system and other POSIX-compatible operating systems organize all files as uniform sequences of characters. However, some other systems make a distinction between files containing text and files containing binary data, and the input and output facilities of ISO C provide for this distinction. This section tells you how to write programs portable to such systems.
When you open a stream, you can specify either a text stream or a
binary stream. You indicate that you want a binary stream by
specifying the `b' modifier in the opentype argument to
fopen; see section Opening Streams. Without this
option, fopen opens the file as a text stream.
Text and binary streams differ in several ways:
'\n') characters, while a binary stream is
simply a long series of characters. A text stream might on some systems
fail to handle lines more than 254 characters long (including the
terminating newline character).
Since a binary stream is always more capable and more predictable than a text stream, you might wonder what purpose text streams serve. Why not simply always use binary streams? The answer is that on these operating systems, text and binary streams use different file formats, and the only way to read or write "an ordinary file of text" that can work with other text-oriented programs is through a text stream.
In the GNU library, and on all POSIX systems, there is no difference between text streams and binary streams. When you open a stream, you get the same kind of stream regardless of whether you ask for binary. This stream can handle any file content, and has none of the restrictions that text streams sometimes have.
The file position of a stream describes where in the file the stream is currently reading or writing. I/O on the stream advances the file position through the file. In the GNU system, the file position is represented as an integer, which counts the number of bytes from the beginning of the file. See section File Position.
During I/O to an ordinary disk file, you can change the file position whenever you wish, so as to read or write any portion of the file. Some other kinds of files may also permit this. Files which support changing the file position are sometimes referred to as random-access files.
You can use the functions in this section to examine or modify the file position indicator associated with a stream. The symbols listed below are declared in the header file `stdio.h'.
This function can fail if the stream doesn't support file positioning,
or if the file position can't be represented in a long int, and
possibly for other reasons as well. If a failure occurs, a value of
-1 is returned.
ftello function is similar to ftell, except that it
returns a value of type off_t. Systems which support this type
use it to describe all file positions, unlike the POSIX specification
which uses a long int. The two are not necessarily the same size.
Therefore, using ftell can lead to problems if the implementation is
written on top of a POSIX compliant low-level I/O implementation, and using
ftello is preferable whenever it is available.
If this function fails it returns (off_t) -1. This can happen due
to missing support for file positioning or internal errors. Otherwise
the return value is the current file position.
The function is an extension defined in the Unix Single Specification version 2.
When the sources are compiled with _FILE_OFFSET_BITS == 64 on a
32 bit system this function is in fact ftello64. I.e., the
LFS interface transparently replaces the old interface.
ftello with the only difference that
the return value is of type off64_t. This also requires that the
stream stream was opened using either fopen64,
freopen64, or tmpfile64 since otherwise the underlying
file operations to position the file pointer beyond the @math{2^31}
bytes limit might fail.
If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32
bits machine this function is available under the name ftello
and so transparently replaces the old interface.
fseek function is used to change the file position of the
stream stream. The value of whence must be one of the
constants SEEK_SET, SEEK_CUR, or SEEK_END, to
indicate whether the offset is relative to the beginning of the
file, the current file position, or the end of the file, respectively.
This function returns a value of zero if the operation was successful,
and a nonzero value to indicate failure. A successful call also clears
the end-of-file indicator of stream and discards any characters
that were "pushed back" by the use of ungetc.
fseek either flushes any buffered output before setting the file
position or else remembers it so it will be written later in its proper
place in the file.
fseek but it corrects a problem with
fseek in a system with POSIX types. Using a value of type
long int for the offset is not compatible with POSIX.
fseeko uses the correct type off_t for the offset
parameter.
For this reason it is a good idea to prefer ftello whenever it is
available since its functionality is (if different at all) closer the
underlying definition.
The functionality and return value is the same as for fseek.
The function is an extension defined in the Unix Single Specification version 2.
When the sources are compiled with _FILE_OFFSET_BITS == 64 on a
32 bit system this function is in fact fseeko64. I.e., the
LFS interface transparently replaces the old interface.
fseeko with the only difference that
the offset parameter is of type off64_t. This also
requires that the stream stream was opened using either
fopen64, freopen64, or tmpfile64 since otherwise
the underlying file operations to position the file pointer beyond the
@math{2^31} bytes limit might fail.
If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32
bits machine this function is available under the name fseeko
and so transparently replaces the old interface.
Portability Note: In non-POSIX systems, ftell,
ftello, fseek and fseeko might work reliably only
on binary streams. See section Text and Binary Streams.
The following symbolic constants are defined for use as the whence
argument to fseek. They are also used with the lseek
function (see section Input and Output Primitives) and to specify offsets for file locks
(see section Control Operations on Files).
fseek or fseeko function, specifies that
the offset provided is relative to the beginning of the file.
fseek or fseeko function, specifies that
the offset provided is relative to the current file position.
fseek or fseeko function, specifies that
the offset provided is relative to the end of the file.
rewind function positions the stream stream at the
beginning of the file. It is equivalent to calling fseek or
fseeko on the stream with an offset argument of
0L and a whence argument of SEEK_SET, except that
the return value is discarded and the error indicator for the stream is
reset.
These three aliases for the `SEEK_...' constants exist for the sake of compatibility with older BSD systems. They are defined in two different header files: `fcntl.h' and `sys/file.h'.
On the GNU system, the file position is truly a character count. You
can specify any character count value as an argument to fseek or
fseeko and get reliable results for any random access file.
However, some ISO C systems do not represent file positions in this
way.
On some systems where text streams truly differ from binary streams, it is impossible to represent the file position of a text stream as a count of characters from the beginning of the file. For example, the file position on some systems must encode both a record offset within the file, and a character offset within the record.
As a consequence, if you want your programs to be portable to these systems, you must observe certain rules:
ftell on a text stream has no predictable
relationship to the number of characters you have read so far. The only
thing you can rely on is that you can use it subsequently as the
offset argument to fseek or fseeko to move back to
the same file position.
fseek or fseeko on a text stream, either the
offset must be zero, or whence must be SEEK_SET and
and the offset must be the result of an earlier call to ftell
on the same stream.
ungetc
that haven't been read or discarded. See section Unreading.
But even if you observe these rules, you may still have trouble for long
files, because ftell and fseek use a long int value
to represent the file position. This type may not have room to encode
all the file positions in a large file. Using the ftello and
fseeko functions might help here since the off_t type is
expected to be able to hold all file position values but this still does
not help to handle additional information which must be associated with
a file position.
So if you do want to support systems with peculiar encodings for the
file positions, it is better to use the functions fgetpos and
fsetpos instead. These functions represent the file position
using the data type fpos_t, whose internal representation varies
from system to system.
These symbols are declared in the header file `stdio.h'.
fgetpos and
fsetpos.
In the GNU system, fpos_t is equivalent to off_t or
long int. In other systems, it might have a different internal
representation.
When compiling with _FILE_OFFSET_BITS == 64 on a 32 bit machine
this type is in fact equivalent to off64_t since the LFS
interface transparently replaced the old interface.
fgetpos64 and
fsetpos64.
In the GNU system, fpos64_t is equivalent to off64_t or
long long int. In other systems, it might have a different internal
representation.
fpos_t object pointed to by
position. If successful, fgetpos returns zero; otherwise
it returns a nonzero value and stores an implementation-defined positive
value in errno.
