@dircategory Programming & development tools. * Gdb: (gdb). The GNU debugger.
Copyright (C) 1988,1989,1990,1991,1992,1993,1994,1995,1996,1998,1999,2000,2001
Free Software Foundation, Inc.
Published by the Free Software Foundation
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ISBN 1-882114-77-9
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being "Free Software" and "Free Software Needs Free Documentation", with the Front-Cover Texts being "A GNU Manual," and with the Back-Cover Texts as in (a) below.
(a) The FSF's Back-Cover Text is: "You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development." @node Top @top Debugging with @value{GDBN} This file describes @value{GDBN}, the @sc{gnu} symbolic debugger. This is the @value{EDITION} Edition, @value{DATE}, for @value{GDBN} Version @value{GDBVN}. Copyright (C) 1988-2000 Free Software Foundation, Inc. @menu * Summary:: Summary of @value{GDBN} * Sample Session:: A sample @value{GDBN} session * Invocation:: Getting in and out of @value{GDBN} * Commands:: @value{GDBN} commands * Running:: Running programs under @value{GDBN} * Stopping:: Stopping and continuing * Stack:: Examining the stack * Source:: Examining source files * Data:: Examining data * Languages:: Using @value{GDBN} with different languages * Symbols:: Examining the symbol table * Altering:: Altering execution * GDB Files:: @value{GDBN} files * Targets:: Specifying a debugging target * Configurations:: Configuration-specific information * Controlling GDB:: Controlling @value{GDBN} * Sequences:: Canned sequences of commands * Emacs:: Using @value{GDBN} under @sc{gnu} Emacs * Annotations:: @value{GDBN}'s annotation interface. * GDB Bugs:: Reporting bugs in @value{GDBN} * Formatting Documentation:: How to format and print @value{GDBN} documentation * Command Line Editing:: Command Line Editing * Using History Interactively:: Using History Interactively * Installing GDB:: Installing GDB * Index:: Index @end menu
The purpose of a debugger such as is to allow you to see what is going on "inside" another program while it executes--or what another program was doing at the moment it crashed.
can do four main kinds of things (plus other things in support of these) to help you catch bugs in the act:
You can use to debug programs written in C and C++. For more information, see section Supported languages. For more information, see section C and C++.
Support for Modula-2 and Chill is partial. For information on Modula-2, see section Modula-2. For information on Chill, see section Chill.
Debugging Pascal programs which use sets, subranges, file variables, or nested functions does not currently work. does not support entering expressions, printing values, or similar features using Pascal syntax.
can be used to debug programs written in Fortran, although it may be necessary to refer to some variables with a trailing underscore.
is free software, protected by the GNU General Public License (GPL). The GPL gives you the freedom to copy or adapt a licensed program--but every person getting a copy also gets with it the freedom to modify that copy (which means that they must get access to the source code), and the freedom to distribute further copies. Typical software companies use copyrights to limit your freedoms; the Free Software Foundation uses the GPL to preserve these freedoms.
Fundamentally, the General Public License is a license which says that you have these freedoms and that you cannot take these freedoms away from anyone else.
The biggest deficiency in the free software community today is not in the software--it is the lack of good free documentation that we can include with the free software. Many of our most important programs do not come with free reference manuals and free introductory texts. Documentation is an essential part of any software package; when an important free software package does not come with a free manual and a free tutorial, that is a major gap. We have many such gaps today.
Consider Perl, for instance. The tutorial manuals that people normally use are non-free. How did this come about? Because the authors of those manuals published them with restrictive terms--no copying, no modification, source files not available--which exclude them from the free software world.
That wasn't the first time this sort of thing happened, and it was far from the last. Many times we have heard a GNU user eagerly describe a manual that he is writing, his intended contribution to the community, only to learn that he had ruined everything by signing a publication contract to make it non-free.
Free documentation, like free software, is a matter of freedom, not price. The problem with the non-free manual is not that publishers charge a price for printed copies--that in itself is fine. (The Free Software Foundation sells printed copies of manuals, too.) The problem is the restrictions on the use of the manual. Free manuals are available in source code form, and give you permission to copy and modify. Non-free manuals do not allow this.
The criteria of freedom for a free manual are roughly the same as for free software. Redistribution (including the normal kinds of commercial redistribution) must be permitted, so that the manual can accompany every copy of the program, both on-line and on paper.
Permission for modification of the technical content is crucial too. When people modify the software, adding or changing features, if they are conscientious they will change the manual too--so they can provide accurate and clear documentation for the modified program. A manual that leaves you no choice but to write a new manual to document a changed version of the program is not really available to our community.
Some kinds of limits on the way modification is handled are acceptable. For example, requirements to preserve the original author's copyright notice, the distribution terms, or the list of authors, are ok. It is also no problem to require modified versions to include notice that they were modified. Even entire sections that may not be deleted or changed are acceptable, as long as they deal with nontechnical topics (like this one). These kinds of restrictions are acceptable because they don't obstruct the community's normal use of the manual.
However, it must be possible to modify all the technical content of the manual, and then distribute the result in all the usual media, through all the usual channels. Otherwise, the restrictions obstruct the use of the manual, it is not free, and we need another manual to replace it.
Please spread the word about this issue. Our community continues to lose manuals to proprietary publishing. If we spread the word that free software needs free reference manuals and free tutorials, perhaps the next person who wants to contribute by writing documentation will realize, before it is too late, that only free manuals contribute to the free software community.
If you are writing documentation, please insist on publishing it under the GNU Free Documentation License or another free documentation license. Remember that this decision requires your approval--you don't have to let the publisher decide. Some commercial publishers will use a free license if you insist, but they will not propose the option; it is up to you to raise the issue and say firmly that this is what you want. If the publisher you are dealing with refuses, please try other publishers. If you're not sure whether a proposed license is free, write to licensing@gnu.org.
You can encourage commercial publishers to sell more free, copylefted manuals and tutorials by buying them, and particularly by buying copies from the publishers that paid for their writing or for major improvements. Meanwhile, try to avoid buying non-free documentation at all. Check the distribution terms of a manual before you buy it, and insist that whoever seeks your business must respect your freedom. Check the history of the book, and try to reward the publishers that have paid or pay the authors to work on it.
The Free Software Foundation maintains a list of free documentation published by other publishers, at http://www.fsf.org/doc/other-free-books.html.
Richard Stallman was the original author of , and of many other GNU programs. Many others have contributed to its development. This section attempts to credit major contributors. One of the virtues of free software is that everyone is free to contribute to it; with regret, we cannot actually acknowledge everyone here. The file `ChangeLog' in the distribution approximates a blow-by-blow account.
Changes much prior to version 2.0 are lost in the mists of time.
Plea: Additions to this section are particularly welcome. If you or your friends (or enemies, to be evenhanded) have been unfairly omitted from this list, we would like to add your names!
So that they may not regard their many labors as thankless, we particularly thank those who shepherded through major releases: Andrew Cagney (releases 5.0 and 5.1); Jim Blandy (release 4.18); Jason Molenda (release 4.17); Stan Shebs (release 4.14); Fred Fish (releases 4.16, 4.15, 4.13, 4.12, 4.11, 4.10, and 4.9); Stu Grossman and John Gilmore (releases 4.8, 4.7, 4.6, 4.5, and 4.4); John Gilmore (releases 4.3, 4.2, 4.1, 4.0, and 3.9); Jim Kingdon (releases 3.5, 3.4, and 3.3); and Randy Smith (releases 3.2, 3.1, and 3.0).
