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This is gdb.info, produced by makeinfo version 4.8 from ./gdb.texinfo.
INFO-DIR-SECTION Software development
START-INFO-DIR-ENTRY
* Gdb: (gdb). The GNU debugger.
END-INFO-DIR-ENTRY
Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996,
1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009,
2010 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "Free Software" and "Free Software Needs Free
Documentation", with the Front-Cover Texts being "A GNU Manual," and
with the Back-Cover Texts as in (a) below.
(a) The FSF's Back-Cover Text is: "You are free to copy and modify
this GNU Manual. Buying copies from GNU Press supports the FSF in
developing GNU and promoting software freedom."
This file documents the GNU debugger GDB.
This is the Ninth Edition, of `Debugging with GDB: the GNU
Source-Level Debugger' for GDB (GDB) Version 7.2.
Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996,
1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009,
2010 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "Free Software" and "Free Software Needs Free
Documentation", with the Front-Cover Texts being "A GNU Manual," and
with the Back-Cover Texts as in (a) below.
(a) The FSF's Back-Cover Text is: "You are free to copy and modify
this GNU Manual. Buying copies from GNU Press supports the FSF in
developing GNU and promoting software freedom."

File: gdb.info, Node: Top, Next: Summary, Prev: (dir), Up: (dir)
Debugging with GDB
******************
This file describes GDB, the GNU symbolic debugger.
This is the Ninth Edition, for GDB (GDB) Version 7.2.
Copyright (C) 1988-2010 Free Software Foundation, Inc.
This edition of the GDB manual is dedicated to the memory of Fred
Fish. Fred was a long-standing contributor to GDB and to Free software
in general. We will miss him.
* Menu:
* Summary:: Summary of GDB
* Sample Session:: A sample GDB session
* Invocation:: Getting in and out of GDB
* Commands:: GDB commands
* Running:: Running programs under GDB
* Stopping:: Stopping and continuing
* Reverse Execution:: Running programs backward
* Process Record and Replay:: Recording inferior's execution and replaying it
* Stack:: Examining the stack
* Source:: Examining source files
* Data:: Examining data
* Optimized Code:: Debugging optimized code
* Macros:: Preprocessor Macros
* Tracepoints:: Debugging remote targets non-intrusively
* Overlays:: Debugging programs that use overlays
* Languages:: Using GDB with different languages
* Symbols:: Examining the symbol table
* Altering:: Altering execution
* GDB Files:: GDB files
* Targets:: Specifying a debugging target
* Remote Debugging:: Debugging remote programs
* Configurations:: Configuration-specific information
* Controlling GDB:: Controlling GDB
* Extending GDB:: Extending GDB
* Interpreters:: Command Interpreters
* TUI:: GDB Text User Interface
* Emacs:: Using GDB under GNU Emacs
* GDB/MI:: GDB's Machine Interface.
* Annotations:: GDB's annotation interface.
* JIT Interface:: Using the JIT debugging interface.
* GDB Bugs:: Reporting bugs in GDB
* Command Line Editing:: Command Line Editing
* Using History Interactively:: Using History Interactively
* Formatting Documentation:: How to format and print GDB documentation
* Installing GDB:: Installing GDB
* Maintenance Commands:: Maintenance Commands
* Remote Protocol:: GDB Remote Serial Protocol
* Agent Expressions:: The GDB Agent Expression Mechanism
* Target Descriptions:: How targets can describe themselves to
GDB
* Operating System Information:: Getting additional information from
the operating system
* Trace File Format:: GDB trace file format
* Copying:: GNU General Public License says
how you can copy and share GDB
* GNU Free Documentation License:: The license for this documentation
* Index:: Index

File: gdb.info, Node: Summary, Next: Sample Session, Prev: Top, Up: Top
Summary of GDB
**************
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:
* Start your program, specifying anything that might affect its
behavior.
* Make your program stop on specified conditions.
* Examine what has happened, when your program has stopped.
* Change things in your program, so you can experiment with
correcting the effects of one bug and go on to learn about another.
You can use GDB to debug programs written in C and C++. For more
information, see *Note Supported Languages: Supported Languages. For
more information, see *Note C and C++: C.
Support for D is partial. For information on D, see *Note D: D.
Support for Modula-2 is partial. For information on Modula-2, see
*Note Modula-2: 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.
* Menu:
* Free Software:: Freely redistributable software
* Contributors:: Contributors to GDB

File: gdb.info, Node: Free Software, Next: Contributors, Up: Summary
Free Software
=============
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.
Free Software Needs Free Documentation
======================================
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'.

File: gdb.info, Node: Contributors, Prev: Free Software, Up: Summary
Contributors to GDB
===================
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 6.3, 6.2, 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.
Andrew Cagney completely re-designed and re-implemented GDB's
unwinder framework, this consisting of a fresh new design featuring
frame IDs, independent frame sniffers, and the sentinel frame. Mark
Kettenis implemented the DWARF 2 unwinder, Jeff Johnston the libunwind
unwinder, and Andrew Cagney the dummy, sentinel, tramp, and trad
unwinders. The architecture-specific changes, each involving a
complete rewrite of the architecture's frame code, were carried out by
Jim Blandy, Joel Brobecker, Kevin Buettner, Andrew Cagney, Stephane
Carrez, Randolph Chung, Orjan Friberg, Richard Henderson, Daniel
Jacobowitz, Jeff Johnston, Mark Kettenis, Theodore A. Roth, Kei
Sakamoto, Yoshinori Sato, Michael Snyder, Corinna Vinschen, and Ulrich
Weigand.
Christian Zankel, Ross Morley, Bob Wilson, and Maxim Grigoriev from
Tensilica, Inc. contributed support for Xtensa processors. Others who
have worked on the Xtensa port of GDB in the past include Steve Tjiang,
John Newlin, and Scott Foehner.
Michael Eager and staff of Xilinx, Inc., contributed support for the
Xilinx MicroBlaze architecture.

File: gdb.info, Node: Sample Session, Next: Invocation, Prev: Summary, Up: Top
1 A Sample GDB Session
**********************
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
Ctrl-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 7.2, 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:
Ctrl-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

File: gdb.info, Node: Invocation, Next: Commands, Prev: Sample Session, Up: Top
2 Getting In and Out of GDB
***************************
This chapter discusses how to start GDB, and how to get out of it. The
essentials are:
* type `gdb' to start GDB.
* type `quit' or `Ctrl-d' to exit.
* Menu:
* Invoking GDB:: How to start GDB
* Quitting GDB:: How to quit GDB
* Shell Commands:: How to use shell commands inside GDB
* Logging Output:: How to log GDB's output to a file

File: gdb.info, Node: Invoking GDB, Next: Quitting GDB, Up: Invocation
2.1 Invoking GDB
================
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 (*note 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.
* Menu:
* File Options:: Choosing files
* Mode Options:: Choosing modes
* Startup:: What GDB does during startup

File: gdb.info, Node: File Options, Next: Mode Options, Up: Invoking GDB
2.1.1 Choosing Files
--------------------
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 `./', e.g. `./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'
Read symbol table from file FILE.
`-exec FILE'
`-e FILE'
Use file FILE as the executable file to execute when appropriate,
and for examining pure data in conjunction with a core dump.
`-se FILE'
Read symbol table from file FILE and use it as the executable file.
`-core FILE'
`-c FILE'
Use file FILE as a core dump to examine.
`-pid NUMBER'
`-p NUMBER'
Connect to process ID NUMBER, as with the `attach' command.
`-command FILE'
`-x FILE'
Execute commands from file FILE. The contents of this file is
evaluated exactly as the `source' command would. *Note Command
files: Command Files.
`-eval-command COMMAND'
`-ex COMMAND'
Execute a single GDB command.
This option may be used multiple times to call multiple commands.
It may also be interleaved with `-command' as required.
gdb -ex 'target sim' -ex 'load' \
-x setbreakpoints -ex 'run' a.out
`-directory DIRECTORY'
`-d DIRECTORY'
Add DIRECTORY to the path to search for source and script files.
`-r'
`-readnow'
Read each symbol file's entire symbol table immediately, rather
than the default, which is to read it incrementally as it is
needed. This makes startup slower, but makes future operations
faster.

File: gdb.info, Node: Mode Options, Next: Startup, Prev: File Options, Up: Invoking GDB
2.1.2 Choosing Modes
--------------------
You can run GDB in various alternative modes--for example, in batch
mode or quiet mode.
`-nx'
`-n'
Do not execute commands found in any initialization files.
Normally, GDB executes the commands in these files after all the
command options and arguments have been processed. *Note Command
Files: Command Files.
`-quiet'
`-silent'
`-q'
"Quiet". Do not print the introductory and copyright messages.
These messages are also suppressed in batch mode.
`-batch'
Run in batch mode. Exit with status `0' after processing all the
command files specified with `-x' (and all commands from
initialization files, if not inhibited with `-n'). Exit with
nonzero status if an error occurs in executing the GDB commands in
the command files. Batch mode also disables pagination, sets
unlimited terminal width and height *note Screen Size::, and acts
as if `set confirm off' were in effect (*note Messages/Warnings::).
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.
`-batch-silent'
Run in batch mode exactly like `-batch', but totally silently. All
GDB output to `stdout' is prevented (`stderr' is unaffected).
This is much quieter than `-silent' and would be useless for an
interactive session.
This is particularly useful when using targets that give `Loading
section' messages, for example.
Note that targets that give their output via GDB, as opposed to
writing directly to `stdout', will also be made silent.
`-return-child-result'
The return code from GDB will be the return code from the child
process (the process being debugged), with the following
exceptions:
* GDB exits abnormally. E.g., due to an incorrect argument or
an internal error. In this case the exit code is the same as
it would have been without `-return-child-result'.
* The user quits with an explicit value. E.g., `quit 1'.
* The child process never runs, or is not allowed to terminate,
in which case the exit code will be -1.
This option is useful in conjunction with `-batch' or
`-batch-silent', when GDB is being used as a remote program loader
or simulator interface.
`-nowindows'
`-nw'
"No windows". If GDB comes with a graphical user interface (GUI)
built in, then this option tells GDB to only use the command-line
interface. If no GUI is available, this option has no effect.
`-windows'
`-w'
If GDB includes a GUI, then this option requires it to be used if
possible.
`-cd DIRECTORY'
Run GDB using DIRECTORY as its working directory, instead of the
current directory.
`-fullname'
`-f'
GNU Emacs sets this option when it runs GDB as a subprocess. It
tells GDB to output the full file name and line number in a
standard, recognizable fashion each time a stack frame is
displayed (which includes each time your program stops). This
recognizable format looks like two `\032' characters, followed by
the file name, line number and character position separated by
colons, and a newline. The Emacs-to-GDB interface program uses
the two `\032' characters as a signal to display the source code
for the frame.
`-epoch'
The Epoch Emacs-GDB interface sets this option when it runs GDB as
a subprocess. It tells GDB to modify its print routines so as to
allow Epoch to display values of expressions in a separate window.
`-annotate LEVEL'
This option sets the "annotation level" inside GDB. Its effect is
identical to using `set annotate LEVEL' (*note Annotations::).
The annotation LEVEL controls how much information GDB prints
together with its prompt, values of expressions, source lines, and
other types of output. Level 0 is the normal, level 1 is for use
when GDB is run as a subprocess of GNU Emacs, level 3 is the
maximum annotation suitable for programs that control GDB, and
level 2 has been deprecated.
The annotation mechanism has largely been superseded by GDB/MI
(*note GDB/MI::).
`--args'
Change interpretation of command line so that arguments following
the executable file are passed as command line arguments to the
inferior. This option stops option processing.
`-baud BPS'
`-b BPS'
Set the line speed (baud rate or bits per second) of any serial
interface used by GDB for remote debugging.
`-l TIMEOUT'
Set the timeout (in seconds) of any communication used by GDB for
remote debugging.
`-tty DEVICE'
`-t DEVICE'
Run using DEVICE for your program's standard input and output.
`-tui'
Activate the "Text User Interface" when starting. The Text User
Interface manages several text windows on the terminal, showing
source, assembly, registers and GDB command outputs (*note GDB
Text User Interface: TUI.). Alternatively, the Text User
Interface can be enabled by invoking the program `gdbtui'. Do not
use this option if you run GDB from Emacs (*note Using GDB under
GNU Emacs: Emacs.).
`-interpreter INTERP'
Use the interpreter INTERP for interface with the controlling
program or device. This option is meant to be set by programs
which communicate with GDB using it as a back end. *Note Command
Interpreters: Interpreters.
`--interpreter=mi' (or `--interpreter=mi2') causes GDB to use the
"GDB/MI interface" (*note The GDB/MI Interface: GDB/MI.) 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'
Open the executable and core files for both reading and writing.
This is equivalent to the `set write on' command inside GDB (*note
Patching::).
`-statistics'
This option causes GDB to print statistics about time and memory
usage after it completes each command and returns to the prompt.
`-version'
This option causes GDB to print its version number and no-warranty
blurb, and exit.

File: gdb.info, Node: Startup, Prev: Mode Options, Up: Invoking GDB
2.1.3 What GDB Does During Startup
----------------------------------
Here's the description of what GDB does during session startup:
1. Sets up the command interpreter as specified by the command line
(*note interpreter: Mode Options.).
2. Reads the system-wide "init file" (if `--with-system-gdbinit' was
used when building GDB; *note System-wide configuration and
settings: System-wide configuration.) and executes all the
commands in that file.
3. Reads the init file (if any) in your home directory(1) and
executes all the commands in that file.
4. Processes command line options and operands.
5. Reads and executes the commands from init file (if any) in the
current working directory. This is only done if the current
directory is different from your home directory. Thus, you can
have more than one init file, one generic in your home directory,
and another, specific to the program you are debugging, in the
directory where you invoke GDB.
6. Reads command files specified by the `-x' option. *Note Command
Files::, for more details about GDB command files.
7. Reads the command history recorded in the "history file". *Note
Command History::, for more details about the command history and
the files where GDB records it.
Init files use the same syntax as "command files" (*note 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 (*note Choosing
Modes: Mode Options.).
To display the list of init files loaded by gdb at startup, you can
use `gdb --help'.
The GDB init files are normally called `.gdbinit'. The DJGPP port
of GDB uses the name `gdb.ini', due to the limitations of file names
imposed by DOS filesystems. The Windows ports of GDB use the standard
name, but if they find a `gdb.ini' file, they warn you about that and
suggest to rename the file to the standard name.
---------- Footnotes ----------
(1) On DOS/Windows systems, the home directory is the one pointed to
by the `HOME' environment variable.

File: gdb.info, Node: Quitting GDB, Next: Shell Commands, Prev: Invoking GDB, Up: Invocation
2.2 Quitting GDB
================
`quit [EXPRESSION]'
`q'
To exit GDB, use the `quit' command (abbreviated `q'), or type an
end-of-file character (usually `Ctrl-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 `Ctrl-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 (*note Debugging an
Already-running Process: Attach.).

File: gdb.info, Node: Shell Commands, Next: Logging Output, Prev: Quitting GDB, Up: Invocation
2.3 Shell Commands
==================
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 STRING'
Invoke a standard shell to execute COMMAND STRING. If it exists,
the environment variable `SHELL' 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-ARGS'
Execute the `make' program with the specified arguments. This is
equivalent to `shell make MAKE-ARGS'.

File: gdb.info, Node: Logging Output, Prev: Shell Commands, Up: Invocation
2.4 Logging Output
==================
You may want to save the output of GDB commands to a file. There are
several commands to control GDB's logging.
`set logging on'
Enable logging.
`set logging off'
Disable logging.
`set logging file FILE'
Change the name of the current logfile. The default logfile is
`gdb.txt'.
`set logging overwrite [on|off]'
By default, GDB will append to the logfile. Set `overwrite' if
you want `set logging on' to overwrite the logfile instead.
`set logging redirect [on|off]'
By default, GDB output will go to both the terminal and the
logfile. Set `redirect' if you want output to go only to the log
file.
`show logging'
Show the current values of the logging settings.

File: gdb.info, Node: Commands, Next: Running, Prev: Invocation, Up: Top
3 GDB Commands
**************
You 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).
* Menu:
* Command Syntax:: How to give commands to GDB
* Completion:: Command completion
* Help:: How to ask GDB for help

File: gdb.info, Node: Command Syntax, Next: Completion, Up: Commands
3.1 Command Syntax
==================
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 *Note dont-repeat:
Define.
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' (*note Screen Size:
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 (*note Command Files:
Command Files.).
The `Ctrl-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.

File: gdb.info, Node: Completion, Next: Help, Prev: Command Syntax, Up: Commands
3.2 Command Completion
======================
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 *Note C++
Expressions: C Plus Plus Expressions. You can use the command `set
overload-resolution off' to disable overload resolution; see *Note GDB
Features for C++: Debugging C Plus Plus.
When completing in an expression which looks up a field in a
structure, GDB also tries(1) to limit completions to the field names
available in the type of the left-hand-side:
(gdb) p gdb_stdout.M-?
magic to_delete to_fputs to_put to_rewind
to_data to_flush to_isatty to_read to_write
This is because the `gdb_stdout' is a variable of the type `struct
ui_file' that is defined in GDB sources as follows:
struct ui_file
{
int *magic;
ui_file_flush_ftype *to_flush;
ui_file_write_ftype *to_write;
ui_file_fputs_ftype *to_fputs;
ui_file_read_ftype *to_read;
ui_file_delete_ftype *to_delete;
ui_file_isatty_ftype *to_isatty;
ui_file_rewind_ftype *to_rewind;
ui_file_put_ftype *to_put;
void *to_data;
}
---------- Footnotes ----------
(1) The completer can be confused by certain kinds of invalid
expressions. Also, it only examines the static type of the expression,
not the dynamic type.