When the sources are compiled with _FILE_OFFSET_BITS == 64 on a
32 bit system the function is in fact fgetpos64. I.e., the LFS
interface transparently replaced the old interface.
fgetpos but the file position is
returned in a variable of type fpos64_t to which position
points.
If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32
bits machine this function is available under the name fgetpos
and so transparently replaces the old interface.
fgetpos on the same stream. If successful, fsetpos
clears the end-of-file indicator on the stream, discards any characters
that were "pushed back" by the use of ungetc, and returns a value
of zero. Otherwise, fsetpos returns a nonzero value and stores
an implementation-defined positive value in errno.
When the sources are compiled with _FILE_OFFSET_BITS == 64 on a
32 bit system the function is in fact fsetpos64. I.e., the LFS
interface transparently replaced the old interface.
fsetpos but the file position used
for positioning is provided in a variable of type fpos64_t to
which position points.
If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32
bits machine this function is available under the name fsetpos
and so transparently replaces the old interface.
Characters that are written to a stream are normally accumulated and transmitted asynchronously to the file in a block, instead of appearing as soon as they are output by the application program. Similarly, streams often retrieve input from the host environment in blocks rather than on a character-by-character basis. This is called buffering.
If you are writing programs that do interactive input and output using streams, you need to understand how buffering works when you design the user interface to your program. Otherwise, you might find that output (such as progress or prompt messages) doesn't appear when you intended it to, or displays some other unexpected behavior.
This section deals only with controlling when characters are transmitted between the stream and the file or device, and not with how things like echoing, flow control, and the like are handled on specific classes of devices. For information on common control operations on terminal devices, see section Low-Level Terminal Interface.
You can bypass the stream buffering facilities altogether by using the low-level input and output functions that operate on file descriptors instead. See section Low-Level Input/Output.
There are three different kinds of buffering strategies:
Newly opened streams are normally fully buffered, with one exception: a stream connected to an interactive device such as a terminal is initially line buffered. See section Controlling Which Kind of Buffering, for information on how to select a different kind of buffering. Usually the automatic selection gives you the most convenient kind of buffering for the file or device you open.
The use of line buffering for interactive devices implies that output
messages ending in a newline will appear immediately--which is usually
what you want. Output that doesn't end in a newline might or might not
show up immediately, so if you want them to appear immediately, you
should flush buffered output explicitly with fflush, as described
in section Flushing Buffers.
Flushing output on a buffered stream means transmitting all accumulated characters to the file. There are many circumstances when buffered output on a stream is flushed automatically:
exit.
See section Normal Termination.
If you want to flush the buffered output at another time, call
fflush, which is declared in the header file `stdio.h'.
fflush causes buffered output on all open output streams
to be flushed.
This function returns EOF if a write error occurs, or zero
otherwise.
fflush_unlocked function is equivalent to the fflush
function except that it does not implicitly lock the stream.
The fflush function can be used to flush all streams currently
opened. While this is useful in some situations it does often more than
necessary since it might be done in situations when terminal input is
required and the program wants to be sure that all output is visible on
the terminal. But this means that only line buffered streams have to be
flushed. Solaris introduced a function especially for this. It was
always available in the GNU C library in some form but never officially
exported.
_flushlbf function flushes all line buffered streams
currently opened.
This function is declared in the `stdio_ext.h' header.
Compatibility Note: Some brain-damaged operating systems have been known to be so thoroughly fixated on line-oriented input and output that flushing a line buffered stream causes a newline to be written! Fortunately, this "feature" seems to be becoming less common. You do not need to worry about this in the GNU system.
In some situations it might be useful to not flush the output pending for a stream but instead simply forget it. If transmission is costly and the output is not needed anymore this is valid reasoning. In this situation a non-standard function introduced in Solaris and available in the GNU C library can be used.
__fpurge function causes the buffer of the stream
stream to be emptied. If the stream is currently in read mode all
input in the buffer is lost. If the stream is in output mode the
buffered output is not written to the device (or whatever other
underlying storage) and the buffer the cleared.
This function is declared in `stdio_ext.h'.
After opening a stream (but before any other operations have been
performed on it), you can explicitly specify what kind of buffering you
want it to have using the setvbuf function.
The facilities listed in this section are declared in the header file `stdio.h'.
_IOFBF
(for full buffering), _IOLBF (for line buffering), or
_IONBF (for unbuffered input/output).
If you specify a null pointer as the buf argument, then setvbuf
allocates a buffer itself using malloc. This buffer will be freed
when you close the stream.
Otherwise, buf should be a character array that can hold at least
size characters. You should not free the space for this array as
long as the stream remains open and this array remains its buffer. You
should usually either allocate it statically, or malloc
(see section Unconstrained Allocation) the buffer. Using an automatic array
is not a good idea unless you close the file before exiting the block
that declares the array.
While the array remains a stream buffer, the stream I/O functions will use the buffer for their internal purposes. You shouldn't try to access the values in the array directly while the stream is using it for buffering.
The setvbuf function returns zero on success, or a nonzero value
if the value of mode is not valid or if the request could not
be honored.
setvbuf function to
specify that the stream should be fully buffered.
setvbuf function to
specify that the stream should be line buffered.
setvbuf function to
specify that the stream should be unbuffered.
setvbuf. This value is
guaranteed to be at least 256.
The value of BUFSIZ is chosen on each system so as to make stream
I/O efficient. So it is a good idea to use BUFSIZ as the size
for the buffer when you call setvbuf.
Actually, you can get an even better value to use for the buffer size
by means of the fstat system call: it is found in the
st_blksize field of the file attributes. See section The meaning of the File Attributes.
Sometimes people also use BUFSIZ as the allocation size of
buffers used for related purposes, such as strings used to receive a
line of input with fgets (see section Character Input). There is no
particular reason to use BUFSIZ for this instead of any other
integer, except that it might lead to doing I/O in chunks of an
efficient size.
setvbuf with a mode argument of
_IONBF. Otherwise, it is equivalent to calling setvbuf
with buf, and a mode of _IOFBF and a size
argument of BUFSIZ.
The setbuf function is provided for compatibility with old code;
use setvbuf in all new programs.
This function is provided for compatibility with old BSD code. Use
setvbuf instead.
This function is provided for compatibility with old BSD code. Use
setvbuf instead.
It is possible to query whether a given stream is line buffered or not using a non-standard function introduced in Solaris and available in the GNU C library.
__flbf function will return a nonzero value in case the
stream stream is line buffered. Otherwise the return value is
zero.
This function is declared in the `stdio_ext.h' header.
Two more extensions allow to determine the size of the buffer and how much of it is used. These functions were also introduced in Solaris.
__fbufsize function return the size of the buffer in the
stream stream. This value can be used to optimize the use of the
stream.
This function is declared in the `stdio_ext.h' header.
__fpending
This function is declared in the `stdio_ext.h' header.
The GNU library provides ways for you to define additional kinds of streams that do not necessarily correspond to an open file.
One such type of stream takes input from or writes output to a string.
These kinds of streams are used internally to implement the
sprintf and sscanf functions. You can also create such a
stream explicitly, using the functions described in section String Streams.
More generally, you can define streams that do input/output to arbitrary objects using functions supplied by your program. This protocol is discussed in section Programming Your Own Custom Streams.
Portability Note: The facilities described in this section are specific to GNU. Other systems or C implementations might or might not provide equivalent functionality.