Richard Stallman, assisted at various times by Peter TerMaat, Chris Hanson, and Richard Mlynarik, handled releases through 2.8.
Michael Tiemann is the author of most of the GNU C++ support in , with significant additional contributions from Per Bothner and Daniel Berlin. James Clark wrote the GNU C++ demangler. Early work on C++ was by Peter TerMaat (who also did much general update work leading to release 3.0).
uses the BFD subroutine library to examine multiple object-file formats; BFD was a joint project of David V. Henkel-Wallace, Rich Pixley, Steve Chamberlain, and John Gilmore.
David Johnson wrote the original COFF support; Pace Willison did the original support for encapsulated COFF.
Brent Benson of Harris Computer Systems contributed DWARF2 support.
Adam de Boor and Bradley Davis contributed the ISI Optimum V support. Per Bothner, Noboyuki Hikichi, and Alessandro Forin contributed MIPS support. Jean-Daniel Fekete contributed Sun 386i support. Chris Hanson improved the HP9000 support. Noboyuki Hikichi and Tomoyuki Hasei contributed Sony/News OS 3 support. David Johnson contributed Encore Umax support. Jyrki Kuoppala contributed Altos 3068 support. Jeff Law contributed HP PA and SOM support. Keith Packard contributed NS32K support. Doug Rabson contributed Acorn Risc Machine support. Bob Rusk contributed Harris Nighthawk CX-UX support. Chris Smith contributed Convex support (and Fortran debugging). Jonathan Stone contributed Pyramid support. Michael Tiemann contributed SPARC support. Tim Tucker contributed support for the Gould NP1 and Gould Powernode. Pace Willison contributed Intel 386 support. Jay Vosburgh contributed Symmetry support.
Andreas Schwab contributed M68K Linux support.
Rich Schaefer and Peter Schauer helped with support of SunOS shared libraries.
Jay Fenlason and Roland McGrath ensured that and GAS agree about several machine instruction sets.
Patrick Duval, Ted Goldstein, Vikram Koka and Glenn Engel helped develop remote debugging. Intel Corporation, Wind River Systems, AMD, and ARM contributed remote debugging modules for the i960, VxWorks, A29K UDI, and RDI targets, respectively.
Brian Fox is the author of the readline libraries providing command-line editing and command history.
Andrew Beers of SUNY Buffalo wrote the language-switching code, the Modula-2 support, and contributed the Languages chapter of this manual.
Fred Fish wrote most of the support for Unix System Vr4. He also enhanced the command-completion support to cover C++ overloaded symbols.
Hitachi America, Ltd. sponsored the support for H8/300, H8/500, and Super-H processors.
NEC sponsored the support for the v850, Vr4xxx, and Vr5xxx processors.
Mitsubishi sponsored the support for D10V, D30V, and M32R/D processors.
Toshiba sponsored the support for the TX39 Mips processor.
Matsushita sponsored the support for the MN10200 and MN10300 processors.
Fujitsu sponsored the support for SPARClite and FR30 processors.
Kung Hsu, Jeff Law, and Rick Sladkey added support for hardware watchpoints.
Michael Snyder added support for tracepoints.
Stu Grossman wrote gdbserver.
Jim Kingdon, Peter Schauer, Ian Taylor, and Stu Grossman made nearly innumerable bug fixes and cleanups throughout .
The following people at the Hewlett-Packard Company contributed support for the PA-RISC 2.0 architecture, HP-UX 10.20, 10.30, and 11.0 (narrow mode), HP's implementation of kernel threads, HP's aC++ compiler, and the terminal user interface: Ben Krepp, Richard Title, John Bishop, Susan Macchia, Kathy Mann, Satish Pai, India Paul, Steve Rehrauer, and Elena Zannoni. Kim Haase provided HP-specific information in this manual.
DJ Delorie ported to MS-DOS, for the DJGPP project. Robert Hoehne made significant contributions to the DJGPP port.
Cygnus Solutions has sponsored maintenance and much of its development since 1991. Cygnus engineers who have worked on fulltime include Mark Alexander, Jim Blandy, Per Bothner, Kevin Buettner, Edith Epstein, Chris Faylor, Fred Fish, Martin Hunt, Jim Ingham, John Gilmore, Stu Grossman, Kung Hsu, Jim Kingdon, John Metzler, Fernando Nasser, Geoffrey Noer, Dawn Perchik, Rich Pixley, Zdenek Radouch, Keith Seitz, Stan Shebs, David Taylor, and Elena Zannoni. In addition, Dave Brolley, Ian Carmichael, Steve Chamberlain, Nick Clifton, JT Conklin, Stan Cox, DJ Delorie, Ulrich Drepper, Frank Eigler, Doug Evans, Sean Fagan, David Henkel-Wallace, Richard Henderson, Jeff Holcomb, Jeff Law, Jim Lemke, Tom Lord, Bob Manson, Michael Meissner, Jason Merrill, Catherine Moore, Drew Moseley, Ken Raeburn, Gavin Romig-Koch, Rob Savoye, Jamie Smith, Mike Stump, Ian Taylor, Angela Thomas, Michael Tiemann, Tom Tromey, Ron Unrau, Jim Wilson, and David Zuhn have made contributions both large and small.
You can use this manual at your leisure to read all about . However, a handful of commands are enough to get started using the debugger. This chapter illustrates those commands.
In this sample session, we emphasize user input like this: input, to make it easier to pick out from the surrounding output.
One of the preliminary versions of GNU m4 (a generic macro
processor) exhibits the following bug: sometimes, when we change its
quote strings from the default, the commands used to capture one macro
definition within another stop working. In the following short m4
session, we define a macro foo which expands to 0000; we
then use the m4 built-in defn to define bar as the
same thing. However, when we change the open quote string to
<QUOTE> and the close quote string to <UNQUOTE>, the same
procedure fails to define a new synonym baz:
$ cd gnu/m4 $ ./m4 define(foo,0000) foo 0000 define(bar,defn(`foo')) bar 0000 changequote(<QUOTE>,<UNQUOTE>) define(baz,defn(<QUOTE>foo<UNQUOTE>)) baz C-d m4: End of input: 0: fatal error: EOF in string
Let us use to try to see what is going on.
$ m4 is free software and you are welcome to distribute copies of it under certain conditions; type "show copying" to see the conditions. There is absolutely no warranty for ; type "show warranty" for details. , Copyright 1999 Free Software Foundation, Inc... ()
reads only enough symbol data to know where to find the rest when needed; as a result, the first prompt comes up very quickly. We now tell to use a narrower display width than usual, so that examples fit in this manual.
() set width 70
We need to see how the m4 built-in changequote works.
Having looked at the source, we know the relevant subroutine is
m4_changequote, so we set a breakpoint there with the
break command.
() break m4_changequote Breakpoint 1 at 0x62f4: file builtin.c, line 879.
