File: gdb.info, Node: Top, Next: Summary, Prev: (dir), Up: (dir)
Debugging with GDB
******************
This file describes GDB, the GNU symbolic debugger.
This is the Ninth Edition, for GDB Version 6.8.
Copyright (C) 1988-2006 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
* Stack:: Examining the stack
* Source:: Examining source files
* Data:: Examining data
* 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
* Sequences:: Canned sequences of commands
* 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.
* GDB Bugs:: Reporting bugs in GDB
* Command Line Editing: (rluserman). Command Line Editing
* Using History Interactively: (history). Using History Interactively
* Formatting Documentation:: How to format and print GDB documentation
* Installing GDB:: Installing GDB
* Maintenance Commands:: Maintenance Commands
* Remote Protocol:: GDB Remote Serial Protocol
* Agent Expressions:: The GDB Agent Expression Mechanism
* Target Descriptions:: How targets can describe themselves to
GDB
* Copying:: GNU General Public License says
how you can copy and share GDB
* GNU Free Documentation License:: The license for this documentation
* Index:: Index
File: gdb.info, Node: Summary, Next: Sample Session, Prev: Top, Up: Top
Summary of GDB
**************
The purpose of a debugger such as GDB is to allow you to see what is
going on "inside" another program while it executes--or what another
program was doing at the moment it crashed.
GDB can do four main kinds of things (plus other things in support of
these) to help you catch bugs in the act:
* Start your program, specifying anything that might affect its
behavior.
* Make your program stop on specified conditions.
* Examine what has happened, when your program has stopped.
* Change things in your program, so you can experiment with
correcting the effects of one bug and go on to learn about another.
You can use GDB to debug programs written in C and C++. For more
information, see *Note Supported Languages: Supported Languages. For
more information, see *Note C and C++: C.
Support for Modula-2 is partial. For information on Modula-2, see
*Note Modula-2: Modula-2.
Debugging Pascal programs which use sets, subranges, file variables,
or nested functions does not currently work. GDB does not support
entering expressions, printing values, or similar features using Pascal
syntax.
GDB can be used to debug programs written in Fortran, although it
may be necessary to refer to some variables with a trailing underscore.
GDB can be used to debug programs written in Objective-C, using
either the Apple/NeXT or the GNU Objective-C runtime.
* Menu:
* Free Software:: Freely redistributable software
* Contributors:: Contributors to GDB
File: gdb.info, Node: Free Software, Next: Contributors, Up: Summary
Free Software
=============
GDB is "free software", protected by the GNU General Public License
(GPL). The GPL gives you the freedom to copy or adapt a licensed
program--but every person getting a copy also gets with it the freedom
to modify that copy (which means that they must get access to the
source code), and the freedom to distribute further copies. Typical
software companies use copyrights to limit your freedoms; the Free
Software Foundation uses the GPL to preserve these freedoms.
Fundamentally, the General Public License is a license which says
that you have these freedoms and that you cannot take these freedoms
away from anyone else.
Free Software Needs Free Documentation
======================================
The biggest deficiency in the free software community today is not in
the software--it is the lack of good free documentation that we can
include with the free software. Many of our most important programs do
not come with free reference manuals and free introductory texts.
Documentation is an essential part of any software package; when an
important free software package does not come with a free manual and a
free tutorial, that is a major gap. We have many such gaps today.
Consider Perl, for instance. The tutorial manuals that people
normally use are non-free. How did this come about? Because the
authors of those manuals published them with restrictive terms--no
copying, no modification, source files not available--which exclude
them from the free software world.
That wasn't the first time this sort of thing happened, and it was
far from the last. Many times we have heard a GNU user eagerly
describe a manual that he is writing, his intended contribution to the
community, only to learn that he had ruined everything by signing a
publication contract to make it non-free.
Free documentation, like free software, is a matter of freedom, not
price. The problem with the non-free manual is not that publishers
charge a price for printed copies--that in itself is fine. (The Free
Software Foundation sells printed copies of manuals, too.) The problem
is the restrictions on the use of the manual. Free manuals are
available in source code form, and give you permission to copy and
modify. Non-free manuals do not allow this.
The criteria of freedom for a free manual are roughly the same as for
free software. Redistribution (including the normal kinds of
commercial redistribution) must be permitted, so that the manual can
accompany every copy of the program, both on-line and on paper.
Permission for modification of the technical content is crucial too.
When people modify the software, adding or changing features, if they
are conscientious they will change the manual too--so they can provide
accurate and clear documentation for the modified program. A manual
that leaves you no choice but to write a new manual to document a
changed version of the program is not really available to our community.
Some kinds of limits on the way modification is handled are
acceptable. For example, requirements to preserve the original
author's copyright notice, the distribution terms, or the list of
authors, are ok. It is also no problem to require modified versions to
include notice that they were modified. Even entire sections that may
not be deleted or changed are acceptable, as long as they deal with
nontechnical topics (like this one). These kinds of restrictions are
acceptable because they don't obstruct the community's normal use of
the manual.
However, it must be possible to modify all the _technical_ content
of the manual, and then distribute the result in all the usual media,
through all the usual channels. Otherwise, the restrictions obstruct
the use of the manual, it is not free, and we need another manual to
replace it.
Please spread the word about this issue. Our community continues to
lose manuals to proprietary publishing. If we spread the word that
free software needs free reference manuals and free tutorials, perhaps
the next person who wants to contribute by writing documentation will
realize, before it is too late, that only free manuals contribute to
the free software community.
If you are writing documentation, please insist on publishing it
under the GNU Free Documentation License or another free documentation
license. Remember that this decision requires your approval--you don't
have to let the publisher decide. Some commercial publishers will use
a free license if you insist, but they will not propose the option; it
is up to you to raise the issue and say firmly that this is what you
want. If the publisher you are dealing with refuses, please try other
publishers. If you're not sure whether a proposed license is free,
write to <licensing AT gnu.org>.
You can encourage commercial publishers to sell more free, copylefted
manuals and tutorials by buying them, and particularly by buying copies
from the publishers that paid for their writing or for major
improvements. Meanwhile, try to avoid buying non-free documentation at
all. Check the distribution terms of a manual before you buy it, and
insist that whoever seeks your business must respect your freedom.
Check the history of the book, and try to reward the publishers that
have paid or pay the authors to work on it.
The Free Software Foundation maintains a list of free documentation
published by other publishers, at
`http://www.fsf.org/doc/other-free-books.html'.
File: gdb.info, Node: Contributors, Prev: Free Software, Up: Summary
Contributors to GDB
===================
Richard Stallman was the original author of GDB, and of many other GNU
programs. Many others have contributed to its development. This
section attempts to credit major contributors. One of the virtues of
free software is that everyone is free to contribute to it; with
regret, we cannot actually acknowledge everyone here. The file
`ChangeLog' in the GDB distribution approximates a blow-by-blow account.
Changes much prior to version 2.0 are lost in the mists of time.
_Plea:_ Additions to this section are particularly welcome. If you
or your friends (or enemies, to be evenhanded) have been unfairly
omitted from this list, we would like to add your names!
So that they may not regard their many labors as thankless, we
particularly thank those who shepherded GDB through major releases:
Andrew Cagney (releases 6.3, 6.2, 6.1, 6.0, 5.3, 5.2, 5.1 and 5.0); Jim
Blandy (release 4.18); Jason Molenda (release 4.17); Stan Shebs
(release 4.14); Fred Fish (releases 4.16, 4.15, 4.13, 4.12, 4.11, 4.10,
and 4.9); Stu Grossman and John Gilmore (releases 4.8, 4.7, 4.6, 4.5,
and 4.4); John Gilmore (releases 4.3, 4.2, 4.1, 4.0, and 3.9); Jim
Kingdon (releases 3.5, 3.4, and 3.3); and Randy Smith (releases 3.2,
3.1, and 3.0).
Richard Stallman, assisted at various times by Peter TerMaat, Chris
Hanson, and Richard Mlynarik, handled releases through 2.8.
Michael Tiemann is the author of most of the GNU C++ support in GDB,
with significant additional contributions from Per Bothner and Daniel
Berlin. James Clark wrote the GNU C++ demangler. Early work on C++
was by Peter TerMaat (who also did much general update work leading to
release 3.0).
GDB uses the BFD subroutine library to examine multiple object-file
formats; BFD was a joint project of David V. Henkel-Wallace, Rich
Pixley, Steve Chamberlain, and John Gilmore.
David Johnson wrote the original COFF support; Pace Willison did the
original support for encapsulated COFF.
Brent Benson of Harris Computer Systems contributed DWARF 2 support.
Adam de Boor and Bradley Davis contributed the ISI Optimum V support.
Per Bothner, Noboyuki Hikichi, and Alessandro Forin contributed MIPS
support. Jean-Daniel Fekete contributed Sun 386i support. Chris
Hanson improved the HP9000 support. Noboyuki Hikichi and Tomoyuki
Hasei contributed Sony/News OS 3 support. David Johnson contributed
Encore Umax support. Jyrki Kuoppala contributed Altos 3068 support.
Jeff Law contributed HP PA and SOM support. Keith Packard contributed
NS32K support. Doug Rabson contributed Acorn Risc Machine support.
Bob Rusk contributed Harris Nighthawk CX-UX support. Chris Smith
contributed Convex support (and Fortran debugging). Jonathan Stone
contributed Pyramid support. Michael Tiemann contributed SPARC support.
Tim Tucker contributed support for the Gould NP1 and Gould Powernode.
Pace Willison contributed Intel 386 support. Jay Vosburgh contributed
Symmetry support. Marko Mlinar contributed OpenRISC 1000 support.
Andreas Schwab contributed M68K GNU/Linux support.
Rich Schaefer and Peter Schauer helped with support of SunOS shared
libraries.
Jay Fenlason and Roland McGrath ensured that GDB and GAS agree about
several machine instruction sets.
Patrick Duval, Ted Goldstein, Vikram Koka and Glenn Engel helped
develop remote debugging. Intel Corporation, Wind River Systems, AMD,
and ARM contributed remote debugging modules for the i960, VxWorks,
A29K UDI, and RDI targets, respectively.
Brian Fox is the author of the readline libraries providing
command-line editing and command history.
Andrew Beers of SUNY Buffalo wrote the language-switching code, the
Modula-2 support, and contributed the Languages chapter of this manual.
Fred Fish wrote most of the support for Unix System Vr4. He also
enhanced the command-completion support to cover C++ overloaded symbols.
Hitachi America (now Renesas America), Ltd. sponsored the support for
H8/300, H8/500, and Super-H processors.
NEC sponsored the support for the v850, Vr4xxx, and Vr5xxx
processors.
Mitsubishi (now Renesas) sponsored the support for D10V, D30V, and
M32R/D processors.
Toshiba sponsored the support for the TX39 Mips processor.
Matsushita sponsored the support for the MN10200 and MN10300
processors.
Fujitsu sponsored the support for SPARClite and FR30 processors.
Kung Hsu, Jeff Law, and Rick Sladkey added support for hardware
watchpoints.
Michael Snyder added support for tracepoints.
Stu Grossman wrote gdbserver.
Jim Kingdon, Peter Schauer, Ian Taylor, and Stu Grossman made nearly
innumerable bug fixes and cleanups throughout GDB.
The following people at the Hewlett-Packard Company contributed
support for the PA-RISC 2.0 architecture, HP-UX 10.20, 10.30, and 11.0
(narrow mode), HP's implementation of kernel threads, HP's aC++
compiler, and the Text User Interface (nee Terminal User Interface):
Ben Krepp, Richard Title, John Bishop, Susan Macchia, Kathy Mann,
Satish Pai, India Paul, Steve Rehrauer, and Elena Zannoni. Kim Haase
provided HP-specific information in this manual.
DJ Delorie ported GDB to MS-DOS, for the DJGPP project. Robert
Hoehne made significant contributions to the DJGPP port.
Cygnus Solutions has sponsored GDB maintenance and much of its
development since 1991. Cygnus engineers who have worked on GDB
fulltime include Mark Alexander, Jim Blandy, Per Bothner, Kevin
Buettner, Edith Epstein, Chris Faylor, Fred Fish, Martin Hunt, Jim
Ingham, John Gilmore, Stu Grossman, Kung Hsu, Jim Kingdon, John Metzler,
Fernando Nasser, Geoffrey Noer, Dawn Perchik, Rich Pixley, Zdenek
Radouch, Keith Seitz, Stan Shebs, David Taylor, and Elena Zannoni. In
addition, Dave Brolley, Ian Carmichael, Steve Chamberlain, Nick Clifton,
JT Conklin, Stan Cox, DJ Delorie, Ulrich Drepper, Frank Eigler, Doug
Evans, Sean Fagan, David Henkel-Wallace, Richard Henderson, Jeff
Holcomb, Jeff Law, Jim Lemke, Tom Lord, Bob Manson, Michael Meissner,
Jason Merrill, Catherine Moore, Drew Moseley, Ken Raeburn, Gavin
Romig-Koch, Rob Savoye, Jamie Smith, Mike Stump, Ian Taylor, Angela
Thomas, Michael Tiemann, Tom Tromey, Ron Unrau, Jim Wilson, and David
Zuhn have made contributions both large and small.
Andrew Cagney, Fernando Nasser, and Elena Zannoni, while working for
Cygnus Solutions, implemented the original GDB/MI interface.
Jim Blandy added support for preprocessor macros, while working for
Red Hat.
Andrew Cagney designed GDB's architecture vector. Many people
including Andrew Cagney, Stephane Carrez, Randolph Chung, Nick Duffek,
Richard Henderson, Mark Kettenis, Grace Sainsbury, Kei Sakamoto,
Yoshinori Sato, Michael Snyder, Andreas Schwab, Jason Thorpe, Corinna
Vinschen, Ulrich Weigand, and Elena Zannoni, helped with the migration
of old architectures to this new framework.
Andrew Cagney completely re-designed and re-implemented GDB's
unwinder framework, this consisting of a fresh new design featuring
frame IDs, independent frame sniffers, and the sentinel frame. Mark
Kettenis implemented the DWARF 2 unwinder, Jeff Johnston the libunwind
unwinder, and Andrew Cagney the dummy, sentinel, tramp, and trad
unwinders. The architecture-specific changes, each involving a
complete rewrite of the architecture's frame code, were carried out by
Jim Blandy, Joel Brobecker, Kevin Buettner, Andrew Cagney, Stephane
Carrez, Randolph Chung, Orjan Friberg, Richard Henderson, Daniel
Jacobowitz, Jeff Johnston, Mark Kettenis, Theodore A. Roth, Kei
Sakamoto, Yoshinori Sato, Michael Snyder, Corinna Vinschen, and Ulrich
Weigand.
Christian Zankel, Ross Morley, Bob Wilson, and Maxim Grigoriev from
Tensilica, Inc. contributed support for Xtensa processors. Others who
have worked on the Xtensa port of GDB in the past include Steve Tjiang,
John Newlin, and Scott Foehner.
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 6.8, Copyright 1999 Free Software Foundation, Inc...
(gdb)
GDB reads only enough symbol data to know where to find the rest when
needed; as a result, the first prompt comes up very quickly. We now
tell GDB to use a narrower display width than usual, so that examples
fit in this manual.
(gdb) set width 70
We need to see how the `m4' built-in `changequote' works. Having
looked at the source, we know the relevant subroutine is
`m4_changequote', so we set a breakpoint there with the GDB `break'
command.
(gdb) break m4_changequote
Breakpoint 1 at 0x62f4: file builtin.c, line 879.
Using the `run' command, we start `m4' running under GDB control; as
long as control does not reach the `m4_changequote' subroutine, the
program runs as usual:
(gdb) run
Starting program: /work/Editorial/gdb/gnu/m4/m4
define(foo,0000)
foo
0000
To trigger the breakpoint, we call `changequote'. GDB suspends
execution of `m4', displaying information about the context where it
stops.
changequote(<QUOTE>,<UNQUOTE>)
Breakpoint 1, m4_changequote (argc=3, argv=0x33c70)
at builtin.c:879
879 if (bad_argc(TOKEN_DATA_TEXT(argv[0]),argc,1,3))
Now we use the command `n' (`next') to advance execution to the next
line of the current function.
(gdb) n
882 set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\
: nil,
`set_quotes' looks like a promising subroutine. We can go into it by
using the command `s' (`step') instead of `next'. `step' goes to the
next line to be executed in _any_ subroutine, so it steps into
`set_quotes'.
(gdb) s
set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
at input.c:530
530 if (lquote != def_lquote)
The display that shows the subroutine where `m4' is now suspended (and
its arguments) is called a stack frame display. It shows a summary of
the stack. We can use the `backtrace' command (which can also be
spelled `bt'), to see where we are in the stack as a whole: the
`backtrace' command displays a stack frame for each active subroutine.
(gdb) bt
#0 set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
at input.c:530
#1 0x6344 in m4_changequote (argc=3, argv=0x33c70)
at builtin.c:882
#2 0x8174 in expand_macro (sym=0x33320) at macro.c:242
#3 0x7a88 in expand_token (obs=0x0, t=209696, td=0xf7fffa30)
at macro.c:71
#4 0x79dc in expand_input () at macro.c:40
#5 0x2930 in main (argc=0, argv=0xf7fffb20) at m4.c:195
We step through a few more lines to see what happens. The first two
times, we can use `s'; the next two times we use `n' to avoid falling
into the `xstrdup' subroutine.
(gdb) s
0x3b5c 532 if (rquote != def_rquote)
(gdb) s
0x3b80 535 lquote = (lq == nil || *lq == '\0') ? \
def_lquote : xstrdup(lq);
(gdb) n
536 rquote = (rq == nil || *rq == '\0') ? def_rquote\
: xstrdup(rq);
(gdb) n
538 len_lquote = strlen(rquote);
The last line displayed looks a little odd; we can examine the variables
`lquote' and `rquote' to see if they are in fact the new left and right
quotes we specified. We use the command `p' (`print') to see their
values.
(gdb) p lquote
$1 = 0x35d40 "<QUOTE>"
(gdb) p rquote
$2 = 0x35d50 "<UNQUOTE>"
`lquote' and `rquote' are indeed the new left and right quotes. To
look at some context, we can display ten lines of source surrounding
the current line with the `l' (`list') command.
(gdb) l
533 xfree(rquote);
534
535 lquote = (lq == nil || *lq == '\0') ? def_lquote\
: xstrdup (lq);
536 rquote = (rq == nil || *rq == '\0') ? def_rquote\
: xstrdup (rq);
537
538 len_lquote = strlen(rquote);
539 len_rquote = strlen(lquote);
540 }
541
542 void
Let us step past the two lines that set `len_lquote' and `len_rquote',
and then examine the values of those variables.
(gdb) n
539 len_rquote = strlen(lquote);
(gdb) n
540 }
(gdb) p len_lquote
$3 = 9
(gdb) p len_rquote
$4 = 7
That certainly looks wrong, assuming `len_lquote' and `len_rquote' are
meant to be the lengths of `lquote' and `rquote' respectively. We can
set them to better values using the `p' command, since it can print the
value of any expression--and that expression can include subroutine
calls and assignments.
(gdb) p len_lquote=strlen(lquote)
$5 = 7
(gdb) p len_rquote=strlen(rquote)
$6 = 9
Is that enough to fix the problem of using the new quotes with the `m4'
built-in `defn'? We can allow `m4' to continue executing with the `c'
(`continue') command, and then try the example that caused trouble
initially:
(gdb) c
Continuing.
define(baz,defn(<QUOTE>foo<UNQUOTE>))
baz
0000
Success! The new quotes now work just as well as the default ones. The
problem seems to have been just the two typos defining the wrong
lengths. We allow `m4' exit by giving it an EOF as input:
Ctrl-d
Program exited normally.
The message `Program exited normally.' is from GDB; it indicates `m4'
has finished executing. We can end our GDB session with the GDB `quit'
command.
(gdb) quit
File: gdb.info, Node: Invocation, Next: Commands, Prev: Sample Session, Up: Top
2 Getting In and Out of GDB
***************************
This chapter discusses how to start GDB, and how to get out of it. The
essentials are:
* type `gdb' to start GDB.
* type `quit' or `Ctrl-d' to exit.
* Menu:
* Invoking GDB:: How to start GDB
* Quitting GDB:: How to quit GDB
* Shell Commands:: How to use shell commands inside GDB
* Logging Output:: How to log GDB's output to a file
File: gdb.info, Node: Invoking GDB, Next: Quitting GDB, Up: Invocation
2.1 Invoking GDB
================
Invoke GDB by running the program `gdb'. Once started, GDB reads
commands from the terminal until you tell it to exit.
You can also run `gdb' with a variety of arguments and options, to
specify more of your debugging environment at the outset.
The command-line options described here are designed to cover a
variety of situations; in some environments, some of these options may
effectively be unavailable.
The most usual way to start GDB is with one argument, specifying an
executable program:
gdb PROGRAM
You can also start with both an executable program and a core file
specified:
gdb PROGRAM CORE
You can, instead, specify a process ID as a second argument, if you
want to debug a running process:
gdb PROGRAM 1234
would attach GDB to process `1234' (unless you also have a file named
`1234'; GDB does check for a core file first).
Taking advantage of the second command-line argument requires a
fairly complete operating system; when you use GDB as a remote debugger
attached to a bare board, there may not be any notion of "process", and
there is often no way to get a core dump. GDB will warn you if it is
unable to attach or to read core dumps.
You can optionally have `gdb' pass any arguments after the
executable file to the inferior using `--args'. This option stops
option processing.
gdb --args gcc -O2 -c foo.c
This will cause `gdb' to debug `gcc', and to set `gcc''s
command-line arguments (*note Arguments::) to `-O2 -c foo.c'.
You can run `gdb' without printing the front material, which
describes GDB's non-warranty, by specifying `-silent':
gdb -silent
You can further control how GDB starts up by using command-line
options. GDB itself can remind you of the options available.
Type
gdb -help
to display all available options and briefly describe their use (`gdb
-h' is a shorter equivalent).
All options and command line arguments you give are processed in
sequential order. The order makes a difference when the `-x' option is
used.
* Menu:
* File Options:: Choosing files
* Mode Options:: Choosing modes
* Startup:: What GDB does during startup
File: gdb.info, Node: File Options, Next: Mode Options, Up: Invoking GDB
2.1.1 Choosing Files
--------------------
When GDB starts, it reads any arguments other than options as
specifying an executable file and core file (or process ID). This is
the same as if the arguments were specified by the `-se' and `-c' (or
`-p') options respectively. (GDB reads the first argument that does
not have an associated option flag as equivalent to the `-se' option
followed by that argument; and the second argument that does not have
an associated option flag, if any, as equivalent to the `-c'/`-p'
option followed by that argument.) If the second argument begins with
a decimal digit, GDB will first attempt to attach to it as a process,
and if that fails, attempt to open it as a corefile. If you have a
corefile whose name begins with a digit, you can prevent GDB from
treating it as a pid by prefixing it with `./', e.g. `./12345'.
If GDB has not been configured to included core file support, such
as for most embedded targets, then it will complain about a second
argument and ignore it.
Many options have both long and short forms; both are shown in the
following list. GDB also recognizes the long forms if you truncate
them, so long as enough of the option is present to be unambiguous.
(If you prefer, you can flag option arguments with `--' rather than
`-', though we illustrate the more usual convention.)
`-symbols FILE'
`-s FILE'
Read symbol table from file FILE.
`-exec FILE'
`-e FILE'
Use file FILE as the executable file to execute when appropriate,
and for examining pure data in conjunction with a core dump.
`-se FILE'
Read symbol table from file FILE and use it as the executable file.
`-core FILE'
`-c FILE'
Use file FILE as a core dump to examine.
`-pid NUMBER'
`-p NUMBER'
Connect to process ID NUMBER, as with the `attach' command.
`-command FILE'
`-x FILE'
Execute GDB commands from file FILE. *Note Command files: Command
Files.
`-eval-command COMMAND'
`-ex COMMAND'
Execute a single GDB command.
This option may be used multiple times to call multiple commands.
It may also be interleaved with `-command' as required.
gdb -ex 'target sim' -ex 'load' \
-x setbreakpoints -ex 'run' a.out
`-directory DIRECTORY'
`-d DIRECTORY'
Add DIRECTORY to the path to search for source and script files.
`-r'
`-readnow'
Read each symbol file's entire symbol table immediately, rather
than the default, which is to read it incrementally as it is
needed. This makes startup slower, but makes future operations
faster.
`--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.
File: gdb.info, Node: Mode Options, Next: Startup, Prev: File Options, Up: Invoking GDB
2.1.2 Choosing Modes
--------------------
You can run GDB in various alternative modes--for example, in batch
mode or quiet mode.
`-nx'
`-n'
Do not execute commands found in any initialization files.
Normally, GDB executes the commands in these files after all the
command options and arguments have been processed. *Note Command
Files: Command Files.
`-quiet'
`-silent'
`-q'
"Quiet". Do not print the introductory and copyright messages.
These messages are also suppressed in batch mode.
`-batch'
Run in batch mode. Exit with status `0' after processing all the
command files specified with `-x' (and all commands from
initialization files, if not inhibited with `-n'). Exit with
nonzero status if an error occurs in executing the GDB commands in
the command files.
Batch mode may be useful for running GDB as a filter, for example
to download and run a program on another computer; in order to
make this more useful, the message
Program exited normally.
(which is ordinarily issued whenever a program running under GDB
control terminates) is not issued when running in batch mode.
`-batch-silent'
Run in batch mode exactly like `-batch', but totally silently. All
GDB output to `stdout' is prevented (`stderr' is unaffected).
This is much quieter than `-silent' and would be useless for an
interactive session.
This is particularly useful when using targets that give `Loading
section' messages, for example.
Note that targets that give their output via GDB, as opposed to
writing directly to `stdout', will also be made silent.
`-return-child-result'
The return code from GDB will be the return code from the child
process (the process being debugged), with the following
exceptions:
* GDB exits abnormally. E.g., due to an incorrect argument or
an internal error. In this case the exit code is the same as
it would have been without `-return-child-result'.
* The user quits with an explicit value. E.g., `quit 1'.
* The child process never runs, or is not allowed to terminate,
in which case the exit code will be -1.
This option is useful in conjunction with `-batch' or
`-batch-silent', when GDB is being used as a remote program loader
or simulator interface.
`-nowindows'
`-nw'
"No windows". If GDB comes with a graphical user interface (GUI)
built in, then this option tells GDB to only use the command-line
interface. If no GUI is available, this option has no effect.
`-windows'
`-w'
If GDB includes a GUI, then this option requires it to be used if
possible.
`-cd DIRECTORY'
Run GDB using DIRECTORY as its working directory, instead of the
current directory.
`-fullname'
`-f'
GNU Emacs sets this option when it runs GDB as a subprocess. It
tells GDB to output the full file name and line number in a
standard, recognizable fashion each time a stack frame is
displayed (which includes each time your program stops). This
recognizable format looks like two `\032' characters, followed by
the file name, line number and character position separated by
colons, and a newline. The Emacs-to-GDB interface program uses
the two `\032' characters as a signal to display the source code
for the frame.
`-epoch'
The Epoch Emacs-GDB interface sets this option when it runs GDB as
a subprocess. It tells GDB to modify its print routines so as to
allow Epoch to display values of expressions in a separate window.
`-annotate LEVEL'
This option sets the "annotation level" inside GDB. Its effect is
identical to using `set annotate LEVEL' (*note Annotations::).
The annotation LEVEL controls how much information GDB prints
together with its prompt, values of expressions, source lines, and
other types of output. Level 0 is the normal, level 1 is for use
when GDB is run as a subprocess of GNU Emacs, level 3 is the
maximum annotation suitable for programs that control GDB, and
level 2 has been deprecated.
The annotation mechanism has largely been superseded by GDB/MI
(*note GDB/MI::).
`--args'
Change interpretation of command line so that arguments following
the executable file are passed as command line arguments to the
inferior. This option stops option processing.
`-baud BPS'
`-b BPS'
Set the line speed (baud rate or bits per second) of any serial
interface used by GDB for remote debugging.
`-l TIMEOUT'
Set the timeout (in seconds) of any communication used by GDB for
remote debugging.
`-tty DEVICE'
`-t DEVICE'
Run using DEVICE for your program's standard input and output.
`-tui'
Activate the "Text User Interface" when starting. The Text User
Interface manages several text windows on the terminal, showing
source, assembly, registers and GDB command outputs (*note GDB
Text User Interface: TUI.). Alternatively, the Text User
Interface can be enabled by invoking the program `gdbtui'. Do not
use this option if you run GDB from Emacs (*note Using GDB under
GNU Emacs: Emacs.).
`-interpreter INTERP'
Use the interpreter INTERP for interface with the controlling
program or device. This option is meant to be set by programs
which communicate with GDB using it as a back end. *Note Command
Interpreters: Interpreters.
`--interpreter=mi' (or `--interpreter=mi2') causes GDB to use the
"GDB/MI interface" (*note The GDB/MI Interface: GDB/MI.) included
since GDB version 6.0. The previous GDB/MI interface, included in
GDB version 5.3 and selected with `--interpreter=mi1', is
deprecated. Earlier GDB/MI interfaces are no longer supported.
`-write'
Open the executable and core files for both reading and writing.
This is equivalent to the `set write on' command inside GDB (*note
Patching::).
`-statistics'
This option causes GDB to print statistics about time and memory
usage after it completes each command and returns to the prompt.
`-version'
This option causes GDB to print its version number and no-warranty
blurb, and exit.
File: gdb.info, Node: Startup, Prev: Mode Options, Up: Invoking GDB
2.1.3 What GDB Does During Startup
----------------------------------
Here's the description of what GDB does during session startup:
1. Sets up the command interpreter as specified by the command line
(*note interpreter: Mode Options.).
2. Reads the "init file" (if any) in your home directory(1) and
executes all the commands in that file.
3. Processes command line options and operands.
4. Reads and executes the commands from init file (if any) in the
current working directory. This is only done if the current
directory is different from your home directory. Thus, you can
have more than one init file, one generic in your home directory,
and another, specific to the program you are debugging, in the
directory where you invoke GDB.
5. Reads command files specified by the `-x' option. *Note Command
Files::, for more details about GDB command files.
6. 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.).
The GDB init files are normally called `.gdbinit'. The DJGPP port
of GDB uses the name `gdb.ini', due to the limitations of file names
imposed by DOS filesystems. The Windows ports of GDB use the standard
name, but if they find a `gdb.ini' file, they warn you about that and
suggest to rename the file to the standard name.
---------- Footnotes ----------
(1) On DOS/Windows systems, the home directory is the one pointed to
by the `HOME' environment variable.
File: gdb.info, Node: Quitting GDB, Next: Shell Commands, Prev: Invoking GDB, Up: Invocation
2.2 Quitting GDB
================
`quit [EXPRESSION]'
`q'
To exit GDB, use the `quit' command (abbreviated `q'), or type an
end-of-file character (usually `Ctrl-d'). If you do not supply
EXPRESSION, GDB will terminate normally; otherwise it will
terminate using the result of EXPRESSION as the error code.
An interrupt (often `Ctrl-c') does not exit from GDB, but rather
terminates the action of any GDB command that is in progress and
returns to GDB command level. It is safe to type the interrupt
character at any time because GDB does not allow it to take effect
until a time when it is safe.
If you have been using GDB to control an attached process or device,
you can release it with the `detach' command (*note Debugging an
Already-running Process: Attach.).
File: gdb.info, Node: Shell Commands, Next: Logging Output, Prev: Quitting GDB, Up: Invocation
2.3 Shell Commands
==================
If you need to execute occasional shell commands during your debugging
session, there is no need to leave or suspend GDB; you can just use the
`shell' command.
`shell COMMAND STRING'
Invoke a standard shell to execute COMMAND STRING. If it exists,
the environment variable `SHELL' determines which shell to run.
Otherwise GDB uses the default shell (`/bin/sh' on Unix systems,
`COMMAND.COM' on MS-DOS, etc.).
The utility `make' is often needed in development environments. You
do not have to use the `shell' command for this purpose in GDB:
`make MAKE-ARGS'
Execute the `make' program with the specified arguments. This is
equivalent to `shell make MAKE-ARGS'.
File: gdb.info, Node: Logging Output, Prev: Shell Commands, Up: Invocation
2.4 Logging Output
==================
You may want to save the output of GDB commands to a file. There are
several commands to control GDB's logging.
`set logging on'
Enable logging.
`set logging off'
Disable logging.
`set logging file FILE'
Change the name of the current logfile. The default logfile is
`gdb.txt'.
`set logging overwrite [on|off]'
By default, GDB will append to the logfile. Set `overwrite' if
you want `set logging on' to overwrite the logfile instead.
`set logging redirect [on|off]'
By default, GDB output will go to both the terminal and the
logfile. Set `redirect' if you want output to go only to the log
file.
`show logging'
Show the current values of the logging settings.
File: gdb.info, Node: Commands, Next: Running, Prev: Invocation, Up: Top
3 GDB Commands
**************
You can abbreviate a GDB command to the first few letters of the command
name, if that abbreviation is unambiguous; and you can repeat certain
GDB commands by typing just <RET>. You can also use the <TAB> key to
get GDB to fill out the rest of a word in a command (or to show you the
alternatives available, if there is more than one possibility).
* Menu:
* Command Syntax:: How to give commands to GDB
* Completion:: Command completion
* Help:: How to ask GDB for help
File: gdb.info, Node: Command Syntax, Next: Completion, Up: Commands
3.1 Command Syntax
==================
A GDB command is a single line of input. There is no limit on how long
it can be. It starts with a command name, which is followed by
arguments whose meaning depends on the command name. For example, the
command `step' accepts an argument which is the number of times to
step, as in `step 5'. You can also use the `step' command with no
arguments. Some commands do not allow any arguments.
GDB command names may always be truncated if that abbreviation is
unambiguous. Other possible command abbreviations are listed in the
documentation for individual commands. In some cases, even ambiguous
abbreviations are allowed; for example, `s' is specially defined as
equivalent to `step' even though there are other commands whose names
start with `s'. You can test abbreviations by using them as arguments
to the `help' command.
A blank line as input to GDB (typing just <RET>) means to repeat the
previous command. Certain commands (for example, `run') will not
repeat this way; these are commands whose unintentional repetition
might cause trouble and which you are unlikely to want to repeat.
User-defined commands can disable this feature; see *Note dont-repeat:
Define.
The `list' and `x' commands, when you repeat them with <RET>,
construct new arguments rather than repeating exactly as typed. This
permits easy scanning of source or memory.
GDB can also use <RET> in another way: to partition lengthy output,
in a way similar to the common utility `more' (*note Screen Size:
Screen Size.). Since it is easy to press one <RET> too many in this
situation, GDB disables command repetition after any command that
generates this sort of display.
Any text from a `#' to the end of the line is a comment; it does
nothing. This is useful mainly in command files (*note Command Files:
Command Files.).
The `Ctrl-o' binding is useful for repeating a complex sequence of
commands. This command accepts the current line, like <RET>, and then
fetches the next line relative to the current line from the history for
editing.
File: gdb.info, Node: Completion, Next: Help, Prev: Command Syntax, Up: Commands
3.2 Command Completion
======================
GDB can fill in the rest of a word in a command for you, if there is
only one possibility; it can also show you what the valid possibilities
are for the next word in a command, at any time. This works for GDB
commands, GDB subcommands, and the names of symbols in your program.
Press the <TAB> key whenever you want GDB to fill out the rest of a
word. If there is only one possibility, GDB fills in the word, and
waits for you to finish the command (or press <RET> to enter it). For
example, if you type
(gdb) info bre <TAB>
GDB fills in the rest of the word `breakpoints', since that is the only
`info' subcommand beginning with `bre':
(gdb) info breakpoints
You can either press <RET> at this point, to run the `info breakpoints'
command, or backspace and enter something else, if `breakpoints' does
not look like the command you expected. (If you were sure you wanted
`info breakpoints' in the first place, you might as well just type
<RET> immediately after `info bre', to exploit command abbreviations
rather than command completion).
If there is more than one possibility for the next word when you
press <TAB>, GDB sounds a bell. You can either supply more characters
and try again, or just press <TAB> a second time; GDB displays all the
possible completions for that word. For example, you might want to set
a breakpoint on a subroutine whose name begins with `make_', but when
you type `b make_<TAB>' GDB just sounds the bell. Typing <TAB> again
displays all the function names in your program that begin with those
characters, for example:
(gdb) b make_ <TAB>
GDB sounds bell; press <TAB> again, to see:
make_a_section_from_file make_environ
make_abs_section make_function_type
make_blockvector make_pointer_type
make_cleanup make_reference_type
make_command make_symbol_completion_list
(gdb) b make_
After displaying the available possibilities, GDB copies your partial
input (`b make_' in the example) so you can finish the command.
If you just want to see the list of alternatives in the first place,
you can press `M-?' rather than pressing <TAB> twice. `M-?' means
`<META> ?'. You can type this either by holding down a key designated
as the <META> shift on your keyboard (if there is one) while typing
`?', or as <ESC> followed by `?'.
Sometimes the string you need, while logically a "word", may contain
parentheses or other characters that GDB normally excludes from its
notion of a word. To permit word completion to work in this situation,
you may enclose words in `'' (single quote marks) in GDB commands.
The most likely situation where you might need this is in typing the
name of a C++ function. This is because C++ allows function
overloading (multiple definitions of the same function, distinguished
by argument type). For example, when you want to set a breakpoint you
may need to distinguish whether you mean the version of `name' that
takes an `int' parameter, `name(int)', or the version that takes a
`float' parameter, `name(float)'. To use the word-completion
facilities in this situation, type a single quote `'' at the beginning
of the function name. This alerts GDB that it may need to consider
more information than usual when you press <TAB> or `M-?' to request
word completion:
(gdb) b 'bubble( M-?
bubble(double,double) bubble(int,int)
(gdb) b 'bubble(
In some cases, GDB can tell that completing a name requires using
quotes. When this happens, GDB inserts the quote for you (while
completing as much as it can) if you do not type the quote in the first
place:
(gdb) b bub <TAB>
GDB alters your input line to the following, and rings a bell:
(gdb) b 'bubble(
In general, GDB can tell that a quote is needed (and inserts it) if you
have not yet started typing the argument list when you ask for
completion on an overloaded symbol.
For more information about overloaded functions, see *Note C++
Expressions: C Plus Plus Expressions. You can use the command `set
overload-resolution off' to disable overload resolution; see *Note GDB
Features for C++: Debugging C Plus Plus.
File: gdb.info, Node: Help, Prev: Completion, Up: Commands
3.3 Getting Help
================
You can always ask GDB itself for information on its commands, using
the command `help'.
`help'
`h'
You can use `help' (abbreviated `h') with no arguments to display
a short list of named classes of commands:
(gdb) help
List of classes of commands:
aliases -- Aliases of other commands
breakpoints -- Making program stop at certain points
data -- Examining data
files -- Specifying and examining files
internals -- Maintenance commands
obscure -- Obscure features
running -- Running the program
stack -- Examining the stack
status -- Status inquiries
support -- Support facilities
tracepoints -- Tracing of program execution without
stopping the program
user-defined -- User-defined commands
Type "help" followed by a class name for a list of
commands in that class.
Type "help" followed by command name for full
documentation.
Command name abbreviations are allowed if unambiguous.
(gdb)
`help CLASS'
Using one of the general help classes as an argument, you can get a
list of the individual commands in that class. For example, here
is the help display for the class `status':
(gdb) help status
Status inquiries.
List of commands:
info -- Generic command for showing things
about the program being debugged
show -- Generic command for showing things
about the debugger
Type "help" followed by command name for full
documentation.
Command name abbreviations are allowed if unambiguous.
(gdb)
`help COMMAND'
With a command name as `help' argument, GDB displays a short
paragraph on how to use that command.
`apropos ARGS'
The `apropos' command searches through all of the GDB commands,
and their documentation, for the regular expression specified in
ARGS. It prints out all matches found. For example:
apropos reload
results in:
set symbol-reloading -- Set dynamic symbol table reloading
multiple times in one run
show symbol-reloading -- Show dynamic symbol table reloading
multiple times in one run
`complete ARGS'
The `complete ARGS' command lists all the possible completions for
the beginning of a command. Use ARGS to specify the beginning of
the command you want completed. For example:
complete i
results in:
if
ignore
info
inspect
This is intended for use by GNU Emacs.
In addition to `help', you can use the GDB commands `info' and
`show' to inquire about the state of your program, or the state of GDB
itself. Each command supports many topics of inquiry; this manual
introduces each of them in the appropriate context. The listings under
`info' and under `show' in the Index point to all the sub-commands.
*Note Index::.
`info'
This command (abbreviated `i') is for describing the state of your
program. For example, you can show the arguments passed to a
function with `info args', list the registers currently in use
with `info registers', or list the breakpoints you have set with
`info breakpoints'. You can get a complete list of the `info'
sub-commands with `help info'.
`set'
You can assign the result of an expression to an environment
variable with `set'. For example, you can set the GDB prompt to a
$-sign with `set prompt $'.
`show'
In contrast to `info', `show' is for describing the state of GDB
itself. You can change most of the things you can `show', by
using the related command `set'; for example, you can control what
number system is used for displays with `set radix', or simply
inquire which is currently in use with `show radix'.
To display all the settable parameters and their current values,
you can use `show' with no arguments; you may also use `info set'.
Both commands produce the same display.
Here are three miscellaneous `show' subcommands, all of which are
exceptional in lacking corresponding `set' commands:
`show version'
Show what version of GDB is running. You should include this
information in GDB bug-reports. If multiple versions of GDB are
in use at your site, you may need to determine which version of
GDB you are running; as GDB evolves, new commands are introduced,
and old ones may wither away. Also, many system vendors ship
variant versions of GDB, and there are variant versions of GDB in
GNU/Linux distributions as well. The version number is the same
as the one announced when you start GDB.
`show copying'
`info copying'
Display information about permission for copying GDB.
`show warranty'
`info warranty'
Display the GNU "NO WARRANTY" statement, or a warranty, if your
version of GDB comes with one.
File: gdb.info, Node: Running, Next: Stopping, Prev: Commands, Up: Top
4 Running Programs Under GDB
****************************
When you run a program under GDB, you must first generate debugging
information when you compile it.
You may start GDB with its arguments, if any, in an environment of
your choice. If you are doing native debugging, you may redirect your
program's input and output, debug an already running process, or kill a
child process.
* Menu:
* Compilation:: Compiling for debugging
* Starting:: Starting your program
* Arguments:: Your program's arguments
* Environment:: Your program's environment
* Working Directory:: Your program's working directory
* Input/Output:: Your program's input and output
* Attach:: Debugging an already-running process
* Kill Process:: Killing the child process
* Threads:: Debugging programs with multiple threads
* Processes:: Debugging programs with multiple processes
* 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, many compilers
are unable to handle the `-g' and `-O' options together. Using those
compilers, you cannot generate optimized executables containing
debugging information.
GCC, the GNU C/C++ compiler, supports `-g' with or without `-O',
making it possible to debug optimized code. We recommend that you
_always_ use `-g' whenever you compile a program. You may think your
program is correct, but there is no sense in pushing your luck.
When you debug a program compiled with `-g -O', remember that the
optimizer is rearranging your code; the debugger shows you what is
really there. Do not be too surprised when the execution path does not
exactly match your source file! An extreme example: if you define a
variable, but never use it, GDB never sees that variable--because the
compiler optimizes it out of existence.
Some things do not work as well with `-g -O' as with just `-g',
particularly on machines with instruction scheduling. If in doubt,
recompile with `-g' alone, and if this fixes the problem, please report
it to us as a bug (including a test case!). *Note Variables::, for
more information about debugging optimized code.
Older versions of the GNU C compiler permitted a variant option
`-gg' for debugging information. GDB no longer supports this format;
if your GNU C compiler has this option, do not use it.
GDB knows about preprocessor macros and can show you their expansion
(*note Macros::). Most compilers do not include information about
preprocessor macros in the debugging information if you specify the
`-g' flag alone, because this information is rather large. Version 3.1
and later of GCC, the GNU C compiler, provides macro information if you
specify the options `-gdwarf-2' and `-g3'; the former option requests
debugging information in the Dwarf 2 format, and the latter requests
"extra information". In the future, we hope to find more compact ways
to represent macro information, so that it can be included with `-g'
alone.
File: gdb.info, Node: Starting, Next: Arguments, Prev: Compilation, Up: Running
4.2 Starting your Program
=========================
`run'
`r'
Use the `run' command to start your program under GDB. You must
first specify the program name (except on VxWorks) with an
argument to GDB (*note Getting In and Out of GDB: Invocation.), or
by using the `file' or `exec-file' command (*note Commands to
Specify Files: Files.).
If you are running your program in an execution environment that
supports processes, `run' creates an inferior process and makes that
process run your program. (In environments without processes, `run'
jumps to the start of your program.)
The execution of a program is affected by certain information it
receives from its superior. GDB provides ways to specify this
information, which you must do _before_ starting your program. (You
can change it after starting your program, but such changes only affect
your program the next time you start it.) This information may be
divided into four categories:
The _arguments._
Specify the arguments to give your program as the arguments of the
`run' command. If a shell is available on your target, the shell
is used to pass the arguments, so that you may use normal
conventions (such as wildcard expansion or variable substitution)
in describing the arguments. In Unix systems, you can control
which shell is used with the `SHELL' environment variable. *Note
Your Program's Arguments: Arguments.
The _environment._
Your program normally inherits its environment from GDB, but you
can use the GDB commands `set environment' and `unset environment'
to change parts of the environment that affect your program.
*Note Your Program's Environment: Environment.
The _working directory._
Your program inherits its working directory from GDB. You can set
the GDB working directory with the `cd' command in GDB. *Note
Your Program's Working Directory: Working Directory.
The _standard input and output._
Your program normally uses the same device for standard input and
standard output as GDB is using. You can redirect input and output
in the `run' command line, or you can use the `tty' command to set
a different device for your program. *Note Your Program's Input
and Output: Input/Output.
_Warning:_ While input and output redirection work, you cannot use
pipes to pass the output of the program you are debugging to
another program; if you attempt this, GDB is likely to wind up
debugging the wrong program.
When you issue the `run' command, your program begins to execute
immediately. *Note Stopping and Continuing: Stopping, for discussion
of how to arrange for your program to stop. Once your program has
stopped, you may call functions in your program, using the `print' or
`call' commands. *Note Examining Data: Data.
If the modification time of your symbol file has changed since the
last time GDB read its symbols, GDB discards its symbol table, and
reads it again. When it does this, GDB tries to retain your current
breakpoints.
`start'
The name of the main procedure can vary from language to language.
With C or C++, the main procedure name is always `main', but other
languages such as Ada do not require a specific name for their
main procedure. The debugger provides a convenient way to start
the execution of the program and to stop at the beginning of the
main procedure, depending on the language used.
The `start' command does the equivalent of setting a temporary
breakpoint at the beginning of the main procedure and then invoking
the `run' command.
Some programs contain an "elaboration" phase where some startup
code is executed before the main procedure is called. This
depends on the languages used to write your program. In C++, for
instance, constructors for static and global objects are executed
before `main' is called. It is therefore possible that the
debugger stops before reaching the main procedure. However, the
temporary breakpoint will remain to halt execution.
Specify the arguments to give to your program as arguments to the
`start' command. These arguments will be given verbatim to the
underlying `run' command. Note that the same arguments will be
reused if no argument is provided during subsequent calls to
`start' or `run'.
It is sometimes necessary to debug the program during elaboration.
In these cases, using the `start' command would stop the
execution of your program too late, as the program would have
already completed the elaboration phase. Under these
circumstances, insert breakpoints in your elaboration code before
running your program.
File: gdb.info, Node: Arguments, Next: Environment, Prev: Starting, Up: Running
4.3 Your Program's Arguments
============================
The arguments to your program can be specified by the arguments of the
`run' command. They are passed to a shell, which expands wildcard
characters and performs redirection of I/O, and thence to your program.
Your `SHELL' environment variable (if it exists) specifies what shell
GDB uses. If you do not define `SHELL', GDB uses the default shell
(`/bin/sh' on Unix).
On non-Unix systems, the program is usually invoked directly by GDB,
which emulates I/O redirection via the appropriate system calls, and
the wildcard characters are expanded by the startup code of the
program, not by the shell.
`run' with no arguments uses the same arguments used by the previous
`run', or those set by the `set args' command.
`set args'
Specify the arguments to be used the next time your program is
run. If `set args' has no arguments, `run' executes your program
with no arguments. Once you have run your program with arguments,
using `set args' before the next `run' is the only way to run it
again without arguments.
`show args'
Show the arguments to give your program when it is started.
File: gdb.info, Node: Environment, Next: Working Directory, Prev: Arguments, Up: Running
4.4 Your Program's Environment
==============================
The "environment" consists of a set of environment variables and their
values. Environment variables conventionally record such things as
your user name, your home directory, your terminal type, and your search
path for programs to run. Usually you set up environment variables with
the shell and they are inherited by all the other programs you run.
When debugging, it can be useful to try running your program with a
modified environment without having to start GDB over again.
`path DIRECTORY'
Add DIRECTORY to the front of the `PATH' environment variable (the
search path for executables) that will be passed to your program.
The value of `PATH' used by GDB does not change. You may specify
several directory names, separated by whitespace or by a
system-dependent separator character (`:' on Unix, `;' on MS-DOS
and MS-Windows). If DIRECTORY is already in the path, it is moved
to the front, so it is searched sooner.
You can use the string `$cwd' to refer to whatever is the current
working directory at the time GDB searches the path. If you use
`.' instead, it refers to the directory where you executed the
`path' command. GDB replaces `.' in the DIRECTORY argument (with
the current path) before adding DIRECTORY to the search path.
`show paths'
Display the list of search paths for executables (the `PATH'
environment variable).
`show environment [VARNAME]'
Print the value of environment variable VARNAME to be given to
your program when it starts. If you do not supply VARNAME, print
the names and values of all environment variables to be given to
your program. You can abbreviate `environment' as `env'.
`set environment VARNAME [=VALUE]'
Set environment variable VARNAME to VALUE. The value changes for
your program only, not for GDB itself. VALUE may be any string;
the values of environment variables are just strings, and any
interpretation is supplied by your program itself. The VALUE
parameter is optional; if it is eliminated, the variable is set to
a null value.
For example, this command:
set env USER = foo
tells the debugged program, when subsequently run, that its user
is named `foo'. (The spaces around `=' are used for clarity here;
they are not actually required.)
`unset environment VARNAME'
Remove variable VARNAME from the environment to be passed to your
program. This is different from `set env VARNAME ='; `unset
environment' removes the variable from the environment, rather
than assigning it an empty value.
_Warning:_ On Unix systems, GDB runs your program using the shell
indicated by your `SHELL' environment variable if it exists (or
`/bin/sh' if not). If your `SHELL' variable names a shell that runs an
initialization file--such as `.cshrc' for C-shell, or `.bashrc' for
BASH--any variables you set in that file affect your program. You may
wish to move setting of environment variables to files that are only
run when you sign on, such as `.login' or `.profile'.
File: gdb.info, Node: Working Directory, Next: Input/Output, Prev: Environment, Up: Running
4.5 Your Program's Working Directory
====================================
Each time you start your program with `run', it inherits its working
directory from the current working directory of GDB. The GDB working
directory is initially whatever it inherited from its parent process
(typically the shell), but you can specify a new working directory in
GDB with the `cd' command.
The GDB working directory also serves as a default for the commands
that specify files for GDB to operate on. *Note Commands to Specify
Files: Files.
`cd DIRECTORY'
Set the GDB working directory to DIRECTORY.
`pwd'
Print the GDB working directory.
It is generally impossible to find the current working directory of
the process being debugged (since a program can change its directory
during its run). If you work on a system where GDB is configured with
the `/proc' support, you can use the `info proc' command (*note SVR4
Process Information::) to find out the current working directory of the
debuggee.
File: gdb.info, Node: Input/Output, Next: Attach, Prev: Working Directory, Up: Running
4.6 Your Program's Input and Output
===================================
By default, the program you run under GDB does input and output to the
same terminal that GDB uses. GDB switches the terminal to its own
terminal modes to interact with you, but it records the terminal modes
your program was using and switches back to them when you continue
running your program.
`info terminal'
Displays information recorded by GDB about the terminal modes your
program is using.
You can redirect your program's input and/or output using shell
redirection with the `run' command. For example,
run > outfile
starts your program, diverting its output to the file `outfile'.
Another way to specify where your program should do input and output
is with the `tty' command. This command accepts a file name as
argument, and causes this file to be the default for future `run'
commands. It also resets the controlling terminal for the child
process, for future `run' commands. For example,
tty /dev/ttyb
directs that processes started with subsequent `run' commands default
to do input and output on the terminal `/dev/ttyb' and have that as
their controlling terminal.
An explicit redirection in `run' overrides the `tty' command's
effect on the input/output device, but not its effect on the controlling
terminal.
When you use the `tty' command or redirect input in the `run'
command, only the input _for your program_ is affected. The input for
GDB still comes from your terminal. `tty' is an alias for `set
inferior-tty'.
You can use the `show inferior-tty' command to tell GDB to display
the name of the terminal that will be used for future runs of your
program.
`set inferior-tty /dev/ttyb'
Set the tty for the program being debugged to /dev/ttyb.
`show inferior-tty'
Show the current tty for the program being debugged.
File: gdb.info, Node: Attach, Next: Kill Process, Prev: Input/Output, Up: Running
4.7 Debugging an Already-running Process
========================================
`attach PROCESS-ID'
This command attaches to a running process--one that was started
outside GDB. (`info files' shows your active targets.) The
command takes as argument a process ID. The usual way to find out
the PROCESS-ID of a Unix process is with the `ps' utility, or with
the `jobs -l' shell command.
`attach' does not repeat if you press <RET> a second time after
executing the command.
To use `attach', your program must be running in an environment
which supports processes; for example, `attach' does not work for
programs on bare-board targets that lack an operating system. You must
also have permission to send the process a signal.
When you use `attach', the debugger finds the program running in the
process first by looking in the current working directory, then (if the
program is not found) by using the source file search path (*note
Specifying Source Directories: Source Path.). You can also use the
`file' command to load the program. *Note Commands to Specify Files:
Files.
The first thing GDB does after arranging to debug the specified
process is to stop it. You can examine and modify an attached process
with all the GDB commands that are ordinarily available when you start
processes with `run'. You can insert breakpoints; you can step and
continue; you can modify storage. If you would rather the process
continue running, you may use the `continue' command after attaching
GDB to the process.
`detach'
When you have finished debugging the attached process, you can use
the `detach' command to release it from GDB control. Detaching
the process continues its execution. After the `detach' command,
that process and GDB become completely independent once more, and
you are ready to `attach' another process or start one with `run'.
`detach' does not repeat if you press <RET> again after executing
the command.
If you exit GDB while you have an attached process, you detach that
process. If you use the `run' command, you kill that process. By
default, GDB asks for confirmation if you try to do either of these
things; you can control whether or not you need to confirm by using the
`set confirm' command (*note Optional Warnings and Messages:
Messages/Warnings.).
File: gdb.info, Node: Kill Process, Next: Threads, 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: Threads, Next: Processes, Prev: Kill Process, Up: Running
4.9 Debugging Programs with Multiple Threads
============================================
In some operating systems, such as HP-UX and Solaris, a single program
may have more than one "thread" of execution. The precise semantics of
threads differ from one operating system to another, but in general the
threads of a single program are akin to multiple processes--except that
they share one address space (that is, they can all examine and modify
the same variables). On the other hand, each thread has its own
registers and execution stack, and perhaps private memory.
GDB provides these facilities for debugging multi-thread programs:
* automatic notification of new threads
* `thread THREADNO', a command to switch among threads
* `info threads', a command to inquire about existing threads
* `thread apply [THREADNO] [ALL] ARGS', a command to apply a command
to a list of threads
* thread-specific breakpoints
* `set print thread-events', which controls printing of messages on
thread start and exit.
_Warning:_ These facilities are not yet available on every GDB
configuration where the operating system supports threads. If
your GDB does not support threads, these commands have no effect.
For example, a system without thread support shows no output from
`info threads', and always rejects the `thread' command, like this:
(gdb) info threads
(gdb) thread 1
Thread ID 1 not known. Use the "info threads" command to
see the IDs of currently known threads.
The GDB thread debugging facility allows you to observe all threads
while your program runs--but whenever GDB takes control, one thread in
particular is always the focus of debugging. This thread is called the
"current thread". Debugging commands show program information from the
perspective of the current thread.
Whenever GDB detects a new thread in your program, it displays the
target system's identification for the thread with a message in the
form `[New SYSTAG]'. SYSTAG is a thread identifier whose form varies
depending on the particular system. For example, on GNU/Linux, you
might see
[New Thread 46912507313328 (LWP 25582)]
when GDB notices a new thread. In contrast, on an SGI system, the
SYSTAG is simply something like `process 368', with no further
qualifier.
For debugging purposes, GDB associates its own thread number--always
a single integer--with each thread in your program.
`info threads'
Display a summary of all threads currently in your program. GDB
displays for each thread (in this order):
1. the thread number assigned by GDB
2. the target system's thread identifier (SYSTAG)
3. the current stack frame summary for that thread
An asterisk `*' to the left of the GDB thread number indicates the
current thread.
For example,
(gdb) info threads
3 process 35 thread 27 0x34e5 in sigpause ()
2 process 35 thread 23 0x34e5 in sigpause ()
* 1 process 35 thread 13 main (argc=1, argv=0x7ffffff8)
at threadtest.c:68
On HP-UX systems:
For debugging purposes, GDB associates its own thread number--a
small integer assigned in thread-creation order--with each thread in
your program.
Whenever GDB detects a new thread in your program, it displays both
GDB's thread number and the target system's identification for the
thread with a message in the form `[New SYSTAG]'. SYSTAG is a thread
identifier whose form varies depending on the particular system. For
example, on HP-UX, you see
[New thread 2 (system thread 26594)]
when GDB notices a new thread.
`info threads'
Display a summary of all threads currently in your program. GDB
displays for each thread (in this order):
1. the thread number assigned by GDB
2. the target system's thread identifier (SYSTAG)
3. the current stack frame summary for that thread
An asterisk `*' to the left of the GDB thread number indicates the
current thread.
For example,
(gdb) info threads
* 3 system thread 26607 worker (wptr=0x7b09c318 "@") \
at quicksort.c:137
2 system thread 26606 0x7b0030d8 in __ksleep () \
from /usr/lib/libc.2
1 system thread 27905 0x7b003498 in _brk () \
from /usr/lib/libc.2
On Solaris, you can display more information about user threads with
a Solaris-specific command:
`maint info sol-threads'
Display info on Solaris user threads.
`thread THREADNO'
Make thread number THREADNO the current thread. The command
argument THREADNO is the internal GDB thread number, as shown in
the first field of the `info threads' display. GDB responds by
displaying the system identifier of the thread you selected, and
its current stack frame summary:
(gdb) thread 2
[Switching to process 35 thread 23]
0x34e5 in sigpause ()
As with the `[New ...]' message, the form of the text after
`Switching to' depends on your system's conventions for identifying
threads.
`thread apply [THREADNO] [ALL] COMMAND'
The `thread apply' command allows you to apply the named COMMAND
to one or more threads. Specify the numbers of the threads that
you want affected with the command argument THREADNO. It can be a
single thread number, one of the numbers shown in the first field
of the `info threads' display; or it could be a range of thread
numbers, as in `2-4'. To apply a command to all threads, type
`thread apply all COMMAND'.
`set print thread-events'
`set print thread-events on'
`set print thread-events off'
The `set print thread-events' command allows you to enable or
disable printing of messages when GDB notices that new threads have
started or that threads have exited. By default, these messages
will be printed if detection of these events is supported by the
target. Note that these messages cannot be disabled on all
targets.
`show print thread-events'
Show whether messages will be printed when GDB detects that threads
have started and exited.
Whenever GDB stops your program, due to a breakpoint or a signal, it
automatically selects the thread where that breakpoint or signal
happened. GDB alerts you to the context switch with a message of the
form `[Switching to SYSTAG]' to identify the thread.
*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.
File: gdb.info, Node: Processes, Next: Checkpoint/Restart, Prev: Threads, Up: Running
4.10 Debugging Programs with Multiple Processes
===============================================
On most systems, GDB has no special support for debugging programs
which create additional processes using the `fork' function. When a
program forks, GDB will continue to debug the parent process and the
child process will run unimpeded. If you have set a breakpoint in any
code which the child then executes, the child will get a `SIGTRAP'
signal which (unless it catches the signal) will cause it to terminate.
However, if you want to debug the child process there is a workaround
which isn't too painful. Put a call to `sleep' in the code which the
child process executes after the fork. It may be useful to sleep only
if a certain environment variable is set, or a certain file exists, so
that the delay need not occur when you don't want to run GDB on the
child. While the child is sleeping, use the `ps' program to get its
process ID. Then tell GDB (a new invocation of GDB if you are also
debugging the parent process) to attach to the child process (*note
Attach::). From that point on you can debug the child process just
like any other process which you attached to.
On some systems, GDB provides support for debugging programs that
create additional processes using the `fork' or `vfork' functions.
Currently, the only platforms with this feature are HP-UX (11.x and
later only?) and GNU/Linux (kernel version 2.5.60 and later).
By default, when a program forks, GDB will continue to debug the
parent process and the child process will run unimpeded.
If you want to follow the child process instead of the parent
process, use the command `set follow-fork-mode'.
`set follow-fork-mode MODE'
Set the debugger response to a program call of `fork' or `vfork'.
A call to `fork' or `vfork' creates a new process. The MODE
argument can be:
`parent'
The original process is debugged after a fork. The child
process runs unimpeded. This is the default.
`child'
The new process is debugged after a fork. The parent process
runs unimpeded.
`show follow-fork-mode'
Display the current debugger response to a `fork' or `vfork' call.
On Linux, if you want to debug both the parent and child processes,
use the command `set detach-on-fork'.
`set detach-on-fork MODE'
Tells gdb whether to detach one of the processes after a fork, or
retain debugger control over them both.
`on'
The child process (or parent process, depending on the value
of `follow-fork-mode') will be detached and allowed to run
independently. This is the default.
`off'
Both processes will be held under the control of GDB. One
process (child or parent, depending on the value of
`follow-fork-mode') is debugged as usual, while the other is
held suspended.
`show detach-on-fork'
Show whether detach-on-fork mode is on/off.
If you choose to set `detach-on-fork' mode off, then GDB will retain
control of all forked processes (including nested forks). You can list
the forked processes under the control of GDB by using the `info forks'
command, and switch from one fork to another by using the `fork'
command.
`info forks'
Print a list of all forked processes under the control of GDB.
The listing will include a fork id, a process id, and the current
position (program counter) of the process.
`fork FORK-ID'
Make fork number FORK-ID the current process. The argument
FORK-ID is the internal fork number assigned by GDB, as shown in
the first field of the `info forks' display.
`process PROCESS-ID'
Make process number PROCESS-ID the current process. The argument
PROCESS-ID must be one that is listed in the output of `info
forks'.
To quit debugging one of the forked processes, you can either detach
from it by using the `detach fork' command (allowing it to run
independently), or delete (and kill) it using the `delete fork' command.
`detach fork FORK-ID'
Detach from the process identified by GDB fork number FORK-ID, and
remove it from the fork list. The process will be allowed to run
independently.
`delete fork FORK-ID'
Kill the process identified by GDB fork number FORK-ID, and remove
it from the fork list.
If you ask to debug a child process and a `vfork' is followed by an
`exec', GDB executes the new target up to the first breakpoint in the
new target. If you have a breakpoint set on `main' in your original
program, the breakpoint will also be set on the child process's `main'.
When a child process is spawned by `vfork', you cannot debug the
child or parent until an `exec' call completes.
If you issue a `run' command to GDB after an `exec' call executes,
the new target restarts. To restart the parent process, use the `file'
command with the parent executable name as its argument.
You can use the `catch' command to make GDB stop whenever a `fork',
`vfork', or `exec' call is made. *Note Setting Catchpoints: Set
Catchpoints.
File: gdb.info, Node: Checkpoint/Restart, Prev: Processes, Up: Running
4.11 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.
4.11.1 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: Stack, Prev: Running, Up: Top
5 Stopping and Continuing
*************************
The principal purposes of using a debugger are so that you can stop your
program before it terminates; or so that, if your program runs into
trouble, you can investigate and find out why.
Inside GDB, your program may stop for any of several reasons, such
as a signal, a breakpoint, or reaching a new line after a GDB command
such as `step'. You may then examine and change variables, set new
breakpoints or remove old ones, and then continue execution. Usually,
the messages shown by GDB provide ample explanation of the status of
your program--but you can also explicitly request this information at
any time.
`info program'
Display information about the status of your program: whether it is
running or not, what process it is, and why it stopped.
* Menu:
* Breakpoints:: Breakpoints, watchpoints, and catchpoints
* Continuing and Stepping:: Resuming execution
* Signals:: Signals
* Thread Stops:: Stopping and starting multi-thread programs
File: gdb.info, Node: Breakpoints, Next: Continuing and Stepping, Up: Stopping
5.1 Breakpoints, Watchpoints, and Catchpoints
=============================================
A "breakpoint" makes your program stop whenever a certain point in the
program is reached. For each breakpoint, you can add conditions to
control in finer detail whether your program stops. You can set
breakpoints with the `break' command and its variants (*note Setting
Breakpoints: Set Breaks.), to specify the place where your program
should stop by line number, function name or exact address in the
program.
On some systems, you can set breakpoints in shared libraries before
the executable is run. There is a minor limitation on HP-UX systems:
you must wait until the executable is run in order to set breakpoints
in shared library routines that are not called directly by the program
(for example, routines that are arguments in a `pthread_create' call).
A "watchpoint" is a special breakpoint that stops your program when
the value of an expression changes. The expression may be a value of a
variable, or it could involve values of one or more variables combined
by operators, such as `a + b'. This is sometimes called "data
breakpoints". You must use a different command to set watchpoints
(*note Setting Watchpoints: Set Watchpoints.), but aside from that, you
can manage a watchpoint like any other breakpoint: you enable, disable,
and delete both breakpoints and watchpoints using the same commands.
You can arrange to have values from your program displayed
automatically whenever GDB stops at a breakpoint. *Note Automatic
Display: Auto Display.
A "catchpoint" is another special breakpoint that stops your program
when a certain kind of event occurs, such as the throwing of a C++
exception or the loading of a library. As with watchpoints, you use a
different command to set a catchpoint (*note Setting Catchpoints: Set
Catchpoints.), but aside from that, you can manage a catchpoint like any
other breakpoint. (To stop when your program receives a signal, use the
`handle' command; see *Note Signals: Signals.)
GDB assigns a number to each breakpoint, watchpoint, or catchpoint
when you create it; these numbers are successive integers starting with
one. In many of the commands for controlling various features of
breakpoints you use the breakpoint number to say which breakpoint you
want to change. Each breakpoint may be "enabled" or "disabled"; if
disabled, it has no effect on your program until you enable it again.
Some GDB commands accept a range of breakpoints on which to operate.
A breakpoint range is either a single breakpoint number, like `5', or
two such numbers, in increasing order, separated by a hyphen, like
`5-7'. When a breakpoint range is given to a command, all breakpoints
in that range are operated on.
* Menu:
* Set Breaks:: Setting breakpoints
* Set Watchpoints:: Setting watchpoints
* Set Catchpoints:: Setting catchpoints
* Delete Breaks:: Deleting breakpoints
* Disabling:: Disabling breakpoints
* Conditions:: Break conditions
* Break Commands:: Breakpoint command lists
* Breakpoint Menus:: Breakpoint menus
* 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 Breakpoint Menus: Breakpoint Menus, for a
discussion of that situation.
`break'
When called without any arguments, `break' sets a breakpoint at
the next instruction to be executed in the selected stack frame
(*note Examining the Stack: Stack.). In any selected frame but the
innermost, this makes your program stop as soon as control returns
to that frame. This is similar to the effect of a `finish'
command in the frame inside the selected frame--except that
`finish' does not leave an active breakpoint. If you use `break'
without an argument in the innermost frame, GDB stops the next
time it reaches the current location; this may be useful inside
loops.
GDB normally ignores breakpoints when it resumes execution, until
at least one instruction has been executed. If it did not do
this, you would be unable to proceed past a breakpoint without
first disabling the breakpoint. This rule applies whether or not
the breakpoint already existed when your program stopped.
`break ... if COND'
Set a breakpoint with condition COND; evaluate the expression COND
each time the breakpoint is reached, and stop only if the value is
nonzero--that is, if COND evaluates as true. `...' stands for one
of the possible arguments described above (or no argument)
specifying where to break. *Note Break Conditions: Conditions,
for more information on breakpoint conditions.
`tbreak ARGS'
Set a breakpoint enabled only for one stop. ARGS are the same as
for the `break' command, and the breakpoint is set in the same
way, but the breakpoint is automatically deleted after the first
time your program stops there. *Note Disabling Breakpoints:
Disabling.
`hbreak ARGS'
Set a hardware-assisted breakpoint. ARGS are the same as for the
`break' command and the breakpoint is set in the same way, but the
breakpoint requires hardware support and some target hardware may
not have this support. The main purpose of this is EPROM/ROM code
debugging, so you can set a breakpoint at an instruction without
changing the instruction. This can be used with the new
trap-generation provided by SPARClite DSU and most x86-based
targets. These targets will generate traps when a program
accesses some data or instruction address that is assigned to the
debug registers. However the hardware breakpoint registers can
take a limited number of breakpoints. For example, on the DSU,
only two data breakpoints can be set at a time, and GDB will
reject this command if more than two are used. Delete or disable
unused hardware breakpoints before setting new ones (*note
Disabling Breakpoints: Disabling.). *Note Break Conditions:
Conditions. For remote targets, you can restrict the number of
hardware breakpoints GDB will use, see *Note set remote
hardware-breakpoint-limit::.
`thbreak ARGS'
Set a hardware-assisted breakpoint enabled only for one stop. ARGS
are the same as for the `hbreak' command and the breakpoint is set
in the same way. However, like the `tbreak' command, the
breakpoint is automatically deleted after the first time your
program stops there. Also, like the `hbreak' command, the
breakpoint requires hardware support and some target hardware may
not have this support. *Note Disabling Breakpoints: Disabling.
See also *Note Break Conditions: Conditions.
`rbreak REGEX'
Set breakpoints on all functions matching the regular expression
REGEX. This command sets an unconditional breakpoint on all
matches, printing a list of all breakpoints it set. Once these
breakpoints are set, they are treated just like the breakpoints
set with the `break' command. You can delete them, disable them,
or make them conditional the same way as any other breakpoint.
The syntax of the regular expression is the standard one used with
tools like `grep'. Note that this is different from the syntax
used by shells, so for instance `foo*' matches all functions that
include an `fo' followed by zero or more `o's. There is an
implicit `.*' leading and trailing the regular expression you
supply, so to match only functions that begin with `foo', use
`^foo'.
When debugging C++ programs, `rbreak' is useful for setting
breakpoints on overloaded functions that are not members of any
special classes.
The `rbreak' command can be used to set breakpoints in *all* the
functions in a program, like this:
(gdb) rbreak .
`info breakpoints [N]'
`info break [N]'
`info watchpoints [N]'
Print a table of all breakpoints, watchpoints, and catchpoints set
and not deleted. Optional argument N means print information only
about the specified breakpoint (or watchpoint or catchpoint). For
each breakpoint, following columns are printed:
_Breakpoint Numbers_
_Type_
Breakpoint, watchpoint, or catchpoint.
_Disposition_
Whether the breakpoint is marked to be disabled or deleted
when hit.
_Enabled or Disabled_
Enabled breakpoints are marked with `y'. `n' marks
breakpoints that are not enabled.
_Address_
Where the breakpoint is in your program, as a memory address.
For a pending breakpoint whose address is not yet known,
this field will contain `<PENDING>'. Such breakpoint won't
fire until a shared library that has the symbol or line
referred by breakpoint is loaded. See below for details. A
breakpoint with several locations will have `<MULTIPLE>' in
this field--see below for details.
_What_
Where the breakpoint is in the source for your program, as a
file and line number. For a pending breakpoint, the original
string passed to the breakpoint command will be listed as it
cannot be resolved until the appropriate shared library is
loaded in the future.
If a breakpoint is conditional, `info break' shows the condition on
the line following the affected breakpoint; breakpoint commands,
if any, are listed after that. A pending breakpoint is allowed to
have a condition specified for it. The condition is not parsed
for validity until a shared library is loaded that allows the
pending breakpoint to resolve to a valid location.
`info break' with a breakpoint number N as argument lists only
that breakpoint. The convenience variable `$_' and the default
examining-address for the `x' command are set to the address of
the last breakpoint listed (*note Examining Memory: Memory.).
`info break' displays a count of the number of times the breakpoint
has been hit. This is especially useful in conjunction with the
`ignore' command. You can ignore a large number of breakpoint
hits, look at the breakpoint info to see how many times the
breakpoint was hit, and then run again, ignoring one less than
that number. This will get you quickly to the last hit of that
breakpoint.
GDB allows you to set any number of breakpoints at the same place in
your program. There is nothing silly or meaningless about this. When
the breakpoints are conditional, this is even useful (*note Break
Conditions: Conditions.).
It is possible that a breakpoint corresponds to several locations in
your program. Examples of this situation are:
* For a C++ constructor, the GCC compiler generates several
instances of the function body, used in different cases.
* For a C++ template function, a given line in the function can
correspond to any number of instantiations.
* For an inlined function, a given source line can correspond to
several places where that function is inlined.
In all those cases, GDB will insert a breakpoint at all the relevant
locations.
A breakpoint with multiple locations is displayed in the breakpoint
table using several rows--one header row, followed by one row for each
breakpoint location. The header row has `<MULTIPLE>' in the address
column. The rows for individual locations contain the actual addresses
for locations, and show the functions to which those locations belong.
The number column for a location is of the form
BREAKPOINT-NUMBER.LOCATION-NUMBER.
For example:
Num Type Disp Enb Address What
1 breakpoint keep y <MULTIPLE>
stop only if i==1
breakpoint already hit 1 time
1.1 y 0x080486a2 in void foo<int>() at t.cc:8
1.2 y 0x080486ca in void foo<double>() at t.cc:8
Each location can be individually enabled or disabled by passing
BREAKPOINT-NUMBER.LOCATION-NUMBER as argument to the `enable' and
`disable' commands. Note that you cannot delete the individual
locations from the list, you can only delete the entire list of
locations that belong to their parent breakpoint (with the `delete NUM'
command, where NUM is the number of the parent breakpoint, 1 in the
above example). Disabling or enabling the parent breakpoint (*note
Disabling::) affects all of the locations that belong to that
breakpoint.
It's quite common to have a breakpoint inside a shared library.
Shared libraries can be loaded and unloaded explicitly, and possibly
repeatedly, as the program is executed. To support this use case, GDB
updates breakpoint locations whenever any shared library is loaded or
unloaded. Typically, you would set a breakpoint in a shared library at
the beginning of your debugging session, when the library is not
loaded, and when the symbols from the library are not available. When
you try to set breakpoint, GDB will ask you if you want to set a so
called "pending breakpoint"--breakpoint whose address is not yet
resolved.
After the program is run, whenever a new shared library is loaded,
GDB reevaluates all the breakpoints. When a newly loaded shared
library contains the symbol or line referred to by some pending
breakpoint, that breakpoint is resolved and becomes an ordinary
breakpoint. When a library is unloaded, all breakpoints that refer to
its symbols or source lines become pending again.
This logic works for breakpoints with multiple locations, too. For
example, if you have a breakpoint in a C++ template function, and a
newly loaded shared library has an instantiation of that template, a
new location is added to the list of locations for the breakpoint.
Except for having unresolved address, pending breakpoints do not
differ from regular breakpoints. You can set conditions or commands,
enable and disable them and perform other breakpoint operations.
GDB provides some additional commands for controlling what happens
when the `break' command cannot resolve breakpoint address
specification to an address:
`set breakpoint pending auto'
This is the default behavior. When GDB cannot find the breakpoint
location, it queries you whether a pending breakpoint should be
created.
`set breakpoint pending on'
This indicates that an unrecognized breakpoint location should
automatically result in a pending breakpoint being created.
`set breakpoint pending off'
This indicates that pending breakpoints are not to be created. Any
unrecognized breakpoint location results in an error. This
setting does not affect any pending breakpoints previously created.
`show breakpoint pending'
Show the current behavior setting for creating pending breakpoints.
The settings above only affect the `break' command and its variants.
Once breakpoint is set, it will be automatically updated as shared
libraries are loaded and unloaded.
For some targets, GDB can automatically decide if hardware or
software breakpoints should be used, depending on whether the
breakpoint address is read-only or read-write. This applies to
breakpoints set with the `break' command as well as to internal
breakpoints set by commands like `next' and `finish'. For breakpoints
set with `hbreak', GDB will always use hardware breakpoints.
You can control this automatic behaviour with the following
commands::
`set breakpoint auto-hw on'
This is the default behavior. When GDB sets a breakpoint, it will
try to use the target memory map to decide if software or hardware
breakpoint must be used.
`set breakpoint auto-hw off'
This indicates GDB should not automatically select breakpoint
type. If the target provides a memory map, GDB will warn when
trying to set software breakpoint at a read-only address.
GDB 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::).
Depending on your system, watchpoints may be implemented in software
or hardware. GDB does software watchpointing by single-stepping your
program and testing the variable's value each time, which is hundreds of
times slower than normal execution. (But this may still be worth it, to
catch errors where you have no clue what part of your program is the
culprit.)
On some systems, such as HP-UX, PowerPC, GNU/Linux and most other
x86-based targets, GDB includes support for hardware watchpoints, which
do not slow down the running of your program.
`watch EXPR [thread THREADNUM]'
Set a watchpoint for an expression. GDB will break when the
expression EXPR is written into by the program and its value
changes. The simplest (and the most popular) use of this command
is to watch the value of a single variable:
(gdb) watch foo
If the command includes a `[thread THREADNUM]' clause, GDB breaks
only when the thread identified by THREADNUM changes the value of
EXPR. If any other threads change the value of EXPR, GDB will not
break. Note that watchpoints restricted to a single thread in
this way only work with Hardware Watchpoints.
`rwatch EXPR [thread THREADNUM]'
Set a watchpoint that will break when the value of EXPR is read by
the program.
`awatch EXPR [thread THREADNUM]'
Set a watchpoint that will break when EXPR is either read from or
written into by the program.
`info watchpoints'
This command prints a list of watchpoints, breakpoints, and
catchpoints; it is the same as `info break' (*note Set Breaks::).
GDB sets a "hardware watchpoint" if possible. Hardware watchpoints
execute very quickly, and the debugger reports a change in value at the
exact instruction where the change occurs. If GDB cannot set a
hardware watchpoint, it sets a software watchpoint, which executes more
slowly and reports the change in value at the next _statement_, not the
instruction, after the change occurs.
You can force GDB to use only software watchpoints with the `set
can-use-hw-watchpoints 0' command. With this variable set to zero, GDB
will never try to use hardware watchpoints, even if the underlying
system supports them. (Note that hardware-assisted watchpoints that
were set _before_ setting `can-use-hw-watchpoints' to zero will still
use the hardware mechanism of watching 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.
The SPARClite DSU will generate traps when a program accesses some
data or instruction address that is assigned to the debug registers.
For the data addresses, DSU facilitates the `watch' command. However
the hardware breakpoint registers can only take two data watchpoints,
and both watchpoints must be the same kind. For example, you can set
two watchpoints with `watch' commands, two with `rwatch' commands, *or*
two with `awatch' commands, but you cannot set one watchpoint with one
command and the other with a different command. GDB will reject the
command if you try to mix watchpoints. Delete or disable unused
watchpoint commands before setting new ones.
If you call a function interactively using `print' or `call', any
watchpoints you have set will be inactive until GDB reaches another
kind of breakpoint or the call completes.
GDB automatically deletes watchpoints that watch local (automatic)
variables, or expressions that involve such variables, when they go out
of scope, that is, when the execution leaves the block in which these
variables were defined. In particular, when the program being debugged
terminates, _all_ local variables go out of scope, and so only
watchpoints that watch global variables remain set. If you rerun the
program, you will need to set all such watchpoints again. One way of
doing that would be to set a code breakpoint at the entry to the `main'
function and when it breaks, set all the watchpoints.
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.)
Software watchpoints single-step the current thread to track the
changes. Other threads are left freely running on `continue';
therefore, their changes cannot be caught. To get more reliable
software watchpoints, please use `set scheduler-locking on'. The
default for Red Hat/Fedora GDB is `set scheduler-locking step',
which makes the software watchpoints safe for the `step' command,
but not for the `continue' command. *Note Thread Stops::.
*Note set remote hardware-watchpoint-limit::.
File: gdb.info, Node: Set Catchpoints, Next: Delete Breaks, Prev: Set Watchpoints, Up: Breakpoints
5.1.3 Setting Catchpoints
-------------------------
You can use "catchpoints" to cause the debugger to stop for certain
kinds of program events, such as C++ exceptions or the loading of a
shared library. Use the `catch' command to set a catchpoint.
`catch EVENT'
Stop when EVENT occurs. EVENT can be any of the following:
`throw'
The throwing of a C++ exception.
`catch'
The catching of a C++ exception.
`exception'
An Ada exception being raised. If an exception name is
specified at the end of the command (eg `catch exception
Program_Error'), the debugger will stop only when this
specific exception is raised. Otherwise, the debugger stops
execution when any Ada exception is raised.
`exception unhandled'
An exception that was raised but is not handled by the
program.
`assert'
A failed Ada assertion.
`exec'
A call to `exec'. This is currently only available for HP-UX
and GNU/Linux.
`fork'
A call to `fork'. This is currently only available for HP-UX
and GNU/Linux.
`vfork'
A call to `vfork'. This is currently only available for HP-UX
and GNU/Linux.
`load'
`load LIBNAME'
The dynamic loading of any shared library, or the loading of
the library LIBNAME. This is currently only available for
HP-UX.
`unload'
`unload LIBNAME'
The unloading of any dynamically loaded shared library, or
the unloading of the library LIBNAME. This is currently only
available for HP-UX.
`tcatch EVENT'
Set a catchpoint that is enabled only for one stop. The
catchpoint is automatically deleted after the first time the event
is caught.
Use the `info break' command to list the current catchpoints.
There are currently some limitations to C++ exception handling
(`catch throw' and `catch catch') in GDB:
* If you call a function interactively, GDB normally returns control
to you when the function has finished executing. If the call
raises an exception, however, the call may bypass the mechanism
that returns control to you and cause your program either to abort
or to simply continue running until it hits a breakpoint, catches
a signal that GDB is listening for, or exits. This is the case
even if you set a catchpoint for the exception; catchpoints on
exceptions are disabled within interactive calls.
* You cannot raise an exception interactively.
* You cannot install an exception handler interactively.
Sometimes `catch' is not the best way to debug exception handling:
if you need to know exactly where an exception is raised, it is better
to stop _before_ the exception handler is called, since that way you
can see the stack before any unwinding takes place. If you set a
breakpoint in an exception handler instead, it may not be easy to find
out where the exception was raised.
To stop just before an exception handler is called, you need some
knowledge of the implementation. In the case of GNU C++, exceptions are
raised by calling a library function named `__raise_exception' which
has the following ANSI C interface:
/* ADDR is where the exception identifier is stored.
ID is the exception identifier. */
void __raise_exception (void **addr, void *id);
To make the debugger catch all exceptions before any stack unwinding
takes place, set a breakpoint on `__raise_exception' (*note
Breakpoints; Watchpoints; and Exceptions: Breakpoints.).
With a conditional breakpoint (*note Break Conditions: Conditions.)
that depends on the value of ID, you can stop your program when a
specific exception is raised. You can use multiple conditional
breakpoints to stop your program when any of a number of exceptions are
raised.
File: gdb.info, Node: Delete Breaks, Next: Disabling, Prev: Set Catchpoints, Up: Breakpoints
5.1.4 Deleting Breakpoints
--------------------------
It is often necessary to eliminate a breakpoint, watchpoint, or
catchpoint once it has done its job and you no longer want your program
to stop there. This is called "deleting" the breakpoint. A breakpoint
that has been deleted no longer exists; it is forgotten.
With the `clear' command you can delete breakpoints according to
where they are in your program. With the `delete' command you can
delete individual breakpoints, watchpoints, or catchpoints by specifying
their breakpoint numbers.
It is not necessary to delete a breakpoint to proceed past it. GDB
automatically ignores breakpoints on the first instruction to be
executed when you continue execution without changing the execution
address.
`clear'
Delete any breakpoints at the next instruction to be executed in
the selected stack frame (*note Selecting a Frame: Selection.).
When the innermost frame is selected, this is a good way to delete
a breakpoint where your program just stopped.
`clear LOCATION'
Delete any breakpoints set at the specified LOCATION. *Note
Specify Location::, for the various forms of LOCATION; the most
useful ones are listed below:
`clear FUNCTION'
`clear FILENAME:FUNCTION'
Delete any breakpoints set at entry to the named FUNCTION.
`clear LINENUM'
`clear FILENAME:LINENUM'
Delete any breakpoints set at or within the code of the
specified LINENUM of the specified FILENAME.
`delete [breakpoints] [RANGE...]'
Delete the breakpoints, watchpoints, or catchpoints of the
breakpoint ranges specified as arguments. If no argument is
specified, delete all breakpoints (GDB asks confirmation, unless
you have `set confirm off'). You can abbreviate this command as
`d'.
File: gdb.info, Node: Disabling, Next: Conditions, Prev: Delete Breaks, Up: Breakpoints
5.1.5 Disabling Breakpoints
---------------------------
Rather than deleting a breakpoint, watchpoint, or catchpoint, you might
prefer to "disable" it. This makes the breakpoint inoperative as if it
had been deleted, but remembers the information on the breakpoint so
that you can "enable" it again later.
You disable and enable breakpoints, watchpoints, and catchpoints with
the `enable' and `disable' commands, optionally specifying one or more
breakpoint numbers as arguments. Use `info break' or `info watch' to
print a list of breakpoints, watchpoints, and catchpoints if you do not
know which numbers to use.
Disabling and enabling a breakpoint that has multiple locations
affects all of its locations.
A breakpoint, watchpoint, or catchpoint can have any of four
different states of enablement:
* Enabled. The breakpoint stops your program. A breakpoint set
with the `break' command starts out in this state.
* Disabled. The breakpoint has no effect on your program.
* Enabled once. The breakpoint stops your program, but then becomes
disabled.
* Enabled for deletion. The breakpoint stops your program, but
immediately after it does so it is deleted permanently. A
breakpoint set with the `tbreak' command starts out in this state.
You can use the following commands to enable or disable breakpoints,
watchpoints, and catchpoints:
`disable [breakpoints] [RANGE...]'
Disable the specified breakpoints--or all breakpoints, if none are
listed. A disabled breakpoint has no effect but is not forgotten.
All options such as ignore-counts, conditions and commands are
remembered in case the breakpoint is enabled again later. You may
abbreviate `disable' as `dis'.
`enable [breakpoints] [RANGE...]'
Enable the specified breakpoints (or all defined breakpoints).
They become effective once again in stopping your program.
`enable [breakpoints] once RANGE...'
Enable the specified breakpoints temporarily. GDB disables any of
these breakpoints immediately after stopping your program.
`enable [breakpoints] delete RANGE...'
Enable the specified breakpoints to work once, then die. GDB
deletes any of these breakpoints as soon as your program stops
there. Breakpoints set by the `tbreak' command start out in this
state.
Except for a breakpoint set with `tbreak' (*note Setting
Breakpoints: Set Breaks.), breakpoints that you set are initially
enabled; subsequently, they become disabled or enabled only when you
use one of the commands above. (The command `until' can set and delete
a breakpoint of its own, but it does not change the state of your other
breakpoints; see *Note Continuing and Stepping: Continuing and
Stepping.)
File: gdb.info, Node: Conditions, Next: Break Commands, Prev: Disabling, Up: Breakpoints
5.1.6 Break Conditions
----------------------
The simplest sort of breakpoint breaks every time your program reaches a
specified place. You can also specify a "condition" for a breakpoint.
A condition is just a Boolean expression in your programming language
(*note Expressions: Expressions.). A breakpoint with a condition
evaluates the expression each time your program reaches it, and your
program stops only if the condition is _true_.
This is the converse of using assertions for program validation; in
that situation, you want to stop when the assertion is violated--that
is, when the condition is false. In C, if you want to test an
assertion expressed by the condition ASSERT, you should set the
condition `! ASSERT' on the appropriate breakpoint.
Conditions are also accepted for watchpoints; you may not need them,
since a watchpoint is inspecting the value of an expression anyhow--but
it might be simpler, say, to just set a watchpoint on a variable name,
and specify a condition that tests whether the new value is an
interesting one.
Break conditions can have side effects, and may even call functions
in your program. This can be useful, for example, to activate functions
that log program progress, or to use your own print functions to format
special data structures. The effects are completely predictable unless
there is another enabled breakpoint at the same address. (In that
case, GDB might see the other breakpoint first and stop your program
without checking the condition of this one.) Note that breakpoint
commands are usually more convenient and flexible than break conditions
for the purpose of performing side effects when a breakpoint is reached
(*note Breakpoint Command Lists: Break Commands.).
Break conditions can be specified when a breakpoint is set, by using
`if' in the arguments to the `break' command. *Note Setting
Breakpoints: Set Breaks. They can also be changed at any time with the
`condition' command.
You can also use the `if' keyword with the `watch' command. The
`catch' command does not recognize the `if' keyword; `condition' is the
only way to impose a further condition on a catchpoint.
`condition BNUM EXPRESSION'
Specify EXPRESSION as the break condition for breakpoint,
watchpoint, or catchpoint number BNUM. After you set a condition,
breakpoint BNUM stops your program only if the value of EXPRESSION
is true (nonzero, in C). When you use `condition', GDB checks
EXPRESSION immediately for syntactic correctness, and to determine
whether symbols in it have referents in the context of your
breakpoint. If EXPRESSION uses symbols not referenced in the
context of the breakpoint, GDB prints an error message:
No symbol "foo" in current context.
GDB does not actually evaluate EXPRESSION at the time the
`condition' command (or a command that sets a breakpoint with a
condition, like `break if ...') is given, however. *Note
Expressions: Expressions.
`condition BNUM'
Remove the condition from breakpoint number BNUM. It becomes an
ordinary unconditional breakpoint.
A special case of a breakpoint condition is to stop only when the
breakpoint has been reached a certain number of times. This is so
useful that there is a special way to do it, using the "ignore count"
of the breakpoint. Every breakpoint has an ignore count, which is an
integer. Most of the time, the ignore count is zero, and therefore has
no effect. But if your program reaches a breakpoint whose ignore count
is positive, then instead of stopping, it just decrements the ignore
count by one and continues. As a result, if the ignore count value is
N, the breakpoint does not stop the next N times your program reaches
it.
`ignore BNUM COUNT'
Set the ignore count of breakpoint number BNUM to COUNT. The next
COUNT times the breakpoint is reached, your program's execution
does not stop; other than to decrement the ignore count, GDB takes
no action.
To make the breakpoint stop the next time it is reached, specify a
count of zero.
When you use `continue' to resume execution of your program from a
breakpoint, you can specify an ignore count directly as an
argument to `continue', rather than using `ignore'. *Note
Continuing and Stepping: Continuing and Stepping.
If a breakpoint has a positive ignore count and a condition, the
condition is not checked. Once the ignore count reaches zero, GDB
resumes checking the condition.
You could achieve the effect of the ignore count with a condition
such as `$foo-- <= 0' using a debugger convenience variable that
is decremented each time. *Note Convenience Variables:
Convenience Vars.
Ignore counts apply to breakpoints, watchpoints, and catchpoints.
File: gdb.info, Node: Break Commands, Next: Breakpoint Menus, 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 [BNUM]'
`... COMMAND-LIST ...'
`end'
Specify a list of commands for breakpoint number BNUM. 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 BNUM argument, `commands' refers to the last breakpoint,
watchpoint, or catchpoint set (not to the breakpoint most recently
encountered).
Pressing <RET> as a means of repeating the last GDB command is
disabled within a COMMAND-LIST.
You can use breakpoint commands to start your program up again.
Simply use the `continue' command, or `step', or any other command that
resumes execution.
Any other commands in the command list, after a command that resumes
execution, are ignored. This is because any time you resume execution
(even with a simple `next' or `step'), you may encounter another
breakpoint--which could have its own command list, leading to
ambiguities about which list to execute.
If the first command you specify in a command list is `silent', the
usual message about stopping at a breakpoint is not printed. This may
be desirable for breakpoints that are to print a specific message and
then continue. If none of the remaining commands print anything, you
see no sign that the breakpoint was reached. `silent' is meaningful
only at the beginning of a breakpoint command list.
The commands `echo', `output', and `printf' allow you to print
precisely controlled output, and are often useful in silent
breakpoints. *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: Breakpoint Menus, Next: Error in Breakpoints, Prev: Break Commands, Up: Breakpoints
5.1.8 Breakpoint Menus
----------------------
Some programming languages (notably C++ and Objective-C) permit a
single function name to be defined several times, for application in
different contexts. This is called "overloading". When a function
name is overloaded, `break FUNCTION' is not enough to tell GDB where
you want a breakpoint. You can use explicit signature of the function,
as in `break FUNCTION(TYPES)', to specify which particular version of
the function you want. Otherwise, GDB offers you a menu of numbered
choices for different possible breakpoints, and waits for your
selection with the prompt `>'. The first two options are always `[0]
cancel' and `[1] all'. Typing `1' sets a breakpoint at each definition
of FUNCTION, and typing `0' aborts the `break' command without setting
any new breakpoints.
For example, the following session excerpt shows an attempt to set a
breakpoint at the overloaded symbol `String::after'. We choose three
particular definitions of that function name:
(gdb) b String::after
[0] cancel
[1] all
[2] file:String.cc; line number:867
[3] file:String.cc; line number:860
[4] file:String.cc; line number:875
[5] file:String.cc; line number:853
[6] file:String.cc; line number:846
[7] file:String.cc; line number:735
> 2 4 6
Breakpoint 1 at 0xb26c: file String.cc, line 867.
Breakpoint 2 at 0xb344: file String.cc, line 875.
Breakpoint 3 at 0xafcc: file String.cc, line 846.
Multiple breakpoints were set.
Use the "delete" command to delete unwanted
breakpoints.
(gdb)
File: gdb.info, Node: Error in Breakpoints, Next: Breakpoint-related Warnings, Prev: Breakpoint Menus, Up: Breakpoints
5.1.9 "Cannot insert breakpoints"
---------------------------------
Under some operating systems, breakpoints cannot be used in a program if
any other process is running that program. In this situation,
attempting to run or continue a program with a breakpoint causes GDB to
print an error message:
Cannot insert breakpoints.
The same program may be running in another process.
When this happens, you have three ways to proceed:
1. Remove or disable the breakpoints, then continue.
2. Suspend GDB, and copy the file containing your program to a new
name. Resume GDB and use the `exec-file' command to specify that
GDB should run your program under that name. Then start your
program again.
3. Relink your program so that the text segment is nonsharable, using
the linker option `-N'. The operating system limitation may not
apply to nonsharable executables.
A similar message can be printed if you request too many active
hardware-assisted breakpoints and watchpoints:
Stopped; cannot insert breakpoints.
You may have requested too many hardware breakpoints and watchpoints.
This message is printed when you attempt to resume the program, since
only then GDB knows exactly how many hardware breakpoints and
watchpoints it needs to insert.
When this message is printed, you need to disable or remove some of
the hardware-assisted breakpoints and watchpoints, and then continue.
File: gdb.info, Node: Breakpoint-related Warnings, Prev: Error in Breakpoints, Up: Breakpoints
5.1.10 "Breakpoint address adjusted..."
---------------------------------------
Some processor architectures place constraints on the addresses at
which breakpoints may be placed. For architectures thus constrained,
GDB will attempt to adjust the breakpoint's address to comply with the
constraints dictated by the architecture.
One example of such an architecture is the Fujitsu FR-V. The FR-V is
a VLIW architecture in which a number of RISC-like instructions may be
bundled together for parallel execution. The FR-V architecture
constrains the location of a breakpoint instruction within such a
bundle to the instruction with the lowest address. GDB honors this
constraint by adjusting a breakpoint's address to the first in the
bundle.
It is not uncommon for optimized code to have bundles which contain
instructions from different source statements, thus it may happen that
a breakpoint's address will be adjusted from one source statement to
another. Since this adjustment may significantly alter GDB's
breakpoint related behavior from what the user expects, a warning is
printed when the breakpoint is first set and also when the breakpoint
is hit.
A warning like the one below is printed when setting a breakpoint
that's been subject to address adjustment:
warning: Breakpoint address adjusted from 0x00010414 to 0x00010410.
Such warnings are printed both for user settable and GDB's internal
breakpoints. If you see one of these warnings, you should verify that
a breakpoint set at the adjusted address will have the desired affect.
If not, the breakpoint in question may be removed and other breakpoints
may be set which will have the desired behavior. E.g., it may be
sufficient to place the breakpoint at a later instruction. A
conditional breakpoint may also be useful in some cases to prevent the
breakpoint from triggering too often.
GDB will also issue a warning when stopping at one of these adjusted
breakpoints:
warning: Breakpoint 1 address previously adjusted from 0x00010414
to 0x00010410.
When this warning is encountered, it may be too late to take remedial
action except in cases where the breakpoint is hit earlier or more
frequently than expected.
File: gdb.info, Node: Continuing and Stepping, Next: Signals, Prev: Breakpoints, Up: Stopping
5.2 Continuing and Stepping
===========================
"Continuing" means resuming program execution until your program
completes normally. In contrast, "stepping" means executing just one
more "step" of your program, where "step" may mean either one line of
source code, or one machine instruction (depending on what particular
command you use). Either when continuing or when stepping, your
program may stop even sooner, due to a breakpoint or a signal. (If it
stops due to a signal, you may want to use `handle', or use `signal 0'
to resume execution. *Note Signals: Signals.)
`continue [IGNORE-COUNT]'
`c [IGNORE-COUNT]'
`fg [IGNORE-COUNT]'
Resume program execution, at the address where your program last
stopped; any breakpoints set at that address are bypassed. The
optional argument IGNORE-COUNT allows you to specify a further
number of times to ignore a breakpoint at this location; its
effect is like that of `ignore' (*note Break Conditions:
Conditions.).
The argument IGNORE-COUNT is meaningful only when your program
stopped due to a breakpoint. At other times, the argument to
`continue' is ignored.
The synonyms `c' and `fg' (for "foreground", as the debugged
program is deemed to be the foreground program) are provided
purely for convenience, and have exactly the same behavior as
`continue'.
To resume execution at a different place, you can use `return'
(*note Returning from a Function: Returning.) to go back to the calling
function; or `jump' (*note Continuing at a Different Address: Jumping.)
to go to an arbitrary location in your program.
A typical technique for using stepping is to set a breakpoint (*note
Breakpoints; Watchpoints; and Catchpoints: Breakpoints.) at the
beginning of the function or the section of your program where a problem
is believed to lie, run your program until it stops at that breakpoint,
and then step through the suspect area, examining the variables that are
interesting, until you see the problem happen.
`step'
Continue running your program until control reaches a different
source line, then stop it and return control to GDB. This command
is abbreviated `s'.
_Warning:_ If you use the `step' command while control is
within a function that was compiled without debugging
information, execution proceeds until control reaches a
function that does have debugging information. Likewise, it
will not step into a function which is compiled without
debugging information. To step through functions without
debugging information, use the `stepi' command, described
below.
The `step' command only stops at the first instruction of a source
line. This prevents the multiple stops that could otherwise occur
in `switch' statements, `for' loops, etc. `step' continues to
stop if a function that has debugging information is called within
the line. In other words, `step' _steps inside_ any functions
called within the line.
Also, the `step' command only enters a function if there is line
number information for the function. Otherwise it acts like the
`next' command. This avoids problems when using `cc -gl' on MIPS
machines. Previously, `step' entered subroutines if there was any
debugging information about the routine.
`step COUNT'
Continue running as in `step', but do so COUNT times. If a
breakpoint is reached, or a signal not related to stepping occurs
before COUNT steps, stepping stops right away.
`next [COUNT]'
Continue to the next source line in the current (innermost) stack
frame. This is similar to `step', but function calls that appear
within the line of code are executed without stopping. Execution
stops when control reaches a different line of code at the
original stack level that was executing when you gave the `next'
command. This command is abbreviated `n'.
An argument COUNT is a repeat count, as for `step'.
The `next' command only stops at the first instruction of a source
line. This prevents multiple stops that could otherwise occur in
`switch' statements, `for' loops, etc.
`set step-mode'
`set step-mode on'
The `set step-mode on' command causes the `step' command to stop
at the first instruction of a function which contains no debug line
information rather than stepping over it.
This is useful in cases where you may be interested in inspecting
the machine instructions of a function which has no symbolic info
and do not want GDB to automatically skip over this function.
`set step-mode off'
Causes the `step' command to step over any functions which
contains no debug information. This is the default.
`show step-mode'
Show whether GDB will stop in or step over functions without
source line debug information.
`finish'
Continue running until just after function in the selected stack
frame returns. Print the returned value (if any).
Contrast this with the `return' command (*note Returning from a
Function: Returning.).
`until'
`u'
Continue running until a source line past the current line, in the
current stack frame, is reached. This command is used to avoid
single stepping through a loop more than once. It is like the
`next' command, except that when `until' encounters a jump, it
automatically continues execution until the program counter is
greater than the address of the jump.
This means that when you reach the end of a loop after single
stepping though it, `until' makes your program continue execution
until it exits the loop. In contrast, a `next' command at the end
of a loop simply steps back to the beginning of the loop, which
forces you to step through the next iteration.
`until' always stops your program if it attempts to exit the
current stack frame.
`until' may produce somewhat counterintuitive results if the order
of machine code does not match the order of the source lines. For
example, in the following excerpt from a debugging session, the `f'
(`frame') command shows that execution is stopped at line `206';
yet when we use `until', we get to line `195':
(gdb) f
#0 main (argc=4, argv=0xf7fffae8) at m4.c:206
206 expand_input();
(gdb) until
195 for ( ; argc > 0; NEXTARG) {
This happened because, for execution efficiency, the compiler had
generated code for the loop closure test at the end, rather than
the start, of the loop--even though the test in a C `for'-loop is
written before the body of the loop. The `until' command appeared
to step back to the beginning of the loop when it advanced to this
expression; however, it has not really gone to an earlier
statement--not in terms of the actual machine code.
`until' with no argument works by means of single instruction
stepping, and hence is slower than `until' with an argument.
`until LOCATION'
`u LOCATION'
Continue running your program until either the specified location
is reached, or the current stack frame returns. LOCATION is any of
the forms described in *Note Specify Location::. This form of the
command uses temporary breakpoints, and hence is quicker than
`until' without an argument. The specified location is actually
reached only if it is in the current frame. This implies that
`until' can be used to skip over recursive function invocations.
For instance in the code below, if the current location is line
`96', issuing `until 99' will execute the program up to line `99'
in the same invocation of factorial, i.e., after the inner
invocations have returned.
94 int factorial (int value)
95 {
96 if (value > 1) {
97 value *= factorial (value - 1);
98 }
99 return (value);
100 }
`advance LOCATION'
Continue running the program up to the given LOCATION. An
argument is required, which should be of one of the forms
described in *Note Specify Location::. Execution will also stop
upon exit from the current stack frame. This command is similar
to `until', but `advance' will not skip over recursive function
calls, and the target location doesn't have to be in the same
frame as the current one.
`stepi'
`stepi ARG'
`si'
Execute one machine instruction, then stop and return to the
debugger.
It is often useful to do `display/i $pc' when stepping by machine
instructions. This makes GDB automatically display the next
instruction to be executed, each time your program stops. *Note
Automatic Display: Auto Display.
An argument is a repeat count, as in `step'.
`nexti'
`nexti ARG'
`ni'
Execute one machine instruction, but if it is a function call,
proceed until the function returns.
An argument is a repeat count, as in `next'.
File: gdb.info, Node: Signals, Next: Thread Stops, Prev: Continuing and Stepping, Up: Stopping
5.3 Signals
===========
A signal is an asynchronous event that can happen in a program. The
operating system defines the possible kinds of signals, and gives each
kind a name and a number. For example, in Unix `SIGINT' is the signal
a program gets when you type an interrupt character (often `Ctrl-c');
`SIGSEGV' is the signal a program gets from referencing a place in
memory far away from all the areas in use; `SIGALRM' occurs when the
alarm clock timer goes off (which happens only if your program has
requested an alarm).
Some signals, including `SIGALRM', are a normal part of the
functioning of your program. Others, such as `SIGSEGV', indicate
errors; these signals are "fatal" (they kill your program immediately)
if the program has not specified in advance some other way to handle
the signal. `SIGINT' does not indicate an error in your program, but
it is normally fatal so it can carry out the purpose of the interrupt:
to kill the program.
GDB has the ability to detect any occurrence of a signal in your
program. You can tell GDB in advance what to do for each kind of
signal.
Normally, GDB is set up to let the non-erroneous signals like
`SIGALRM' be silently passed to your program (so as not to interfere
with their role in the program's functioning) but to stop your program
immediately whenever an error signal happens. You can change these
settings with the `handle' command.
`info signals'
`info handle'
Print a table of all the kinds of signals and how GDB has been
told to handle each one. You can use this to see the signal
numbers of all the defined types of signals.
`info signals SIG'
Similar, but print information only about the specified signal
number.
`info handle' is an alias for `info signals'.
`handle SIGNAL [KEYWORDS...]'
Change the way GDB handles signal SIGNAL. SIGNAL can be the
number of a signal or its name (with or without the `SIG' at the
beginning); a list of signal numbers of the form `LOW-HIGH'; or
the word `all', meaning all the known signals. Optional arguments
KEYWORDS, described below, say what change to make.
The keywords allowed by the `handle' command can be abbreviated.
Their full names are:
`nostop'
GDB should not stop your program when this signal happens. It may
still print a message telling you that the signal has come in.
`stop'
GDB should stop your program when this signal happens. This
implies the `print' keyword as well.
`print'
GDB should print a message when this signal happens.
`noprint'
GDB should not mention the occurrence of the signal at all. This
implies the `nostop' keyword as well.
`pass'
`noignore'
GDB should allow your program to see this signal; your program can
handle the signal, or else it may terminate if the signal is fatal
and not handled. `pass' and `noignore' are synonyms.
`nopass'
`ignore'
GDB should not allow your program to see this signal. `nopass'
and `ignore' are synonyms.
When a signal stops your program, the signal is not visible to the
program until you continue. Your program sees the signal then, if
`pass' is in effect for the signal in question _at that time_. In
other words, after GDB reports a signal, you can use the `handle'
command with `pass' or `nopass' to control whether your program sees
that signal when you continue.
The default is set to `nostop', `noprint', `pass' for non-erroneous
signals such as `SIGALRM', `SIGWINCH' and `SIGCHLD', and to `stop',
`print', `pass' for the erroneous signals.
You can also use the `signal' command to prevent your program from
seeing a signal, or cause it to see a signal it normally would not see,
or to give it any signal at any time. For example, if your program
stopped due to some sort of memory reference error, you might store
correct values into the erroneous variables and continue, hoping to see
more execution; but your program would probably terminate immediately as
a result of the fatal signal once it saw the signal. To prevent this,
you can continue with `signal 0'. *Note Giving your Program a Signal:
Signaling.
File: gdb.info, Node: Thread Stops, Prev: Signals, Up: Stopping
5.4 Stopping and Starting Multi-thread Programs
===============================================
When your program has multiple threads (*note Debugging Programs with
Multiple Threads: Threads.), you can choose whether to set breakpoints
on all threads, or on a particular thread.
`break LINESPEC thread THREADNO'
`break LINESPEC thread THREADNO if ...'
LINESPEC specifies source lines; there are several ways of writing
them (*note Specify Location::), but the effect is always to
specify some source line.
Use the qualifier `thread THREADNO' with a breakpoint command to
specify that you only want GDB to stop the program when a
particular thread reaches this breakpoint. THREADNO is one of the
numeric thread identifiers assigned by GDB, shown in the first
column of the `info threads' display.
If you do not specify `thread THREADNO' when you set a breakpoint,
the breakpoint applies to _all_ threads of your program.
You can use the `thread' qualifier on conditional breakpoints as
well; in this case, place `thread THREADNO' before the breakpoint
condition, like this:
(gdb) break frik.c:13 thread 28 if bartab > lim
Whenever your program stops under GDB for any reason, _all_ threads
of execution stop, not just the current thread. This allows you to
examine the overall state of the program, including switching between
threads, without worrying that things may change underfoot.
There is an unfortunate side effect. If one thread stops for a
breakpoint, or for some other reason, and another thread is blocked in a
system call, then the system call may return prematurely. This is a
consequence of the interaction between multiple threads and the signals
that GDB uses to implement breakpoints and other events that stop
execution.
To handle this problem, your program should check the return value of
each system call and react appropriately. This is good programming
style anyways.
For example, do not write code like this:
sleep (10);
The call to `sleep' will return early if a different thread stops at
a breakpoint or for some other reason.
Instead, write this:
int unslept = 10;
while (unslept > 0)
unslept = sleep (unslept);
A system call is allowed to return early, so the system is still
conforming to its specification. But GDB does cause your
multi-threaded program to behave differently than it would without GDB.
Also, GDB uses internal breakpoints in the thread library to monitor
certain events such as thread creation and thread destruction. When
such an event happens, a system call in another thread may return
prematurely, even though your program does not appear to stop.
Conversely, whenever you restart the program, _all_ threads start
executing. _This is true even when single-stepping_ with commands like
`step' or `next'.
In particular, GDB cannot single-step all threads in lockstep.
Since thread scheduling is up to your debugging target's operating
system (not controlled by GDB), other threads may execute more than one
statement while the current thread completes a single step. Moreover,
in general other threads stop in the middle of a statement, rather than
at a clean statement boundary, when the program stops.
You might even find your program stopped in another thread after
continuing or even single-stepping. This happens whenever some other
thread runs into a breakpoint, a signal, or an exception before the
first thread completes whatever you requested.
On some OSes, you can lock the OS scheduler and thus allow only a
single thread to run.
`set scheduler-locking MODE'
Set the scheduler locking mode. If it is `off', then there is no
locking and any thread may run at any time. If `on', then only the
current thread may run when the inferior is resumed. The `step'
mode optimizes for single-stepping. It stops other threads from
"seizing the prompt" by preempting the current thread while you are
stepping. Other threads will only rarely (or never) get a chance
to run when you step. They are more likely to run when you `next'
over a function call, and they are completely free to run when you
use commands like `continue', `until', or `finish'. However,
unless another thread hits a breakpoint during its timeslice, they
will never steal the GDB prompt away from the thread that you are
debugging.
`show scheduler-locking'
Display the current scheduler locking mode.
File: gdb.info, Node: Stack, Next: Source, Prev: Stopping, Up: Top
6 Examining the Stack
*********************
When your program has stopped, the first thing you need to know is
where it stopped and how it got there.
Each time your program performs a function call, information about
the call is generated. That information includes the location of the
call in your program, the arguments of the call, and the local
variables of the function being called. The information is saved in a
block of data called a "stack frame". The stack frames are allocated
in a region of memory called the "call stack".
When your program stops, the GDB commands for examining the stack
allow you to see all of this information.
One of the stack frames is "selected" by GDB and many GDB commands
refer implicitly to the selected frame. In particular, whenever you
ask GDB for the value of a variable in your program, the value is found
in the selected frame. There are special GDB commands to select
whichever frame you are interested in. *Note Selecting a Frame:
Selection.
When your program stops, GDB automatically selects the currently
executing frame and describes it briefly, similar to the `frame'
command (*note Information about a Frame: Frame Info.).
* Menu:
* Frames:: Stack frames
* Backtrace:: Backtraces
* Selection:: Selecting a frame
* Frame Info:: Information on a frame
File: gdb.info, Node: Frames, Next: Backtrace, Up: Stack
6.1 Stack Frames
================
The call stack is divided up into contiguous pieces called "stack
frames", or "frames" for short; each frame is the data associated with
one call to one function. The frame contains the arguments given to
the function, the function's local variables, and the address at which
the function is executing.
When your program is started, the stack has only one frame, that of
the function `main'. This is called the "initial" frame or the
"outermost" frame. Each time a function is called, a new frame is
made. Each time a function returns, the frame for that function
invocation is eliminated. If a function is recursive, there can be
many frames for the same function. The frame for the function in which
execution is actually occurring is called the "innermost" frame. This
is the most recently created of all the stack frames that still exist.
Inside your program, stack frames are identified by their addresses.
A stack frame consists of many bytes, each of which has its own
address; each kind of computer has a convention for choosing one byte
whose address serves as the address of the frame. Usually this address
is kept in a register called the "frame pointer register" (*note $fp:
Registers.) while execution is going on in that frame.
GDB assigns numbers to all existing stack frames, starting with zero
for the innermost frame, one for the frame that called it, and so on
upward. These numbers do not really exist in your program; they are
assigned by GDB to give you a way of designating stack frames in GDB
commands.
Some compilers provide a way to compile functions so that they
operate without stack frames. (For example, the GCC option
`-fomit-frame-pointer'
generates functions without a frame.) This is occasionally done
with heavily used library functions to save the frame setup time. GDB
has limited facilities for dealing with these function invocations. If
the innermost function invocation has no stack frame, GDB nevertheless
regards it as though it had a separate frame, which is numbered zero as
usual, allowing correct tracing of the function call chain. However,
GDB has no provision for frameless functions elsewhere in the stack.
`frame ARGS'
The `frame' command allows you to move from one stack frame to
another, and to print the stack frame you select. ARGS may be
either the address of the frame or the stack frame number.
Without an argument, `frame' prints the current stack frame.
`select-frame'
The `select-frame' command allows you to move from one stack frame
to another without printing the frame. This is the silent version
of `frame'.
File: gdb.info, Node: Backtrace, Next: Selection, Prev: Frames, Up: Stack
6.2 Backtraces
==============
A backtrace is a summary of how your program got where it is. It shows
one line per frame, for many frames, starting with the currently
executing frame (frame zero), followed by its caller (frame one), and
on up the stack.
`backtrace'
`bt'
Print a backtrace of the entire stack: one line per frame for all
frames in the stack.
You can stop the backtrace at any time by typing the system
interrupt character, normally `Ctrl-c'.
`backtrace N'
`bt N'
Similar, but print only the innermost N frames.
`backtrace -N'
`bt -N'
Similar, but print only the outermost N frames.
`backtrace full'
`bt full'
`bt full N'
`bt full -N'
Print the values of the local variables also. N specifies the
number of frames to print, as described above.
The names `where' and `info stack' (abbreviated `info s') are
additional aliases for `backtrace'.
In a multi-threaded program, GDB by default shows the backtrace only
for the current thread. To display the backtrace for several or all of
the threads, use the command `thread apply' (*note thread apply:
Threads.). For example, if you type `thread apply all backtrace', GDB
will display the backtrace for all the threads; this is handy when you
debug a core dump of a multi-threaded program.
Each line in the backtrace shows the frame number and the function
name. The program counter value is also shown--unless you use `set
print address off'. The backtrace also shows the source file name and
line number, as well as the arguments to the function. The program
counter value is omitted if it is at the beginning of the code for that
line number.
Here is an example of a backtrace. It was made with the command `bt
3', so it shows the innermost three frames.
#0 m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8)
at builtin.c:993
#1 0x6e38 in expand_macro (sym=0x2b600) at macro.c:242
#2 0x6840 in expand_token (obs=0x0, t=177664, td=0xf7fffb08)
at macro.c:71
(More stack frames follow...)
The display for frame zero does not begin with a program counter value,
indicating that your program has stopped at the beginning of the code
for line `993' of `builtin.c'.
If your program was compiled with optimizations, some compilers will
optimize away arguments passed to functions if those arguments are
never used after the call. Such optimizations generate code that
passes arguments through registers, but doesn't store those arguments
in the stack frame. GDB has no way of displaying such arguments in
stack frames other than the innermost one. Here's what such a
backtrace might look like:
#0 m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8)
at builtin.c:993
#1 0x6e38 in expand_macro (sym=<value optimized out>) at macro.c:242
#2 0x6840 in expand_token (obs=0x0, t=<value optimized out>, td=0xf7fffb08)
at macro.c:71
(More stack frames follow...)
The values of arguments that were not saved in their stack frames are
shown as `<value optimized out>'.
If you need to display the values of such optimized-out arguments,
either deduce that from other variables whose values depend on the one
you are interested in, or recompile without optimizations.
Most programs have a standard user entry point--a place where system
libraries and startup code transition into user code. For C this is
`main'(1). When GDB finds the entry function in a backtrace it will
terminate the backtrace, to avoid tracing into highly system-specific
(and generally uninteresting) code.
If you need to examine the startup code, or limit the number of
levels in a backtrace, you can change this behavior:
`set backtrace past-main'
`set backtrace past-main on'
Backtraces will continue past the user entry point.
`set backtrace past-main off'
Backtraces will stop when they encounter the user entry point.
This is the default.
`show backtrace past-main'
Display the current user entry point backtrace policy.
`set backtrace past-entry'
`set backtrace past-entry on'
Backtraces will continue past the internal entry point of an
application. This entry point is encoded by the linker when the
application is built, and is likely before the user entry point
`main' (or equivalent) is called.
`set backtrace past-entry off'
Backtraces will stop when they encounter the internal entry point
of an application. This is the default.
`show backtrace past-entry'
Display the current internal entry point backtrace policy.
`set backtrace limit N'
`set backtrace limit 0'
Limit the backtrace to N levels. A value of zero means unlimited.
`show backtrace limit'
Display the current limit on backtrace levels.
---------- Footnotes ----------
(1) Note that embedded programs (the so-called "free-standing"
environment) are not required to have a `main' function as the entry
point. They could even have multiple entry points.
File: gdb.info, Node: Selection, Next: Frame Info, Prev: Backtrace, Up: Stack
6.3 Selecting a Frame
=====================
Most commands for examining the stack and other data in your program
work on whichever stack frame is selected at the moment. Here are the
commands for selecting a stack frame; all of them finish by printing a
brief description of the stack frame just selected.
`frame N'
`f N'
Select frame number N. Recall that frame zero is the innermost
(currently executing) frame, frame one is the frame that called the
innermost one, and so on. The highest-numbered frame is the one
for `main'.
`frame ADDR'
`f ADDR'
Select the frame at address ADDR. This is useful mainly if the
chaining of stack frames has been damaged by a bug, making it
impossible for GDB to assign numbers properly to all frames. In
addition, this can be useful when your program has multiple stacks
and switches between them.
On the SPARC architecture, `frame' needs two addresses to select
an arbitrary frame: a frame pointer and a stack pointer.
On the MIPS and Alpha architecture, it needs two addresses: a stack
pointer and a program counter.
On the 29k architecture, it needs three addresses: a register stack
pointer, a program counter, and a memory stack pointer.
`up N'
Move N frames up the stack. For positive numbers N, this advances
toward the outermost frame, to higher frame numbers, to frames
that have existed longer. N defaults to one.
`down N'
Move N frames down the stack. For positive numbers N, this
advances toward the innermost frame, to lower frame numbers, to
frames that were created more recently. N defaults to one. You
may abbreviate `down' as `do'.
All of these commands end by printing two lines of output describing
the frame. The first line shows the frame number, the function name,
the arguments, and the source file and line number of execution in that
frame. The second line shows the text of that source line.
For example:
(gdb) up
#1 0x22f0 in main (argc=1, argv=0xf7fffbf4, env=0xf7fffbfc)
at env.c:10
10 read_input_file (argv[i]);
After such a printout, the `list' command with no arguments prints
ten lines centered on the point of execution in the frame. You can
also edit the program at the point of execution with your favorite
editing program by typing `edit'. *Note Printing Source Lines: List,
for details.
`up-silently N'
`down-silently N'
These two commands are variants of `up' and `down', respectively;
they differ in that they do their work silently, without causing
display of the new frame. They are intended primarily for use in
GDB command scripts, where the output might be unnecessary and
distracting.
File: gdb.info, Node: Frame Info, Prev: Selection, Up: Stack
6.4 Information About a Frame
=============================
There are several other commands to print information about the selected
stack frame.
`frame'
`f'
When used without any argument, this command does not change which
frame is selected, but prints a brief description of the currently
selected stack frame. It can be abbreviated `f'. With an
argument, this command is used to select a stack frame. *Note
Selecting a Frame: Selection.
`info frame'
`info f'
This command prints a verbose description of the selected stack
frame, including:
* the address of the frame
* the address of the next frame down (called by this frame)
* the address of the next frame up (caller of this frame)
* the language in which the source code corresponding to this
frame is written
* the address of the frame's arguments
* the address of the frame's local variables
* the program counter saved in it (the address of execution in
the caller frame)
* which registers were saved in the frame
The verbose description is useful when something has gone wrong
that has made the stack format fail to fit the usual conventions.
`info frame ADDR'
`info f ADDR'
Print a verbose description of the frame at address ADDR, without
selecting that frame. The selected frame remains unchanged by this
command. This requires the same kind of address (more than one
for some architectures) that you specify in the `frame' command.
*Note Selecting a Frame: Selection.
`info args'
Print the arguments of the selected frame, each on a separate line.
`info locals'
Print the local variables of the selected frame, each on a separate
line. These are all variables (declared either static or
automatic) accessible at the point of execution of the selected
frame.
`info catch'
Print a list of all the exception handlers that are active in the
current stack frame at the current point of execution. To see
other exception handlers, visit the associated frame (using the
`up', `down', or `frame' commands); then type `info catch'. *Note
Setting Catchpoints: Set Catchpoints.
File: gdb.info, Node: Source, Next: Data, Prev: Stack, Up: Top
7 Examining Source Files
************************
GDB can print parts of your program's source, since the debugging
information recorded in the program tells GDB what source files were
used to build it. When your program stops, GDB spontaneously prints
the line where it stopped. Likewise, when you select a stack frame
(*note Selecting a Frame: Selection.), GDB prints the line where
execution in that frame has stopped. You can print other portions of
source files by explicit command.
If you use GDB through its GNU Emacs interface, you may prefer to
use Emacs facilities to view source; see *Note Using GDB under GNU
Emacs: Emacs.
* Menu:
* List:: Printing source lines
* Specify Location:: How to specify code locations
* Edit:: Editing source files
* Search:: Searching source files
* Source Path:: Specifying source directories
* Machine Code:: Source and machine code
File: gdb.info, Node: List, Next: Specify Location, Up: Source
7.1 Printing Source Lines
=========================
To print lines from a source file, use the `list' command (abbreviated
`l'). By default, ten lines are printed. There are several ways to
specify what part of the file you want to print; see *Note Specify
Location::, for the full list.
Here are the forms of the `list' command most commonly used:
`list LINENUM'
Print lines centered around line number LINENUM in the current
source file.
`list FUNCTION'
Print lines centered around the beginning of function FUNCTION.
`list'
Print more lines. If the last lines printed were printed with a
`list' command, this prints lines following the last lines
printed; however, if the last line printed was a solitary line
printed as part of displaying a stack frame (*note Examining the
Stack: Stack.), this prints lines centered around that line.
`list -'
Print lines just before the lines last printed.
By default, GDB prints ten source lines with any of these forms of
the `list' command. You can change this using `set listsize':
`set listsize COUNT'
Make the `list' command display COUNT source lines (unless the
`list' argument explicitly specifies some other number).
`show listsize'
Display the number of lines that `list' prints.
Repeating a `list' command with <RET> discards the argument, so it
is equivalent to typing just `list'. This is more useful than listing
the same lines again. An exception is made for an argument of `-';
that argument is preserved in repetition so that each repetition moves
up in the source file.
In general, the `list' command expects you to supply zero, one or two
"linespecs". Linespecs specify source lines; there are several ways of
writing them (*note Specify Location::), but the effect is always to
specify some source line.
Here is a complete description of the possible arguments for `list':
`list LINESPEC'
Print lines centered around the line specified by LINESPEC.
`list FIRST,LAST'
Print lines from FIRST to LAST. Both arguments are linespecs.
When a `list' command has two linespecs, and the source file of
the second linespec is omitted, this refers to the same source
file as the first linespec.
`list ,LAST'
Print lines ending with LAST.
`list FIRST,'
Print lines starting with FIRST.
`list +'
Print lines just after the lines last printed.
`list -'
Print lines just before the lines last printed.
`list'
As described in the preceding table.
File: gdb.info, Node: Specify Location, Next: Edit, Prev: List, Up: Source
7.2 Specifying a Location
=========================
Several GDB commands accept arguments that specify a location of your
program's code. Since GDB is a source-level debugger, a location
usually specifies some line in the source code; for that reason,
locations are also known as "linespecs".
Here are all the different ways of specifying a code location that
GDB understands:
`LINENUM'
Specifies the line number LINENUM of the current source file.
`-OFFSET'
`+OFFSET'
Specifies the line OFFSET lines before or after the "current
line". For the `list' command, the current line is the last one
printed; for the breakpoint commands, this is the line at which
execution stopped in the currently selected "stack frame" (*note
Frames: Frames, for a description of stack frames.) When used as
the second of the two linespecs in a `list' command, this
specifies the line OFFSET lines up or down from the first linespec.
`FILENAME:LINENUM'
Specifies the line LINENUM in the source file FILENAME.
`FUNCTION'
Specifies the line that begins the body of the function FUNCTION.
For example, in C, this is the line with the open brace.
`FILENAME:FUNCTION'
Specifies the line that begins the body of the function FUNCTION
in the file FILENAME. You only need the file name with a function
name to avoid ambiguity when there are identically named functions
in different source files.
`*ADDRESS'
Specifies the program address ADDRESS. For line-oriented
commands, such as `list' and `edit', this specifies a source line
that contains ADDRESS. For `break' and other breakpoint oriented
commands, this can be used to set breakpoints in parts of your
program which do not have debugging information or source files.
Here ADDRESS may be any expression valid in the current working
language (*note working language: Languages.) that specifies a code
address. In addition, as a convenience, GDB extends the semantics
of expressions used in locations to cover the situations that
frequently happen during debugging. Here are the various forms of
ADDRESS:
`EXPRESSION'
Any expression valid in the current working language.
`FUNCADDR'
An address of a function or procedure derived from its name.
In C, C++, Java, Objective-C, Fortran, minimal, and assembly,
this is simply the function's name FUNCTION (and actually a
special case of a valid expression). In Pascal and Modula-2,
this is `&FUNCTION'. In Ada, this is `FUNCTION'Address'
(although the Pascal form also works).
This form specifies the address of the function's first
instruction, before the stack frame and arguments have been
set up.
`'FILENAME'::FUNCADDR'
Like FUNCADDR above, but also specifies the name of the source
file explicitly. This is useful if the name of the function
does not specify the function unambiguously, e.g., if there
are several functions with identical names in different
source files.
File: gdb.info, Node: Edit, Next: Search, Prev: Specify Location, Up: Source
7.3 Editing Source Files
========================
To edit the lines in a source file, use the `edit' command. The
editing program of your choice is invoked with the current line set to
the active line in the program. Alternatively, there are several ways
to specify what part of the file you want to print if you want to see
other parts of the program:
`edit LOCATION'
Edit the source file specified by `location'. Editing starts at
that LOCATION, e.g., at the specified source line of the specified
file. *Note Specify Location::, for all the possible forms of the
LOCATION argument; here are the forms of the `edit' command most
commonly used:
`edit NUMBER'
Edit the current source file with NUMBER as the active line
number.
`edit FUNCTION'
Edit the file containing FUNCTION at the beginning of its
definition.
7.3.1 Choosing your Editor
--------------------------
You can customize GDB to use any editor you want (1). By default, it
is `/bin/ex', but you can change this by setting the environment
variable `EDITOR' before using GDB. For example, to configure GDB to
use the `vi' editor, you could use these commands with the `sh' shell:
EDITOR=/usr/bin/vi
export EDITOR
gdb ...
or in the `csh' shell,
setenv EDITOR /usr/bin/vi
gdb ...
---------- Footnotes ----------
(1) The only restriction is that your editor (say `ex'), recognizes
the following command-line syntax:
ex +NUMBER file
The optional numeric value +NUMBER specifies the number of the line
in the file where to start editing.
File: gdb.info, Node: Search, Next: Source Path, Prev: Edit, Up: Source
7.4 Searching Source Files
==========================
There are two commands for searching through the current source file
for a regular expression.
`forward-search REGEXP'
`search REGEXP'
The command `forward-search REGEXP' checks each line, starting
with the one following the last line listed, for a match for
REGEXP. It lists the line that is found. You can use the synonym
`search REGEXP' or abbreviate the command name as `fo'.
`reverse-search REGEXP'
The command `reverse-search REGEXP' checks each line, starting
with the one before the last line listed and going backward, for a
match for REGEXP. It lists the line that is found. You can
abbreviate this command as `rev'.
File: gdb.info, Node: Source Path, Next: Machine Code, Prev: Search, Up: Source
7.5 Specifying Source Directories
=================================
Executable programs sometimes do not record the directories of the
source files from which they were compiled, just the names. Even when
they do, the directories could be moved between the compilation and
your debugging session. GDB has a list of directories to search for
source files; this is called the "source path". Each time GDB wants a
source file, it tries all the directories in the list, in the order
they are present in the list, until it finds a file with the desired
name.
For example, suppose an executable references the file
`/usr/src/foo-1.0/lib/foo.c', and our source path is `/mnt/cross'. The
file is first looked up literally; if this fails,
`/mnt/cross/usr/src/foo-1.0/lib/foo.c' is tried; if this fails,
`/mnt/cross/foo.c' is opened; if this fails, an error message is
printed. GDB does not look up the parts of the source file name, such
as `/mnt/cross/src/foo-1.0/lib/foo.c'. Likewise, the subdirectories of
the source path are not searched: if the source path is `/mnt/cross',
and the binary refers to `foo.c', GDB would not find it under
`/mnt/cross/usr/src/foo-1.0/lib'.
Plain file names, relative file names with leading directories, file
names containing dots, etc. are all treated as described above; for
instance, if the source path is `/mnt/cross', and the source file is
recorded as `../lib/foo.c', GDB would first try `../lib/foo.c', then
`/mnt/cross/../lib/foo.c', and after that--`/mnt/cross/foo.c'.
Note that the executable search path is _not_ used to locate the
source files.
Whenever you reset or rearrange the source path, GDB clears out any
information it has cached about where source files are found and where
each line is in the file.
When you start GDB, its source path includes only `cdir' and `cwd',
in that order. To add other directories, use the `directory' command.
The search path is used to find both program source files and GDB
script files (read using the `-command' option and `source' command).
In addition to the source path, GDB provides a set of commands that
manage a list of source path substitution rules. A "substitution rule"
specifies how to rewrite source directories stored in the program's
debug information in case the sources were moved to a different
directory between compilation and debugging. A rule is made of two
strings, the first specifying what needs to be rewritten in the path,
and the second specifying how it should be rewritten. In *Note set
substitute-path::, we name these two parts FROM and TO respectively.
GDB does a simple string replacement of FROM with TO at the start of
the directory part of the source file name, and uses that result
instead of the original file name to look up the sources.
Using the previous example, suppose the `foo-1.0' tree has been
moved from `/usr/src' to `/mnt/cross', then you can tell GDB to replace
`/usr/src' in all source path names with `/mnt/cross'. The first
lookup will then be `/mnt/cross/foo-1.0/lib/foo.c' in place of the
original location of `/usr/src/foo-1.0/lib/foo.c'. To define a source
path substitution rule, use the `set substitute-path' command (*note
set substitute-path::).
To avoid unexpected substitution results, a rule is applied only if
the FROM part of the directory name ends at a directory separator. For
instance, a rule substituting `/usr/source' into `/mnt/cross' will be
applied to `/usr/source/foo-1.0' but not to `/usr/sourceware/foo-2.0'.
And because the substitution is applied only at the beginning of the
directory name, this rule will not be applied to
`/root/usr/source/baz.c' either.
In many cases, you can achieve the same result using the `directory'
command. However, `set substitute-path' can be more efficient in the
case where the sources are organized in a complex tree with multiple
subdirectories. With the `directory' command, you need to add each
subdirectory of your project. If you moved the entire tree while
preserving its internal organization, then `set substitute-path' allows
you to direct the debugger to all the sources with one single command.
`set substitute-path' is also more than just a shortcut command.
The source path is only used if the file at the original location no
longer exists. On the other hand, `set substitute-path' modifies the
debugger behavior to look at the rewritten location instead. So, if
for any reason a source file that is not relevant to your executable is
located at the original location, a substitution rule is the only
method available to point GDB at the new location.
`directory DIRNAME ...'
`dir DIRNAME ...'
Add directory DIRNAME to the front of the source path. Several
directory names may be given to this command, separated by `:'
(`;' on MS-DOS and MS-Windows, where `:' usually appears as part
of absolute file names) or whitespace. You may specify a
directory that is already in the source path; this moves it
forward, so GDB searches it sooner.
You can use the string `$cdir' to refer to the compilation
directory (if one is recorded), and `$cwd' to refer to the current
working directory. `$cwd' is not the same as `.'--the former
tracks the current working directory as it changes during your GDB
session, while the latter is immediately expanded to the current
directory at the time you add an entry to the source path.
`directory'
Reset the source path to its default value (`$cdir:$cwd' on Unix
systems). This requires confirmation.
`show directories'
Print the source path: show which directories it contains.
`set substitute-path FROM TO'
Define a source path substitution rule, and add it at the end of
the current list of existing substitution rules. If a rule with
the same FROM was already defined, then the old rule is also
deleted.
For example, if the file `/foo/bar/baz.c' was moved to
`/mnt/cross/baz.c', then the command
(gdb) set substitute-path /usr/src /mnt/cross
will tell GDB to replace `/usr/src' with `/mnt/cross', which will
allow GDB to find the file `baz.c' even though it was moved.
In the case when more than one substitution rule have been defined,
the rules are evaluated one by one in the order where they have
been defined. The first one matching, if any, is selected to
perform the substitution.
For instance, if we had entered the following commands:
(gdb) set substitute-path /usr/src/include /mnt/include
(gdb) set substitute-path /usr/src /mnt/src
GDB would then rewrite `/usr/src/include/defs.h' into
`/mnt/include/defs.h' by using the first rule. However, it would
use the second rule to rewrite `/usr/src/lib/foo.c' into
`/mnt/src/lib/foo.c'.
`unset substitute-path [path]'
If a path is specified, search the current list of substitution
rules for a rule that would rewrite that path. Delete that rule
if found. A warning is emitted by the debugger if no rule could
be found.
If no path is specified, then all substitution rules are deleted.
`show substitute-path [path]'
If a path is specified, then print the source path substitution
rule which would rewrite that path, if any.
If no path is specified, then print all existing source path
substitution rules.
If your source path is cluttered with directories that are no longer
of interest, GDB may sometimes cause confusion by finding the wrong
versions of source. You can correct the situation as follows:
1. Use `directory' with no argument to reset the source path to its
default value.
2. Use `directory' with suitable arguments to reinstall the
directories you want in the source path. You can add all the
directories in one command.
File: gdb.info, Node: Machine Code, Prev: Source Path, Up: Source
7.6 Source and Machine Code
===========================
You can use the command `info line' to map source lines to program
addresses (and vice versa), and the command `disassemble' to display a
range of addresses as machine instructions. When run under GNU Emacs
mode, the `info line' command causes the arrow to point to the line
specified. Also, `info line' prints addresses in symbolic form as well
as hex.
`info line LINESPEC'
Print the starting and ending addresses of the compiled code for
source line LINESPEC. You can specify source lines in any of the
ways documented in *Note Specify Location::.
For example, we can use `info line' to discover the location of the
object code for the first line of function `m4_changequote':
(gdb) info line m4_changequote
Line 895 of "builtin.c" starts at pc 0x634c and ends at 0x6350.
We can also inquire (using `*ADDR' as the form for LINESPEC) what
source line covers a particular address:
(gdb) info line *0x63ff
Line 926 of "builtin.c" starts at pc 0x63e4 and ends at 0x6404.
After `info line', the default address for the `x' command is
changed to the starting address of the line, so that `x/i' is
sufficient to begin examining the machine code (*note Examining Memory:
Memory.). Also, this address is saved as the value of the convenience
variable `$_' (*note Convenience Variables: Convenience Vars.).
`disassemble'
This specialized command dumps a range of memory as machine
instructions. The default memory range is the function
surrounding the program counter of the selected frame. A single
argument to this command is a program counter value; GDB dumps the
function surrounding this value. Two arguments specify a range of
addresses (first inclusive, second exclusive) to dump.
The following example shows the disassembly of a range of addresses
of HP PA-RISC 2.0 code:
(gdb) disas 0x32c4 0x32e4
Dump of assembler code from 0x32c4 to 0x32e4:
0x32c4 <main+204>: addil 0,dp
0x32c8 <main+208>: ldw 0x22c(sr0,r1),r26
0x32cc <main+212>: ldil 0x3000,r31
0x32d0 <main+216>: ble 0x3f8(sr4,r31)
0x32d4 <main+220>: ldo 0(r31),rp
0x32d8 <main+224>: addil -0x800,dp
0x32dc <main+228>: ldo 0x588(r1),r26
0x32e0 <main+232>: ldil 0x3000,r31
End of assembler dump.
Some architectures have more than one commonly-used set of
instruction mnemonics or other syntax.
For programs that were dynamically linked and use shared libraries,
instructions that call functions or branch to locations in the shared
libraries might show a seemingly bogus location--it's actually a
location of the relocation table. On some architectures, GDB might be
able to resolve these to actual function names.
`set disassembly-flavor INSTRUCTION-SET'
Select the instruction set to use when disassembling the program
via the `disassemble' or `x/i' commands.
Currently this command is only defined for the Intel x86 family.
You can set INSTRUCTION-SET to either `intel' or `att'. The
default is `att', the AT&T flavor used by default by Unix
assemblers for x86-based targets.
`show disassembly-flavor'
Show the current setting of the disassembly flavor.
File: gdb.info, Node: Data, Next: Macros, Prev: Source, Up: Top
8 Examining Data
****************
The usual way to examine data in your program is with the `print'
command (abbreviated `p'), or its synonym `inspect'. It evaluates and
prints the value of an expression of the language your program is
written in (*note Using GDB with Different Languages: Languages.).
`print EXPR'
`print /F EXPR'
EXPR is an expression (in the source language). By default the
value of EXPR is printed in a format appropriate to its data type;
you can choose a different format by specifying `/F', where F is a
letter specifying the format; see *Note Output Formats: Output
Formats.
`print'
`print /F'
If you omit EXPR, GDB displays the last value again (from the
"value history"; *note Value History: Value History.). This
allows you to conveniently inspect the same value in an
alternative format.
A more low-level way of examining data is with the `x' command. It
examines data in memory at a specified address and prints it in a
specified format. *Note Examining Memory: Memory.
If you are interested in information about types, or about how the
fields of a struct or a class are declared, use the `ptype EXP' command
rather than `print'. *Note Examining the Symbol Table: Symbols.
* Menu:
* Expressions:: Expressions
* Variables:: Program variables
* Arrays:: Artificial arrays
* Output Formats:: Output formats
* Memory:: Examining memory
* Auto Display:: Automatic display
* Print Settings:: Print settings
* Value History:: Value history
* Convenience Vars:: Convenience variables
* Registers:: Registers
* Floating Point Hardware:: Floating point hardware
* Vector Unit:: Vector Unit
* OS Information:: Auxiliary data provided by operating system
* Memory Region Attributes:: Memory region attributes
* Dump/Restore Files:: Copy between memory and a file
* Core File Generation:: Cause a program dump its core
* Character Sets:: Debugging programs that use a different
character set than GDB does
* Caching Remote Data:: Data caching for remote targets
File: gdb.info, Node: Expressions, Next: Variables, Up: Data
8.1 Expressions
===============
`print' and many other GDB commands accept an expression and compute
its value. Any kind of constant, variable or operator defined by the
programming language you are using is valid in an expression in GDB.
This includes conditional expressions, function calls, casts, and
string constants. It also includes preprocessor macros, if you
compiled your program to include this information; see *Note
Compilation::.
GDB supports array constants in expressions input by the user. The
syntax is {ELEMENT, ELEMENT...}. For example, you can use the command
`print {1, 2, 3}' to build up an array in memory that is `malloc'ed in
the target program.
Because C is so widespread, most of the expressions shown in
examples in this manual are in C. *Note Using GDB with Different
Languages: Languages, for information on how to use expressions in other
languages.
In this section, we discuss operators that you can use in GDB
expressions regardless of your programming language.
Casts are supported in all languages, not just in C, because it is so
useful to cast a number into a pointer in order to examine a structure
at that address in memory.
GDB supports these operators, in addition to those common to
programming languages:
`@'
`@' is a binary operator for treating parts of memory as arrays.
*Note Artificial Arrays: Arrays, for more information.
`::'
`::' allows you to specify a variable in terms of the file or
function where it is defined. *Note Program Variables: Variables.
`{TYPE} ADDR'
Refers to an object of type TYPE stored at address ADDR in memory.
ADDR may be any expression whose value is an integer or pointer
(but parentheses are required around binary operators, just as in
a cast). This construct is allowed regardless of what kind of
data is normally supposed to reside at ADDR.
File: gdb.info, Node: Variables, Next: Arrays, Prev: Expressions, Up: Data
8.2 Program Variables
=====================
The most common kind of expression to use is the name of a variable in
your program.
Variables in expressions are understood in the selected stack frame
(*note Selecting a Frame: Selection.); they must be either:
* global (or file-static)
or
* visible according to the scope rules of the programming language
from the point of execution in that frame
This means that in the function
foo (a)
int a;
{
bar (a);
{
int b = test ();
bar (b);
}
}
you can examine and use the variable `a' whenever your program is
executing within the function `foo', but you can only use or examine
the variable `b' while your program is executing inside the block where
`b' is declared.
There is an exception: you can refer to a variable or function whose
scope is a single source file even if the current execution point is not
in this file. But it is possible to have more than one such variable or
function with the same name (in different source files). If that
happens, referring to that name has unpredictable effects. If you wish,
you can specify a static variable in a particular function or file,
using the colon-colon (`::') notation:
FILE::VARIABLE
FUNCTION::VARIABLE
Here FILE or FUNCTION is the name of the context for the static
VARIABLE. In the case of file names, you can use quotes to make sure
GDB parses the file name as a single word--for example, to print a
global value of `x' defined in `f2.c':
(gdb) p 'f2.c'::x
This use of `::' is very rarely in conflict with the very similar
use of the same notation in C++. GDB also supports use of the C++
scope resolution operator in GDB expressions.
_Warning:_ Occasionally, a local variable may appear to have the
wrong value at certain points in a function--just after entry to a
new scope, and just before exit.
You may see this problem when you are stepping by machine
instructions. This is because, on most machines, it takes more than
one instruction to set up a stack frame (including local variable
definitions); if you are stepping by machine instructions, variables
may appear to have the wrong values until the stack frame is completely
built. On exit, it usually also takes more than one machine
instruction to destroy a stack frame; after you begin stepping through
that group of instructions, local variable definitions may be gone.
This may also happen when the compiler does significant
optimizations. To be sure of always seeing accurate values, turn off
all optimization when compiling.
Another possible effect of compiler optimizations is to optimize
unused variables out of existence, or assign variables to registers (as
opposed to memory addresses). Depending on the support for such cases
offered by the debug info format used by the compiler, GDB might not be
able to display values for such local variables. If that happens, GDB
will print a message like this:
No symbol "foo" in current context.
To solve such problems, either recompile without optimizations, or
use a different debug info format, if the compiler supports several such
formats. For example, GCC, the GNU C/C++ compiler, usually supports
the `-gstabs+' option. `-gstabs+' produces debug info in a format that
is superior to formats such as COFF. You may be able to use DWARF 2
(`-gdwarf-2'), which is also an effective form for debug info. *Note
Options for Debugging Your Program or GCC: (gcc.info)Debugging Options.
*Note C and C++: C, for more information about debug info formats that
are best suited to C++ programs.
If you ask to print an object whose contents are unknown to GDB,
e.g., because its data type is not completely specified by the debug
information, GDB will say `<incomplete type>'. *Note incomplete type:
Symbols, for more about this.
Strings are identified as arrays of `char' values without specified
signedness. Arrays of either `signed char' or `unsigned char' get
printed as arrays of 1 byte sized integers. `-fsigned-char' or
`-funsigned-char' GCC options have no effect as GDB defines literal
string type `"char"' as `char' without a sign. For program code
char var0[] = "A";
signed char var1[] = "A";
You get during debugging
(gdb) print var0
$1 = "A"
(gdb) print var1
$2 = {65 'A', 0 '\0'}
File: gdb.info, Node: Arrays, Next: Output Formats, Prev: Variables, Up: Data
8.3 Artificial Arrays
=====================
It is often useful to print out several successive objects of the same
type in memory; a section of an array, or an array of dynamically
determined size for which only a pointer exists in the program.
You can do this by referring to a contiguous span of memory as an
"artificial array", using the binary operator `@'. The left operand of
`@' should be the first element of the desired array and be an
individual object. The right operand should be the desired length of
the array. The result is an array value whose elements are all of the
type of the left argument. The first element is actually the left
argument; the second element comes from bytes of memory immediately
following those that hold the first element, and so on. Here is an
example. If a program says
int *array = (int *) malloc (len * sizeof (int));
you can print the contents of `array' with
p *array@len
The left operand of `@' must reside in memory. Array values made
with `@' in this way behave just like other arrays in terms of
subscripting, and are coerced to pointers when used in expressions.
Artificial arrays most often appear in expressions via the value history
(*note Value History: Value History.), after printing one out.
Another way to create an artificial array is to use a cast. This
re-interprets a value as if it were an array. The value need not be in
memory:
(gdb) p/x (short[2])0x12345678
$1 = {0x1234, 0x5678}
As a convenience, if you leave the array length out (as in
`(TYPE[])VALUE') GDB calculates the size to fill the value (as
`sizeof(VALUE)/sizeof(TYPE)':
(gdb) p/x (short[])0x12345678
$2 = {0x1234, 0x5678}
Sometimes the artificial array mechanism is not quite enough; in
moderately complex data structures, the elements of interest may not
actually be adjacent--for example, if you are interested in the values
of pointers in an array. One useful work-around in this situation is
to use a convenience variable (*note Convenience Variables: Convenience
Vars.) as a counter in an expression that prints the first interesting
value, and then repeat that expression via <RET>. For instance,
suppose you have an array `dtab' of pointers to structures, and you are
interested in the values of a field `fv' in each structure. Here is an
example of what you might type:
set $i = 0
p dtab[$i++]->fv
<RET>
<RET>
...
File: gdb.info, Node: Output Formats, Next: Memory, Prev: Arrays, Up: Data
8.4 Output Formats
==================
By default, GDB prints a value according to its data type. Sometimes
this is not what you want. For example, you might want to print a
number in hex, or a pointer in decimal. Or you might want to view data
in memory at a certain address as a character string or as an
instruction. To do these things, specify an "output format" when you
print a value.
The simplest use of output formats is to say how to print a value
already computed. This is done by starting the arguments of the
`print' command with a slash and a format letter. The format letters
supported are:
`x'
Regard the bits of the value as an integer, and print the integer
in hexadecimal.
`d'
Print as integer in signed decimal.
`u'
Print as integer in unsigned decimal.
`o'
Print as integer in octal.
`t'
Print as integer in binary. The letter `t' stands for "two". (1)
`a'
Print as an address, both absolute in hexadecimal and as an offset
from the nearest preceding symbol. You can use this format used
to discover where (in what function) an unknown address is located:
(gdb) p/a 0x54320
$3 = 0x54320 <_initialize_vx+396>
The command `info symbol 0x54320' yields similar results. *Note
info symbol: Symbols.
`c'
Regard as an integer and print it as a character constant. This
prints both the numerical value and its character representation.
The character representation is replaced with the octal escape
`\nnn' for characters outside the 7-bit ASCII range.
Without this format, GDB displays `char', `unsigned char', and
`signed char' data as character constants. Single-byte members of
vectors are displayed as integer data.
`f'
Regard the bits of the value as a floating point number and print
using typical floating point syntax.
`s'
Regard as a string, if possible. With this format, pointers to
single-byte data are displayed as null-terminated strings and
arrays of single-byte data are displayed as fixed-length strings.
Other values are displayed in their natural types.
Without this format, GDB displays pointers to and arrays of
`char', `unsigned char', and `signed char' as strings.
Single-byte members of a vector are displayed as an integer array.
For example, to print the program counter in hex (*note
Registers::), type
p/x $pc
Note that no space is required before the slash; this is because command
names in GDB cannot contain a slash.
To reprint the last value in the value history with a different
format, you can use the `print' command with just a format and no
expression. For example, `p/x' reprints the last value in hex.
---------- Footnotes ----------
(1) `b' cannot be used because these format letters are also used
with the `x' command, where `b' stands for "byte"; see *Note Examining
Memory: Memory.
File: gdb.info, Node: Memory, Next: Auto Display, Prev: Output Formats, Up: Data
8.5 Examining Memory
====================
You can use the command `x' (for "examine") to examine memory in any of
several formats, independently of your program's data types.
`x/NFU ADDR'
`x ADDR'
`x'
Use the `x' command to examine memory.
N, F, and U are all optional parameters that specify how much memory
to display and how to format it; ADDR is an expression giving the
address where you want to start displaying memory. If you use defaults
for NFU, you need not type the slash `/'. Several commands set
convenient defaults for ADDR.
N, the repeat count
The repeat count is a decimal integer; the default is 1. It
specifies how much memory (counting by units U) to display.
F, the display format
The display format is one of the formats used by `print' (`x',
`d', `u', `o', `t', `a', `c', `f', `s'), and in addition `i' (for
machine instructions). The default is `x' (hexadecimal)
initially. The default changes each time you use either `x' or
`print'.
U, the unit size
The unit size is any of
`b'
Bytes.
`h'
Halfwords (two bytes).
`w'
Words (four bytes). This is the initial default.
`g'
Giant words (eight bytes).
Each time you specify a unit size with `x', that size becomes the
default unit the next time you use `x'. (For the `s' and `i'
formats, the unit size is ignored and is normally not written.)
ADDR, starting display address
ADDR is the address where you want GDB to begin displaying memory.
The expression need not have a pointer value (though it may); it
is always interpreted as an integer address of a byte of memory.
*Note Expressions: Expressions, for more information on
expressions. The default for ADDR is usually just after the last
address examined--but several other commands also set the default
address: `info breakpoints' (to the address of the last breakpoint
listed), `info line' (to the starting address of a line), and
`print' (if you use it to display a value from memory).
For example, `x/3uh 0x54320' is a request to display three halfwords
(`h') of memory, formatted as unsigned decimal integers (`u'), starting
at address `0x54320'. `x/4xw $sp' prints the four words (`w') of
memory above the stack pointer (here, `$sp'; *note Registers:
Registers.) in hexadecimal (`x').
Since the letters indicating unit sizes are all distinct from the
letters specifying output formats, you do not have to remember whether
unit size or format comes first; either order works. The output
specifications `4xw' and `4wx' mean exactly the same thing. (However,
the count N must come first; `wx4' does not work.)
Even though the unit size U is ignored for the formats `s' and `i',
you might still want to use a count N; for example, `3i' specifies that
you want to see three machine instructions, including any operands.
For convenience, especially when used with the `display' command, the
`i' format also prints branch delay slot instructions, if any, beyond
the count specified, which immediately follow the last instruction that
is within the count. The command `disassemble' gives an alternative
way of inspecting machine instructions; see *Note Source and Machine
Code: Machine Code.
All the defaults for the arguments to `x' are designed to make it
easy to continue scanning memory with minimal specifications each time
you use `x'. For example, after you have inspected three machine
instructions with `x/3i ADDR', you can inspect the next seven with just
`x/7'. If you use <RET> to repeat the `x' command, the repeat count N
is used again; the other arguments default as for successive uses of
`x'.
The addresses and contents printed by the `x' command are not saved
in the value history because there is often too much of them and they
would get in the way. Instead, GDB makes these values available for
subsequent use in expressions as values of the convenience variables
`$_' and `$__'. After an `x' command, the last address examined is
available for use in expressions in the convenience variable `$_'. The
contents of that address, as examined, are available in the convenience
variable `$__'.
If the `x' command has a repeat count, the address and contents saved
are from the last memory unit printed; this is not the same as the last
address printed if several units were printed on the last line of
output.
When you are debugging a program running on a remote target machine
(*note Remote Debugging::), you may wish to verify the program's image
in the remote machine's memory against the executable file you
downloaded to the target. The `compare-sections' command is provided
for such situations.
`compare-sections [SECTION-NAME]'
Compare the data of a loadable section SECTION-NAME in the
executable file of the program being debugged with the same
section in the remote machine's memory, and report any mismatches.
With no arguments, compares all loadable sections. This command's
availability depends on the target's support for the `"qCRC"'
remote request.
File: gdb.info, Node: Auto Display, Next: Print Settings, Prev: Memory, Up: Data
8.6 Automatic Display
=====================
If you find that you want to print the value of an expression frequently
(to see how it changes), you might want to add it to the "automatic
display list" so that GDB prints its value each time your program stops.
Each expression added to the list is given a number to identify it; to
remove an expression from the list, you specify that number. The
automatic display looks like this:
2: foo = 38
3: bar[5] = (struct hack *) 0x3804
This display shows item numbers, expressions and their current values.
As with displays you request manually using `x' or `print', you can
specify the output format you prefer; in fact, `display' decides
whether to use `print' or `x' depending your format specification--it
uses `x' if you specify either the `i' or `s' format, or a unit size;
otherwise it uses `print'.
`display EXPR'
Add the expression EXPR to the list of expressions to display each
time your program stops. *Note Expressions: Expressions.
`display' does not repeat if you press <RET> again after using it.
`display/FMT EXPR'
For FMT specifying only a display format and not a size or count,
add the expression EXPR to the auto-display list but arrange to
display it each time in the specified format FMT. *Note Output
Formats: Output Formats.
`display/FMT ADDR'
For FMT `i' or `s', or including a unit-size or a number of units,
add the expression ADDR as a memory address to be examined each
time your program stops. Examining means in effect doing `x/FMT
ADDR'. *Note Examining Memory: Memory.
For example, `display/i $pc' can be helpful, to see the machine
instruction about to be executed each time execution stops (`$pc' is a
common name for the program counter; *note Registers: Registers.).
`undisplay DNUMS...'
`delete display DNUMS...'
Remove item numbers DNUMS from the list of expressions to display.
`undisplay' does not repeat if you press <RET> after using it.
(Otherwise you would just get the error `No display number ...'.)
`disable display DNUMS...'
Disable the display of item numbers DNUMS. A disabled display
item is not printed automatically, but is not forgotten. It may be
enabled again later.
`enable display DNUMS...'
Enable display of item numbers DNUMS. It becomes effective once
again in auto display of its expression, until you specify
otherwise.
`display'
Display the current values of the expressions on the list, just as
is done when your program stops.
`info display'
Print the list of expressions previously set up to display
automatically, each one with its item number, but without showing
the values. This includes disabled expressions, which are marked
as such. It also includes expressions which would not be
displayed right now because they refer to automatic variables not
currently available.
If a display expression refers to local variables, then it does not
make sense outside the lexical context for which it was set up. Such an
expression is disabled when execution enters a context where one of its
variables is not defined. For example, if you give the command
`display last_char' while inside a function with an argument
`last_char', GDB displays this argument while your program continues to
stop inside that function. When it stops elsewhere--where there is no
variable `last_char'--the display is disabled automatically. The next
time your program stops where `last_char' is meaningful, you can enable
the display expression once again.
File: gdb.info, Node: Print Settings, Next: Value History, Prev: Auto Display, Up: Data
8.7 Print Settings
==================
GDB provides the following ways to control how arrays, structures, and
symbols are printed.
These settings are useful for debugging programs in any language:
`set print address'
`set print address on'
GDB prints memory addresses showing the location of stack traces,
structure values, pointer values, breakpoints, and so forth, even
when it also displays the contents of those addresses. The default
is `on'. For example, this is what a stack frame display looks
like with `set print address on':
(gdb) f
#0 set_quotes (lq=0x34c78 "<<", rq=0x34c88 ">>")
at input.c:530
530 if (lquote != def_lquote)
`set print address off'
Do not print addresses when displaying their contents. For
example, this is the same stack frame displayed with `set print
address off':
(gdb) set print addr off
(gdb) f
#0 set_quotes (lq="<<", rq=">>") at input.c:530
530 if (lquote != def_lquote)
You can use `set print address off' to eliminate all machine
dependent displays from the GDB interface. For example, with
`print address off', you should get the same text for backtraces on
all machines--whether or not they involve pointer arguments.
`show print address'
Show whether or not addresses are to be printed.
When GDB prints a symbolic address, it normally prints the closest
earlier symbol plus an offset. If that symbol does not uniquely
identify the address (for example, it is a name whose scope is a single
source file), you may need to clarify. One way to do this is with
`info line', for example `info line *0x4537'. Alternately, you can set
GDB to print the source file and line number when it prints a symbolic
address:
`set print symbol-filename on'
Tell GDB to print the source file name and line number of a symbol
in the symbolic form of an address.
`set print symbol-filename off'
Do not print source file name and line number of a symbol. This
is the default.
`show print symbol-filename'
Show whether or not GDB will print the source file name and line
number of a symbol in the symbolic form of an address.
Another situation where it is helpful to show symbol filenames and
line numbers is when disassembling code; GDB shows you the line number
and source file that corresponds to each instruction.
Also, you may wish to see the symbolic form only if the address being
printed is reasonably close to the closest earlier symbol:
`set print max-symbolic-offset MAX-OFFSET'
Tell GDB to only display the symbolic form of an address if the
offset between the closest earlier symbol and the address is less
than MAX-OFFSET. The default is 0, which tells GDB to always
print the symbolic form of an address if any symbol precedes it.
`show print max-symbolic-offset'
Ask how large the maximum offset is that GDB prints in a symbolic
address.
If you have a pointer and you are not sure where it points, try `set
print symbol-filename on'. Then you can determine the name and source
file location of the variable where it points, using `p/a POINTER'.
This interprets the address in symbolic form. For example, here GDB
shows that a variable `ptt' points at another variable `t', defined in
`hi2.c':
(gdb) set print symbol-filename on
(gdb) p/a ptt
$4 = 0xe008 <t in hi2.c>
_Warning:_ For pointers that point to a local variable, `p/a' does
not show the symbol name and filename of the referent, even with
the appropriate `set print' options turned on.
Other settings control how different kinds of objects are printed:
`set print array'
`set print array on'
Pretty print arrays. This format is more convenient to read, but
uses more space. The default is off.
`set print array off'
Return to compressed format for arrays.
`show print array'
Show whether compressed or pretty format is selected for displaying
arrays.
`set print array-indexes'
`set print array-indexes on'
Print the index of each element when displaying arrays. May be
more convenient to locate a given element in the array or quickly
find the index of a given element in that printed array. The
default is off.
`set print array-indexes off'
Stop printing element indexes when displaying arrays.
`show print array-indexes'
Show whether the index of each element is printed when displaying
arrays.
`set print elements NUMBER-OF-ELEMENTS'
Set a limit on how many elements of an array GDB will print. If
GDB is printing a large array, it stops printing after it has
printed the number of elements set by the `set print elements'
command. This limit also applies to the display of strings. When
GDB starts, this limit is set to 200. Setting NUMBER-OF-ELEMENTS
to zero means that the printing is unlimited.
`show print elements'
Display the number of elements of a large array that GDB will
print. If the number is 0, then the printing is unlimited.
`set print frame-arguments VALUE'
This command allows to control how the values of arguments are
printed when the debugger prints a frame (*note Frames::). The
possible values are:
`all'
The values of all arguments are printed. This is the default.
`scalars'
Print the value of an argument only if it is a scalar. The
value of more complex arguments such as arrays, structures,
unions, etc, is replaced by `...'. Here is an example where
only scalar arguments are shown:
#1 0x08048361 in call_me (i=3, s=..., ss=0xbf8d508c, u=..., e=green)
at frame-args.c:23
`none'
None of the argument values are printed. Instead, the value
of each argument is replaced by `...'. In this case, the
example above now becomes:
#1 0x08048361 in call_me (i=..., s=..., ss=..., u=..., e=...)
at frame-args.c:23
By default, all argument values are always printed. But this
command can be useful in several cases. For instance, it can be
used to reduce the amount of information printed in each frame,
making the backtrace more readable. Also, this command can be
used to improve performance when displaying Ada frames, because
the computation of large arguments can sometimes be CPU-intensive,
especiallly in large applications. Setting `print
frame-arguments' to `scalars' or `none' avoids this computation,
thus speeding up the display of each Ada frame.
`show print frame-arguments'
Show how the value of arguments should be displayed when printing
a frame.
`set print repeats'
Set the threshold for suppressing display of repeated array
elements. When the number of consecutive identical elements of an
array exceeds the threshold, GDB prints the string `"<repeats N
times>"', where N is the number of identical repetitions, instead
of displaying the identical elements themselves. Setting the
threshold to zero will cause all elements to be individually
printed. The default threshold is 10.
`show print repeats'
Display the current threshold for printing repeated identical
elements.
`set print null-stop'
Cause GDB to stop printing the characters of an array when the
first NULL is encountered. This is useful when large arrays
actually contain only short strings. The default is off.
`show print null-stop'
Show whether GDB stops printing an array on the first NULL
character.
`set print pretty on'
Cause GDB to print structures in an indented format with one member
per line, like this:
$1 = {
next = 0x0,
flags = {
sweet = 1,
sour = 1
},
meat = 0x54 "Pork"
}
`set print pretty off'
Cause GDB to print structures in a compact format, like this:
$1 = {next = 0x0, flags = {sweet = 1, sour = 1}, \
meat = 0x54 "Pork"}
This is the default format.
`show print pretty'
Show which format GDB is using to print structures.
`set print sevenbit-strings on'
Print using only seven-bit characters; if this option is set, GDB
displays any eight-bit characters (in strings or character values)
using the notation `\'NNN. This setting is best if you are
working in English (ASCII) and you use the high-order bit of
characters as a marker or "meta" bit.
`set print sevenbit-strings off'
Print full eight-bit characters. This allows the use of more
international character sets, and is the default.
`show print sevenbit-strings'
Show whether or not GDB is printing only seven-bit characters.
`set print union on'
Tell GDB to print unions which are contained in structures and
other unions. This is the default setting.
`set print union off'
Tell GDB not to print unions which are contained in structures and
other unions. GDB will print `"{...}"' instead.
`show print union'
Ask GDB whether or not it will print unions which are contained in
structures and other unions.
For example, given the declarations
typedef enum {Tree, Bug} Species;
typedef enum {Big_tree, Acorn, Seedling} Tree_forms;
typedef enum {Caterpillar, Cocoon, Butterfly}
Bug_forms;
struct thing {
Species it;
union {
Tree_forms tree;
Bug_forms bug;
} form;
};
struct thing foo = {Tree, {Acorn}};
with `set print union on' in effect `p foo' would print
$1 = {it = Tree, form = {tree = Acorn, bug = Cocoon}}
and with `set print union off' in effect it would print
$1 = {it = Tree, form = {...}}
`set print union' affects programs written in C-like languages and
in Pascal.
These settings are of interest when debugging C++ programs:
`set print demangle'
`set print demangle on'
Print C++ names in their source form rather than in the encoded
("mangled") form passed to the assembler and linker for type-safe
linkage. The default is on.
`show print demangle'
Show whether C++ names are printed in mangled or demangled form.
`set print asm-demangle'
`set print asm-demangle on'
Print C++ names in their source form rather than their mangled
form, even in assembler code printouts such as instruction
disassemblies. The default is off.
`show print asm-demangle'
Show whether C++ names in assembly listings are printed in mangled
or demangled form.
`set demangle-style STYLE'
Choose among several encoding schemes used by different compilers
to represent C++ names. The choices for STYLE are currently:
`auto'
Allow GDB to choose a decoding style by inspecting your
program.
`gnu'
Decode based on the GNU C++ compiler (`g++') encoding
algorithm. This is the default.
`hp'
Decode based on the HP ANSI C++ (`aCC') encoding algorithm.
`lucid'
Decode based on the Lucid C++ compiler (`lcc') encoding
algorithm.
`arm'
Decode using the algorithm in the `C++ Annotated Reference
Manual'. *Warning:* this setting alone is not sufficient to
allow debugging `cfront'-generated executables. GDB would
require further enhancement to permit that.
If you omit STYLE, you will see a list of possible formats.
`show demangle-style'
Display the encoding style currently in use for decoding C++
symbols.
`set print object'
`set print object on'
When displaying a pointer to an object, identify the _actual_
(derived) type of the object rather than the _declared_ type, using
the virtual function table.
`set print object off'
Display only the declared type of objects, without reference to the
virtual function table. This is the default setting.
`show print object'
Show whether actual, or declared, object types are displayed.
`set print static-members'
`set print static-members on'
Print static members when displaying a C++ object. The default is
on.
`set print static-members off'
Do not print static members when displaying a C++ object.
`show print static-members'
Show whether C++ static members are printed or not.
`set print pascal_static-members'
`set print pascal_static-members on'
Print static members when displaying a Pascal object. The default
is on.
`set print pascal_static-members off'
Do not print static members when displaying a Pascal object.
`show print pascal_static-members'
Show whether Pascal static members are printed or not.
`set print vtbl'
`set print vtbl on'
Pretty print C++ virtual function tables. The default is off.
(The `vtbl' commands do not work on programs compiled with the HP
ANSI C++ compiler (`aCC').)
`set print vtbl off'
Do not pretty print C++ virtual function tables.
`show print vtbl'
Show whether C++ virtual function tables are pretty printed, or
not.
File: gdb.info, Node: Value History, Next: Convenience Vars, Prev: Print Settings, Up: Data
8.8 Value History
=================
Values printed by the `print' command are saved in the GDB "value
history". This allows you to refer to them in other expressions.
Values are kept until the symbol table is re-read or discarded (for
example with the `file' or `symbol-file' commands). When the symbol
table changes, the value history is discarded, since the values may
contain pointers back to the types defined in the symbol table.
The values printed are given "history numbers" by which you can
refer to them. These are successive integers starting with one.
`print' shows you the history number assigned to a value by printing
`$NUM = ' before the value; here NUM is the history number.
To refer to any previous value, use `$' followed by the value's
history number. The way `print' labels its output is designed to
remind you of this. Just `$' refers to the most recent value in the
history, and `$$' refers to the value before that. `$$N' refers to the
Nth value from the end; `$$2' is the value just prior to `$$', `$$1' is
equivalent to `$$', and `$$0' is equivalent to `$'.
For example, suppose you have just printed a pointer to a structure
and want to see the contents of the structure. It suffices to type
p *$
If you have a chain of structures where the component `next' points
to the next one, you can print the contents of the next one with this:
p *$.next
You can print successive links in the chain by repeating this
command--which you can do by just typing <RET>.
Note that the history records values, not expressions. If the value
of `x' is 4 and you type these commands:
print x
set x=5
then the value recorded in the value history by the `print' command
remains 4 even though the value of `x' has changed.
`show values'
Print the last ten values in the value history, with their item
numbers. This is like `p $$9' repeated ten times, except that
`show values' does not change the history.
`show values N'
Print ten history values centered on history item number N.
`show values +'
Print ten history values just after the values last printed. If
no more values are available, `show values +' produces no display.
Pressing <RET> to repeat `show values N' has exactly the same effect
as `show values +'.
File: gdb.info, Node: Convenience Vars, Next: Registers, Prev: Value History, Up: Data
8.9 Convenience Variables
=========================
GDB provides "convenience variables" that you can use within GDB to
hold on to a value and refer to it later. These variables exist
entirely within GDB; they are not part of your program, and setting a
convenience variable has no direct effect on further execution of your
program. That is why you can use them freely.
Convenience variables are prefixed with `$'. Any name preceded by
`$' can be used for a convenience variable, unless it is one of the
predefined machine-specific register names (*note Registers:
Registers.). (Value history references, in contrast, are _numbers_
preceded by `$'. *Note Value History: Value History.)
You can save a value in a convenience variable with an assignment
expression, just as you would set a variable in your program. For
example:
set $foo = *object_ptr
would save in `$foo' the value contained in the object pointed to by
`object_ptr'.
Using a convenience variable for the first time creates it, but its
value is `void' until you assign a new value. You can alter the value
with another assignment at any time.
Convenience variables have no fixed types. You can assign a
convenience variable any type of value, including structures and
arrays, even if that variable already has a value of a different type.
The convenience variable, when used as an expression, has the type of
its current value.
`show convenience'
Print a list of convenience variables used so far, and their
values. Abbreviated `show conv'.
`init-if-undefined $VARIABLE = EXPRESSION'
Set a convenience variable if it has not already been set. This
is useful for user-defined commands that keep some state. It is
similar, in concept, to using local static variables with
initializers in C (except that convenience variables are global).
It can also be used to allow users to override default values used
in a command script.
If the variable is already defined then the expression is not
evaluated so any side-effects do not occur.
One of the ways to use a convenience variable is as a counter to be
incremented or a pointer to be advanced. For example, to print a field
from successive elements of an array of structures:
set $i = 0
print bar[$i++]->contents
Repeat that command by typing <RET>.
Some convenience variables are created automatically by GDB and given
values likely to be useful.
`$_'
The variable `$_' is automatically set by the `x' command to the
last address examined (*note Examining Memory: Memory.). Other
commands which provide a default address for `x' to examine also
set `$_' to that address; these commands include `info line' and
`info breakpoint'. The type of `$_' is `void *' except when set
by the `x' command, in which case it is a pointer to the type of
`$__'.
`$__'
The variable `$__' is automatically set by the `x' command to the
value found in the last address examined. Its type is chosen to
match the format in which the data was printed.
`$_exitcode'
The variable `$_exitcode' is automatically set to the exit code
when the program being debugged terminates.
On HP-UX systems, if you refer to a function or variable name that
begins with a dollar sign, GDB searches for a user or system name
first, before it searches for a convenience variable.
File: gdb.info, Node: Registers, Next: Floating Point Hardware, Prev: Convenience Vars, Up: Data
8.10 Registers
==============
You can refer to machine register contents, in expressions, as variables
with names starting with `$'. The names of registers are different for
each machine; use `info registers' to see the names used on your
machine.
`info registers'
Print the names and values of all registers except floating-point
and vector registers (in the selected stack frame).
`info all-registers'
Print the names and values of all registers, including
floating-point and vector registers (in the selected stack frame).
`info registers REGNAME ...'
Print the "relativized" value of each specified register REGNAME.
As discussed in detail below, register values are normally
relative to the selected stack frame. REGNAME may be any register
name valid on the machine you are using, with or without the
initial `$'.
GDB has four "standard" register names that are available (in
expressions) on most machines--whenever they do not conflict with an
architecture's canonical mnemonics for registers. The register names
`$pc' and `$sp' are used for the program counter register and the stack
pointer. `$fp' is used for a register that contains a pointer to the
current stack frame, and `$ps' is used for a register that contains the
processor status. For example, you could print the program counter in
hex with
p/x $pc
or print the instruction to be executed next with
x/i $pc
or add four to the stack pointer(1) with
set $sp += 4
Whenever possible, these four standard register names are available
on your machine even though the machine has different canonical
mnemonics, so long as there is no conflict. The `info registers'
command shows the canonical names. For example, on the SPARC, `info
registers' displays the processor status register as `$psr' but you can
also refer to it as `$ps'; and on x86-based machines `$ps' is an alias
for the EFLAGS register.
GDB always considers the contents of an ordinary register as an
integer when the register is examined in this way. Some machines have
special registers which can hold nothing but floating point; these
registers are considered to have floating point values. There is no way
to refer to the contents of an ordinary register as floating point value
(although you can _print_ it as a floating point value with `print/f
$REGNAME').
Some registers have distinct "raw" and "virtual" data formats. This
means that the data format in which the register contents are saved by
the operating system is not the same one that your program normally
sees. For example, the registers of the 68881 floating point
coprocessor are always saved in "extended" (raw) format, but all C
programs expect to work with "double" (virtual) format. In such cases,
GDB normally works with the virtual format only (the format that makes
sense for your program), but the `info registers' command prints the
data in both formats.
Some machines have special registers whose contents can be
interpreted in several different ways. For example, modern x86-based
machines have SSE and MMX registers that can hold several values packed
together in several different formats. GDB refers to such registers in
`struct' notation:
(gdb) print $xmm1
$1 = {
v4_float = {0, 3.43859137e-038, 1.54142831e-044, 1.821688e-044},
v2_double = {9.92129282474342e-303, 2.7585945287983262e-313},
v16_int8 = "\000\000\000\000\3706;\001\v\000\000\000\r\000\000",
v8_int16 = {0, 0, 14072, 315, 11, 0, 13, 0},
v4_int32 = {0, 20657912, 11, 13},
v2_int64 = {88725056443645952, 55834574859},
uint128 = 0x0000000d0000000b013b36f800000000
}
To set values of such registers, you need to tell GDB which view of the
register you wish to change, as if you were assigning value to a
`struct' member:
(gdb) set $xmm1.uint128 = 0x000000000000000000000000FFFFFFFF
Normally, register values are relative to the selected stack frame
(*note Selecting a Frame: Selection.). This means that you get the
value that the register would contain if all stack frames farther in
were exited and their saved registers restored. In order to see the
true contents of hardware registers, you must select the innermost
frame (with `frame 0').
However, GDB must deduce where registers are saved, from the machine
code generated by your compiler. If some registers are not saved, or if
GDB is unable to locate the saved registers, the selected stack frame
makes no difference.
---------- Footnotes ----------
(1) This is a way of removing one word from the stack, on machines
where stacks grow downward in memory (most machines, nowadays). This
assumes that the innermost stack frame is selected; setting `$sp' is
not allowed when other stack frames are selected. To pop entire frames
off the stack, regardless of machine architecture, use `return'; see
*Note Returning from a Function: Returning.
File: gdb.info, Node: Floating Point Hardware, Next: Vector Unit, Prev: Registers, Up: Data
8.11 Floating Point Hardware
============================
Depending on the configuration, GDB may be able to give you more
information about the status of the floating point hardware.
`info float'
Display hardware-dependent information about the floating point
unit. The exact contents and layout vary depending on the
floating point chip. Currently, `info float' is supported on the
ARM and x86 machines.
File: gdb.info, Node: Vector Unit, Next: OS Information, Prev: Floating Point Hardware, Up: Data
8.12 Vector Unit
================
Depending on the configuration, GDB may be able to give you more
information about the status of the vector unit.
`info vector'
Display information about the vector unit. The exact contents and
layout vary depending on the hardware.
File: gdb.info, Node: OS Information, Next: Memory Region Attributes, Prev: Vector Unit, Up: Data
8.13 Operating System Auxiliary Information
===========================================
GDB provides interfaces to useful OS facilities that can help you debug
your program.
When GDB runs on a "Posix system" (such as GNU or Unix machines), it
interfaces with the inferior via the `ptrace' system call. The
operating system creates a special sata structure, called `struct
user', for this interface. You can use the command `info udot' to
display the contents of this data structure.
`info udot'
Display the contents of the `struct user' maintained by the OS
kernel for the program being debugged. GDB displays the contents
of `struct user' as a list of hex numbers, similar to the
`examine' command.
Some operating systems supply an "auxiliary vector" to programs at
startup. This is akin to the arguments and environment that you
specify for a program, but contains a system-dependent variety of
binary values that tell system libraries important details about the
hardware, operating system, and process. Each value's purpose is
identified by an integer tag; the meanings are well-known but
system-specific. Depending on the configuration and operating system
facilities, GDB may be able to show you this information. For remote
targets, this functionality may further depend on the remote stub's
support of the `qXfer:auxv:read' packet, see *Note qXfer auxiliary
vector read::.
`info auxv'
Display the auxiliary vector of the inferior, which can be either a
live process or a core dump file. GDB prints each tag value
numerically, and also shows names and text descriptions for
recognized tags. Some values in the vector are numbers, some bit
masks, and some pointers to strings or other data. GDB displays
each value in the most appropriate form for a recognized tag, and
in hexadecimal for an unrecognized tag.
File: gdb.info, Node: Memory Region Attributes, Next: Dump/Restore Files, Prev: OS Information, Up: Data
8.14 Memory Region Attributes
=============================
"Memory region attributes" allow you to describe special handling
required by regions of your target's memory. GDB uses attributes to
determine whether to allow certain types of memory accesses; whether to
use specific width accesses; and whether to cache target memory. By
default the description of memory regions is fetched from the target
(if the current target supports this), but the user can override the
fetched regions.
Defined memory regions can be individually enabled and disabled.
When a memory region is disabled, GDB uses the default attributes when
accessing memory in that region. Similarly, if no memory regions have
been defined, GDB uses the default attributes when accessing all memory.
When a memory region is defined, it is given a number to identify it;
to enable, disable, or remove a memory region, you specify that number.
`mem LOWER UPPER ATTRIBUTES...'
Define a memory region bounded by LOWER and UPPER with attributes
ATTRIBUTES..., and add it to the list of regions monitored by GDB.
Note that UPPER == 0 is a special case: it is treated as the
target's maximum memory address. (0xffff on 16 bit targets,
0xffffffff on 32 bit targets, etc.)
`mem auto'
Discard any user changes to the memory regions and use
target-supplied regions, if available, or no regions if the target
does not support.
`delete mem NUMS...'
Remove memory regions NUMS... from the list of regions monitored
by GDB.
`disable mem NUMS...'
Disable monitoring of memory regions NUMS.... A disabled memory
region is not forgotten. It may be enabled again later.
`enable mem NUMS...'
Enable monitoring of memory regions NUMS....
`info mem'
Print a table of all defined memory regions, with the following
columns for each region:
_Memory Region Number_
_Enabled or Disabled._
Enabled memory regions are marked with `y'. Disabled memory
regions are marked with `n'.
_Lo Address_
The address defining the inclusive lower bound of the memory
region.
_Hi Address_
The address defining the exclusive upper bound of the memory
region.
_Attributes_
The list of attributes set for this memory region.
8.14.1 Attributes
-----------------
8.14.1.1 Memory Access Mode
...........................
The access mode attributes set whether GDB may make read or write
accesses to a memory region.
While these attributes prevent GDB from performing invalid memory
accesses, they do nothing to prevent the target system, I/O DMA, etc.
from accessing memory.
`ro'
Memory is read only.
`wo'
Memory is write only.
`rw'
Memory is read/write. This is the default.
8.14.1.2 Memory Access Size
...........................
The access size attribute tells GDB to use specific sized accesses in
the memory region. Often memory mapped device registers require
specific sized accesses. If no access size attribute is specified, GDB
may use accesses of any size.
`8'
Use 8 bit memory accesses.
`16'
Use 16 bit memory accesses.
`32'
Use 32 bit memory accesses.
`64'
Use 64 bit memory accesses.
8.14.1.3 Data Cache
...................
The data cache attributes set whether GDB will cache target memory.
While this generally improves performance by reducing debug protocol
overhead, it can lead to incorrect results because GDB does not know
about volatile variables or memory mapped device registers.
`cache'
Enable GDB to cache target memory.
`nocache'
Disable GDB from caching target memory. This is the default.
8.14.2 Memory Access Checking
-----------------------------
GDB can be instructed to refuse accesses to memory that is not
explicitly described. This can be useful if accessing such regions has
undesired effects for a specific target, or to provide better error
checking. The following commands control this behaviour.
`set mem inaccessible-by-default [on|off]'
If `on' is specified, make GDB treat memory not explicitly
described by the memory ranges as non-existent and refuse accesses
to such memory. The checks are only performed if there's at least
one memory range defined. If `off' is specified, make GDB treat
the memory not explicitly described by the memory ranges as RAM.
The default value is `on'.
`show mem inaccessible-by-default'
Show the current handling of accesses to unknown memory.
File: gdb.info, Node: Dump/Restore Files, Next: Core File Generation, Prev: Memory Region Attributes, Up: Data
8.15 Copy Between Memory and a File
===================================
You can use the commands `dump', `append', and `restore' to copy data
between target memory and a file. The `dump' and `append' commands
write data to a file, and the `restore' command reads data from a file
back into the inferior's memory. Files may be in binary, Motorola
S-record, Intel hex, or Tektronix Hex format; however, GDB can only
append to binary files.
`dump [FORMAT] memory FILENAME START_ADDR END_ADDR'
`dump [FORMAT] value FILENAME EXPR'
Dump the contents of memory from START_ADDR to END_ADDR, or the
value of EXPR, to FILENAME in the given format.
The FORMAT parameter may be any one of:
`binary'
Raw binary form.
`ihex'
Intel hex format.
`srec'
Motorola S-record format.
`tekhex'
Tektronix Hex format.
GDB uses the same definitions of these formats as the GNU binary
utilities, like `objdump' and `objcopy'. If FORMAT is omitted,
GDB dumps the data in raw binary form.
`append [binary] memory FILENAME START_ADDR END_ADDR'
`append [binary] value FILENAME EXPR'
Append the contents of memory from START_ADDR to END_ADDR, or the
value of EXPR, to the file FILENAME, in raw binary form. (GDB can
only append data to files in raw binary form.)
`restore FILENAME [binary] BIAS START END'
Restore the contents of file FILENAME into memory. The `restore'
command can automatically recognize any known BFD file format,
except for raw binary. To restore a raw binary file you must
specify the optional keyword `binary' after the filename.
If BIAS is non-zero, its value will be added to the addresses
contained in the file. Binary files always start at address zero,
so they will be restored at address BIAS. Other bfd files have a
built-in location; they will be restored at offset BIAS from that
location.
If START and/or END are non-zero, then only data between file
offset START and file offset END will be restored. These offsets
are relative to the addresses in the file, before the BIAS
argument is applied.
File: gdb.info, Node: Core File Generation, Next: Character Sets, Prev: Dump/Restore Files, Up: Data
8.16 How to Produce a Core File from Your Program
=================================================
A "core file" or "core dump" is a file that records the memory image of
a running process and its process status (register values etc.). Its
primary use is post-mortem debugging of a program that crashed while it
ran outside a debugger. A program that crashes automatically produces
a core file, unless this feature is disabled by the user. *Note
Files::, for information on invoking GDB in the post-mortem debugging
mode.
Occasionally, you may wish to produce a core file of the program you
are debugging in order to preserve a snapshot of its state. GDB has a
special command for that.
`generate-core-file [FILE]'
`gcore [FILE]'
Produce a core dump of the inferior process. The optional argument
FILE specifies the file name where to put the core dump. If not
specified, the file name defaults to `core.PID', where PID is the
inferior process ID.
Note that this command is implemented only for some systems (as of
this writing, GNU/Linux, FreeBSD, Solaris, Unixware, and S390).
File: gdb.info, Node: Character Sets, Next: Caching Remote Data, Prev: Core File Generation, Up: Data
8.17 Character Sets
===================
If the program you are debugging uses a different character set to
represent characters and strings than the one GDB uses itself, GDB can
automatically translate between the character sets for you. The
character set GDB uses we call the "host character set"; the one the
inferior program uses we call the "target character set".
For example, if you are running GDB on a GNU/Linux system, which
uses the ISO Latin 1 character set, but you are using GDB's remote
protocol (*note Remote Debugging::) to debug a program running on an
IBM mainframe, which uses the EBCDIC character set, then the host
character set is Latin-1, and the target character set is EBCDIC. If
you give GDB the command `set target-charset EBCDIC-US', then GDB
translates between EBCDIC and Latin 1 as you print character or string
values, or use character and string literals in expressions.
GDB has no way to automatically recognize which character set the
inferior program uses; you must tell it, using the `set target-charset'
command, described below.
Here are the commands for controlling GDB's character set support:
`set target-charset CHARSET'
Set the current target character set to CHARSET. We list the
character set names GDB recognizes below, but if you type `set
target-charset' followed by <TAB><TAB>, GDB will list the target
character sets it supports.
`set host-charset CHARSET'
Set the current host character set to CHARSET.
By default, GDB uses a host character set appropriate to the
system it is running on; you can override that default using the
`set host-charset' command.
GDB can only use certain character sets as its host character set.
We list the character set names GDB recognizes below, and
indicate which can be host character sets, but if you type `set
target-charset' followed by <TAB><TAB>, GDB will list the host
character sets it supports.
`set charset CHARSET'
Set the current host and target character sets to CHARSET. As
above, if you type `set charset' followed by <TAB><TAB>, GDB will
list the name of the character sets that can be used for both host
and target.
`show charset'
Show the names of the current host and target charsets.
`show host-charset'
Show the name of the current host charset.
`show target-charset'
Show the name of the current target charset.
GDB currently includes support for the following character sets:
`ASCII'
Seven-bit U.S. ASCII. GDB can use this as its host character set.
`ISO-8859-1'
The ISO Latin 1 character set. This extends ASCII with accented
characters needed for French, German, and Spanish. GDB can use
this as its host character set.
`EBCDIC-US'
`IBM1047'
Variants of the EBCDIC character set, used on some of IBM's
mainframe operating systems. (GNU/Linux on the S/390 uses U.S.
ASCII.) GDB cannot use these as its host character set.
Note that these are all single-byte character sets. More work inside
GDB is needed to support multi-byte or variable-width character
encodings, like the UTF-8 and UCS-2 encodings of Unicode.
Here is an example of GDB's character set support in action. Assume
that the following source code has been placed in the file
`charset-test.c':
#include <stdio.h>
char ascii_hello[]
= {72, 101, 108, 108, 111, 44, 32, 119,
111, 114, 108, 100, 33, 10, 0};
char ibm1047_hello[]
= {200, 133, 147, 147, 150, 107, 64, 166,
150, 153, 147, 132, 90, 37, 0};
main ()
{
printf ("Hello, world!\n");
}
In this program, `ascii_hello' and `ibm1047_hello' are arrays
containing the string `Hello, world!' followed by a newline, encoded in
the ASCII and IBM1047 character sets.
We compile the program, and invoke the debugger on it:
$ gcc -g charset-test.c -o charset-test
$ gdb -nw charset-test
GNU gdb 2001-12-19-cvs
Copyright 2001 Free Software Foundation, Inc.
...
(gdb)
We can use the `show charset' command to see what character sets GDB
is currently using to interpret and display characters and strings:
(gdb) show charset
The current host and target character set is `ISO-8859-1'.
(gdb)
For the sake of printing this manual, let's use ASCII as our initial
character set:
(gdb) set charset ASCII
(gdb) show charset
The current host and target character set is `ASCII'.
(gdb)
Let's assume that ASCII is indeed the correct character set for our
host system -- in other words, let's assume that if GDB prints
characters using the ASCII character set, our terminal will display
them properly. Since our current target character set is also ASCII,
the contents of `ascii_hello' print legibly:
(gdb) print ascii_hello
$1 = 0x401698 "Hello, world!\n"
(gdb) print ascii_hello[0]
$2 = 72 'H'
(gdb)
GDB uses the target character set for character and string literals
you use in expressions:
(gdb) print '+'
$3 = 43 '+'
(gdb)
The ASCII character set uses the number 43 to encode the `+'
character.
GDB relies on the user to tell it which character set the target
program uses. If we print `ibm1047_hello' while our target character
set is still ASCII, we get jibberish:
(gdb) print ibm1047_hello
$4 = 0x4016a8 "\310\205\223\223\226k@\246\226\231\223\204Z%"
(gdb) print ibm1047_hello[0]
$5 = 200 '\310'
(gdb)
If we invoke the `set target-charset' followed by <TAB><TAB>, GDB
tells us the character sets it supports:
(gdb) set target-charset
ASCII EBCDIC-US IBM1047 ISO-8859-1
(gdb) set target-charset
We can select IBM1047 as our target character set, and examine the
program's strings again. Now the ASCII string is wrong, but GDB
translates the contents of `ibm1047_hello' from the target character
set, IBM1047, to the host character set, ASCII, and they display
correctly:
(gdb) set target-charset IBM1047
(gdb) show charset
The current host character set is `ASCII'.
The current target character set is `IBM1047'.
(gdb) print ascii_hello
$6 = 0x401698 "\110\145%%?\054\040\167?\162%\144\041\012"
(gdb) print ascii_hello[0]
$7 = 72 '\110'
(gdb) print ibm1047_hello
$8 = 0x4016a8 "Hello, world!\n"
(gdb) print ibm1047_hello[0]
$9 = 200 'H'
(gdb)
As above, GDB uses the target character set for character and string
literals you use in expressions:
(gdb) print '+'
$10 = 78 '+'
(gdb)
The IBM1047 character set uses the number 78 to encode the `+'
character.
File: gdb.info, Node: Caching Remote Data, Prev: Character Sets, Up: Data
8.18 Caching Data of Remote Targets
===================================
GDB can cache data exchanged between the debugger and a remote target
(*note Remote Debugging::). Such caching generally improves
performance, because it reduces the overhead of the remote protocol by
bundling memory reads and writes into large chunks. Unfortunately, GDB
does not currently know anything about volatile registers, and thus
data caching will produce incorrect results when volatile registers are
in use.
`set remotecache on'
`set remotecache off'
Set caching state for remote targets. When `ON', use data
caching. By default, this option is `OFF'.
`show remotecache'
Show the current state of data caching for remote targets.
`info dcache'
Print the information about the data cache performance. The
information displayed includes: the dcache width and depth; and for
each cache line, how many times it was referenced, and its data and
state (dirty, bad, ok, etc.). This command is useful for debugging
the data cache operation.
File: gdb.info, Node: Macros, Next: Tracepoints, Prev: Data, Up: Top
9 C Preprocessor Macros
***********************
Some languages, such as C and C++, provide a way to define and invoke
"preprocessor macros" which expand into strings of tokens. GDB can
evaluate expressions containing macro invocations, show the result of
macro expansion, and show a macro's definition, including where it was
defined.
You may need to compile your program specially to provide GDB with
information about preprocessor macros. Most compilers do not include
macros in their debugging information, even when you compile with the
`-g' flag. *Note Compilation::.
A program may define a macro at one point, remove that definition
later, and then provide a different definition after that. Thus, at
different points in the program, a macro may have different
definitions, or have no definition at all. If there is a current stack
frame, GDB uses the macros in scope at that frame's source code line.
Otherwise, GDB uses the macros in scope at the current listing location;
see *Note List::.
At the moment, GDB does not support the `##' token-splicing
operator, the `#' stringification operator, or variable-arity macros.
Whenever GDB evaluates an expression, it always expands any macro
invocations present in the expression. GDB also provides the following
commands for working with macros explicitly.
`macro expand EXPRESSION'
`macro exp EXPRESSION'
Show the results of expanding all preprocessor macro invocations in
EXPRESSION. Since GDB simply expands macros, but does not parse
the result, EXPRESSION need not be a valid expression; it can be
any string of tokens.
`macro expand-once EXPRESSION'
`macro exp1 EXPRESSION'
(This command is not yet implemented.) Show the results of
expanding those preprocessor macro invocations that appear
explicitly in EXPRESSION. Macro invocations appearing in that
expansion are left unchanged. This command allows you to see the
effect of a particular macro more clearly, without being confused
by further expansions. Since GDB simply expands macros, but does
not parse the result, EXPRESSION need not be a valid expression; it
can be any string of tokens.
`info macro MACRO'
Show the definition of the macro named MACRO, and describe the
source location where that definition was established.
`macro define MACRO REPLACEMENT-LIST'
`macro define MACRO(ARGLIST) REPLACEMENT-LIST'
(This command is not yet implemented.) Introduce a definition for
a preprocessor macro named MACRO, invocations of which are replaced
by the tokens given in REPLACEMENT-LIST. The first form of this
command defines an "object-like" macro, which takes no arguments;
the second form defines a "function-like" macro, which takes the
arguments given in ARGLIST.
A definition introduced by this command is in scope in every
expression evaluated in GDB, until it is removed with the `macro
undef' command, described below. The definition overrides all
definitions for MACRO present in the program being debugged, as
well as any previous user-supplied definition.
`macro undef MACRO'
(This command is not yet implemented.) Remove any user-supplied
definition for the macro named MACRO. This command only affects
definitions provided with the `macro define' command, described
above; it cannot remove definitions present in the program being
debugged.
`macro list'
(This command is not yet implemented.) List all the macros
defined using the `macro define' command.
Here is a transcript showing the above commands in action. First, we
show our source files:
$ cat sample.c
#include <stdio.h>
#include "sample.h"
#define M 42
#define ADD(x) (M + x)
main ()
{
#define N 28
printf ("Hello, world!\n");
#undef N
printf ("We're so creative.\n");
#define N 1729
printf ("Goodbye, world!\n");
}
$ cat sample.h
#define Q <
$
Now, we compile the program using the GNU C compiler, GCC. We pass
the `-gdwarf-2' and `-g3' flags to ensure the compiler includes
information about preprocessor macros in the debugging information.
$ gcc -gdwarf-2 -g3 sample.c -o sample
$
Now, we start GDB on our sample program:
$ gdb -nw sample
GNU gdb 2002-05-06-cvs
Copyright 2002 Free Software Foundation, Inc.
GDB is free software, ...
(gdb)
We can expand macros and examine their definitions, even when the
program is not running. GDB uses the current listing position to
decide which macro definitions are in scope:
(gdb) list main
3
4 #define M 42
5 #define ADD(x) (M + x)
6
7 main ()
8 {
9 #define N 28
10 printf ("Hello, world!\n");
11 #undef N
12 printf ("We're so creative.\n");
(gdb) info macro ADD
Defined at /home/jimb/gdb/macros/play/sample.c:5
#define ADD(x) (M + x)
(gdb) info macro Q
Defined at /home/jimb/gdb/macros/play/sample.h:1
included at /home/jimb/gdb/macros/play/sample.c:2
#define Q <
(gdb) macro expand ADD(1)
expands to: (42 + 1)
(gdb) macro expand-once ADD(1)
expands to: once (M + 1)
(gdb)
In the example above, note that `macro expand-once' expands only the
macro invocation explicit in the original text -- the invocation of
`ADD' -- but does not expand the invocation of the macro `M', which was
introduced by `ADD'.
Once the program is running, GDB uses the macro definitions in force
at the source line of the current stack frame:
(gdb) break main
Breakpoint 1 at 0x8048370: file sample.c, line 10.
(gdb) run
Starting program: /home/jimb/gdb/macros/play/sample
Breakpoint 1, main () at sample.c:10
10 printf ("Hello, world!\n");
(gdb)
At line 10, the definition of the macro `N' at line 9 is in force:
(gdb) info macro N
Defined at /home/jimb/gdb/macros/play/sample.c:9
#define N 28
(gdb) macro expand N Q M
expands to: 28 < 42
(gdb) print N Q M
$1 = 1
(gdb)
As we step over directives that remove `N''s definition, and then
give it a new definition, GDB finds the definition (or lack thereof) in
force at each point:
(gdb) next
Hello, world!
12 printf ("We're so creative.\n");
(gdb) info macro N
The symbol `N' has no definition as a C/C++ preprocessor macro
at /home/jimb/gdb/macros/play/sample.c:12
(gdb) next
We're so creative.
14 printf ("Goodbye, world!\n");
(gdb) info macro N
Defined at /home/jimb/gdb/macros/play/sample.c:13
#define N 1729
(gdb) macro expand N Q M
expands to: 1729 < 42
(gdb) print N Q M
$2 = 0
(gdb)
File: gdb.info, Node: Tracepoints, Next: Overlays, Prev: Macros, Up: Top
10 Tracepoints
**************
In some applications, it is not feasible for the debugger to interrupt
the program's execution long enough for the developer to learn anything
helpful about its behavior. If the program's correctness depends on
its real-time behavior, delays introduced by a debugger might cause the
program to change its behavior drastically, or perhaps fail, even when
the code itself is correct. It is useful to be able to observe the
program's behavior without interrupting it.
Using GDB's `trace' and `collect' commands, you can specify
locations in the program, called "tracepoints", and arbitrary
expressions to evaluate when those tracepoints are reached. Later,
using the `tfind' command, you can examine the values those expressions
had when the program hit the tracepoints. The expressions may also
denote objects in memory--structures or arrays, for example--whose
values GDB should record; while visiting a particular tracepoint, you
may inspect those objects as if they were in memory at that moment.
However, because GDB records these values without interacting with you,
it can do so quickly and unobtrusively, hopefully not disturbing the
program's behavior.
The tracepoint facility is currently available only for remote
targets. *Note Targets::. In addition, your remote target must know
how to collect trace data. This functionality is implemented in the
remote stub; however, none of the stubs distributed with GDB support
tracepoints as of this writing. The format of the remote packets used
to implement tracepoints are described in *Note Tracepoint Packets::.
This chapter describes the tracepoint commands and features.
* Menu:
* Set Tracepoints::
* Analyze Collected Data::
* Tracepoint Variables::
File: gdb.info, Node: Set Tracepoints, Next: Analyze Collected Data, Up: Tracepoints
10.1 Commands to Set Tracepoints
================================
Before running such a "trace experiment", an arbitrary number of
tracepoints can be set. Like a breakpoint (*note Set Breaks::), a
tracepoint has a number assigned to it by GDB. Like with breakpoints,
tracepoint numbers are successive integers starting from one. Many of
the commands associated with tracepoints take the tracepoint number as
their argument, to identify which tracepoint to work on.
For each tracepoint, you can specify, in advance, some arbitrary set
of data that you want the target to collect in the trace buffer when it
hits that tracepoint. The collected data can include registers, local
variables, or global data. Later, you can use GDB commands to examine
the values these data had at the time the tracepoint was hit.
This section describes commands to set tracepoints and associated
conditions and actions.
* Menu:
* Create and Delete Tracepoints::
* Enable and Disable Tracepoints::
* Tracepoint Passcounts::
* Tracepoint Actions::
* Listing Tracepoints::
* Starting and Stopping Trace Experiments::
File: gdb.info, Node: Create and Delete Tracepoints, Next: Enable and Disable Tracepoints, Up: Set Tracepoints
10.1.1 Create and Delete Tracepoints
------------------------------------
`trace'
The `trace' command is very similar to the `break' command. Its
argument can be a source line, a function name, or an address in
the target program. *Note Set Breaks::. The `trace' command
defines a tracepoint, which is a point in the target program where
the debugger will briefly stop, collect some data, and then allow
the program to continue. Setting a tracepoint or changing its
commands doesn't take effect until the next `tstart' command;
thus, you cannot change the tracepoint attributes once a trace
experiment is running.
Here are some examples of using the `trace' command:
(gdb) trace foo.c:121 // a source file and line number
(gdb) trace +2 // 2 lines forward
(gdb) trace my_function // first source line of function
(gdb) trace *my_function // EXACT start address of function
(gdb) trace *0x2117c4 // an address
You can abbreviate `trace' as `tr'.
The convenience variable `$tpnum' records the tracepoint number of
the most recently set tracepoint.
`delete tracepoint [NUM]'
Permanently delete one or more tracepoints. With no argument, the
default is to delete all tracepoints.
Examples:
(gdb) delete trace 1 2 3 // remove three tracepoints
(gdb) delete trace // remove all tracepoints
You can abbreviate this command as `del tr'.
File: gdb.info, Node: Enable and Disable Tracepoints, Next: Tracepoint Passcounts, Prev: Create and Delete Tracepoints, Up: Set Tracepoints
10.1.2 Enable and Disable Tracepoints
-------------------------------------
`disable tracepoint [NUM]'
Disable tracepoint NUM, or all tracepoints if no argument NUM is
given. A disabled tracepoint will have no effect during the next
trace experiment, but it is not forgotten. You can re-enable a
disabled tracepoint using the `enable tracepoint' command.
`enable tracepoint [NUM]'
Enable tracepoint NUM, or all tracepoints. The enabled
tracepoints will become effective the next time a trace experiment
is run.
File: gdb.info, Node: Tracepoint Passcounts, Next: Tracepoint Actions, Prev: Enable and Disable Tracepoints, Up: Set Tracepoints
10.1.3 Tracepoint Passcounts
----------------------------
`passcount [N [NUM]]'
Set the "passcount" of a tracepoint. The passcount is a way to
automatically stop a trace experiment. If a tracepoint's
passcount is N, then the trace experiment will be automatically
stopped on the N'th time that tracepoint is hit. If the
tracepoint number NUM is not specified, the `passcount' command
sets the passcount of the most recently defined tracepoint. If no
passcount is given, the trace experiment will run until stopped
explicitly by the user.
Examples:
(gdb) passcount 5 2 // Stop on the 5th execution of
`// tracepoint 2'
(gdb) passcount 12 // Stop on the 12th execution of the
`// most recently defined tracepoint.'
(gdb) trace foo
(gdb) pass 3
(gdb) trace bar
(gdb) pass 2
(gdb) trace baz
(gdb) pass 1 // Stop tracing when foo has been
`// executed 3 times OR when bar has'
`// been executed 2 times'
`// OR when baz has been executed 1 time.'
File: gdb.info, Node: Tracepoint Actions, Next: Listing Tracepoints, Prev: Tracepoint Passcounts, Up: Set Tracepoints
10.1.4 Tracepoint Action Lists
------------------------------
`actions [NUM]'
This command will prompt for a list of actions to be taken when the
tracepoint is hit. If the tracepoint number NUM is not specified,
this command sets the actions for the one that was most recently
defined (so that you can define a tracepoint and then say
`actions' without bothering about its number). You specify the
actions themselves on the following lines, one action at a time,
and terminate the actions list with a line containing just `end'.
So far, the only defined actions are `collect' and
`while-stepping'.
To remove all actions from a tracepoint, type `actions NUM' and
follow it immediately with `end'.
(gdb) collect DATA // collect some data
(gdb) while-stepping 5 // single-step 5 times, collect data
(gdb) end // signals the end of actions.
In the following example, the action list begins with `collect'
commands indicating the things to be collected when the tracepoint
is hit. Then, in order to single-step and collect additional data
following the tracepoint, a `while-stepping' command is used,
followed by the list of things to be collected while stepping. The
`while-stepping' command is terminated by its own separate `end'
command. Lastly, the action list is terminated by an `end'
command.
(gdb) trace foo
(gdb) actions
Enter actions for tracepoint 1, one per line:
> collect bar,baz
> collect $regs
> while-stepping 12
> collect $fp, $sp
> end
end
`collect EXPR1, EXPR2, ...'
Collect values of the given expressions when the tracepoint is hit.
This command accepts a comma-separated list of any valid
expressions. In addition to global, static, or local variables,
the following special arguments are supported:
`$regs'
collect all registers
`$args'
collect all function arguments
`$locals'
collect all local variables.
You can give several consecutive `collect' commands, each one with
a single argument, or one `collect' command with several arguments
separated by commas: the effect is the same.
The command `info scope' (*note info scope: Symbols.) is
particularly useful for figuring out what data to collect.
`while-stepping N'
Perform N single-step traces after the tracepoint, collecting new
data at each step. The `while-stepping' command is followed by
the list of what to collect while stepping (followed by its own
`end' command):
> while-stepping 12
> collect $regs, myglobal
> end
>
You may abbreviate `while-stepping' as `ws' or `stepping'.
File: gdb.info, Node: Listing Tracepoints, Next: Starting and Stopping Trace Experiments, Prev: Tracepoint Actions, Up: Set Tracepoints
10.1.5 Listing Tracepoints
--------------------------
`info tracepoints [NUM]'
Display information about the tracepoint NUM. If you don't specify
a tracepoint number, displays information about all the tracepoints
defined so far. For each tracepoint, the following information is
shown:
* its number
* whether it is enabled or disabled
* its address
* its passcount as given by the `passcount N' command
* its step count as given by the `while-stepping N' command
* where in the source files is the tracepoint set
* its action list as given by the `actions' command
(gdb) info trace
Num Enb Address PassC StepC What
1 y 0x002117c4 0 0 <gdb_asm>
2 y 0x0020dc64 0 0 in g_test at g_test.c:1375
3 y 0x0020b1f4 0 0 in get_data at ../foo.c:41
(gdb)
This command can be abbreviated `info tp'.
File: gdb.info, Node: Starting and Stopping Trace Experiments, Prev: Listing Tracepoints, Up: Set Tracepoints
10.1.6 Starting and Stopping Trace Experiments
----------------------------------------------
`tstart'
This command takes no arguments. It starts the trace experiment,
and begins collecting data. This has the side effect of
discarding all the data collected in the trace buffer during the
previous trace experiment.
`tstop'
This command takes no arguments. It ends the trace experiment, and
stops collecting data.
*Note*: a trace experiment and data collection may stop
automatically if any tracepoint's passcount is reached (*note
Tracepoint Passcounts::), or if the trace buffer becomes full.
`tstatus'
This command displays the status of the current trace data
collection.
Here is an example of the commands we described so far:
(gdb) trace gdb_c_test
(gdb) actions
Enter actions for tracepoint #1, one per line.
> collect $regs,$locals,$args
> while-stepping 11
> collect $regs
> end
> end
(gdb) tstart
[time passes ...]
(gdb) tstop
File: gdb.info, Node: Analyze Collected Data, Next: Tracepoint Variables, Prev: Set Tracepoints, Up: Tracepoints
10.2 Using the Collected Data
=============================
After the tracepoint experiment ends, you use GDB commands for
examining the trace data. The basic idea is that each tracepoint
collects a trace "snapshot" every time it is hit and another snapshot
every time it single-steps. All these snapshots are consecutively
numbered from zero and go into a buffer, and you can examine them
later. The way you examine them is to "focus" on a specific trace
snapshot. When the remote stub is focused on a trace snapshot, it will
respond to all GDB requests for memory and registers by reading from
the buffer which belongs to that snapshot, rather than from _real_
memory or registers of the program being debugged. This means that
*all* GDB commands (`print', `info registers', `backtrace', etc.) will
behave as if we were currently debugging the program state as it was
when the tracepoint occurred. Any requests for data that are not in
the buffer will fail.
* Menu:
* tfind:: How to select a trace snapshot
* tdump:: How to display all data for a snapshot
* save-tracepoints:: How to save tracepoints for a future run
File: gdb.info, Node: tfind, Next: tdump, Up: Analyze Collected Data
10.2.1 `tfind N'
----------------
The basic command for selecting a trace snapshot from the buffer is
`tfind N', which finds trace snapshot number N, counting from zero. If
no argument N is given, the next snapshot is selected.
Here are the various forms of using the `tfind' command.
`tfind start'
Find the first snapshot in the buffer. This is a synonym for
`tfind 0' (since 0 is the number of the first snapshot).
`tfind none'
Stop debugging trace snapshots, resume _live_ debugging.
`tfind end'
Same as `tfind none'.
`tfind'
No argument means find the next trace snapshot.
`tfind -'
Find the previous trace snapshot before the current one. This
permits retracing earlier steps.
`tfind tracepoint NUM'
Find the next snapshot associated with tracepoint NUM. Search
proceeds forward from the last examined trace snapshot. If no
argument NUM is given, it means find the next snapshot collected
for the same tracepoint as the current snapshot.
`tfind pc ADDR'
Find the next snapshot associated with the value ADDR of the
program counter. Search proceeds forward from the last examined
trace snapshot. If no argument ADDR is given, it means find the
next snapshot with the same value of PC as the current snapshot.
`tfind outside ADDR1, ADDR2'
Find the next snapshot whose PC is outside the given range of
addresses.
`tfind range ADDR1, ADDR2'
Find the next snapshot whose PC is between ADDR1 and ADDR2.
`tfind line [FILE:]N'
Find the next snapshot associated with the source line N. If the
optional argument FILE is given, refer to line N in that source
file. Search proceeds forward from the last examined trace
snapshot. If no argument N is given, it means find the next line
other than the one currently being examined; thus saying `tfind
line' repeatedly can appear to have the same effect as stepping
from line to line in a _live_ debugging session.
The default arguments for the `tfind' commands are specifically
designed to make it easy to scan through the trace buffer. For
instance, `tfind' with no argument selects the next trace snapshot, and
`tfind -' with no argument selects the previous trace snapshot. So, by
giving one `tfind' command, and then simply hitting <RET> repeatedly
you can examine all the trace snapshots in order. Or, by saying `tfind
-' and then hitting <RET> repeatedly you can examine the snapshots in
reverse order. The `tfind line' command with no argument selects the
snapshot for the next source line executed. The `tfind pc' command with
no argument selects the next snapshot with the same program counter
(PC) as the current frame. The `tfind tracepoint' command with no
argument selects the next trace snapshot collected by the same
tracepoint as the current one.
In addition to letting you scan through the trace buffer manually,
these commands make it easy to construct GDB scripts that scan through
the trace buffer and print out whatever collected data you are
interested in. Thus, if we want to examine the PC, FP, and SP
registers from each trace frame in the buffer, we can say this:
(gdb) tfind start
(gdb) while ($trace_frame != -1)
> printf "Frame %d, PC = %08X, SP = %08X, FP = %08X\n", \
$trace_frame, $pc, $sp, $fp
> tfind
> end
Frame 0, PC = 0020DC64, SP = 0030BF3C, FP = 0030BF44
Frame 1, PC = 0020DC6C, SP = 0030BF38, FP = 0030BF44
Frame 2, PC = 0020DC70, SP = 0030BF34, FP = 0030BF44
Frame 3, PC = 0020DC74, SP = 0030BF30, FP = 0030BF44
Frame 4, PC = 0020DC78, SP = 0030BF2C, FP = 0030BF44
Frame 5, PC = 0020DC7C, SP = 0030BF28, FP = 0030BF44
Frame 6, PC = 0020DC80, SP = 0030BF24, FP = 0030BF44
Frame 7, PC = 0020DC84, SP = 0030BF20, FP = 0030BF44
Frame 8, PC = 0020DC88, SP = 0030BF1C, FP = 0030BF44
Frame 9, PC = 0020DC8E, SP = 0030BF18, FP = 0030BF44
Frame 10, PC = 00203F6C, SP = 0030BE3C, FP = 0030BF14
Or, if we want to examine the variable `X' at each source line in
the buffer:
(gdb) tfind start
(gdb) while ($trace_frame != -1)
> printf "Frame %d, X == %d\n", $trace_frame, X
> tfind line
> end
Frame 0, X = 1
Frame 7, X = 2
Frame 13, X = 255
File: gdb.info, Node: tdump, Next: save-tracepoints, Prev: tfind, Up: Analyze Collected Data
10.2.2 `tdump'
--------------
This command takes no arguments. It prints all the data collected at
the current trace snapshot.
(gdb) trace 444
(gdb) actions
Enter actions for tracepoint #2, one per line:
> collect $regs, $locals, $args, gdb_long_test
> end
(gdb) tstart
(gdb) tfind line 444
#0 gdb_test (p1=0x11, p2=0x22, p3=0x33, p4=0x44, p5=0x55, p6=0x66)
at gdb_test.c:444
444 printp( "%s: arguments = 0x%X 0x%X 0x%X 0x%X 0x%X 0x%X\n", )
(gdb) tdump
Data collected at tracepoint 2, trace frame 1:
d0 0xc4aa0085 -995491707
d1 0x18 24
d2 0x80 128
d3 0x33 51
d4 0x71aea3d 119204413
d5 0x22 34
d6 0xe0 224
d7 0x380035 3670069
a0 0x19e24a 1696330
a1 0x3000668 50333288
a2 0x100 256
a3 0x322000 3284992
a4 0x3000698 50333336
a5 0x1ad3cc 1758156
fp 0x30bf3c 0x30bf3c
sp 0x30bf34 0x30bf34
ps 0x0 0
pc 0x20b2c8 0x20b2c8
fpcontrol 0x0 0
fpstatus 0x0 0
fpiaddr 0x0 0
p = 0x20e5b4 "gdb-test"
p1 = (void *) 0x11
p2 = (void *) 0x22
p3 = (void *) 0x33
p4 = (void *) 0x44
p5 = (void *) 0x55
p6 = (void *) 0x66
gdb_long_test = 17 '\021'
(gdb)
File: gdb.info, Node: save-tracepoints, Prev: tdump, Up: Analyze Collected Data
10.2.3 `save-tracepoints FILENAME'
----------------------------------
This command saves all current tracepoint definitions together with
their actions and passcounts, into a file `FILENAME' suitable for use
in a later debugging session. To read the saved tracepoint
definitions, use the `source' command (*note Command Files::).
File: gdb.info, Node: Tracepoint Variables, Prev: Analyze Collected Data, Up: Tracepoints
10.3 Convenience Variables for Tracepoints
==========================================
`(int) $trace_frame'
The current trace snapshot (a.k.a. "frame") number, or -1 if no
snapshot is selected.
`(int) $tracepoint'
The tracepoint for the current trace snapshot.
`(int) $trace_line'
The line number for the current trace snapshot.
`(char []) $trace_file'
The source file for the current trace snapshot.
`(char []) $trace_func'
The name of the function containing `$tracepoint'.
Note: `$trace_file' is not suitable for use in `printf', use
`output' instead.
Here's a simple example of using these convenience variables for
stepping through all the trace snapshots and printing some of their
data.
(gdb) tfind start
(gdb) while $trace_frame != -1
> output $trace_file
> printf ", line %d (tracepoint #%d)\n", $trace_line, $tracepoint
> tfind
> end
File: gdb.info, Node: Overlays, Next: Languages, Prev: Tracepoints, Up: Top
11 Debugging Programs That Use Overlays
***************************************
If your program is too large to fit completely in your target system's
memory, you can sometimes use "overlays" to work around this problem.
GDB provides some support for debugging programs that use overlays.
* Menu:
* How Overlays Work:: A general explanation of overlays.
* Overlay Commands:: Managing overlays in GDB.
* Automatic Overlay Debugging:: GDB can find out which overlays are
mapped by asking the inferior.
* Overlay Sample Program:: A sample program using overlays.
File: gdb.info, Node: How Overlays Work, Next: Overlay Commands, Up: Overlays
11.1 How Overlays Work
======================
Suppose you have a computer whose instruction address space is only 64
kilobytes long, but which has much more memory which can be accessed by
other means: special instructions, segment registers, or memory
management hardware, for example. Suppose further that you want to
adapt a program which is larger than 64 kilobytes to run on this system.
One solution is to identify modules of your program which are
relatively independent, and need not call each other directly; call
these modules "overlays". Separate the overlays from the main program,
and place their machine code in the larger memory. Place your main
program in instruction memory, but leave at least enough space there to
hold the largest overlay as well.
Now, to call a function located in an overlay, you must first copy
that overlay's machine code from the large memory into the space set
aside for it in the instruction memory, and then jump to its entry point
there.
Data Instruction Larger
Address Space Address Space Address Space
+-----------+ +-----------+ +-----------+
| | | | | |
+-----------+ +-----------+ +-----------+<-- overlay 1
| program | | main | .----| overlay 1 | load address
| variables | | program | | +-----------+
| and heap | | | | | |
+-----------+ | | | +-----------+<-- overlay 2
| | +-----------+ | | | load address
+-----------+ | | | .-| overlay 2 |
| | | | | |
mapped --->+-----------+ | | +-----------+
address | | | | | |
| overlay | <-' | | |
| area | <---' +-----------+<-- overlay 3
| | <---. | | load address
+-----------+ `--| overlay 3 |
| | | |
+-----------+ | |
+-----------+
| |
+-----------+
A code overlay
The diagram (*note A code overlay::) shows a system with separate
data and instruction address spaces. To map an overlay, the program
copies its code from the larger address space to the instruction
address space. Since the overlays shown here all use the same mapped
address, only one may be mapped at a time. For a system with a single
address space for data and instructions, the diagram would be similar,
except that the program variables and heap would share an address space
with the main program and the overlay area.
An overlay loaded into instruction memory and ready for use is
called a "mapped" overlay; its "mapped address" is its address in the
instruction memory. An overlay not present (or only partially present)
in instruction memory is called "unmapped"; its "load address" is its
address in the larger memory. The mapped address is also called the
"virtual memory address", or "VMA"; the load address is also called the
"load memory address", or "LMA".
Unfortunately, overlays are not a completely transparent way to
adapt a program to limited instruction memory. They introduce a new
set of global constraints you must keep in mind as you design your
program:
* Before calling or returning to a function in an overlay, your
program must make sure that overlay is actually mapped.
Otherwise, the call or return will transfer control to the right
address, but in the wrong overlay, and your program will probably
crash.
* If the process of mapping an overlay is expensive on your system,
you will need to choose your overlays carefully to minimize their
effect on your program's performance.
* The executable file you load onto your system must contain each
overlay's instructions, appearing at the overlay's load address,
not its mapped address. However, each overlay's instructions must
be relocated and its symbols defined as if the overlay were at its
mapped address. You can use GNU linker scripts to specify
different load and relocation addresses for pieces of your
program; see *Note Overlay Description: (ld.info)Overlay
Description.
* The procedure for loading executable files onto your system must
be able to load their contents into the larger address space as
well as the instruction and data spaces.
The overlay system described above is rather simple, and could be
improved in many ways:
* If your system has suitable bank switch registers or memory
management hardware, you could use those facilities to make an
overlay's load area contents simply appear at their mapped address
in instruction space. This would probably be faster than copying
the overlay to its mapped area in the usual way.
* If your overlays are small enough, you could set aside more than
one overlay area, and have more than one overlay mapped at a time.
* You can use overlays to manage data, as well as instructions. In
general, data overlays are even less transparent to your design
than code overlays: whereas code overlays only require care when
you call or return to functions, data overlays require care every
time you access the data. Also, if you change the contents of a
data overlay, you must copy its contents back out to its load
address before you can copy a different data overlay into the same
mapped area.
File: gdb.info, Node: Overlay Commands, Next: Automatic Overlay Debugging, Prev: How Overlays Work, Up: Overlays
11.2 Overlay Commands
=====================
To use GDB's overlay support, each overlay in your program must
correspond to a separate section of the executable file. The section's
virtual memory address and load memory address must be the overlay's
mapped and load addresses. Identifying overlays with sections allows
GDB to determine the appropriate address of a function or variable,
depending on whether the overlay is mapped or not.
GDB's overlay commands all start with the word `overlay'; you can
abbreviate this as `ov' or `ovly'. The commands are:
`overlay off'
Disable GDB's overlay support. When overlay support is disabled,
GDB assumes that all functions and variables are always present at
their mapped addresses. By default, GDB's overlay support is
disabled.
`overlay manual'
Enable "manual" overlay debugging. In this mode, GDB relies on
you to tell it which overlays are mapped, and which are not, using
the `overlay map-overlay' and `overlay unmap-overlay' commands
described below.
`overlay map-overlay OVERLAY'
`overlay map OVERLAY'
Tell GDB that OVERLAY is now mapped; OVERLAY must be the name of
the object file section containing the overlay. When an overlay
is mapped, GDB assumes it can find the overlay's functions and
variables at their mapped addresses. GDB assumes that any other
overlays whose mapped ranges overlap that of OVERLAY are now
unmapped.
`overlay unmap-overlay OVERLAY'
`overlay unmap OVERLAY'
Tell GDB that OVERLAY is no longer mapped; OVERLAY must be the
name of the object file section containing the overlay. When an
overlay is unmapped, GDB assumes it can find the overlay's
functions and variables at their load addresses.
`overlay auto'
Enable "automatic" overlay debugging. In this mode, GDB consults
a data structure the overlay manager maintains in the inferior to
see which overlays are mapped. For details, see *Note Automatic
Overlay Debugging::.
`overlay load-target'
`overlay load'
Re-read the overlay table from the inferior. Normally, GDB
re-reads the table GDB automatically each time the inferior stops,
so this command should only be necessary if you have changed the
overlay mapping yourself using GDB. This command is only useful
when using automatic overlay debugging.
`overlay list-overlays'
`overlay list'
Display a list of the overlays currently mapped, along with their
mapped addresses, load addresses, and sizes.
Normally, when GDB prints a code address, it includes the name of
the function the address falls in:
(gdb) print main
$3 = {int ()} 0x11a0 <main>
When overlay debugging is enabled, GDB recognizes code in unmapped
overlays, and prints the names of unmapped functions with asterisks
around them. For example, if `foo' is a function in an unmapped
overlay, GDB prints it this way:
(gdb) overlay list
No sections are mapped.
(gdb) print foo
$5 = {int (int)} 0x100000 <*foo*>
When `foo''s overlay is mapped, GDB prints the function's name
normally:
(gdb) overlay list
Section .ov.foo.text, loaded at 0x100000 - 0x100034,
mapped at 0x1016 - 0x104a
(gdb) print foo
$6 = {int (int)} 0x1016 <foo>
When overlay debugging is enabled, GDB can find the correct address
for functions and variables in an overlay, whether or not the overlay
is mapped. This allows most GDB commands, like `break' and
`disassemble', to work normally, even on unmapped code. However, GDB's
breakpoint support has some limitations:
* You can set breakpoints in functions in unmapped overlays, as long
as GDB can write to the overlay at its load address.
* GDB can not set hardware or simulator-based breakpoints in
unmapped overlays. However, if you set a breakpoint at the end of
your overlay manager (and tell GDB which overlays are now mapped,
if you are using manual overlay management), GDB will re-set its
breakpoints properly.
File: gdb.info, Node: Automatic Overlay Debugging, Next: Overlay Sample Program, Prev: Overlay Commands, Up: Overlays
11.3 Automatic Overlay Debugging
================================
GDB can automatically track which overlays are mapped and which are
not, given some simple co-operation from the overlay manager in the
inferior. If you enable automatic overlay debugging with the `overlay
auto' command (*note Overlay Commands::), GDB looks in the inferior's
memory for certain variables describing the current state of the
overlays.
Here are the variables your overlay manager must define to support
GDB's automatic overlay debugging:
`_ovly_table':
This variable must be an array of the following structures:
struct
{
/* The overlay's mapped address. */
unsigned long vma;
/* The size of the overlay, in bytes. */
unsigned long size;
/* The overlay's load address. */
unsigned long lma;
/* Non-zero if the overlay is currently mapped;
zero otherwise. */
unsigned long mapped;
}
`_novlys':
This variable must be a four-byte signed integer, holding the total
number of elements in `_ovly_table'.
To decide whether a particular overlay is mapped or not, GDB looks
for an entry in `_ovly_table' whose `vma' and `lma' members equal the
VMA and LMA of the overlay's section in the executable file. When GDB
finds a matching entry, it consults the entry's `mapped' member to
determine whether the overlay is currently mapped.
In addition, your overlay manager may define a function called
`_ovly_debug_event'. If this function is defined, GDB will silently
set a breakpoint there. If the overlay manager then calls this
function whenever it has changed the overlay table, this will enable
GDB to accurately keep track of which overlays are in program memory,
and update any breakpoints that may be set in overlays. This will
allow breakpoints to work even if the overlays are kept in ROM or other
non-writable memory while they are not being executed.
File: gdb.info, Node: Overlay Sample Program, Prev: Automatic Overlay Debugging, Up: Overlays
11.4 Overlay Sample Program
===========================
When linking a program which uses overlays, you must place the overlays
at their load addresses, while relocating them to run at their mapped
addresses. To do this, you must write a linker script (*note Overlay
Description: (ld.info)Overlay Description.). Unfortunately, since
linker scripts are specific to a particular host system, target
architecture, and target memory layout, this manual cannot provide
portable sample code demonstrating GDB's overlay support.
However, the GDB source distribution does contain an overlaid
program, with linker scripts for a few systems, as part of its test
suite. The program consists of the following files from
`gdb/testsuite/gdb.base':
`overlays.c'
The main program file.
`ovlymgr.c'
A simple overlay manager, used by `overlays.c'.
`foo.c'
`bar.c'
`baz.c'
`grbx.c'
Overlay modules, loaded and used by `overlays.c'.
`d10v.ld'
`m32r.ld'
Linker scripts for linking the test program on the `d10v-elf' and
`m32r-elf' targets.
You can build the test program using the `d10v-elf' GCC
cross-compiler like this:
$ d10v-elf-gcc -g -c overlays.c
$ d10v-elf-gcc -g -c ovlymgr.c
$ d10v-elf-gcc -g -c foo.c
$ d10v-elf-gcc -g -c bar.c
$ d10v-elf-gcc -g -c baz.c
$ d10v-elf-gcc -g -c grbx.c
$ d10v-elf-gcc -g overlays.o ovlymgr.o foo.o bar.o \
baz.o grbx.o -Wl,-Td10v.ld -o overlays
The build process is identical for any other architecture, except
that you must substitute the appropriate compiler and linker script for
the target system for `d10v-elf-gcc' and `d10v.ld'.
File: gdb.info, Node: Languages, Next: Symbols, Prev: Overlays, Up: Top
12 Using GDB with Different Languages
*************************************
Although programming languages generally have common aspects, they are
rarely expressed in the same manner. For instance, in ANSI C,
dereferencing a pointer `p' is accomplished by `*p', but in Modula-2,
it is accomplished by `p^'. Values can also be represented (and
displayed) differently. Hex numbers in C appear as `0x1ae', while in
Modula-2 they appear as `1AEH'.
Language-specific information is built into GDB for some languages,
allowing you to express operations like the above in your program's
native language, and allowing GDB to output values in a manner
consistent with the syntax of your program's native language. The
language you use to build expressions is called the "working language".
* Menu:
* Setting:: Switching between source languages
* Show:: Displaying the language
* Checks:: Type and range checks
* Supported Languages:: Supported languages
* Unsupported Languages:: Unsupported languages
File: gdb.info, Node: Setting, Next: Show, Up: Languages
12.1 Switching Between Source Languages
=======================================
There are two ways to control the working language--either have GDB set
it automatically, or select it manually yourself. You can use the `set
language' command for either purpose. On startup, GDB defaults to
setting the language automatically. The working language is used to
determine how expressions you type are interpreted, how values are
printed, etc.
In addition to the working language, every source file that GDB
knows about has its own working language. For some object file
formats, the compiler might indicate which language a particular source
file is in. However, most of the time GDB infers the language from the
name of the file. The language of a source file controls whether C++
names are demangled--this way `backtrace' can show each frame
appropriately for its own language. There is no way to set the
language of a source file from within GDB, but you can set the language
associated with a filename extension. *Note Displaying the Language:
Show.
This is most commonly a problem when you use a program, such as
`cfront' or `f2c', that generates C but is written in another language.
In that case, make the program use `#line' directives in its C output;
that way GDB will know the correct language of the source code of the
original program, and will display that source code, not the generated
C code.
* Menu:
* Filenames:: Filename extensions and languages.
* Manually:: Setting the working language manually
* Automatically:: Having GDB infer the source language
File: gdb.info, Node: Filenames, Next: Manually, Up: Setting
12.1.1 List of Filename Extensions and Languages
------------------------------------------------
If a source file name ends in one of the following extensions, then GDB
infers that its language is the one indicated.
`.ada'
`.ads'
`.adb'
`.a'
Ada source file.
`.c'
C source file
`.C'
`.cc'
`.cp'
`.cpp'
`.cxx'
`.c++'
C++ source file
`.m'
Objective-C source file
`.f'
`.F'
Fortran source file
`.mod'
Modula-2 source file
`.s'
`.S'
Assembler source file. This actually behaves almost like C, but
GDB does not skip over function prologues when stepping.
In addition, you may set the language associated with a filename
extension. *Note Displaying the Language: Show.
File: gdb.info, Node: Manually, Next: Automatically, Prev: Filenames, Up: Setting
12.1.2 Setting the Working Language
-----------------------------------
If you allow GDB to set the language automatically, expressions are
interpreted the same way in your debugging session and your program.
If you wish, you may set the language manually. To do this, issue
the command `set language LANG', where LANG is the name of a language,
such as `c' or `modula-2'. For a list of the supported languages, type
`set language'.
Setting the language manually prevents GDB from updating the working
language automatically. This can lead to confusion if you try to debug
a program when the working language is not the same as the source
language, when an expression is acceptable to both languages--but means
different things. For instance, if the current source file were
written in C, and GDB was parsing Modula-2, a command such as:
print a = b + c
might not have the effect you intended. In C, this means to add `b'
and `c' and place the result in `a'. The result printed would be the
value of `a'. In Modula-2, this means to compare `a' to the result of
`b+c', yielding a `BOOLEAN' value.
File: gdb.info, Node: Automatically, Prev: Manually, Up: Setting
12.1.3 Having GDB Infer the Source Language
-------------------------------------------
To have GDB set the working language automatically, use `set language
local' or `set language auto'. GDB then infers the working language.
That is, when your program stops in a frame (usually by encountering a
breakpoint), GDB sets the working language to the language recorded for
the function in that frame. If the language for a frame is unknown
(that is, if the function or block corresponding to the frame was
defined in a source file that does not have a recognized extension),
the current working language is not changed, and GDB issues a warning.
This may not seem necessary for most programs, which are written
entirely in one source language. However, program modules and libraries
written in one source language can be used by a main program written in
a different source language. Using `set language auto' in this case
frees you from having to set the working language manually.
File: gdb.info, Node: Show, Next: Checks, Prev: Setting, Up: Languages
12.2 Displaying the Language
============================
The following commands help you find out which language is the working
language, and also what language source files were written in.
`show language'
Display the current working language. This is the language you
can use with commands such as `print' to build and compute
expressions that may involve variables in your program.
`info frame'
Display the source language for this frame. This language becomes
the working language if you use an identifier from this frame.
*Note Information about a Frame: Frame Info, to identify the other
information listed here.
`info source'
Display the source language of this source file. *Note Examining
the Symbol Table: Symbols, to identify the other information
listed here.
In unusual circumstances, you may have source files with extensions
not in the standard list. You can then set the extension associated
with a language explicitly:
`set extension-language EXT LANGUAGE'
Tell GDB that source files with extension EXT are to be assumed as
written in the source language LANGUAGE.
`info extensions'
List all the filename extensions and the associated languages.
File: gdb.info, Node: Checks, Next: Supported Languages, Prev: Show, Up: Languages
12.3 Type and Range Checking
============================
_Warning:_ In this release, the GDB commands for type and range
checking are included, but they do not yet have any effect. This
section documents the intended facilities.
Some languages are designed to guard you against making seemingly
common errors through a series of compile- and run-time checks. These
include checking the type of arguments to functions and operators, and
making sure mathematical overflows are caught at run time. Checks such
as these help to ensure a program's correctness once it has been
compiled by eliminating type mismatches, and providing active checks
for range errors when your program is running.
GDB can check for conditions like the above if you wish. Although
GDB does not check the statements in your program, it can check
expressions entered directly into GDB for evaluation via the `print'
command, for example. As with the working language, GDB can also
decide whether or not to check automatically based on your program's
source language. *Note Supported Languages: Supported Languages, for
the default settings of supported languages.
* Menu:
* Type Checking:: An overview of type checking
* Range Checking:: An overview of range checking
File: gdb.info, Node: Type Checking, Next: Range Checking, Up: Checks
12.3.1 An Overview of Type Checking
-----------------------------------
Some languages, such as Modula-2, are strongly typed, meaning that the
arguments to operators and functions have to be of the correct type,
otherwise an error occurs. These checks prevent type mismatch errors
from ever causing any run-time problems. For example,
1 + 2 => 3
but
error--> 1 + 2.3
The second example fails because the `CARDINAL' 1 is not
type-compatible with the `REAL' 2.3.
For the expressions you use in GDB commands, you can tell the GDB
type checker to skip checking; to treat any mismatches as errors and
abandon the expression; or to only issue warnings when type mismatches
occur, but evaluate the expression anyway. When you choose the last of
these, GDB evaluates expressions like the second example above, but
also issues a warning.
Even if you turn type checking off, there may be other reasons
related to type that prevent GDB from evaluating an expression. For
instance, GDB does not know how to add an `int' and a `struct foo'.
These particular type errors have nothing to do with the language in
use, and usually arise from expressions, such as the one described
above, which make little sense to evaluate anyway.
Each language defines to what degree it is strict about type. For
instance, both Modula-2 and C require the arguments to arithmetical
operators to be numbers. In C, enumerated types and pointers can be
represented as numbers, so that they are valid arguments to mathematical
operators. *Note Supported Languages: Supported Languages, for further
details on specific languages.
GDB provides some additional commands for controlling the type
checker:
`set check type auto'
Set type checking on or off based on the current working language.
*Note Supported Languages: Supported Languages, for the default
settings for each language.
`set check type on'
`set check type off'
Set type checking on or off, overriding the default setting for the
current working language. Issue a warning if the setting does not
match the language default. If any type mismatches occur in
evaluating an expression while type checking is on, GDB prints a
message and aborts evaluation of the expression.
`set check type warn'
Cause the type checker to issue warnings, but to always attempt to
evaluate the expression. Evaluating the expression may still be
impossible for other reasons. For example, GDB cannot add numbers
and structures.
`show type'
Show the current setting of the type checker, and whether or not
GDB is setting it automatically.
File: gdb.info, Node: Range Checking, Prev: Type Checking, Up: Checks
12.3.2 An Overview of Range Checking
------------------------------------
In some languages (such as Modula-2), it is an error to exceed the
bounds of a type; this is enforced with run-time checks. Such range
checking is meant to ensure program correctness by making sure
computations do not overflow, or indices on an array element access do
not exceed the bounds of the array.
For expressions you use in GDB commands, you can tell GDB to treat
range errors in one of three ways: ignore them, always treat them as
errors and abandon the expression, or issue warnings but evaluate the
expression anyway.
A range error can result from numerical overflow, from exceeding an
array index bound, or when you type a constant that is not a member of
any type. Some languages, however, do not treat overflows as an error.
In many implementations of C, mathematical overflow causes the result
to "wrap around" to lower values--for example, if M is the largest
integer value, and S is the smallest, then
M + 1 => S
This, too, is specific to individual languages, and in some cases
specific to individual compilers or machines. *Note Supported
Languages: Supported Languages, for further details on specific
languages.
GDB provides some additional commands for controlling the range
checker:
`set check range auto'
Set range checking on or off based on the current working language.
*Note Supported Languages: Supported Languages, for the default
settings for each language.
`set check range on'
`set check range off'
Set range checking on or off, overriding the default setting for
the current working language. A warning is issued if the setting
does not match the language default. If a range error occurs and
range checking is on, then a message is printed and evaluation of
the expression is aborted.
`set check range warn'
Output messages when the GDB range checker detects a range error,
but attempt to evaluate the expression anyway. Evaluating the
expression may still be impossible for other reasons, such as
accessing memory that the process does not own (a typical example
from many Unix systems).
`show range'
Show the current setting of the range checker, and whether or not
it is being set automatically by GDB.
File: gdb.info, Node: Supported Languages, Next: Unsupported Languages, Prev: Checks, Up: Languages
12.4 Supported Languages
========================
GDB supports C, C++, Objective-C, Fortran, Java, Pascal, assembly,
Modula-2, and Ada. Some GDB features may be used in expressions
regardless of the language you use: the GDB `@' and `::' operators, and
the `{type}addr' construct (*note Expressions: Expressions.) can be
used with the constructs of any supported language.
The following sections detail to what degree each source language is
supported by GDB. These sections are not meant to be language
tutorials or references, but serve only as a reference guide to what the
GDB expression parser accepts, and what input and output formats should
look like for different languages. There are many good books written
on each of these languages; please look to these for a language
reference or tutorial.
* Menu:
* C:: C and C++
* Objective-C:: Objective-C
* Fortran:: Fortran
* Pascal:: Pascal
* Modula-2:: Modula-2
* Ada:: Ada
File: gdb.info, Node: C, Next: Objective-C, Up: Supported Languages
12.4.1 C and C++
----------------
Since C and C++ are so closely related, many features of GDB apply to
both languages. Whenever this is the case, we discuss those languages
together.
The C++ debugging facilities are jointly implemented by the C++
compiler and GDB. Therefore, to debug your C++ code effectively, you
must compile your C++ programs with a supported C++ compiler, such as
GNU `g++', or the HP ANSI C++ compiler (`aCC').
For best results when using GNU C++, use the DWARF 2 debugging
format; if it doesn't work on your system, try the stabs+ debugging
format. You can select those formats explicitly with the `g++'
command-line options `-gdwarf-2' and `-gstabs+'. *Note Options for
Debugging Your Program or GCC: (gcc.info)Debugging Options.
* Menu:
* C Operators:: C and C++ operators
* C Constants:: C and C++ constants
* C Plus Plus Expressions:: C++ expressions
* C Defaults:: Default settings for C and C++
* C Checks:: C and C++ type and range checks
* Debugging C:: GDB and C
* Debugging C Plus Plus:: GDB features for C++
* Decimal Floating Point:: Numbers in Decimal Floating Point format
File: gdb.info, Node: C Operators, Next: C Constants, Up: C
12.4.1.1 C and C++ Operators
............................
Operators must be defined on values of specific types. For instance,
`+' is defined on numbers, but not on structures. Operators are often
defined on groups of types.
For the purposes of C and C++, the following definitions hold:
* _Integral types_ include `int' with any of its storage-class
specifiers; `char'; `enum'; and, for C++, `bool'.
* _Floating-point types_ include `float', `double', and `long
double' (if supported by the target platform).
* _Pointer types_ include all types defined as `(TYPE *)'.
* _Scalar types_ include all of the above.
The following operators are supported. They are listed here in order
of increasing precedence:
`,'
The comma or sequencing operator. Expressions in a
comma-separated list are evaluated from left to right, with the
result of the entire expression being the last expression
evaluated.
`='
Assignment. The value of an assignment expression is the value
assigned. Defined on scalar types.
`OP='
Used in an expression of the form `A OP= B', and translated to
`A = A OP B'. `OP=' and `=' have the same precedence. OP is any
one of the operators `|', `^', `&', `<<', `>>', `+', `-', `*',
`/', `%'.
`?:'
The ternary operator. `A ? B : C' can be thought of as: if A
then B else C. A should be of an integral type.
`||'
Logical OR. Defined on integral types.
`&&'
Logical AND. Defined on integral types.
`|'
Bitwise OR. Defined on integral types.
`^'
Bitwise exclusive-OR. Defined on integral types.
`&'
Bitwise AND. Defined on integral types.
`==, !='
Equality and inequality. Defined on scalar types. The value of
these expressions is 0 for false and non-zero for true.
`<, >, <=, >='
Less than, greater than, less than or equal, greater than or equal.
Defined on scalar types. The value of these expressions is 0 for
false and non-zero for true.
`<<, >>'
left shift, and right shift. Defined on integral types.
`@'
The GDB "artificial array" operator (*note Expressions:
Expressions.).
`+, -'
Addition and subtraction. Defined on integral types,
floating-point types and pointer types.
`*, /, %'
Multiplication, division, and modulus. Multiplication and
division are defined on integral and floating-point types.
Modulus is defined on integral types.
`++, --'
Increment and decrement. When appearing before a variable, the
operation is performed before the variable is used in an
expression; when appearing after it, the variable's value is used
before the operation takes place.
`*'
Pointer dereferencing. Defined on pointer types. Same precedence
as `++'.
`&'
Address operator. Defined on variables. Same precedence as `++'.
For debugging C++, GDB implements a use of `&' beyond what is
allowed in the C++ language itself: you can use `&(&REF)' to
examine the address where a C++ reference variable (declared with
`&REF') is stored.
`-'
Negative. Defined on integral and floating-point types. Same
precedence as `++'.
`!'
Logical negation. Defined on integral types. Same precedence as
`++'.
`~'
Bitwise complement operator. Defined on integral types. Same
precedence as `++'.
`., ->'
Structure member, and pointer-to-structure member. For
convenience, GDB regards the two as equivalent, choosing whether
to dereference a pointer based on the stored type information.
Defined on `struct' and `union' data.
`.*, ->*'
Dereferences of pointers to members.
`[]'
Array indexing. `A[I]' is defined as `*(A+I)'. Same precedence
as `->'.
`()'
Function parameter list. Same precedence as `->'.
`::'
C++ scope resolution operator. Defined on `struct', `union', and
`class' types.
`::'
Doubled colons also represent the GDB scope operator (*note
Expressions: Expressions.). Same precedence as `::', above.
If an operator is redefined in the user code, GDB usually attempts
to invoke the redefined version instead of using the operator's
predefined meaning.
File: gdb.info, Node: C Constants, Next: C Plus Plus Expressions, Prev: C Operators, Up: C
12.4.1.2 C and C++ Constants
............................
GDB allows you to express the constants of C and C++ in the following
ways:
* Integer constants are a sequence of digits. Octal constants are
specified by a leading `0' (i.e. zero), and hexadecimal constants
by a leading `0x' or `0X'. Constants may also end with a letter
`l', specifying that the constant should be treated as a `long'
value.
* Floating point constants are a sequence of digits, followed by a
decimal point, followed by a sequence of digits, and optionally
followed by an exponent. An exponent is of the form:
`e[[+]|-]NNN', where NNN is another sequence of digits. The `+'
is optional for positive exponents. A floating-point constant may
also end with a letter `f' or `F', specifying that the constant
should be treated as being of the `float' (as opposed to the
default `double') type; or with a letter `l' or `L', which
specifies a `long double' constant.
* Enumerated constants consist of enumerated identifiers, or their
integral equivalents.
* Character constants are a single character surrounded by single
quotes (`''), or a number--the ordinal value of the corresponding
character (usually its ASCII value). Within quotes, the single
character may be represented by a letter or by "escape sequences",
which are of the form `\NNN', where NNN is the octal representation
of the character's ordinal value; or of the form `\X', where `X'
is a predefined special character--for example, `\n' for newline.
* String constants are a sequence of character constants surrounded
by double quotes (`"'). Any valid character constant (as described
above) may appear. Double quotes within the string must be
preceded by a backslash, so for instance `"a\"b'c"' is a string of
five characters.
* Pointer constants are an integral value. You can also write
pointers to constants using the C operator `&'.
* Array constants are comma-separated lists surrounded by braces `{'
and `}'; for example, `{1,2,3}' is a three-element array of
integers, `{{1,2}, {3,4}, {5,6}}' is a three-by-two array, and
`{&"hi", &"there", &"fred"}' is a three-element array of pointers.
File: gdb.info, Node: C Plus Plus Expressions, Next: C Defaults, Prev: C Constants, Up: C
12.4.1.3 C++ Expressions
........................
GDB expression handling can interpret most C++ expressions.
_Warning:_ GDB can only debug C++ code if you use the proper
compiler and the proper debug format. Currently, GDB works best
when debugging C++ code that is compiled with GCC 2.95.3 or with
GCC 3.1 or newer, using the options `-gdwarf-2' or `-gstabs+'.
DWARF 2 is preferred over stabs+. Most configurations of GCC emit
either DWARF 2 or stabs+ as their default debug format, so you
usually don't need to specify a debug format explicitly. Other
compilers and/or debug formats are likely to work badly or not at
all when using GDB to debug C++ code.
1. Member function calls are allowed; you can use expressions like
count = aml->GetOriginal(x, y)
2. While a member function is active (in the selected stack frame),
your expressions have the same namespace available as the member
function; that is, GDB allows implicit references to the class
instance pointer `this' following the same rules as C++.
3. You can call overloaded functions; GDB resolves the function call
to the right definition, with some restrictions. GDB does not
perform overload resolution involving user-defined type
conversions, calls to constructors, or instantiations of templates
that do not exist in the program. It also cannot handle ellipsis
argument lists or default arguments.
It does perform integral conversions and promotions, floating-point
promotions, arithmetic conversions, pointer conversions,
conversions of class objects to base classes, and standard
conversions such as those of functions or arrays to pointers; it
requires an exact match on the number of function arguments.
Overload resolution is always performed, unless you have specified
`set overload-resolution off'. *Note GDB Features for C++:
Debugging C Plus Plus.
You must specify `set overload-resolution off' in order to use an
explicit function signature to call an overloaded function, as in
p 'foo(char,int)'('x', 13)
The GDB command-completion facility can simplify this; see *Note
Command Completion: Completion.
4. GDB understands variables declared as C++ references; you can use
them in expressions just as you do in C++ source--they are
automatically dereferenced.
In the parameter list shown when GDB displays a frame, the values
of reference variables are not displayed (unlike other variables);
this avoids clutter, since references are often used for large
structures. The _address_ of a reference variable is always
shown, unless you have specified `set print address off'.
5. GDB supports the C++ name resolution operator `::'--your
expressions can use it just as expressions in your program do.
Since one scope may be defined in another, you can use `::'
repeatedly if necessary, for example in an expression like
`SCOPE1::SCOPE2::NAME'. GDB also allows resolving name scope by
reference to source files, in both C and C++ debugging (*note
Program Variables: Variables.).
In addition, when used with HP's C++ compiler, GDB supports calling
virtual functions correctly, printing out virtual bases of objects,
calling functions in a base subobject, casting objects, and invoking
user-defined operators.
File: gdb.info, Node: C Defaults, Next: C Checks, Prev: C Plus Plus Expressions, Up: C
12.4.1.4 C and C++ Defaults
...........................
If you allow GDB to set type and range checking automatically, they
both default to `off' whenever the working language changes to C or
C++. This happens regardless of whether you or GDB selects the working
language.
If you allow GDB to set the language automatically, it recognizes
source files whose names end with `.c', `.C', or `.cc', etc, and when
GDB enters code compiled from one of these files, it sets the working
language to C or C++. *Note Having GDB Infer the Source Language:
Automatically, for further details.
File: gdb.info, Node: C Checks, Next: Debugging C, Prev: C Defaults, Up: C
12.4.1.5 C and C++ Type and Range Checks
........................................
By default, when GDB parses C or C++ expressions, type checking is not
used. However, if you turn type checking on, GDB considers two
variables type equivalent if:
* The two variables are structured and have the same structure,
union, or enumerated tag.
* The two variables have the same type name, or types that have been
declared equivalent through `typedef'.
Range checking, if turned on, is done on mathematical operations.
Array indices are not checked, since they are often used to index a
pointer that is not itself an array.
File: gdb.info, Node: Debugging C, Next: Debugging C Plus Plus, Prev: C Checks, Up: C
12.4.1.6 GDB and C
..................
The `set print union' and `show print union' commands apply to the
`union' type. When set to `on', any `union' that is inside a `struct'
or `class' is also printed. Otherwise, it appears as `{...}'.
The `@' operator aids in the debugging of dynamic arrays, formed
with pointers and a memory allocation function. *Note Expressions:
Expressions.
File: gdb.info, Node: Debugging C Plus Plus, Next: Decimal Floating Point, Prev: Debugging C, Up: C
12.4.1.7 GDB Features for C++
.............................
Some GDB commands are particularly useful with C++, and some are
designed specifically for use with C++. Here is a summary:
`breakpoint menus'
When you want a breakpoint in a function whose name is overloaded,
GDB breakpoint menus help you specify which function definition
you want. *Note Breakpoint Menus: Breakpoint Menus.
`rbreak REGEX'
Setting breakpoints using regular expressions is helpful for
setting breakpoints on overloaded functions that are not members
of any special classes. *Note Setting Breakpoints: Set Breaks.
`catch throw'
`catch catch'
Debug C++ exception handling using these commands. *Note Setting
Catchpoints: Set Catchpoints.
`ptype TYPENAME'
Print inheritance relationships as well as other information for
type TYPENAME. *Note Examining the Symbol Table: Symbols.
`set print demangle'
`show print demangle'
`set print asm-demangle'
`show print asm-demangle'
Control whether C++ symbols display in their source form, both when
displaying code as C++ source and when displaying disassemblies.
*Note Print Settings: Print Settings.
`set print object'
`show print object'
Choose whether to print derived (actual) or declared types of
objects. *Note Print Settings: Print Settings.
`set print vtbl'
`show print vtbl'
Control the format for printing virtual function tables. *Note
Print Settings: Print Settings. (The `vtbl' commands do not work
on programs compiled with the HP ANSI C++ compiler (`aCC').)
`set overload-resolution on'
Enable overload resolution for C++ expression evaluation. The
default is on. For overloaded functions, GDB evaluates the
arguments and searches for a function whose signature matches the
argument types, using the standard C++ conversion rules (see *Note
C++ Expressions: C Plus Plus Expressions, for details). If it
cannot find a match, it emits a message.
`set overload-resolution off'
Disable overload resolution for C++ expression evaluation. For
overloaded functions that are not class member functions, GDB
chooses the first function of the specified name that it finds in
the symbol table, whether or not its arguments are of the correct
type. For overloaded functions that are class member functions,
GDB searches for a function whose signature _exactly_ matches the
argument types.
`show overload-resolution'
Show the current setting of overload resolution.
`Overloaded symbol names'
You can specify a particular definition of an overloaded symbol,
using the same notation that is used to declare such symbols in
C++: type `SYMBOL(TYPES)' rather than just SYMBOL. You can also
use the GDB command-line word completion facilities to list the
available choices, or to finish the type list for you. *Note
Command Completion: Completion, for details on how to do this.
File: gdb.info, Node: Decimal Floating Point, Prev: Debugging C Plus Plus, Up: C
12.4.1.8 Decimal Floating Point format
......................................
GDB can examine, set and perform computations with numbers in decimal
floating point format, which in the C language correspond to the
`_Decimal32', `_Decimal64' and `_Decimal128' types as specified by the
extension to support decimal floating-point arithmetic.
There are two encodings in use, depending on the architecture: BID
(Binary Integer Decimal) for x86 and x86-64, and DPD (Densely Packed
Decimal) for PowerPC. GDB will use the appropriate encoding for the
configured target.
Because of a limitation in `libdecnumber', the library used by GDB
to manipulate decimal floating point numbers, it is not possible to
convert (using a cast, for example) integers wider than 32-bit to
decimal float.
In addition, in order to imitate GDB's behaviour with binary floating
point computations, error checking in decimal float operations ignores
underflow, overflow and divide by zero exceptions.
In the PowerPC architecture, GDB provides a set of pseudo-registers
to inspect `_Decimal128' values stored in floating point registers. See
*Note PowerPC: PowerPC. for more details.
File: gdb.info, Node: Objective-C, Next: Fortran, Prev: C, Up: Supported Languages
12.4.2 Objective-C
------------------
This section provides information about some commands and command
options that are useful for debugging Objective-C code. See also *Note
info classes: Symbols, and *Note info selectors: Symbols, for a few
more commands specific to Objective-C support.
* Menu:
* Method Names in Commands::
* The Print Command with Objective-C::
File: gdb.info, