Go to the first, previous, next, last section, table of contents.

Library Maintenance

How to Install the GNU C Library

Installation of the GNU C library is relatively simple, but usually requires several GNU tools to be installed already. (see section Recommended Tools to Install the GNU C Library, below.)

To configure the GNU C library for your system, run the shell script `configure' with sh. Use an argument which is the conventional GNU name for your system configuration--for example, `sparc-sun-sunos4.1', for a Sun 4 running SunOS 4.1. See section `Installing GNU CC' in Using and Porting GNU CC, for a full description of standard GNU configuration names. If you omit the configuration name, `configure' will try to guess one for you by inspecting the system it is running on. It may or may not be able to come up with a guess, and the its guess might be wrong. `configure' will tell you the canonical name of the chosen configuration before proceeding.

Here are some options that you should specify (if appropriate) when you run configure:

Use this option if you plan to use GNU ld to link programs with the GNU C Library. (We strongly recommend that you do.) This option enables use of features that exist only in GNU ld; so if you configure for GNU ld you must use GNU ld every time you link with the GNU C Library, and when building it.
Use this option if you plan to use the GNU assembler, gas, when building the GNU C Library. On some systems, the library may not build properly if you do not use gas.
This option implies both `--with-gnu-ld' and `--with-gnu-as'. On systems where GNU tools are the system tools, there is no need to specify this option. These include GNU, GNU/Linux, and free BSD systems.
Use this option if your computer lacks hardware floating-point support.
Install machine-independent data files in subdirectories of `directory'. (You can also set this in `configparms'; see below.)
Install the library and other machine-dependent files in subdirectories of `directory'. (You can also set this in `configparms'; see below.)
Enable or disable building of an ELF shared library on systems that support it. The default is to build the shared library on systems using ELF when the GNU binutils are available.
Enable or disable building of the profiled C library, `-lc_p'. The default is to build the profiled library. You may wish to disable it if you don't plan to do profiling, because it doubles the build time of compiling just the unprofiled static library.
Enable building a highly-optimized but possibly undebuggable static C library. This causes the normal static and shared (if enabled) C libraries to be compiled with maximal optimization, including the `-fomit-frame-pointer' switch that makes debugging impossible on many machines, and without debugging information (which makes the binaries substantially smaller). An additional static library is compiled with no optimization and full debugging information, and installed as `-lc_g'.

The simplest way to run configure is to do it in the directory that contains the library sources. This prepares to build the library in that very directory.

You can prepare to build the library in some other directory by going to that other directory to run configure. In order to run configure, you will have to specify a directory for it, like this:

mkdir sun4
cd sun4
../configure sparc-sun-sunos4.1

configure looks for the sources in whatever directory you specified for finding configure itself. It does not matter where in the file system the source and build directories are--as long as you specify the source directory when you run configure, you will get the proper results.

This feature lets you keep sources and binaries in different directories, and that makes it easy to build the library for several different machines from the same set of sources. Simply create a build directory for each target machine, and run configure in that directory specifying the target machine's configuration name.

The library has a number of special-purpose configuration parameters. These are defined in the file `Makeconfig'; see the comments in that file for the details.

But don't edit the file `Makeconfig' yourself--instead, create a file `configparms' in the directory where you are building the library, and define in that file the parameters you want to specify. `configparms' should not be an edited copy of `Makeconfig'; specify only the parameters that you want to override. To see how to set these parameters, find the section of `Makeconfig' that says "These are the configuration variables." Then for each parameter that you want to change, copy the definition from `Makeconfig' to your new `configparms' file, and change the value as appropriate for your system.

It is easy to configure the GNU C library for cross-compilation by setting a few variables in `configparms'. Set CC to the cross-compiler for the target you configured the library for; it is important to use this same CC value when running configure, like this: `CC=target-gcc configure target'. Set BUILD_CC to the compiler to use for for programs run on the build system as part of compiling the library. You may need to set AR and RANLIB to cross-compiling versions of ar and ranlib if the native tools are not configured to work with object files for the target you configured for.

