This file describes gdb, the gnu symbolic debugger.
This is the Ninth Edition, for gdb Version 6.3.50.20050707.
Copyright (C) 1988-2005 Free Software Foundation, Inc.
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The purpose of a debugger such as gdb 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.
gdb 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 gdb to debug programs written in C and C++. For more information, see Supported languages. For more information, see C and C++.
Support for Modula-2 is partial. For information on Modula-2, see Modula-2.
Debugging Pascal programs which use sets, subranges, file variables, or nested functions does not currently work. gdb does not support entering expressions, printing values, or similar features using Pascal syntax.
gdb can be used to debug programs written in Fortran, although it may be necessary to refer to some variables with a trailing underscore.
gdb can be used to debug programs written in Objective-C, using either the Apple/NeXT or the GNU Objective-C runtime.
gdb 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 gdb, 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 gdb 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 gdb through major releases: Andrew Cagney (releases 6.1, 6.0, 5.3, 5.2, 5.1 and 5.0); 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 gdb, 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).
gdb 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 DWARF 2 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. Marko Mlinar contributed OpenRISC 1000 support.
Andreas Schwab contributed M68K gnu/Linux support.
Rich Schaefer and Peter Schauer helped with support of SunOS shared libraries.
Jay Fenlason and Roland McGrath ensured that gdb 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 (now Renesas 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 (now Renesas) 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 gdb.
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 Text User Interface (nee 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 gdb to MS-DOS, for the DJGPP project. Robert Hoehne made significant contributions to the DJGPP port.
Cygnus Solutions has sponsored gdb maintenance and much of its development since 1991. Cygnus engineers who have worked on gdb 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.
Andrew Cagney, Fernando Nasser, and Elena Zannoni, while working for Cygnus Solutions, implemented the original gdb/mi interface.
Jim Blandy added support for preprocessor macros, while working for Red Hat.
Andrew Cagney designed gdb's architecture vector. Many people including Andrew Cagney, Stephane Carrez, Randolph Chung, Nick Duffek, Richard Henderson, Mark Kettenis, Grace Sainsbury, Kei Sakamoto, Yoshinori Sato, Michael Snyder, Andreas Schwab, Jason Thorpe, Corinna Vinschen, Ulrich Weigand, and Elena Zannoni, helped with the migration of old architectures to this new framework.
You can use this manual at your leisure to read all about gdb. However, a handful of commands are enough to get started using the debugger. This chapter illustrates those commands.
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 gdb to try to see what is going on.
$ gdb m4
gdb 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 gdb; type "show warranty"
for details.
gdb 6.3.50.20050707, Copyright 1999 Free Software Foundation, Inc...
(gdb)
gdb 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 gdb to use a narrower display width than usual, so that examples fit in this manual.
(gdb) 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 gdb
break command.
(gdb) break m4_changequote
Breakpoint 1 at 0x62f4: file builtin.c, line 879.
Using the run command, we start m4 running under gdb
control; as long as control does not reach the m4_changequote
subroutine, the program runs as usual:
(gdb) run
Starting program: /work/Editorial/gdb/gnu/m4/m4
define(foo,0000)
foo
0000
To trigger the breakpoint, we call changequote. gdb
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.
(gdb) 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.
(gdb) 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.
(gdb) 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.
(gdb) s
0x3b5c 532 if (rquote != def_rquote)
(gdb) s
0x3b80 535 lquote = (lq == nil || *lq == '\0') ? \
def_lquote : xstrdup(lq);
(gdb) n
536 rquote = (rq == nil || *rq == '\0') ? def_rquote\
: xstrdup(rq);
(gdb) 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.
(gdb) p lquote
$1 = 0x35d40 "<QUOTE>"
(gdb) 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.
(gdb) 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.
(gdb) n
539 len_rquote = strlen(lquote);
(gdb) n
540 }
(gdb) p len_lquote
$3 = 9
(gdb) 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.
(gdb) p len_lquote=strlen(lquote)
$5 = 7
(gdb) 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:
(gdb) 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 gdb; it
indicates m4 has finished executing. We can end our gdb
session with the gdb quit command.
(gdb) quit
This chapter discusses how to start gdb, and how to get out of it. The essentials are:
Invoke gdb by running the program gdb. Once started,
gdb reads commands from the terminal until you tell it to exit.
You can also run gdb 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 gdb is with one argument, specifying an executable program:
gdb program
You can also start with both an executable program and a core file specified:
gdb program core
You can, instead, specify a process ID as a second argument, if you want to debug a running process:
gdb program 1234
would attach gdb to process 1234 (unless you also have a file
named 1234; gdb does check for a core file first).
Taking advantage of the second command-line argument requires a fairly complete operating system; when you use gdb 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. gdb will warn you if it is unable to attach or to read core dumps.
You can optionally have gdb pass any arguments after the
executable file to the inferior using --args. This option stops
option processing.
gdb --args gcc -O2 -c foo.c
This will cause gdb to debug gcc, and to set
gcc's command-line arguments (see Arguments) to `-O2 -c foo.c'.
You can run gdb without printing the front material, which describes
gdb's non-warranty, by specifying -silent:
gdb -silent
You can further control how gdb starts up by using command-line options. gdb itself can remind you of the options available.
Type
gdb -help
to display all available options and briefly describe their use (`gdb -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 gdb 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' (or `-p' options respectively. (gdb 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'/`-p' option followed by that argument.) If the second argument begins with a decimal digit, gdb will first attempt to attach to it as a process, and if that fails, attempt to open it as a corefile. If you have a corefile whose name begins with a digit, you can prevent gdb from treating it as a pid by prefixing it with ./, eg. ./12345.
If gdb 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. gdb 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-pid number-p numberattach command.
If there is no such process, gdb will attempt to open a core
file named number.
-command file-x file-directory directory-d directory-m-mappedmmap
system call, you can use this option
to have gdb 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 gdb 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 gdb
is run. It holds an exact image of the internal gdb symbol
table. It cannot be shared across multiple host platforms.
-r-readnowYou typically combine the -mapped and -readnow options in
order to build a .syms file that contains complete symbol
information. (See Commands to specify files, for information
on .syms files.) A simple gdb invocation to do nothing
but build a .syms file for future use is:
gdb -batch -nx -mapped -readnow programname
You can run gdb in various alternative modes—for example, in batch mode or quiet mode.
-nx-n-quiet-silent-q-batch0 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 gdb commands
in the command files.
Batch mode may be useful for running gdb 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
gdb control terminates) is not issued when running in batch
mode.
-nowindows-nw-windows-w-cd directory-fullname-f-epoch-annotate levelThe annotation mechanism has largely been superseeded by gdb/mi
(see GDB/MI).
--args-baud bps-b bps-l timeout-tty device-t device-tui-interpreter interp`--interpreter=mi' (or `--interpreter=mi2') causes
gdb to use the gdb/mi interface (see The gdb/mi Interface) included since gdb version 6.0. The
previous gdb/mi interface, included in gdb version 5.3 and
selected with `--interpreter=mi1', is deprecated. Earlier
gdb/mi interfaces are no longer supported.
-write-statistics-versionHere's the description of what gdb does during session startup:
Init files use the same syntax as command files (see Command Files) and are processed by gdb in the same way. The init file in your home directory can set options (such as `set complaints') that affect subsequent processing of command line options and operands. Init files are not executed if you use the `-nx' option (see Choosing modes).
The gdb init files are normally called .gdbinit. On some configurations of gdb, the init file is known by a different name (these are typically environments where a specialized form of gdb may need to coexist with other forms, hence a different name for the specialized version's init file). These are the environments with special init file names:
quit [expression]qquit command (abbreviated
q), or type an end-of-file character (usually C-d). If you
do not supply expression, gdb 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 gdb, but rather terminates the action of any gdb command that is in progress and returns to gdb command level. It is safe to type the interrupt character at any time because gdb does not allow it to take effect until a time when it is safe.
If you have been using gdb to control an attached process or
device, you can release it with the detach command
(see 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 gdb; you can
just use the shell command.
shell command stringSHELL determines which
shell to run. Otherwise gdb 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
gdb:
make make-argsmake program with the specified
arguments. This is equivalent to `shell make make-args'.
You may want to save the output of gdb commands to a file. There are several commands to control gdb's logging.
set logging onset logging offset logging file fileset logging overwrite [on|off]overwrite if
you want set logging on to overwrite the logfile instead.
set logging redirect [on|off]redirect if you want output to go only to the log file.
show loggingYou can abbreviate a gdb command to the first few letters of the command name, if that abbreviation is unambiguous; and you can repeat certain gdb commands by typing just <RET>. You can also use the <TAB> key to get gdb 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 gdb 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.
gdb 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 gdb (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. User-defined commands can disable this feature; see
dont-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.
gdb can also use <RET> in another way: to partition lengthy
output, in a way similar to the common utility more
(see Screen size). Since it is easy to press one
<RET> too many in this situation, gdb 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 Command files).
The C-o binding is useful for repeating a complex sequence of commands. This command accepts the current line, like RET, and then fetches the next line relative to the current line from the history for editing.
gdb 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 gdb commands, gdb subcommands, and the names of symbols in your program.
Press the <TAB> key whenever you want gdb to fill out the rest of a word. If there is only one possibility, gdb fills in the word, and waits for you to finish the command (or press <RET> to enter it). For example, if you type
(gdb) info bre <TAB>
gdb fills in the rest of the word `breakpoints', since that is
the only info subcommand beginning with `bre':
(gdb) 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>, gdb sounds a bell. You can either supply more characters and try again, or just press <TAB> a second time; gdb 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> gdb just sounds the bell. Typing <TAB> again displays all the function names in your program that begin with those characters, for example:
(gdb) b make_ <TAB>
gdb 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 (gdb) b make_
After displaying the available possibilities, gdb 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 gdb 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
gdb 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
gdb that it may need to consider more information than usual
when you press <TAB> or M-? to request word completion:
(gdb) b 'bubble( M-?
bubble(double,double) bubble(int,int)
(gdb) b 'bubble(
In some cases, gdb can tell that completing a name requires using quotes. When this happens, gdb inserts the quote for you (while completing as much as it can) if you do not type the quote in the first place:
(gdb) b bub <TAB>
gdb alters your input line to the following, and rings a bell:
(gdb) b 'bubble(
In general, gdb 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 C++ expressions. You can use the command set
overload-resolution off to disable overload resolution;
see gdb features for C++.
You can always ask gdb itself for information on its commands,
using the command help.
helphhelp (abbreviated h) with no arguments to
display a short list of named classes of commands:
(gdb) 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.
(gdb)
help classstatus:
(gdb) 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.
(gdb)
help commandhelp argument, gdb displays a
short paragraph on how to use that command.
apropos argsapropos command searches through all of the gdb
commands, and their documentation, for the regular expression specified in
args. It prints out all matches found. For example:
apropos reload
results 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 argscomplete 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 i
results in:
if
ignore
info
inspect
This is intended for use by gnu Emacs.
In addition to help, you can use the gdb commands info
and show to inquire about the state of your program, or the state
of gdb 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 Index.
infoi) 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.
setset. For example, you can set the gdb prompt to a $-sign with
set prompt $.
showinfo, show is for describing the state of
gdb 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 versionshow copyinginfo copyingshow warrantyinfo warrantyWhen you run a program under gdb, you must first generate debugging information when you compile it.
You may start gdb 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.
Programs that are to be shipped to your customers are compiled with optimizations, using the `-O' compiler option. However, many compilers are unable to handle the `-g' and `-O' options together. Using those compilers, you cannot generate optimized executables containing debugging information.
gcc, the gnu C/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, gdb 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!). See Variables, for more information about debugging optimized code.
Older versions of the gnu C compiler permitted a variant option `-gg' for debugging information. gdb no longer supports this format; if your gnu C compiler has this option, do not use it.
gdb knows about preprocessor macros and can show you their expansion (see Macros). Most compilers do not include information about preprocessor macros in the debugging information if you specify the -g flag alone, because this information is rather large. Version 3.1 and later of gcc, the gnu C compiler, provides macro information if you specify the options -gdwarf-2 and -g3; the former option requests debugging information in the Dwarf 2 format, and the latter requests “extra information”. In the future, we hope to find more compact ways to represent macro information, so that it can be included with -g alone.
runrrun command to start your program under gdb.
You must first specify the program name (except on VxWorks) with an
argument to gdb (see Getting In and Out of gdb), or by using the file or exec-file command
(see 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. gdb 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 Your program's arguments.
set environment and unset
environment to change parts of the environment that affect
your program. See Your program's environment.
cd command in gdb.
See Your program's working directory.
run command line, or you can use the tty command to
set a different device for your program.
See 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, gdb is likely to wind up debugging the wrong program.
When you issue the run command, your program begins to execute
immediately. See 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 Examining Data.
If the modification time of your symbol file has changed since the last time gdb read its symbols, gdb discards its symbol table, and reads it again. When it does this, gdb tries to retain your current breakpoints.
startmain, but
other languages such as Ada do not require a specific name for their
main procedure. The debugger provides a convenient way to start the
execution of the program and to stop at the beginning of the main
procedure, depending on the language used.
The `start' command does the equivalent of setting a temporary breakpoint at the beginning of the main procedure and then invoking the `run' command.
Some programs contain an elaboration phase where some startup code is
executed before the main procedure is called. This depends on the
languages used to write your program. In C++, for instance,
constructors for static and global objects are executed before
main is called. It is therefore possible that the debugger stops
before reaching the main procedure. However, the temporary breakpoint
will remain to halt execution.
Specify the arguments to give to your program as arguments to the `start' command. These arguments will be given verbatim to the underlying `run' command. Note that the same arguments will be reused if no argument is provided during subsequent calls to `start' or `run'.
It is sometimes necessary to debug the program during elaboration. In
these cases, using the start command would stop the execution of
your program too late, as the program would have already completed the
elaboration phase. Under these circumstances, insert breakpoints in your
elaboration code before running your program.
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
gdb uses. If you do not define SHELL, gdb uses
the default shell (/bin/sh on Unix).
On non-Unix systems, the program is usually invoked directly by gdb, 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 argsset 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 argsThe 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 gdb over again.
path directoryPATH environment variable
(the search path for executables) that will be passed to your program.
The value of PATH used by gdb 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 gdb searches the path. If you
use `.' instead, it refers to the directory where you executed the
path command. gdb replaces `.' in the
directory argument (with the current path) before adding
directory to the search path.
show pathsPATH
environment variable).
show environment [varname]environment as env.
set environment varname [=value]For example, this command:
set env USER = foo
tells 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 varnameunset environment removes the variable from the environment,
rather than assigning it an empty value.
Warning: On Unix systems, gdb 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 gdb.
The gdb working directory is initially whatever it inherited
from its parent process (typically the shell), but you can specify a new
working directory in gdb with the cd command.
The gdb working directory also serves as a default for the commands that specify files for gdb to operate on. See Commands to specify files.
It is generally impossible to find the current working directory of
the process being debugged (since a program can change its directory
during its run). If you work on a system where gdb is
configured with the /proc support, you can use the info
proc command (see SVR4 Process Information) to find out the
current working directory of the debuggee.
By default, the program you run under gdb does input and output to the same terminal that gdb uses. gdb 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 terminalYou 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 gdb still comes from your terminal. tty is an alias
for set inferior-tty.
You can use the show inferior-tty command to tell gdb to
display the name of the terminal that will be used for future runs of your
program.
set inferior-tty /dev/ttybshow inferior-ttyattach process-idinfo 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 Specifying source directories). You can also use
the file command to load the program. See Commands to Specify Files.
The first thing gdb does after arranging to debug the specified
process is to stop it. You can examine and modify an attached process
with all the gdb 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 gdb to the process.
detachdetach command to release it from gdb control. Detaching
the process continues its execution. After the detach command,
that process and gdb 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 gdb or use the run command while you have an
attached process, you kill that process. By default, gdb 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 Optional warnings and messages).
killThis command is useful if you wish to debug a core dump instead of a running process. gdb ignores any core dump file while your program is running.
On some operating systems, a program cannot be executed outside gdb
while you have breakpoints set on it inside gdb. 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, gdb 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.
gdb provides these facilities for debugging multi-thread programs:
Warning: These facilities are not yet available on every gdb configuration where the operating system supports threads. If your gdb 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 thethreadcommand, like this:(gdb) info threads (gdb) thread 1 Thread ID 1 not known. Use the "info threads" command to see the IDs of currently known threads.
