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This is gdb.info, produced by makeinfo version 6.5 from gdb.texinfo.
Copyright (C) 1988-2020 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."
INFO-DIR-SECTION Software development
START-INFO-DIR-ENTRY
* Gdb: (gdb). The GNU debugger.
* gdbserver: (gdb) Server. The GNU debugging server.
END-INFO-DIR-ENTRY
This file documents the GNU debugger GDB.
This is the Tenth Edition, of 'Debugging with GDB: the GNU
Source-Level Debugger' for GDB (GDB) Version 9.2.
Copyright (C) 1988-2020 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 Tenth Edition, for GDB (GDB) Version 9.2.
Copyright (C) 1988-2020 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.
* In-Process Agent:: In-Process Agent
* GDB Bugs:: Reporting bugs in GDB
* Command Line Editing:: Command Line Editing
* Using History Interactively:: Using History Interactively
* In Memoriam:: In Memoriam
* 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
* Index Section Format:: .gdb_index section format
* Man Pages:: Manual pages
* Copying:: GNU General Public License says
how you can copy and share GDB
* GNU Free Documentation License:: The license for this documentation
* Concept Index:: Index of GDB concepts
* Command and Variable Index:: Index of GDB commands, variables,
functions, and Python data types

File: gdb.info, Node: Summary, Next: Sample Session, 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.
Support for OpenCL C is partial. For information on OpenCL C, see
*note OpenCL C: OpenCL C.
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
* Free Documentation:: Free Software Needs Free Documentation
* Contributors:: Contributors to GDB

File: gdb.info, Node: Free Software, Next: Free Documentation, 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.

File: gdb.info, Node: Free Documentation, Next: Contributors, Prev: Free Software, Up: Summary
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 Documentation, 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.
Initial support for the FreeBSD/mips target and native configuration
was developed by SRI International and the University of Cambridge
Computer Laboratory under DARPA/AFRL contract FA8750-10-C-0237
("CTSRD"), as part of the DARPA CRASH research programme.
Initial support for the FreeBSD/riscv target and native configuration
was developed by SRI International and the University of Cambridge
Computer Laboratory (Department of Computer Science and Technology)
under DARPA contract HR0011-18-C-0016 ("ECATS"), as part of the DARPA
SSITH research programme.
The original port to the OpenRISC 1000 is believed to be due to
Alessandro Forin and Per Bothner. More recent ports have been the work
of Jeremy Bennett, Franck Jullien, Stefan Wallentowitz and Stafford
Horne.

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 9.2, 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 or use
option '-p', if you want to debug a running process:
gdb PROGRAM 1234
gdb -p 1234
would attach GDB to process '1234'. With option '-p' you can omit the
PROGRAM filename.
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' (or
'-q'/'--quiet'):
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
'-init-command FILE'
'-ix FILE'
Execute commands from file FILE before loading the inferior (but
after loading gdbinit files). *Note Startup::.
'-init-eval-command COMMAND'
'-iex COMMAND'
Execute a single GDB command before loading the inferior (but after
loading gdbinit files). *Note Startup::.
'-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.
'--readnever'
Do not read each symbol file's symbolic debug information. This
makes startup faster but at the expense of not being able to
perform symbolic debugging. DWARF unwind information is also not
read, meaning backtraces may become incomplete or inaccurate. One
use of this is when a user simply wants to do the following
sequence: attach, dump core, detach. Loading the debugging
information in this case is an unnecessary cause of delay.

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 file. There
are three init files, loaded in the following order:
'system.gdbinit'
This is the system-wide init file. Its location is specified
with the '--with-system-gdbinit' configure option (*note
System-wide configuration::). It is loaded first when GDB
starts, before command line options have been processed.
'system.gdbinit.d'
This is the system-wide init directory. Its location is
specified with the '--with-system-gdbinit-dir' configure
option (*note System-wide configuration::). Files in this
directory are loaded in alphabetical order immediately after
system.gdbinit (if enabled) when GDB starts, before command
line options have been processed. Files need to have a
recognized scripting language extension ('.py'/'.scm') or be
named with a '.gdb' extension to be interpreted as regular GDB
commands. GDB will not recurse into any subdirectories of
this directory.
'~/.gdbinit'
This is the init file in your home directory. It is loaded
next, after 'system.gdbinit', and before command options have
been processed.
'./.gdbinit'
This is the init file in the current directory. It is loaded
last, after command line options other than '-x' and '-ex'
have been processed. Command line options '-x' and '-ex' are
processed last, after './.gdbinit' has been loaded.
For further documentation on startup processing, *Note Startup::.
For documentation on how to write command files, *Note Command
Files: Command Files.
'-nh'
Do not execute commands found in '~/.gdbinit', the init file in
your home directory. *Note Startup::.
'-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.
'-data-directory DIRECTORY'
'-D DIRECTORY'
Run GDB using DIRECTORY as its data directory. The data directory
is where GDB searches for its auxiliary files. *Note Data Files::.
'-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.
'-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.). 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=mi3') causes GDB to use the
"GDB/MI interface" version 3 (*note The GDB/MI Interface: GDB/MI.)
included since GDB version 9.1. GDB/MI version 2 ('mi2'), included
in GDB 6.0 and version 1 ('mi1'), included in GDB 5.3, are also
available. 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.
'-configuration'
This option causes GDB to print details about its build-time
configuration parameters, and then exit. These details can be
important when reporting GDB bugs (*note GDB Bugs::).

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 the files in the
system-wide gdbinit directory (if '--with-system-gdbinit-dir' was
used) and executes all the commands in those files. The files need
to be named with a '.gdb' extension to be interpreted as GDB
commands, or they can be written in a supported scripting language
with an appropriate file extension.
3. Reads the init file (if any) in your home directory(1) and executes
all the commands in that file.
4. Executes commands and command files specified by the '-iex' and
'-ix' options in their specified order. Usually you should use the
'-ex' and '-x' options instead, but this way you can apply settings
before GDB init files get executed and before inferior gets loaded.
5. Processes command line options and operands.
6. Reads and executes the commands from init file (if any) in the
current working directory as long as 'set auto-load local-gdbinit'
is set to 'on' (*note Init File in the Current 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.
7. If the command line specified a program to debug, or a process to
attach to, or a core file, GDB loads any auto-loaded scripts
provided for the program or for its loaded shared libraries. *Note
Auto-loading::.
If you wish to disable the auto-loading during startup, you must do
something like the following:
$ gdb -iex "set auto-load python-scripts off" myprogram
Option '-ex' does not work because the auto-loading is then turned
off too late.
8. Executes commands and command files specified by the '-ex' and '-x'
options in their specified order. *Note Command Files::, for more
details about GDB command files.
9. 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 port of GDB uses the standard
name, but if it finds a 'gdb.ini' file in your home directory, it warns
you about that and suggests 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'
'!COMMAND-STRING'
Invoke a standard shell to execute COMMAND-STRING. Note that no
space is needed between '!' and 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'.
'pipe [COMMAND] | SHELL_COMMAND'
'| [COMMAND] | SHELL_COMMAND'
'pipe -d DELIM COMMAND DELIM SHELL_COMMAND'
'| -d DELIM COMMAND DELIM SHELL_COMMAND'
Executes COMMAND and sends its output to SHELL_COMMAND. Note that
no space is needed around '|'. If no COMMAND is provided, the last
command executed is repeated.
In case the COMMAND contains a '|', the option '-d DELIM' can be
used to specify an alternate delimiter string DELIM that separates
the COMMAND from the SHELL_COMMAND.
Example:
(gdb) p var
$1 = {
black = 144,
red = 233,
green = 377,
blue = 610,
white = 987
}
(gdb) pipe p var|wc
7 19 80
(gdb) |p var|wc -l
7
(gdb) p /x var
$4 = {
black = 0x90,
red = 0xe9,
green = 0x179,
blue = 0x262,
white = 0x3db
}
(gdb) ||grep red
red => 0xe9,
(gdb) | -d ! echo this contains a | char\n ! sed -e 's/|/PIPE/'
this contains a PIPE char
(gdb) | -d xxx echo this contains a | char!\n xxx sed -e 's/|/PIPE/'
this contains a PIPE char!
(gdb)
The convenience variables '$_shell_exitcode' and '$_shell_exitsignal'
can be used to examine the exit status of the last shell command
launched by 'shell', 'make', 'pipe' and '|'. *Note Convenience
Variables: Convenience Vars.

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.
'set logging debugredirect [on|off]'
By default, GDB debug output will go to both the terminal and the
logfile. Set 'debugredirect' if you want debug output to go only
to the log file.
'show logging'
Show the current values of the logging settings.
You can also redirect the output of a GDB command to a shell command.
*Note pipe::.

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
* Command Settings:: How to change default behavior of commands
* Completion:: Command completion
* Command Options:: Command options
* Help:: How to ask GDB for help

File: gdb.info, Node: Command Syntax, Next: Command Settings, 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: Command Settings, Next: Completion, Prev: Command Syntax, Up: Commands
3.2 Command Settings
====================
Many commands change their behavior according to command-specific
variables or settings. These settings can be changed with the 'set'
subcommands. For example, the 'print' command (*note Examining Data:
Data.) prints arrays differently depending on settings changeable with
the commands 'set print elements NUMBER-OF-ELEMENTS' and 'set print
array-indexes', among others.
You can change these settings to your preference in the gdbinit files
loaded at GDB startup. *Note Startup::.
The settings can also be changed interactively during the debugging
session. For example, to change the limit of array elements to print,
you can do the following:
(GDB) set print elements 10
(GDB) print some_array
$1 = {0, 10, 20, 30, 40, 50, 60, 70, 80, 90...}
The above 'set print elements 10' command changes the number of
elements to print from the default of 200 to 10. If you only intend
this limit of 10 to be used for printing 'some_array', then you must
restore the limit back to 200, with 'set print elements 200'.
Some commands allow overriding settings with command options. For
example, the 'print' command supports a number of options that allow
overriding relevant global print settings as set by 'set print'
subcommands. *Note print options::. The example above could be
rewritten as:
(GDB) print -elements 10 -- some_array
$1 = {0, 10, 20, 30, 40, 50, 60, 70, 80, 90...}
Alternatively, you can use the 'with' command to change a setting
temporarily, for the duration of a command invocation.
'with SETTING [VALUE] [-- COMMAND]'
'w SETTING [VALUE] [-- COMMAND]'
Temporarily set SETTING to VALUE for the duration of COMMAND.
SETTING is any setting you can change with the 'set' subcommands.
VALUE is the value to assign to 'setting' while running 'command'.
If no COMMAND is provided, the last command executed is repeated.
If a COMMAND is provided, it must be preceded by a double dash
('--') separator. This is required because some settings accept
free-form arguments, such as expressions or filenames.
For example, the command
(GDB) with print array on -- print some_array
is equivalent to the following 3 commands:
(GDB) set print array on
(GDB) print some_array
(GDB) set print array off
The 'with' command is particularly useful when you want to override
a setting while running user-defined commands, or commands defined
in Python or Guile. *Note Extending GDB: Extending GDB.
(GDB) with print pretty on -- my_complex_command
To change several settings for the same command, you can nest
'with' commands. For example, 'with language ada -- with print
elements 10' temporarily changes the language to Ada and sets a
limit of 10 elements to print for arrays and strings.