Some of the machine-dependent code for some machines uses extensions in the GNU C compiler, so you may need to compile the library with GCC. (In fact, all of the existing complete ports require GCC.)

To build the library and related programs, type make. This will produce a lot of output, some of which may look like errors from make (but isn't). Look for error messages from make containing `***'. Those indicate that something is really wrong.

To build and run some test programs which exercise some of the library facilities, type make check. This will produce several files with names like `program.out'.

To format the GNU C Library Reference Manual for printing, type make dvi. You need a working TeX installation to do this.

To install the library and its header files, and the Info files of the manual, type make install. This will build things if necessary, before installing them. If you want to install the files in a different place than the one specified at configuration time you can specify a value for the Makefile variable install_root on the command line. This is useful to create chroot'ed environment or to prepare binary releases.

Recommended Tools to Install the GNU C Library

We recommend installing the following GNU tools before attempting to build the GNU C library:

Supported Configurations

The GNU C Library currently supports configurations that match the following patterns:


Former releases of this library (version 1.09.1 and perhaps earlier versions) used to run on the following configurations:


Since no one has volunteered to test and fix the above configurations, these are not supported at the moment. It's expected that these don't work anymore. Porting the library is not hard. If you are interested in doing a port, please contact the glibc maintainers by sending electronic mail to [email protected].

Each case of `ix86' can be `i386', `i486', `i586', or `i686'. All of those configurations produce a library that can run on any of these processors. The library will be optimized for the specified processor, but will not use instructions not available on all of them.

While no other configurations are supported, there are handy aliases for these few. (These aliases work in other GNU software as well.)

hp320-bsd4.3 hp300bsd
sun3-sunos4.n sun3
sun4-solaris2.n sun4-sunos5.n
sun4-sunos4.n sun4

Reporting Bugs

There are probably bugs in the GNU C library. There are certainly errors and omissions in this manual. If you report them, they will get fixed. If you don't, no one will ever know about them and they will remain unfixed for all eternity, if not longer.

To report a bug, first you must find it. Hopefully, this will be the hard part. Once you've found a bug, make sure it's really a bug. A good way to do this is to see if the GNU C library behaves the same way some other C library does. If so, probably you are wrong and the libraries are right (but not necessarily). If not, one of the libraries is probably wrong.

Once you're sure you've found a bug, try to narrow it down to the smallest test case that reproduces the problem. In the case of a C library, you really only need to narrow it down to one library function call, if possible. This should not be too difficult.

The final step when you have a simple test case is to report the bug. When reporting a bug, send your test case, the results you got, the results you expected, what you think the problem might be (if you've thought of anything), your system type, and the version of the GNU C library which you are using. Also include the files `config.status' and `config.make' which are created by running `configure'; they will be in whatever directory was current when you ran `configure'.

If you think you have found some way in which the GNU C library does not conform to the ISO and POSIX standards (see section Standards and Portability), that is definitely a bug. Report it!

Send bug reports to the Internet address [email protected] or the UUCP path mit-eddie!prep.ai.mit.edu!bug-glibc. If you have other problems with installation or use, please report those as well.

If you are not sure how a function should behave, and this manual doesn't tell you, that's a bug in the manual. Report that too! If the function's behavior disagrees with the manual, then either the library or the manual has a bug, so report the disagreement. If you find any errors or omissions in this manual, please report them to the Internet address [email protected] or the UUCP path mit-eddie!prep.ai.mit.edu!bug-glibc-manual.

Adding New Functions

The process of building the library is driven by the makefiles, which make heavy use of special features of GNU make. The makefiles are very complex, and you probably don't want to try to understand them. But what they do is fairly straightforward, and only requires that you define a few variables in the right places.

The library sources are divided into subdirectories, grouped by topic.

The `string' subdirectory has all the string-manipulation functions, `math' has all the mathematical functions, etc.