The gdb thread debugging facility allows you to observe all threads while your program runs—but whenever gdb 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 gdb 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 gdb 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, gdb associates its own thread number—always a single integer—with each thread in your program.
info threadsAn asterisk `*' to the left of the gdb thread number indicates the current thread.
For example,
(gdb) 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, gdb associates its own thread number—a small integer assigned in thread-creation order—with each thread in your program.
Whenever gdb detects a new thread in your program, it displays both gdb'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 gdb notices a new thread.
info threadsAn asterisk `*' to the left of the gdb thread number indicates the current thread.
For example,
(gdb) 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
On Solaris, you can display more information about user threads with a Solaris-specific command:
maint info sol-threadsthread threadno
(gdb) 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] argsthread 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
gdb 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 gdb stops your program, due to a breakpoint or a signal, it automatically selects the thread where that breakpoint or signal happened. gdb alerts you to the context switch with a message of the form `[Switching to systag]' to identify the thread.
See Stopping and starting multi-thread programs, for more information about how gdb behaves when you stop and start programs with multiple threads.
See Setting watchpoints, for information about watchpoints in programs with multiple threads.
On most systems, gdb has no special support for debugging
programs which create additional processes using the fork
function. When a program forks, gdb 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 gdb
on the child. While the child is sleeping, use the ps program to
get its process ID. Then tell gdb (a new invocation of
gdb if you are also debugging the parent process) to attach to
the child process (see Attach). From that point on you can debug
the child process just like any other process which you attached to.
On some systems, gdb provides support for debugging programs that
create additional processes using the fork or vfork functions.
Currently, the only platforms with this feature are HP-UX (11.x and later
only?) and GNU/Linux (kernel version 2.5.60 and later).
By default, when a program forks, gdb 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 modefork or
vfork. A call to fork or vfork creates a new
process. The mode argument can be:
parentchildshow follow-fork-modefork or vfork call.
If you ask to debug a child process and a vfork is followed by an
exec, gdb 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 gdb 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 gdb stop whenever
a fork, vfork, or exec call is made. See 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 gdb, your program may stop for any of several reasons,
such as a signal, a breakpoint, or reaching a new line after a
gdb 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 gdb provide
ample explanation of the status of your program—but you can also
explicitly request this information at any time.
info programA 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 Setting breakpoints), to specify the place where your program
should stop by line number, function name or exact address in the
program.
On some systems, 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 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 gdb stops at a breakpoint. See 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 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 Signals.)
gdb 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 gdb 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 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 functionbreak +offsetbreak -offsetbreak linenumbreak filename:linenumbreak filename:functionbreak *addressbreakbreak sets a breakpoint at
the next instruction to be executed in the selected stack frame
(see 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, gdb stops
the next time it reaches the current location; this may be useful
inside loops.
gdb 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 condtbreak argsbreak 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 Disabling breakpoints.
hbreak argsbreak 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 most 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
gdb will reject this command if more than two are used. Delete
or disable unused hardware breakpoints before setting new ones
(see Disabling). See Break conditions.
For remote targets, you can restrict the number of hardware
breakpoints gdb will use, see set remote hardware-breakpoint-limit.
thbreak argshbreak 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 Disabling breakpoints.
See also Break conditions.
rbreak regexbreak 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.
The rbreak command can be used to set breakpoints in
all the functions in a program, like this:
(gdb) rbreak .
info breakpoints [n]info break [n]info watchpoints [n]If a breakpoint is conditional, info break shows the condition on
the line following the affected breakpoint; breakpoint commands, if any,
are listed after that. A pending breakpoint is allowed to have a condition
specified for it. The condition is not parsed for validity until a shared
library is loaded that allows the pending breakpoint to resolve to a
valid location.
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 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.
gdb 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 Break conditions).
If a specified breakpoint location cannot be found, it may be due to the fact that the location is in a shared library that is yet to be loaded. In such a case, you may want gdb to create a special breakpoint (known as a pending breakpoint) that attempts to resolve itself in the future when an appropriate shared library gets loaded.
Pending breakpoints are useful to set at the start of your gdb session for locations that you know will be dynamically loaded later by the program being debugged. When shared libraries are loaded, a check is made to see if the load resolves any pending breakpoint locations. If a pending breakpoint location gets resolved, a regular breakpoint is created and the original pending breakpoint is removed.
gdb provides some additional commands for controlling pending breakpoint support:
set breakpoint pending autoset breakpoint pending onset breakpoint pending offshow breakpoint pendingNormal breakpoint operations apply to pending breakpoints as well. You may specify a condition for a pending breakpoint and/or commands to run when the breakpoint is reached. You can also enable or disable the pending breakpoint. When you specify a condition for a pending breakpoint, the parsing of the condition will be deferred until the point where the pending breakpoint location is resolved. Disabling a pending breakpoint tells gdb to not attempt to resolve the breakpoint on any subsequent shared library load. When a pending breakpoint is re-enabled, gdb checks to see if the location is already resolved. This is done because any number of shared library loads could have occurred since the time the breakpoint was disabled and one or more of these loads could resolve the location.
gdb 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 gdb maintenance command
`maint info breakpoints' (see maint info breakpoints).
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. gdb 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, gnu/Linux and most other x86-based targets, gdb includes support for hardware watchpoints, which do not slow down the running of your program.
watch exprrwatch exprawatch exprinfo watchpointsinfo break (see Set Breaks).
gdb 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 gdb 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.
You can force gdb to use only software watchpoints with the
set can-use-hw-watchpoints 0 command. With this variable set to
zero, gdb will never try to use hardware watchpoints, even if
the underlying system supports them. (Note that hardware-assisted
watchpoints that were set before setting
can-use-hw-watchpoints to zero will still use the hardware
mechanism of watching expressiion values.)
set can-use-hw-watchpointsshow can-use-hw-watchpointsFor remote targets, you can restrict the number of hardware watchpoints gdb will use, see set remote hardware-breakpoint-limit.
When you issue the watch command, gdb 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 gdb does not do
that currently. If gdb 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, gdb 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, gdb 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, gdb 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.
gdb 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 gdb reaches another
kind of breakpoint or the call completes.
gdb 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, gdb 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, gdb 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 gdb 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, gdb may not notice when a non-current thread's activity changes the expression. (Hardware watchpoints, in contrast, watch an expression in all threads.)
See set remote hardware-watchpoint-limit.
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 eventthrowcatchexecexec. This is currently only available for HP-UX.
forkfork. This is currently only available for HP-UX.
vforkvfork. This is currently only available for HP-UX.
loadload libnameunloadunload libnametcatch eventUse the info break command to list the current catchpoints.
There are currently some limitations to C++ exception handling
(catch throw and catch catch) in gdb:
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 Breakpoints; watchpoints; and exceptions).
With a conditional breakpoint (see 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. gdb automatically ignores breakpoints on the first instruction to be executed when you continue execution without changing the execution address.
clearclear functionclear filename:functionclear linenumclear filename:linenumdelete [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...tbreak command start out in this state.
Except for a breakpoint set with tbreak (see 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 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 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, gdb 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 Breakpoint command lists).
Break conditions can be specified when a breakpoint is set, by using
`if' in the arguments to the break command. See 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 expressioncondition, gdb 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, gdb
prints an error message:
No symbol "foo" in current context.
gdb 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 Expressions.
condition bnumA 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 countTo make the breakpoint stop the next time it is reached, specify a count of zero.
When you use 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 Continuing and stepping.
If a breakpoint has a positive ignore count and a condition, the condition is not checked. Once the ignore count reaches zero, gdb resumes checking the condition.
You could achieve the effect of the ignore count with a condition such as `$foo-- <= 0' using a debugger convenience variable that is decremented each time. See Convenience variables.
Ignore counts apply to breakpoints, watchpoints, and catchpoints.
You can give any breakpoint (or watchpoint or catchpoint) a series of commands to execute when your program stops due to that breakpoint. For example, you might want to print the values of certain expressions, or enable other breakpoints.
commands [bnum]... command-list ...endend to terminate the commands.
To remove all commands from a breakpoint, type commands and
follow it immediately with end; that is, give no commands.
With no bnum argument, commands refers to the last
breakpoint, watchpoint, or catchpoint set (not to the breakpoint most
recently encountered).
Pressing <RET> as a means of repeating the last gdb command is disabled within a command-list.
You can use breakpoint commands to start your program up again. Simply
use the continue command, or step, or any other command
that resumes execution.
Any other commands in the command list, after a command that resumes
execution, are ignored. This is because any time you resume execution
(even with a simple next or step), you may encounter
another breakpoint—which could have its own command list, leading to
ambiguities about which list to execute.
If the first command you specify in a command list is silent, the
usual message about stopping at a breakpoint is not printed. This may
be desirable for breakpoints that are to print a specific message and
then continue. If none of the remaining commands print anything, you
see no sign that the breakpoint was reached. silent is
meaningful only at the beginning of a breakpoint command list.
The commands echo, output, and printf allow you to
print precisely controlled output, and are often useful in silent
breakpoints. See Commands for controlled output.
For example, here is how you could use breakpoint commands to print the
value of x at entry to foo whenever x is positive.
break foo if x>0
commands
silent
printf "x is %d\n",x
cont
end
One application for breakpoint commands is to compensate for one bug so
you can test for another. Put a breakpoint just after the erroneous line
of code, give it a condition to detect the case in which something
erroneous has been done, and give it commands to assign correct values
to any variables that need them. End with the continue command
so that your program does not stop, and start with the silent
command so that no output is produced. Here is an example:
break 403
commands
silent
set x = y + 4
cont
end
Some programming languages (notably C++ and Objective-C) permit a
single function name
to be defined several times, for application in different contexts.
This is called overloading. When a function name is overloaded,
`break function' is not enough to tell gdb where you want
a breakpoint. If you realize this is a problem, you can use
something like `break function(types)' to specify which
particular version of the function you want. Otherwise, gdb offers
you a menu of numbered choices for different possible breakpoints, and
waits for your selection with the prompt `>'. The first two
options are always `[0] cancel' and `[1] all'. Typing 1
sets a breakpoint at each definition of function, and typing
0 aborts the break command without setting any new
breakpoints.
For example, the following session excerpt shows an attempt to set a
breakpoint at the overloaded symbol String::after.
We choose three particular definitions of that function name:
(gdb) b String::after
[0] cancel
[1] all
[2] file:String.cc; line number:867
[3] file:String.cc; line number:860
[4] file:String.cc; line number:875
[5] file:String.cc; line number:853
[6] file:String.cc; line number:846
[7] file:String.cc; line number:735
> 2 4 6
Breakpoint 1 at 0xb26c: file String.cc, line 867.
Breakpoint 2 at 0xb344: file String.cc, line 875.
Breakpoint 3 at 0xafcc: file String.cc, line 846.
Multiple breakpoints were set.
Use the "delete" command to delete unwanted
breakpoints.
(gdb)
Under some operating systems, breakpoints cannot be used in a program if any other process is running that program. In this situation, attempting to run or continue a program with a breakpoint causes gdb to print an error message:
Cannot insert breakpoints.
The same program may be running in another process.
When this happens, you have three ways to proceed:
exec-file command to specify
that gdb should run your program under that name.
Then start your program again.
A similar message can be printed if you request too many active hardware-assisted breakpoints and watchpoints:
Stopped; cannot insert breakpoints.
You may have requested too many hardware breakpoints and watchpoints.
This message is printed when you attempt to resume the program, since only then gdb knows exactly how many hardware breakpoints and watchpoints it needs to insert.
When this message is printed, you need to disable or remove some of the hardware-assisted breakpoints and watchpoints, and then continue.
Some processor architectures place constraints on the addresses at which breakpoints may be placed. For architectures thus constrained, gdb will attempt to adjust the breakpoint's address to comply with the constraints dictated by the architecture.
One example of such an architecture is the Fujitsu FR-V. The FR-V is a VLIW architecture in which a number of RISC-like instructions may be bundled together for parallel execution. The FR-V architecture constrains the location of a breakpoint instruction within such a bundle to the instruction with the lowest address. gdb honors this constraint by adjusting a breakpoint's address to the first in the bundle.
It is not uncommon for optimized code to have bundles which contain instructions from different source statements, thus it may happen that a breakpoint's address will be adjusted from one source statement to another. Since this adjustment may significantly alter gdb's breakpoint related behavior from what the user expects, a warning is printed when the breakpoint is first set and also when the breakpoint is hit.
A warning like the one below is printed when setting a breakpoint that's been subject to address adjustment:
warning: Breakpoint address adjusted from 0x00010414 to 0x00010410.
Such warnings are printed both for user settable and gdb's internal breakpoints. If you see one of these warnings, you should verify that a breakpoint set at the adjusted address will have the desired affect. If not, the breakpoint in question may be removed and other breakpoints may be set which will have the desired behavior. E.g., it may be sufficient to place the breakpoint at a later instruction. A conditional breakpoint may also be useful in some cases to prevent the breakpoint from triggering too often.
gdb will also issue a warning when stopping at one of these adjusted breakpoints:
warning: Breakpoint 1 address previously adjusted from 0x00010414
to 0x00010410.
When this warning is encountered, it may be too late to take remedial action except in cases where the breakpoint is hit earlier or more frequently than expected.
Continuing means resuming program execution until your program
completes normally. In contrast, stepping means executing just
one more “step” of your program, where “step” may mean either one
line of source code, or one machine instruction (depending on what
particular command you use). Either when continuing or when stepping,
your program may stop even sooner, due to a breakpoint or a signal. (If
it stops due to a signal, you may want to use handle, or use
`signal 0' to resume execution. See Signals.)
continue [ignore-count]c [ignore-count]fg [ignore-count]ignore (see Break conditions).
The argument ignore-count is meaningful only when your program
stopped due to a breakpoint. At other times, the argument to
continue is ignored.
The synonyms c and fg (for foreground, as the
debugged program is deemed to be the foreground program) are provided
purely for convenience, and have exactly the same behavior as
continue.
To resume execution at a different place, you can use return
(see Returning from a function) to go back to the
calling function; or jump (see Continuing at a different address) to go to an arbitrary location in your program.
A typical technique for using stepping is to set a breakpoint (see Breakpoints; watchpoints; and catchpoints) at the beginning of the function or the section of your program where a problem is believed to lie, run your program until it stops at that breakpoint, and then step through the suspect area, examining the variables that are interesting, until you see the problem happen.
steps.
Warning: If you use thestepcommand while control is within a function that was compiled without debugging information, execution proceeds until control reaches a function that does have debugging information. Likewise, it will not step into a function which is compiled without debugging information. To step through functions without debugging information, use thestepicommand, described below.
The step command only stops at the first instruction of a source
line. This prevents the multiple stops that could otherwise occur in
switch statements, for loops, etc. step continues
to stop if a function that has debugging information is called within
the line. In other words, step steps inside any functions
called within the line.
Also, the step command only enters a function if there is line
number information for the function. Otherwise it acts like the
next command. This avoids problems when using cc -gl
on MIPS machines. Previously, step entered subroutines if there
was any debugging information about the routine.
step countstep, but do so count times. If a
breakpoint is reached, or a signal not related to stepping occurs before
count steps, stepping stops right away.
next [count]step, but function calls that appear within
the line of code are executed without stopping. Execution stops when
control reaches a different line of code at the original stack level
that was executing when you gave the next command. This command
is abbreviated n.
An argument count is a repeat count, as for step.
The next command only stops at the first instruction of a
source line. This prevents multiple stops that could otherwise occur in
switch statements, for loops, etc.
set step-modeset step-mode onset step-mode on command causes the step command to
stop at the first instruction of a function which contains no debug line
information rather than stepping over it.
This is useful in cases where you may be interested in inspecting the
machine instructions of a function which has no symbolic info and do not
want gdb to automatically skip over this function.
set step-mode offstep command to step over any functions which contains no
debug information. This is the default.
show step-modefinishContrast this with the return command (see Returning from a function).
untilunext
command, except that when until encounters a jump, it
automatically continues execution until the program counter is greater
than the address of the jump.
This means that when you reach the end of a loop after single stepping
though it, until makes your program continue execution until it
exits the loop. In contrast, a next command at the end of a loop
simply steps back to the beginning of the loop, which forces you to step
through the next iteration.
until always stops your program if it attempts to exit the current
stack frame.
until may produce somewhat counterintuitive results if the order
of machine code does not match the order of the source lines. For
example, in the following excerpt from a debugging session, the f
(frame) command shows that execution is stopped at line
206; yet when we use until, we get to line 195:
(gdb) f
#0 main (argc=4, argv=0xf7fffae8) at m4.c:206
206 expand_input();
(gdb) until
195 for ( ; argc > 0; NEXTARG) {
This happened because, for execution efficiency, the compiler had
generated code for the loop closure test at the end, rather than the
start, of the loop—even though the test in a C for-loop is
written before the body of the loop. The until command appeared
to step back to the beginning of the loop when it advanced to this
expression; however, it has not really gone to an earlier
statement—not in terms of the actual machine code.
until with no argument works by means of single
instruction stepping, and hence is slower than until with an
argument.
until locationu locationbreak (see Setting breakpoints). This form of the command uses breakpoints, and
hence is quicker than until without an argument. The specified
location is actually reached only if it is in the current frame. This
implies that until can be used to skip over recursive function
invocations. For instance in the code below, if the current location is
line 96, issuing until 99 will execute the program up to
line 99 in the same invocation of factorial, i.e. after the inner
invocations have returned.
94 int factorial (int value)
95 {
96 if (value > 1) {
97 value *= factorial (value - 1);
98 }
99 return (value);
100 }
advance locationbreak
command. Execution will also stop upon exit from the current stack
frame. This command is similar to until, but advance will
not skip over recursive function calls, and the target location doesn't
have to be in the same frame as the current one.
stepistepi argsiIt is often useful to do `display/i $pc' when stepping by machine instructions. This makes gdb automatically display the next instruction to be executed, each time your program stops. See Automatic display.
An argument is a repeat count, as in step.
nextinexti argniAn argument is a repeat count, as in next.
A signal is an asynchronous event that can happen in a program. The
operating system defines the possible kinds of signals, and gives each
kind a name and a number. For example, in Unix SIGINT is the
signal a program gets when you type an interrupt character (often C-c);
SIGSEGV is the signal a program gets from referencing a place in
memory far away from all the areas in use; SIGALRM occurs when
the alarm clock timer goes off (which happens only if your program has
requested an alarm).
Some signals, including SIGALRM, are a normal part of the
functioning of your program. Others, such as SIGSEGV, indicate
errors; these signals are fatal (they kill your program immediately) if the
program has not specified in advance some other way to handle the signal.
SIGINT does not indicate an error in your program, but it is normally
fatal so it can carry out the purpose of the interrupt: to kill the program.
gdb has the ability to detect any occurrence of a signal in your program. You can tell gdb in advance what to do for each kind of signal.
Normally, gdb is set up to let the non-erroneous signals like
SIGALRM be silently passed to your program
(so as not to interfere with their role in the program's functioning)
but to stop your program immediately whenever an error signal happens.
You can change these settings with the handle command.
info signalsinfo handleinfo handle is an alias for info signals.
handle signal keywords...The keywords allowed by the handle command can be abbreviated.
Their full names are:
nostopstopprint keyword as well.
printnoprintnostop keyword as well.
passnoignorepass and noignore are synonyms.
nopassignorenopass and ignore are synonyms.
When a signal stops your program, the signal is not visible to the
program until you
continue. Your program sees the signal then, if pass is in
effect for the signal in question at that time. In other words,
after gdb reports a signal, you can use the handle
command with pass or nopass to control whether your
program sees that signal when you continue.
The default is set to nostop, noprint, pass for
non-erroneous signals such as SIGALRM, SIGWINCH and
SIGCHLD, and to stop, print, pass for the
erroneous signals.
You can also use the signal command to prevent your program from
seeing a signal, or cause it to see a signal it normally would not see,
or to give it any signal at any time. For example, if your program stopped
due to some sort of memory reference error, you might store correct
values into the erroneous variables and continue, hoping to see more
execution; but your program would probably terminate immediately as
a result of the fatal signal once it saw the signal. To prevent this,
you can continue with `signal 0'. See Giving your program a signal.
When your program has multiple threads (see Debugging programs with multiple threads), you can choose whether to set breakpoints on all threads, or on a particular thread.
break linespec thread threadnobreak linespec thread threadno if ...Use the qualifier `thread threadno' with a breakpoint command to specify that you only want gdb to stop the program when a particular thread reaches this breakpoint. threadno is one of the numeric thread identifiers assigned by gdb, shown in the first column of the `info threads' display.
If you do not specify `thread threadno' when you set a breakpoint, the breakpoint applies to all threads of your program.
You can use the thread qualifier on conditional breakpoints as
well; in this case, place `thread threadno' before the
breakpoint condition, like this:
(gdb) break frik.c:13 thread 28 if bartab > lim
Whenever your program stops under gdb for any reason, all threads of execution stop, not just the current thread. This allows you to examine the overall state of the program, including switching between threads, without worrying that things may change underfoot.
There is an unfortunate side effect. If one thread stops for a breakpoint, or for some other reason, and another thread is blocked in a system call, then the system call may return prematurely. This is a consequence of the interaction between multiple threads and the signals that gdb uses to implement breakpoints and other events that stop execution.
To handle this problem, your program should check the return value of each system call and react appropriately. This is good programming style anyways.
For example, do not write code like this:
sleep (10);
The call to sleep will return early if a different thread stops
at a breakpoint or for some other reason.
Instead, write this:
int unslept = 10;
while (unslept > 0)
unslept = sleep (unslept);
A system call is allowed to return early, so the system is still conforming to its specification. But gdb does cause your multi-threaded program to behave differently than it would without gdb.
Also, gdb uses internal breakpoints in the thread library to monitor certain events such as thread creation and thread destruction. When such an event happens, a system call in another thread may return prematurely, even though your program does not appear to stop.
Conversely, whenever you restart the program, all threads start
executing. This is true even when single-stepping with commands
like step or next.
In particular, gdb cannot single-step all threads in lockstep. Since thread scheduling is up to your debugging target's operating system (not controlled by gdb), other threads may execute more than one statement while the current thread completes a single step. Moreover, in general other threads stop in the middle of a statement, rather than at a clean statement boundary, when the program stops.
You might even find your program stopped in another thread after continuing or even single-stepping. This happens whenever some other thread runs into a breakpoint, a signal, or an exception before the first thread completes whatever you requested.
On some OSes, you can lock the OS scheduler and thus allow only a single thread to run.
set scheduler-locking modeoff, then there is no
locking and any thread may run at any time. If on, then only the
current thread may run when the inferior is resumed. The step
mode optimizes for single-stepping. It stops other threads from
“seizing the prompt” by preempting the current thread while you are
stepping. Other threads will only rarely (or never) get a chance to run
when you step. They are more likely to run when you `next' over a
function call, and they are completely free to run when you use commands
like `continue', `until', or `finish'. However, unless another
thread hits a breakpoint during its timeslice, they will never steal the
gdb prompt away from the thread that you are debugging.
show scheduler-lockingWhen your program has stopped, the first thing you need to know is where it stopped and how it got there.
Each time your program performs a function call, information about the call is generated. That information includes the location of the call in your program, the arguments of the call, and the local variables of the function being called. The information is saved in a block of data called a stack frame. The stack frames are allocated in a region of memory called the call stack.
When your program stops, the gdb commands for examining the stack allow you to see all of this information.
One of the stack frames is selected by gdb and many gdb commands refer implicitly to the selected frame. In particular, whenever you ask gdb for the value of a variable in your program, the value is found in the selected frame. There are special gdb commands to select whichever frame you are interested in. See Selecting a frame.
When your program stops, gdb automatically selects the
currently executing frame and describes it briefly, similar to the
frame command (see Information about a frame).
The call stack is divided up into contiguous pieces called stack frames, or frames for short; each frame is the data associated with one call to one function. The frame contains the arguments given to the function, the function's local variables, and the address at which the function is executing.
When your program is started, the stack has only one frame, that of the
function main. This is called the initial frame or the
outermost frame. Each time a function is called, a new frame is
made. Each time a function returns, the frame for that function invocation
is eliminated. If a function is recursive, there can be many frames for
the same function. The frame for the function in which execution is
actually occurring is called the innermost frame. This is the most
recently created of all the stack frames that still exist.
Inside your program, stack frames are identified by their addresses. A stack frame consists of many bytes, each of which has its own address; each kind of computer has a convention for choosing one byte whose address serves as the address of the frame. Usually this address is kept in a register called the frame pointer register (see $fp) while execution is going on in that frame.
gdb assigns numbers to all existing stack frames, starting with zero for the innermost frame, one for the frame that called it, and so on upward. These numbers do not really exist in your program; they are assigned by gdb to give you a way of designating stack frames in gdb commands.
Some compilers provide a way to compile functions so that they operate without stack frames. (For example, the gcc option
`-fomit-frame-pointer'
generates functions without a frame.) This is occasionally done with heavily used library functions to save the frame setup time. gdb has limited facilities for dealing with these function invocations. If the innermost function invocation has no stack frame, gdb nevertheless regards it as though it had a separate frame, which is numbered zero as usual, allowing correct tracing of the function call chain. However, gdb has no provision for frameless functions elsewhere in the stack.
frame argsframe command allows you to move from one stack frame to another,
and to print the stack frame you select. args may be either the
address of the frame or the stack frame number. Without an argument,
frame prints the current stack frame.
select-frameselect-frame command allows you to move from one stack frame
to another without printing the frame. This is the silent version of
frame.
A backtrace is a summary of how your program got where it is. It shows one line per frame, for many frames, starting with the currently executing frame (frame zero), followed by its caller (frame one), and on up the stack.
backtracebtYou can stop the backtrace at any time by typing the system interrupt
character, normally C-c.
backtrace nbt nbacktrace -nbt -nbacktrace fullbt fullThe names where and info stack (abbreviated info s)
are additional aliases for backtrace.
Each line in the backtrace shows the frame number and the function name.
The program counter value is also shown—unless you use set
print address off. The backtrace also shows the source file name and
line number, as well as the arguments to the function. The program
counter value is omitted if it is at the beginning of the code for that
line number.
Here is an example of a backtrace. It was made with the command `bt 3', so it shows the innermost three frames.
#0 m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8)
at builtin.c:993
#1 0x6e38 in expand_macro (sym=0x2b600) at macro.c:242
#2 0x6840 in expand_token (obs=0x0, t=177664, td=0xf7fffb08)
at macro.c:71
(More stack frames follow...)
The display for frame zero does not begin with a program counter
value, indicating that your program has stopped at the beginning of the
code for line 993 of builtin.c.
If your program was compiled with optimizations, some compilers will optimize away arguments passed to functions if those arguments are never used after the call. Such optimizations generate code that passes arguments through registers, but doesn't store those arguments in the stack frame. gdb has no way of displaying such arguments in stack frames other than the innermost one. Here's what such a backtrace might look like:
#0 m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8)
at builtin.c:993
#1 0x6e38 in expand_macro (sym=<value optimized out>) at macro.c:242
#2 0x6840 in expand_token (obs=0x0, t=<value optimized out>, td=0xf7fffb08)
at macro.c:71
(More stack frames follow...)
The values of arguments that were not saved in their stack frames are shown as `<value optimized out>'.
If you need to display the values of such optimized-out arguments, either deduce that from other variables whose values depend on the one you are interested in, or recompile without optimizations.
Most programs have a standard user entry point—a place where system
libraries and startup code transition into user code. For C this is
main2.
When gdb finds the entry function in a backtrace
it will terminate the backtrace, to avoid tracing into highly
system-specific (and generally uninteresting) code.
If you need to examine the startup code, or limit the number of levels in a backtrace, you can change this behavior:
set backtrace past-mainset backtrace past-main onset backtrace past-main offshow backtrace past-mainset backtrace past-entryset backtrace past-entry onmain (or equivalent) is called.
set backtrace past-entry offshow backtrace past-entryset backtrace limit nset backtrace limit 0show backtrace limitMost commands for examining the stack and other data in your program work on whichever stack frame is selected at the moment. Here are the commands for selecting a stack frame; all of them finish by printing a brief description of the stack frame just selected.
frame nf nmain.
frame addrf addrOn the SPARC architecture, frame needs two addresses to
select an arbitrary frame: a frame pointer and a stack pointer.
On the MIPS and Alpha architecture, it needs two addresses: a stack pointer and a program counter.
On the 29k architecture, it needs three addresses: a register stack pointer, a program counter, and a memory stack pointer.
up ndown ndown as do.
All of these commands end by printing two lines of output describing the frame. The first line shows the frame number, the function name, the arguments, and the source file and line number of execution in that frame. The second line shows the text of that source line.
For example:
(gdb) up
#1 0x22f0 in main (argc=1, argv=0xf7fffbf4, env=0xf7fffbfc)
at env.c:10
10 read_input_file (argv[i]);
After such a printout, the list command with no arguments
prints ten lines centered on the point of execution in the frame.
You can also edit the program at the point of execution with your favorite
editing program by typing edit.
See Printing source lines,
for details.
up-silently ndown-silently nup and down,
respectively; they differ in that they do their work silently, without
causing display of the new frame. They are intended primarily for use
in gdb command scripts, where the output might be unnecessary and
distracting.
There are several other commands to print information about the selected stack frame.
frameff. With an
argument, this command is used to select a stack frame.
See Selecting a frame.
info frameinfo fThe verbose description is useful when
something has gone wrong that has made the stack format fail to fit
the usual conventions.
info frame addrinfo f addrframe command.
See Selecting a frame.
info argsinfo localsinfo catchup,
down, or frame commands); then type info catch.
See Setting catchpoints.
gdb can print parts of your program's source, since the debugging information recorded in the program tells gdb what source files were used to build it. When your program stops, gdb spontaneously prints the line where it stopped. Likewise, when you select a stack frame (see Selecting a frame), gdb prints the line where execution in that frame has stopped. You can print other portions of source files by explicit command.
If you use gdb through its gnu Emacs interface, you may prefer to use Emacs facilities to view source; see Using gdb under gnu Emacs.
To print lines from a source file, use the list command
(abbreviated l). By default, ten lines are printed.
There are several ways to specify what part of the file you want to print.
Here are the forms of the list command most commonly used:
list linenumlist functionlistlist command, this prints lines following the last lines
printed; however, if the last line printed was a solitary line printed
as part of displaying a stack frame (see Examining the Stack), this prints lines centered around that line.
list -By default, gdb prints ten source lines with any of these forms of
the list command. You can change this using set listsize:
set listsize countlist command display count source lines (unless
the list argument explicitly specifies some other number).
show listsizelist prints.
Repeating a list command with <RET> discards the argument,
so it is equivalent to typing just list. This is more useful
than listing the same lines again. An exception is made for an
argument of `-'; that argument is preserved in repetition so that
each repetition moves up in the source file.
In general, the list command expects you to supply zero, one or two
linespecs. Linespecs specify source lines; there are several ways
of writing them, but the effect is always to specify some source line.
Here is a complete description of the possible arguments for list:
list linespeclist first,lastlist ,lastlist first,list +list -listHere are the ways of specifying a single source line—all the kinds of linespec.
list command has two linespecs, this refers to
the same source file as the first linespec.
+offsetlist command that has
two, this specifies the line offset lines down from the
first linespec.
-offset:number:function*address
To edit the lines in a source file, use the edit command.
The editing program of your choice
is invoked with the current line set to
the active line in the program.
Alternatively, there are several ways to specify what part of the file you
want to print if you want to see other parts of the program.
Here are the forms of the edit command most commonly used:
editedit numberedit functionedit filename:numberedit filename:functionedit *addressYou can customize gdb to use any editor you want
3.
By default, it is /bin/ex, but you can change this
by setting the environment variable EDITOR before using
gdb. For example, to configure gdb to use the
vi editor, you could use these commands with the sh shell:
EDITOR=/usr/bin/vi
export EDITOR
gdb ...
or in the csh shell,
setenv EDITOR /usr/bin/vi
gdb ...
There are two commands for searching through the current source file for a regular expression.
forward-search regexpsearch regexpfo.
reverse-search regexprev.
Executable programs sometimes do not record the directories of the source files from which they were compiled, just the names. Even when they do, the directories could be moved between the compilation and your debugging session. gdb has a list of directories to search for source files; this is called the source path. Each time gdb wants a source file, it tries all the directories in the list, in the order they are present in the list, until it finds a file with the desired name.
For example, suppose an executable references the file /usr/src/foo-1.0/lib/foo.c, and our source path is /mnt/cross. The file is first looked up literally; if this fails, /mnt/cross/usr/src/foo-1.0/lib/foo.c is tried; if this fails, /mnt/cross/foo.c is opened; if this fails, an error message is printed. gdb does not look up the parts of the source file name, such as /mnt/cross/src/foo-1.0/lib/foo.c. Likewise, the subdirectories of the source path are not searched: if the source path is /mnt/cross, and the binary refers to foo.c, gdb would not find it under /mnt/cross/usr/src/foo-1.0/lib.
Plain file names, relative file names with leading directories, file names containing dots, etc. are all treated as described above; for instance, if the source path is /mnt/cross, and the source file is recorded as ../lib/foo.c, gdb would first try ../lib/foo.c, then /mnt/cross/../lib/foo.c, and after that—/mnt/cross/foo.c.
Note that the executable search path is not used to locate the source files. Neither is the current working directory, unless it happens to be in the source path.
Whenever you reset or rearrange the source path, gdb clears out any information it has cached about where source files are found and where each line is in the file.
When you start gdb, its source path includes only `cdir'
and `cwd', in that order.
To add other directories, use the directory command.
directory dirname ...dir dirname ...You can use the string `$cdir' to refer to the compilation
directory (if one is recorded), and `$cwd' to refer to the current
working directory. `$cwd' is not the same as `.'—the former
tracks the current working directory as it changes during your gdb
session, while the latter is immediately expanded to the current
directory at the time you add an entry to the source path.
directoryshow directoriesIf your source path is cluttered with directories that are no longer of interest, gdb may sometimes cause confusion by finding the wrong versions of source. You can correct the situation as follows:
directory with no argument to reset the source path to empty.
directory with suitable arguments to reinstall the
directories you want in the source path. You can add all the
directories in one command.
You can use the command info line to map source lines to program
addresses (and vice versa), and the command disassemble to display
a range of addresses as machine instructions. When run under gnu Emacs
mode, the info line command causes the arrow to point to the
line specified. Also, info line prints addresses in symbolic form as
well as hex.
info line linespeclist command (see Printing source lines).
For example, we can use info line to discover the location of
the object code for the first line of function
m4_changequote:
(gdb) info line m4_changequote
Line 895 of "builtin.c" starts at pc 0x634c and ends at 0x6350.
We can also inquire (using *addr as the form for
linespec) what source line covers a particular address:
(gdb) info line *0x63ff
Line 926 of "builtin.c" starts at pc 0x63e4 and ends at 0x6404.
After info line, the default address for the x command
is changed to the starting address of the line, so that `x/i' is
sufficient to begin examining the machine code (see Examining memory). Also, this address is saved as the value of the
convenience variable $_ (see Convenience variables).
disassembleThe following example shows the disassembly of a range of addresses of HP PA-RISC 2.0 code:
(gdb) disas 0x32c4 0x32e4
Dump of assembler code from 0x32c4 to 0x32e4:
0x32c4 <main+204>: addil 0,dp
0x32c8 <main+208>: ldw 0x22c(sr0,r1),r26
0x32cc <main+212>: ldil 0x3000,r31
0x32d0 <main+216>: ble 0x3f8(sr4,r31)
0x32d4 <main+220>: ldo 0(r31),rp
0x32d8 <main+224>: addil -0x800,dp
0x32dc <main+228>: ldo 0x588(r1),r26
0x32e0 <main+232>: ldil 0x3000,r31
End of assembler dump.
Some architectures have more than one commonly-used set of instruction mnemonics or other syntax.
For programs that were dynamically linked and use shared libraries, instructions that call functions or branch to locations in the shared libraries might show a seemingly bogus location—it's actually a location of the relocation table. On some architectures, gdb might be able to resolve these to actual function names.
set disassembly-flavor instruction-setdisassemble or x/i commands.
Currently this command is only defined for the Intel x86 family. You
can set instruction-set to either intel or att.
The default is att, the AT&T flavor used by default by Unix
assemblers for x86-based targets.
show disassembly-flavor
The usual way to examine data in your program is with the print
command (abbreviated p), or its synonym inspect. It
evaluates and prints the value of an expression of the language your
program is written in (see Using gdb with Different Languages).
print exprprint /f exprprintprint /fA more low-level way of examining data is with the x command.
It examines data in memory at a specified address and prints it in a
specified format. See Examining memory.
If you are interested in information about types, or about how the
fields of a struct or a class are declared, use the ptype exp
command rather than print. See Examining the Symbol Table.
print and many other gdb commands accept an expression and
compute its value. Any kind of constant, variable or operator defined
by the programming language you are using is valid in an expression in
gdb. This includes conditional expressions, function calls,
casts, and string constants. It also includes preprocessor macros, if
you compiled your program to include this information; see
Compilation.
gdb supports array constants in expressions input by
the user. The syntax is {element, element...}. For example,
you can use the command print {1, 2, 3} to build up an array in
memory that is malloced in the target program.
Because C is so widespread, most of the expressions shown in examples in this manual are in C. See Using gdb with Different Languages, for information on how to use expressions in other languages.
In this section, we discuss operators that you can use in gdb expressions regardless of your programming language.
Casts are supported in all languages, not just in C, because it is so useful to cast a number into a pointer in order to examine a structure at that address in memory.
gdb supports these operators, in addition to those common to programming languages:
@::{type} addrThe most common kind of expression to use is the name of a variable in your program.
Variables in expressions are understood in the selected stack frame (see Selecting a frame); they must be either:
or
This means that in the function
foo (a)
int a;
{
bar (a);
{
int b = test ();
bar (b);
}
}
you can examine and use the variable a whenever your program is
executing within the function foo, but you can only use or
examine the variable b while your program is executing inside
the block where b is declared.
There is an exception: you can refer to a variable or function whose
scope is a single source file even if the current execution point is not
in this file. But it is possible to have more than one such variable or
function with the same name (in different source files). If that
happens, referring to that name has unpredictable effects. If you wish,
you can specify a static variable in a particular function or file,
using the colon-colon (::) notation:
file::variable
function::variable
Here file or function is the name of the context for the
static variable. In the case of file names, you can use quotes to
make sure gdb parses the file name as a single word—for example,
to print a global value of x defined in f2.c:
(gdb) p 'f2.c'::x
This use of `::' is very rarely in conflict with the very similar use of the same notation in C++. gdb also supports use of the C++ scope resolution operator in gdb expressions.
Warning: Occasionally, a local variable may appear to have the wrong value at certain points in a function—just after entry to a new scope, and just before exit.You may see this problem when you are stepping by machine instructions. This is because, on most machines, it takes more than one instruction to set up a stack frame (including local variable definitions); if you are stepping by machine instructions, variables may appear to have the wrong values until the stack frame is completely built. On exit, it usually also takes more than one machine instruction to destroy a stack frame; after you begin stepping through that group of instructions, local variable definitions may be gone.
This may also happen when the compiler does significant optimizations. To be sure of always seeing accurate values, turn off all optimization when compiling.
Another possible effect of compiler optimizations is to optimize unused variables out of existence, or assign variables to registers (as opposed to memory addresses). Depending on the support for such cases offered by the debug info format used by the compiler, gdb might not be able to display values for such local variables. If that happens, gdb will print a message like this:
No symbol "foo" in current context.
To solve such problems, either recompile without optimizations, or use a different debug info format, if the compiler supports several such formats. For example, gcc, the gnu C/C++ compiler, usually supports the -gstabs+ option. -gstabs+ produces debug info in a format that is superior to formats such as COFF. You may be able to use DWARF 2 (-gdwarf-2), which is also an effective form for debug info. See Options for Debugging Your Program or gnu CC. See Debugging C++, for more info about debug info formats that are best suited to C++ programs.
It is often useful to print out several successive objects of the same type in memory; a section of an array, or an array of dynamically determined size for which only a pointer exists in the program.
You can do this by referring to a contiguous span of memory as an artificial array, using the binary operator `@'. The left operand of `@' should be the first element of the desired array and be an individual object. The right operand should be the desired length of the array. The result is an array value whose elements are all of the type of the left argument. The first element is actually the left argument; the second element comes from bytes of memory immediately following those that hold the first element, and so on. Here is an example. If a program says
int *array = (int *) malloc (len * sizeof (int));
you can print the contents of array with
p *array@len
The left operand of `@' must reside in memory. Array values made with `@' in this way behave just like other arrays in terms of subscripting, and are coerced to pointers when used in expressions. Artificial arrays most often appear in expressions via the value history (see Value history), after printing one out.
Another way to create an artificial array is to use a cast. This re-interprets a value as if it were an array. The value need not be in memory:
(gdb) p/x (short[2])0x12345678
$1 = {0x1234, 0x5678}
As a convenience, if you leave the array length out (as in `(type[])value') gdb calculates the size to fill the value (as `sizeof(value)/sizeof(type)':
(gdb) p/x (short[])0x12345678
$2 = {0x1234, 0x5678}
Sometimes the artificial array mechanism is not quite enough; in
moderately complex data structures, the elements of interest may not
actually be adjacent—for example, if you are interested in the values
of pointers in an array. One useful work-around in this situation is
to use a convenience variable (see Convenience variables) as a counter in an expression that prints the first
interesting value, and then repeat that expression via <RET>. For
instance, suppose you have an array dtab of pointers to
structures, and you are interested in the values of a field fv
in each structure. Here is an example of what you might type:
set $i = 0
p dtab[$i++]->fv
<RET>
<RET>
...
By default, gdb prints a value according to its data type. Sometimes this is not what you want. For example, you might want to print a number in hex, or a pointer in decimal. Or you might want to view data in memory at a certain address as a character string or as an instruction. To do these things, specify an output format when you print a value.
The simplest use of output formats is to say how to print a value
already computed. This is done by starting the arguments of the
print command with a slash and a format letter. The format
letters supported are:
xduota (gdb) p/a 0x54320
$3 = 0x54320 <_initialize_vx+396>
The command info symbol 0x54320 yields similar results.
See info symbol.
cfFor example, to print the program counter in hex (see Registers), type
p/x $pc
Note that no space is required before the slash; this is because command names in gdb cannot contain a slash.
To reprint the last value in the value history with a different format,
you can use the print command with just a format and no
expression. For example, `p/x' reprints the last value in hex.
You can use the command x (for “examine”) to examine memory in
any of several formats, independently of your program's data types.
x/nfu addrx addrxx command to examine memory.
n, f, and u are all optional parameters that specify how much memory to display and how to format it; addr is an expression giving the address where you want to start displaying memory. If you use defaults for nfu, you need not type the slash `/'. Several commands set convenient defaults for addr.
print
(`x', `d', `u', `o', `t', `a', `c',
`f'), and in addition `s' (for null-terminated strings) and
`i' (for machine instructions). The default is `x'
(hexadecimal) initially. The default changes each time you use either
x or print.
bhwgEach time you specify a unit size with x, that size becomes the
default unit the next time you use x. (For the `s' and
`i' formats, the unit size is ignored and is normally not written.)
info breakpoints (to
the address of the last breakpoint listed), info line (to the
starting address of a line), and print (if you use it to display
a value from memory).
For example, `x/3uh 0x54320' is a request to display three halfwords
(h) of memory, formatted as unsigned decimal integers (`u'),
starting at address 0x54320. `x/4xw $sp' prints the four
words (`w') of memory above the stack pointer (here, `$sp';
see Registers) in hexadecimal (`x').
Since the letters indicating unit sizes are all distinct from the letters specifying output formats, you do not have to remember whether unit size or format comes first; either order works. The output specifications `4xw' and `4wx' mean exactly the same thing. (However, the count n must come first; `wx4' does not work.)
Even though the unit size u is ignored for the formats `s'
and `i', you might still want to use a count n; for example,
`3i' specifies that you want to see three machine instructions,
including any operands. The command disassemble gives an
alternative way of inspecting machine instructions; see Source and machine code.
All the defaults for the arguments to x are designed to make it
easy to continue scanning memory with minimal specifications each time
you use x. For example, after you have inspected three machine
instructions with `x/3i addr', you can inspect the next seven
with just `x/7'. If you use <RET> to repeat the x command,
the repeat count n is used again; the other arguments default as
for successive uses of x.
The addresses and contents printed by the x command are not saved
in the value history because there is often too much of them and they
would get in the way. Instead, gdb makes these values available for
subsequent use in expressions as values of the convenience variables
$_ and $__. After an x command, the last address
examined is available for use in expressions in the convenience variable
$_. The contents of that address, as examined, are available in
the convenience variable $__.
If the x command has a repeat count, the address and contents saved
are from the last memory unit printed; this is not the same as the last
address printed if several units were printed on the last line of output.
When you are debugging a program running on a remote target machine
(see Remote), you may wish to verify the program's image in the
remote machine's memory against the executable file you downloaded to
the target. The compare-sections command is provided for such
situations.
compare-sections [section-name]"qCRC"
remote request.
If you find that you want to print the value of an expression frequently (to see how it changes), you might want to add it to the automatic display list so that gdb prints its value each time your program stops. Each expression added to the list is given a number to identify it; to remove an expression from the list, you specify that number. The automatic display looks like this:
2: foo = 38
3: bar[5] = (struct hack *) 0x3804
This display shows item numbers, expressions and their current values. As with
displays you request manually using x or print, you can
specify the output format you prefer; in fact, display decides
whether to use print or x depending on how elaborate your
format specification is—it uses x if you specify a unit size,
or one of the two formats (`i' and `s') that are only
supported by x; otherwise it uses print.
display exprdisplay does not repeat if you press <RET> again after using it.
display/fmt exprdisplay/fmt addrFor example, `display/i $pc' can be helpful, to see the machine instruction about to be executed each time execution stops (`$pc' is a common name for the program counter; see Registers).
undisplay dnums...delete display dnums...undisplay does not repeat if you press <RET> after using it.
(Otherwise you would just get the error `No display number ...'.)
disable display dnums...enable display dnums...displayinfo displayIf a display expression refers to local variables, then it does not make
sense outside the lexical context for which it was set up. Such an
expression is disabled when execution enters a context where one of its
variables is not defined. For example, if you give the command
display last_char while inside a function with an argument
last_char, gdb displays this argument while your program
continues to stop inside that function. When it stops elsewhere—where
there is no variable last_char—the display is disabled
automatically. The next time your program stops where last_char
is meaningful, you can enable the display expression once again.
gdb provides the following ways to control how arrays, structures, and symbols are printed.
These settings are useful for debugging programs in any language:
set print addressset print address onon. For example, this is what a stack frame display looks like with
set print address on:
(gdb) f
#0 set_quotes (lq=0x34c78 "<<", rq=0x34c88 ">>")
at input.c:530
530 if (lquote != def_lquote)
set print address offset print address off:
(gdb) set print addr off
(gdb) f
#0 set_quotes (lq="<<", rq=">>") at input.c:530
530 if (lquote != def_lquote)
You can use `set print address off' to eliminate all machine
dependent displays from the gdb interface. For example, with
print address off, you should get the same text for backtraces on
all machines—whether or not they involve pointer arguments.
show print addressWhen gdb prints a symbolic address, it normally prints the
closest earlier symbol plus an offset. If that symbol does not uniquely
identify the address (for example, it is a name whose scope is a single
source file), you may need to clarify. One way to do this is with
info line, for example `info line *0x4537'. Alternately,
you can set gdb to print the source file and line number when
it prints a symbolic address:
set print symbol-filename onset print symbol-filename offshow print symbol-filenameAnother situation where it is helpful to show symbol filenames and line numbers is when disassembling code; gdb shows you the line number and source file that corresponds to each instruction.
Also, you may wish to see the symbolic form only if the address being printed is reasonably close to the closest earlier symbol:
set print max-symbolic-offset max-offsetshow print max-symbolic-offsetIf you have a pointer and you are not sure where it points, try
`set print symbol-filename on'. Then you can determine the name
and source file location of the variable where it points, using
`p/a pointer'. This interprets the address in symbolic form.
For example, here gdb shows that a variable ptt points
at another variable t, defined in hi2.c:
(gdb) set print symbol-filename on
(gdb) p/a ptt
$4 = 0xe008 <t in hi2.c>
Warning: For pointers that point to a local variable, `p/a'
does not show the symbol name and filename of the referent, even with
the appropriate set print options turned on.
Other settings control how different kinds of objects are printed:
set print arrayset print array onset print array offshow print arrayset print elements number-of-elementsset print elements command.
This limit also applies to the display of strings.
When gdb starts, this limit is set to 200.
Setting number-of-elements to zero means that the printing is unlimited.
show print elementsset print repeats"<repeats n times>", where n is the number of
identical repetitions, instead of displaying the identical elements
themselves. Setting the threshold to zero will cause all elements to
be individually printed. The default threshold is 10.
show print repeatsset print null-stopshow print null-stopset print pretty on $1 = {
next = 0x0,
flags = {
sweet = 1,
sour = 1
},
meat = 0x54 "Pork"
}
set print pretty off $1 = {next = 0x0, flags = {sweet = 1, sour = 1}, \
meat = 0x54 "Pork"}
This is the default format.
show print prettyset print sevenbit-strings on\nnn. This setting is
best if you are working in English (ascii) and you use the
high-order bit of characters as a marker or “meta” bit.
set print sevenbit-strings offshow print sevenbit-stringsset print union onset print union off"{...}"
instead.
show print unionFor example, given the declarations
typedef enum {Tree, Bug} Species;
typedef enum {Big_tree, Acorn, Seedling} Tree_forms;
typedef enum {Caterpillar, Cocoon, Butterfly}
Bug_forms;
struct thing {
Species it;
union {
Tree_forms tree;
Bug_forms bug;
} form;
};
struct thing foo = {Tree, {Acorn}};
with set print union on in effect `p foo' would print
$1 = {it = Tree, form = {tree = Acorn, bug = Cocoon}}
and with set print union off in effect it would print
$1 = {it = Tree, form = {...}}
set print union affects programs written in C-like languages
and in Pascal.
These settings are of interest when debugging C++ programs:
set print demangleset print demangle onshow print demangleset print asm-demangleset print asm-demangle onshow print asm-demangleset demangle-style styleautognug++) encoding algorithm.
This is the default.
hpaCC) encoding algorithm.
lucidlcc) encoding algorithm.
armcfront-generated executables. gdb would
require further enhancement to permit that.
show demangle-styleset print objectset print object onset print object offshow print objectset print static-membersset print static-members onset print static-members offshow print static-membersset print pascal_static-membersset print pascal_static-members onset print pascal_static-members offshow print pascal_static-membersset print vtblset print vtbl onvtbl commands do not work on programs compiled with the HP
ANSI C++ compiler (aCC).)
set print vtbl offshow print vtblValues printed by the print command are saved in the gdb
value history. This allows you to refer to them in other expressions.
Values are kept until the symbol table is re-read or discarded
(for example with the file or symbol-file commands).
When the symbol table changes, the value history is discarded,
since the values may contain pointers back to the types defined in the
symbol table.
The values printed are given history numbers by which you can
refer to them. These are successive integers starting with one.
print shows you the history number assigned to a value by
printing `$num = ' before the value; here num is the
history number.
To refer to any previous value, use `$' followed by the value's
history number. The way print labels its output is designed to
remind you of this. Just $ refers to the most recent value in
the history, and $$ refers to the value before that.
$$n refers to the nth value from the end; $$2
is the value just prior to $$, $$1 is equivalent to
$$, and $$0 is equivalent to $.
For example, suppose you have just printed a pointer to a structure and want to see the contents of the structure. It suffices to type
p *$
If you have a chain of structures where the component next points
to the next one, you can print the contents of the next one with this:
p *$.next
You can print successive links in the chain by repeating this command—which you can do by just typing <RET>.
Note that the history records values, not expressions. If the value of
x is 4 and you type these commands:
print x
set x=5
then the value recorded in the value history by the print command
remains 4 even though the value of x has changed.
show valuesshow
values does not change the history.
show values nshow values +show values + produces no display.
Pressing <RET> to repeat show values n has exactly the
same effect as `show values +'.
gdb provides convenience variables that you can use within gdb to hold on to a value and refer to it later. These variables exist entirely within gdb; they are not part of your program, and setting a convenience variable has no direct effect on further execution of your program. That is why you can use them freely.
Convenience variables are prefixed with `$'. Any name preceded by `$' can be used for a convenience variable, unless it is one of the predefined machine-specific register names (see Registers). (Value history references, in contrast, are numbers preceded by `$'. See Value history.)
You can save a value in a convenience variable with an assignment expression, just as you would set a variable in your program. For example:
set $foo = *object_ptr
would save in $foo the value contained in the object pointed to by
object_ptr.
Using a convenience variable for the first time creates it, but its
value is void until you assign a new value. You can alter the
value with another assignment at any time.
Convenience variables have no fixed types. You can assign a convenience variable any type of value, including structures and arrays, even if that variable already has a value of a different type. The convenience variable, when used as an expression, has the type of its current value.
show convenienceshow conv.
One of the ways to use a convenience variable is as a counter to be incremented or a pointer to be advanced. For example, to print a field from successive elements of an array of structures:
set $i = 0
print bar[$i++]->contents
Repeat that command by typing <RET>.
Some convenience variables are created automatically by gdb and given values likely to be useful.
$_$_ is automatically set by the x command to
the last address examined (see Examining memory). Other
commands which provide a default address for x to examine also
set $_ to that address; these commands include info line
and info breakpoint. The type of $_ is void *
except when set by the x command, in which case it is a pointer
to the type of $__.
$__$__ is automatically set by the x command
to the value found in the last address examined. Its type is chosen
to match the format in which the data was printed.
$_exitcode$_exitcode is automatically set to the exit code when
the program being debugged terminates.
On HP-UX systems, if you refer to a function or variable name that begins with a dollar sign, gdb searches for a user or system name first, before it searches for a convenience variable.
You can refer to machine register contents, in expressions, as variables
with names starting with `$'. The names of registers are different
for each machine; use info registers to see the names used on
your machine.
info registersinfo all-registersinfo registers regname ...gdb has four “standard” register names that are available (in
expressions) on most machines—whenever they do not conflict with an
architecture's canonical mnemonics for registers. The register names
$pc and $sp are used for the program counter register and
the stack pointer. $fp is used for a register that contains a
pointer to the current stack frame, and $ps is used for a
register that contains the processor status. For example,
you could print the program counter in hex with
p/x $pc
or print the instruction to be executed next with
x/i $pc
or add four to the stack pointer5 with
set $sp += 4
Whenever possible, these four standard register names are available on
your machine even though the machine has different canonical mnemonics,
so long as there is no conflict. The info registers command
shows the canonical names. For example, on the SPARC, info
registers displays the processor status register as $psr but you
can also refer to it as $ps; and on x86-based machines $ps
is an alias for the eflags register.
gdb always considers the contents of an ordinary register as an integer when the register is examined in this way. Some machines have special registers which can hold nothing but floating point; these registers are considered to have floating point values. There is no way to refer to the contents of an ordinary register as floating point value (although you can print it as a floating point value with `print/f $regname').
Some registers have distinct “raw” and “virtual” data formats. This
means that the data format in which the register contents are saved by
the operating system is not the same one that your program normally
sees. For example, the registers of the 68881 floating point
coprocessor are always saved in “extended” (raw) format, but all C
programs expect to work with “double” (virtual) format. In such
cases, gdb normally works with the virtual format only (the format
that makes sense for your program), but the info registers command
prints the data in both formats.
Normally, register values are relative to the selected stack frame (see Selecting a frame). This means that you get the value that the register would contain if all stack frames farther in were exited and their saved registers restored. In order to see the true contents of hardware registers, you must select the innermost frame (with `frame 0').
However, gdb must deduce where registers are saved, from the machine code generated by your compiler. If some registers are not saved, or if gdb is unable to locate the saved registers, the selected stack frame makes no difference.
Depending on the configuration, gdb may be able to give you more information about the status of the floating point hardware.
info floatDepending on the configuration, gdb may be able to give you more information about the status of the vector unit.
info vectorgdb provides interfaces to useful OS facilities that can help you debug your program.
When gdb runs on a Posix system (such as GNU or Unix
machines), it interfaces with the inferior via the ptrace
system call. The operating system creates a special sata structure,
called struct user, for this interface. You can use the
command info udot to display the contents of this data
structure.
info udotstruct user maintained by the OS
kernel for the program being debugged. gdb displays the
contents of struct user as a list of hex numbers, similar to
the examine command.
Some operating systems supply an auxiliary vector to programs at startup. This is akin to the arguments and environment that you specify for a program, but contains a system-dependent variety of binary values that tell system libraries important details about the hardware, operating system, and process. Each value's purpose is identified by an integer tag; the meanings are well-known but system-specific. Depending on the configuration and operating system facilities, gdb may be able to show you this information. For remote targets, this functionality may further depend on the remote stub's support of the `qPart:auxv:read' packet, see auxiliary vector.
info auxvMemory region attributes allow you to describe special handling required by regions of your target's memory. gdb uses attributes to determine whether to allow certain types of memory accesses; whether to use specific width accesses; and whether to cache target memory.
Defined memory regions can be individually enabled and disabled. When a memory region is disabled, gdb uses the default attributes when accessing memory in that region. Similarly, if no memory regions have been defined, gdb uses the default attributes when accessing all memory.
When a memory region is defined, it is given a number to identify it; to enable, disable, or remove a memory region, you specify that number.
mem lower upper attributes...delete mem nums...disable mem nums...enable mem nums...info memThe access mode attributes set whether gdb may make read or write accesses to a memory region.
While these attributes prevent gdb from performing invalid memory accesses, they do nothing to prevent the target system, I/O DMA, etc. from accessing memory.
roworwThe acccess size attributes tells gdb to use specific sized accesses in the memory region. Often memory mapped device registers require specific sized accesses. If no access size attribute is specified, gdb may use accesses of any size.
8163264The data cache attributes set whether gdb will cache target memory. While this generally improves performance by reducing debug protocol overhead, it can lead to incorrect results because gdb does not know about volatile variables or memory mapped device registers.
cachenocache
You can use the commands dump, append, and
restore to copy data between target memory and a file. The
dump and append commands write data to a file, and the
restore command reads data from a file back into the inferior's
memory. Files may be in binary, Motorola S-record, Intel hex, or
Tektronix Hex format; however, gdb can only append to binary
files.
dump [format] memory filename start_addr end_addrdump [format] value filename exprThe format parameter may be any one of:
binaryihexsrectekhexgdb uses the same definitions of these formats as the gnu binary utilities, like `objdump' and `objcopy'. If format is omitted, gdb dumps the data in raw binary form.
append [binary] memory filename start_addr end_addrappend [binary] value filename exprrestore filename [binary] bias start endrestore command can automatically recognize any known bfd
file format, except for raw binary. To restore a raw binary file you
must specify the optional keyword binary after the filename.
If bias is non-zero, its value will be added to the addresses contained in the file. Binary files always start at address zero, so they will be restored at address bias. Other bfd files have a built-in location; they will be restored at offset bias from that location.
If start and/or end are non-zero, then only data between file offset start and file offset end will be restored. These offsets are relative to the addresses in the file, before the bias argument is applied.
A core file or core dump is a file that records the memory image of a running process and its process status (register values etc.). Its primary use is post-mortem debugging of a program that crashed while it ran outside a debugger. A program that crashes automatically produces a core file, unless this feature is disabled by the user. See Files, for information on invoking gdb in the post-mortem debugging mode.
Occasionally, you may wish to produce a core file of the program you are debugging in order to preserve a snapshot of its state. gdb has a special command for that.
generate-core-file [file]gcore [file]Note that this command is implemented only for some systems (as of this writing, gnu/Linux, FreeBSD, Solaris, Unixware, and S390).
If the program you are debugging uses a different character set to represent characters and strings than the one gdb uses itself, gdb can automatically translate between the character sets for you. The character set gdb uses we call the host character set; the one the inferior program uses we call the target character set.
For example, if you are running gdb on a gnu/Linux system, which
uses the ISO Latin 1 character set, but you are using gdb's
remote protocol (see Remote Debugging) to debug a program
running on an IBM mainframe, which uses the ebcdic character set,
then the host character set is Latin-1, and the target character set is
ebcdic. If you give gdb the command set
target-charset EBCDIC-US, then gdb translates between
ebcdic and Latin 1 as you print character or string values, or use
character and string literals in expressions.
gdb has no way to automatically recognize which character set
the inferior program uses; you must tell it, using the set
target-charset command, described below.
Here are the commands for controlling gdb's character set support:
set target-charset charsetset target-charset followed by <TAB><TAB>, gdb will
list the target character sets it supports.
set host-charset charsetBy default, gdb uses a host character set appropriate to the
system it is running on; you can override that default using the
set host-charset command.
gdb can only use certain character sets as its host character
set. We list the character set names gdb recognizes below, and
indicate which can be host character sets, but if you type
set target-charset followed by <TAB><TAB>, gdb will
list the host character sets it supports.
set charset charsetset charset followed by <TAB><TAB>,
gdb will list the name of the character sets that can be used
for both host and target.
show charsetshow host-charsetshow target-charsetgdb currently includes support for the following character sets:
ASCIIISO-8859-1EBCDIC-USIBM1047Note that these are all single-byte character sets. More work inside GDB is needed to support multi-byte or variable-width character encodings, like the UTF-8 and UCS-2 encodings of Unicode.
Here is an example of gdb's character set support in action. Assume that the following source code has been placed in the file charset-test.c:
#include <stdio.h>
char ascii_hello[]
= {72, 101, 108, 108, 111, 44, 32, 119,
111, 114, 108, 100, 33, 10, 0};
char ibm1047_hello[]
= {200, 133, 147, 147, 150, 107, 64, 166,
150, 153, 147, 132, 90, 37, 0};
main ()
{
printf ("Hello, world!\n");
}
In this program, ascii_hello and ibm1047_hello are arrays
containing the string `Hello, world!' followed by a newline,
encoded in the ascii and ibm1047 character sets.
We compile the program, and invoke the debugger on it:
$ gcc -g charset-test.c -o charset-test
$ gdb -nw charset-test
GNU gdb 2001-12-19-cvs
Copyright 2001 Free Software Foundation, Inc.
...
(gdb)
We can use the show charset command to see what character sets
gdb is currently using to interpret and display characters and
strings:
(gdb) show charset
The current host and target character set is `ISO-8859-1'.
(gdb)
For the sake of printing this manual, let's use ascii as our initial character set:
(gdb) set charset ASCII
(gdb) show charset
The current host and target character set is `ASCII'.
(gdb)
Let's assume that ascii is indeed the correct character set for our
host system — in other words, let's assume that if gdb prints
characters using the ascii character set, our terminal will display
them properly. Since our current target character set is also
ascii, the contents of ascii_hello print legibly:
(gdb) print ascii_hello
$1 = 0x401698 "Hello, world!\n"
(gdb) print ascii_hello[0]
$2 = 72 'H'
(gdb)
gdb uses the target character set for character and string literals you use in expressions:
(gdb) print '+'
$3 = 43 '+'
(gdb)
The ascii character set uses the number 43 to encode the `+' character.
gdb relies on the user to tell it which character set the
target program uses. If we print ibm1047_hello while our target
character set is still ascii, we get jibberish:
(gdb) print ibm1047_hello
$4 = 0x4016a8 "\310\205\223\223\226k@\246\226\231\223\204Z%"
(gdb) print ibm1047_hello[0]
$5 = 200 '\310'
(gdb)
If we invoke the set target-charset followed by <TAB><TAB>,
gdb tells us the character sets it supports:
(gdb) set target-charset
ASCII EBCDIC-US IBM1047 ISO-8859-1
(gdb) set target-charset
We can select ibm1047 as our target character set, and examine the
program's strings again. Now the ascii string is wrong, but
gdb translates the contents of ibm1047_hello from the
target character set, ibm1047, to the host character set,
ascii, and they display correctly:
(gdb) set target-charset IBM1047
(gdb) show charset
The current host character set is `ASCII'.
The current target character set is `IBM1047'.
(gdb) print ascii_hello
$6 = 0x401698 "\110\145%%?\054\040\167?\162%\144\041\012"
(gdb) print ascii_hello[0]
$7 = 72 '\110'
(gdb) print ibm1047_hello
$8 = 0x4016a8 "Hello, world!\n"
(gdb) print ibm1047_hello[0]
$9 = 200 'H'
(gdb)
As above, gdb uses the target character set for character and string literals you use in expressions:
(gdb) print '+'
$10 = 78 '+'
(gdb)
The ibm1047 character set uses the number 78 to encode the `+' character.
gdb can cache data exchanged between the debugger and a remote target (see Remote). Such caching generally improves performance, because it reduces the overhead of the remote protocol by bundling memory reads and writes into large chunks. Unfortunately, gdb does not currently know anything about volatile registers, and thus data caching will produce incorrect results when volatile registers are in use.
set remotecache onset remotecache offON, use data
caching. By default, this option is OFF.
show remotecacheinfo dcacheSome languages, such as C and C++, provide a way to define and invoke “preprocessor macros” which expand into strings of tokens. gdb can evaluate expressions containing macro invocations, show the result of macro expansion, and show a macro's definition, including where it was defined.