File: gdb.info, Node: Help, Prev: Completion, Up: Commands
3.3 Getting Help
================
You can always ask GDB itself for information on its commands, using
the command `help'.
`help'
`h'
You can use `help' (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 CLASS'
Using one of the general help classes as an argument, you can get a
list of the individual commands in that class. For example, here
is the help display for the class `status':
(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 COMMAND'
With a command name as `help' argument, GDB displays a short
paragraph on how to use that command.
`apropos ARGS'
The `apropos' 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 ARGS'
The `complete ARGS' command lists all the possible completions for
the beginning of a command. Use ARGS to specify the beginning of
the command you want completed. For example:
complete 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.
*Note Index::.
`info'
This command (abbreviated `i') is for describing the state of your
program. For example, you can show the arguments passed to a
function with `info args', list the registers currently in use
with `info registers', or list the breakpoints you have set with
`info breakpoints'. You can get a complete list of the `info'
sub-commands with `help info'.
`set'
You can assign the result of an expression to an environment
variable with `set'. For example, you can set the GDB prompt to a
$-sign with `set prompt $'.
`show'
In contrast to `info', `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 version'
Show what version of GDB is running. You should include this
information in GDB bug-reports. If multiple versions of GDB are
in use at your site, you may need to determine which version of
GDB you are running; as GDB evolves, new commands are introduced,
and old ones may wither away. Also, many system vendors ship
variant versions of GDB, and there are variant versions of GDB in
GNU/Linux distributions as well. The version number is the same
as the one announced when you start GDB.
`show copying'
`info copying'
Display information about permission for copying GDB.
`show warranty'
`info warranty'
Display the GNU "NO WARRANTY" statement, or a warranty, if your
version of GDB comes with one.

File: gdb.info, Node: Running, Next: Stopping, Prev: Commands, Up: Top
4 Running Programs Under GDB
****************************
When 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.
* Menu:
* Compilation:: Compiling for debugging
* Starting:: Starting your program
* Arguments:: Your program's arguments
* Environment:: Your program's environment
* Working Directory:: Your program's working directory
* Input/Output:: Your program's input and output
* Attach:: Debugging an already-running process
* Kill Process:: Killing the child process
* Inferiors and Programs:: Debugging multiple inferiors and programs
* Threads:: Debugging programs with multiple threads
* Forks:: Debugging forks
* Checkpoint/Restart:: Setting a _bookmark_ to return to later

File: gdb.info, Node: Compilation, Next: Starting, Up: Running
4.1 Compiling for Debugging
===========================
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, some 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. For
more information, see *Note 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
(*note 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.

File: gdb.info, Node: Starting, Next: Arguments, Prev: Compilation, Up: Running
4.2 Starting your Program
=========================
`run'
`r'
Use the `run' command to start your program under GDB. You must
first specify the program name (except on VxWorks) with an
argument to GDB (*note Getting In and Out of GDB: Invocation.), or
by using the `file' or `exec-file' command (*note Commands to
Specify Files: 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 some environments without processes,
`run' jumps to the start of your program. Other targets, like
`remote', are always running. If you get an error message like this
one:
The "remote" target does not support "run".
Try "help target" or "continue".
then use `continue' to run your program. You may need `load' first
(*note load::).
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:
The _arguments._
Specify the arguments to give your program as the arguments of the
`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. *Note
Your Program's Arguments: Arguments.
The _environment._
Your program normally inherits its environment from GDB, but you
can use the GDB commands `set environment' and `unset environment'
to change parts of the environment that affect your program.
*Note Your Program's Environment: Environment.
The _working directory._
Your program inherits its working directory from GDB. You can set
the GDB working directory with the `cd' command in GDB. *Note
Your Program's Working Directory: Working Directory.
The _standard input and output._
Your program normally uses the same device for standard input and
standard output as GDB is using. You can redirect input and output
in the `run' command line, or you can use the `tty' command to set
a different device for your program. *Note Your Program's Input
and Output: Input/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. *Note Stopping and Continuing: Stopping, 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. *Note Examining Data: 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.
`start'
The name of the main procedure can vary from language to language.
With C or C++, the main procedure name is always `main', 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.
`set exec-wrapper WRAPPER'
`show exec-wrapper'
`unset exec-wrapper'
When `exec-wrapper' is set, the specified wrapper is used to
launch programs for debugging. GDB starts your program with a
shell command of the form `exec WRAPPER PROGRAM'. Quoting is
added to PROGRAM and its arguments, but not to WRAPPER, so you
should add quotes if appropriate for your shell. The wrapper runs
until it executes your program, and then GDB takes control.
You can use any program that eventually calls `execve' with its
arguments as a wrapper. Several standard Unix utilities do this,
e.g. `env' and `nohup'. Any Unix shell script ending with `exec
"$@"' will also work.
For example, you can use `env' to pass an environment variable to
the debugged program, without setting the variable in your shell's
environment:
(gdb) set exec-wrapper env 'LD_PRELOAD=libtest.so'
(gdb) run
This command is available when debugging locally on most targets,
excluding DJGPP, Cygwin, MS Windows, and QNX Neutrino.
`set disable-randomization'
`set disable-randomization on'
This option (enabled by default in GDB) will turn off the native
randomization of the virtual address space of the started program.
This option is useful for multiple debugging sessions to make the
execution better reproducible and memory addresses reusable across
debugging sessions.
This feature is implemented only on GNU/Linux. You can get the
same behavior using
(gdb) set exec-wrapper setarch `uname -m` -R
`set disable-randomization off'
Leave the behavior of the started executable unchanged. Some bugs
rear their ugly heads only when the program is loaded at certain
addresses. If your bug disappears when you run the program under
GDB, that might be because GDB by default disables the address
randomization on platforms, such as GNU/Linux, which do that for
stand-alone programs. Use `set disable-randomization off' to try
to reproduce such elusive bugs.
The virtual address space randomization is implemented only on
GNU/Linux. It protects the programs against some kinds of
security attacks. In these cases the attacker needs to know the
exact location of a concrete executable code. Randomizing its
location makes it impossible to inject jumps misusing a code at
its expected addresses.
Prelinking shared libraries provides a startup performance
advantage but it makes addresses in these libraries predictable
for privileged processes by having just unprivileged access at the
target system. Reading the shared library binary gives enough
information for assembling the malicious code misusing it. Still
even a prelinked shared library can get loaded at a new random
address just requiring the regular relocation process during the
startup. Shared libraries not already prelinked are always loaded
at a randomly chosen address.
Position independent executables (PIE) contain position
independent code similar to the shared libraries and therefore
such executables get loaded at a randomly chosen address upon
startup. PIE executables always load even already prelinked
shared libraries at a random address. You can build such
executable using `gcc -fPIE -pie'.
Heap (malloc storage), stack and custom mmap areas are always
placed randomly (as long as the randomization is enabled).
`show disable-randomization'
Show the current setting of the explicit disable of the native
randomization of the virtual address space of the started program.

File: gdb.info, Node: Arguments, Next: Environment, Prev: Starting, Up: Running
4.3 Your Program's Arguments
============================
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 args'
Specify the arguments to be used the next time your program is
run. If `set args' has no arguments, `run' executes your program
with no arguments. Once you have run your program with arguments,
using `set args' before the next `run' is the only way to run it
again without arguments.
`show args'
Show the arguments to give your program when it is started.

File: gdb.info, Node: Environment, Next: Working Directory, Prev: Arguments, Up: Running
4.4 Your Program's Environment
==============================
The "environment" consists of a set of environment variables and their
values. Environment variables conventionally record such things as
your user name, your home directory, your terminal type, and your search
path for programs to run. Usually you set up environment variables with
the shell and they are inherited by all the other programs you run.
When debugging, it can be useful to try running your program with a
modified environment without having to start GDB over again.
`path DIRECTORY'
Add DIRECTORY to the front of the `PATH' 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 paths'
Display the list of search paths for executables (the `PATH'
environment variable).
`show environment [VARNAME]'
Print the value of environment variable VARNAME to be given to
your program when it starts. If you do not supply VARNAME, print
the names and values of all environment variables to be given to
your program. You can abbreviate `environment' as `env'.
`set environment VARNAME [=VALUE]'
Set environment variable VARNAME to VALUE. The value changes for
your program only, not for GDB itself. VALUE may be any string;
the values of environment variables are just strings, and any
interpretation is supplied by your program itself. The VALUE
parameter is optional; if it is eliminated, the variable is set to
a null 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 VARNAME'
Remove variable VARNAME from the environment to be passed to your
program. This is different from `set env VARNAME ='; `unset
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'.

File: gdb.info, Node: Working Directory, Next: Input/Output, Prev: Environment, Up: Running
4.5 Your Program's Working Directory
====================================
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. *Note Commands to Specify
Files: Files.
`cd DIRECTORY'
Set the GDB working directory to DIRECTORY.
`pwd'
Print the GDB working directory.
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 (*note SVR4
Process Information::) to find out the current working directory of the
debuggee.

File: gdb.info, Node: Input/Output, Next: Attach, Prev: Working Directory, Up: Running
4.6 Your Program's Input and Output
===================================
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 terminal'
Displays information recorded by GDB about the terminal modes your
program is using.
You can redirect your program's input and/or output using shell
redirection with the `run' command. For example,
run > outfile
starts your program, diverting its output to the file `outfile'.
Another way to specify where your program should do input and output
is with the `tty' command. This command accepts a file name as
argument, and causes this file to be the default for future `run'
commands. It also resets the controlling terminal for the child
process, for future `run' commands. For example,
tty /dev/ttyb
directs that processes started with subsequent `run' commands default
to do input and output on the terminal `/dev/ttyb' and have that as
their controlling terminal.
An explicit redirection in `run' overrides the `tty' command's
effect on the input/output device, but not its effect on the controlling
terminal.
When you use the `tty' command or redirect input in the `run'
command, only the input _for your program_ is affected. The input for
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/ttyb'
Set the tty for the program being debugged to /dev/ttyb.
`show inferior-tty'
Show the current tty for the program being debugged.

File: gdb.info, Node: Attach, Next: Kill Process, Prev: Input/Output, Up: Running
4.7 Debugging an Already-running Process
========================================
`attach PROCESS-ID'
This command attaches to a running process--one that was started
outside GDB. (`info files' shows your active targets.) The
command takes as argument a process ID. The usual way to find out
the PROCESS-ID of a Unix process is with the `ps' utility, or with
the `jobs -l' shell command.
`attach' does not repeat if you press <RET> a second time after
executing the command.
To use `attach', your program must be running in an environment
which supports processes; for example, `attach' does not work for
programs on bare-board targets that lack an operating system. You must
also have permission to send the process a signal.
When you use `attach', the debugger finds the program running in the
process first by looking in the current working directory, then (if the
program is not found) by using the source file search path (*note
Specifying Source Directories: Source Path.). You can also use the
`file' command to load the program. *Note Commands to Specify Files:
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.
`detach'
When you have finished debugging the attached process, you can use
the `detach' 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 while you have an attached process, you detach that
process. If you use the `run' command, 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 (*note Optional Warnings and Messages:
Messages/Warnings.).

File: gdb.info, Node: Kill Process, Next: Inferiors and Programs, Prev: Attach, Up: Running
4.8 Killing the Child Process
=============================
`kill'
Kill the child process in which your program is running under GDB.
This 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).

File: gdb.info, Node: Inferiors and Programs, Next: Threads, Prev: Kill Process, Up: Running
4.9 Debugging Multiple Inferiors and Programs
=============================================
GDB lets you run and debug multiple programs in a single session. In
addition, GDB on some systems may let you run several programs
simultaneously (otherwise you have to exit from one before starting
another). In the most general case, you can have multiple threads of
execution in each of multiple processes, launched from multiple
executables.
GDB represents the state of each program execution with an object
called an "inferior". An inferior typically corresponds to a process,
but is more general and applies also to targets that do not have
processes. Inferiors may be created before a process runs, and may be
retained after a process exits. Inferiors have unique identifiers that
are different from process ids. Usually each inferior will also have
its own distinct address space, although some embedded targets may have
several inferiors running in different parts of a single address space.
Each inferior may in turn have multiple threads running in it.
To find out what inferiors exist at any moment, use `info inferiors':
`info inferiors'
Print a list of all inferiors currently being managed by GDB.
GDB displays for each inferior (in this order):
1. the inferior number assigned by GDB
2. the target system's inferior identifier
3. the name of the executable the inferior is running.
An asterisk `*' preceding the GDB inferior number indicates the
current inferior.
For example,
(gdb) info inferiors
Num Description Executable
2 process 2307 hello
* 1 process 3401 goodbye
To switch focus between inferiors, use the `inferior' command:
`inferior INFNO'
Make inferior number INFNO the current inferior. The argument
INFNO is the inferior number assigned by GDB, as shown in the
first field of the `info inferiors' display.
You can get multiple executables into a debugging session via the
`add-inferior' and `clone-inferior' commands. On some systems GDB can
add inferiors to the debug session automatically by following calls to
`fork' and `exec'. To remove inferiors from the debugging session use
the `remove-inferior' command.
`add-inferior [ -copies N ] [ -exec EXECUTABLE ]'
Adds N inferiors to be run using EXECUTABLE as the executable. N
defaults to 1. If no executable is specified, the inferiors
begins empty, with no program. You can still assign or change the
program assigned to the inferior at any time by using the `file'
command with the executable name as its argument.
`clone-inferior [ -copies N ] [ INFNO ]'
Adds N inferiors ready to execute the same program as inferior
INFNO. N defaults to 1. INFNO defaults to the number of the
current inferior. This is a convenient command when you want to
run another instance of the inferior you are debugging.
(gdb) info inferiors
Num Description Executable
* 1 process 29964 helloworld
(gdb) clone-inferior
Added inferior 2.
1 inferiors added.
(gdb) info inferiors
Num Description Executable
2 <null> helloworld
* 1 process 29964 helloworld
You can now simply switch focus to inferior 2 and run it.
`remove-inferior INFNO'
Removes the inferior INFNO. It is not possible to remove an
inferior that is running with this command. For those, use the
`kill' or `detach' command first.
To quit debugging one of the running inferiors that is not the
current inferior, you can either detach from it by using the
`detach inferior' command (allowing it to run independently), or kill it
using the `kill inferior' command:
`detach inferior INFNO'
Detach from the inferior identified by GDB inferior number INFNO.
Note that the inferior's entry still stays on the list of
inferiors shown by `info inferiors', but its Description will show
`<null>'.
`kill inferior INFNO'
Kill the inferior identified by GDB inferior number INFNO. Note
that the inferior's entry still stays on the list of inferiors
shown by `info inferiors', but its Description will show `<null>'.
After the successful completion of a command such as `detach',
`detach inferior', `kill' or `kill inferior', or after a normal process
exit, the inferior is still valid and listed with `info inferiors',
ready to be restarted.
To be notified when inferiors are started or exit under GDB's
control use `set print inferior-events':
`set print inferior-events'
`set print inferior-events on'
`set print inferior-events off'
The `set print inferior-events' command allows you to enable or
disable printing of messages when GDB notices that new inferiors
have started or that inferiors have exited or have been detached.
By default, these messages will not be printed.
`show print inferior-events'
Show whether messages will be printed when GDB detects that
inferiors have started, exited or have been detached.
Many commands will work the same with multiple programs as with a
single program: e.g., `print myglobal' will simply display the value of
`myglobal' in the current inferior.
Occasionaly, when debugging GDB itself, it may be useful to get more
info about the relationship of inferiors, programs, address spaces in a
debug session. You can do that with the `maint info program-spaces'
command.
`maint info program-spaces'
Print a list of all program spaces currently being managed by GDB.
GDB displays for each program space (in this order):
1. the program space number assigned by GDB
2. the name of the executable loaded into the program space,
with e.g., the `file' command.
An asterisk `*' preceding the GDB program space number indicates
the current program space.
In addition, below each program space line, GDB prints extra
information that isn't suitable to display in tabular form. For
example, the list of inferiors bound to the program space.
(gdb) maint info program-spaces
Id Executable
2 goodbye
Bound inferiors: ID 1 (process 21561)
* 1 hello
Here we can see that no inferior is running the program `hello',
while `process 21561' is running the program `goodbye'. On some
targets, it is possible that multiple inferiors are bound to the
same program space. The most common example is that of debugging
both the parent and child processes of a `vfork' call. For
example,
(gdb) maint info program-spaces
Id Executable
* 1 vfork-test
Bound inferiors: ID 2 (process 18050), ID 1 (process 18045)
Here, both inferior 2 and inferior 1 are running in the same
program space as a result of inferior 1 having executed a `vfork'
call.