Each subdirectory contains a simple makefile, called `Makefile', which defines a few make variables and then includes the global makefile `Rules' with a line like:

include ../Rules

The basic variables that a subdirectory makefile defines are:

The name of the subdirectory, for example `stdio'. This variable must be defined.
The names of the header files in this section of the library, such as `stdio.h'.
The names of the modules (source files) in this section of the library. These should be simple names, such as `strlen' (rather than complete file names, such as `strlen.c'). Use routines for modules that define functions in the library, and aux for auxiliary modules containing things like data definitions. But the values of routines and aux are just concatenated, so there really is no practical difference.
The names of test programs for this section of the library. These should be simple names, such as `tester' (rather than complete file names, such as `tester.c'). `make tests' will build and run all the test programs. If a test program needs input, put the test data in a file called `test-program.input'; it will be given to the test program on its standard input. If a test program wants to be run with arguments, put the arguments (all on a single line) in a file called `test-program.args'. Test programs should exit with zero status when the test passes, and nonzero status when the test indicates a bug in the library or error in building.
The names of "other" programs associated with this section of the library. These are programs which are not tests per se, but are other small programs included with the library. They are built by `make others'.
Files to be installed by `make install'. Files listed in `install-lib' are installed in the directory specified by `libdir' in `configparms' or `Makeconfig' (see section How to Install the GNU C Library). Files listed in install-data are installed in the directory specified by `datadir' in `configparms' or `Makeconfig'. Files listed in install are installed in the directory specified by `bindir' in `configparms' or `Makeconfig'.
Other files from this subdirectory which should be put into a distribution tar file. You need not list here the makefile itself or the source and header files listed in the other standard variables. Only define distribute if there are files used in an unusual way that should go into the distribution.
Files which are generated by `Makefile' in this subdirectory. These files will be removed by `make clean', and they will never go into a distribution.
Extra object files which are built by `Makefile' in this subdirectory. This should be a list of file names like `foo.o'; the files will actually be found in whatever directory object files are being built in. These files will be removed by `make clean'. This variable is used for secondary object files needed to build others or tests.

Porting the GNU C Library

The GNU C library is written to be easily portable to a variety of machines and operating systems. Machine- and operating system-dependent functions are well separated to make it easy to add implementations for new machines or operating systems. This section describes the layout of the library source tree and explains the mechanisms used to select machine-dependent code to use.

All the machine-dependent and operating system-dependent files in the library are in the subdirectory `sysdeps' under the top-level library source directory. This directory contains a hierarchy of subdirectories (see section Layout of the `sysdeps' Directory Hierarchy).

Each subdirectory of `sysdeps' contains source files for a particular machine or operating system, or for a class of machine or operating system (for example, systems by a particular vendor, or all machines that use IEEE 754 floating-point format). A configuration specifies an ordered list of these subdirectories. Each subdirectory implicitly appends its parent directory to the list. For example, specifying the list `unix/bsd/vax' is equivalent to specifying the list `unix/bsd/vax unix/bsd unix'. A subdirectory can also specify that it implies other subdirectories which are not directly above it in the directory hierarchy. If the file `Implies' exists in a subdirectory, it lists other subdirectories of `sysdeps' which are appended to the list, appearing after the subdirectory containing the `Implies' file. Lines in an `Implies' file that begin with a `#' character are ignored as comments. For example, `unix/bsd/Implies' contains:

# BSD has Internet-related things.

and `unix/Implies' contains:


So the final list is `unix/bsd/vax unix/bsd unix/inet unix posix'.

`sysdeps' has two "special" subdirectories, called `generic' and `stub'. These two are always implicitly appended to the list of subdirectories (in that order), so you needn't put them in an `Implies' file, and you should not create any subdirectories under them intended to be new specific categories. `generic' is for things that can be implemented in machine-independent C, using only other machine-independent functions in the C library. `stub' is for stub versions of functions which cannot be implemented on a particular machine or operating system. The stub functions always return an error, and set errno to ENOSYS (Function not implemented). See section Error Reporting.