File: gdb.info, Node: Completion, Next: Command Options, Prev: Command Settings, Up: Commands
3.3 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, command options, 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 '?'.
If the number of possible completions is large, GDB will print as
much of the list as it has collected, as well as a message indicating
that the list may be truncated.
(gdb) b m<TAB><TAB>
main
<... the rest of the possible completions ...>
*** List may be truncated, max-completions reached. ***
(gdb) b m
This behavior can be controlled with the following commands:
'set max-completions LIMIT'
'set max-completions unlimited'
Set the maximum number of completion candidates. GDB will stop
looking for more completions once it collects this many candidates.
This is useful when completing on things like function names as
collecting all the possible candidates can be time consuming. The
default value is 200. A value of zero disables tab-completion.
Note that setting either no limit or a very large limit can make
completion slow.
'show max-completions'
Show the maximum number of candidates that GDB will collect and
show during completion.
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.
A likely situation where you might need this is in typing an
expression that involves a C++ symbol name with template parameters.
This is because when completing expressions, GDB treats the '<'
character as word delimiter, assuming that it's the less-than comparison
operator (*note C and C++ Operators: C Operators.).
For example, when you want to call a C++ template function
interactively using the 'print' or 'call' commands, you may need to
distinguish whether you mean the version of 'name' that was specialized
for 'int', 'name<int>()', or the version that was specialized for
'float', '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) p 'func< M-?
func<int>() func<float>()
(gdb) p 'func<
When setting breakpoints however (*note Specify Location::), you
don't usually need to type a quote before the function name, because GDB
understands that you want to set a breakpoint on a function:
(gdb) b func< M-?
func<int>() func<float>()
(gdb) b func<
This is true even in the case of typing the name of C++ overloaded
functions (multiple definitions of the same function, distinguished by
argument type). For example, when you want to set a breakpoint you
don't 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)'.
(gdb) b bubble( M-?
bubble(int) bubble(double)
(gdb) b bubble(dou M-?
bubble(double)
See *note quoting names:: for a description of other scenarios that
require quoting.
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_fputs to_rewind
to_data to_isatty to_write
to_delete to_put to_write_async_safe
to_flush to_read
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_write_async_safe_ftype *to_write_async_safe;
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: Command Options, Next: Help, Prev: Completion, Up: Commands
3.4 Command options
===================
Some commands accept options starting with a leading dash. For example,
'print -pretty'. Similarly to command names, you can abbreviate a GDB
option to the first few letters of the option name, if that abbreviation
is unambiguous, and you can also use the <TAB> key to get GDB to fill
out the rest of a word in an option (or to show you the alternatives
available, if there is more than one possibility).
Some commands take raw input as argument. For example, the print
command processes arbitrary expressions in any of the languages
supported by GDB. With such commands, because raw input may start with
a leading dash that would be confused with an option or any of its
abbreviations, e.g. 'print -p' (short for 'print -pretty' or printing
negative 'p'?), if you specify any command option, then you must use a
double-dash ('--') delimiter to indicate the end of options.
Some options are described as accepting an argument which can be
either 'on' or 'off'. These are known as "boolean options". Similarly
to boolean settings commands--'on' and 'off' are the typical values, but
any of '1', 'yes' and 'enable' can also be used as "true" value, and any
of '0', 'no' and 'disable' can also be used as "false" value. You can
also omit a "true" value, as it is implied by default.
For example, these are equivalent:
(gdb) print -object on -pretty off -element unlimited -- *myptr
(gdb) p -o -p 0 -e u -- *myptr
You can discover the set of options some command accepts by
completing on '-' after the command name. For example:
(gdb) print -<TAB><TAB>
-address -max-depth -raw-values -union
-array -null-stop -repeats -vtbl
-array-indexes -object -static-members
-elements -pretty -symbol
Completion will in some cases guide you with a suggestion of what
kind of argument an option expects. For example:
(gdb) print -elements <TAB><TAB>
NUMBER unlimited
Here, the option expects a number (e.g., '100'), not literal
'NUMBER'. Such metasyntactical arguments are always presented in
uppercase.
(For more on using the 'print' command, see *note Examining Data:
Data.)

File: gdb.info, Node: Help, Prev: Command Options, Up: Commands
3.5 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 [-v] REGEXP'
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. The optional flag '-v', which
stands for 'verbose', indicates to output the full documentation of
the matching commands and highlight the parts of the documentation
matching REGEXP. For example:
apropos alias
results in:
alias -- Define a new command that is an alias of an existing command
aliases -- Aliases of other commands
d -- Delete some breakpoints or auto-display expressions
del -- Delete some breakpoints or auto-display expressions
delete -- Delete some breakpoints or auto-display expressions
while
apropos -v cut.*thread apply
results in the below output, where 'cut for 'thread apply' is
highlighted if styling is enabled.
taas -- Apply a command to all threads (ignoring errors
and empty output).
Usage: taas COMMAND
shortcut for 'thread apply all -s COMMAND'
tfaas -- Apply a command to all frames of all threads
(ignoring errors and empty output).
Usage: tfaas COMMAND
shortcut for 'thread apply all -s frame apply all -s COMMAND'
'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 Command, Variable, and Function Index point to all
the sub-commands. *Note Command and Variable 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 several 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.
'show configuration'
Display detailed information about the way GDB was configured when
it was built. This displays the optional arguments passed to the
'configure' script and also configuration parameters detected
automatically by 'configure'. When reporting a GDB bug (*note GDB
Bugs::), it is important to include this information in your
report.

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. Version 3.1 and later of GCC, the GNU C compiler, provides
macro information if you are using the DWARF debugging format, and
specify the option '-g3'.
*Note Options for Debugging Your Program or GCC: (gcc)Debugging
Options, for more information on GCC options affecting debug
information.
You will have the best debugging experience if you use the latest
version of the DWARF debugging format that your compiler supports.
DWARF is currently the most expressive and best supported debugging
format in GDB.