You may need to compile your program specially to provide gdb with information about preprocessor macros. Most compilers do not include macros in their debugging information, even when you compile with the -g flag. See Compilation.
A program may define a macro at one point, remove that definition later, and then provide a different definition after that. Thus, at different points in the program, a macro may have different definitions, or have no definition at all. If there is a current stack frame, gdb uses the macros in scope at that frame's source code line. Otherwise, gdb uses the macros in scope at the current listing location; see List.
At the moment, gdb does not support the ##
token-splicing operator, the # stringification operator, or
variable-arity macros.
Whenever gdb evaluates an expression, it always expands any macro invocations present in the expression. gdb also provides the following commands for working with macros explicitly.
macro expand expressionmacro exp expressionmacro expand-once expressionmacro exp1 expressioninfo macro macromacro define macro replacement-listmacro define macro(arglist) replacement-listA definition introduced by this command is in scope in every expression evaluated in gdb, until it is removed with the macro undef command, described below. The definition overrides all definitions for macro present in the program being debugged, as well as any previous user-supplied definition.
macro undef macromacro listmacro define command.
Here is a transcript showing the above commands in action. First, we show our source files:
$ cat sample.c
#include <stdio.h>
#include "sample.h"
#define M 42
#define ADD(x) (M + x)
main ()
{
#define N 28
printf ("Hello, world!\n");
#undef N
printf ("We're so creative.\n");
#define N 1729
printf ("Goodbye, world!\n");
}
$ cat sample.h
#define Q <
$
Now, we compile the program using the gnu C compiler, gcc. We pass the -gdwarf-2 and -g3 flags to ensure the compiler includes information about preprocessor macros in the debugging information.
$ gcc -gdwarf-2 -g3 sample.c -o sample
$
Now, we start gdb on our sample program:
$ gdb -nw sample
GNU gdb 2002-05-06-cvs
Copyright 2002 Free Software Foundation, Inc.
GDB is free software, ...
(gdb)
We can expand macros and examine their definitions, even when the program is not running. gdb uses the current listing position to decide which macro definitions are in scope:
(gdb) list main
3
4 #define M 42
5 #define ADD(x) (M + x)
6
7 main ()
8 {
9 #define N 28
10 printf ("Hello, world!\n");
11 #undef N
12 printf ("We're so creative.\n");
(gdb) info macro ADD
Defined at /home/jimb/gdb/macros/play/sample.c:5
#define ADD(x) (M + x)
(gdb) info macro Q
Defined at /home/jimb/gdb/macros/play/sample.h:1
included at /home/jimb/gdb/macros/play/sample.c:2
#define Q <
(gdb) macro expand ADD(1)
expands to: (42 + 1)
(gdb) macro expand-once ADD(1)
expands to: once (M + 1)
(gdb)
In the example above, note that macro expand-once expands only
the macro invocation explicit in the original text — the invocation of
ADD — but does not expand the invocation of the macro M,
which was introduced by ADD.
Once the program is running, GDB uses the macro definitions in force at the source line of the current stack frame:
(gdb) break main
Breakpoint 1 at 0x8048370: file sample.c, line 10.
(gdb) run
Starting program: /home/jimb/gdb/macros/play/sample
Breakpoint 1, main () at sample.c:10
10 printf ("Hello, world!\n");
(gdb)
At line 10, the definition of the macro N at line 9 is in force:
(gdb) info macro N
Defined at /home/jimb/gdb/macros/play/sample.c:9
#define N 28
(gdb) macro expand N Q M
expands to: 28 < 42
(gdb) print N Q M
$1 = 1
(gdb)
As we step over directives that remove N's definition, and then
give it a new definition, gdb finds the definition (or lack
thereof) in force at each point:
(gdb) next
Hello, world!
12 printf ("We're so creative.\n");
(gdb) info macro N
The symbol `N' has no definition as a C/C++ preprocessor macro
at /home/jimb/gdb/macros/play/sample.c:12
(gdb) next
We're so creative.
14 printf ("Goodbye, world!\n");
(gdb) info macro N
Defined at /home/jimb/gdb/macros/play/sample.c:13
#define N 1729
(gdb) macro expand N Q M
expands to: 1729 < 42
(gdb) print N Q M
$2 = 0
(gdb)
In some applications, it is not feasible for the debugger to interrupt the program's execution long enough for the developer to learn anything helpful about its behavior. If the program's correctness depends on its real-time behavior, delays introduced by a debugger might cause the program to change its behavior drastically, or perhaps fail, even when the code itself is correct. It is useful to be able to observe the program's behavior without interrupting it.
Using gdb's trace and collect commands, you can
specify locations in the program, called tracepoints, and
arbitrary expressions to evaluate when those tracepoints are reached.
Later, using the tfind command, you can examine the values
those expressions had when the program hit the tracepoints. The
expressions may also denote objects in memory—structures or arrays,
for example—whose values gdb should record; while visiting
a particular tracepoint, you may inspect those objects as if they were
in memory at that moment. However, because gdb records these
values without interacting with you, it can do so quickly and
unobtrusively, hopefully not disturbing the program's behavior.
The tracepoint facility is currently available only for remote targets. See Targets. In addition, your remote target must know how to collect trace data. This functionality is implemented in the remote stub; however, none of the stubs distributed with gdb support tracepoints as of this writing.
This chapter describes the tracepoint commands and features.
Before running such a trace experiment, an arbitrary number of tracepoints can be set. Like a breakpoint (see Set Breaks), a tracepoint has a number assigned to it by gdb. Like with breakpoints, tracepoint numbers are successive integers starting from one. Many of the commands associated with tracepoints take the tracepoint number as their argument, to identify which tracepoint to work on.
For each tracepoint, you can specify, in advance, some arbitrary set of data that you want the target to collect in the trace buffer when it hits that tracepoint. The collected data can include registers, local variables, or global data. Later, you can use gdb commands to examine the values these data had at the time the tracepoint was hit.
This section describes commands to set tracepoints and associated conditions and actions.
tracetrace command is very similar to the break command.
Its argument can be a source line, a function name, or an address in
the target program. See Set Breaks. The trace command
defines a tracepoint, which is a point in the target program where the
debugger will briefly stop, collect some data, and then allow the
program to continue. Setting a tracepoint or changing its commands
doesn't take effect until the next tstart command; thus, you
cannot change the tracepoint attributes once a trace experiment is
running.
Here are some examples of using the trace command:
(gdb) trace foo.c:121 // a source file and line number
(gdb) trace +2 // 2 lines forward
(gdb) trace my_function // first source line of function
(gdb) trace *my_function // EXACT start address of function
(gdb) trace *0x2117c4 // an address
You can abbreviate trace as tr.
The convenience variable $tpnum records the tracepoint number
of the most recently set tracepoint.
delete tracepoint [num]Examples:
(gdb) delete trace 1 2 3 // remove three tracepoints
(gdb) delete trace // remove all tracepoints
You can abbreviate this command as del tr.
disable tracepoint [num]enable tracepoint command.
enable tracepoint [num]passcount [n [num]]passcount command sets the
passcount of the most recently defined tracepoint. If no passcount is
given, the trace experiment will run until stopped explicitly by the
user.
Examples:
(gdb) passcount 5 2 // Stop on the 5th execution of
// tracepoint 2
(gdb) passcount 12 // Stop on the 12th execution of the
// most recently defined tracepoint.
(gdb) trace foo
(gdb) pass 3
(gdb) trace bar
(gdb) pass 2
(gdb) trace baz
(gdb) pass 1 // Stop tracing when foo has been
// executed 3 times OR when bar has
// been executed 2 times
// OR when baz has been executed 1 time.
actions [num]actions without bothering about its number). You specify the
actions themselves on the following lines, one action at a time, and
terminate the actions list with a line containing just end. So
far, the only defined actions are collect and
while-stepping.
To remove all actions from a tracepoint, type `actions num' and follow it immediately with `end'.
(gdb) collect data // collect some data
(gdb) while-stepping 5 // single-step 5 times, collect data
(gdb) end // signals the end of actions.
In the following example, the action list begins with collect
commands indicating the things to be collected when the tracepoint is
hit. Then, in order to single-step and collect additional data
following the tracepoint, a while-stepping command is used,
followed by the list of things to be collected while stepping. The
while-stepping command is terminated by its own separate
end command. Lastly, the action list is terminated by an
end command.
(gdb) trace foo
(gdb) actions
Enter actions for tracepoint 1, one per line:
> collect bar,baz
> collect $regs
> while-stepping 12
> collect $fp, $sp
> end
end
collect expr1, expr2, ...$regs$args$localsYou can give several consecutive collect commands, each one
with a single argument, or one collect command with several
arguments separated by commas: the effect is the same.
The command info scope (see info scope) is
particularly useful for figuring out what data to collect.
while-stepping nwhile-stepping command is
followed by the list of what to collect while stepping (followed by
its own end command):
> while-stepping 12
> collect $regs, myglobal
> end
>
You may abbreviate while-stepping as ws or
stepping.
info tracepoints [num]passcount n command
while-stepping n command
actions command
(gdb) info trace
Num Enb Address PassC StepC What
1 y 0x002117c4 0 0 <gdb_asm>
2 y 0x0020dc64 0 0 in g_test at g_test.c:1375
3 y 0x0020b1f4 0 0 in get_data at ../foo.c:41
(gdb)
This command can be abbreviated info tp.
tstarttstopNote: a trace experiment and data collection may stop automatically if any tracepoint's passcount is reached (see Tracepoint Passcounts), or if the trace buffer becomes full.
tstatusHere is an example of the commands we described so far:
(gdb) trace gdb_c_test
(gdb) actions
Enter actions for tracepoint #1, one per line.
> collect $regs,$locals,$args
> while-stepping 11
> collect $regs
> end
> end
(gdb) tstart
[time passes ...]
(gdb) tstop
After the tracepoint experiment ends, you use gdb commands
for examining the trace data. The basic idea is that each tracepoint
collects a trace snapshot every time it is hit and another
snapshot every time it single-steps. All these snapshots are
consecutively numbered from zero and go into a buffer, and you can
examine them later. The way you examine them is to focus on a
specific trace snapshot. When the remote stub is focused on a trace
snapshot, it will respond to all gdb requests for memory and
registers by reading from the buffer which belongs to that snapshot,
rather than from real memory or registers of the program being
debugged. This means that all gdb commands
(print, info registers, backtrace, etc.) will
behave as if we were currently debugging the program state as it was
when the tracepoint occurred. Any requests for data that are not in
the buffer will fail.
tfind nThe basic command for selecting a trace snapshot from the buffer is
tfind n, which finds trace snapshot number n,
counting from zero. If no argument n is given, the next
snapshot is selected.
Here are the various forms of using the tfind command.
tfind starttfind 0 (since 0 is the number of the first snapshot).
tfind nonetfind endtfindtfind -tfind tracepoint numtfind pc addrtfind outside addr1, addr2tfind range addr1, addr2tfind line [file:]ntfind line repeatedly can appear to have the same effect as
stepping from line to line in a live debugging session.
The default arguments for the tfind commands are specifically
designed to make it easy to scan through the trace buffer. For
instance, tfind with no argument selects the next trace
snapshot, and tfind - with no argument selects the previous
trace snapshot. So, by giving one tfind command, and then
simply hitting <RET> repeatedly you can examine all the trace
snapshots in order. Or, by saying tfind - and then hitting
<RET> repeatedly you can examine the snapshots in reverse order.
The tfind line command with no argument selects the snapshot
for the next source line executed. The tfind pc command with
no argument selects the next snapshot with the same program counter
(PC) as the current frame. The tfind tracepoint command with
no argument selects the next trace snapshot collected by the same
tracepoint as the current one.
In addition to letting you scan through the trace buffer manually, these commands make it easy to construct gdb scripts that scan through the trace buffer and print out whatever collected data you are interested in. Thus, if we want to examine the PC, FP, and SP registers from each trace frame in the buffer, we can say this:
(gdb) tfind start
(gdb) while ($trace_frame != -1)
> printf "Frame %d, PC = %08X, SP = %08X, FP = %08X\n", \
$trace_frame, $pc, $sp, $fp
> tfind
> end
Frame 0, PC = 0020DC64, SP = 0030BF3C, FP = 0030BF44
Frame 1, PC = 0020DC6C, SP = 0030BF38, FP = 0030BF44
Frame 2, PC = 0020DC70, SP = 0030BF34, FP = 0030BF44
Frame 3, PC = 0020DC74, SP = 0030BF30, FP = 0030BF44
Frame 4, PC = 0020DC78, SP = 0030BF2C, FP = 0030BF44
Frame 5, PC = 0020DC7C, SP = 0030BF28, FP = 0030BF44
Frame 6, PC = 0020DC80, SP = 0030BF24, FP = 0030BF44
Frame 7, PC = 0020DC84, SP = 0030BF20, FP = 0030BF44
Frame 8, PC = 0020DC88, SP = 0030BF1C, FP = 0030BF44
Frame 9, PC = 0020DC8E, SP = 0030BF18, FP = 0030BF44
Frame 10, PC = 00203F6C, SP = 0030BE3C, FP = 0030BF14
Or, if we want to examine the variable X at each source line in
the buffer:
(gdb) tfind start
(gdb) while ($trace_frame != -1)
> printf "Frame %d, X == %d\n", $trace_frame, X
> tfind line
> end
Frame 0, X = 1
Frame 7, X = 2
Frame 13, X = 255
tdumpThis command takes no arguments. It prints all the data collected at the current trace snapshot.
(gdb) trace 444
(gdb) actions
Enter actions for tracepoint #2, one per line:
> collect $regs, $locals, $args, gdb_long_test
> end
(gdb) tstart
(gdb) tfind line 444
#0 gdb_test (p1=0x11, p2=0x22, p3=0x33, p4=0x44, p5=0x55, p6=0x66)
at gdb_test.c:444
444 printp( "%s: arguments = 0x%X 0x%X 0x%X 0x%X 0x%X 0x%X\n", )
(gdb) tdump
Data collected at tracepoint 2, trace frame 1:
d0 0xc4aa0085 -995491707
d1 0x18 24
d2 0x80 128
d3 0x33 51
d4 0x71aea3d 119204413
d5 0x22 34
d6 0xe0 224
d7 0x380035 3670069
a0 0x19e24a 1696330
a1 0x3000668 50333288
a2 0x100 256
a3 0x322000 3284992
a4 0x3000698 50333336
a5 0x1ad3cc 1758156
fp 0x30bf3c 0x30bf3c
sp 0x30bf34 0x30bf34
ps 0x0 0
pc 0x20b2c8 0x20b2c8
fpcontrol 0x0 0
fpstatus 0x0 0
fpiaddr 0x0 0
p = 0x20e5b4 "gdb-test"
p1 = (void *) 0x11
p2 = (void *) 0x22
p3 = (void *) 0x33
p4 = (void *) 0x44
p5 = (void *) 0x55
p6 = (void *) 0x66
gdb_long_test = 17 '\021'
(gdb)
save-tracepoints filename
This command saves all current tracepoint definitions together with
their actions and passcounts, into a file filename
suitable for use in a later debugging session. To read the saved
tracepoint definitions, use the source command (see Command Files).
(int) $trace_frame(int) $tracepoint(int) $trace_line(char []) $trace_file(char []) $trace_func$tracepoint.
Note: $trace_file is not suitable for use in printf,
use output instead.
Here's a simple example of using these convenience variables for stepping through all the trace snapshots and printing some of their data.
(gdb) tfind start
(gdb) while $trace_frame != -1
> output $trace_file
> printf ", line %d (tracepoint #%d)\n", $trace_line, $tracepoint
> tfind
> end
If your program is too large to fit completely in your target system's memory, you can sometimes use overlays to work around this problem. gdb provides some support for debugging programs that use overlays.
Suppose you have a computer whose instruction address space is only 64 kilobytes long, but which has much more memory which can be accessed by other means: special instructions, segment registers, or memory management hardware, for example. Suppose further that you want to adapt a program which is larger than 64 kilobytes to run on this system.
One solution is to identify modules of your program which are relatively independent, and need not call each other directly; call these modules overlays. Separate the overlays from the main program, and place their machine code in the larger memory. Place your main program in instruction memory, but leave at least enough space there to hold the largest overlay as well.
Now, to call a function located in an overlay, you must first copy that overlay's machine code from the large memory into the space set aside for it in the instruction memory, and then jump to its entry point there.