The fmemopen and open_memstream functions allow you to do
I/O to a string or memory buffer. These facilities are declared in
`stdio.h'.
If you specify a null pointer as the buf argument, fmemopen
dynamically allocates an array size bytes long (as with malloc;
see section Unconstrained Allocation). This is really only useful
if you are going to write things to the buffer and then read them back
in again, because you have no way of actually getting a pointer to the
buffer (for this, try open_memstream, below). The buffer is
freed when the stream is open.
The argument opentype is the same as in fopen
(see section Opening Streams). If the opentype specifies
append mode, then the initial file position is set to the first null
character in the buffer. Otherwise the initial file position is at the
beginning of the buffer.
When a stream open for writing is flushed or closed, a null character (zero byte) is written at the end of the buffer if it fits. You should add an extra byte to the size argument to account for this. Attempts to write more than size bytes to the buffer result in an error.
For a stream open for reading, null characters (zero bytes) in the buffer do not count as "end of file". Read operations indicate end of file only when the file position advances past size bytes. So, if you want to read characters from a null-terminated string, you should supply the length of the string as the size argument.
Here is an example of using fmemopen to create a stream for
reading from a string:
#include <stdio.h>
static char buffer[] = "foobar";
int
main (void)
{
int ch;
FILE *stream;
stream = fmemopen (buffer, strlen (buffer), "r");
while ((ch = fgetc (stream)) != EOF)
printf ("Got %c\n", ch);
fclose (stream);
return 0;
}
This program produces the following output:
Got f Got o Got o Got b Got a Got r
malloc; see section Unconstrained Allocation) and grown as necessary.
When the stream is closed with fclose or flushed with
fflush, the locations ptr and sizeloc are updated to
contain the pointer to the buffer and its size. The values thus stored
remain valid only as long as no further output on the stream takes
place. If you do more output, you must flush the stream again to store
new values before you use them again.
A null character is written at the end of the buffer. This null character is not included in the size value stored at sizeloc.
You can move the stream's file position with fseek or
fseeko (see section File Positioning). Moving the file position past
the end of the data already written fills the intervening space with
zeroes.
Here is an example of using open_memstream:
#include <stdio.h>
int
main (void)
{
char *bp;
size_t size;
FILE *stream;
stream = open_memstream (&bp, &size);
fprintf (stream, "hello");
fflush (stream);
printf ("buf = `%s', size = %d\n", bp, size);
fprintf (stream, ", world");
fclose (stream);
printf ("buf = `%s', size = %d\n", bp, size);
return 0;
}
This program produces the following output:
buf = `hello', size = 5 buf = `hello, world', size = 12
You can open an output stream that puts it data in an obstack. See section Obstacks.
Calling fflush on this stream updates the current size of the
object to match the amount of data that has been written. After a call
to fflush, you can examine the object temporarily.
You can move the file position of an obstack stream with fseek or
fseeko (see section File Positioning). Moving the file position past
the end of the data written fills the intervening space with zeros.
To make the object permanent, update the obstack with fflush, and
then use obstack_finish to finalize the object and get its address.
The following write to the stream starts a new object in the obstack,
and later writes add to that object until you do another fflush
and obstack_finish.
But how do you find out how long the object is? You can get the length
in bytes by calling obstack_object_size (see section Status of an Obstack), or you can null-terminate the object like this:
obstack_1grow (obstack, 0);
Whichever one you do, you must do it before calling
obstack_finish. (You can do both if you wish.)
Here is a sample function that uses open_obstack_stream:
char *
make_message_string (const char *a, int b)
{
FILE *stream = open_obstack_stream (&message_obstack);
output_task (stream);
fprintf (stream, ": ");
fprintf (stream, a, b);
fprintf (stream, "\n");
fclose (stream);
obstack_1grow (&message_obstack, 0);
return obstack_finish (&message_obstack);
}
This section describes how you can make a stream that gets input from an arbitrary data source or writes output to an arbitrary data sink programmed by you. We call these custom streams. The functions and types described here are all GNU extensions.
Inside every custom stream is a special object called the cookie.
This is an object supplied by you which records where to fetch or store
the data read or written. It is up to you to define a data type to use
for the cookie. The stream functions in the library never refer
directly to its contents, and they don't even know what the type is;
they record its address with type void *.
To implement a custom stream, you must specify how to fetch or store the data in the specified place. You do this by defining hook functions to read, write, change "file position", and close the stream. All four of these functions will be passed the stream's cookie so they can tell where to fetch or store the data. The library functions don't know what's inside the cookie, but your functions will know.
When you create a custom stream, you must specify the cookie pointer,
and also the four hook functions stored in a structure of type
cookie_io_functions_t.
These facilities are declared in `stdio.h'.
cookie_read_function_t *read
EOF.
cookie_write_function_t *write
cookie_seek_function_t *seek
fseek or fseeko on this stream can only seek to
locations within the buffer; any attempt to seek outside the buffer will
return an ESPIPE error.
cookie_close_function_t *close
fopen;
see section Opening Streams. (But note that the "truncate on
open" option is ignored.) The new stream is fully buffered.
The fopencookie function returns the newly created stream, or a null
pointer in case of an error.
Here are more details on how you should define the four hook functions that a custom stream needs.
You should define the function to read data from the cookie as:
ssize_t reader (void *cookie, char *buffer, size_t size)
This is very similar to the read function; see section Input and Output Primitives. Your function should transfer up to size bytes into
the buffer, and return the number of bytes read, or zero to
indicate end-of-file. You can return a value of -1 to indicate
an error.
You should define the function to write data to the cookie as:
ssize_t writer (void *cookie, const char *buffer, size_t size)
This is very similar to the write function; see section Input and Output Primitives. Your function should transfer up to size bytes from
the buffer, and return the number of bytes written. You can return a
value of -1 to indicate an error.
You should define the function to perform seek operations on the cookie as:
int seeker (void *cookie, fpos_t *position, int whence)
For this function, the position and whence arguments are
interpreted as for fgetpos; see section Portable File-Position Functions. In
the GNU library, fpos_t is equivalent to off_t or
long int, and simply represents the number of bytes from the
beginning of the file.
After doing the seek operation, your function should store the resulting
file position relative to the beginning of the file in position.
Your function should return a value of 0 on success and -1
to indicate an error.
You should define the function to do cleanup operations on the cookie appropriate for closing the stream as:
int cleaner (void *cookie)
Your function should return -1 to indicate an error, and 0
otherwise.
On systems which are based on System V messages of programs (especially
the system tools) are printed in a strict form using the fmtmsg
function. The uniformity sometimes helps the user to interpret messages
and the strictness tests of the fmtmsg function ensure that the
programmer follows some minimal requirements.
Messages can be printed to standard error and/or to the console. To
select the destination the programmer can use the following two values,
bitwise OR combined if wanted, for the classification parameter of
fmtmsg:
MM_PRINT
MM_CONSOLE
The erroneous piece of the system can be signalled by exactly one of the
following values which also is bitwise ORed with the
classification parameter to fmtmsg:
MM_HARD
MM_SOFT
MM_FIRM
A third component of the classification parameter to fmtmsg
can describe the part of the system which detects the problem. This is
done by using exactly one of the following values:
MM_APPL
MM_UTIL
MM_OPSYS
A last component of classification can signal the results of this message. Exactly one of the following values can be used:
Each of the parameters can be a special value which means this value is to be omitted. The symbolic names for these values are:
MM_NULLLBL
MM_NULLSEV
MM_NULLMC
MM_NULLTXT
MM_NULLACT
MM_NULLTAG
There is another way certain fields can be omitted from the output to standard error. This is described below in the description of environment variables influencing the behaviour.