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 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. If you
do not define 'SHELL', GDB uses the default shell ('/bin/sh'). You
can disable use of any shell with the 'set startup-with-shell'
command (see below for details).
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._
You can set your program's working directory with the command 'set
cwd'. If you do not set any working directory with this command,
your program will inherit GDB's working directory if native
debugging, or the remote server's working directory if remote
debugging. *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, either
insert breakpoints in your elaboration code before running your
program or use the 'starti' command.
'starti'
The 'starti' command does the equivalent of setting a temporary
breakpoint at the first instruction of a program's execution and
then invoking the 'run' command. For programs containing an
elaboration phase, the 'starti' command will stop execution at the
start of the elaboration phase.
'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 startup-with-shell'
'set startup-with-shell on'
'set startup-with-shell off'
'show startup-with-shell'
On Unix systems, by default, if a shell is available on your
target, GDB) uses it to start your program. Arguments of the 'run'
command are passed to the shell, which does variable substitution,
expands wildcard characters and performs redirection of I/O. In
some circumstances, it may be useful to disable such use of a
shell, for example, when debugging the shell itself or diagnosing
startup failures such as:
(gdb) run
Starting program: ./a.out
During startup program terminated with signal SIGSEGV, Segmentation fault.
which indicates the shell or the wrapper specified with
'exec-wrapper' crashed, not your program. Most often, this is
caused by something odd in your shell's non-interactive mode
initialization file--such as '.cshrc' for C-shell, $'.zshenv' for
the Z shell, or the file specified in the 'BASH_ENV' environment
variable for BASH.
'set auto-connect-native-target'
'set auto-connect-native-target on'
'set auto-connect-native-target off'
'show auto-connect-native-target'
By default, if not connected to any target yet (e.g., with 'target
remote'), the 'run' command starts your program as a native process
under GDB, on your local machine. If you're sure you don't want to
debug programs on your local machine, you can tell GDB to not
connect to the native target automatically with the 'set
auto-connect-native-target off' command.
If 'on', which is the default, and if GDB is not connected to a
target already, the 'run' command automaticaly connects to the
native target, if one is available.
If 'off', and if GDB is not connected to a target already, the
'run' command fails with an error:
(gdb) run
Don't know how to run. Try "help target".
If GDB is already connected to a target, GDB always uses it with
the 'run' command.
In any case, you can explicitly connect to the native target with
the 'target native' command. For example,
(gdb) set auto-connect-native-target off
(gdb) run
Don't know how to run. Try "help target".
(gdb) target native
(gdb) run
Starting program: ./a.out
[Inferior 1 (process 10421) exited normally]
In case you connected explicitly to the 'native' target, GDB
remains connected even if all inferiors exit, ready for the next
'run' command. Use the 'disconnect' command to disconnect.
Examples of other commands that likewise respect the
'auto-connect-native-target' setting: 'attach', 'info proc', 'info
os'.
'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 certain targets, including
GNU/Linux. 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.
On targets where it is available, virtual address space
randomization protects the programs against certain 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 (and the shell GDB uses to launch it), not for GDB
itself. The 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.)
Note that on Unix systems, GDB runs your program via a shell, which
also inherits the environment set with 'set environment'. If
necessary, you can avoid that by using the 'env' program as a
wrapper instead of using 'set environment'. *Note set
exec-wrapper::, for an example doing just that.
Environment variables that are set by the user are also transmitted
to 'gdbserver' to be used when starting the remote inferior. *note
QEnvironmentHexEncoded::.
'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.
Environment variables that are unset by the user are also unset on
'gdbserver' when starting the remote inferior. *note
QEnvironmentUnset::.
_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 when started non-interactively--such as '.cshrc' for
C-shell, $'.zshenv' for the Z shell, or the file specified in the
'BASH_ENV' environment variable 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', the inferior will be
initialized with the current working directory specified by the 'set
cwd' command. If no directory has been specified by this command, then
the inferior will inherit GDB's current working directory as its working
directory if native debugging, or it will inherit the remote server's
current working directory if remote debugging.
'set cwd [DIRECTORY]'
Set the inferior's working directory to DIRECTORY, which will be
'glob'-expanded in order to resolve tildes ('~'). If no argument
has been specified, the command clears the setting and resets it to
an empty state. This setting has no effect on GDB's working
directory, and it only takes effect the next time you start the
inferior. The '~' in DIRECTORY is a short for the "home
directory", usually pointed to by the 'HOME' environment variable.
On MS-Windows, if 'HOME' is not defined, GDB uses the concatenation
of 'HOMEDRIVE' and 'HOMEPATH' as fallback.
You can also change GDB's current working directory by using the
'cd' command. *Note cd command::.
'show cwd'
Show the inferior's working directory. If no directory has been
specified by 'set cwd', then the default inferior's working
directory is the same as GDB's working directory.
'cd [DIRECTORY]'
Set the GDB working directory to DIRECTORY. If not given,
DIRECTORY uses ''~''.
The GDB working directory serves as a default for the commands that
specify files for GDB to operate on. *Note Commands to Specify
Files: Files. *Note set cwd command::.
'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 supports the 'info
proc' command (*note Process Information::), you can use the 'info proc'
command 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 [ TTY ]'
Set the tty for the program being debugged to TTY. Omitting TTY
restores the default behavior, which is to use the same terminal as
GDB.
'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. By
default all inferiors are printed, but the argument ID... - a space
separated list of inferior numbers - can be used to limit the
display to just the requested inferiors.
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.
The debugger convenience variable '$_inferior' contains the number of
the current inferior. You may find this useful in writing breakpoint
conditional expressions, command scripts, and so forth. *Note
Convenience Variables: Convenience Vars, for general information on
convenience variables.
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-inferiors' 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, and 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-inferiors INFNO...'
Removes the inferior or inferiors 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 inferiors' command:
'detach inferior INFNO...'
Detach from the inferior or inferiors identified by GDB inferior
number(s) 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 inferiors INFNO...'
Kill the inferior or inferiors identified by GDB inferior number(s)
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 inferiors', 'kill' or 'kill inferiors', 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.
Occasionally, 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
* 1 hello
2 goodbye
Bound inferiors: ID 1 (process 21561)
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 GNU/Linux 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 THREAD-ID', a command to switch among threads
* 'info threads', a command to inquire about existing threads
* 'thread apply [THREAD-ID-LIST | 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.
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]', where SYSTAG is a thread identifier whose form varies
depending on the particular system. For example, on GNU/Linux, you
might see
[New Thread 0x41e02940 (LWP 25582)]
when GDB notices a new thread. In contrast, on other systems, 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 of an inferior. This number is
unique between all threads of an inferior, but not unique between
threads of different inferiors.
You can refer to a given thread in an inferior using the qualified
INFERIOR-NUM.THREAD-NUM syntax, also known as "qualified thread ID",
with INFERIOR-NUM being the inferior number and THREAD-NUM being the
thread number of the given inferior. For example, thread '2.3' refers
to thread number 3 of inferior 2. If you omit INFERIOR-NUM (e.g.,
'thread 3'), then GDB infers you're referring to a thread of the current
inferior.
Until you create a second inferior, GDB does not show the
INFERIOR-NUM part of thread IDs, even though you can always use the full
INFERIOR-NUM.THREAD-NUM form to refer to threads of inferior 1, the
initial inferior.
Some commands accept a space-separated "thread ID list" as argument.
A list element can be:
1. A thread ID as shown in the first field of the 'info threads'
display, with or without an inferior qualifier. E.g., '2.1' or
'1'.
2. A range of thread numbers, again with or without an inferior
qualifier, as in INF.THR1-THR2 or THR1-THR2. E.g., '1.2-4' or
'2-4'.
3. All threads of an inferior, specified with a star wildcard, with or
without an inferior qualifier, as in INF.'*' (e.g., '1.*') or '*'.
The former refers to all threads of the given inferior, and the
latter form without an inferior qualifier refers to all threads of
the current inferior.
For example, if the current inferior is 1, and inferior 7 has one
thread with ID 7.1, the thread list '1 2-3 4.5 6.7-9 7.*' includes
threads 1 to 3 of inferior 1, thread 5 of inferior 4, threads 7 to 9 of
inferior 6 and all threads of inferior 7. That is, in expanded
qualified form, the same as '1.1 1.2 1.3 4.5 6.7 6.8 6.9 7.1'.
In addition to a _per-inferior_ number, each thread is also assigned
a unique _global_ number, also known as "global thread ID", a single
integer. Unlike the thread number component of the thread ID, no two
threads have the same global ID, even when you're debugging multiple
inferiors.
From GDB's perspective, a process always has at least one thread. In
other words, GDB assigns a thread number to the program's "main thread"
even if the program is not multi-threaded.
The debugger convenience variables '$_thread' and '$_gthread'
contain, respectively, the per-inferior thread number and the global
thread number of the current thread. You may find this useful in
writing breakpoint conditional expressions, command scripts, and so
forth. *Note Convenience Variables: Convenience Vars, for general
information on convenience variables.
If GDB detects the program is multi-threaded, it augments the usual
message about stopping at a breakpoint with the ID and name of the
thread that hit the breakpoint.
Thread 2 "client" hit Breakpoint 1, send_message () at client.c:68
Likewise when the program receives a signal:
Thread 1 "main" received signal SIGINT, Interrupt.
'info threads [THREAD-ID-LIST]'
Display information about one or more threads. With no arguments
displays information about all threads. You can specify the list
of threads that you want to display using the thread ID list syntax
(*note thread ID lists::).
GDB displays for each thread (in this order):
1. the per-inferior thread number assigned by GDB
2. the global thread number assigned by GDB, if the '-gid' option
was specified
3. the target system's thread identifier (SYSTAG)
4. the thread's name, if one is known. A thread can either be
named by the user (see 'thread name', below), or, in some
cases, by the program itself.
5. 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
Id Target Id Frame
* 1 process 35 thread 13 main (argc=1, argv=0x7ffffff8)
2 process 35 thread 23 0x34e5 in sigpause ()
3 process 35 thread 27 0x34e5 in sigpause ()
at threadtest.c:68
If you're debugging multiple inferiors, GDB displays thread IDs using
the qualified INFERIOR-NUM.THREAD-NUM format. Otherwise, only
THREAD-NUM is shown.
If you specify the '-gid' option, GDB displays a column indicating
each thread's global thread ID:
(gdb) info threads
Id GId Target Id Frame
1.1 1 process 35 thread 13 main (argc=1, argv=0x7ffffff8)
1.2 3 process 35 thread 23 0x34e5 in sigpause ()
1.3 4 process 35 thread 27 0x34e5 in sigpause ()
* 2.1 2 process 65 thread 1 main (argc=1, argv=0x7ffffff8)
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 THREAD-ID'
Make thread ID THREAD-ID the current thread. The command argument
THREAD-ID is the GDB thread ID, as shown in the first field of the
'info threads' display, with or without an inferior qualifier
(e.g., '2.1' or '1').
GDB responds by displaying the system identifier of the thread you
selected, and its current stack frame summary:
(gdb) thread 2
[Switching to thread 2 (Thread 0xb7fdab70 (LWP 12747))]
#0 some_function (ignore=0x0) at example.c:8
8 printf ("hello\n");
As with the '[New ...]' message, the form of the text after
'Switching to' depends on your system's conventions for identifying
threads.
'thread apply [THREAD-ID-LIST | all [-ascending]] [FLAG]... COMMAND'
The 'thread apply' command allows you to apply the named COMMAND to
one or more threads. Specify the threads that you want affected
using the thread ID list syntax (*note thread ID lists::), or
specify 'all' to apply to all threads. To apply a command to all
threads in descending order, type 'thread apply all COMMAND'. To
apply a command to all threads in ascending order, type 'thread
apply all -ascending COMMAND'.
The FLAG arguments control what output to produce and how to handle
errors raised when applying COMMAND to a thread. FLAG must start
with a '-' directly followed by one letter in 'qcs'. If several
flags are provided, they must be given individually, such as '-c
-q'.
By default, GDB displays some thread information before the output
produced by COMMAND, and an error raised during the execution of a
COMMAND will abort 'thread apply'. The following flags can be used
to fine-tune this behavior:
'-c'
The flag '-c', which stands for 'continue', causes any errors
in COMMAND to be displayed, and the execution of 'thread
apply' then continues.
'-s'
The flag '-s', which stands for 'silent', causes any errors or
empty output produced by a COMMAND to be silently ignored.
That is, the execution continues, but the thread information
and errors are not printed.
'-q'
The flag '-q' ('quiet') disables printing the thread
information.
Flags '-c' and '-s' cannot be used together.
'taas [OPTION]... COMMAND'
Shortcut for 'thread apply all -s [OPTION]... COMMAND'. Applies
COMMAND on all threads, ignoring errors and empty output.
The 'taas' command accepts the same options as the 'thread apply
all' command. *Note thread apply all::.
'tfaas [OPTION]... COMMAND'
Shortcut for 'thread apply all -s -- frame apply all -s [OPTION]...
COMMAND'. Applies COMMAND on all frames of all threads, ignoring
errors and empty output. Note that the flag '-s' is specified
twice: The first '-s' ensures that 'thread apply' only shows the
thread information of the threads for which 'frame apply' produces
some output. The second '-s' is needed to ensure that 'frame
apply' shows the frame information of a frame only if the COMMAND
successfully produced some output.
It can for example be used to print a local variable or a function
argument without knowing the thread or frame where this variable or
argument is, using:
(gdb) tfaas p some_local_var_i_do_not_remember_where_it_is
The 'tfaas' command accepts the same options as the 'frame apply'
command. *Note frame apply::.
'thread name [NAME]'
This command assigns a name to the current thread. If no argument
is given, any existing user-specified name is removed. The thread
name appears in the 'info threads' display.
On some systems, such as GNU/Linux, GDB is able to determine the
name of the thread as given by the OS. On these systems, a name
specified with 'thread name' will override the system-give name,
and removing the user-specified name will cause GDB to once again
display the system-specified name.
'thread find [REGEXP]'
Search for and display thread ids whose name or SYSTAG matches the
supplied regular expression.
As well as being the complement to the 'thread name' command, this
command also allows you to identify a thread by its target SYSTAG.
For instance, on GNU/Linux, the target SYSTAG is the LWP id.
(GDB) thread find 26688
Thread 4 has target id 'Thread 0x41e02940 (LWP 26688)'
(GDB) info thread 4
Id Target Id Frame
4 Thread 0x41e02940 (LWP 26688) 0x00000031ca6cd372 in select ()
'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 its default value
('$sdir:$pdir' on GNU/Linux and Solaris systems). Internally, the
default value comes from the 'LIBTHREAD_DB_SEARCH_PATH' macro.
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'. GDB also consults first if inferior specific
thread debugging library loading is enabled by 'set auto-load
libthread-db' (*note libthread_db.so.1 file::).
A special entry '$sdir' for 'libthread-db-search-path' refers to
the default system directories that are normally searched for
loading shared libraries. The '$sdir' entry is the only kind not
needing to be enabled by 'set auto-load libthread-db' (*note
libthread_db.so.1 file::).
A special entry '$pdir' for 'libthread-db-search-path' refers to
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.
'set debug libthread-db'
'show debug libthread-db'
Turns on or off display of 'libthread_db'-related events. Use '1'
to enable, '0' to disable.

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. On
GNU/Linux platforms, this feature is supported with kernel version
2.5.46 and later.
The fork debugging commands are supported in native mode and when
connected to 'gdbserver' in either 'target remote' mode or 'target
extended-remote' mode.
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 inferiors' command (allowing it to run
independently), or kill it using the 'kill inferiors' 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
1 <null> prog1
* 2 <null> prog2
'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
'follow-exec-mode' is supported in native mode and 'target
extended-remote' mode.
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
* Skipping Over Functions and Files::
Skipping over functions and files
* 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.
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 space-separated list of breakpoints on
which to operate. A list element can be either a single breakpoint
number, like '5', or a range of such numbers, like '5-7'. When a
breakpoint list is given to a command, all breakpoints in that list 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
* Dynamic Printf:: Dynamic printf
* Save Breakpoints:: How to save breakpoints in a file
* Static Probe Points:: Listing static probe points
* 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. The 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. The 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. The
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.
In programs using different languages, GDB chooses the syntax to
print the list of all breakpoints it sets according to the 'set
language' value: using 'set language auto' (see *note Set Language
Automatically: Automatically.) means to use the language of the
breakpoint's function, other values mean to use the manually
specified language (see *note Set Language Manually: Manually.).
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 [LIST...]'
'info break [LIST...]'
Print a table of all breakpoints, watchpoints, and catchpoints set
and not deleted. Optional argument N means print information only
about the specified breakpoint(s) (or watchpoint(s) or
catchpoint(s)). 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, there are two evaluation modes:
"host" and "target". If mode is "host", breakpoint condition
evaluation is done by GDB on the host's side. If it is "target",
then the condition is evaluated by the target. The 'info break'
command shows the condition on the line following the affected
breakpoint, together with its condition evaluation mode in between
parentheses.
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.
For a breakpoints with an enable count (xref) greater than 1, 'info
break' also displays that count.
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:
* Multiple functions in the program may have the same name.
* 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.
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
You cannot delete the individual locations from a breakpoint.
However, each location can be individually enabled or disabled by
passing BREAKPOINT-NUMBER.LOCATION-NUMBER as argument to the 'enable'
and 'disable' commands. It's also possible to 'enable' and 'disable' a
range of LOCATION-NUMBER locations using a BREAKPOINT-NUMBER and two
LOCATION-NUMBERs, in increasing order, separated by a hyphen, like
'BREAKPOINT-NUMBER.LOCATION-NUMBER1-LOCATION-NUMBER2', in which case GDB
acts on all the locations in the range (inclusive). 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. This is the default mode.
'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 deleted.
GDB handles conditional breakpoints by evaluating these conditions
when a breakpoint breaks. If the condition is true, then the process
being debugged stops, otherwise the process is resumed.
If the target supports evaluating conditions on its end, GDB may
download the breakpoint, together with its conditions, to it.
This feature can be controlled via the following commands:
'set breakpoint condition-evaluation host'
This option commands GDB to evaluate the breakpoint conditions on
the host's side. Unconditional breakpoints are sent to the target
which in turn receives the triggers and reports them back to GDB
for condition evaluation. This is the standard evaluation mode.
'set breakpoint condition-evaluation target'
This option commands GDB to download breakpoint conditions to the
target at the moment of their insertion. The target is responsible
for evaluating the conditional expression and reporting breakpoint
stop events back to GDB whenever the condition is true. Due to
limitations of target-side evaluation, some conditions cannot be
evaluated there, e.g., conditions that depend on local data that is
only known to the host. Examples include conditional expressions
involving convenience variables, complex types that cannot be
handled by the agent expression parser and expressions that are too
long to be sent over to the target, specially when the target is a
remote system. In these cases, the conditions will be evaluated by
GDB.
'set breakpoint condition-evaluation auto'
This is the default mode. If the target supports evaluating
breakpoint conditions on its end, GDB will download breakpoint
conditions to the target (limitations mentioned previously apply).
If the target does not support breakpoint condition evaluation,
then GDB will fallback to evaluating all these conditions on the
host's side.
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::).

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 most PowerPC or x86-based targets, GDB
includes support for hardware watchpoints, which do not slow down the
running of your program.
'watch [-l|-location] EXPR [thread THREAD-ID] [mask MASKVALUE]'
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 THREAD-ID]' argument, GDB breaks
only when the thread identified by THREAD-ID 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.
Ordinarily a watchpoint respects the scope of variables in EXPR
(see below). The '-location' argument tells GDB to instead watch
the memory referred to by EXPR. In this case, GDB will evaluate
EXPR, take the address of the result, and watch the memory at that
address. The type of the result is used to determine the size of
the watched memory. If the expression's result does not have an
address, then GDB will print an error.
The '[mask MASKVALUE]' argument allows creation of masked
watchpoints, if the current architecture supports this feature
(e.g., PowerPC Embedded architecture, see *note PowerPC
Embedded::.) A "masked watchpoint" specifies a mask in addition to
an address to watch. The mask specifies that some bits of an
address (the bits which are reset in the mask) should be ignored
when matching the address accessed by the inferior against the
watchpoint address. Thus, a masked watchpoint watches many
addresses simultaneously--those addresses whose unmasked bits are
identical to the unmasked bits in the watchpoint address. The
'mask' argument implies '-location'. Examples:
(gdb) watch foo mask 0xffff00ff
(gdb) watch *0xdeadbeef mask 0xffffff00
'rwatch [-l|-location] EXPR [thread THREAD-ID] [mask MASKVALUE]'
Set a watchpoint that will break when the value of EXPR is read by
the program.
'awatch [-l|-location] EXPR [thread THREAD-ID] [mask MASKVALUE]'
Set a watchpoint that will break when EXPR is either read from or
written into by the program.
'info watchpoints [LIST...]'
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. The EVENT can be any of the following:
'throw [REGEXP]'
'rethrow [REGEXP]'
'catch [REGEXP]'
The throwing, re-throwing, or catching of a C++ exception.
If REGEXP is given, then only exceptions whose type matches
the regular expression will be caught.
The convenience variable '$_exception' is available at an
exception-related catchpoint, on some systems. This holds the
exception being thrown.
There are currently some limitations to C++ exception handling
in GDB:
* The support for these commands is system-dependent.
Currently, only systems using the 'gnu-v3' C++ ABI (*note
ABI::) are supported.
* The regular expression feature and the '$_exception'
convenience variable rely on the presence of some SDT
probes in 'libstdc++'. If these probes are not present,
then these features cannot be used. These probes were
first available in the GCC 4.8 release, but whether or
not they are available in your GCC also depends on how it
was built.
* The '$_exception' convenience variable is only valid at
the instruction at which an exception-related catchpoint
is set.
* When an exception-related catchpoint is hit, GDB stops at
a location in the system library which implements runtime
exception support for C++, usually 'libstdc++'. You can
use 'up' (*note Selection::) to get to your code.
* If you call a function interactively, GDB normally
returns control to you when the function has finished
executing. If the call raises an exception, however, the
call may bypass the mechanism that returns control to you
and cause your program either to abort or to simply
continue running until it hits a breakpoint, catches a
signal that GDB is listening for, or exits. This is the
case even if you set a catchpoint for the exception;
catchpoints on exceptions are disabled within interactive
calls. *Note Calling::, for information on controlling
this with 'set unwind-on-terminating-exception'.
* You cannot raise an exception interactively.
* You cannot install an exception handler interactively.
'exception [NAME]'
An Ada exception being raised. If an exception name is
specified at the end of the command (eg 'catch exception
Program_Error'), the debugger will stop only when this
specific exception is raised. Otherwise, the debugger stops
execution when any Ada exception is raised.
When inserting an exception catchpoint on a user-defined
exception whose name is identical to one of the exceptions
defined by the language, the fully qualified name must be used
as the exception name. Otherwise, GDB will assume that it
should stop on the pre-defined exception rather than the
user-defined one. For instance, assuming an exception called
'Constraint_Error' is defined in package 'Pck', then the
command to use to catch such exceptions is 'catch exception
Pck.Constraint_Error'.
The convenience variable '$_ada_exception' holds the address
of the exception being thrown. This can be useful when
setting a condition for such a catchpoint.
'exception unhandled'
An exception that was raised but is not handled by the
program. The convenience variable '$_ada_exception' is set as
for 'catch exception'.
'handlers [NAME]'
An Ada exception being handled. If an exception name is
specified at the end of the command (eg 'catch handlers
Program_Error'), the debugger will stop only when this
specific exception is handled. Otherwise, the debugger stops
execution when any Ada exception is handled.
When inserting a handlers catchpoint on a user-defined
exception whose name is identical to one of the exceptions
defined by the language, the fully qualified name must be used
as the exception name. Otherwise, GDB will assume that it
should stop on the pre-defined exception rather than the
user-defined one. For instance, assuming an exception called
'Constraint_Error' is defined in package 'Pck', then the
command to use to catch such exceptions handling is 'catch
handlers Pck.Constraint_Error'.
The convenience variable '$_ada_exception' is set as for
'catch exception'.
'assert'
A failed Ada assertion. Note that the convenience variable
'$_ada_exception' is _not_ set by this catchpoint.
'exec'
A call to 'exec'.
'syscall'
'syscall [NAME | NUMBER | group:GROUPNAME | g:GROUPNAME] ...'
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).
You may specify a group of related syscalls to be caught at
once using the 'group:' syntax ('g:' is a shorter equivalent).
For instance, on some platforms GDB allows you to catch all
network related syscalls, by passing the argument
'group:network' to 'catch syscall'. Note that not all syscall
groups are available in every system. You can use the command
completion facilities (*note command completion: Completion.)
to list the syscall groups available on your environment.
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)
Here is an example of catching a syscall group:
(gdb) catch syscall group:process
Catchpoint 1 (syscalls 'exit' [1] 'fork' [2] 'waitpid' [7]
'execve' [11] 'wait4' [114] 'clone' [120] 'vfork' [190]
'exit_group' [252] 'waitid' [284] 'unshare' [310])
(gdb) r
Starting program: /tmp/catch-syscall
Catchpoint 1 (call to syscall fork), 0x00007ffff7df4e27 in open64 ()
from /lib64/ld-linux-x86-64.so.2
(gdb) c
Continuing.
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'.
'vfork'
A call to 'vfork'.
'load [REGEXP]'
'unload [REGEXP]'
The loading or unloading of a shared library. If REGEXP is
given, then the catchpoint will stop only if the regular
expression matches one of the affected libraries.
'signal [SIGNAL... | 'all']'
The delivery of a signal.
With no arguments, this catchpoint will catch any signal that
is not used internally by GDB, specifically, all signals
except 'SIGTRAP' and 'SIGINT'.
With the argument 'all', all signals, including those used by
GDB, will be caught. This argument cannot be used with other
signal names.
Otherwise, the arguments are a list of signal names as given
to 'handle' (*note Signals::). Only signals specified in this
list will be caught.
One reason that 'catch signal' can be more useful than
'handle' is that you can attach commands and conditions to the
catchpoint.
When a signal is caught by a catchpoint, the signal's 'stop'
and 'print' settings, as specified by 'handle', are ignored.
However, whether the signal is still delivered to the inferior
depends on the 'pass' setting; this can be changed in the
catchpoint's commands.
'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.

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] [LIST...]'
Delete the breakpoints, watchpoints, or catchpoints of the
breakpoint list specified as argument. 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 several
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 a count. The breakpoint stops your program for the
next N times, 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] [LIST...]'
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] [LIST...]'
Enable the specified breakpoints (or all defined breakpoints).
They become effective once again in stopping your program.
'enable [breakpoints] once LIST...'
Enable the specified breakpoints temporarily. GDB disables any of
these breakpoints immediately after stopping your program.
'enable [breakpoints] count COUNT LIST...'
Enable the specified breakpoints temporarily. GDB records COUNT
with each of the specified breakpoints, and decrements a
breakpoint's count when it is hit. When any count reaches 0, GDB
disables that breakpoint. If a breakpoint has an ignore count
(*note Break Conditions: Conditions.), that will be decremented to
0 before COUNT is affected.
'enable [breakpoints] delete LIST...'
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.).
Breakpoint conditions can also be evaluated on the target's side if
the target supports it. Instead of evaluating the conditions locally,
GDB encodes the expression into an agent expression (*note Agent
Expressions::) suitable for execution on the target, independently of
GDB. Global variables become raw memory locations, locals become stack
accesses, and so forth.
In this case, GDB will only be notified of a breakpoint trigger when
its condition evaluates to true. This mechanism may provide faster
response times depending on the performance characteristics of the
target since it does not need to keep GDB informed about every
breakpoint trigger, even those with false conditions.
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: Dynamic Printf, 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 [LIST...]'
'... 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: Dynamic Printf, Next: Save Breakpoints, Prev: Break Commands, Up: Breakpoints
5.1.8 Dynamic Printf
--------------------
The dynamic printf command 'dprintf' combines a breakpoint with
formatted printing of your program's data to give you the effect of
inserting 'printf' calls into your program on-the-fly, without having to
recompile it.
In its most basic form, the output goes to the GDB console. However,
you can set the variable 'dprintf-style' for alternate handling. For
instance, you can ask to format the output by calling your program's
'printf' function. This has the advantage that the characters go to the
program's output device, so they can recorded in redirects to files and
so forth.
If you are doing remote debugging with a stub or agent, you can also
ask to have the printf handled by the remote agent. In addition to
ensuring that the output goes to the remote program's device along with
any other output the program might produce, you can also ask that the
dprintf remain active even after disconnecting from the remote target.
Using the stub/agent is also more efficient, as it can do everything
without needing to communicate with GDB.
'dprintf LOCATION,TEMPLATE,EXPRESSION[,EXPRESSION...]'
Whenever execution reaches LOCATION, print the values of one or
more EXPRESSIONS under the control of the string TEMPLATE. To
print several values, separate them with commas.
'set dprintf-style STYLE'
Set the dprintf output to be handled in one of several different
styles enumerated below. A change of style affects all existing
dynamic printfs immediately. (If you need individual control over
the print commands, simply define normal breakpoints with
explicitly-supplied command lists.)
'gdb'
Handle the output using the GDB 'printf' command.
'call'
Handle the output by calling a function in your program
(normally 'printf').
'agent'
Have the remote debugging agent (such as 'gdbserver') handle
the output itself. This style is only available for agents
that support running commands on the target.
'set dprintf-function FUNCTION'
Set the function to call if the dprintf style is 'call'. By
default its value is 'printf'. You may set it to any expression.
that GDB can evaluate to a function, as per the 'call' command.
'set dprintf-channel CHANNEL'
Set a "channel" for dprintf. If set to a non-empty value, GDB will
evaluate it as an expression and pass the result as a first
argument to the 'dprintf-function', in the manner of 'fprintf' and
similar functions. Otherwise, the dprintf format string will be
the first argument, in the manner of 'printf'.
As an example, if you wanted 'dprintf' output to go to a logfile
that is a standard I/O stream assigned to the variable 'mylog', you
could do the following:
(gdb) set dprintf-style call
(gdb) set dprintf-function fprintf
(gdb) set dprintf-channel mylog
(gdb) dprintf 25,"at line 25, glob=%d\n",glob
Dprintf 1 at 0x123456: file main.c, line 25.
(gdb) info break
1 dprintf keep y 0x00123456 in main at main.c:25
call (void) fprintf (mylog,"at line 25, glob=%d\n",glob)
continue
(gdb)
Note that the 'info break' displays the dynamic printf commands as
normal breakpoint commands; you can thus easily see the effect of
the variable settings.
'set disconnected-dprintf on'
'set disconnected-dprintf off'
Choose whether 'dprintf' commands should continue to run if GDB has
disconnected from the target. This only applies if the
'dprintf-style' is 'agent'.
'show disconnected-dprintf off'
Show the current choice for disconnected 'dprintf'.
GDB does not check the validity of function and channel, relying on
you to supply values that are meaningful for the contexts in which they
are being used. For instance, the function and channel may be the
values of local variables, but if that is the case, then all enabled
dynamic prints must be at locations within the scope of those locals.
If evaluation fails, GDB will report an error.

File: gdb.info, Node: Save Breakpoints, Next: Static Probe Points, Prev: Dynamic Printf, Up: Breakpoints
5.1.9 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: Static Probe Points, Next: Error in Breakpoints, Prev: Save Breakpoints, Up: Breakpoints
5.1.10 Static Probe Points
--------------------------
GDB supports "SDT" probes in the code. SDT stands for Statically
Defined Tracing, and the probes are designed to have a tiny runtime code
and data footprint, and no dynamic relocations.
Currently, the following types of probes are supported on
ELF-compatible systems:
* 'SystemTap' (<http://sourceware.org/systemtap/>) SDT probes(1).
'SystemTap' probes are usable from assembly, C and C++
languages(2).
* 'DTrace' (<http://oss.oracle.com/projects/DTrace>) USDT probes.
'DTrace' probes are usable from C and C++ languages.
Some 'SystemTap' probes have an associated semaphore variable; for
instance, this happens automatically if you defined your probe using a
DTrace-style '.d' file. If your probe has a semaphore, GDB will
automatically enable it when you specify a breakpoint using the
'-probe-stap' notation. But, if you put a breakpoint at a probe's
location by some other method (e.g., 'break file:line'), then GDB will
not automatically set the semaphore. 'DTrace' probes do not support
semaphores.
You can examine the available static static probes using 'info
probes', with optional arguments:
'info probes [TYPE] [PROVIDER [NAME [OBJFILE]]]'
If given, TYPE is either 'stap' for listing 'SystemTap' probes or
'dtrace' for listing 'DTrace' probes. If omitted all probes are
listed regardless of their types.
If given, PROVIDER is a regular expression used to match against
provider names when selecting which probes to list. If omitted,
probes by all probes from all providers are listed.
If given, NAME is a regular expression to match against probe names
when selecting which probes to list. If omitted, probe names are
not considered when deciding whether to display them.
If given, OBJFILE is a regular expression used to select which
object files (executable or shared libraries) to examine. If not
given, all object files are considered.
'info probes all'
List the available static probes, from all types.
Some probe points can be enabled and/or disabled. The effect of
enabling or disabling a probe depends on the type of probe being
handled. Some 'DTrace' probes can be enabled or disabled, but
'SystemTap' probes cannot be disabled.
You can enable (or disable) one or more probes using the following
commands, with optional arguments:
'enable probes [PROVIDER [NAME [OBJFILE]]]'
If given, PROVIDER is a regular expression used to match against
provider names when selecting which probes to enable. If omitted,
all probes from all providers are enabled.
If given, NAME is a regular expression to match against probe names
when selecting which probes to enable. If omitted, probe names are
not considered when deciding whether to enable them.
If given, OBJFILE is a regular expression used to select which
object files (executable or shared libraries) to examine. If not
given, all object files are considered.
'disable probes [PROVIDER [NAME [OBJFILE]]]'
See the 'enable probes' command above for a description of the
optional arguments accepted by this command.
A probe may specify up to twelve arguments. These are available at
the point at which the probe is defined--that is, when the current PC is
at the probe's location. The arguments are available using the
convenience variables (*note Convenience Vars::)
'$_probe_arg0'...'$_probe_arg11'. In 'SystemTap' probes each probe
argument is an integer of the appropriate size; types are not preserved.
In 'DTrace' probes types are preserved provided that they are recognized
as such by GDB; otherwise the value of the probe argument will be a long
integer. The convenience variable '$_probe_argc' holds the number of
arguments at the current probe point.
These variables are always available, but attempts to access them at
any location other than a probe point will cause GDB to give an error
message.
---------- Footnotes ----------
(1) See
<http://sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps> for
more information on how to add 'SystemTap' SDT probes in your
applications.
(2) See
<http://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation> for
a good reference on how the SDT probes are implemented.

File: gdb.info, Node: Error in Breakpoints, Next: Breakpoint-related Warnings, Prev: Static Probe Points, Up: Breakpoints
5.1.11 "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
5.1.12 "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: Skipping Over Functions and Files, 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.), or you may step into the signal's
handler (*note stepping and signal handlers::).)
'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.).
'set print finish [on|off]'
'show print finish'
By default the 'finish' command will show the value that is
returned by the function. This can be disabled using 'set print
finish off'. When disabled, the value is still entered into the
value history (*note Value History::), but not displayed.
'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. The 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'.
By default, and if available, GDB makes use of target-assisted "range
stepping". In other words, whenever you use a stepping command (e.g.,
'step', 'next'), GDB tells the target to step the corresponding range of
instruction addresses instead of issuing multiple single-steps. This
speeds up line stepping, particularly for remote targets. Ideally,
there should be no reason you would want to turn range stepping off.
However, it's possible that a bug in the debug info, a bug in the remote
stub (for remote targets), or even a bug in GDB could make line stepping
behave incorrectly when target-assisted range stepping is enabled. You
can use the following command to turn off range stepping if necessary:
'set range-stepping'
'show range-stepping'
Control whether range stepping is enabled.
If 'on', and the target supports it, GDB tells the target to step a
range of addresses itself, instead of issuing multiple
single-steps. If 'off', GDB always issues single-steps, even if
range stepping is supported by the target. The default is 'on'.

File: gdb.info, Node: Skipping Over Functions and Files, Next: Signals, Prev: Continuing and Stepping, Up: Stopping
5.3 Skipping Over Functions and Files
=====================================
The program you are debugging may contain some functions which are
uninteresting to debug. The 'skip' command lets you tell GDB to skip a
function, all functions in a file or a particular function in a
particular file when stepping.
For example, consider the following C function:
101 int func()
102 {
103 foo(boring());
104 bar(boring());
105 }
Suppose you wish to step into the functions 'foo' and 'bar', but you are
not interested in stepping through 'boring'. If you run 'step' at line
103, you'll enter 'boring()', but if you run 'next', you'll step over
both 'foo' and 'boring'!
One solution is to 'step' into 'boring' and use the 'finish' command
to immediately exit it. But this can become tedious if 'boring' is
called from many places.
A more flexible solution is to execute 'skip boring'. This instructs
GDB never to step into 'boring'. Now when you execute 'step' at line
103, you'll step over 'boring' and directly into 'foo'.
Functions may be skipped by providing either a function name,
linespec (*note Specify Location::), regular expression that matches the
function's name, file name or a 'glob'-style pattern that matches the
file name.
On Posix systems the form of the regular expression is "Extended
Regular Expressions". See for example 'man 7 regex' on GNU/Linux
systems. On non-Posix systems the form of the regular expression is
whatever is provided by the 'regcomp' function of the underlying system.
See for example 'man 7 glob' on GNU/Linux systems for a description of
'glob'-style patterns.
'skip [OPTIONS]'
The basic form of the 'skip' command takes zero or more options
that specify what to skip. The OPTIONS argument is any useful
combination of the following:
'-file FILE'
'-fi FILE'
Functions in FILE will be skipped over when stepping.
'-gfile FILE-GLOB-PATTERN'
'-gfi FILE-GLOB-PATTERN'
Functions in files matching FILE-GLOB-PATTERN will be skipped
over when stepping.
(gdb) skip -gfi utils/*.c
'-function LINESPEC'
'-fu LINESPEC'
Functions named by LINESPEC or the function containing the
line named by LINESPEC will be skipped over when stepping.
*Note Specify Location::.
'-rfunction REGEXP'
'-rfu REGEXP'
Functions whose name matches REGEXP will be skipped over when
stepping.
This form is useful for complex function names. For example,
there is generally no need to step into C++ 'std::string'
constructors or destructors. Plus with C++ templates it can
be hard to write out the full name of the function, and often
it doesn't matter what the template arguments are. Specifying
the function to be skipped as a regular expression makes this
easier.
(gdb) skip -rfu ^std::(allocator|basic_string)<.*>::~?\1 *\(
If you want to skip every templated C++ constructor and
destructor in the 'std' namespace you can do:
(gdb) skip -rfu ^std::([a-zA-z0-9_]+)<.*>::~?\1 *\(
If no options are specified, the function you're currently
debugging will be skipped.
'skip function [LINESPEC]'
After running this command, the function named by LINESPEC or the
function containing the line named by LINESPEC will be skipped over
when stepping. *Note Specify Location::.
If you do not specify LINESPEC, the function you're currently
debugging will be skipped.
(If you have a function called 'file' that you want to skip, use
'skip function file'.)
'skip file [FILENAME]'
After running this command, any function whose source lives in
FILENAME will be skipped over when stepping.
(gdb) skip file boring.c
File boring.c will be skipped when stepping.
If you do not specify FILENAME, functions whose source lives in the
file you're currently debugging will be skipped.
Skips can be listed, deleted, disabled, and enabled, much like
breakpoints. These are the commands for managing your list of skips:
'info skip [RANGE]'
Print details about the specified skip(s). If RANGE is not
specified, print a table with details about all functions and files
marked for skipping. 'info skip' prints the following information
about each skip:
_Identifier_
A number identifying this skip.
_Enabled or Disabled_
Enabled skips are marked with 'y'. Disabled skips are marked
with 'n'.
_Glob_
If the file name is a 'glob' pattern this is 'y'. Otherwise
it is 'n'.
_File_
The name or 'glob' pattern of the file to be skipped. If no
file is specified this is '<none>'.
_RE_
If the function name is a 'regular expression' this is 'y'.
Otherwise it is 'n'.
_Function_
The name or regular expression of the function to skip. If no
function is specified this is '<none>'.
'skip delete [RANGE]'
Delete the specified skip(s). If RANGE is not specified, delete
all skips.
'skip enable [RANGE]'
Enable the specified skip(s). If RANGE is not specified, enable
all skips.
'skip disable [RANGE]'
Disable the specified skip(s). If RANGE is not specified, disable
all skips.
'set debug skip [on|off]'
Set whether to print the debug output about skipping files and
functions.
'show debug skip'
Show whether the debug output about skipping files and functions is
printed.

File: gdb.info, Node: Signals, Next: Thread Stops, Prev: Skipping Over Functions and Files, Up: Stopping
5.4 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'.
'catch signal [SIGNAL... | 'all']'
Set a catchpoint for the indicated signals. *Note Set
Catchpoints::, for details about this command.
'handle SIGNAL [KEYWORDS...]'
Change the way GDB handles signal SIGNAL. The 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.
GDB optimizes for stepping the mainline code. If a signal that has
'handle nostop' and 'handle pass' set arrives while a stepping command
(e.g., 'stepi', 'step', 'next') is in progress, GDB lets the signal
handler run and then resumes stepping the mainline code once the signal
handler returns. In other words, GDB steps over the signal handler.
This prevents signals that you've specified as not interesting (with
'handle nostop') from changing the focus of debugging unexpectedly.
Note that the signal handler itself may still hit a breakpoint, stop for
another signal that has 'handle stop' in effect, or for any other event
that normally results in stopping the stepping command sooner. Also
note that GDB still informs you that the program received a signal if
'handle print' is set.
If you set 'handle pass' for a signal, and your program sets up a
handler for it, then issuing a stepping command, such as 'step' or
'stepi', when your program is stopped due to the signal will step _into_
the signal handler (if the target supports that).
Likewise, if you use the 'queue-signal' command to queue a signal to
be delivered to the current thread when execution of the thread resumes
(*note Giving your Program a Signal: Signaling.), then a stepping
command will step into the signal handler.
Here's an example, using 'stepi' to step to the first instruction of
'SIGUSR1''s handler:
(gdb) handle SIGUSR1
Signal Stop Print Pass to program Description
SIGUSR1 Yes Yes Yes User defined signal 1
(gdb) c
Continuing.
Program received signal SIGUSR1, User defined signal 1.
main () sigusr1.c:28
28 p = 0;
(gdb) si
sigusr1_handler () at sigusr1.c:9
9 {
The same, but using 'queue-signal' instead of waiting for the program
to receive the signal first:
(gdb) n
28 p = 0;
(gdb) queue-signal SIGUSR1
(gdb) si
sigusr1_handler () at sigusr1.c:9
9 {
(gdb)
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.
On some targets, a 'SIGSEGV' can be caused by a boundary violation,
i.e., accessing an address outside of the allowed range. In those cases
GDB may displays additional information, depending on how GDB has been
told to handle the signal. With 'handle stop SIGSEGV', GDB displays the
violation kind: "Upper" or "Lower", the memory address accessed and the
bounds, while with 'handle nostop SIGSEGV' no additional information is
displayed.
The usual output of a segfault is:
Program received signal SIGSEGV, Segmentation fault
0x0000000000400d7c in upper () at i386-mpx-sigsegv.c:68
68 value = *(p + len);
While a bound violation is presented as:
Program received signal SIGSEGV, Segmentation fault
Upper bound violation while accessing address 0x7fffffffc3b3
Bounds: [lower = 0x7fffffffc390, upper = 0x7fffffffc3a3]
0x0000000000400d7c in upper () at i386-mpx-sigsegv.c:68
68 value = *(p + len);

File: gdb.info, Node: Thread Stops, Prev: Signals, Up: Stopping
5.5 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
5.5.1 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. It applies to normal execution,
record mode, and replay 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
never get a chance to run when you step, 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. The 'replay' mode behaves like
'off' in record mode and like 'on' in replay mode.
'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
5.5.2 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:
# 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
5.5.3 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.
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
5.5.4 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 LOCATION thread THREAD-ID'
'break LOCATION thread THREAD-ID if ...'
LOCATION 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 THREAD-ID' with a breakpoint command to
specify that you only want GDB to stop the program when a
particular thread reaches this breakpoint. The THREAD-ID specifier
is one of the thread identifiers assigned by GDB, shown in the
first column of the 'info threads' display.
If you do not specify 'thread THREAD-ID' 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 THREAD-ID' before or after the
breakpoint condition, like this:
(gdb) break frik.c:13 thread 28 if bartab > lim
Thread-specific breakpoints are automatically deleted when GDB
detects the corresponding thread is no longer in the thread list. For
example:
(gdb) c
Thread-specific breakpoint 3 deleted - thread 28 no longer in the thread list.
There are several ways for a thread to disappear, such as a regular
thread exit, but also when you detach from the process with the 'detach'
command (*note Debugging an Already-running Process: Attach.), or if GDB
loses the remote connection (*note Remote Debugging::), etc. Note that
with some targets, GDB is only able to detect a thread has exited when
the user explictly asks for the thread list with the 'info threads'
command.

File: gdb.info, Node: Interrupted System Calls, Next: Observer Mode, Prev: Thread-Specific Breakpoints, Up: Thread Stops
5.5.5 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
5.5.6 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).
On some platforms, GDB has built-in support for reverse execution,
activated with the 'record' or 'record btrace' commands. *Note Process
Record and Replay::. Some remote targets, typically full system
emulators, support reverse execution directly without requiring any
special command.
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
consistent 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.
Currently, process record and replay is supported on ARM, Aarch64,
Moxie, PowerPC, PowerPC64, S/390, and x86 (i386/amd64) running
GNU/Linux. Process record and replay can be used both when native
debugging, and when remote debugging via 'gdbserver'.
For architecture environments that support process record and replay,
GDB provides the following commands:
'record METHOD'
This command starts the process record and replay target. The
recording method can be specified as parameter. Without a
parameter the command uses the 'full' recording method. The
following recording methods are available:
'full'
Full record/replay recording using GDB's software record and
replay implementation. This method allows replaying and
reverse execution.
'btrace FORMAT'
Hardware-supported instruction recording, supported on Intel
processors. This method does not record data. Further, the
data is collected in a ring buffer so old data will be
overwritten when the buffer is full. It allows limited
reverse execution. Variables and registers are not available
during reverse execution. In remote debugging, recording
continues on disconnect. Recorded data can be inspected after
reconnecting. The recording may be stopped using 'record
stop'.
The recording format can be specified as parameter. Without a
parameter the command chooses the recording format. The
following recording formats are available:
'bts'
Use the "Branch Trace Store" (BTS) recording format. In
this format, the processor stores a from/to record for
each executed branch in the btrace ring buffer.
'pt'
Use the "Intel Processor Trace" recording format. In
this format, the processor stores the execution trace in
a compressed form that is afterwards decoded by GDB.
The trace can be recorded with very low overhead. The
compressed trace format also allows small trace buffers
to already contain a big number of instructions compared
to BTS.
Decoding the recorded execution trace, on the other hand,
is more expensive than decoding BTS trace. This is
mostly due to the increased number of instructions to
process. You should increase the buffer-size with care.
Not all recording formats may be available on all processors.
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 'record METHOD' command.
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::),
not all recording methods are available. The 'full' recording
method does not 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 goto'
Go to a specific location in the execution log. There are several
ways to specify the location to go to:
'record goto begin'
'record goto start'
Go to the beginning of the execution log.
'record goto end'
Go to the end of the execution log.
'record goto N'
Go to instruction number N in the execution log.
'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.
This command may not be available for all recording methods.
'record restore FILENAME'
Restore the execution log from a file 'FILENAME'. File must have
been created with 'record save'.
'set record full insn-number-max LIMIT'
'set record full insn-number-max unlimited'
Set the limit of instructions to be recorded for the 'full'
recording method. 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 'unlimited' or zero, GDB will never delete recorded
instructions from the execution log. The number of recorded
instructions is limited only by the available memory.
'show record full insn-number-max'
Show the limit of instructions to be recorded with the 'full'
recording method.
'set record full stop-at-limit'
Control the behavior of the 'full' recording method 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 full stop-at-limit'
Show the current setting of 'stop-at-limit'.
'set record full memory-query'
Control the behavior when GDB is unable to record memory changes
caused by an instruction for the 'full' recording method. 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 full memory-query'
Show the current setting of 'memory-query'.
The 'btrace' record target does not trace data. As a convenience,
when replaying, GDB reads read-only memory off the live program
directly, assuming that the addresses of the read-only areas don't
change. This for example makes it possible to disassemble code
while replaying, but not to print variables. In some cases, being
able to inspect variables might be useful. You can use the
following command for that:
'set record btrace replay-memory-access'
Control the behavior of the 'btrace' recording method when
accessing memory during replay. If 'read-only' (the default), GDB
will only allow accesses to read-only memory. If 'read-write', GDB
will allow accesses to read-only and to read-write memory. Beware
that the accessed memory corresponds to the live target and not
necessarily to the current replay position.
'set record btrace cpu IDENTIFIER'
Set the processor to be used for enabling workarounds for processor
errata when decoding the trace.
Processor errata are defects in processor operation, caused by its
design or manufacture. They can cause a trace not to match the
specification. This, in turn, may cause trace decode to fail. GDB
can detect erroneous trace packets and correct them, thus avoiding
the decoding failures. These corrections are known as "errata
workarounds", and are enabled based on the processor on which the
trace was recorded.
By default, GDB attempts to detect the processor automatically, and
apply the necessary workarounds for it. However, you may need to
specify the processor if GDB does not yet support it. This command
allows you to do that, and also allows to disable the workarounds.
The argument IDENTIFIER identifies the CPU and is of the form:
'VENDOR:PROCESSOR IDENTIFIER'. In addition, there are two special
identifiers, 'none' and 'auto' (default).
The following vendor identifiers and corresponding processor
identifiers are currently supported:
'intel' FAMILY/MODEL[/STEPPING]
On GNU/Linux systems, the processor FAMILY, MODEL, and STEPPING can
be obtained from '/proc/cpuinfo'.
If IDENTIFIER is 'auto', enable errata workarounds for the
processor on which the trace was recorded. If IDENTIFIER is
'none', errata workarounds are disabled.
For example, when using an old GDB on a new system, decode may fail
because GDB does not support the new processor. It often suffices
to specify an older processor that GDB supports.
(gdb) info record
Active record target: record-btrace
Recording format: Intel Processor Trace.
Buffer size: 16kB.
Failed to configure the Intel Processor Trace decoder: unknown cpu.
(gdb) set record btrace cpu intel:6/158
(gdb) info record
Active record target: record-btrace
Recording format: Intel Processor Trace.
Buffer size: 16kB.
Recorded 84872 instructions in 3189 functions (0 gaps) for thread 1 (...).
'show record btrace replay-memory-access'
Show the current setting of 'replay-memory-access'.
'show record btrace cpu'
Show the processor to be used for enabling trace decode errata
workarounds.
'set record btrace bts buffer-size SIZE'
'set record btrace bts buffer-size unlimited'
Set the requested ring buffer size for branch tracing in BTS
format. Default is 64KB.
If SIZE is a positive number, then GDB will try to allocate a
buffer of at least SIZE bytes for each new thread that uses the
btrace recording method and the BTS format. The actually obtained
buffer size may differ from the requested SIZE. Use the 'info
record' command to see the actual buffer size for each thread that
uses the btrace recording method and the BTS format.
If LIMIT is 'unlimited' or zero, GDB will try to allocate a buffer
of 4MB.
Bigger buffers mean longer traces. On the other hand, GDB will
also need longer to process the branch trace data before it can be
used.
'show record btrace bts buffer-size SIZE'
Show the current setting of the requested ring buffer size for
branch tracing in BTS format.
'set record btrace pt buffer-size SIZE'
'set record btrace pt buffer-size unlimited'
Set the requested ring buffer size for branch tracing in Intel
Processor Trace format. Default is 16KB.
If SIZE is a positive number, then GDB will try to allocate a
buffer of at least SIZE bytes for each new thread that uses the
btrace recording method and the Intel Processor Trace format. The
actually obtained buffer size may differ from the requested SIZE.
Use the 'info record' command to see the actual buffer size for
each thread.
If LIMIT is 'unlimited' or zero, GDB will try to allocate a buffer
of 4MB.
Bigger buffers mean longer traces. On the other hand, GDB will
also need longer to process the branch trace data before it can be
used.
'show record btrace pt buffer-size SIZE'
Show the current setting of the requested ring buffer size for
branch tracing in Intel Processor Trace format.
'info record'
Show various statistics about the recording depending on the
recording method:
'full'
For the 'full' recording method, it shows 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.
'btrace'
For the 'btrace' recording method, it shows:
* Recording format.
* Number of instructions that have been recorded.
* Number of blocks of sequential control-flow formed by the
recorded instructions.
* Whether in record mode or replay mode.
For the 'bts' recording format, it also shows:
* Size of the perf ring buffer.
For the 'pt' recording format, it also shows:
* Size of the perf ring buffer.
'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".
'record instruction-history'
Disassembles instructions from the recorded execution log. By
default, ten instructions are disassembled. This can be changed
using the 'set record instruction-history-size' command.
Instructions are printed in execution order.
It can also print mixed source+disassembly if you specify the the
'/m' or '/s' modifier, and print the raw instructions in hex as
well as in symbolic form by specifying the '/r' modifier.
The current position marker is printed for the instruction at the
current program counter value. This instruction can appear
multiple times in the trace and the current position marker will be
printed every time. To omit the current position marker, specify
the '/p' modifier.
To better align the printed instructions when the trace contains
instructions from more than one function, the function name may be
omitted by specifying the '/f' modifier.
Speculatively executed instructions are prefixed with '?'. This
feature is not available for all recording formats.
There are several ways to specify what part of the execution log to
disassemble:
'record instruction-history INSN'
Disassembles ten instructions starting from instruction number
INSN.
'record instruction-history INSN, +/-N'
Disassembles N instructions around instruction number INSN.
If N is preceded with '+', disassembles N instructions after
instruction number INSN. If N is preceded with '-',
disassembles N instructions before instruction number INSN.
'record instruction-history'
Disassembles ten more instructions after the last disassembly.
'record instruction-history -'
Disassembles ten more instructions before the last
disassembly.
'record instruction-history BEGIN, END'
Disassembles instructions beginning with instruction number
BEGIN until instruction number END. The instruction number
END is included.
This command may not be available for all recording methods.
'set record instruction-history-size SIZE'
'set record instruction-history-size unlimited'
Define how many instructions to disassemble in the 'record
instruction-history' command. The default value is 10. A SIZE of
'unlimited' means unlimited instructions.
'show record instruction-history-size'
Show how many instructions to disassemble in the 'record
instruction-history' command.
'record function-call-history'
Prints the execution history at function granularity. It prints
one line for each sequence of instructions that belong to the same
function giving the name of that function, the source lines for
this instruction sequence (if the '/l' modifier is specified), and
the instructions numbers that form the sequence (if the '/i'
modifier is specified). The function names are indented to reflect
the call stack depth if the '/c' modifier is specified. The '/l',
'/i', and '/c' modifiers can be given together.
(gdb) list 1, 10
1 void foo (void)
2 {
3 }
4
5 void bar (void)
6 {
7 ...
8 foo ();
9 ...
10 }
(gdb) record function-call-history /ilc
1 bar inst 1,4 at foo.c:6,8
2 foo inst 5,10 at foo.c:2,3
3 bar inst 11,13 at foo.c:9,10
By default, ten lines are printed. This can be changed using the
'set record function-call-history-size' command. Functions are
printed in execution order. There are several ways to specify what
to print:
'record function-call-history FUNC'
Prints ten functions starting from function number FUNC.
'record function-call-history FUNC, +/-N'
Prints N functions around function number FUNC. If N is
preceded with '+', prints N functions after function number
FUNC. If N is preceded with '-', prints N functions before
function number FUNC.
'record function-call-history'
Prints ten more functions after the last ten-line print.
'record function-call-history -'
Prints ten more functions before the last ten-line print.
'record function-call-history BEGIN, END'
Prints functions beginning with function number BEGIN until
function number END. The function number END is included.
This command may not be available for all recording methods.
'set record function-call-history-size SIZE'
'set record function-call-history-size unlimited'
Define how many lines to print in the 'record
function-call-history' command. The default value is 10. A size
of 'unlimited' means unlimited lines.
'show record function-call-history-size'
Show how many lines to print in the 'record function-call-history'
command.