Data Instruction Larger
Address Space Address Space Address Space
+-----------+ +-----------+ +-----------+
| | | | | |
+-----------+ +-----------+ +-----------+<-- overlay 1
| program | | main | .----| overlay 1 | load address
| variables | | program | | +-----------+
| and heap | | | | | |
+-----------+ | | | +-----------+<-- overlay 2
| | +-----------+ | | | load address
+-----------+ | | | .-| overlay 2 |
| | | | | |
mapped --->+-----------+ | | +-----------+
address | | | | | |
| overlay | <-' | | |
| area | <---' +-----------+<-- overlay 3
| | <---. | | load address
+-----------+ `--| overlay 3 |
| | | |
+-----------+ | |
+-----------+
| |
+-----------+
A code overlay
The diagram (see A code overlay) shows a system with separate data and instruction address spaces. To map an overlay, the program copies its code from the larger address space to the instruction address space. Since the overlays shown here all use the same mapped address, only one may be mapped at a time. For a system with a single address space for data and instructions, the diagram would be similar, except that the program variables and heap would share an address space with the main program and the overlay area.
An overlay loaded into instruction memory and ready for use is called a mapped overlay; its mapped address is its address in the instruction memory. An overlay not present (or only partially present) in instruction memory is called unmapped; its load address is its address in the larger memory. The mapped address is also called the virtual memory address, or VMA; the load address is also called the load memory address, or LMA.
Unfortunately, overlays are not a completely transparent way to adapt a program to limited instruction memory. They introduce a new set of global constraints you must keep in mind as you design your program:
The overlay system described above is rather simple, and could be improved in many ways:
To use gdb's overlay support, each overlay in your program must correspond to a separate section of the executable file. The section's virtual memory address and load memory address must be the overlay's mapped and load addresses. Identifying overlays with sections allows gdb to determine the appropriate address of a function or variable, depending on whether the overlay is mapped or not.
gdb's overlay commands all start with the word overlay;
you can abbreviate this as ov or ovly. The commands are:
overlay offoverlay manualoverlay map-overlay and overlay unmap-overlay
commands described below.
overlay map-overlay overlayoverlay map overlayoverlay unmap-overlay overlayoverlay unmap overlayoverlay autooverlay load-targetoverlay loadoverlay list-overlaysoverlay listNormally, when gdb prints a code address, it includes the name of the function the address falls in:
(gdb) print main
$3 = {int ()} 0x11a0 <main>
When overlay debugging is enabled, gdb recognizes code in
unmapped overlays, and prints the names of unmapped functions with
asterisks around them. For example, if foo is a function in an
unmapped overlay, gdb prints it this way:
(gdb) overlay list
No sections are mapped.
(gdb) print foo
$5 = {int (int)} 0x100000 <*foo*>
When foo's overlay is mapped, gdb prints the function's
name normally:
(gdb) overlay list
Section .ov.foo.text, loaded at 0x100000 - 0x100034,
mapped at 0x1016 - 0x104a
(gdb) print foo
$6 = {int (int)} 0x1016 <foo>
When overlay debugging is enabled, gdb can find the correct
address for functions and variables in an overlay, whether or not the
overlay is mapped. This allows most gdb commands, like
break and disassemble, to work normally, even on unmapped
code. However, gdb's breakpoint support has some limitations:
gdb can automatically track which overlays are mapped and which
are not, given some simple co-operation from the overlay manager in the
inferior. If you enable automatic overlay debugging with the
overlay auto command (see Overlay Commands), gdb
looks in the inferior's memory for certain variables describing the
current state of the overlays.
Here are the variables your overlay manager must define to support gdb's automatic overlay debugging:
_ovly_table: struct
{
/* The overlay's mapped address. */
unsigned long vma;
/* The size of the overlay, in bytes. */
unsigned long size;
/* The overlay's load address. */
unsigned long lma;
/* Non-zero if the overlay is currently mapped;
zero otherwise. */
unsigned long mapped;
}
_novlys:_ovly_table.
To decide whether a particular overlay is mapped or not, gdb
looks for an entry in _ovly_table whose vma and
lma members equal the VMA and LMA of the overlay's section in the
executable file. When gdb finds a matching entry, it consults
the entry's mapped member to determine whether the overlay is
currently mapped.
In addition, your overlay manager may define a function called
_ovly_debug_event. If this function is defined, gdb
will silently set a breakpoint there. If the overlay manager then
calls this function whenever it has changed the overlay table, this
will enable gdb to accurately keep track of which overlays
are in program memory, and update any breakpoints that may be set
in overlays. This will allow breakpoints to work even if the
overlays are kept in ROM or other non-writable memory while they
are not being executed.
When linking a program which uses overlays, you must place the overlays at their load addresses, while relocating them to run at their mapped addresses. To do this, you must write a linker script (see Overlay Description). Unfortunately, since linker scripts are specific to a particular host system, target architecture, and target memory layout, this manual cannot provide portable sample code demonstrating gdb's overlay support.
However, the gdb source distribution does contain an overlaid program, with linker scripts for a few systems, as part of its test suite. The program consists of the following files from gdb/testsuite/gdb.base:
d10v-elf
and m32r-elf targets.
You can build the test program using the d10v-elf GCC
cross-compiler like this:
$ d10v-elf-gcc -g -c overlays.c
$ d10v-elf-gcc -g -c ovlymgr.c
$ d10v-elf-gcc -g -c foo.c
$ d10v-elf-gcc -g -c bar.c
$ d10v-elf-gcc -g -c baz.c
$ d10v-elf-gcc -g -c grbx.c
$ d10v-elf-gcc -g overlays.o ovlymgr.o foo.o bar.o \
baz.o grbx.o -Wl,-Td10v.ld -o overlays
The build process is identical for any other architecture, except that
you must substitute the appropriate compiler and linker script for the
target system for d10v-elf-gcc and d10v.ld.
Although programming languages generally have common aspects, they are
rarely expressed in the same manner. For instance, in ANSI C,
dereferencing a pointer p is accomplished by *p, but in
Modula-2, it is accomplished by p^. Values can also be
represented (and displayed) differently. Hex numbers in C appear as
`0x1ae', while in Modula-2 they appear as `1AEH'.
Language-specific information is built into gdb for some languages, allowing you to express operations like the above in your program's native language, and allowing gdb to output values in a manner consistent with the syntax of your program's native language. The language you use to build expressions is called the working language.
There are two ways to control the working language—either have gdb
set it automatically, or select it manually yourself. You can use the
set language command for either purpose. On startup, gdb
defaults to setting the language automatically. The working language is
used to determine how expressions you type are interpreted, how values
are printed, etc.
In addition to the working language, every source file that
gdb knows about has its own working language. For some object
file formats, the compiler might indicate which language a particular
source file is in. However, most of the time gdb infers the
language from the name of the file. The language of a source file
controls whether C++ names are demangled—this way backtrace can
show each frame appropriately for its own language. There is no way to
set the language of a source file from within gdb, but you can
set the language associated with a filename extension. See Displaying the language.
This is most commonly a problem when you use a program, such
as cfront or f2c, that generates C but is written in
another language. In that case, make the
program use #line directives in its C output; that way
gdb will know the correct language of the source code of the original
program, and will display that source code, not the generated C code.
If a source file name ends in one of the following extensions, then gdb infers that its language is the one indicated.
In addition, you may set the language associated with a filename extension. See Displaying the language.
If you allow gdb to set the language automatically, expressions are interpreted the same way in your debugging session and your program.
If you wish, you may set the language manually. To do this, issue the
command `set language lang', where lang is the name of
a language, such as
c or modula-2.
For a list of the supported languages, type `set language'.
Setting the language manually prevents gdb from updating the working language automatically. This can lead to confusion if you try to debug a program when the working language is not the same as the source language, when an expression is acceptable to both languages—but means different things. For instance, if the current source file were written in C, and gdb was parsing Modula-2, a command such as:
print a = b + c
might not have the effect you intended. In C, this means to add
b and c and place the result in a. The result
printed would be the value of a. In Modula-2, this means to compare
a to the result of b+c, yielding a BOOLEAN value.
To have gdb set the working language automatically, use `set language local' or `set language auto'. gdb then infers the working language. That is, when your program stops in a frame (usually by encountering a breakpoint), gdb sets the working language to the language recorded for the function in that frame. If the language for a frame is unknown (that is, if the function or block corresponding to the frame was defined in a source file that does not have a recognized extension), the current working language is not changed, and gdb issues a warning.
This may not seem necessary for most programs, which are written entirely in one source language. However, program modules and libraries written in one source language can be used by a main program written in a different source language. Using `set language auto' in this case frees you from having to set the working language manually.
The following commands help you find out which language is the working language, and also what language source files were written in.
show languageprint to
build and compute expressions that may involve variables in your program.
info frameinfo sourceIn unusual circumstances, you may have source files with extensions not in the standard list. You can then set the extension associated with a language explicitly:
set extension-language ext languageinfo extensionsWarning: In this release, the gdb commands for type and range checking are included, but they do not yet have any effect. This section documents the intended facilities.
Some languages are designed to guard you against making seemingly common errors through a series of compile- and run-time checks. These include checking the type of arguments to functions and operators, and making sure mathematical overflows are caught at run time. Checks such as these help to ensure a program's correctness once it has been compiled by eliminating type mismatches, and providing active checks for range errors when your program is running.
gdb can check for conditions like the above if you wish.
Although gdb does not check the statements in your program,
it can check expressions entered directly into gdb for
evaluation via the print command, for example. As with the
working language, gdb can also decide whether or not to check
automatically based on your program's source language.
See Supported languages, for the default
settings of supported languages.
Some languages, such as Modula-2, are strongly typed, meaning that the arguments to operators and functions have to be of the correct type, otherwise an error occurs. These checks prevent type mismatch errors from ever causing any run-time problems. For example,
1 + 2 => 3
but
error--> 1 + 2.3
The second example fails because the CARDINAL 1 is not
type-compatible with the REAL 2.3.
For the expressions you use in gdb commands, you can tell the gdb type checker to skip checking; to treat any mismatches as errors and abandon the expression; or to only issue warnings when type mismatches occur, but evaluate the expression anyway. When you choose the last of these, gdb evaluates expressions like the second example above, but also issues a warning.
Even if you turn type checking off, there may be other reasons
related to type that prevent gdb from evaluating an expression.
For instance, gdb does not know how to add an int and
a struct foo. These particular type errors have nothing to do
with the language in use, and usually arise from expressions, such as
the one described above, which make little sense to evaluate anyway.
Each language defines to what degree it is strict about type. For instance, both Modula-2 and C require the arguments to arithmetical operators to be numbers. In C, enumerated types and pointers can be represented as numbers, so that they are valid arguments to mathematical operators. See Supported languages, for further details on specific languages.
gdb provides some additional commands for controlling the type checker:
set check type autoset check type onset check type offset check type warnshow typeIn some languages (such as Modula-2), it is an error to exceed the bounds of a type; this is enforced with run-time checks. Such range checking is meant to ensure program correctness by making sure computations do not overflow, or indices on an array element access do not exceed the bounds of the array.
For expressions you use in gdb commands, you can tell gdb to treat range errors in one of three ways: ignore them, always treat them as errors and abandon the expression, or issue warnings but evaluate the expression anyway.
A range error can result from numerical overflow, from exceeding an array index bound, or when you type a constant that is not a member of any type. Some languages, however, do not treat overflows as an error. In many implementations of C, mathematical overflow causes the result to “wrap around” to lower values—for example, if m is the largest integer value, and s is the smallest, then
m + 1 => s
This, too, is specific to individual languages, and in some cases specific to individual compilers or machines. See Supported languages, for further details on specific languages.
gdb provides some additional commands for controlling the range checker:
set check range autoset check range onset check range offset check range warnshow rangegdb supports C, C++, Objective-C, Fortran, Java, Pascal,
assembly, Modula-2, and Ada.
Some gdb features may be used in expressions regardless of the
language you use: the gdb @ and :: operators,
and the `{type}addr' construct (see Expressions) can be used with the constructs of any supported
language.
The following sections detail to what degree each source language is supported by gdb. These sections are not meant to be language tutorials or references, but serve only as a reference guide to what the gdb expression parser accepts, and what input and output formats should look like for different languages. There are many good books written on each of these languages; please look to these for a language reference or tutorial.
Since C and C++ are so closely related, many features of gdb apply to both languages. Whenever this is the case, we discuss those languages together.
The C++ debugging facilities are jointly implemented by the C++
compiler and gdb. Therefore, to debug your C++ code
effectively, you must compile your C++ programs with a supported
C++ compiler, such as gnu g++, or the HP ANSI C++
compiler (aCC).
For best results when using gnu C++, use the DWARF 2 debugging
format; if it doesn't work on your system, try the stabs+ debugging
format. You can select those formats explicitly with the g++
command-line options -gdwarf-2 and -gstabs+.
See Options for Debugging Your Program or gnu CC.
Operators must be defined on values of specific types. For instance,
+ is defined on numbers, but not on structures. Operators are
often defined on groups of types.
For the purposes of C and C++, the following definitions hold:
int with any of its storage-class
specifiers; char; enum; and, for C++, bool.
float, double, and
long double (if supported by the target platform).
(type *).
The following operators are supported. They are listed here in order of increasing precedence:
,== op= b,
and translated to a = a op b.
op= and = have the same precedence.
op is any one of the operators |, ^, &,
<<, >>, +, -, *, /, %.
?: ? b : c can be thought
of as: if a then b else c. a should be of an
integral type.
||&&|^&==, !=<, >, <=, >=<<, >>@+, -*, /, %++, --*++.
&++.
For debugging C++, gdb implements a use of `&' beyond what is
allowed in the C++ language itself: you can use `&(&ref)'
(or, if you prefer, simply `&&ref') to examine the address
where a C++ reference variable (declared with `&ref') is
stored.
-++.
!++.
~++.
., ->struct and union data.
.*, ->*[][i] is defined as
*(a+i). Same precedence as ->.
()->.
::struct, union,
and class types.
::::,
above.
If an operator is redefined in the user code, gdb usually attempts to invoke the redefined version instead of using the operator's predefined meaning.
gdb allows you to express the constants of C and C++ in the following ways:
long value.
float (as opposed to the default double) type; or with
a letter `l' or `L', which specifies a long double
constant.
'), or a number—the ordinal value of the corresponding character
(usually its ascii value). Within quotes, the single character may
be represented by a letter or by escape sequences, which are of
the form `\nnn', where nnn is the octal representation
of the character's ordinal value; or of the form `\x', where
`x' is a predefined special character—for example,
`\n' for newline.
"). Any valid character constant (as described
above) may appear. Double quotes within the string must be preceded by
a backslash, so for instance `"a\"b'c"' is a string of five
characters.
gdb expression handling can interpret most C++ expressions.
Warning: gdb can only debug C++ code if you use the proper compiler and the proper debug format. Currently, gdb works best when debugging C++ code that is compiled with gcc 2.95.3 or with gcc 3.1 or newer, using the options -gdwarf-2 or -gstabs+. DWARF 2 is preferred over stabs+. Most configurations of gcc emit either DWARF 2 or stabs+ as their default debug format, so you usually don't need to specify a debug format explicitly. Other compilers and/or debug formats are likely to work badly or not at all when using gdb to debug C++ code.
count = aml->GetOriginal(x, y)
this following the same rules as C++.
It does perform integral conversions and promotions, floating-point promotions, arithmetic conversions, pointer conversions, conversions of class objects to base classes, and standard conversions such as those of functions or arrays to pointers; it requires an exact match on the number of function arguments.
Overload resolution is always performed, unless you have specified
set overload-resolution off. See gdb features for C++.
You must specify set overload-resolution off in order to use an
explicit function signature to call an overloaded function, as in
p 'foo(char,int)'('x', 13)
The gdb command-completion facility can simplify this; see Command completion.
In the parameter list shown when gdb displays a frame, the values of reference variables are not displayed (unlike other variables); this avoids clutter, since references are often used for large structures. The address of a reference variable is always shown, unless you have specified `set print address off'.
::—your
expressions can use it just as expressions in your program do. Since
one scope may be defined in another, you can use :: repeatedly if
necessary, for example in an expression like
`scope1::scope2::name'. gdb also allows
resolving name scope by reference to source files, in both C and C++
debugging (see Program variables).
In addition, when used with HP's C++ compiler, gdb supports calling virtual functions correctly, printing out virtual bases of objects, calling functions in a base subobject, casting objects, and invoking user-defined operators.