The severity parameter can have one of the values in the following table:
MM_NOSEV
MM_NULLSEV.
MM_HALT
HALT.
MM_ERROR
ERROR.
MM_WARNING
WARNING.
MM_INFO
INFO.
The numeric value of these five macros are between 0 and
4. Using the environment variable SEV_LEVEL or using the
addseverity function one can add more severity levels with their
corresponding string to print. This is described below
(see section Adding Severity Classes).
If no parameter is ignored the output looks like this:
label: severity-string: text TO FIX: action tag
The colons, new line characters and the TO FIX string are
inserted if necessary, i.e., if the corresponding parameter is not
ignored.
This function is specified in the X/Open Portability Guide. It is also available on all systems derived from System V.
The function returns the value MM_OK if no error occurred. If
only the printing to standard error failed, it returns MM_NOMSG.
If printing to the console fails, it returns MM_NOCON. If
nothing is printed MM_NOTOK is returned. Among situations where
all outputs fail this last value is also returned if a parameter value
is incorrect.
There are two environment variables which influence the behaviour of
fmtmsg. The first is MSGVERB. It is used to control the
output actually happening on standard error (not the console
output). Each of the five fields can explicitly be enabled. To do
this the user has to put the MSGVERB variable with a format like
the following in the environment before calling the fmtmsg function
the first time:
MSGVERB=keyword[:keyword[:...]]
Valid keywords are label, severity, text,
action, and tag. If the environment variable is not given
or is the empty string, a not supported keyword is given or the value is
somehow else invalid, no part of the message is masked out.
The second environment variable which influences the behaviour of
fmtmsg is SEV_LEVEL. This variable and the change in the
behaviour of fmtmsg is not specified in the X/Open Portability
Guide. It is available in System V systems, though. It can be used to
introduce new severity levels. By default, only the five severity levels
described above are available. Any other numeric value would make
fmtmsg print nothing.
If the user puts SEV_LEVEL with a format like
SEV_LEVEL=[description[:description[:...]]]
in the environment of the process before the first call to
fmtmsg, where description has a value of the form
severity-keyword,level,printstring
The severity-keyword part is not used by fmtmsg but it has
to be present. The level part is a string representation of a
number. The numeric value must be a number greater than 4. This value
must be used in the severity parameter of fmtmsg to select
this class. It is not possible to overwrite any of the predefined
classes. The printstring is the string printed when a message of
this class is processed by fmtmsg (see above, fmtsmg does
not print the numeric value but instead the string representation).
There is another possibility to introduce severity classes besides using
the environment variable SEV_LEVEL. This simplifies the task of
introducing new classes in a running program. One could use the
setenv or putenv function to set the environment variable,
but this is toilsome.
fmtmsg function.
The severity parameter of addseverity must match the value
for the parameter with the same name of fmtmsg, and string
is the string printed in the actual messages instead of the numeric
value.
If string is NULL the severity class with the numeric value
according to severity is removed.
It is not possible to overwrite or remove one of the default severity
classes. All calls to addseverity with severity set to one
of the values for the default classes will fail.
The return value is MM_OK if the task was successfully performed.
If the return value is MM_NOTOK something went wrong. This could
mean that no more memory is available or a class is not available when
it has to be removed.
This function is not specified in the X/Open Portability Guide although
the fmtsmg function is. It is available on System V systems.
fmtmsg and addseverityHere is a simple example program to illustrate the use of the both functions described in this section.
#include <fmtmsg.h>
int
main (void)
{
addseverity (5, "NOTE2");
fmtmsg (MM_PRINT, "only1field", MM_INFO, "text2", "action2", "tag2");
fmtmsg (MM_PRINT, "UX:cat", 5, "invalid syntax", "refer to manual",
"UX:cat:001");
fmtmsg (MM_PRINT, "label:foo", 6, "text", "action", "tag");
return 0;
}
The second call to fmtmsg illustrates a use of this function as
it usually occurs on System V systems, which heavily use this function.
It seems worthwhile to give a short explanation here of how this system
works on System V. The value of the
label field (UX:cat) says that the error occured in the
Unix program cat. The explanation of the error follows and the
value for the action parameter is "refer to manual". One
could be more specific here, if necessary. The tag field contains,
as proposed above, the value of the string given for the label
parameter, and additionally a unique ID (001 in this case). For
a GNU environment this string could contain a reference to the
corresponding node in the Info page for the program.
Running this program without specifying the MSGVERB and
SEV_LEVEL function produces the following output:
UX:cat: NOTE2: invalid syntax TO FIX: refer to manual UX:cat:001
We see the different fields of the message and how the extra glue (the
colons and the TO FIX string) are printed. But only one of the
three calls to fmtmsg produced output. The first call does not
print anything because the label parameter is not in the correct
form. The string must contain two fields, separated by a colon
(see section Printing Formatted Messages). The third fmtmsg call
produced no output since the class with the numeric value 6 is
not defined. Although a class with numeric value 5 is also not
defined by default, the call to addseverity introduces it and
the second call to fmtmsg produces the above output.
When we change the environment of the program to contain
SEV_LEVEL=XXX,6,NOTE when running it we get a different result:
UX:cat: NOTE2: invalid syntax TO FIX: refer to manual UX:cat:001 label:foo: NOTE: text TO FIX: action tag
Now the third call to fmtmsg produced some output and we see how
the string NOTE from the environment variable appears in the
message.
Now we can reduce the output by specifying which fields we are
interested in. If we additionally set the environment variable
MSGVERB to the value severity:label:action we get the
following output:
UX:cat: NOTE2 TO FIX: refer to manual label:foo: NOTE TO FIX: action
I.e., the output produced by the text and the tag parameters
to fmtmsg vanished. Please also note that now there is no colon
after the NOTE and NOTE2 strings in the output. This is
not necessary since there is no more output on this line because the text
is missing.
This chapter describes functions for performing low-level input/output operations on file descriptors. These functions include the primitives for the higher-level I/O functions described in section Input/Output on Streams, as well as functions for performing low-level control operations for which there are no equivalents on streams.
Stream-level I/O is more flexible and usually more convenient; therefore, programmers generally use the descriptor-level functions only when necessary. These are some of the usual reasons:
fileno to get the descriptor
corresponding to a stream.)
This section describes the primitives for opening and closing files
using file descriptors. The open and creat functions are
declared in the header file `fcntl.h', while close is
declared in `unistd.h'.
open function creates and returns a new file descriptor
for the file named by filename. Initially, the file position
indicator for the file is at the beginning of the file. The argument
mode is used only when a file is created, but it doesn't hurt
to supply the argument in any case.
The flags argument controls how the file is to be opened. This is a bit mask; you create the value by the bitwise OR of the appropriate parameters (using the `|' operator in C). See section File Status Flags, for the parameters available.
The normal return value from open is a non-negative integer file
descriptor. In the case of an error, a value of @math{-1} is returned
instead. In addition to the usual file name errors (see section File Name Errors), the following errno error conditions are defined
for this function:
EACCES
EEXIST
O_CREAT and O_EXCL are set, and the named file already
exists.