Using the run command, we start m4 running under
control; as long as control does not reach the m4_changequote
subroutine, the program runs as usual:
() run Starting program: /work/Editorial/gdb/gnu/m4/m4 define(foo,0000) foo 0000
To trigger the breakpoint, we call changequote.
suspends execution of m4, displaying information about the
context where it stops.
changequote(<QUOTE>,<UNQUOTE>)
Breakpoint 1, m4_changequote (argc=3, argv=0x33c70)
at builtin.c:879
879 if (bad_argc(TOKEN_DATA_TEXT(argv[0]),argc,1,3))
Now we use the command n (next) to advance execution to
the next line of the current function.
() n 882 set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\ : nil,
set_quotes looks like a promising subroutine. We can go into it
by using the command s (step) instead of next.
step goes to the next line to be executed in any
subroutine, so it steps into set_quotes.
() s
set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
at input.c:530
530 if (lquote != def_lquote)
The display that shows the subroutine where m4 is now
suspended (and its arguments) is called a stack frame display. It
shows a summary of the stack. We can use the backtrace
command (which can also be spelled bt), to see where we are
in the stack as a whole: the backtrace command displays a
stack frame for each active subroutine.
() bt
#0 set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
at input.c:530
#1 0x6344 in m4_changequote (argc=3, argv=0x33c70)
at builtin.c:882
#2 0x8174 in expand_macro (sym=0x33320) at macro.c:242
#3 0x7a88 in expand_token (obs=0x0, t=209696, td=0xf7fffa30)
at macro.c:71
#4 0x79dc in expand_input () at macro.c:40
#5 0x2930 in main (argc=0, argv=0xf7fffb20) at m4.c:195
We step through a few more lines to see what happens. The first two
times, we can use `s'; the next two times we use n to avoid
falling into the xstrdup subroutine.
() s 0x3b5c 532 if (rquote != def_rquote) () s 0x3b80 535 lquote = (lq == nil || *lq == '\0') ? \ def_lquote : xstrdup(lq); () n 536 rquote = (rq == nil || *rq == '\0') ? def_rquote\ : xstrdup(rq); () n 538 len_lquote = strlen(rquote);
The last line displayed looks a little odd; we can examine the variables
lquote and rquote to see if they are in fact the new left
and right quotes we specified. We use the command p
(print) to see their values.
() p lquote $1 = 0x35d40 "<QUOTE>" () p rquote $2 = 0x35d50 "<UNQUOTE>"
lquote and rquote are indeed the new left and right quotes.
To look at some context, we can display ten lines of source
surrounding the current line with the l (list) command.
() l 533 xfree(rquote); 534 535 lquote = (lq == nil || *lq == '\0') ? def_lquote\ : xstrdup (lq); 536 rquote = (rq == nil || *rq == '\0') ? def_rquote\ : xstrdup (rq); 537 538 len_lquote = strlen(rquote); 539 len_rquote = strlen(lquote); 540 } 541 542 void
Let us step past the two lines that set len_lquote and
len_rquote, and then examine the values of those variables.
() n 539 len_rquote = strlen(lquote); () n 540 } () p len_lquote $3 = 9 () p len_rquote $4 = 7
That certainly looks wrong, assuming len_lquote and
len_rquote are meant to be the lengths of lquote and
rquote respectively. We can set them to better values using
the p command, since it can print the value of
any expression--and that expression can include subroutine calls and
assignments.
() p len_lquote=strlen(lquote) $5 = 7 () p len_rquote=strlen(rquote) $6 = 9
Is that enough to fix the problem of using the new quotes with the
m4 built-in defn? We can allow m4 to continue
executing with the c (continue) command, and then try the
example that caused trouble initially:
() c Continuing. define(baz,defn(<QUOTE>foo<UNQUOTE>)) baz 0000
Success! The new quotes now work just as well as the default ones. The
problem seems to have been just the two typos defining the wrong
lengths. We allow m4 exit by giving it an EOF as input:
C-d Program exited normally.
The message `Program exited normally.' is from ; it
indicates m4 has finished executing. We can end our
session with the quit command.
() quit
This chapter discusses how to start , and how to get out of it. The essentials are:
Invoke by running the program . Once started,
reads commands from the terminal until you tell it to exit.
You can also run with a variety of arguments and options,
to specify more of your debugging environment at the outset.
The command-line options described here are designed to cover a variety of situations; in some environments, some of these options may effectively be unavailable.
The most usual way to start is with one argument, specifying an executable program:
program
You can also start with both an executable program and a core file specified:
program core
You can, instead, specify a process ID as a second argument, if you want to debug a running process:
program 1234
would attach to process 1234 (unless you also have a file
named `1234'; does check for a core file first).
Taking advantage of the second command-line argument requires a fairly complete operating system; when you use as a remote debugger attached to a bare board, there may not be any notion of "process", and there is often no way to get a core dump. will warn you if it is unable to attach or to read core dumps.
You can run without printing the front material, which describes
's non-warranty, by specifying -silent:
-silent
You can further control how starts up by using command-line options. itself can remind you of the options available.
Type
-help
to display all available options and briefly describe their use (` -h' is a shorter equivalent).
All options and command line arguments you give are processed in sequential order. The order makes a difference when the `-x' option is used.
When starts, it reads any arguments other than options as specifying an executable file and core file (or process ID). This is the same as if the arguments were specified by the `-se' and `-c' options respectively. ( reads the first argument that does not have an associated option flag as equivalent to the `-se' option followed by that argument; and the second argument that does not have an associated option flag, if any, as equivalent to the `-c' option followed by that argument.)
If has not been configured to included core file support, such as for most embedded targets, then it will complain about a second argument and ignore it.
Many options have both long and short forms; both are shown in the following list. also recognizes the long forms if you truncate them, so long as enough of the option is present to be unambiguous. (If you prefer, you can flag option arguments with `--' rather than `-', though we illustrate the more usual convention.)
-symbols file
-s file
-exec file
-e file
-se file
-core file
-c file
-c number
attach command
(unless there is a file in core-dump format named number, in which
case `-c' specifies that file as a core dump to read).
-command file
-x file
-directory directory
-d directory
-m
-mapped
mmap
system call, you can use this option
to have write the symbols from your
program into a reusable file in the current directory. If the program you are debugging is
called `/tmp/fred', the mapped symbol file is `/tmp/fred.syms'.
Future debugging sessions notice the presence of this file,
and can quickly map in symbol information from it, rather than reading
the symbol table from the executable program.
The `.syms' file is specific to the host machine where
is run. It holds an exact image of the internal symbol
table. It cannot be shared across multiple host platforms.
-r
-readnow
You typically combine the -mapped and -readnow options in
order to build a `.syms' file that contains complete symbol
information. (See section Commands to specify files, for information
on `.syms' files.) A simple invocation to do nothing
but build a `.syms' file for future use is:
gdb -batch -nx -mapped -readnow programname
You can run in various alternative modes--for example, in batch mode or quiet mode.
-nx
-n
-quiet
-silent
-q
-batch
0 after processing all the
command files specified with `-x' (and all commands from
initialization files, if not inhibited with `-n'). Exit with
nonzero status if an error occurs in executing the commands
in the command files.
Batch mode may be useful for running as a filter, for
example to download and run a program on another computer; in order to
make this more useful, the message
Program exited normally.(which is ordinarily issued whenever a program running under control terminates) is not issued when running in batch mode.