A source file is known to be system-dependent by its having a version in `generic' or `stub'; every generally-available function whose implementation is system-dependent in should have either a generic or stub implementation (there is no point in having both). Some rare functions are only useful on specific systems and aren't defined at all on others; these do not appear anywhere in the system-independent source code or makefiles (including the `generic' and `stub' directories), only in the system-dependent `Makefile' in the specific system's subdirectory.

If you come across a file that is in one of the main source directories (`string', `stdio', etc.), and you want to write a machine- or operating system-dependent version of it, move the file into `sysdeps/generic' and write your new implementation in the appropriate system-specific subdirectory. Note that if a file is to be system-dependent, it must not appear in one of the main source directories.

There are a few special files that may exist in each subdirectory of `sysdeps':

A makefile for this machine or operating system, or class of machine or operating system. This file is included by the library makefile `Makerules', which is used by the top-level makefile and the subdirectory makefiles. It can change the variables set in the including makefile or add new rules. It can use GNU make conditional directives based on the variable `subdir' (see above) to select different sets of variables and rules for different sections of the library. It can also set the make variable `sysdep-routines', to specify extra modules to be included in the library. You should use `sysdep-routines' rather than adding modules to `routines' because the latter is used in determining what to distribute for each subdirectory of the main source tree. Each makefile in a subdirectory in the ordered list of subdirectories to be searched is included in order. Since several system-dependent makefiles may be included, each should append to `sysdep-routines' rather than simply setting it:
sysdep-routines := $(sysdep-routines) foo bar
This file contains the names of new whole subdirectories under the top-level library source tree that should be included for this system. These subdirectories are treated just like the system-independent subdirectories in the library source tree, such as `stdio' and `math'. Use this when there are completely new sets of functions and header files that should go into the library for the system this subdirectory of `sysdeps' implements. For example, `sysdeps/unix/inet/Subdirs' contains `inet'; the `inet' directory contains various network-oriented operations which only make sense to put in the library on systems that support the Internet.
This file contains the names of files (relative to the subdirectory of `sysdeps' in which it appears) which should be included in the distribution. List any new files used by rules in the `Makefile' in the same directory, or header files used by the source files in that directory. You don't need to list files that are implementations (either C or assembly source) of routines whose names are given in the machine-independent makefiles in the main source tree.
This file is a shell script fragment to be run at configuration time. The top-level `configure' script uses the shell . command to read the `configure' file in each system-dependent directory chosen, in order. The `configure' files are often generated from `configure.in' files using Autoconf. A system-dependent `configure' script will usually add things to the shell variables `DEFS' and `config_vars'; see the top-level `configure' script for details. The script can check for `--with-package' options that were passed to the top-level `configure'. For an option `--with-package=value' `configure' sets the shell variable `with_package' (with any dashes in package converted to underscores) to value; if the option is just `--with-package' (no argument), then it sets `with_package' to `yes'.
This file is an Autoconf input fragment to be processed into the file `configure' in this subdirectory. See section `Introduction' in Autoconf: Generating Automatic Configuration Scripts, for a description of Autoconf. You should write either `configure' or `configure.in', but not both. The first line of `configure.in' should invoke the m4 macro `GLIBC_PROVIDES'. This macro does several AC_PROVIDE calls for Autoconf macros which are used by the top-level `configure' script; without this, those macros might be invoked again unnecessarily by Autoconf.

That is the general system for how system-dependencies are isolated. The next section explains how to decide what directories in `sysdeps' to use. section Porting the GNU C Library to Unix Systems, has some tips on porting the library to Unix variants.

Layout of the `sysdeps' Directory Hierarchy

A GNU configuration name has three parts: the CPU type, the manufacturer's name, and the operating system. `configure' uses these to pick the list of system-dependent directories to look for. If the `--nfp' option is not passed to `configure', the directory `machine/fpu' is also used. The operating system often has a base operating system; for example, if the operating system is `sunos4.1', the base operating system is `unix/bsd'. The algorithm used to pick the list of directories is simple: `configure' makes a list of the base operating system, manufacturer, CPU type, and operating system, in that order. It then concatenates all these together with slashes in between, to produce a directory name; for example, the configuration `sparc-sun-sunos4.1' results in `unix/bsd/sun/sparc/sunos4.1'. `configure' then tries removing each element of the list in turn, so `unix/bsd/sparc' and `sun/sparc' are also tried, among others. Since the precise version number of the operating system is often not important, and it would be very inconvenient, for example, to have identical `sunos4.1.1' and `sunos4.1.2' directories, `configure' tries successively less specific operating system names by removing trailing suffixes starting with a period.

As an example, here is the complete list of directories that would be tried for the configuration `sparc-sun-sunos4.1' (without the `--nfp' option):


Different machine architectures are conventionally subdirectories at the top level of the `sysdeps' directory tree. For example, `sysdeps/sparc' and `sysdeps/m68k'. These contain files specific to those machine architectures, but not specific to any particular operating system. There might be subdirectories for specializations of those architectures, such as `sysdeps/m68k/68020'. Code which is specific to the floating-point coprocessor used with a particular machine should go in `sysdeps/machine/fpu'.

There are a few directories at the top level of the `sysdeps' hierarchy that are not for particular machine architectures.

As described above (see section Porting the GNU C Library), these are the two subdirectories that every configuration implicitly uses after all others.
This directory is for code using the IEEE 754 floating-point format, where the C type float is IEEE 754 single-precision format, and double is IEEE 754 double-precision format. Usually this directory is referred to in the `Implies' file in a machine architecture-specific directory, such as `m68k/Implies'.
This directory contains implementations of things in the library in terms of POSIX.1 functions. This includes some of the POSIX.1 functions themselves. Of course, POSIX.1 cannot be completely implemented in terms of itself, so a configuration using just `posix' cannot be complete.
This is the directory for Unix-like things. See section Porting the GNU C Library to Unix Systems. `unix' implies `posix'. There are some special-purpose subdirectories of `unix':
This directory is for things common to both BSD and System V release 4. Both `unix/bsd' and `unix/sysv/sysv4' imply `unix/common'.
This directory is for socket and related functions on Unix systems. The `inet' top-level subdirectory is enabled by `unix/inet/Subdirs'. `unix/common' implies `unix/inet'.
This is the directory for things based on the Mach microkernel from CMU (including the GNU operating system). Other basic operating systems (VMS, for example) would have their own directories at the top level of the `sysdeps' hierarchy, parallel to `unix' and `mach'.

Porting the GNU C Library to Unix Systems

Most Unix systems are fundamentally very similar. There are variations between different machines, and variations in what facilities are provided by the kernel. But the interface to the operating system facilities is, for the most part, pretty uniform and simple.

The code for Unix systems is in the directory `unix', at the top level of the `sysdeps' hierarchy. This directory contains subdirectories (and subdirectory trees) for various Unix variants.

The functions which are system calls in most Unix systems are implemented in assembly code in files in `sysdeps/unix'. These files are named with a suffix of `.S'; for example, `__open.S'. Files ending in `.S' are run through the C preprocessor before being fed to the assembler.

These files all use a set of macros that should be defined in `sysdep.h'. The `sysdep.h' file in `sysdeps/unix' partially defines them; a `sysdep.h' file in another directory must finish defining them for the particular machine and operating system variant. See `sysdeps/unix/sysdep.h' and the machine-specific `sysdep.h' implementations to see what these macros are and what they should do.

The system-specific makefile for the `unix' directory (that is, the file `sysdeps/unix/Makefile') gives rules to generate several files from the Unix system you are building the library on (which is assumed to be the target system you are building the library for). All the generated files are put in the directory where the object files are kept; they should not affect the source tree itself. The files generated are `ioctls.h', `errnos.h', `sys/param.h', and `errlist.c' (for the `stdio' section of the library).

Contributors to the GNU C Library

The GNU C library was written originally by Roland McGrath. Some parts of the library were contributed or worked on by other people.

Go to the first, previous, next, last section, table of contents.