EINTR
open operation was interrupted by a signal.
See section Primitives Interrupted by Signals.
EISDIR
EMFILE
RLIMIT_NOFILE resource limit; see section Limiting Resource Usage.
ENFILE
ENOENT
O_CREAT is not specified.
ENOSPC
ENXIO
O_NONBLOCK and O_WRONLY are both set in the flags
argument, the file named by filename is a FIFO (see section Pipes and FIFOs), and no process has the file open for reading.
EROFS
O_WRONLY,
O_RDWR, and O_TRUNC are set in the flags argument,
or O_CREAT is set and the file does not already exist.
If on a 32 bit machine the sources are translated with
_FILE_OFFSET_BITS == 64 the function open returns a file
descriptor opened in the large file mode which enables the file handling
functions to use files up to @math{2^63} bytes in size and offset from
@math{-2^63} to @math{2^63}. This happens transparently for the user
since all of the lowlevel file handling functions are equally replaced.
This function is a cancellation point in multi-threaded programs. This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time open is
called. If the thread gets cancelled these resources stay allocated
until the program ends. To avoid this calls to open should be
protected using cancellation handlers.
The open function is the underlying primitive for the fopen
and freopen functions, that create streams.
open. It returns a file descriptor
which can be used to access the file named by filename. The only
difference is that on 32 bit systems the file is opened in the
large file mode. I.e., file length and file offsets can exceed 31 bits.
When the sources are translated with _FILE_OFFSET_BITS == 64 this
function is actually available under the name open. I.e., the
new, extended API using 64 bit file sizes and offsets transparently
replaces the old API.
creat (filename, mode)
is equivalent to:
open (filename, O_WRONLY | O_CREAT | O_TRUNC, mode)
If on a 32 bit machine the sources are translated with
_FILE_OFFSET_BITS == 64 the function creat returns a file
descriptor opened in the large file mode which enables the file handling
functions to use files up to @math{2^63} in size and offset from
@math{-2^63} to @math{2^63}. This happens transparently for the user
since all of the lowlevel file handling functions are equally replaced.
creat. It returns a file descriptor
which can be used to access the file named by filename. The only
the difference is that on 32 bit systems the file is opened in the
large file mode. I.e., file length and file offsets can exceed 31 bits.
To use this file descriptor one must not use the normal operations but
instead the counterparts named *64, e.g., read64.
When the sources are translated with _FILE_OFFSET_BITS == 64 this
function is actually available under the name open. I.e., the
new, extended API using 64 bit file sizes and offsets transparently
replaces the old API.
close closes the file descriptor filedes.
Closing a file has the following consequences:
This function is a cancellation point in multi-threaded programs. This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time close is
called. If the thread gets cancelled these resources stay allocated
until the program ends. To avoid this, calls to close should be
protected using cancellation handlers.
The normal return value from close is @math{0}; a value of @math{-1}
is returned in case of failure. The following errno error
conditions are defined for this function:
EBADF
EINTR
close call was interrupted by a signal.
See section Primitives Interrupted by Signals.
Here is an example of how to handle EINTR properly:
TEMP_FAILURE_RETRY (close (desc));
ENOSPC
EIO
EDQUOT
write can sometimes
not be detected until close. See section Input and Output Primitives, for details
on their meaning.
Please note that there is no separate close64 function.
This is not necessary since this function does not determine nor depend
on the mode of the file. The kernel which performs the close
operation knows which mode the descriptor is used for and can handle
this situation.
To close a stream, call fclose (see section Closing Streams) instead
of trying to close its underlying file descriptor with close.
This flushes any buffered output and updates the stream object to
indicate that it is closed.
This section describes the functions for performing primitive input and
output operations on file descriptors: read, write, and
lseek. These functions are declared in the header file
`unistd.h'.
size_t,
but must be a signed type.
read function reads up to size bytes from the file
with descriptor filedes, storing the results in the buffer.
(This is not necessarily a character string, and no terminating null
character is added.)
The return value is the number of bytes actually read. This might be less than size; for example, if there aren't that many bytes left in the file or if there aren't that many bytes immediately available. The exact behavior depends on what kind of file it is. Note that reading less than size bytes is not an error.
A value of zero indicates end-of-file (except if the value of the
size argument is also zero). This is not considered an error.
If you keep calling read while at end-of-file, it will keep
returning zero and doing nothing else.
If read returns at least one character, there is no way you can
tell whether end-of-file was reached. But if you did reach the end, the
next read will return zero.
In case of an error, read returns @math{-1}. The following
errno error conditions are defined for this function:
EAGAIN
read waits for
some input. But if the O_NONBLOCK flag is set for the file
(see section File Status Flags), read returns immediately without
reading any data, and reports this error.
Compatibility Note: Most versions of BSD Unix use a different
error code for this: EWOULDBLOCK. In the GNU library,
EWOULDBLOCK is an alias for EAGAIN, so it doesn't matter
which name you use.
On some systems, reading a large amount of data from a character special
file can also fail with EAGAIN if the kernel cannot find enough
physical memory to lock down the user's pages. This is limited to
devices that transfer with direct memory access into the user's memory,
which means it does not include terminals, since they always use
separate buffers inside the kernel. This problem never happens in the
GNU system.
Any condition that could result in EAGAIN can instead result in a
successful read which returns fewer bytes than requested.
Calling read again immediately would result in EAGAIN.
EBADF
EINTR
read was interrupted by a signal while it was waiting for input.
See section Primitives Interrupted by Signals. A signal will not necessary cause
read to return EINTR; it may instead result in a
successful read which returns fewer bytes than requested.
EIO
EIO also occurs when a background process tries to read from the
controlling terminal, and the normal action of stopping the process by
sending it a SIGTTIN signal isn't working. This might happen if
the signal is being blocked or ignored, or because the process group is
orphaned. See section Job Control, for more information about job control,
and section Signal Handling, for information about signals.
Please note that there is no function named read64. This is not
necessary since this function does not directly modify or handle the
possibly wide file offset. Since the kernel handles this state
internally, the read function can be used for all cases.
This function is a cancellation point in multi-threaded programs. This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time read is
called. If the thread gets cancelled these resources stay allocated
until the program ends. To avoid this, calls to read should be
protected using cancellation handlers.
The read function is the underlying primitive for all of the
functions that read from streams, such as fgetc.
pread function is similar to the read function. The
first three arguments are identical, and the return values and error
codes also correspond.
The difference is the fourth argument and its handling. The data block
is not read from the current position of the file descriptor
filedes. Instead the data is read from the file starting at
position offset. The position of the file descriptor itself is
not affected by the operation. The value is the same as before the call.
When the source file is compiled with _FILE_OFFSET_BITS == 64 the
pread function is in fact pread64 and the type
off_t has 64 bits, which makes it possible to handle files up to
@math{2^63} bytes in length.
The return value of pread describes the number of bytes read.
In the error case it returns @math{-1} like read does and the
error codes are also the same, with these additions:
EINVAL
ESPIPE
The function is an extension defined in the Unix Single Specification version 2.
pread function. The difference
is that the offset parameter is of type off64_t instead of
off_t which makes it possible on 32 bit machines to address
files larger than @math{2^31} bytes and up to @math{2^63} bytes. The
file descriptor filedes must be opened using open64 since
otherwise the large offsets possible with off64_t will lead to
errors with a descriptor in small file mode.
When the source file is compiled with _FILE_OFFSET_BITS == 64 on a
32 bit machine this function is actually available under the name
pread and so transparently replaces the 32 bit interface.
write function writes up to size bytes from
buffer to the file with descriptor filedes. The data in
buffer is not necessarily a character string and a null character is
output like any other character.
The return value is the number of bytes actually written. This may be
size, but can always be smaller. Your program should always call
write in a loop, iterating until all the data is written.
Once write returns, the data is enqueued to be written and can be
read back right away, but it is not necessarily written out to permanent
storage immediately. You can use fsync when you need to be sure
your data has been permanently stored before continuing. (It is more
efficient for the system to batch up consecutive writes and do them all
at once when convenient. Normally they will always be written to disk
within a minute or less.) Modern systems provide another function
fdatasync which guarantees integrity only for the file data and
is therefore faster.
You can use the O_FSYNC open mode to make write always
store the data to disk before returning; see section I/O Operating Modes.
In the case of an error, write returns @math{-1}. The following
errno error conditions are defined for this function:
EAGAIN
write blocks until the write operation is complete.
But if the O_NONBLOCK flag is set for the file (see section Control Operations on Files), it returns immediately without writing any data and
reports this error. An example of a situation that might cause the
process to block on output is writing to a terminal device that supports
flow control, where output has been suspended by receipt of a STOP
character.
Compatibility Note: Most versions of BSD Unix use a different
error code for this: EWOULDBLOCK. In the GNU library,
EWOULDBLOCK is an alias for EAGAIN, so it doesn't matter
which name you use.
On some systems, writing a large amount of data from a character special
file can also fail with EAGAIN if the kernel cannot find enough
physical memory to lock down the user's pages. This is limited to
devices that transfer with direct memory access into the user's memory,
which means it does not include terminals, since they always use
separate buffers inside the kernel. This problem does not arise in the
GNU system.
EBADF
EFBIG
EINTR
write operation was interrupted by a signal while it was
blocked waiting for completion. A signal will not necessarily cause
write to return EINTR; it may instead result in a
successful write which writes fewer bytes than requested.
See section Primitives Interrupted by Signals.
EIO
ENOSPC
EPIPE
SIGPIPE
signal is also sent to the process; see section Signal Handling.
Unless you have arranged to prevent EINTR failures, you should
check errno after each failing call to write, and if the
error was EINTR, you should simply repeat the call.
See section Primitives Interrupted by Signals. The easy way to do this is with the
macro TEMP_FAILURE_RETRY, as follows:
nbytes = TEMP_FAILURE_RETRY (write (desc, buffer, count));
Please note that there is no function named write64. This is not
necessary since this function does not directly modify or handle the
possibly wide file offset. Since the kernel handles this state
internally the write function can be used for all cases.
This function is a cancellation point in multi-threaded programs. This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time write is
called. If the thread gets cancelled these resources stay allocated
until the program ends. To avoid this, calls to write should be
protected using cancellation handlers.
The write function is the underlying primitive for all of the
functions that write to streams, such as fputc.
pwrite function is similar to the write function. The
first three arguments are identical, and the return values and error codes
also correspond.
The difference is the fourth argument and its handling. The data block
is not written to the current position of the file descriptor
filedes. Instead the data is written to the file starting at
position offset. The position of the file descriptor itself is
not affected by the operation. The value is the same as before the call.
When the source file is compiled with _FILE_OFFSET_BITS == 64 the
pwrite function is in fact pwrite64 and the type
off_t has 64 bits, which makes it possible to handle files up to
@math{2^63} bytes in length.
The return value of pwrite describes the number of written bytes.
In the error case it returns @math{-1} like write does and the
error codes are also the same, with these additions:
EINVAL
ESPIPE
The function is an extension defined in the Unix Single Specification version 2.
pwrite function. The difference
is that the offset parameter is of type off64_t instead of
off_t which makes it possible on 32 bit machines to address
files larger than @math{2^31} bytes and up to @math{2^63} bytes. The
file descriptor filedes must be opened using open64 since
otherwise the large offsets possible with off64_t will lead to
errors with a descriptor in small file mode.
When the source file is compiled using _FILE_OFFSET_BITS == 64 on a
32 bit machine this function is actually available under the name
pwrite and so transparently replaces the 32 bit interface.
Just as you can set the file position of a stream with fseek, you
can set the file position of a descriptor with lseek. This
specifies the position in the file for the next read or
write operation. See section File Positioning, for more information
on the file position and what it means.
To read the current file position value from a descriptor, use
lseek (desc, 0, SEEK_CUR).
lseek function is used to change the file position of the
file with descriptor filedes.
The whence argument specifies how the offset should be
interpreted, in the same way as for the fseek function, and it must
be one of the symbolic constants SEEK_SET, SEEK_CUR, or
SEEK_END.
SEEK_SET
SEEK_CUR
SEEK_END
The return value from lseek is normally the resulting file
position, measured in bytes from the beginning of the file.
You can use this feature together with SEEK_CUR to read the
current file position.
If you want to append to the file, setting the file position to the
current end of file with SEEK_END is not sufficient. Another
process may write more data after you seek but before you write,
extending the file so the position you write onto clobbers their data.
Instead, use the O_APPEND operating mode; see section I/O Operating Modes.
You can set the file position past the current end of the file. This
does not by itself make the file longer; lseek never changes the
file. But subsequent output at that position will extend the file.
Characters between the previous end of file and the new position are
filled with zeros. Extending the file in this way can create a
"hole": the blocks of zeros are not actually allocated on disk, so the
file takes up less space than it appears to; it is then called a
"sparse file".
If the file position cannot be changed, or the operation is in some way
invalid, lseek returns a value of @math{-1}. The following
errno error conditions are defined for this function:
EBADF
EINVAL
ESPIPE
ESPIPE if the object is not seekable.)
When the source file is compiled with _FILE_OFFSET_BITS == 64 the
lseek function is in fact lseek64 and the type
off_t has 64 bits which makes it possible to handle files up to
@math{2^63} bytes in length.
This function is a cancellation point in multi-threaded programs. This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time lseek is
called. If the thread gets cancelled these resources stay allocated
until the program ends. To avoid this calls to lseek should be
protected using cancellation handlers.
The lseek function is the underlying primitive for the
fseek, fseeko, ftell, ftello and
rewind functions, which operate on streams instead of file
descriptors.
lseek function. The difference
is that the offset parameter is of type off64_t instead of
off_t which makes it possible on 32 bit machines to address
files larger than @math{2^31} bytes and up to @math{2^63} bytes. The
file descriptor filedes must be opened using open64 since
otherwise the large offsets possible with off64_t will lead to
errors with a descriptor in small file mode.
When the source file is compiled with _FILE_OFFSET_BITS == 64 on a
32 bits machine this function is actually available under the name
lseek and so transparently replaces the 32 bit interface.
You can have multiple descriptors for the same file if you open the file
more than once, or if you duplicate a descriptor with dup.
Descriptors that come from separate calls to open have independent
file positions; using lseek on one descriptor has no effect on the
other. For example,
{
int d1, d2;
char buf[4];
d1 = open ("foo", O_RDONLY);
d2 = open ("foo", O_RDONLY);
lseek (d1, 1024, SEEK_SET);
read (d2, buf, 4);
}
will read the first four characters of the file `foo'. (The error-checking code necessary for a real program has been omitted here for brevity.)
By contrast, descriptors made by duplication share a common file position with the original descriptor that was duplicated. Anything which alters the file position of one of the duplicates, including reading or writing data, affects all of them alike. Thus, for example,
{
int d1, d2, d3;
char buf1[4], buf2[4];
d1 = open ("foo", O_RDONLY);
d2 = dup (d1);
d3 = dup (d2);
lseek (d3, 1024, SEEK_SET);
read (d1, buf1, 4);
read (d2, buf2, 4);
}
will read four characters starting with the 1024'th character of `foo', and then four more characters starting with the 1028'th character.
fpos_t or long int.
If the source is compiled with _FILE_OFFSET_BITS == 64 this type
is transparently replaced by off64_t.
off_t. The difference is that even
on 32 bit machines, where the off_t type would have 32 bits,
off64_t has 64 bits and so is able to address files up to
@math{2^63} bytes in length.
When compiling with _FILE_OFFSET_BITS == 64 this type is
available under the name off_t.
These aliases for the `SEEK_...' constants exist for the sake of compatibility with older BSD systems. They are defined in two different header files: `fcntl.h' and `sys/file.h'.
L_SET
SEEK_SET.
L_INCR
SEEK_CUR.
L_XTND
SEEK_END.
Given an open file descriptor, you can create a stream for it with the
fdopen function. You can get the underlying file descriptor for
an existing stream with the fileno function. These functions are
declared in the header file `stdio.h'.
fdopen function returns a new stream for the file descriptor
filedes.
The opentype argument is interpreted in the same way as for the
fopen function (see section Opening Streams), except that
the `b' option is not permitted; this is because GNU makes no
distinction between text and binary files. Also, "w" and
"w+" do not cause truncation of the file; these have an effect only
when opening a file, and in this case the file has already been opened.
You must make sure that the opentype argument matches the actual
mode of the open file descriptor.
The return value is the new stream. If the stream cannot be created (for example, if the modes for the file indicated by the file descriptor do not permit the access specified by the opentype argument), a null pointer is returned instead.
In some other systems, fdopen may fail to detect that the modes
for file descriptor do not permit the access specified by
opentype. The GNU C library always checks for this.
For an example showing the use of the fdopen function,
see section Creating a Pipe.
fileno returns @math{-1}.
fileno_unlocked function is equivalent to the fileno
function except that it does not implicitly lock the stream if the state
is FSETLOCKING_INTERNAL.
This function is a GNU extension.
There are also symbolic constants defined in `unistd.h' for the
file descriptors belonging to the standard streams stdin,
stdout, and stderr; see section Standard Streams.
STDIN_FILENO
0, which is the file descriptor for
standard input.
STDOUT_FILENO
1, which is the file descriptor for
standard output.
STDERR_FILENO
2, which is the file descriptor for
standard error output.
You can have multiple file descriptors and streams (let's call both streams and descriptors "channels" for short) connected to the same file, but you must take care to avoid confusion between channels. There are two cases to consider: linked channels that share a single file position value, and independent channels that have their own file positions.
It's best to use just one channel in your program for actual data
transfer to any given file, except when all the access is for input.
For example, if you open a pipe (something you can only do at the file
descriptor level), either do all I/O with the descriptor, or construct a
stream from the descriptor with fdopen and then do all I/O with
the stream.
Channels that come from a single opening share the same file position;
we call them linked channels. Linked channels result when you
make a stream from a descriptor using fdopen, when you get a
descriptor from a stream with fileno, when you copy a descriptor
with dup or dup2, and when descriptors are inherited
during fork. For files that don't support random access, such as
terminals and pipes, all channels are effectively linked. On
random-access files, all append-type output streams are effectively
linked to each other.
If you have been using a stream for I/O, and you want to do I/O using another channel (either a stream or a descriptor) that is linked to it, you must first clean up the stream that you have been using. See section Cleaning Streams.
Terminating a process, or executing a new program in the process, destroys all the streams in the process. If descriptors linked to these streams persist in other processes, their file positions become undefined as a result. To prevent this, you must clean up the streams before destroying them.
When you open channels (streams or descriptors) separately on a seekable file, each channel has its own file position. These are called independent channels.
The system handles each channel independently. Most of the time, this is quite predictable and natural (especially for input): each channel can read or write sequentially at its own place in the file. However, if some of the channels are streams, you must take these precautions:
If you do output to one channel at the end of the file, this will certainly leave the other independent channels positioned somewhere before the new end. You cannot reliably set their file positions to the new end of file before writing, because the file can always be extended by another process between when you set the file position and when you write the data. Instead, use an append-type descriptor or stream; they always output at the current end of the file. In order to make the end-of-file position accurate, you must clean the output channel you were using, if it is a stream.
It's impossible for two channels to have separate file pointers for a file that doesn't support random access. Thus, channels for reading or writing such files are always linked, never independent. Append-type channels are also always linked. For these channels, follow the rules for linked channels; see section Linked Channels.
On the GNU system, you can clean up any stream with fclean:
On other systems, you can use fflush to clean a stream in most
cases.
You can skip the fclean or fflush if you know the stream
is already clean. A stream is clean whenever its buffer is empty. For
example, an unbuffered stream is always clean. An input stream that is
at end-of-file is clean. A line-buffered stream is clean when the last
character output was a newline.
There is one case in which cleaning a stream is impossible on most
systems. This is when the stream is doing input from a file that is not
random-access. Such streams typically read ahead, and when the file is
not random access, there is no way to give back the excess data already
read. When an input stream reads from a random-access file,
fflush does clean the stream, but leaves the file pointer at an
unpredictable place; you must set the file pointer before doing any
further I/O. On the GNU system, using fclean avoids both of
these problems.
Closing an output-only stream also does fflush, so this is a
valid way of cleaning an output stream. On the GNU system, closing an
input stream does fclean.
You need not clean a stream before using its descriptor for control operations such as setting terminal modes; these operations don't affect the file position and are not affected by it. You can use any descriptor for these operations, and all channels are affected simultaneously. However, text already "output" to a stream but still buffered by the stream will be subject to the new terminal modes when subsequently flushed. To make sure "past" output is covered by the terminal settings that were in effect at the time, flush the output streams for that terminal before setting the modes. See section Terminal Modes.
Some applications may need to read or write data to multiple buffers,
which are separated in memory. Although this can be done easily enough
with multiple calls to read and write, it is inefficent
because there is overhead associated with each kernel call.
Instead, many platforms provide special high-speed primitives to perform
these scatter-gather operations in a single kernel call. The GNU C
library will provide an emulation on any system that lacks these
primitives, so they are not a portability threat. They are defined in
sys/uio.h.
These functions are controlled with arrays of iovec structures,
which describe the location and size of each buffer.
The iovec structure describes a buffer. It contains two fields:
void *iov_base
size_t iov_len
The readv function reads data from filedes and scatters it
into the buffers described in vector, which is taken to be
count structures long. As each buffer is filled, data is sent to the
next.
Note that readv is not guaranteed to fill all the buffers.
It may stop at any point, for the same reasons read would.
The return value is a count of bytes (not buffers) read, @math{0}
indicating end-of-file, or @math{-1} indicating an error. The possible
errors are the same as in read.