If you allow gdb to set type and range checking automatically, they
both default to off whenever the working language changes to
C or C++. This happens regardless of whether you or gdb
selects the working language.
If you allow gdb to set the language automatically, it recognizes source files whose names end with .c, .C, or .cc, etc, and when gdb enters code compiled from one of these files, it sets the working language to C or C++. See Having gdb infer the source language, for further details.
By default, when gdb parses C or C++ expressions, type checking is not used. However, if you turn type checking on, gdb considers two variables type equivalent if:
typedef.
Range checking, if turned on, is done on mathematical operations. Array indices are not checked, since they are often used to index a pointer that is not itself an array.
The set print union and show print union commands apply to
the union type. When set to `on', any union that is
inside a struct or class is also printed. Otherwise, it
appears as `{...}'.
The @ operator aids in the debugging of dynamic arrays, formed
with pointers and a memory allocation function. See Expressions.
Some gdb commands are particularly useful with C++, and some are designed specifically for use with C++. Here is a summary:
rbreak regexcatch throwcatch catchptype typenameset print demangleshow print demangleset print asm-demangleshow print asm-demangleset print objectshow print objectset print vtblshow print vtblvtbl commands do not work on programs compiled with the HP
ANSI C++ compiler (aCC).)
set overload-resolution onset overload-resolution offshow overload-resolution(types) rather than just symbol. You can
also use the gdb command-line word completion facilities to list the
available choices, or to finish the type list for you.
See Command completion, for details on how to do this.
This section provides information about some commands and command options that are useful for debugging Objective-C code. See also info classes, and info selectors, for a few more commands specific to Objective-C support.
The following commands have been extended to accept Objective-C method names as line specifications:
clear
break
info line
jump
list
A fully qualified Objective-C method name is specified as
-[Class methodName]
where the minus sign is used to indicate an instance method and a
plus sign (not shown) is used to indicate a class method. The class
name Class and method name methodName are enclosed in
brackets, similar to the way messages are specified in Objective-C
source code. For example, to set a breakpoint at the create
instance method of class Fruit in the program currently being
debugged, enter:
break -[Fruit create]
To list ten program lines around the initialize class method,
enter:
list +[NSText initialize]
In the current version of gdb, the plus or minus sign is required. In future versions of gdb, the plus or minus sign will be optional, but you can use it to narrow the search. It is also possible to specify just a method name:
break create
You must specify the complete method name, including any colons. If
your program's source files contain more than one create method,
you'll be presented with a numbered list of classes that implement that
method. Indicate your choice by number, or type `0' to exit if
none apply.
As another example, to clear a breakpoint established at the
makeKeyAndOrderFront: method of the NSWindow class, enter:
clear -[NSWindow makeKeyAndOrderFront:]
The print command has also been extended to accept methods. For example:
print -[object hash]
will tell gdb to send the hash message to object
and print the result. Also, an additional command has been added,
print-object or po for short, which is meant to print
the description of an object. However, this command may only work
with certain Objective-C libraries that have a particular hook
function, _NSPrintForDebugger, defined.
info common [common-name]COMMON
block whose name is common-name. With no argument, the names of
all COMMON blocks visible at current program location are
printed.
Fortran symbols are usually case-insensitive, so gdb by default uses case-insensitive matches for Fortran symbols. You can change that with the `set case-insensitive' command, see Symbols, for the details.
Debugging Pascal programs which use sets, subranges, file variables, or nested functions does not currently work. gdb does not support entering expressions, printing values, or similar features using Pascal syntax.
The Pascal-specific command set print pascal_static-members
controls whether static members of Pascal objects are displayed.
See pascal_static-members.
The extensions made to gdb to support Modula-2 only support output from the gnu Modula-2 compiler (which is currently being developed). Other Modula-2 compilers are not currently supported, and attempting to debug executables produced by them is most likely to give an error as gdb reads in the executable's symbol table.
Operators must be defined on values of specific types. For instance,
+ is defined on numbers, but not on structures. Operators are
often defined on groups of types. For the purposes of Modula-2, the
following definitions hold:
INTEGER, CARDINAL, and
their subranges.
CHAR and its subranges.
REAL.
POINTER TO
type.
SET and BITSET types.
BOOLEAN.
The following operators are supported, and appear in order of increasing precedence:
,:=:= value is
value.
<, ><=, >=<.
=, <>, #<. In gdb scripts, only <> is
available for inequality, since # conflicts with the script
comment character.
IN<.
ORAND, &@+, -*/*.
DIV, MOD*.
-INTEGER and REAL data.
^NOT^.
.RECORD field selector. Defined on RECORD data. Same
precedence as ^.
[]ARRAY data. Same precedence as ^.
()PROCEDURE objects. Same precedence
as ^.
::, .Warning: Sets and their operations are not yet supported, so gdb treats the use of the operatorIN, or the use of operators+,-,*,/,=, ,<>,#,<=, and>=on sets as an error.
Modula-2 also makes available several built-in procedures and functions. In describing these, the following metavariables are used:
ARRAY variable.
CHAR constant or variable.
SET OF mtype (where mtype is the type of m).
All Modula-2 built-in procedures also return a result, described below.
ABS(n)CAP(c)CHR(i)DEC(v)DEC(v,i)EXCL(m,s)FLOAT(i)HIGH(a)INC(v)INC(v,i)INCL(m,s)MAX(t)MIN(t)ODD(i)ORD(x)SIZE(x)TRUNC(r)VAL(t,i)Warning: Sets and their operations are not yet supported, so gdb treats the use of proceduresINCLandEXCLas an error.
gdb allows you to express the constants of Modula-2 in the following ways:
') or double ("). They may
also be expressed by their ordinal value (their ascii value, usually)
followed by a `C'.
') or double (").
Escape sequences in the style of C are also allowed. See C and C++ constants, for a brief explanation of escape
sequences.
TRUE and
FALSE.
If type and range checking are set automatically by gdb, they
both default to on whenever the working language changes to
Modula-2. This happens regardless of whether you or gdb
selected the working language.
If you allow gdb to set the language automatically, then entering code compiled from a file whose name ends with .mod sets the working language to Modula-2. See Having gdb set the language automatically, for further details.
A few changes have been made to make Modula-2 programs easier to debug. This is done primarily via loosening its type strictness:
:=) returns the value of its right-hand
argument.
Warning: in this release, gdb does not yet perform type or range checking.
gdb considers two Modula-2 variables type equivalent if:
TYPE
t1 = t2 statement
As long as type checking is enabled, any attempt to combine variables whose types are not equivalent is an error.
Range checking is done on all mathematical operations, assignment, array index bounds, and all built-in functions and procedures.
:: and .
There are a few subtle differences between the Modula-2 scope operator
(.) and the gdb scope operator (::). The two have
similar syntax:
module . id
scope :: id
where scope is the name of a module or a procedure, module the name of a module, and id is any declared identifier within your program, except another module.
Using the :: operator makes gdb search the scope
specified by scope for the identifier id. If it is not
found in the specified scope, then gdb searches all scopes
enclosing the one specified by scope.
Using the . operator makes gdb search the current scope for
the identifier specified by id that was imported from the
definition module specified by module. With this operator, it is
an error if the identifier id was not imported from definition
module module, or if id is not an identifier in
module.
Some gdb commands have little use when debugging Modula-2 programs.
Five subcommands of set print and show print apply
specifically to C and C++: `vtbl', `demangle',
`asm-demangle', `object', and `union'. The first four
apply to C++, and the last to the C union type, which has no direct
analogue in Modula-2.
The @ operator (see Expressions), while available
with any language, is not useful with Modula-2. Its
intent is to aid the debugging of dynamic arrays, which cannot be
created in Modula-2 as they can in C or C++. However, because an
address can be specified by an integral constant, the construct
`{type}adrexp' is still useful.
In gdb scripts, the Modula-2 inequality operator # is
interpreted as the beginning of a comment. Use <> instead.
The extensions made to gdb for Ada only support output from the gnu Ada (GNAT) compiler. Other Ada compilers are not currently supported, and attempting to debug executables produced by them is most likely to be difficult.
The Ada mode of gdb supports a fairly large subset of Ada expression syntax, with some extensions. The philosophy behind the design of this subset is
Thus, for brevity, the debugger acts as if there were
implicit with and use clauses in effect for all user-written
packages, making it unnecessary to fully qualify most names with
their packages, regardless of context. Where this causes ambiguity,
gdb asks the user's intent.
The debugger will start in Ada mode if it detects an Ada main program. As for other languages, it will enter Ada mode when stopped in a program that was translated from an Ada source file.
While in Ada mode, you may use `–' for comments. This is useful mostly for documenting command files. The standard gdb comment (`#') still works at the beginning of a line in Ada mode, but not in the middle (to allow based literals).
The debugger supports limited overloading. Given a subprogram call in which
the function symbol has multiple definitions, it will use the number of
actual parameters and some information about their types to attempt to narrow
the set of definitions. It also makes very limited use of context, preferring
procedures to functions in the context of the call command, and
functions to procedures elsewhere.
Here are the notable omissions from the subset:
in) operator.
Characters.Latin_1 are not available and
concatenation is not implemented. Thus, escape characters in strings are
not currently available.
and, or,
xor, not, and relational tests other than equality)
are not implemented.
new operator is not implemented.
As it does for other languages, gdb makes certain generic extensions to Ada (see Expressions):
@N displays the values of
E and the N-1 adjacent variables following it in memory as an array.
In Ada, this operator is generally not necessary, since its prime use
is in displaying parts of an array, and slicing will usually do this in Ada.
However, there are occasional uses when debugging programs
in which certain debugging information has been optimized away.
::var means “the variable named var that appears
in function or file B.” When B is a file name, you must typically
surround it in single quotes.
{type} addr means “the variable of type
type that appears at address addr.”
In addition, gdb provides a few other shortcuts and outright additions specific to Ada:
set x := y + 3
print A(tmp := y + 1)
break f
condition 1 (report(i); k += 1; A(k) > 100)
"One line.["0a"]Next line.["0a"]"
contains an ASCII newline character (Ada.Characters.Latin_1.LF) after each
period.
print 'max(x, y)
(3 => 10, 17, 1)
That is, in contrast to valid Ada, only the first component has a =>
clause.
gdb print <JMPBUF_SAVE>[0]
It is sometimes necessary to debug the program during elaboration, and
before reaching the main procedure.
As defined in the Ada Reference
Manual, the elaboration code is invoked from a procedure called
adainit. To run your program up to the beginning of
elaboration, simply use the following two commands:
tbreak adainit and run.
Besides the omissions listed previously (see Omissions from Ada), we know of several problems with and limitations of Ada mode in gdb, some of which will be fixed with planned future releases of the debugger and the GNU Ada compiler.
.all after an expression
to get it printed properly.
System.Address.
Standard for any of
the standard symbols defined by the Ada language. gdb knows about
this: it will strip the prefix from names when you use it, and will never
look for a name you have so qualified among local symbols, nor match against
symbols in other packages or subprograms. If you have
defined entities anywhere in your program other than parameters and
local variables whose simple names match names in Standard,
GNAT's lack of qualification here can cause confusion. When this happens,
you can usually resolve the confusion
by qualifying the problematic names with package
Standard explicitly.
In addition to the other fully-supported programming languages,
gdb also provides a pseudo-language, called minimal.
It does not represent a real programming language, but provides a set
of capabilities close to what the C or assembly languages provide.
This should allow most simple operations to be performed while debugging
an application that uses a language currently not supported by gdb.
If the language is set to auto, gdb will automatically
select this language if the current frame corresponds to an unsupported
language.
The commands described in this chapter allow you to inquire about the symbols (names of variables, functions and types) defined in your program. This information is inherent in the text of your program and does not change as your program executes. gdb finds it in your program's symbol table, in the file indicated when you started gdb (see Choosing files), or by one of the file-management commands (see Commands to specify files).
Occasionally, you may need to refer to symbols that contain unusual characters, which gdb ordinarily treats as word delimiters. The most frequent case is in referring to static variables in other source files (see Program variables). File names are recorded in object files as debugging symbols, but gdb would ordinarily parse a typical file name, like foo.c, as the three words `foo' `.' `c'. To allow gdb to recognize `foo.c' as a single symbol, enclose it in single quotes; for example,
p 'foo.c'::x
looks up the value of x in the scope of the file foo.c.
set case-sensitive onset case-sensitive offset case-sensitive autoset
case-sensitive lets you do that by specifying on for
case-sensitive matches or off for case-insensitive ones. If
you specify auto, case sensitivity is reset to the default
suitable for the source language. The default is case-sensitive
matches for all languages except for Fortran, for which the default is
case-insensitive matches.
show case-sensitiveinfo address symbolNote the contrast with `print &symbol', which does not work at all for a register variable, and for a stack local variable prints the exact address of the current instantiation of the variable.
info symbol addr (gdb) info symbol 0x54320
_initialize_vx + 396 in section .text
This is the opposite of the info address command. You can use
it to find out the name of a variable or a function given its address.
whatis exprwhatis$, the last value in the value history.
ptype typenameptype exprptypeptype
differs from whatis by printing a detailed description, instead
of just the name of the type.
For example, for this variable declaration:
struct complex {double real; double imag;} v;
the two commands give this output:
(gdb) whatis v
type = struct complex
(gdb) ptype v
type = struct complex {
double real;
double imag;
}
As with whatis, using ptype without an argument refers to
the type of $, the last value in the value history.
info types regexpinfo typesvalue, but
`i type ^value$' gives information only on types whose complete
name is value.
This command differs from ptype in two ways: first, like
whatis, it does not print a detailed description; second, it
lists all source files where a type is defined.
info scope location (gdb) info scope command_line_handler
Scope for command_line_handler:
Symbol rl is an argument at stack/frame offset 8, length 4.
Symbol linebuffer is in static storage at address 0x150a18, length 4.
Symbol linelength is in static storage at address 0x150a1c, length 4.
Symbol p is a local variable in register $esi, length 4.
Symbol p1 is a local variable in register $ebx, length 4.
Symbol nline is a local variable in register $edx, length 4.
Symbol repeat is a local variable at frame offset -8, length 4.
This command is especially useful for determining what data to collect during a trace experiment, see collect.
info sourceinfo sourcesinfo functionsinfo functions regexpstep; `info fun ^step' finds those whose names
start with step. If a function name contains characters
that conflict with the regular expression language (eg.
`operator*()'), they may be quoted with a backslash.
info variablesinfo variables regexpinfo classesinfo classes regexpinfo selectorsinfo selectors regexpSome systems allow individual object files that make up your program to be replaced without stopping and restarting your program. For example, in VxWorks you can simply recompile a defective object file and keep on running. If you are running on one of these systems, you can allow gdb to reload the symbols for automatically relinked modules:
set symbol-reloading onset symbol-reloading offsymbol-reloading off, since otherwise gdb
may discard symbols when linking large programs, that may contain
several modules (from different directories or libraries) with the same
name.
show symbol-reloadingon or off setting.
set opaque-type-resolution onstruct, class, or
union—for example, struct MyType *—that is used in one
source file although the full declaration of struct MyType is in
another source file. The default is on.
A change in the setting of this subcommand will not take effect until
the next time symbols for a file are loaded.
set opaque-type-resolution off {<no data fields>}
show opaque-type-resolutionmaint print symbols filenamemaint print psymbols filenamemaint print msymbols filenameinfo sources to find out which files these are. If you
use `maint print psymbols' instead, the dump shows information about
symbols that gdb only knows partially—that is, symbols defined in
files that gdb has skimmed, but not yet read completely. Finally,
`maint print msymbols' dumps just the minimal symbol information
required for each object file from which gdb has read some symbols.
See Commands to specify files, for a discussion of how
gdb reads symbols (in the description of symbol-file).
maint info symtabs [ regexp ]maint info psymtabs [ regexp ]struct symtab or struct partial_symtab
structures whose names match regexp. If regexp is not
given, list them all. The output includes expressions which you can
copy into a gdb debugging this one to examine a particular
structure in more detail. For example:
(gdb) maint info psymtabs dwarf2read
{ objfile /home/gnu/build/gdb/gdb
((struct objfile *) 0x82e69d0)
{ psymtab /home/gnu/src/gdb/dwarf2read.c
((struct partial_symtab *) 0x8474b10)
readin no
fullname (null)
text addresses 0x814d3c8 -- 0x8158074
globals (* (struct partial_symbol **) 0x8507a08 @ 9)
sta