-nowindows
-nw
-windows
-w
-cd directory
-fullname
-f
-epoch
-annotate level
-async
-noasync
-baud bps
-b bps
-tty device
-t device
-tui
-interpreter interp
-write
-statistics
-version
quit [expression]
q
quit command (abbreviated
q), or type an end-of-file character (usually C-d). If you
do not supply expression, will terminate normally;
otherwise it will terminate using the result of expression as the
error code.
An interrupt (often C-c) does not exit from , but rather terminates the action of any command that is in progress and returns to command level. It is safe to type the interrupt character at any time because does not allow it to take effect until a time when it is safe.
If you have been using to control an attached process or
device, you can release it with the detach command
(see section Debugging an already-running process).
If you need to execute occasional shell commands during your
debugging session, there is no need to leave or suspend ; you can
just use the shell command.
shell command string
SHELL determines which
shell to run. Otherwise uses the default shell
(`/bin/sh' on Unix systems, `COMMAND.COM' on MS-DOS, etc.).
The utility make is often needed in development environments.
You do not have to use the shell command for this purpose in
:
make make-args
make program with the specified
arguments. This is equivalent to `shell make make-args'.
You can abbreviate a command to the first few letters of the command name, if that abbreviation is unambiguous; and you can repeat certain commands by typing just RET. You can also use the TAB key to get to fill out the rest of a word in a command (or to show you the alternatives available, if there is more than one possibility).
A command is a single line of input. There is no limit on
how long it can be. It starts with a command name, which is followed by
arguments whose meaning depends on the command name. For example, the
command step accepts an argument which is the number of times to
step, as in `step 5'. You can also use the step command
with no arguments. Some commands do not allow any arguments.
command names may always be truncated if that abbreviation is
unambiguous. Other possible command abbreviations are listed in the
documentation for individual commands. In some cases, even ambiguous
abbreviations are allowed; for example, s is specially defined as
equivalent to step even though there are other commands whose
names start with s. You can test abbreviations by using them as
arguments to the help command.
A blank line as input to (typing just RET) means to
repeat the previous command. Certain commands (for example, run)
will not repeat this way; these are commands whose unintentional
repetition might cause trouble and which you are unlikely to want to
repeat.
The list and x commands, when you repeat them with
RET, construct new arguments rather than repeating
exactly as typed. This permits easy scanning of source or memory.
can also use RET in another way: to partition lengthy
output, in a way similar to the common utility more
(see section Screen size). Since it is easy to press one
RET too many in this situation, disables command
repetition after any command that generates this sort of display.
Any text from a # to the end of the line is a comment; it does nothing. This is useful mainly in command files (see section Command files).
can fill in the rest of a word in a command for you, if there is only one possibility; it can also show you what the valid possibilities are for the next word in a command, at any time. This works for commands, subcommands, and the names of symbols in your program.
Press the TAB key whenever you want to fill out the rest of a word. If there is only one possibility, fills in the word, and waits for you to finish the command (or press RET to enter it). For example, if you type
() info bre TAB
fills in the rest of the word `breakpoints', since that is
the only info subcommand beginning with `bre':
() info breakpoints
You can either press RET at this point, to run the info
breakpoints command, or backspace and enter something else, if
`breakpoints' does not look like the command you expected. (If you
were sure you wanted info breakpoints in the first place, you
might as well just type RET immediately after `info bre',
to exploit command abbreviations rather than command completion).
If there is more than one possibility for the next word when you press TAB, sounds a bell. You can either supply more characters and try again, or just press TAB a second time; displays all the possible completions for that word. For example, you might want to set a breakpoint on a subroutine whose name begins with `make_', but when you type b make_TAB just sounds the bell. Typing TAB again displays all the function names in your program that begin with those characters, for example:
() b make_ TAB sounds bell; press TAB again, to see: make_a_section_from_file make_environ make_abs_section make_function_type make_blockvector make_pointer_type make_cleanup make_reference_type make_command make_symbol_completion_list () b make_
After displaying the available possibilities, copies your partial input (`b make_' in the example) so you can finish the command.
If you just want to see the list of alternatives in the first place, you can press M-? rather than pressing TAB twice. M-? means META ?. You can type this either by holding down a key designated as the META shift on your keyboard (if there is one) while typing ?, or as ESC followed by ?.
Sometimes the string you need, while logically a "word", may contain
parentheses or other characters that normally excludes from
its notion of a word. To permit word completion to work in this
situation, you may enclose words in ' (single quote marks) in
commands.
The most likely situation where you might need this is in typing the
name of a C++ function. This is because C++ allows function
overloading (multiple definitions of the same function, distinguished
by argument type). For example, when you want to set a breakpoint you
may need to distinguish whether you mean the version of name
that takes an int parameter, name(int), or the version
that takes a float parameter, name(float). To use the
word-completion facilities in this situation, type a single quote
' at the beginning of the function name. This alerts
that it may need to consider more information than usual
when you press TAB or M-? to request word completion:
() b 'bubble( M-? bubble(double,double) bubble(int,int) () b 'bubble(
In some cases, can tell that completing a name requires using quotes. When this happens, inserts the quote for you (while completing as much as it can) if you do not type the quote in the first place:
() b bub TAB alters your input line to the following, and rings a bell: () b 'bubble(
In general, can tell that a quote is needed (and inserts it) if you have not yet started typing the argument list when you ask for completion on an overloaded symbol.
For more information about overloaded functions, see section C++ expressions. You can use the command set
overload-resolution off to disable overload resolution;
see section features for C++.
You can always ask itself for information on its commands,
using the command help.
help
h
help (abbreviated h) with no arguments to
display a short list of named classes of commands:
() help List of classes of commands: aliases -- Aliases of other commands breakpoints -- Making program stop at certain points data -- Examining data files -- Specifying and examining files internals -- Maintenance commands obscure -- Obscure features running -- Running the program stack -- Examining the stack status -- Status inquiries support -- Support facilities tracepoints -- Tracing of program execution without
stopping the program user-defined -- User-defined commands Type "help" followed by a class name for a list of commands in that class. Type "help" followed by command name for full documentation. Command name abbreviations are allowed if unambiguous. ()
help class
status:
() help status Status inquiries. List of commands: info -- Generic command for showing things about the program being debugged show -- Generic command for showing things about the debugger Type "help" followed by command name for full documentation. Command name abbreviations are allowed if unambiguous. ()
help command
help argument, displays a
short paragraph on how to use that command.
apropos args
apropos args command searches through all of the
commands, and their documentation, for the regular expression specified in
args. It prints out all matches found. For example:
apropos reloadresults in:
set symbol-reloading -- Set dynamic symbol table reloading
multiple times in one run
show symbol-reloading -- Show dynamic symbol table reloading
multiple times in one run
complete args
complete args command lists all the possible completions
for the beginning of a command. Use args to specify the beginning of the
command you want completed. For example:
complete iresults in:
if ignore info inspectThis is intended for use by GNU Emacs.
In addition to help, you can use the commands info
and show to inquire about the state of your program, or the state
of itself. Each command supports many topics of inquiry; this
manual introduces each of them in the appropriate context. The listings
under info and under show in the Index point to
all the sub-commands. See section Index.
info
i) is for describing the state of your
program. For example, you can list the arguments given to your program
with info args, list the registers currently in use with info
registers, or list the breakpoints you have set with info breakpoints.
You can get a complete list of the info sub-commands with
help info.
set
set. For example, you can set the prompt to a $-sign with
set prompt $.
show
info, show is for describing the state of
itself.