The writev function gathers data from the buffers described in
vector, which is taken to be count structures long, and writes
them to filedes. As each buffer is written, it moves on to the
next.
Like readv, writev may stop midstream under the same
conditions write would.
The return value is a count of bytes written, or @math{-1} indicating an
error. The possible errors are the same as in write.
Note that if the buffers are small (under about 1kB), high-level streams
may be easier to use than these functions. However, readv and
writev are more efficient when the individual buffers themselves
(as opposed to the total output), are large. In that case, a high-level
stream would not be able to cache the data effectively.
On modern operating systems, it is possible to mmap (pronounced "em-map") a file to a region of memory. When this is done, the file can be accessed just like an array in the program.
This is more efficent than read or write, as only the regions
of the file that a program actually accesses are loaded. Accesses to
not-yet-loaded parts of the mmapped region are handled in the same way as
swapped out pages.
Since mmapped pages can be stored back to their file when physical memory is low, it is possible to mmap files orders of magnitude larger than both the physical memory and swap space. The only limit is address space. The theoretical limit is 4GB on a 32-bit machine - however, the actual limit will be smaller since some areas will be reserved for other purposes. If the LFS interface is used the file size on 32-bit systems is not limited to 2GB (offsets are signed which reduces the addressable area of 4GB by half); the full 64-bit are available.
Memory mapping only works on entire pages of memory. Thus, addresses for mapping must be page-aligned, and length values will be rounded up. To determine the size of a page the machine uses one should use
size_t page_size = (size_t) sysconf (_SC_PAGESIZE);
These functions are declared in `sys/mman.h'.
The mmap function creates a new mapping, connected to bytes
(offset) to (offset + length) in the file open on
filedes.
address gives a preferred starting address for the mapping.
NULL expresses no preference. Any previous mapping at that
address is automatically removed. The address you give may still be
changed, unless you use the MAP_FIXED flag.
protect contains flags that control what kind of access is
permitted. They include PROT_READ, PROT_WRITE, and
PROT_EXEC, which permit reading, writing, and execution,
respectively. Inappropriate access will cause a segfault (see section Program Error Signals).
Note that most hardware designs cannot support write permission without
read permission, and many do not distinguish read and execute permission.
Thus, you may receive wider permissions than you ask for, and mappings of
write-only files may be denied even if you do not use PROT_READ.
flags contains flags that control the nature of the map.
One of MAP_SHARED or MAP_PRIVATE must be specified.
They include:
MAP_PRIVATE
PROT_WRITE.
MAP_SHARED
msync, described below, if it is important that other processes
using conventional I/O get a consistent view of the file.
MAP_FIXED
MAP_ANONYMOUS
MAP_ANON
malloc for large blocks. This is not an issue with the GNU C library,
as the included malloc automatically uses mmap where appropriate.
mmap returns the address of the new mapping, or @math{-1} for an
error.
Possible errors include:
EINVAL
EACCES
ENOMEM
ENODEV
ENOEXEC
mmap64 function is equivalent to the mmap function but
the offset parameter is of type off64_t. On 32-bit systems
this allows the file associated with the filedes descriptor to be
larger than 2GB. filedes must be a descriptor returned from a
call to open64 or fopen64 and freopen64 where the
descriptor is retrieved with fileno.
When the sources are translated with _FILE_OFFSET_BITS == 64 this
function is actually available under the name mmap. I.e., the
new, extended API using 64 bit file sizes and offsets transparently
replaces the old API.
munmap removes any memory maps from (addr) to (addr +
length). length should be the length of the mapping.
It is safe to unmap multiple mappings in one command, or include unmapped space in the range. It is also possible to unmap only part of an existing mapping. However, only entire pages can be removed. If length is not an even number of pages, it will be rounded up.
It returns @math{0} for success and @math{-1} for an error.
One error is possible:
EINVAL
When using shared mappings, the kernel can write the file at any time before the mapping is removed. To be certain data has actually been written to the file and will be accessible to non-memory-mapped I/O, it is necessary to use this function.
It operates on the region address to (address + length). It may be used on part of a mapping or multiple mappings, however the region given should not contain any unmapped space.
flags can contain some options:
MS_SYNC
msync only makes sure that accesses to a file with
conventional I/O reflect the recent changes.
MS_ASYNC
msync to begin the synchronization, but not to wait for
it to complete.
msync returns @math{0} for success and @math{-1} for
error. Errors include:
EINVAL
EFAULT
This function can be used to change the size of an existing memory
area. address and length must cover a region entirely mapped
in the same mmap statement. A new mapping with the same
characteristics will be returned with the length new_length.
One option is possible, MREMAP_MAYMOVE. If it is given in
flags, the system may remove the existing mapping and create a new
one of the desired length in another location.
The address of the resulting mapping is returned, or @math{-1}. Possible error codes include:
EFAULT
EINVAL
EAGAIN
ENOMEM
MREMAP_MAYMOVE is not given and the extension would collide with
another mapped region.
This function is only available on a few systems. Except for performing optional optimizations one should not rely on this function.
Not all file descriptors may be mapped. Sockets, pipes, and most devices
only allow sequential access and do not fit into the mapping abstraction.
In addition, some regular files may not be mmapable, and older kernels may
not support mapping at all. Thus, programs using mmap should
have a fallback method to use should it fail. See section `Mmap' in GNU Coding Standards.
Sometimes a program needs to accept input on multiple input channels whenever input arrives. For example, some workstations may have devices such as a digitizing tablet, function button box, or dial box that are connected via normal asynchronous serial interfaces; good user interface style requires responding immediately to input on any device. Another example is a program that acts as a server to several other processes via pipes or sockets.
You cannot normally use read for this purpose, because this
blocks the program until input is available on one particular file
descriptor; input on other channels won't wake it up. You could set
nonblocking mode and poll each file descriptor in turn, but this is very
inefficient.
A better solution is to use the select function. This blocks the
program until input or output is ready on a specified set of file
descriptors, or until a timer expires, whichever comes first. This
facility is declared in the header file `sys/types.h'.
In the case of a server socket (see section Listening for Connections), we say that
"input" is available when there are pending connections that could be
accepted (see section Accepting Connections). accept for server
sockets blocks and interacts with select just as read does
for normal input.
The file descriptor sets for the select function are specified
as fd_set objects. Here is the description of the data type
and some macros for manipulating these objects.
fd_set data type represents file descriptor sets for the
select function. It is actually a bit array.
fd_set object can hold information about. On systems with a
fixed maximum number, FD_SETSIZE is at least that number. On
some systems, including GNU, there is no absolute limit on the number of
descriptors open, but this macro still has a constant value which
controls the number of bits in an fd_set; if you get a file
descriptor with a value as high as FD_SETSIZE, you cannot put
that descriptor into an fd_set.
Next, here is the description of the select function itself.
select function blocks the calling process until there is
activity on any of the specified sets of file descriptors, or until the
timeout period has expired.
The file descriptors specified by the read-fds argument are checked to see if they are ready for reading; the write-fds file descriptors are checked to see if they are ready for writing; and the except-fds file descriptors are checked for exceptional conditions. You can pass a null pointer for any of these arguments if you are not interested in checking for that kind of condition.
A file descriptor is considered ready for reading if it is not at end of
file. A server socket is considered ready for reading if there is a
pending connection which can be accepted with accept;
see section Accepting Connections. A client socket is ready for writing when
its connection is fully established;