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
* Frame Apply:: Applying a command to several frames
* Frame Filter Management:: Managing frame filters

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 labels each existing stack frame with a "level", a number that is
zero for the innermost frame, one for the frame that called it, and so
on upward. These level numbers give you a way of designating stack
frames in GDB commands. The terms "frame number" and "frame level" can
be used interchangeably to describe this number.
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.

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.
To print a backtrace of the entire stack, use the 'backtrace'
command, or its alias 'bt'. This command will print one line per frame
for frames in the stack. By default, all stack frames are printed. You
can stop the backtrace at any time by typing the system interrupt
character, normally 'Ctrl-c'.
'backtrace [OPTION]... [QUALIFIER]... [COUNT]'
'bt [OPTION]... [QUALIFIER]... [COUNT]'
Print the backtrace of the entire stack.
The optional COUNT can be one of the following:
'N'
'N'
Print only the innermost N frames, where N is a positive
number.
'-N'
'-N'
Print only the outermost N frames, where N is a positive
number.
Options:
'-full'
Print the values of the local variables also. This can be
combined with the optional COUNT to limit the number of frames
shown.
'-no-filters'
Do not run Python frame filters on this backtrace. *Note
Frame Filter API::, for more information. Additionally use
*note disable frame-filter all:: to turn off all frame
filters. This is only relevant when GDB has been configured
with 'Python' support.
'-hide'
A Python frame filter might decide to "elide" some frames.
Normally such elided frames are still printed, but they are
indented relative to the filtered frames that cause them to be
elided. The '-hide' option causes elided frames to not be
printed at all.
The 'backtrace' command also supports a number of options that
allow overriding relevant global print settings as set by 'set
backtrace' and 'set print' subcommands:
'-past-main [on|off]'
Set whether backtraces should continue past 'main'. Related
setting: *note set backtrace past-main::.
'-past-entry [on|off]'
Set whether backtraces should continue past the entry point of
a program. Related setting: *note set backtrace past-entry::.
'-entry-values no|only|preferred|if-needed|both|compact|default'
Set printing of function arguments at function entry. Related
setting: *note set print entry-values::.
'-frame-arguments all|scalars|none'
Set printing of non-scalar frame arguments. Related setting:
*note set print frame-arguments::.
'-raw-frame-arguments [on|off]'
Set whether to print frame arguments in raw form. Related
setting: *note set print raw-frame-arguments::.
'-frame-info auto|source-line|location|source-and-location|location-and-address|short-location'
Set printing of frame information. Related setting: *note set
print frame-info::.
The optional QUALIFIER is maintained for backward compatibility.
It can be one of the following:
'full'
Equivalent to the '-full' option.
'no-filters'
Equivalent to the '-no-filters' option.
'hide'
Equivalent to the '-hide' option.
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. The command
'set print frame-info' (*note Print Settings::) controls what frame
information is 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=<optimized out>) at macro.c:242
#2 0x6840 in expand_token (obs=0x0, t=<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 '<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'
'set backtrace limit unlimited'
Limit the backtrace to N levels. A value of 'unlimited' or zero
means unlimited levels.
'show backtrace limit'
Display the current limit on backtrace levels.
You can control how file names are displayed.
'set filename-display'
'set filename-display relative'
Display file names relative to the compilation directory. This is
the default.
'set filename-display basename'
Display only basename of a filename.
'set filename-display absolute'
Display an absolute filename.
'show filename-display'
Show the current way to display filenames.
---------- 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 [ FRAME-SELECTION-SPEC ]'
'f [ FRAME-SELECTION-SPEC ]'
The 'frame' command allows different stack frames to be selected.
The FRAME-SELECTION-SPEC can be any of the following:
'NUM'
'level NUM'
Select frame level NUM. 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 level
frame is usually the one for 'main'.
As this is the most common method of navigating the frame
stack, the string 'level' can be omitted. For example, the
following two commands are equivalent:
(gdb) frame 3
(gdb) frame level 3
'address STACK-ADDRESS'
Select the frame with stack address STACK-ADDRESS. The
STACK-ADDRESS for a frame can be seen in the output of 'info
frame', for example:
(gdb) info frame
Stack level 1, frame at 0x7fffffffda30:
rip = 0x40066d in b (amd64-entry-value.cc:59); saved rip 0x4004c5
tail call frame, caller of frame at 0x7fffffffda30
source language c++.
Arglist at unknown address.
Locals at unknown address, Previous frame's sp is 0x7fffffffda30
The STACK-ADDRESS for this frame is '0x7fffffffda30' as
indicated by the line:
Stack level 1, frame at 0x7fffffffda30:
'function FUNCTION-NAME'
Select the stack frame for function FUNCTION-NAME. If there
are multiple stack frames for function FUNCTION-NAME then the
inner most stack frame is selected.
'view STACK-ADDRESS [ PC-ADDR ]'
View a frame that is not part of GDB's backtrace. The frame
viewed has stack address STACK-ADDR, and optionally, a program
counter address of PC-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.
When viewing a frame outside the current backtrace using
'frame view' then you can always return to the original stack
using one of the previous stack frame selection instructions,
for example 'frame level 0'.
'up N'
Move N frames up the stack; N defaults to 1. For positive numbers
N, this advances toward the outermost frame, to higher frame
numbers, to frames that have existed longer.
'down N'
Move N frames down the stack; N defaults to 1. For positive
numbers N, this advances toward the innermost frame, to lower frame
numbers, to frames that were created more recently. 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.
'select-frame [ FRAME-SELECTION-SPEC ]'
The 'select-frame' command is a variant of 'frame' that does not
display the new frame after selecting it. This command is intended
primarily for use in GDB command scripts, where the output might be
unnecessary and distracting. The FRAME-SELECTION-SPEC is as for
the 'frame' command described in *note Selecting a Frame:
Selection.
'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, Next: Frame Apply, 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 [ FRAME-SELECTION-SPEC ]'
'info f [ FRAME-SELECTION-SPEC ]'
Print a verbose description of the frame selected by
FRAME-SELECTION-SPEC. The FRAME-SELECTION-SPEC is the same as for
the 'frame' command (*note Selecting a Frame: Selection.). The
selected frame remains unchanged by this command.
'info args [-q]'
Print the arguments of the selected frame, each on a separate line.
The optional flag '-q', which stands for 'quiet', disables printing
header information and messages explaining why no argument have
been printed.
'info args [-q] [-t TYPE_REGEXP] [REGEXP]'
Like 'info args', but only print the arguments selected with the
provided regexp(s).
If REGEXP is provided, print only the arguments whose names match
the regular expression REGEXP.
If TYPE_REGEXP is provided, print only the arguments whose types,
as printed by the 'whatis' command, match the regular expression
TYPE_REGEXP. If TYPE_REGEXP contains space(s), it should be
enclosed in quote characters. If needed, use backslash to escape
the meaning of special characters or quotes.
If both REGEXP and TYPE_REGEXP are provided, an argument is printed
only if its name matches REGEXP and its type matches TYPE_REGEXP.
'info locals [-q]'
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.
The optional flag '-q', which stands for 'quiet', disables printing
header information and messages explaining why no local variables
have been printed.
'info locals [-q] [-t TYPE_REGEXP] [REGEXP]'
Like 'info locals', but only print the local variables selected
with the provided regexp(s).
If REGEXP is provided, print only the local variables whose names
match the regular expression REGEXP.
If TYPE_REGEXP is provided, print only the local variables whose
types, as printed by the 'whatis' command, match the regular
expression TYPE_REGEXP. If TYPE_REGEXP contains space(s), it
should be enclosed in quote characters. If needed, use backslash
to escape the meaning of special characters or quotes.
If both REGEXP and TYPE_REGEXP are provided, a local variable is
printed only if its name matches REGEXP and its type matches
TYPE_REGEXP.
The command 'info locals -q -t TYPE_REGEXP' can usefully be
combined with the commands 'frame apply' and 'thread apply'. For
example, your program might use Resource Acquisition Is
Initialization types (RAII) such as 'lock_something_t': each local
variable of type 'lock_something_t' automatically places a lock
that is destroyed when the variable goes out of scope. You can
then list all acquired locks in your program by doing
thread apply all -s frame apply all -s info locals -q -t lock_something_t
or the equivalent shorter form
tfaas i lo -q -t lock_something_t