You can change most of the things you can show, by using the
related command set; for example, you can control what number
system is used for displays with set radix, or simply inquire
which is currently in use with show radix.
To display all the settable parameters and their current
values, you can use show with no arguments; you may also use
info set. Both commands produce the same display.
Here are three miscellaneous show subcommands, all of which are
exceptional in lacking corresponding set commands:
show version
show copying
show warranty
When you run a program under , you must first generate debugging information when you compile it.
You may start with its arguments, if any, in an environment of your choice. If you are doing native debugging, you may redirect your program's input and output, debug an already running process, or kill a child process.
In order to debug a program effectively, you need to generate debugging information when you compile it. This debugging information is stored in the object file; it describes the data type of each variable or function and the correspondence between source line numbers and addresses in the executable code.
To request debugging information, specify the `-g' option when you run the compiler.
Many C compilers are unable to handle the `-g' and `-O' options together. Using those compilers, you cannot generate optimized executables containing debugging information.
, the GNU C compiler, supports `-g' with or without `-O', making it possible to debug optimized code. We recommend that you always use `-g' whenever you compile a program. You may think your program is correct, but there is no sense in pushing your luck.
When you debug a program compiled with `-g -O', remember that the optimizer is rearranging your code; the debugger shows you what is really there. Do not be too surprised when the execution path does not exactly match your source file! An extreme example: if you define a variable, but never use it, never sees that variable--because the compiler optimizes it out of existence.
Some things do not work as well with `-g -O' as with just `-g', particularly on machines with instruction scheduling. If in doubt, recompile with `-g' alone, and if this fixes the problem, please report it to us as a bug (including a test case!).
Older versions of the GNU C compiler permitted a variant option `-gg' for debugging information. no longer supports this format; if your GNU C compiler has this option, do not use it.
run
r
run command to start your program under .
You must first specify the program name (except on VxWorks) with an
argument to (see section Getting In and Out of), or by using the file or exec-file command
(see section Commands to specify files).
If you are running your program in an execution environment that
supports processes, run creates an inferior process and makes
that process run your program. (In environments without processes,
run jumps to the start of your program.)
The execution of a program is affected by certain information it receives from its superior. provides ways to specify this information, which you must do before starting your program. (You can change it after starting your program, but such changes only affect your program the next time you start it.) This information may be divided into four categories:
run command. If a shell is available on your target, the shell
is used to pass the arguments, so that you may use normal conventions
(such as wildcard expansion or variable substitution) in describing
the arguments.
In Unix systems, you can control which shell is used with the
SHELL environment variable.
See section Your program's arguments.
set environment and unset
environment to change parts of the environment that affect
your program. See section Your program's environment.
cd command in .
See section Your program's working directory.
run command line, or you can use the tty command to
set a different device for your program.
See section Your program's input and output.
Warning: While input and output redirection work, you cannot use
pipes to pass the output of the program you are debugging to another
program; if you attempt this, is likely to wind up debugging the
wrong program.
When you issue the run command, your program begins to execute
immediately. See section Stopping and Continuing, for discussion
of how to arrange for your program to stop. Once your program has
stopped, you may call functions in your program, using the print
or call commands. See section Examining Data.
If the modification time of your symbol file has changed since the last time read its symbols, discards its symbol table, and reads it again. When it does this, tries to retain your current breakpoints.
The arguments to your program can be specified by the arguments of the
run command.
They are passed to a shell, which expands wildcard characters and
performs redirection of I/O, and thence to your program. Your
SHELL environment variable (if it exists) specifies what shell
uses. If you do not define SHELL, uses
the default shell (`/bin/sh' on Unix).
On non-Unix systems, the program is usually invoked directly by , which emulates I/O redirection via the appropriate system calls, and the wildcard characters are expanded by the startup code of the program, not by the shell.
run with no arguments uses the same arguments used by the previous
run, or those set by the set args command.
set args
set args has no arguments, run executes your program
with no arguments. Once you have run your program with arguments,
using set args before the next run is the only way to run
it again without arguments.
show args
The environment consists of a set of environment variables and their values. Environment variables conventionally record such things as your user name, your home directory, your terminal type, and your search path for programs to run. Usually you set up environment variables with the shell and they are inherited by all the other programs you run. When debugging, it can be useful to try running your program with a modified environment without having to start over again.
path directory
PATH environment variable
(the search path for executables) that will be passed to your program.
The value of PATH used by does not change.
You may specify several directory names, separated by whitespace or by a
system-dependent separator character (`:' on Unix, `;' on
MS-DOS and MS-Windows). If directory is already in the path, it
is moved to the front, so it is searched sooner.
You can use the string `$cwd' to refer to whatever is the current
working directory at the time searches the path. If you
use `.' instead, it refers to the directory where you executed the
path command. replaces `.' in the
directory argument (with the current path) before adding
directory to the search path.
show paths
PATH
environment variable).
show environment [varname]
environment as env.
set environment varname [=value]
set env USER = footells the debugged program, when subsequently run, that its user is named `foo'. (The spaces around `=' are used for clarity here; they are not actually required.)
unset environment varname
unset environment removes the variable from the environment,
rather than assigning it an empty value.
Warning: On Unix systems, runs your program using
the shell indicated
by your SHELL environment variable if it exists (or
/bin/sh if not). If your SHELL variable names a shell
that runs an initialization file--such as `.cshrc' for C-shell, or
`.bashrc' for BASH--any variables you set in that file affect
your program. You may wish to move setting of environment variables to
files that are only run when you sign on, such as `.login' or
`.profile'.
Each time you start your program with run, it inherits its
working directory from the current working directory of .
The working directory is initially whatever it inherited
from its parent process (typically the shell), but you can specify a new
working directory in with the cd command.
The working directory also serves as a default for the commands that specify files for to operate on. See section Commands to specify files.
cd directory
pwd
By default, the program you run under does input and output to the same terminal that uses. switches the terminal to its own terminal modes to interact with you, but it records the terminal modes your program was using and switches back to them when you continue running your program.
info terminal
You can redirect your program's input and/or output using shell
redirection with the run command. For example,
run > outfile
starts your program, diverting its output to the file `outfile'.
Another way to specify where your program should do input and output is
with the tty command. This command accepts a file name as
argument, and causes this file to be the default for future run
commands. It also resets the controlling terminal for the child
process, for future run commands. For example,
tty /dev/ttyb
directs that processes started with subsequent run commands
default to do input and output on the terminal `/dev/ttyb' and have
that as their controlling terminal.
An explicit redirection in run overrides the tty command's
effect on the input/output device, but not its effect on the controlling
terminal.
When you use the tty command or redirect input in the run
command, only the input for your program is affected. The input
for still comes from your terminal.
attach process-id
info files shows your active
targets.) The command takes as argument a process ID. The usual way to
find out the process-id of a Unix process is with the ps utility,
or with the `jobs -l' shell command.
attach does not repeat if you press RET a second time after
executing the command.
To use attach, your program must be running in an environment
which supports processes; for example, attach does not work for
programs on bare-board targets that lack an operating system. You must
also have permission to send the process a signal.
When you use attach, the debugger finds the program running in
the process first by looking in the current working directory, then (if
the program is not found) by using the source file search path
(see section Specifying source directories). You can also use
the file command to load the program. See section Commands to specify files.