File: gdb.info, Node: Threads, Next: Forks, Prev: Inferiors and Programs, Up: Running
4.10 Debugging Programs with Multiple Threads
=============================================
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:
* automatic notification of new threads
* `thread THREADNO', a command to switch among threads
* `info threads', a command to inquire about existing threads
* `thread apply [THREADNO] [ALL] ARGS', a command to apply a command
to a list of threads
* thread-specific breakpoints
* `set print thread-events', which controls printing of messages on
thread start and exit.
* `set libthread-db-search-path PATH', which lets the user specify
which `libthread_db' to use if the default choice isn't compatible
with the program.
_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 the `thread' command, 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 GNU/Linux, you
might see
[New Thread 46912507313328 (LWP 25582)]
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 threads'
Display a summary of all threads currently in your program. GDB
displays for each thread (in this order):
1. the thread number assigned by GDB
2. the target system's thread identifier (SYSTAG)
3. the current stack frame summary for that thread
An 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 threads'
Display a summary of all threads currently in your program. GDB
displays for each thread (in this order):
1. the thread number assigned by GDB
2. the target system's thread identifier (SYSTAG)
3. the current stack frame summary for that thread
An 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-threads'
Display info on Solaris user threads.
`thread THREADNO'
Make thread number THREADNO the current thread. The command
argument THREADNO is the internal GDB thread number, as shown in
the first field of the `info threads' display. GDB responds by
displaying the system identifier of the thread you selected, and
its current stack frame summary:
(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.
The debugger convenience variable `$_thread' contains the number
of the current thread. You may find this useful in writing
breakpoint conditional expressions, command scripts, and so forth.
See *Note Convenience Variables: Convenience Vars, for general
information on convenience variables.
`thread apply [THREADNO] [ALL] COMMAND'
The `thread apply' command allows you to apply the named COMMAND
to one or more threads. Specify the numbers of the threads that
you want affected with the command argument THREADNO. It can be a
single thread number, one of the numbers shown in the first field
of the `info threads' display; or it could be a range of thread
numbers, as in `2-4'. To apply a command to all threads, type
`thread apply all COMMAND'.
`set print thread-events'
`set print thread-events on'
`set print thread-events off'
The `set print thread-events' command allows you to enable or
disable printing of messages when GDB notices that new threads have
started or that threads have exited. By default, these messages
will be printed if detection of these events is supported by the
target. Note that these messages cannot be disabled on all
targets.
`show print thread-events'
Show whether messages will be printed when GDB detects that threads
have started and exited.
*Note Stopping and Starting Multi-thread Programs: Thread Stops, for
more information about how GDB behaves when you stop and start programs
with multiple threads.
*Note Setting Watchpoints: Set Watchpoints, for information about
watchpoints in programs with multiple threads.
`set libthread-db-search-path [PATH]'
If this variable is set, PATH is a colon-separated list of
directories GDB will use to search for `libthread_db'. If you
omit PATH, `libthread-db-search-path' will be reset to an empty
list.
On GNU/Linux and Solaris systems, GDB uses a "helper"
`libthread_db' library to obtain information about threads in the
inferior process. GDB will use `libthread-db-search-path' to find
`libthread_db'. If that fails, GDB will continue with default
system shared library directories, and finally the directory from
which `libpthread' was loaded in the inferior process.
For any `libthread_db' library GDB finds in above directories, GDB
attempts to initialize it with the current inferior process. If
this initialization fails (which could happen because of a version
mismatch between `libthread_db' and `libpthread'), GDB will unload
`libthread_db', and continue with the next directory. If none of
`libthread_db' libraries initialize successfully, GDB will issue a
warning and thread debugging will be disabled.
Setting `libthread-db-search-path' is currently implemented only
on some platforms.
`show libthread-db-search-path'
Display current libthread_db search path.

File: gdb.info, Node: Forks, Next: Checkpoint/Restart, Prev: Threads, Up: Running
4.11 Debugging Forks
====================
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 (*note
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 MODE'
Set the debugger response to a program call of `fork' or `vfork'.
A call to `fork' or `vfork' creates a new process. The MODE
argument can be:
`parent'
The original process is debugged after a fork. The child
process runs unimpeded. This is the default.
`child'
The new process is debugged after a fork. The parent process
runs unimpeded.
`show follow-fork-mode'
Display the current debugger response to a `fork' or `vfork' call.
On Linux, if you want to debug both the parent and child processes,
use the command `set detach-on-fork'.
`set detach-on-fork MODE'
Tells gdb whether to detach one of the processes after a fork, or
retain debugger control over them both.
`on'
The child process (or parent process, depending on the value
of `follow-fork-mode') will be detached and allowed to run
independently. This is the default.
`off'
Both processes will be held under the control of GDB. One
process (child or parent, depending on the value of
`follow-fork-mode') is debugged as usual, while the other is
held suspended.
`show detach-on-fork'
Show whether detach-on-fork mode is on/off.
If you choose to set `detach-on-fork' mode off, then GDB will retain
control of all forked processes (including nested forks). You can list
the forked processes under the control of GDB by using the
`info inferiors' command, and switch from one fork to another by using
the `inferior' command (*note Debugging Multiple Inferiors and
Programs: Inferiors and Programs.).
To quit debugging one of the forked processes, you can either detach
from it by using the `detach inferior' command (allowing it to run
independently), or kill it using the `kill inferior' command. *Note
Debugging Multiple Inferiors and Programs: Inferiors and Programs.
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'.
On some systems, 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. By default,
after an `exec' call executes, GDB discards the symbols of the previous
executable image. You can change this behaviour with the
`set follow-exec-mode' command.
`set follow-exec-mode MODE'
Set debugger response to a program call of `exec'. An `exec' call
replaces the program image of a process.
`follow-exec-mode' can be:
`new'
GDB creates a new inferior and rebinds the process to this
new inferior. The program the process was running before the
`exec' call can be restarted afterwards by restarting the
original inferior.
For example:
(gdb) info inferiors
(gdb) info inferior
Id Description Executable
* 1 <null> prog1
(gdb) run
process 12020 is executing new program: prog2
Program exited normally.
(gdb) info inferiors
Id Description Executable
* 2 <null> prog2
1 <null> prog1
`same'
GDB keeps the process bound to the same inferior. The new
executable image replaces the previous executable loaded in
the inferior. Restarting the inferior after the `exec' call,
with e.g., the `run' command, restarts the executable the
process was running after the `exec' call. This is the
default mode.
For example:
(gdb) info inferiors
Id Description Executable
* 1 <null> prog1
(gdb) run
process 12020 is executing new program: prog2
Program exited normally.
(gdb) info inferiors
Id Description Executable
* 1 <null> prog2
You can use the `catch' command to make GDB stop whenever a `fork',
`vfork', or `exec' call is made. *Note Setting Catchpoints: Set
Catchpoints.

File: gdb.info, Node: Checkpoint/Restart, Prev: Forks, Up: Running
4.12 Setting a _Bookmark_ to Return to Later
============================================
On certain operating systems(1), GDB is able to save a "snapshot" of a
program's state, called a "checkpoint", and come back to it later.
Returning to a checkpoint effectively undoes everything that has
happened in the program since the `checkpoint' was saved. This
includes changes in memory, registers, and even (within some limits)
system state. Effectively, it is like going back in time to the moment
when the checkpoint was saved.
Thus, if you're stepping thru a program and you think you're getting
close to the point where things go wrong, you can save a checkpoint.
Then, if you accidentally go too far and miss the critical statement,
instead of having to restart your program from the beginning, you can
just go back to the checkpoint and start again from there.
This can be especially useful if it takes a lot of time or steps to
reach the point where you think the bug occurs.
To use the `checkpoint'/`restart' method of debugging:
`checkpoint'
Save a snapshot of the debugged program's current execution state.
The `checkpoint' command takes no arguments, but each checkpoint
is assigned a small integer id, similar to a breakpoint id.
`info checkpoints'
List the checkpoints that have been saved in the current debugging
session. For each checkpoint, the following information will be
listed:
`Checkpoint ID'
`Process ID'
`Code Address'
`Source line, or label'
`restart CHECKPOINT-ID'
Restore the program state that was saved as checkpoint number
CHECKPOINT-ID. All program variables, registers, stack frames
etc. will be returned to the values that they had when the
checkpoint was saved. In essence, gdb will "wind back the clock"
to the point in time when the checkpoint was saved.
Note that breakpoints, GDB variables, command history etc. are
not affected by restoring a checkpoint. In general, a checkpoint
only restores things that reside in the program being debugged,
not in the debugger.
`delete checkpoint CHECKPOINT-ID'
Delete the previously-saved checkpoint identified by CHECKPOINT-ID.
Returning to a previously saved checkpoint will restore the user
state of the program being debugged, plus a significant subset of the
system (OS) state, including file pointers. It won't "un-write" data
from a file, but it will rewind the file pointer to the previous
location, so that the previously written data can be overwritten. For
files opened in read mode, the pointer will also be restored so that the
previously read data can be read again.
Of course, characters that have been sent to a printer (or other
external device) cannot be "snatched back", and characters received
from eg. a serial device can be removed from internal program buffers,
but they cannot be "pushed back" into the serial pipeline, ready to be
received again. Similarly, the actual contents of files that have been
changed cannot be restored (at this time).
However, within those constraints, you actually can "rewind" your
program to a previously saved point in time, and begin debugging it
again -- and you can change the course of events so as to debug a
different execution path this time.
Finally, there is one bit of internal program state that will be
different when you return to a checkpoint -- the program's process id.
Each checkpoint will have a unique process id (or PID), and each will
be different from the program's original PID. If your program has
saved a local copy of its process id, this could potentially pose a
problem.
4.12.1 A Non-obvious Benefit of Using Checkpoints
-------------------------------------------------
On some systems such as GNU/Linux, address space randomization is
performed on new processes for security reasons. This makes it
difficult or impossible to set a breakpoint, or watchpoint, on an
absolute address if you have to restart the program, since the absolute
location of a symbol will change from one execution to the next.
A checkpoint, however, is an _identical_ copy of a process.
Therefore if you create a checkpoint at (eg.) the start of main, and
simply return to that checkpoint instead of restarting the process, you
can avoid the effects of address randomization and your symbols will
all stay in the same place.
---------- Footnotes ----------
(1) Currently, only GNU/Linux.

File: gdb.info, Node: Stopping, Next: Reverse Execution, Prev: Running, Up: Top
5 Stopping and Continuing
*************************
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 program'
Display information about the status of your program: whether it is
running or not, what process it is, and why it stopped.
* Menu:
* Breakpoints:: Breakpoints, watchpoints, and catchpoints
* Continuing and Stepping:: Resuming execution
* Signals:: Signals
* Thread Stops:: Stopping and starting multi-thread programs

File: gdb.info, Node: Breakpoints, Next: Continuing and Stepping, Up: Stopping
5.1 Breakpoints, Watchpoints, and Catchpoints
=============================================
A "breakpoint" makes your program stop whenever a certain point in the
program is reached. For each breakpoint, you can add conditions to
control in finer detail whether your program stops. You can set
breakpoints with the `break' command and its variants (*note Setting
Breakpoints: Set Breaks.), 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. The expression may be a value of a
variable, or it could involve values of one or more variables combined
by operators, such as `a + b'. This is sometimes called "data
breakpoints". You must use a different command to set watchpoints
(*note Setting Watchpoints: Set 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. *Note Automatic
Display: Auto 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 (*note Setting Catchpoints: Set
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 *Note Signals: 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 breakpoints
in that range are operated on.
* Menu:
* Set Breaks:: Setting breakpoints
* Set Watchpoints:: Setting watchpoints
* Set Catchpoints:: Setting catchpoints
* Delete Breaks:: Deleting breakpoints
* Disabling:: Disabling breakpoints
* Conditions:: Break conditions
* Break Commands:: Breakpoint command lists
* Save Breakpoints:: How to save breakpoints in a file
* Error in Breakpoints:: ``Cannot insert breakpoints''
* Breakpoint-related Warnings:: ``Breakpoint address adjusted...''

File: gdb.info, Node: Set Breaks, Next: Set Watchpoints, Up: Breakpoints
5.1.1 Setting Breakpoints
-------------------------
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 *Note Convenience Variables:
Convenience Vars, for a discussion of what you can do with convenience
variables.
`break LOCATION'
Set a breakpoint at the given LOCATION, which can specify a
function name, a line number, or an address of an instruction.
(*Note Specify Location::, for a list of all the possible ways to
specify a LOCATION.) The breakpoint will stop your program just
before it executes any of the code in the specified LOCATION.
When using source languages that permit overloading of symbols,
such as C++, a function name may refer to more than one possible
place to break. *Note Ambiguous Expressions: Ambiguous
Expressions, for a discussion of that situation.
It is also possible to insert a breakpoint that will stop the
program only if a specific thread (*note Thread-Specific
Breakpoints::) or a specific task (*note Ada Tasks::) hits that
breakpoint.
`break'
When called without any arguments, `break' sets a breakpoint at
the next instruction to be executed in the selected stack frame
(*note Examining the Stack: 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 COND'
Set a breakpoint with condition COND; evaluate the expression COND
each time the breakpoint is reached, and stop only if the value is
nonzero--that is, if COND evaluates as true. `...' stands for one
of the possible arguments described above (or no argument)
specifying where to break. *Note Break Conditions: Conditions,
for more information on breakpoint conditions.
`tbreak ARGS'
Set a breakpoint enabled only for one stop. ARGS are the same as
for the `break' command, and the breakpoint is set in the same
way, but the breakpoint is automatically deleted after the first
time your program stops there. *Note Disabling Breakpoints:
Disabling.
`hbreak ARGS'
Set a hardware-assisted breakpoint. ARGS are the same as for the
`break' command and the breakpoint is set in the same way, but the
breakpoint requires hardware support and some target hardware may
not have this support. The main purpose of this is EPROM/ROM code
debugging, so you can set a breakpoint at an instruction without
changing the instruction. This can be used with the new
trap-generation provided by SPARClite DSU and 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 (*note
Disabling Breakpoints: Disabling.). *Note Break Conditions:
Conditions. For remote targets, you can restrict the number of
hardware breakpoints GDB will use, see *Note set remote
hardware-breakpoint-limit::.
`thbreak ARGS'
Set a hardware-assisted breakpoint enabled only for one stop. ARGS
are the same as for the `hbreak' command and the breakpoint is set
in the same way. However, like the `tbreak' command, the
breakpoint is automatically deleted after the first time your
program stops there. Also, like the `hbreak' command, the
breakpoint requires hardware support and some target hardware may
not have this support. *Note Disabling Breakpoints: Disabling.
See also *Note Break Conditions: Conditions.
`rbreak REGEX'
Set breakpoints on all functions matching the regular expression
REGEX. This command sets an unconditional breakpoint on all
matches, printing a list of all breakpoints it set. Once these
breakpoints are set, they are treated just like the breakpoints
set with the `break' command. You can delete them, disable them,
or make them conditional the same way as any other breakpoint.
The syntax of the regular expression is the standard one used with
tools like `grep'. Note that this is different from the syntax
used by shells, so for instance `foo*' matches all functions that
include an `fo' followed by zero or more `o's. 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 .
`rbreak FILE:REGEX'
If `rbreak' is called with a filename qualification, it limits the
search for functions matching the given regular expression to the
specified FILE. This can be used, for example, to set breakpoints
on every function in a given file:
(gdb) rbreak file.c:.
The colon separating the filename qualifier from the regex may
optionally be surrounded by spaces.
`info breakpoints [N]'
`info break [N]'
Print a table of all breakpoints, watchpoints, and catchpoints set
and not deleted. Optional argument N means print information only
about the specified breakpoint (or watchpoint or catchpoint). For
each breakpoint, following columns are printed:
_Breakpoint Numbers_
_Type_
Breakpoint, watchpoint, or catchpoint.
_Disposition_
Whether the breakpoint is marked to be disabled or deleted
when hit.
_Enabled or Disabled_
Enabled breakpoints are marked with `y'. `n' marks
breakpoints that are not enabled.
_Address_
Where the breakpoint is in your program, as a memory address.
For a pending breakpoint whose address is not yet known,
this field will contain `<PENDING>'. Such breakpoint won't
fire until a shared library that has the symbol or line
referred by breakpoint is loaded. See below for details. A
breakpoint with several locations will have `<MULTIPLE>' in
this field--see below for details.
_What_
Where the breakpoint is in the source for your program, as a
file and line number. For a pending breakpoint, the original
string passed to the breakpoint command will be listed as it
cannot be resolved until the appropriate shared library is
loaded in the future.
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 (*note Examining Memory: 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 (*note Break
Conditions: Conditions.).
It is possible that a breakpoint corresponds to several locations in
your program. Examples of this situation are:
* For a C++ constructor, the GCC compiler generates several
instances of the function body, used in different cases.
* For a C++ template function, a given line in the function can
correspond to any number of instantiations.
* For an inlined function, a given source line can correspond to
several places where that function is inlined.
In all those cases, GDB will insert a breakpoint at all the relevant
locations(1).
A breakpoint with multiple locations is displayed in the breakpoint
table using several rows--one header row, followed by one row for each
breakpoint location. The header row has `<MULTIPLE>' in the address
column. The rows for individual locations contain the actual addresses
for locations, and show the functions to which those locations belong.
The number column for a location is of the form
BREAKPOINT-NUMBER.LOCATION-NUMBER.
For example:
Num Type Disp Enb Address What
1 breakpoint keep y <MULTIPLE>
stop only if i==1
breakpoint already hit 1 time
1.1 y 0x080486a2 in void foo<int>() at t.cc:8
1.2 y 0x080486ca in void foo<double>() at t.cc:8
Each location can be individually enabled or disabled by passing
BREAKPOINT-NUMBER.LOCATION-NUMBER as argument to the `enable' and
`disable' commands. Note that you cannot delete the individual
locations from the list, you can only delete the entire list of
locations that belong to their parent breakpoint (with the `delete NUM'
command, where NUM is the number of the parent breakpoint, 1 in the
above example). Disabling or enabling the parent breakpoint (*note
Disabling::) affects all of the locations that belong to that
breakpoint.
It's quite common to have a breakpoint inside a shared library.
Shared libraries can be loaded and unloaded explicitly, and possibly
repeatedly, as the program is executed. To support this use case, GDB
updates breakpoint locations whenever any shared library is loaded or
unloaded. Typically, you would set a breakpoint in a shared library at
the beginning of your debugging session, when the library is not
loaded, and when the symbols from the library are not available. When
you try to set breakpoint, GDB will ask you if you want to set a so
called "pending breakpoint"--breakpoint whose address is not yet
resolved.
After the program is run, whenever a new shared library is loaded,
GDB reevaluates all the breakpoints. When a newly loaded shared
library contains the symbol or line referred to by some pending
breakpoint, that breakpoint is resolved and becomes an ordinary
breakpoint. When a library is unloaded, all breakpoints that refer to
its symbols or source lines become pending again.
This logic works for breakpoints with multiple locations, too. For
example, if you have a breakpoint in a C++ template function, and a
newly loaded shared library has an instantiation of that template, a
new location is added to the list of locations for the breakpoint.
Except for having unresolved address, pending breakpoints do not
differ from regular breakpoints. You can set conditions or commands,
enable and disable them and perform other breakpoint operations.
GDB provides some additional commands for controlling what happens
when the `break' command cannot resolve breakpoint address
specification to an address:
`set breakpoint pending auto'
This is the default behavior. When GDB cannot find the breakpoint
location, it queries you whether a pending breakpoint should be
created.
`set breakpoint pending on'
This indicates that an unrecognized breakpoint location should
automatically result in a pending breakpoint being created.
`set breakpoint pending off'
This indicates that pending breakpoints are not to be created. Any
unrecognized breakpoint location results in an error. This
setting does not affect any pending breakpoints previously created.
`show breakpoint pending'
Show the current behavior setting for creating pending breakpoints.
The settings above only affect the `break' command and its variants.
Once breakpoint is set, it will be automatically updated as shared
libraries are loaded and unloaded.
For some targets, GDB can automatically decide if hardware or
software breakpoints should be used, depending on whether the
breakpoint address is read-only or read-write. This applies to
breakpoints set with the `break' command as well as to internal
breakpoints set by commands like `next' and `finish'. For breakpoints
set with `hbreak', GDB will always use hardware breakpoints.
You can control this automatic behaviour with the following
commands::
`set breakpoint auto-hw on'
This is the default behavior. When GDB sets a breakpoint, it will
try to use the target memory map to decide if software or hardware
breakpoint must be used.
`set breakpoint auto-hw off'
This indicates GDB should not automatically select breakpoint
type. If the target provides a memory map, GDB will warn when
trying to set software breakpoint at a read-only address.
GDB normally implements breakpoints by replacing the program code at
the breakpoint address with a special instruction, which, when
executed, given control to the debugger. By default, the program code
is so modified only when the program is resumed. As soon as the
program stops, GDB restores the original instructions. This behaviour
guards against leaving breakpoints inserted in the target should gdb
abrubptly disconnect. However, with slow remote targets, inserting and
removing breakpoint can reduce the performance. This behavior can be
controlled with the following commands::
`set breakpoint always-inserted off'
All breakpoints, including newly added by the user, are inserted in
the target only when the target is resumed. All breakpoints are
removed from the target when it stops.
`set breakpoint always-inserted on'
Causes all breakpoints to be inserted in the target at all times.
If the user adds a new breakpoint, or changes an existing
breakpoint, the breakpoints in the target are updated immediately.
A breakpoint is removed from the target only when breakpoint
itself is removed.
`set breakpoint always-inserted auto'
This is the default mode. If GDB is controlling the inferior in
non-stop mode (*note Non-Stop Mode::), gdb behaves as if
`breakpoint always-inserted' mode is on. If GDB is controlling
the inferior in all-stop mode, GDB behaves as if `breakpoint
always-inserted' mode is off.
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'
(*note maint info breakpoints::).
---------- Footnotes ----------
(1) As of this writing, multiple-location breakpoints work only if
there's line number information for all the locations. This means that
they will generally not work in system libraries, unless you have debug
info with line numbers for them.

File: gdb.info, Node: Set Watchpoints, Next: Set Catchpoints, Prev: Set Breaks, Up: Breakpoints
5.1.2 Setting Watchpoints
-------------------------
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. (This is sometimes called a "data breakpoint".) The
expression may be as simple as the value of a single variable, or as
complex as many variables combined by operators. Examples include:
* A reference to the value of a single variable.
* An address cast to an appropriate data type. For example, `*(int
*)0x12345678' will watch a 4-byte region at the specified address
(assuming an `int' occupies 4 bytes).
* An arbitrarily complex expression, such as `a*b + c/d'. The
expression can use any operators valid in the program's native
language (*note Languages::).
You can set a watchpoint on an expression even if the expression can
not be evaluated yet. For instance, you can set a watchpoint on
`*global_ptr' before `global_ptr' is initialized. GDB will stop when
your program sets `global_ptr' and the expression produces a valid
value. If the expression becomes valid in some other way than changing
a variable (e.g. if the memory pointed to by `*global_ptr' becomes
readable as the result of a `malloc' call), GDB may not stop until the
next time the expression changes.
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, PowerPC, 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 EXPR [thread THREADNUM]'
Set a watchpoint for an expression. GDB will break when the
expression EXPR is written into by the program and its value
changes. The simplest (and the most popular) use of this command
is to watch the value of a single variable:
(gdb) watch foo
If the command includes a `[thread THREADNUM]' clause, GDB breaks
only when the thread identified by THREADNUM changes the value of
EXPR. If any other threads change the value of EXPR, GDB will not
break. Note that watchpoints restricted to a single thread in
this way only work with Hardware Watchpoints.
`rwatch EXPR [thread THREADNUM]'
Set a watchpoint that will break when the value of EXPR is read by
the program.
`awatch EXPR [thread THREADNUM]'
Set a watchpoint that will break when EXPR is either read from or
written into by the program.
`info watchpoints'
This command prints a list of watchpoints, using the same format as
`info break' (*note Set Breaks::).
If you watch for a change in a numerically entered address you need
to dereference it, as the address itself is just a constant number
which will never change. GDB refuses to create a watchpoint that
watches a never-changing value:
(gdb) watch 0x600850
Cannot watch constant value 0x600850.
(gdb) watch *(int *) 0x600850
Watchpoint 1: *(int *) 6293584
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 expression values.)
`set can-use-hw-watchpoints'
Set whether or not to use hardware watchpoints.
`show can-use-hw-watchpoints'
Show the current mode of using hardware watchpoints.
For remote targets, you can restrict the number of hardware
watchpoints GDB will use, see *Note 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.
Watching complex expressions that reference many variables can also
exhaust the resources available for hardware-assisted watchpoints.
That's because GDB needs to watch every variable in the expression with
separately allocated resources.
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.
In multi-threaded programs, watchpoints will detect changes to the
watched expression from every thread.
_Warning:_ In multi-threaded 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.)
*Note set remote hardware-watchpoint-limit::.

File: gdb.info, Node: Set Catchpoints, Next: Delete Breaks, Prev: Set Watchpoints, Up: Breakpoints
5.1.3 Setting Catchpoints
-------------------------
You can use "catchpoints" to cause the debugger to stop for certain
kinds of program events, such as C++ exceptions or the loading of a
shared library. Use the `catch' command to set a catchpoint.
`catch EVENT'
Stop when EVENT occurs. EVENT can be any of the following:
`throw'
The throwing of a C++ exception.
`catch'
The catching of a C++ exception.
`exception'
An Ada exception being raised. If an exception name is
specified at the end of the command (eg `catch exception
Program_Error'), the debugger will stop only when this
specific exception is raised. Otherwise, the debugger stops
execution when any Ada exception is raised.
When inserting an exception catchpoint on a user-defined
exception whose name is identical to one of the exceptions
defined by the language, the fully qualified name must be
used as the exception name. Otherwise, GDB will assume that
it should stop on the pre-defined exception rather than the
user-defined one. For instance, assuming an exception called
`Constraint_Error' is defined in package `Pck', then the
command to use to catch such exceptions is `catch exception
Pck.Constraint_Error'.
`exception unhandled'
An exception that was raised but is not handled by the
program.
`assert'
A failed Ada assertion.
`exec'
A call to `exec'. This is currently only available for HP-UX
and GNU/Linux.
`syscall'
`syscall [NAME | NUMBER] ...'
A call to or return from a system call, a.k.a. "syscall". A
syscall is a mechanism for application programs to request a
service from the operating system (OS) or one of the OS
system services. GDB can catch some or all of the syscalls
issued by the debuggee, and show the related information for
each syscall. If no argument is specified, calls to and
returns from all system calls will be caught.
NAME can be any system call name that is valid for the
underlying OS. Just what syscalls are valid depends on the
OS. On GNU and Unix systems, you can find the full list of
valid syscall names on `/usr/include/asm/unistd.h'.
Normally, GDB knows in advance which syscalls are valid for
each OS, so you can use the GDB command-line completion
facilities (*note command completion: Completion.) to list the
available choices.
You may also specify the system call numerically. A syscall's
number is the value passed to the OS's syscall dispatcher to
identify the requested service. When you specify the syscall
by its name, GDB uses its database of syscalls to convert the
name into the corresponding numeric code, but using the
number directly may be useful if GDB's database does not have
the complete list of syscalls on your system (e.g., because
GDB lags behind the OS upgrades).
The example below illustrates how this command works if you
don't provide arguments to it:
(gdb) catch syscall
Catchpoint 1 (syscall)
(gdb) r
Starting program: /tmp/catch-syscall
Catchpoint 1 (call to syscall 'close'), \
0xffffe424 in __kernel_vsyscall ()
(gdb) c
Continuing.
Catchpoint 1 (returned from syscall 'close'), \
0xffffe424 in __kernel_vsyscall ()
(gdb)
Here is an example of catching a system call by name:
(gdb) catch syscall chroot
Catchpoint 1 (syscall 'chroot' [61])
(gdb) r
Starting program: /tmp/catch-syscall
Catchpoint 1 (call to syscall 'chroot'), \
0xffffe424 in __kernel_vsyscall ()
(gdb) c
Continuing.
Catchpoint 1 (returned from syscall 'chroot'), \
0xffffe424 in __kernel_vsyscall ()
(gdb)
An example of specifying a system call numerically. In the
case below, the syscall number has a corresponding entry in
the XML file, so GDB finds its name and prints it:
(gdb) catch syscall 252
Catchpoint 1 (syscall(s) 'exit_group')
(gdb) r
Starting program: /tmp/catch-syscall
Catchpoint 1 (call to syscall 'exit_group'), \
0xffffe424 in __kernel_vsyscall ()
(gdb) c
Continuing.
Program exited normally.
(gdb)
However, there can be situations when there is no
corresponding name in XML file for that syscall number. In
this case, GDB prints a warning message saying that it was
not able to find the syscall name, but the catchpoint will be
set anyway. See the example below:
(gdb) catch syscall 764
warning: The number '764' does not represent a known syscall.
Catchpoint 2 (syscall 764)
(gdb)
If you configure GDB using the `--without-expat' option, it
will not be able to display syscall names. Also, if your
architecture does not have an XML file describing its system
calls, you will not be able to see the syscall names. It is
important to notice that these two features are used for
accessing the syscall name database. In either case, you
will see a warning like this:
(gdb) catch syscall
warning: Could not open "syscalls/i386-linux.xml"
warning: Could not load the syscall XML file 'syscalls/i386-linux.xml'.
GDB will not be able to display syscall names.
Catchpoint 1 (syscall)
(gdb)
Of course, the file name will change depending on your
architecture and system.
Still using the example above, you can also try to catch a
syscall by its number. In this case, you would see something
like:
(gdb) catch syscall 252
Catchpoint 1 (syscall(s) 252)
Again, in this case GDB would not be able to display
syscall's names.
`fork'
A call to `fork'. This is currently only available for HP-UX
and GNU/Linux.
`vfork'
A call to `vfork'. This is currently only available for HP-UX
and GNU/Linux.
`tcatch EVENT'
Set a catchpoint that is enabled only for one stop. The
catchpoint is automatically deleted after the first time the event
is caught.
Use the `info break' command to list the current catchpoints.
There are currently some limitations to C++ exception handling
(`catch throw' and `catch catch') in GDB:
* If you call a function interactively, GDB normally returns control
to you when the function has finished executing. If the call
raises an exception, however, the call may bypass the mechanism
that returns control to you and cause your program either to abort
or to simply continue running until it hits a breakpoint, catches
a signal that GDB is listening for, or exits. This is the case
even if you set a catchpoint for the exception; catchpoints on
exceptions are disabled within interactive calls.
* You cannot raise an exception interactively.
* You cannot install an exception handler interactively.
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' (*note
Breakpoints; Watchpoints; and Exceptions: Breakpoints.).
With a conditional breakpoint (*note Break Conditions: 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.

File: gdb.info, Node: Delete Breaks, Next: Disabling, Prev: Set Catchpoints, Up: Breakpoints
5.1.4 Deleting Breakpoints
--------------------------
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.
`clear'
Delete any breakpoints at the next instruction to be executed in
the selected stack frame (*note Selecting a Frame: Selection.).
When the innermost frame is selected, this is a good way to delete
a breakpoint where your program just stopped.
`clear LOCATION'
Delete any breakpoints set at the specified LOCATION. *Note
Specify Location::, for the various forms of LOCATION; the most
useful ones are listed below:
`clear FUNCTION'
`clear FILENAME:FUNCTION'
Delete any breakpoints set at entry to the named FUNCTION.
`clear LINENUM'
`clear FILENAME:LINENUM'
Delete any breakpoints set at or within the code of the
specified LINENUM of the specified FILENAME.
`delete [breakpoints] [RANGE...]'
Delete the breakpoints, watchpoints, or catchpoints of the
breakpoint ranges specified as arguments. If no argument is
specified, delete all breakpoints (GDB asks confirmation, unless
you have `set confirm off'). You can abbreviate this command as
`d'.

File: gdb.info, Node: Disabling, Next: Conditions, Prev: Delete Breaks, Up: Breakpoints
5.1.5 Disabling Breakpoints
---------------------------
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' to print a list of
all breakpoints, watchpoints, and catchpoints if you do not know which
numbers to use.
Disabling and enabling a breakpoint that has multiple locations
affects all of its locations.
A breakpoint, watchpoint, or catchpoint can have any of four
different states of enablement:
* Enabled. The breakpoint stops your program. A breakpoint set
with the `break' command starts out in this state.
* Disabled. The breakpoint has no effect on your program.
* Enabled once. The breakpoint stops your program, but then becomes
disabled.
* Enabled for deletion. The breakpoint stops your program, but
immediately after it does so it is deleted permanently. A
breakpoint set with the `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 the specified breakpoints--or all breakpoints, if none are
listed. A disabled breakpoint has no effect but is not forgotten.
All options such as ignore-counts, conditions and commands are
remembered in case the breakpoint is enabled again later. You may
abbreviate `disable' as `dis'.
`enable [breakpoints] [RANGE...]'
Enable the specified breakpoints (or all defined breakpoints).
They become effective once again in stopping your program.
`enable [breakpoints] once RANGE...'
Enable the specified breakpoints temporarily. GDB disables any of
these breakpoints immediately after stopping your program.
`enable [breakpoints] delete RANGE...'
Enable the specified breakpoints to work once, then die. GDB
deletes any of these breakpoints as soon as your program stops
there. Breakpoints set by the `tbreak' command start out in this
state.
Except for a breakpoint set with `tbreak' (*note Setting
Breakpoints: Set Breaks.), 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 *Note Continuing and Stepping: Continuing and
Stepping.)

File: gdb.info, Node: Conditions, Next: Break Commands, Prev: Disabling, Up: Breakpoints
5.1.6 Break Conditions
----------------------
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
(*note Expressions: 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
(*note Breakpoint Command Lists: Break Commands.).
Break conditions can be specified when a breakpoint is set, by using
`if' in the arguments to the `break' command. *Note Setting
Breakpoints: Set Breaks. They can also be changed at any time with the
`condition' command.
You can also use the `if' keyword with the `watch' command. The
`catch' command does not recognize the `if' keyword; `condition' is the
only way to impose a further condition on a catchpoint.
`condition BNUM EXPRESSION'
Specify EXPRESSION as the break condition for breakpoint,
watchpoint, or catchpoint number BNUM. After you set a condition,
breakpoint BNUM stops your program only if the value of EXPRESSION
is true (nonzero, in C). When you use `condition', 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. *Note
Expressions: Expressions.
`condition BNUM'
Remove the condition from breakpoint number BNUM. It becomes an
ordinary unconditional breakpoint.
A special case of a breakpoint condition is to stop only when the
breakpoint has been reached a certain number of times. This is so
useful that there is a special way to do it, using the "ignore count"
of the breakpoint. Every breakpoint has an ignore count, which is an
integer. Most of the time, the ignore count is zero, and therefore has
no effect. But if your program reaches a breakpoint whose ignore count
is positive, then instead of stopping, it just decrements the ignore
count by one and continues. As a result, if the ignore count value is
N, the breakpoint does not stop the next N times your program reaches
it.
`ignore BNUM COUNT'
Set the ignore count of breakpoint number BNUM to COUNT. The next
COUNT times the breakpoint is reached, your program's execution
does not stop; other than to decrement the ignore count, GDB takes
no action.
To 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'. *Note
Continuing and Stepping: 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. *Note Convenience Variables:
Convenience Vars.
Ignore counts apply to breakpoints, watchpoints, and catchpoints.

File: gdb.info, Node: Break Commands, Next: Save Breakpoints, Prev: Conditions, Up: Breakpoints
5.1.7 Breakpoint Command Lists
------------------------------
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 [RANGE...]'
`... COMMAND-LIST ...'
`end'
Specify a list of commands for the given breakpoints. The commands
themselves appear on the following lines. Type a line containing
just `end' 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 argument, `commands' refers to the last breakpoint,
watchpoint, or catchpoint set (not to the breakpoint most recently
encountered). If the most recent breakpoints were set with a
single command, then the `commands' will apply to all the
breakpoints set by that command. This applies to breakpoints set
by `rbreak', and also applies when a single `break' command
creates multiple breakpoints (*note Ambiguous Expressions:
Ambiguous Expressions.).
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. *Note Commands for Controlled Output: 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

File: gdb.info, Node: Save Breakpoints, Next: Error in Breakpoints, Prev: Break Commands, Up: Breakpoints
5.1.8 How to save breakpoints to a file
---------------------------------------
To save breakpoint definitions to a file use the `save breakpoints'
command.
`save breakpoints [FILENAME]'
This command saves all current breakpoint definitions together with
their commands and ignore counts, into a file `FILENAME' suitable
for use in a later debugging session. This includes all types of
breakpoints (breakpoints, watchpoints, catchpoints, tracepoints).
To read the saved breakpoint definitions, use the `source' command
(*note Command Files::). Note that watchpoints with expressions
involving local variables may fail to be recreated because it may
not be possible to access the context where the watchpoint is
valid anymore. Because the saved breakpoint definitions are
simply a sequence of GDB commands that recreate the breakpoints,
you can edit the file in your favorite editing program, and remove
the breakpoint definitions you're not interested in, or that can
no longer be recreated.

File: gdb.info, Node: Error in Breakpoints, Next: Breakpoint-related Warnings, Prev: Save Breakpoints, Up: Breakpoints
5.1.9 "Cannot insert breakpoints"
---------------------------------
If you request too many active hardware-assisted breakpoints and
watchpoints, you will see this error message:
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.

File: gdb.info, Node: Breakpoint-related Warnings, Prev: Error in Breakpoints, Up: Breakpoints
5.1.10 "Breakpoint address adjusted..."
---------------------------------------
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.

File: gdb.info, Node: Continuing and Stepping, Next: Signals, Prev: Breakpoints, Up: Stopping
5.2 Continuing and Stepping
===========================
"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. *Note Signals: Signals.)
`continue [IGNORE-COUNT]'
`c [IGNORE-COUNT]'
`fg [IGNORE-COUNT]'
Resume program execution, at the address where your program last
stopped; any breakpoints set at that address are bypassed. The
optional argument IGNORE-COUNT allows you to specify a further
number of times to ignore a breakpoint at this location; its
effect is like that of `ignore' (*note Break Conditions:
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'
(*note Returning from a Function: Returning.) to go back to the calling
function; or `jump' (*note Continuing at a Different Address: Jumping.)
to go to an arbitrary location in your program.
A typical technique for using stepping is to set a breakpoint (*note
Breakpoints; Watchpoints; and Catchpoints: Breakpoints.) 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.
`step'
Continue running your program until control reaches a different
source line, then stop it and return control to GDB. This command
is abbreviated `s'.
_Warning:_ If you use the `step' command 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 the `stepi' command, 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 COUNT'
Continue running as in `step', 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]'
Continue to the next source line in the current (innermost) stack
frame. This is similar to `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-mode'
`set step-mode on'
The `set 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 off'
Causes the `step' command to step over any functions which
contains no debug information. This is the default.
`show step-mode'
Show whether GDB will stop in or step over functions without
source line debug information.
`finish'
Continue running until just after function in the selected stack
frame returns. Print the returned value (if any). This command
can be abbreviated as `fin'.
Contrast this with the `return' command (*note Returning from a
Function: Returning.).
`until'
`u'
Continue running until a source line past the current line, in the
current stack frame, is reached. This command is used to avoid
single stepping through a loop more than once. It is like the
`next' 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 LOCATION'
`u LOCATION'
Continue running your program until either the specified location
is reached, or the current stack frame returns. LOCATION is any of
the forms described in *Note Specify Location::. This form of the
command uses temporary 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 LOCATION'
Continue running the program up to the given LOCATION. An
argument is required, which should be of one of the forms
described in *Note Specify Location::. 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.
`stepi'
`stepi ARG'
`si'
Execute one machine instruction, then stop and return to the
debugger.
It 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. *Note
Automatic Display: Auto Display.
An argument is a repeat count, as in `step'.
`nexti'
`nexti ARG'
`ni'
Execute one machine instruction, but if it is a function call,
proceed until the function returns.
An argument is a repeat count, as in `next'.

File: gdb.info, Node: Signals, Next: Thread Stops, Prev: Continuing and Stepping, Up: Stopping
5.3 Signals
===========
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 `Ctrl-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 signals'
`info handle'
Print a table of all the kinds of signals and how GDB has been
told to handle each one. You can use this to see the signal
numbers of all the defined types of signals.
`info signals SIG'
Similar, but print information only about the specified signal
number.
`info handle' is an alias for `info signals'.
`handle SIGNAL [KEYWORDS...]'
Change the way GDB handles signal SIGNAL. SIGNAL can be the
number of a signal or its name (with or without the `SIG' at the
beginning); a list of signal numbers of the form `LOW-HIGH'; or
the word `all', meaning all the known signals. Optional arguments
KEYWORDS, described below, say what change to make.
The keywords allowed by the `handle' command can be abbreviated.
Their full names are:
`nostop'
GDB should not stop your program when this signal happens. It may
still print a message telling you that the signal has come in.
`stop'
GDB should stop your program when this signal happens. This
implies the `print' keyword as well.
`print'
GDB should print a message when this signal happens.
`noprint'
GDB should not mention the occurrence of the signal at all. This
implies the `nostop' keyword as well.
`pass'
`noignore'
GDB should allow your program to see this signal; your program can
handle the signal, or else it may terminate if the signal is fatal
and not handled. `pass' and `noignore' are synonyms.
`nopass'
`ignore'
GDB should not allow your program to see this signal. `nopass'
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'. *Note Giving your Program a Signal:
Signaling.
On some targets, GDB can inspect extra signal information associated
with the intercepted signal, before it is actually delivered to the
program being debugged. This information is exported by the
convenience variable `$_siginfo', and consists of data that is passed
by the kernel to the signal handler at the time of the receipt of a
signal. The data type of the information itself is target dependent.
You can see the data type using the `ptype $_siginfo' command. On Unix
systems, it typically corresponds to the standard `siginfo_t' type, as
defined in the `signal.h' system header.
Here's an example, on a GNU/Linux system, printing the stray
referenced address that raised a segmentation fault.
(gdb) continue
Program received signal SIGSEGV, Segmentation fault.
0x0000000000400766 in main ()
69 *(int *)p = 0;
(gdb) ptype $_siginfo
type = struct {
int si_signo;
int si_errno;
int si_code;
union {
int _pad[28];
struct {...} _kill;
struct {...} _timer;
struct {...} _rt;
struct {...} _sigchld;
struct {...} _sigfault;
struct {...} _sigpoll;
} _sifields;
}
(gdb) ptype $_siginfo._sifields._sigfault
type = struct {
void *si_addr;
}
(gdb) p $_siginfo._sifields._sigfault.si_addr
$1 = (void *) 0x7ffff7ff7000
Depending on target support, `$_siginfo' may also be writable.

File: gdb.info, Node: Thread Stops, Prev: Signals, Up: Stopping
5.4 Stopping and Starting Multi-thread Programs
===============================================
GDB supports debugging programs with multiple threads (*note Debugging
Programs with Multiple Threads: Threads.). There are two modes of
controlling execution of your program within the debugger. In the
default mode, referred to as "all-stop mode", when any thread in your
program stops (for example, at a breakpoint or while being stepped),
all other threads in the program are also stopped by GDB. On some
targets, GDB also supports "non-stop mode", in which other threads can
continue to run freely while you examine the stopped thread in the
debugger.
* Menu:
* All-Stop Mode:: All threads stop when GDB takes control
* Non-Stop Mode:: Other threads continue to execute
* Background Execution:: Running your program asynchronously
* Thread-Specific Breakpoints:: Controlling breakpoints
* Interrupted System Calls:: GDB may interfere with system calls
* Observer Mode:: GDB does not alter program behavior

File: gdb.info, Node: All-Stop Mode, Next: Non-Stop Mode, Up: Thread Stops
5.4.1 All-Stop Mode
-------------------
In all-stop mode, 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.
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.
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 such as
`[Switching to Thread N]' to identify the thread.
On some OSes, you can modify GDB's default behavior by locking the
OS scheduler to allow only a single thread to run.
`set scheduler-locking MODE'
Set the scheduler locking mode. If it is `off', 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 prevents other threads from
preempting the current thread while you are stepping, so that the
focus of debugging does not change unexpectedly. Other threads
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, GDB does not change the
current thread away from the thread that you are debugging.
`show scheduler-locking'
Display the current scheduler locking mode.
By default, when you issue one of the execution commands such as
`continue', `next' or `step', GDB allows only threads of the current
inferior to run. For example, if GDB is attached to two inferiors,
each with two threads, the `continue' command resumes only the two
threads of the current inferior. This is useful, for example, when you
debug a program that forks and you want to hold the parent stopped (so
that, for instance, it doesn't run to exit), while you debug the child.
In other situations, you may not be interested in inspecting the
current state of any of the processes GDB is attached to, and you may
want to resume them all until some breakpoint is hit. In the latter
case, you can instruct GDB to allow all threads of all the inferiors to
run with the `set schedule-multiple' command.
`set schedule-multiple'
Set the mode for allowing threads of multiple processes to be
resumed when an execution command is issued. When `on', all
threads of all processes are allowed to run. When `off', only the
threads of the current process are resumed. The default is `off'.
The `scheduler-locking' mode takes precedence when set to `on',
or while you are stepping and set to `step'.
`show schedule-multiple'
Display the current mode for resuming the execution of threads of
multiple processes.

File: gdb.info, Node: Non-Stop Mode, Next: Background Execution, Prev: All-Stop Mode, Up: Thread Stops
5.4.2 Non-Stop Mode
-------------------
For some multi-threaded targets, GDB supports an optional mode of
operation in which you can examine stopped program threads in the
debugger while other threads continue to execute freely. This
minimizes intrusion when debugging live systems, such as programs where
some threads have real-time constraints or must continue to respond to
external events. This is referred to as "non-stop" mode.
In non-stop mode, when a thread stops to report a debugging event,
_only_ that thread is stopped; GDB does not stop other threads as well,
in contrast to the all-stop mode behavior. Additionally, execution
commands such as `continue' and `step' apply by default only to the
current thread in non-stop mode, rather than all threads as in all-stop
mode. This allows you to control threads explicitly in ways that are
not possible in all-stop mode -- for example, stepping one thread while
allowing others to run freely, stepping one thread while holding all
others stopped, or stepping several threads independently and
simultaneously.
To enter non-stop mode, use this sequence of commands before you run
or attach to your program:
# Enable the async interface.
set target-async 1
# If using the CLI, pagination breaks non-stop.
set pagination off
# Finally, turn it on!
set non-stop on
You can use these commands to manipulate the non-stop mode setting:
`set non-stop on'
Enable selection of non-stop mode.
`set non-stop off'
Disable selection of non-stop mode.
`show non-stop'
Show the current non-stop enablement setting.
Note these commands only reflect whether non-stop mode is enabled,
not whether the currently-executing program is being run in non-stop
mode. In particular, the `set non-stop' preference is only consulted
when GDB starts or connects to the target program, and it is generally
not possible to switch modes once debugging has started. Furthermore,
since not all targets support non-stop mode, even when you have enabled
non-stop mode, GDB may still fall back to all-stop operation by default.
In non-stop mode, all execution commands apply only to the current
thread by default. That is, `continue' only continues one thread. To
continue all threads, issue `continue -a' or `c -a'.
You can use GDB's background execution commands (*note Background
Execution::) to run some threads in the background while you continue
to examine or step others from GDB. The MI execution commands (*note
GDB/MI Program Execution::) are always executed asynchronously in
non-stop mode.
Suspending execution is done with the `interrupt' command when
running in the background, or `Ctrl-c' during foreground execution. In
all-stop mode, this stops the whole process; but in non-stop mode the
interrupt applies only to the current thread. To stop the whole
program, use `interrupt -a'.
Other execution commands do not currently support the `-a' option.
In non-stop mode, when a thread stops, GDB doesn't automatically make
that thread current, as it does in all-stop mode. This is because the
thread stop notifications are asynchronous with respect to GDB's
command interpreter, and it would be confusing if GDB unexpectedly
changed to a different thread just as you entered a command to operate
on the previously current thread.

File: gdb.info, Node: Background Execution, Next: Thread-Specific Breakpoints, Prev: Non-Stop Mode, Up: Thread Stops
5.4.3 Background Execution
--------------------------
GDB's execution commands have two variants: the normal foreground
(synchronous) behavior, and a background (asynchronous) behavior. In
foreground execution, GDB waits for the program to report that some
thread has stopped before prompting for another command. In background
execution, GDB immediately gives a command prompt so that you can issue
other commands while your program runs.
You need to explicitly enable asynchronous mode before you can use
background execution commands. You can use these commands to
manipulate the asynchronous mode setting:
`set target-async on'
Enable asynchronous mode.
`set target-async off'
Disable asynchronous mode.
`show target-async'
Show the current target-async setting.
If the target doesn't support async mode, GDB issues an error
message if you attempt to use the background execution commands.
To specify background execution, add a `&' to the command. For
example, the background form of the `continue' command is `continue&',
or just `c&'. The execution commands that accept background execution
are:
`run'
*Note Starting your Program: Starting.
`attach'
*Note Debugging an Already-running Process: Attach.
`step'
*Note step: Continuing and Stepping.
`stepi'
*Note stepi: Continuing and Stepping.
`next'
*Note next: Continuing and Stepping.
`nexti'
*Note nexti: Continuing and Stepping.
`continue'
*Note continue: Continuing and Stepping.
`finish'
*Note finish: Continuing and Stepping.
`until'
*Note until: Continuing and Stepping.
Background execution is especially useful in conjunction with
non-stop mode for debugging programs with multiple threads; see *Note
Non-Stop Mode::. However, you can also use these commands in the
normal all-stop mode with the restriction that you cannot issue another
execution command until the previous one finishes. Examples of
commands that are valid in all-stop mode while the program is running
include `help' and `info break'.
You can interrupt your program while it is running in the background
by using the `interrupt' command.
`interrupt'
`interrupt -a'
Suspend execution of the running program. In all-stop mode,
`interrupt' stops the whole process, but in non-stop mode, it stops
only the current thread. To stop the whole program in non-stop
mode, use `interrupt -a'.

File: gdb.info, Node: Thread-Specific Breakpoints, Next: Interrupted System Calls, Prev: Background Execution, Up: Thread Stops
5.4.4 Thread-Specific Breakpoints
---------------------------------
When your program has multiple threads (*note Debugging Programs with
Multiple Threads: Threads.), you can choose whether to set breakpoints
on all threads, or on a particular thread.
`break LINESPEC thread THREADNO'
`break LINESPEC thread THREADNO if ...'
LINESPEC specifies source lines; there are several ways of writing
them (*note Specify Location::), but the effect is always to
specify some source line.
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 or after the
breakpoint condition, like this:
(gdb) break frik.c:13 thread 28 if bartab > lim

File: gdb.info, Node: Interrupted System Calls, Next: Observer Mode, Prev: Thread-Specific Breakpoints, Up: Thread Stops
5.4.5 Interrupted System Calls
------------------------------
There is an unfortunate side effect when using GDB to debug
multi-threaded programs. 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.

File: gdb.info, Node: Observer Mode, Prev: Interrupted System Calls, Up: Thread Stops
5.4.6 Observer Mode
-------------------
If you want to build on non-stop mode and observe program behavior
without any chance of disruption by GDB, you can set variables to
disable all of the debugger's attempts to modify state, whether by
writing memory, inserting breakpoints, etc. These operate at a low
level, intercepting operations from all commands.
When all of these are set to `off', then GDB is said to be "observer
mode". As a convenience, the variable `observer' can be set to disable
these, plus enable non-stop mode.
Note that GDB will not prevent you from making nonsensical
combinations of these settings. For instance, if you have enabled
`may-insert-breakpoints' but disabled `may-write-memory', then
breakpoints that work by writing trap instructions into the code stream
will still not be able to be placed.
`set observer on'
`set observer off'
When set to `on', this disables all the permission variables below
(except for `insert-fast-tracepoints'), plus enables non-stop
debugging. Setting this to `off' switches back to normal
debugging, though remaining in non-stop mode.
`show observer'
Show whether observer mode is on or off.
`set may-write-registers on'
`set may-write-registers off'
This controls whether GDB will attempt to alter the values of
registers, such as with assignment expressions in `print', or the
`jump' command. It defaults to `on'.
`show may-write-registers'
Show the current permission to write registers.
`set may-write-memory on'
`set may-write-memory off'
This controls whether GDB will attempt to alter the contents of
memory, such as with assignment expressions in `print'. It
defaults to `on'.
`show may-write-memory'
Show the current permission to write memory.
`set may-insert-breakpoints on'
`set may-insert-breakpoints off'
This controls whether GDB will attempt to insert breakpoints.
This affects all breakpoints, including internal breakpoints
defined by GDB. It defaults to `on'.
`show may-insert-breakpoints'
Show the current permission to insert breakpoints.
`set may-insert-tracepoints on'
`set may-insert-tracepoints off'
This controls whether GDB will attempt to insert (regular)
tracepoints at the beginning of a tracing experiment. It affects
only non-fast tracepoints, fast tracepoints being under the
control of `may-insert-fast-tracepoints'. It defaults to `on'.
`show may-insert-tracepoints'
Show the current permission to insert tracepoints.
`set may-insert-fast-tracepoints on'
`set may-insert-fast-tracepoints off'
This controls whether GDB will attempt to insert fast tracepoints
at the beginning of a tracing experiment. It affects only fast
tracepoints, regular (non-fast) tracepoints being under the
control of `may-insert-tracepoints'. It defaults to `on'.
`show may-insert-fast-tracepoints'
Show the current permission to insert fast tracepoints.
`set may-interrupt on'
`set may-interrupt off'
This controls whether GDB will attempt to interrupt or stop
program execution. When this variable is `off', the `interrupt'
command will have no effect, nor will `Ctrl-c'. It defaults to
`on'.
`show may-interrupt'
Show the current permission to interrupt or stop the program.

File: gdb.info, Node: Reverse Execution, Next: Process Record and Replay, Prev: Stopping, Up: Top
6 Running programs backward
***************************
When you are debugging a program, it is not unusual to realize that you
have gone too far, and some event of interest has already happened. If
the target environment supports it, GDB can allow you to "rewind" the
program by running it backward.
A target environment that supports reverse execution should be able
to "undo" the changes in machine state that have taken place as the
program was executing normally. Variables, registers etc. should
revert to their previous values. Obviously this requires a great deal
of sophistication on the part of the target environment; not all target
environments can support reverse execution.
When a program is executed in reverse, the instructions that have
most recently been executed are "un-executed", in reverse order. The
program counter runs backward, following the previous thread of
execution in reverse. As each instruction is "un-executed", the values
of memory and/or registers that were changed by that instruction are
reverted to their previous states. After executing a piece of source
code in reverse, all side effects of that code should be "undone", and
all variables should be returned to their prior values(1).
If you are debugging in a target environment that supports reverse
execution, GDB provides the following commands.
`reverse-continue [IGNORE-COUNT]'
`rc [IGNORE-COUNT]'
Beginning at the point where your program last stopped, start
executing in reverse. Reverse execution will stop for breakpoints
and synchronous exceptions (signals), just like normal execution.
Behavior of asynchronous signals depends on the target environment.
`reverse-step [COUNT]'
Run the program backward until control reaches the start of a
different source line; then stop it, and return control to GDB.
Like the `step' command, `reverse-step' will only stop at the
beginning of a source line. It "un-executes" the previously
executed source line. If the previous source line included calls
to debuggable functions, `reverse-step' will step (backward) into
the called function, stopping at the beginning of the _last_
statement in the called function (typically a return statement).
Also, as with the `step' command, if non-debuggable functions are
called, `reverse-step' will run thru them backward without
stopping.
`reverse-stepi [COUNT]'
Reverse-execute one machine instruction. Note that the instruction
to be reverse-executed is _not_ the one pointed to by the program
counter, but the instruction executed prior to that one. For
instance, if the last instruction was a jump, `reverse-stepi' will
take you back from the destination of the jump to the jump
instruction itself.
`reverse-next [COUNT]'
Run backward to the beginning of the previous line executed in the
current (innermost) stack frame. If the line contains function
calls, they will be "un-executed" without stopping. Starting from
the first line of a function, `reverse-next' will take you back to
the caller of that function, _before_ the function was called,
just as the normal `next' command would take you from the last
line of a function back to its return to its caller (2).
`reverse-nexti [COUNT]'
Like `nexti', `reverse-nexti' executes a single instruction in
reverse, except that called functions are "un-executed" atomically.
That is, if the previously executed instruction was a return from
another function, `reverse-nexti' will continue to execute in
reverse until the call to that function (from the current stack
frame) is reached.
`reverse-finish'
Just as the `finish' command takes you to the point where the
current function returns, `reverse-finish' takes you to the point
where it was called. Instead of ending up at the end of the
current function invocation, you end up at the beginning.
`set exec-direction'
Set the direction of target execution.
`set exec-direction reverse'
GDB will perform all execution commands in reverse, until the
exec-direction mode is changed to "forward". Affected commands
include `step, stepi, next, nexti, continue, and finish'. The
`return' command cannot be used in reverse mode.
`set exec-direction forward'
GDB will perform all execution commands in the normal fashion.
This is the default.
---------- Footnotes ----------
(1) Note that some side effects are easier to undo than others. For
instance, memory and registers are relatively easy, but device I/O is
hard. Some targets may be able undo things like device I/O, and some
may not.
The contract between GDB and the reverse executing target requires
only that the target do something reasonable when GDB tells it to
execute backwards, and then report the results back to GDB. Whatever
the target reports back to GDB, GDB will report back to the user. GDB
assumes that the memory and registers that the target reports are in a
consistant state, but GDB accepts whatever it is given.
(2) Unless the code is too heavily optimized.

File: gdb.info, Node: Process Record and Replay, Next: Stack, Prev: Reverse Execution, Up: Top
7 Recording Inferior's Execution and Replaying It
*************************************************
On some platforms, GDB provides a special "process record and replay"
target that can record a log of the process execution, and replay it
later with both forward and reverse execution commands.
When this target is in use, if the execution log includes the record
for the next instruction, GDB will debug in "replay mode". In the
replay mode, the inferior does not really execute code instructions.
Instead, all the events that normally happen during code execution are
taken from the execution log. While code is not really executed in
replay mode, the values of registers (including the program counter
register) and the memory of the inferior are still changed as they
normally would. Their contents are taken from the execution log.
If the record for the next instruction is not in the execution log,
GDB will debug in "record mode". In this mode, the inferior executes
normally, and GDB records the execution log for future replay.
The process record and replay target supports reverse execution
(*note Reverse Execution::), even if the platform on which the inferior
runs does not. However, the reverse execution is limited in this case
by the range of the instructions recorded in the execution log. In
other words, reverse execution on platforms that don't support it
directly can only be done in the replay mode.
When debugging in the reverse direction, GDB will work in replay
mode as long as the execution log includes the record for the previous
instruction; otherwise, it will work in record mode, if the platform
supports reverse execution, or stop if not.
For architecture environments that support process record and replay,
GDB provides the following commands:
`target record'
This command starts the process record and replay target. The
process record and replay target can only debug a process that is
already running. Therefore, you need first to start the process
with the `run' or `start' commands, and then start the recording
with the `target record' command.
Both `record' and `rec' are aliases of `target record'.
Displaced stepping (*note displaced stepping: Maintenance
Commands.) will be automatically disabled when process record and
replay target is started. That's because the process record and
replay target doesn't support displaced stepping.
If the inferior is in the non-stop mode (*note Non-Stop Mode::) or
in the asynchronous execution mode (*note Background Execution::),
the process record and replay target cannot be started because it
doesn't support these two modes.
`record stop'
Stop the process record and replay target. When process record and
replay target stops, the entire execution log will be deleted and
the inferior will either be terminated, or will remain in its
final state.
When you stop the process record and replay target in record mode
(at the end of the execution log), the inferior will be stopped at
the next instruction that would have been recorded. In other
words, if you record for a while and then stop recording, the
inferior process will be left in the same state as if the
recording never happened.
On the other hand, if the process record and replay target is
stopped while in replay mode (that is, not at the end of the
execution log, but at some earlier point), the inferior process
will become "live" at that earlier state, and it will then be
possible to continue the usual "live" debugging of the process
from that state.
When the inferior process exits, or GDB detaches from it, process
record and replay target will automatically stop itself.
`record save FILENAME'
Save the execution log to a file `FILENAME'. Default filename is
`gdb_record.PROCESS_ID', where PROCESS_ID is the process ID of the
inferior.
`record restore FILENAME'
Restore the execution log from a file `FILENAME'. File must have
been created with `record save'.
`set record insn-number-max LIMIT'
Set the limit of instructions to be recorded. Default value is
200000.
If LIMIT is a positive number, then GDB will start deleting
instructions from the log once the number of the record
instructions becomes greater than LIMIT. For every new recorded
instruction, GDB will delete the earliest recorded instruction to
keep the number of recorded instructions at the limit. (Since
deleting recorded instructions loses information, GDB lets you
control what happens when the limit is reached, by means of the
`stop-at-limit' option, described below.)
If LIMIT is zero, GDB will never delete recorded instructions from
the execution log. The number of recorded instructions is
unlimited in this case.
`show record insn-number-max'
Show the limit of instructions to be recorded.
`set record stop-at-limit'
Control the behavior when the number of recorded instructions
reaches the limit. If ON (the default), GDB will stop when the
limit is reached for the first time and ask you whether you want
to stop the inferior or continue running it and recording the
execution log. If you decide to continue recording, each new
recorded instruction will cause the oldest one to be deleted.
If this option is OFF, GDB will automatically delete the oldest
record to make room for each new one, without asking.
`show record stop-at-limit'
Show the current setting of `stop-at-limit'.
`set record memory-query'
Control the behavior when GDB is unable to record memory changes
caused by an instruction. If ON, GDB will query whether to stop
the inferior in that case.
If this option is OFF (the default), GDB will automatically ignore
the effect of such instructions on memory. Later, when GDB
replays this execution log, it will mark the log of this
instruction as not accessible, and it will not affect the replay
results.
`show record memory-query'
Show the current setting of `memory-query'.
`info record'
Show various statistics about the state of process record and its
in-memory execution log buffer, including:
* Whether in record mode or replay mode.
* Lowest recorded instruction number (counting from when the
current execution log started recording instructions).
* Highest recorded instruction number.
* Current instruction about to be replayed (if in replay mode).
* Number of instructions contained in the execution log.
* Maximum number of instructions that may be contained in the
execution log.
`record delete'
When record target runs in replay mode ("in the past"), delete the
subsequent execution log and begin to record a new execution log
starting from the current address. This means you will abandon
the previously recorded "future" and begin recording a new
"future".

File: gdb.info, Node: Stack, Next: Source, Prev: Process Record and Replay, Up: Top
8 Examining the Stack
*********************
When 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. *Note Selecting a Frame:
Selection.
When your program stops, GDB automatically selects the currently
executing frame and describes it briefly, similar to the `frame'
command (*note Information about a Frame: Frame Info.).
* Menu:
* Frames:: Stack frames
* Backtrace:: Backtraces
* Selection:: Selecting a frame
* Frame Info:: Information on a frame

File: gdb.info, Node: Frames, Next: Backtrace, Up: Stack
8.1 Stack Frames
================
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" (*note $fp:
Registers.) 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 ARGS'
The `frame' 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-frame'
The `select-frame' command allows you to move from one stack frame
to another without printing the frame. This is the silent version
of `frame'.

File: gdb.info, Node: Backtrace, Next: Selection, Prev: Frames, Up: Stack
8.2 Backtraces
==============
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.
`backtrace'
`bt'
Print a backtrace of the entire stack: one line per frame for all
frames in the stack.
You can stop the backtrace at any time by typing the system
interrupt character, normally `Ctrl-c'.
`backtrace N'
`bt N'
Similar, but print only the innermost N frames.
`backtrace -N'
`bt -N'
Similar, but print only the outermost N frames.
`backtrace full'
`bt full'
`bt full N'
`bt full -N'
Print the values of the local variables also. N specifies the
number of frames to print, as described above.
The names `where' and `info stack' (abbreviated `info s') are
additional aliases for `backtrace'.
In a multi-threaded program, GDB by default shows the backtrace only
for the current thread. To display the backtrace for several or all of
the threads, use the command `thread apply' (*note thread apply:
Threads.). For example, if you type `thread apply all backtrace', GDB
will display the backtrace for all the threads; this is handy when you
debug a core dump of a multi-threaded program.
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, data=...) 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'.
The value of parameter `data' in frame 1 has been replaced by `...'.
By default, GDB prints the value of a parameter only if it is a scalar
(integer, pointer, enumeration, etc). See command `set print
frame-arguments' in *Note Print Settings:: for more details on how to
configure the way function parameter values are printed.
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
`main'(1). 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-main'
`set backtrace past-main on'
Backtraces will continue past the user entry point.
`set backtrace past-main off'
Backtraces will stop when they encounter the user entry point.
This is the default.
`show backtrace past-main'
Display the current user entry point backtrace policy.
`set backtrace past-entry'
`set backtrace past-entry on'
Backtraces will continue past the internal entry point of an
application. This entry point is encoded by the linker when the
application is built, and is likely before the user entry point
`main' (or equivalent) is called.
`set backtrace past-entry off'
Backtraces will stop when they encounter the internal entry point
of an application. This is the default.
`show backtrace past-entry'
Display the current internal entry point backtrace policy.
`set backtrace limit N'
`set backtrace limit 0'
Limit the backtrace to N levels. A value of zero means unlimited.
`show backtrace limit'
Display the current limit on backtrace levels.
---------- Footnotes ----------
(1) Note that embedded programs (the so-called "free-standing"
environment) are not required to have a `main' function as the entry
point. They could even have multiple entry points.

File: gdb.info, Node: Selection, Next: Frame Info, Prev: Backtrace, Up: Stack
8.3 Selecting a Frame
=====================
Most 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 N'
`f N'
Select frame number N. Recall that frame zero is the innermost
(currently executing) frame, frame one is the frame that called the
innermost one, and so on. The highest-numbered frame is the one
for `main'.
`frame ADDR'
`f ADDR'
Select the frame at address ADDR. This is useful mainly if the
chaining of stack frames has been damaged by a bug, making it
impossible for GDB to assign numbers properly to all frames. In
addition, this can be useful when your program has multiple stacks
and switches between them.
On 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 N'
Move N frames up the stack. For positive numbers N, this advances
toward the outermost frame, to higher frame numbers, to frames
that have existed longer. N defaults to one.
`down N'
Move N frames down the stack. For positive numbers N, this
advances toward the innermost frame, to lower frame numbers, to
frames that were created more recently. N defaults to one. You
may abbreviate `down' 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'. *Note Printing Source Lines: List,
for details.
`up-silently N'
`down-silently N'
These two commands are variants of `up' 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.

File: gdb.info, Node: Frame Info, Prev: Selection, Up: Stack
8.4 Information About a Frame
=============================
There are several other commands to print information about the selected
stack frame.
`frame'
`f'
When used without any argument, this command does not change which
frame is selected, but prints a brief description of the currently
selected stack frame. It can be abbreviated `f'. With an
argument, this command is used to select a stack frame. *Note
Selecting a Frame: Selection.
`info frame'
`info f'
This command prints a verbose description of the selected stack
frame, including:
* the address of the frame
* the address of the next frame down (called by this frame)
* the address of the next frame up (caller of this frame)
* the language in which the source code corresponding to this
frame is written
* the address of the frame's arguments
* the address of the frame's local variables
* the program counter saved in it (the address of execution in
the caller frame)
* which registers were saved in the frame
The verbose description is useful when something has gone wrong
that has made the stack format fail to fit the usual conventions.
`info frame ADDR'
`info f ADDR'
Print a verbose description of the frame at address ADDR, without
selecting that frame. The selected frame remains unchanged by this
command. This requires the same kind of address (more than one
for some architectures) that you specify in the `frame' command.
*Note Selecting a Frame: Selection.
`info args'
Print the arguments of the selected frame, each on a separate line.
`info locals'
Print the local variables of the selected frame, each on a separate
line. These are all variables (declared either static or
automatic) accessible at the point of execution of the selected
frame.
`info catch'
Print a list of all the exception handlers that are active in the
current stack frame at the current point of execution. To see
other exception handlers, visit the associated frame (using the
`up', `down', or `frame' commands); then type `info catch'. *Note
Setting Catchpoints: Set Catchpoints.

File: gdb.info, Node: Source, Next: Data, Prev: Stack, Up: Top
9 Examining Source Files
************************
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
(*note Selecting a Frame: Selection.), 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 *Note Using GDB under GNU
Emacs: Emacs.
* Menu:
* List:: Printing source lines
* Specify Location:: How to specify code locations
* Edit:: Editing source files
* Search:: Searching source files
* Source Path:: Specifying source directories
* Machine Code:: Source and machine code

File: gdb.info, Node: List, Next: Specify Location, Up: Source
9.1 Printing Source Lines
=========================
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; see *Note Specify
Location::, for the full list.
Here are the forms of the `list' command most commonly used:
`list LINENUM'
Print lines centered around line number LINENUM in the current
source file.
`list FUNCTION'
Print lines centered around the beginning of function FUNCTION.
`list'
Print more lines. If the last lines printed were printed with a
`list' 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 (*note Examining the
Stack: Stack.), this prints lines centered around that line.
`list -'
Print lines just before the lines last printed.
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 COUNT'
Make the `list' command display COUNT source lines (unless the
`list' argument explicitly specifies some other number).
`show listsize'
Display the number of lines that `list' 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 (*note Specify Location::), but the effect is always to
specify some source line.
Here is a complete description of the possible arguments for `list':
`list LINESPEC'
Print lines centered around the line specified by LINESPEC.
`list FIRST,LAST'
Print lines from FIRST to LAST. Both arguments are linespecs.
When a `list' command has two linespecs, and the source file of
the second linespec is omitted, this refers to the same source
file as the first linespec.
`list ,LAST'
Print lines ending with LAST.
`list FIRST,'
Print lines starting with FIRST.
`list +'
Print lines just after the lines last printed.
`list -'
Print lines just before the lines last printed.
`list'
As described in the preceding table.

File: gdb.info, Node: Specify Location, Next: Edit, Prev: List, Up: Source
9.2 Specifying a Location
=========================
Several GDB commands accept arguments that specify a location of your
program's code. Since GDB is a source-level debugger, a location
usually specifies some line in the source code; for that reason,
locations are also known as "linespecs".
Here are all the different ways of specifying a code location that
GDB understands:
`LINENUM'
Specifies the line number LINENUM of the current source file.
`-OFFSET'
`+OFFSET'
Specifies the line OFFSET lines before or after the "current
line". For the `list' command, the current line is the last one
printed; for the breakpoint commands, this is the line at which
execution stopped in the currently selected "stack frame" (*note
Frames: Frames, for a description of stack frames.) When used as
the second of the two linespecs in a `list' command, this
specifies the line OFFSET lines up or down from the first linespec.
`FILENAME:LINENUM'
Specifies the line LINENUM in the source file FILENAME.
`FUNCTION'
Specifies the line that begins the body of the function FUNCTION.
For example, in C, this is the line with the open brace.
`FILENAME:FUNCTION'
Specifies the line that begins the body of the function FUNCTION
in the file FILENAME. You only need the file name with a function
name to avoid ambiguity when there are identically named functions
in different source files.
`*ADDRESS'
Specifies the program address ADDRESS. For line-oriented
commands, such as `list' and `edit', this specifies a source line
that contains ADDRESS. For `break' and other breakpoint oriented
commands, this can be used to set breakpoints in parts of your
program which do not have debugging information or source files.
Here ADDRESS may be any expression valid in the current working
language (*note working language: Languages.) that specifies a code
address. In addition, as a convenience, GDB extends the semantics
of expressions used in locations to cover the situations that
frequently happen during debugging. Here are the various forms of
ADDRESS:
`EXPRESSION'
Any expression valid in the current working language.
`FUNCADDR'
An address of a function or procedure derived from its name.
In C, C++, Java, Objective-C, Fortran, minimal, and assembly,
this is simply the function's name FUNCTION (and actually a
special case of a valid expression). In Pascal and Modula-2,
this is `&FUNCTION'. In Ada, this is `FUNCTION'Address'
(although the Pascal form also works).
This form specifies the address of the function's first
instruction, before the stack frame and arguments have been
set up.
`'FILENAME'::FUNCADDR'
Like FUNCADDR above, but also specifies the name of the source
file explicitly. This is useful if the name of the function
does not specify the function unambiguously, e.g., if there
are several functions with identical names in different
source files.

File: gdb.info, Node: Edit, Next: Search, Prev: Specify Location, Up: Source
9.3 Editing Source Files
========================
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:
`edit LOCATION'
Edit the source file specified by `location'. Editing starts at
that LOCATION, e.g., at the specified source line of the specified
file. *Note Specify Location::, for all the possible forms of the
LOCATION argument; here are the forms of the `edit' command most
commonly used:
`edit NUMBER'
Edit the current source file with NUMBER as the active line
number.
`edit FUNCTION'
Edit the file containing FUNCTION at the beginning of its
definition.
9.3.1 Choosing your Editor
--------------------------
You can customize GDB to use any editor you want (1). 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 ...
---------- Footnotes ----------
(1) The only restriction is that your editor (say `ex'), recognizes
the following command-line syntax:
ex +NUMBER file
The optional numeric value +NUMBER specifies the number of the line
in the file where to start editing.

File: gdb.info, Node: Search, Next: Source Path, Prev: Edit, Up: Source
9.4 Searching Source Files
==========================
There are two commands for searching through the current source file
for a regular expression.
`forward-search REGEXP'
`search REGEXP'
The command `forward-search REGEXP' checks each line, starting
with the one following the last line listed, for a match for
REGEXP. It lists the line that is found. You can use the synonym
`search REGEXP' or abbreviate the command name as `fo'.
`reverse-search REGEXP'
The command `reverse-search REGEXP' checks each line, starting
with the one before the last line listed and going backward, for a
match for REGEXP. It lists the line that is found. You can
abbreviate this command as `rev'.

File: gdb.info, Node: Source Path, Next: Machine Code, Prev: Search, Up: Source
9.5 Specifying Source Directories
=================================
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.
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.
The search path is used to find both program source files and GDB
script files (read using the `-command' option and `source' command).
In addition to the source path, GDB provides a set of commands that
manage a list of source path substitution rules. A "substitution rule"
specifies how to rewrite source directories stored in the program's
debug information in case the sources were moved to a different
directory between compilation and debugging. A rule is made of two
strings, the first specifying what needs to be rewritten in the path,
and the second specifying how it should be rewritten. In *Note set
substitute-path::, we name these two parts FROM and TO respectively.
GDB does a simple string replacement of FROM with TO at the start of
the directory part of the source file name, and uses that result
instead of the original file name to look up the sources.
Using the previous example, suppose the `foo-1.0' tree has been
moved from `/usr/src' to `/mnt/cross', then you can tell GDB to replace
`/usr/src' in all source path names with `/mnt/cross'. The first
lookup will then be `/mnt/cross/foo-1.0/lib/foo.c' in place of the
original location of `/usr/src/foo-1.0/lib/foo.c'. To define a source
path substitution rule, use the `set substitute-path' command (*note
set substitute-path::).
To avoid unexpected substitution results, a rule is applied only if
the FROM part of the directory name ends at a directory separator. For
instance, a rule substituting `/usr/source' into `/mnt/cross' will be
applied to `/usr/source/foo-1.0' but not to `/usr/sourceware/foo-2.0'.
And because the substitution is applied only at the beginning of the
directory name, this rule will not be applied to
`/root/usr/source/baz.c' either.
In many cases, you can achieve the same result using the `directory'
command. However, `set substitute-path' can be more efficient in the
case where the sources are organized in a complex tree with multiple
subdirectories. With the `directory' command, you need to add each
subdirectory of your project. If you moved the entire tree while
preserving its internal organization, then `set substitute-path' allows
you to direct the debugger to all the sources with one single command.
`set substitute-path' is also more than just a shortcut command.
The source path is only used if the file at the original location no
longer exists. On the other hand, `set substitute-path' modifies the
debugger behavior to look at the rewritten location instead. So, if
for any reason a source file that is not relevant to your executable is
located at the original location, a substitution rule is the only
method available to point GDB at the new location.
You can configure a default source path substitution rule by
configuring GDB with the `--with-relocated-sources=DIR' option. The DIR
should be the name of a directory under GDB's configured prefix (set
with `--prefix' or `--exec-prefix'), and directory names in debug
information under DIR will be adjusted automatically if the installed
GDB is moved to a new location. This is useful if GDB, libraries or
executables with debug information and corresponding source code are
being moved together.
`directory DIRNAME ...'
`dir DIRNAME ...'
Add directory DIRNAME to the front of the source path. Several
directory names may be given to this command, separated by `:'
(`;' on MS-DOS and MS-Windows, where `:' usually appears as part
of absolute file names) or whitespace. You may specify a
directory that is already in the source path; this moves it
forward, so GDB searches it sooner.
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. `$cwd' is not the same as `.'--the former
tracks the current working directory as it changes during your GDB
session, while the latter is immediately expanded to the current
directory at the time you add an entry to the source path.
`directory'
Reset the source path to its default value (`$cdir:$cwd' on Unix
systems). This requires confirmation.
`show directories'
Print the source path: show which directories it contains.
`set substitute-path FROM TO'
Define a source path substitution rule, and add it at the end of
the current list of existing substitution rules. If a rule with
the same FROM was already defined, then the old rule is also
deleted.
For example, if the file `/foo/bar/baz.c' was moved to
`/mnt/cross/baz.c', then the command
(gdb) set substitute-path /usr/src /mnt/cross
will tell GDB to replace `/usr/src' with `/mnt/cross', which will
allow GDB to find the file `baz.c' even though it was moved.
In the case when more than one substitution rule have been defined,
the rules are evaluated one by one in the order where they have
been defined. The first one matching, if any, is selected to
perform the substitution.
For instance, if we had entered the following commands:
(gdb) set substitute-path /usr/src/include /mnt/include
(gdb) set substitute-path /usr/src /mnt/src
GDB would then rewrite `/usr/src/include/defs.h' into
`/mnt/include/defs.h' by using the first rule. However, it would
use the second rule to rewrite `/usr/src/lib/foo.c' into
`/mnt/src/lib/foo.c'.
`unset substitute-path [path]'
If a path is specified, search the current list of substitution
rules for a rule that would rewrite that path. Delete that rule
if found. A warning is emitted by the debugger if no rule could
be found.
If no path is specified, then all substitution rules are deleted.
`show substitute-path [path]'
If a path is specified, then print the source path substitution
rule which would rewrite that path, if any.
If no path is specified, then print all existing source path
substitution rules.
If 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:
1. Use `directory' with no argument to reset the source path to its
default value.
2. Use `directory' with suitable arguments to reinstall the
directories you want in the source path. You can add all the
directories in one command.

File: gdb.info, Node: Machine Code, Prev: Source Path, Up: Source
9.6 Source and Machine Code
===========================
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. You can use the command
`set disassemble-next-line' to set whether to disassemble next source
line when execution stops. 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 LINESPEC'
Print the starting and ending addresses of the compiled code for
source line LINESPEC. You can specify source lines in any of the
ways documented in *Note Specify Location::.
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 (*note Examining Memory:
Memory.). Also, this address is saved as the value of the convenience
variable `$_' (*note Convenience Variables: Convenience Vars.).
`disassemble'
`disassemble /m'
`disassemble /r'
This specialized command dumps a range of memory as machine
instructions. It can also print mixed source+disassembly by
specifying the `/m' modifier and print the raw instructions in hex
as well as in symbolic form by specifying the `/r'. The default
memory range is the function surrounding the program counter of
the selected frame. A single argument to this command is a
program counter value; GDB dumps the function surrounding this
value. When two arguments are given, they should be separated by
a comma, possibly surrounded by whitespace. The arguments specify
a range of addresses to dump, in one of two forms:
`START,END'
the addresses from START (inclusive) to END (exclusive)
`START,+LENGTH'
the addresses from START (inclusive) to `START+LENGTH'
(exclusive).
When 2 arguments are specified, the name of the function is also
printed (since there could be several functions in the given
range).
The argument(s) can be any expression yielding a numeric value,
such as `0x32c4', `&main+10' or `$pc - 8'.
If the range of memory being disassembled contains current program
counter, the instruction at that location is shown with a `=>'
marker.
The 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.
Here is an example showing mixed source+assembly for Intel x86, when
the program is stopped just after function prologue:
(gdb) disas /m main
Dump of assembler code for function main:
5 {
0x08048330 <+0>: push %ebp
0x08048331 <+1>: mov %esp,%ebp
0x08048333 <+3>: sub $0x8,%esp
0x08048336 <+6>: and $0xfffffff0,%esp
0x08048339 <+9>: sub $0x10,%esp
6 printf ("Hello.\n");
=> 0x0804833c <+12>: movl $0x8048440,(%esp)
0x08048343 <+19>: call 0x8048284 <puts@plt>
7 return 0;
8 }
0x08048348 <+24>: mov $0x0,%eax
0x0804834d <+29>: leave
0x0804834e <+30>: ret
End of assembler dump.
Here is another example showing raw instructions in hex for AMD
x86-64,
(gdb) disas /r 0x400281,+10
Dump of assembler code from 0x400281 to 0x40028b:
0x0000000000400281: 38 36 cmp %dh,(%rsi)
0x0000000000400283: 2d 36 34 2e 73 sub $0x732e3436,%eax
0x0000000000400288: 6f outsl %ds:(%rsi),(%dx)
0x0000000000400289: 2e 32 00 xor %cs:(%rax),%al
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-SET'
Select the instruction set to use when disassembling the program
via the `disassemble' 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'
Show the current setting of the disassembly flavor.
`set disassemble-next-line'
`show disassemble-next-line'
Control whether or not GDB will disassemble the next source line
or instruction when execution stops. If ON, GDB will display
disassembly of the next source line when execution of the program
being debugged stops. This is _in addition_ to displaying the
source line itself, which GDB always does if possible. If the
next source line cannot be displayed for some reason (e.g., if GDB
cannot find the source file, or there's no line info in the debug
info), GDB will display disassembly of the next _instruction_
instead of showing the next source line. If AUTO, GDB will
display disassembly of next instruction only if the source line
cannot be displayed. This setting causes GDB to display some
feedback when you step through a function with no line info or
whose source file is unavailable. The default is OFF, which means
never display the disassembly of the next line or instruction.

File: gdb.info, Node: Data, Next: Optimized Code, Prev: Source, Up: Top
10 Examining Data
*****************
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 (*note Using GDB with Different Languages: Languages.). It
may also print the expression using a Python-based pretty-printer
(*note Pretty Printing::).
`print EXPR'
`print /F EXPR'
EXPR is an expression (in the source language). By default the
value of EXPR is printed in a format appropriate to its data type;
you can choose a different format by specifying `/F', where F is a
letter specifying the format; see *Note Output Formats: Output
Formats.
`print'
`print /F'
If you omit EXPR, GDB displays the last value again (from the
"value history"; *note Value History: Value History.). This
allows you to conveniently inspect the same value in an
alternative format.
A 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. *Note Examining Memory: 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'. *Note Examining the Symbol Table: Symbols.
* Menu:
* Expressions:: Expressions
* Ambiguous Expressions:: Ambiguous Expressions
* Variables:: Program variables
* Arrays:: Artificial arrays
* Output Formats:: Output formats
* Memory:: Examining memory
* Auto Display:: Automatic display
* Print Settings:: Print settings
* Pretty Printing:: Python pretty printing
* Value History:: Value history
* Convenience Vars:: Convenience variables
* Registers:: Registers
* Floating Point Hardware:: Floating point hardware
* Vector Unit:: Vector Unit
* OS Information:: Auxiliary data provided by operating system
* Memory Region Attributes:: Memory region attributes
* Dump/Restore Files:: Copy between memory and a file
* Core File Generation:: Cause a program dump its core
* Character Sets:: Debugging programs that use a different
character set than GDB does
* Caching Remote Data:: Data caching for remote targets
* Searching Memory:: Searching memory for a sequence of bytes

File: gdb.info, Node: Expressions, Next: Ambiguous Expressions, Up: Data
10.1 Expressions
================
`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 *Note
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 create an array of three integers. If you pass an
array to a function or assign it to a program variable, GDB copies the
array to memory that is `malloc'ed in the target program.
Because C is so widespread, most of the expressions shown in
examples in this manual are in C. *Note Using GDB with Different
Languages: 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:
`@'
`@' is a binary operator for treating parts of memory as arrays.
*Note Artificial Arrays: Arrays, for more information.
`::'
`::' allows you to specify a variable in terms of the file or
function where it is defined. *Note Program Variables: Variables.
`{TYPE} ADDR'
Refers to an object of type TYPE stored at address ADDR in memory.
ADDR may be any expression whose value is an integer or pointer
(but parentheses are required around binary operators, just as in
a cast). This construct is allowed regardless of what kind of
data is normally supposed to reside at ADDR.

File: gdb.info, Node: Ambiguous Expressions, Next: Variables, Prev: Expressions, Up: Data
10.2 Ambiguous Expressions
==========================
Expressions can sometimes contain some ambiguous elements. For
instance, some programming languages (notably Ada, C++ and Objective-C)
permit a single function name to be defined several times, for
application in different contexts. This is called "overloading".
Another example involving Ada is generics. A "generic package" is
similar to C++ templates and is typically instantiated several times,
resulting in the same function name being defined in different contexts.
In some cases and depending on the language, it is possible to adjust
the expression to remove the ambiguity. For instance in C++, you can
specify the signature of the function you want to break on, as in
`break FUNCTION(TYPES)'. In Ada, using the fully qualified name of
your function often makes the expression unambiguous as well.
When an ambiguity that needs to be resolved is detected, the debugger
has the capability to display a menu of numbered choices for each
possibility, and then waits for the selection with the prompt `>'. The
first option is always `[0] cancel', and typing `0 <RET>' aborts the
current command. If the command in which the expression was used
allows more than one choice to be selected, the next option in the menu
is `[1] all', and typing `1 <RET>' selects all possible choices.
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)
`set multiple-symbols MODE'
This option allows you to adjust the debugger behavior when an
expression is ambiguous.
By default, MODE is set to `all'. If the command with which the
expression is used allows more than one choice, then GDB
automatically selects all possible choices. For instance,
inserting a breakpoint on a function using an ambiguous name
results in a breakpoint inserted on each possible match. However,
if a unique choice must be made, then GDB uses the menu to help
you disambiguate the expression. For instance, printing the
address of an overloaded function will result in the use of the
menu.
When MODE is set to `ask', the debugger always uses the menu when
an ambiguity is detected.
Finally, when MODE is set to `cancel', the debugger reports an
error due to the ambiguity and the command is aborted.
`show multiple-symbols'
Show the current value of the `multiple-symbols' setting.

File: gdb.info, Node: Variables, Next: Arrays, Prev: Ambiguous Expressions, Up: Data
10.3 Program Variables
======================
The 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
(*note Selecting a Frame: Selection.); they must be either:
* global (or file-static)
or
* visible according to the scope rules of the programming language
from the point of execution in that frame
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. *Note
Options for Debugging Your Program or GCC: (gcc.info)Debugging Options.
*Note C and C++: C, for more information about debug info formats that
are best suited to C++ programs.
If you ask to print an object whose contents are unknown to GDB,
e.g., because its data type is not completely specified by the debug
information, GDB will say `<incomplete type>'. *Note incomplete type:
Symbols, for more about this.
Strings are identified as arrays of `char' values without specified
signedness. Arrays of either `signed char' or `unsigned char' get
printed as arrays of 1 byte sized integers. `-fsigned-char' or
`-funsigned-char' GCC options have no effect as GDB defines literal
string type `"char"' as `char' without a sign. For program code
char var0[] = "A";
signed char var1[] = "A";
You get during debugging
(gdb) print var0
$1 = "A"
(gdb) print var1
$2 = {65 'A', 0 '\0'}

File: gdb.info, Node: Arrays, Next: Output Formats, Prev: Variables, Up: Data
10.4 Artificial Arrays
======================
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
(*note Value History: 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 (*note Convenience Variables: Convenience
Vars.) 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>
...

File: gdb.info, Node: Output Formats, Next: Memory, Prev: Arrays, Up: Data
10.5 Output Formats
===================
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:
`x'
Regard the bits of the value as an integer, and print the integer
in hexadecimal.
`d'
Print as integer in signed decimal.
`u'
Print as integer in unsigned decimal.
`o'
Print as integer in octal.
`t'
Print as integer in binary. The letter `t' stands for "two". (1)
`a'
Print as an address, both absolute in hexadecimal and as an offset
from the nearest preceding symbol. You can use this format used
to discover where (in what function) an unknown address is located:
(gdb) p/a 0x54320
$3 = 0x54320 <_initialize_vx+396>
The command `info symbol 0x54320' yields similar results. *Note
info symbol: Symbols.
`c'
Regard as an integer and print it as a character constant. This
prints both the numerical value and its character representation.
The character representation is replaced with the octal escape
`\nnn' for characters outside the 7-bit ASCII range.
Without this format, GDB displays `char', `unsigned char', and
`signed char' data as character constants. Single-byte members of
vectors are displayed as integer data.
`f'
Regard the bits of the value as a floating point number and print
using typical floating point syntax.
`s'
Regard as a string, if possible. With this format, pointers to
single-byte data are displayed as null-terminated strings and
arrays of single-byte data are displayed as fixed-length strings.
Other values are displayed in their natural types.
Without this format, GDB displays pointers to and arrays of
`char', `unsigned char', and `signed char' as strings.
Single-byte members of a vector are displayed as an integer array.
`r'
Print using the `raw' formatting. By default, GDB will use a
Python-based pretty-printer, if one is available (*note Pretty
Printing::). This typically results in a higher-level display of
the value's contents. The `r' format bypasses any Python
pretty-printer which might exist.
For example, to print the program counter in hex (*note
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.
---------- Footnotes ----------
(1) `b' cannot be used because these format letters are also used
with the `x' command, where `b' stands for "byte"; see *Note Examining
Memory: Memory.

File: gdb.info, Node: Memory, Next: Auto Display, Prev: Output Formats, Up: Data
10.6 Examining Memory
=====================
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 ADDR'
`x ADDR'
`x'
Use the `x' 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.
N, the repeat count
The repeat count is a decimal integer; the default is 1. It
specifies how much memory (counting by units U) to display.
F, the display format
The display format is one of the formats used by `print' (`x',
`d', `u', `o', `t', `a', `c', `f', `s'), and in addition `i' (for
machine instructions). The default is `x' (hexadecimal)
initially. The default changes each time you use either `x' or
`print'.
U, the unit size
The unit size is any of
`b'
Bytes.
`h'
Halfwords (two bytes).
`w'
Words (four bytes). This is the initial default.
`g'
Giant words (eight bytes).
Each time you specify a unit size with `x', that size becomes the
default unit the next time you use `x'. For the `i' format, the
unit size is ignored and is normally not written. For the `s'
format, the unit size defaults to `b', unless it is explicitly
given. Use `x /hs' to display 16-bit char strings and `x /ws' to
display 32-bit strings. The next use of `x /s' will again display
8-bit strings. Note that the results depend on the programming
language of the current compilation unit. If the language is C,
the `s' modifier will use the UTF-16 encoding while `w' will use
UTF-32. The encoding is set by the programming language and cannot
be altered.
ADDR, starting display address
ADDR is the address where you want GDB to begin displaying memory.
The expression need not have a pointer value (though it may); it
is always interpreted as an integer address of a byte of memory.
*Note Expressions: Expressions, for more information on
expressions. The default for ADDR is usually just after the last
address examined--but several other commands also set the default
address: `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'; *note Registers:
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.
For convenience, especially when used with the `display' command, the
`i' format also prints branch delay slot instructions, if any, beyond
the count specified, which immediately follow the last instruction that
is within the count. The command `disassemble' gives an alternative
way of inspecting machine instructions; see *Note Source and Machine
Code: 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'.
When examining machine instructions, the instruction at current
program counter is shown with a `=>' marker. For example:
(gdb) x/5i $pc-6
0x804837f <main+11>: mov %esp,%ebp
0x8048381 <main+13>: push %ecx
0x8048382 <main+14>: sub $0x4,%esp
=> 0x8048385 <main+17>: movl $0x8048460,(%esp)
0x804838c <main+24>: call 0x80482d4 <puts@plt>
The addresses and contents printed by the `x' command are not saved
in the value history because there is often too much of them and they
would get in the way. Instead, GDB makes these values available for
subsequent use in expressions as values of the convenience variables
`$_' 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
(*note Remote Debugging::), 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]'
Compare the data of a loadable section SECTION-NAME in the
executable file of the program being debugged with the same
section in the remote machine's memory, and report any mismatches.
With no arguments, compares all loadable sections. This command's
availability depends on the target's support for the `"qCRC"'
remote request.

File: gdb.info, Node: Auto Display, Next: Print Settings, Prev: Memory, Up: Data
10.7 Automatic Display
======================
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 your format specification--it
uses `x' if you specify either the `i' or `s' format, or a unit size;
otherwise it uses `print'.
`display EXPR'
Add the expression EXPR to the list of expressions to display each
time your program stops. *Note Expressions: Expressions.
`display' does not repeat if you press <RET> again after using it.
`display/FMT EXPR'
For FMT specifying only a display format and not a size or count,
add the expression EXPR to the auto-display list but arrange to
display it each time in the specified format FMT. *Note Output
Formats: Output Formats.
`display/FMT ADDR'
For FMT `i' or `s', or including a unit-size or a number of units,
add the expression ADDR as a memory address to be examined each
time your program stops. Examining means in effect doing `x/FMT
ADDR'. *Note Examining Memory: Memory.
For 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; *note Registers: Registers.).
`undisplay DNUMS...'
`delete display DNUMS...'
Remove item numbers DNUMS from the list of expressions to display.
`undisplay' does not repeat if you press <RET> after using it.
(Otherwise you would just get the error `No display number ...'.)
`disable display DNUMS...'
Disable the display of item numbers DNUMS. A disabled display
item is not printed automatically, but is not forgotten. It may be
enabled again later.
`enable display DNUMS...'
Enable display of item numbers DNUMS. It becomes effective once
again in auto display of its expression, until you specify
otherwise.
`display'
Display the current values of the expressions on the list, just as
is done when your program stops.
`info display'
Print the list of expressions previously set up to display
automatically, each one with its item number, but without showing
the values. This includes disabled expressions, which are marked
as such. It also includes expressions which would not be
displayed right now because they refer to automatic variables not
currently available.
If 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.

File: gdb.info, Node: Print Settings, Next: Pretty Printing, Prev: Auto Display, Up: Data
10.8 Print Settings
===================
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 address'
`set print address on'
GDB prints memory addresses showing the location of stack traces,
structure values, pointer values, breakpoints, and so forth, even
when it also displays the contents of those addresses. The default
is `on'. 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 off'
Do not print addresses when displaying their contents. For
example, this is the same stack frame displayed with `set 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 address'
Show whether or not addresses are to be printed.
When 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 on'
Tell GDB to print the source file name and line number of a symbol
in the symbolic form of an address.
`set print symbol-filename off'
Do not print source file name and line number of a symbol. This
is the default.
`show print symbol-filename'
Show whether or not GDB will print the source file name and line
number of a symbol in the symbolic form of an address.
Another 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-OFFSET'
Tell GDB to only display the symbolic form of an address if the
offset between the closest earlier symbol and the address is less
than MAX-OFFSET. The default is 0, which tells GDB to always
print the symbolic form of an address if any symbol precedes it.
`show print max-symbolic-offset'
Ask how large the maximum offset is that GDB prints in a symbolic
address.
If 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 array'
`set print array on'
Pretty print arrays. This format is more convenient to read, but
uses more space. The default is off.
`set print array off'
Return to compressed format for arrays.
`show print array'
Show whether compressed or pretty format is selected for displaying
arrays.
`set print array-indexes'
`set print array-indexes on'
Print the index of each element when displaying arrays. May be
more convenient to locate a given element in the array or quickly
find the index of a given element in that printed array. The
default is off.
`set print array-indexes off'
Stop printing element indexes when displaying arrays.
`show print array-indexes'
Show whether the index of each element is printed when displaying
arrays.
`set print elements NUMBER-OF-ELEMENTS'
Set a limit on how many elements of an array GDB will print. If
GDB is printing a large array, it stops printing after it has
printed the number of elements set by the `set 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 elements'
Display the number of elements of a large array that GDB will
print. If the number is 0, then the printing is unlimited.
`set print frame-arguments VALUE'
This command allows to control how the values of arguments are
printed when the debugger prints a frame (*note Frames::). The
possible values are:
`all'
The values of all arguments are printed.
`scalars'
Print the value of an argument only if it is a scalar. The
value of more complex arguments such as arrays, structures,
unions, etc, is replaced by `...'. This is the default.
Here is an example where only scalar arguments are shown:
#1 0x08048361 in call_me (i=3, s=..., ss=0xbf8d508c, u=..., e=green)
at frame-args.c:23
`none'
None of the argument values are printed. Instead, the value
of each argument is replaced by `...'. In this case, the
example above now becomes:
#1 0x08048361 in call_me (i=..., s=..., ss=..., u=..., e=...)
at frame-args.c:23
By default, only scalar arguments are printed. This command can
be used to configure the debugger to print the value of all
arguments, regardless of their type. However, it is often
advantageous to not print the value of more complex parameters.
For instance, it reduces the amount of information printed in each
frame, making the backtrace more readable. Also, it improves
performance when displaying Ada frames, because the computation of
large arguments can sometimes be CPU-intensive, especially in
large applications. Setting `print frame-arguments' to `scalars'
(the default) or `none' avoids this computation, thus speeding up
the display of each Ada frame.
`show print frame-arguments'
Show how the value of arguments should be displayed when printing
a frame.
`set print repeats'
Set the threshold for suppressing display of repeated array
elements. When the number of consecutive identical elements of an
array exceeds the threshold, GDB prints the string `"<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 repeats'
Display the current threshold for printing repeated identical
elements.
`set print null-stop'
Cause GDB to stop printing the characters of an array when the
first NULL is encountered. This is useful when large arrays
actually contain only short strings. The default is off.
`show print null-stop'
Show whether GDB stops printing an array on the first NULL
character.
`set print pretty on'
Cause GDB to print structures in an indented format with one member
per line, like this:
$1 = {
next = 0x0,
flags = {
sweet = 1,
sour = 1
},
meat = 0x54 "Pork"
}
`set print pretty off'
Cause GDB to print structures in a compact format, like this:
$1 = {next = 0x0, flags = {sweet = 1, sour = 1}, \
meat = 0x54 "Pork"}
This is the default format.
`show print pretty'
Show which format GDB is using to print structures.
`set print sevenbit-strings on'
Print using only seven-bit characters; if this option is set, GDB
displays any eight-bit characters (in strings or character values)
using the notation `\'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 off'
Print full eight-bit characters. This allows the use of more
international character sets, and is the default.
`show print sevenbit-strings'
Show whether or not GDB is printing only seven-bit characters.
`set print union on'
Tell GDB to print unions which are contained in structures and
other unions. This is the default setting.
`set print union off'
Tell GDB not to print unions which are contained in structures and
other unions. GDB will print `"{...}"' instead.
`show print union'
Ask GDB whether or not it will print unions which are contained in
structures and other unions.
For 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 demangle'
`set print demangle on'
Print C++ names in their source form rather than in the encoded
("mangled") form passed to the assembler and linker for type-safe
linkage. The default is on.
`show print demangle'
Show whether C++ names are printed in mangled or demangled form.
`set print asm-demangle'
`set print asm-demangle on'
Print C++ names in their source form rather than their mangled
form, even in assembler code printouts such as instruction
disassemblies. The default is off.
`show print asm-demangle'
Show whether C++ names in assembly listings are printed in mangled
or demangled form.
`set demangle-style STYLE'
Choose among several encoding schemes used by different compilers
to represent C++ names. The choices for STYLE are currently:
`auto'
Allow GDB to choose a decoding style by inspecting your
program.
`gnu'
Decode based on the GNU C++ compiler (`g++') encoding
algorithm. This is the default.
`hp'
Decode based on the HP ANSI C++ (`aCC') encoding algorithm.
`lucid'
Decode based on the Lucid C++ compiler (`lcc') encoding
algorithm.
`arm'
Decode using the algorithm in the `C++ Annotated Reference
Manual'. *Warning:* this setting alone is not sufficient to
allow debugging `cfront'-generated executables. GDB would
require further enhancement to permit that.
If you omit STYLE, you will see a list of possible formats.
`show demangle-style'
Display the encoding style currently in use for decoding C++
symbols.
`set print object'
`set print object on'
When displaying a pointer to an object, identify the _actual_
(derived) type of the object rather than the _declared_ type, using
the virtual function table.
`set print object off'
Display only the declared type of objects, without reference to the
virtual function table. This is the default setting.
`show print object'
Show whether actual, or declared, object types are displayed.
`set print static-members'
`set print static-members on'
Print static members when displaying a C++ object. The default is
on.
`set print static-members off'
Do not print static members when displaying a C++ object.
`show print static-members'
Show whether C++ static members are printed or not.
`set print pascal_static-members'
`set print pascal_static-members on'
Print static members when displaying a Pascal object. The default
is on.
`set print pascal_static-members off'
Do not print static members when displaying a Pascal object.
`show print pascal_static-members'
Show whether Pascal static members are printed or not.
`set print vtbl'
`set print vtbl on'
Pretty print C++ virtual function tables. The default is off.
(The `vtbl' commands do not work on programs compiled with the HP
ANSI C++ compiler (`aCC').)
`set print vtbl off'
Do not pretty print C++ virtual function tables.
`show print vtbl'
Show whether C++ virtual function tables are pretty printed, or
not.

File: gdb.info, Node: Pretty Printing, Next: Value History, Prev: Print Settings, Up: Data
10.9 Pretty Printing
====================
GDB provides a mechanism to allow pretty-printing of values using
Python code. It greatly simplifies the display of complex objects.
This mechanism works for both MI and the CLI.
For example, here is how a C++ `std::string' looks without a
pretty-printer:
(gdb) print s
$1 = {
static npos = 4294967295,
_M_dataplus = {
<std::allocator<char>> = {
<__gnu_cxx::new_allocator<char>> = {
<No data fields>}, <No data fields>
},
members of std::basic_string<char, std::char_traits<char>,
std::allocator<char> >::_Alloc_hider:
_M_p = 0x804a014 "abcd"
}
}
With a pretty-printer for `std::string' only the contents are
printed:
(gdb) print s
$2 = "abcd"
For implementing pretty printers for new types you should read the
Python API details (*note Pretty Printing API::).

File: gdb.info, Node: Value History, Next: Convenience Vars, Prev: Pretty Printing, Up: Data
10.10 Value History
===================
Values 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 `$$0' is equivalent to `$'.
For example, suppose you have just printed a pointer to a structure
and want to see the contents of the structure. It suffices to type
p *$
If you have a chain of structures where the component `next' points
to the next one, you can print the contents of the next one with this:
p *$.next
You can print successive links in the chain by repeating this
command--which you can do by just typing <RET>.
Note that the history records values, not expressions. If the value
of `x' is 4 and you type these commands:
print x
set x=5
then the value recorded in the value history by the `print' command
remains 4 even though the value of `x' has changed.
`show values'
Print the last ten values in the value history, with their item
numbers. This is like `p $$9' repeated ten times, except that
`show values' does not change the history.
`show values N'
Print ten history values centered on history item number N.
`show values +'
Print ten history values just after the values last printed. If
no more values are available, `show values +' produces no display.
Pressing <RET> to repeat `show values N' has exactly the same effect
as `show values +'.

File: gdb.info, Node: Convenience Vars, Next: Registers, Prev: Value History, Up: Data
10.11 Convenience Variables
===========================
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 (*note Registers:
Registers.). (Value history references, in contrast, are _numbers_
preceded by `$'. *Note Value History: 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 convenience'
Print a list of convenience variables used so far, and their
values. Abbreviated `show conv'.
`init-if-undefined $VARIABLE = EXPRESSION'
Set a convenience variable if it has not already been set. This
is useful for user-defined commands that keep some state. It is
similar, in concept, to using local static variables with
initializers in C (except that convenience variables are global).
It can also be used to allow users to override default values used
in a command script.
If the variable is already defined then the expression is not
evaluated so any side-effects do not occur.
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.
`$_'
The variable `$_' is automatically set by the `x' command to the
last address examined (*note Examining Memory: 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
`$__'.
`$__'
The variable `$__' is automatically set by the `x' command to the
value found in the last address examined. Its type is chosen to
match the format in which the data was printed.
`$_exitcode'
The variable `$_exitcode' is automatically set to the exit code
when the program being debugged terminates.
`$_sdata'
The variable `$_sdata' contains extra collected static tracepoint
data. *Note Tracepoint Action Lists: Tracepoint Actions. Note
that `$_sdata' could be empty, if not inspecting a trace buffer, or
if extra static tracepoint data has not been collected.
`$_siginfo'
The variable `$_siginfo' contains extra signal information (*note
extra signal information::). Note that `$_siginfo' could be
empty, if the application has not yet received any signals. For
example, it will be empty before you execute the `run' command.
`$_tlb'
The variable `$_tlb' is automatically set when debugging
applications running on MS-Windows in native mode or connected to
gdbserver that supports the `qGetTIBAddr' request. *Note General
Query Packets::. This variable contains the address of the thread
information block.
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.
GDB also supplies some "convenience functions". These have a syntax
similar to convenience variables. A convenience function can be used
in an expression just like an ordinary function; however, a convenience
function is implemented internally to GDB.
`help function'
Print a list of all convenience functions.