The first thing does after arranging to debug the specified
process is to stop it. You can examine and modify an attached process
with all the commands that are ordinarily available when
you start processes with run. You can insert breakpoints; you
can step and continue; you can modify storage. If you would rather the
process continue running, you may use the continue command after
attaching to the process.
detach
detach command to release it from control. Detaching
the process continues its execution. After the detach command,
that process and become completely independent once more, and you
are ready to attach another process or start one with run.
detach does not repeat if you press RET again after
executing the command.
If you exit or use the run command while you have an
attached process, you kill that process. By default, asks
for confirmation if you try to do either of these things; you can
control whether or not you need to confirm by using the set
confirm command (see section Optional warnings and messages).
kill
This command is useful if you wish to debug a core dump instead of a running process. ignores any core dump file while your program is running.
On some operating systems, a program cannot be executed outside
while you have breakpoints set on it inside . You can use the
kill command in this situation to permit running your program
outside the debugger.
The kill command is also useful if you wish to recompile and
relink your program, since on many systems it is impossible to modify an
executable file while it is running in a process. In this case, when you
next type run, notices that the file has changed, and
reads the symbol table again (while trying to preserve your current
breakpoint settings).
In some operating systems, such as HP-UX and Solaris, a single program may have more than one thread of execution. The precise semantics of threads differ from one operating system to another, but in general the threads of a single program are akin to multiple processes--except that they share one address space (that is, they can all examine and modify the same variables). On the other hand, each thread has its own registers and execution stack, and perhaps private memory.
provides these facilities for debugging multi-thread programs:
Warning: These facilities are not yet available on every configuration where the operating system supports threads. If your does not support threads, these commands have no effect. For example, a system without thread support shows no output from `info threads', and always rejects the
threadcommand, like this:() info threads () thread 1 Thread ID 1 not known. Use the "info threads" command to see the IDs of currently known threads.
The thread debugging facility allows you to observe all threads while your program runs--but whenever takes control, one thread in particular is always the focus of debugging. This thread is called the current thread. Debugging commands show program information from the perspective of the current thread.
Whenever detects a new thread in your program, it displays the target system's identification for the thread with a message in the form `[New systag]'. systag is a thread identifier whose form varies depending on the particular system. For example, on LynxOS, you might see
[New process 35 thread 27]
when notices a new thread. In contrast, on an SGI system, the systag is simply something like `process 368', with no further qualifier.
For debugging purposes, associates its own thread number--always a single integer--with each thread in your program.
info threads
() info threads
3 process 35 thread 27 0x34e5 in sigpause ()
2 process 35 thread 23 0x34e5 in sigpause ()
* 1 process 35 thread 13 main (argc=1, argv=0x7ffffff8)
at threadtest.c:68
On HP-UX systems:
For debugging purposes, associates its own thread number--a small integer assigned in thread-creation order--with each thread in your program.
Whenever detects a new thread in your program, it displays both 's thread number and the target system's identification for the thread with a message in the form `[New systag]'. systag is a thread identifier whose form varies depending on the particular system. For example, on HP-UX, you see
[New thread 2 (system thread 26594)]
when notices a new thread.
info threads
() info threads
* 3 system thread 26607 worker (wptr=0x7b09c318 "@") \
at quicksort.c:137
2 system thread 26606 0x7b0030d8 in __ksleep () \
from /usr/lib/libc.2
1 system thread 27905 0x7b003498 in _brk () \
from /usr/lib/libc.2
thread threadno
() thread 2 [Switching to process 35 thread 23] 0x34e5 in sigpause ()As with the `[New ...]' message, the form of the text after `Switching to' depends on your system's conventions for identifying threads.
thread apply [threadno] [all] args
thread apply command allows you to apply a command to one or
more threads. Specify the numbers of the threads that you want affected
with the command argument threadno. threadno is the internal
thread number, as shown in the first field of the `info
threads' display. To apply a command to all threads, use
thread apply all args.
Whenever stops your program, due to a breakpoint or a signal, it automatically selects the thread where that breakpoint or signal happened. alerts you to the context switch with a message of the form `[Switching to systag]' to identify the thread.
See section Stopping and starting multi-thread programs, for more information about how behaves when you stop and start programs with multiple threads.
See section Setting watchpoints, for information about watchpoints in programs with multiple threads.
On most systems, has no special support for debugging
programs which create additional processes using the fork
function. When a program forks, will continue to debug the
parent process and the child process will run unimpeded. If you have
set a breakpoint in any code which the child then executes, the child
will get a SIGTRAP signal which (unless it catches the signal)
will cause it to terminate.
However, if you want to debug the child process there is a workaround
which isn't too painful. Put a call to sleep in the code which
the child process executes after the fork. It may be useful to sleep
only if a certain environment variable is set, or a certain file exists,
so that the delay need not occur when you don't want to run
on the child. While the child is sleeping, use the ps program to
get its process ID. Then tell (a new invocation of
if you are also debugging the parent process) to attach to
the child process (see section Debugging an already-running process). From that point on you can debug
the child process just like any other process which you attached to.
On HP-UX (11.x and later only?), provides support for
debugging programs that create additional processes using the
fork or vfork function.
By default, when a program forks, will continue to debug the parent process and the child process will run unimpeded.
If you want to follow the child process instead of the parent process,
use the command set follow-fork-mode.
set follow-fork-mode mode
fork or
vfork. A call to fork or vfork creates a new
process. The mode can be:
parent
child
ask
show follow-fork-mode
fork or vfork call.
If you ask to debug a child process and a vfork is followed by an
exec, executes the new target up to the first
breakpoint in the new target. If you have a breakpoint set on
main in your original program, the breakpoint will also be set on
the child process's main.
When a child process is spawned by vfork, you cannot debug the
child or parent until an exec call completes.
If you issue a run command to after an exec
call executes, the new target restarts. To restart the parent process,
use the file command with the parent executable name as its
argument.
You can use the catch command to make stop whenever
a fork, vfork, or exec call is made. See section Setting catchpoints.
The principal purposes of using a debugger are so that you can stop your program before it terminates; or so that, if your program runs into trouble, you can investigate and find out why.
Inside , your program may stop for any of several reasons,
such as a signal, a breakpoint, or reaching a new line after a
command such as step. You may then examine and
change variables, set new breakpoints or remove old ones, and then
continue execution. Usually, the messages shown by provide
ample explanation of the status of your program--but you can also
explicitly request this information at any time.
info program
A breakpoint makes your program stop whenever a certain point in
the program is reached. For each breakpoint, you can add conditions to
control in finer detail whether your program stops. You can set
breakpoints with the break command and its variants (see section Setting breakpoints), to specify the place where your program
should stop by line number, function name or exact address in the
program.
In HP-UX, SunOS 4.x, SVR4, and Alpha OSF/1 configurations, you can set
breakpoints in shared libraries before the executable is run. There is
a minor limitation on HP-UX systems: you must wait until the executable
is run in order to set breakpoints in shared library routines that are
not called directly by the program (for example, routines that are
arguments in a pthread_create call).
A watchpoint is a special breakpoint that stops your program when the value of an expression changes. You must use a different command to set watchpoints (see section Setting watchpoints), but aside from that, you can manage a watchpoint like any other breakpoint: you enable, disable, and delete both breakpoints and watchpoints using the same commands.
You can arrange to have values from your program displayed automatically whenever stops at a breakpoint. See section Automatic display.
A catchpoint is another special breakpoint that stops your program
when a certain kind of event occurs, such as the throwing of a C++
exception or the loading of a library. As with watchpoints, you use a
different command to set a catchpoint (see section Setting catchpoints), but aside from that, you can manage a catchpoint like any
other breakpoint. (To stop when your program receives a signal, use the
handle command; see section Signals.)
assigns a number to each breakpoint, watchpoint, or catchpoint when you create it; these numbers are successive integers starting with one. In many of the commands for controlling various features of breakpoints you use the breakpoint number to say which breakpoint you want to change. Each breakpoint may be enabled or disabled; if disabled, it has no effect on your program until you enable it again.
Some commands accept a range of breakpoints on which to operate. A breakpoint range is either a single breakpoint number, like `5', or two such numbers, in increasing order, separated by a hyphen, like `5-7'. When a breakpoint range is given to a command, all breakpoint in that range are operated on.
Breakpoints are set with the break command (abbreviated
b). The debugger convenience variable `$bpnum' records the
number of the breakpoint you've set most recently; see section Convenience variables, for a discussion of what you can do with
convenience variables.
You have several ways to say where the breakpoint should go.
break function
break +offset
break -offset
break linenum
break filename:linenum
break filename:function
break *address
break
break sets a breakpoint at
the next instruction to be executed in the selected stack frame
(see section Examining the Stack). In any selected frame but the
innermost, this makes your program stop as soon as control
returns to that frame. This is similar to the effect of a
finish command in the frame inside the selected frame--except
that finish does not leave an active breakpoint. If you use
break without an argument in the innermost frame, stops
the next time it reaches the current location; this may be useful
inside loops.
normally ignores breakpoints when it resumes execution, until at
least one instruction has been executed. If it did not do this, you
would be unable to proceed past a breakpoint without first disabling the
breakpoint. This rule applies whether or not the breakpoint already
existed when your program stopped.
break ... if cond
tbreak args
break command, and the breakpoint is set in the same
way, but the breakpoint is automatically deleted after the first time your
program stops there. See section Disabling breakpoints.
hbreak args
break command and the breakpoint is set in the same way, but the
breakpoint requires hardware support and some target hardware may not
have this support. The main purpose of this is EPROM/ROM code
debugging, so you can set a breakpoint at an instruction without
changing the instruction. This can be used with the new trap-generation
provided by SPARClite DSU and some x86-based targets. These targets
will generate traps when a program accesses some data or instruction
address that is assigned to the debug registers. However the hardware
breakpoint registers can take a limited number of breakpoints. For
example, on the DSU, only two data breakpoints can be set at a time, and
will reject this command if more than two are used. Delete
or disable unused hardware breakpoints before setting new ones
(see section Disabling breakpoints). See section Break conditions.
thbreak args
hbreak command and the breakpoint is set in
the same way. However, like the tbreak command,
the breakpoint is automatically deleted after the
first time your program stops there. Also, like the hbreak
command, the breakpoint requires hardware support and some target hardware
may not have this support. See section Disabling breakpoints.
See also section Break conditions.
rbreak regex
break command. You can delete them, disable them, or make
them conditional the same way as any other breakpoint.
The syntax of the regular expression is the standard one used with tools
like `grep'. Note that this is different from the syntax used by
shells, so for instance foo* matches all functions that include
an fo followed by zero or more os. There is an implicit
.* leading and trailing the regular expression you supply, so to
match only functions that begin with foo, use ^foo.
When debugging C++ programs, rbreak is useful for setting
breakpoints on overloaded functions that are not members of any special
classes.
info breakpoints [n]
info break [n]
info watchpoints [n]
info break shows the condition on
the line following the affected breakpoint; breakpoint commands, if any,
are listed after that.
info break with a breakpoint
number n as argument lists only that breakpoint. The
convenience variable $_ and the default examining-address for
the x command are set to the address of the last breakpoint
listed (see section Examining memory).
info break displays a count of the number of times the breakpoint
has been hit. This is especially useful in conjunction with the
ignore command. You can ignore a large number of breakpoint
hits, look at the breakpoint info to see how many times the breakpoint
was hit, and then run again, ignoring one less than that number. This
will get you quickly to the last hit of that breakpoint.
allows you to set any number of breakpoints at the same place in your program. There is nothing silly or meaningless about this. When the breakpoints are conditional, this is even useful (see section Break conditions).
itself sometimes sets breakpoints in your program for special
purposes, such as proper handling of longjmp (in C programs).
These internal breakpoints are assigned negative numbers, starting with
-1; `info breakpoints' does not display them.
You can see these breakpoints with the maintenance command `maint info breakpoints'.
maint info breakpoints
breakpoint
watchpoint
longjmp
longjmp calls.
longjmp resume
longjmp.
until
until command.
finish
finish command.
shlib events
You can use a watchpoint to stop execution whenever the value of an expression changes, without having to predict a particular place where this may happen.
Depending on your system, watchpoints may be implemented in software or hardware. does software watchpointing by single-stepping your program and testing the variable's value each time, which is hundreds of times slower than normal execution. (But this may still be worth it, to catch errors where you have no clue what part of your program is the culprit.)
On some systems, such as HP-UX, Linux and some other x86-based targets, includes support for hardware watchpoints, which do not slow down the running of your program.
watch expr
rwatch expr
awatch expr
info watchpoints
info break.
sets a hardware watchpoint if possible. Hardware watchpoints execute very quickly, and the debugger reports a change in value at the exact instruction where the change occurs. If cannot set a hardware watchpoint, it sets a software watchpoint, which executes more slowly and reports the change in value at the next statement, not the instruction, after the change occurs.
When you issue the watch command, reports
Hardware watchpoint num: expr
if it was able to set a hardware watchpoint.
Currently, the awatch and rwatch commands can only set
hardware watchpoints, because accesses to data that don't change the
value of the watched expression cannot be detected without examining
every instruction as it is being executed, and does not do
that currently. If finds that it is unable to set a
hardware breakpoint with the awatch or rwatch command, it
will print a message like this:
Expression cannot be implemented with read/access watchpoint.
Sometimes, cannot set a hardware watchpoint because the data type of the watched expression is wider than what a hardware watchpoint on the target machine can handle. For example, some systems can only watch regions that are up to 4 bytes wide; on such systems you cannot set hardware watchpoints for an expression that yields a double-precision floating-point number (which is typically 8 bytes wide). As a work-around, it might be possible to break the large region into a series of smaller ones and watch them with separate watchpoints.
If you set too many hardware watchpoints, might be unable to insert all of them when you resume the execution of your program. Since the precise number of active watchpoints is unknown until such time as the program is about to be resumed, might not be able to warn you about this when you set the watchpoints, and the warning will be printed only when the program is resumed:
Hardware watchpoint num: Could not insert watchpoint
If this happens, delete or disable some of the watchpoints.
The SPARClite DSU will generate traps when a program accesses some data
or instruction address that is assigned to the debug registers. For the
data addresses, DSU facilitates the watch command. However the
hardware breakpoint registers can only take two data watchpoints, and
both watchpoints must be the same kind. For example, you can set two
watchpoints with watch commands, two with rwatch commands,
or two with awatch commands, but you cannot set one
watchpoint with one command and the other with a different command.
will reject the command if you try to mix watchpoints.
Delete or disable unused watchpoint commands before setting new ones.
If you call a function interactively using print or call,
any watchpoints you have set will be inactive until reaches another
kind of breakpoint or the call completes.
automatically deletes watchpoints that watch local
(automatic) variables, or expressions that involve such variables, when
they go out of scope, that is, when the execution leaves the block in
which these variables were defined. In particular, when the program
being debugged terminates, all local variables go out of scope,
and so only watchpoints that watch global variables remain set. If you
rerun the program, you will need to set all such watchpoints again. One
way of doing that would be to set a code breakpoint at the entry to the
main function and when it breaks, set all the watchpoints.
Warning: In multi-thread programs, watchpoints have only limited usefulness. With the current watchpoint implementation, can only watch the value of an expression in a single thread. If you are confident that the expression can only change due to the current thread's activity (and if you are also confident that no other thread can become current), then you can use watchpoints as usual. However, may not notice when a non-current thread's activity changes the expression.
HP-UX Warning: In multi-thread programs, software watchpoints have only limited usefulness. If creates a software watchpoint, it can only watch the value of an expression in a single thread. If you are confident that the expression can only change due to the current thread's activity (and if you are also confident that no other thread can become current), then you can use software watchpoints as usual. However, may not notice when a non-current thread's activity changes the expression. (Hardware watchpoints, in contrast, watch an expression in all threads.)
You can use catchpoints to cause the debugger to stop for certain
kinds of program events, such as C++ exceptions or the loading of a
shared library. Use the catch command to set a catchpoint.
catch event
throw
catch
exec
exec. This is currently only available for HP-UX.
fork
fork. This is currently only available for HP-UX.
vfork
vfork. This is currently only available for HP-UX.
load
load libname
unload
unload libname
tcatch event
Use the info break command to list the current catchpoints.
There are currently some limitations to C++ exception handling
(catch throw and catch catch) in :
Sometimes catch is not the best way to debug exception handling:
if you need to know exactly where an exception is raised, it is better to
stop before the exception handler is called, since that way you
can see the stack before any unwinding takes place. If you set a
breakpoint in an exception handler instead, it may not be easy to find
out where the exception was raised.
To stop just before an exception handler is called, you need some
knowledge of the implementation. In the case of GNU C++, exceptions are
raised by calling a library function named __raise_exception
which has the following ANSI C interface:
/* addr is where the exception identifier is stored.
id is the exception identifier. */
void __raise_exception (void **addr, void *id);
To make the debugger catch all exceptions before any stack
unwinding takes place, set a breakpoint on __raise_exception
(see section Breakpoints, watchpoints, and catchpoints).
With a conditional breakpoint (see section Break conditions) that depends on the value of id, you can stop your program when a specific exception is raised. You can use multiple conditional breakpoints to stop your program when any of a number of exceptions are raised.
It is often necessary to eliminate a breakpoint, watchpoint, or catchpoint once it has done its job and you no longer want your program to stop there. This is called deleting the breakpoint. A breakpoint that has been deleted no longer exists; it is forgotten.
With the clear command you can delete breakpoints according to
where they are in your program. With the delete command you can
delete individual breakpoints, watchpoints, or catchpoints by specifying
their breakpoint numbers.
It is not necessary to delete a breakpoint to proceed past it. automatically ignores breakpoints on the first instruction to be executed when you continue execution without changing the execution address.
clear
clear function
clear filename:function
clear linenum
clear filename:linenum
delete [breakpoints] [range...]
set
confirm off). You can abbreviate this command as d.
Rather than deleting a breakpoint, watchpoint, or catchpoint, you might prefer to disable it. This makes the breakpoint inoperative as if it had been deleted, but remembers the information on the breakpoint so that you can enable it again later.
You disable and enable breakpoints, watchpoints, and catchpoints with
the enable and disable commands, optionally specifying one
or more breakpoint numbers as arguments. Use info break or
info watch to print a list of breakpoints, watchpoints, and
catchpoints if you do not know which numbers to use.
A breakpoint, watchpoint, or catchpoint can have any of four different states of enablement:
break command starts out in this state.
tbreak command starts out in this state.
You can use the following commands to enable or disable breakpoints, watchpoints, and catchpoints:
disable [breakpoints] [range...]
disable as dis.
enable [breakpoints] [range...]
enable [breakpoints] once range...
enable [breakpoints] delete range...
Except for a breakpoint set with tbreak (see section Setting breakpoints), breakpoints that you set are initially enabled;
subsequently, they become disabled or enabled only when you use one of
the commands above. (The command until can set and delete a
breakpoint of its own, but it does not change the state of your other
breakpoints; see section Continuing and stepping.)
The simplest sort of breakpoint breaks every time your program reaches a specified place. You can also specify a condition for a breakpoint. A condition is just a Boolean expression in your programming language (see section Expressions). A breakpoint with a condition evaluates the expression each time your program reaches it, and your program stops only if the condition is true.
This is the converse of using assertions for program validation; in that situation, you want to stop when the assertion is violated--that is, when the condition is false. In C, if you want to test an assertion expressed by the condition assert, you should set the condition `! assert' on the appropriate breakpoint.
Conditions are also accepted for watchpoints; you may not need them, since a watchpoint is inspecting the value of an expression anyhow--but it might be simpler, say, to just set a watchpoint on a variable name, and specify a condition that tests whether the new value is an interesting one.
Break conditions can have side effects, and may even call functions in your program. This can be useful, for example, to activate functions that log program progress, or to use your own print functions to format special data structures. The effects are completely predictable unless there is another enabled breakpoint at the same address. (In that case, might see the other breakpoint first and stop your program without checking the condition of this one.) Note that breakpoint commands are usually more convenient and flexible than break conditions for the purpose of performing side effects when a breakpoint is reached (see section Breakpoint command lists).
Break conditions can be specified when a breakpoint is set, by using
`if' in the arguments to the break command. See section Setting breakpoints. They can also be changed at any time
with the condition command.
You can also use the if keyword with the watch command.
The catch command does not recognize the if keyword;
condition is the only way to impose a further condition on a
catchpoint.
condition bnum expression
condition, checks expression immediately for
syntactic correctness, and to determine whether symbols in it have
referents in the context of your breakpoint. If expression uses
symbols not referenced in the context of the breakpoint,
prints an error message:
No symbol "foo" in current context.does not actually evaluate expression at the time the
condition
command (or a command that sets a breakpoint with a condition, like
break if ...) is given, however. See section Expressions.
condition bnum
A special case of a breakpoint condition is to stop only when the breakpoint has been reached a certain number of times. This is so useful that there is a special way to do it, using the ignore count of the breakpoint. Every breakpoint has an ignore count, which is an integer. Most of the time, the ignore count is zero, and therefore has no effect. But if your program reaches a breakpoint whose ignore count is positive, then instead of stopping, it just decrements the ignore count by one and continues. As a result, if the ignore count value is n, the breakpoint does not stop the next n times your program reaches it.
ignore bnum count
continue to resume execution of your program from a
breakpoint, you can specify an ignore count directly as an argument to
continue, rather than using ignore. See section Continuing and stepping.
If a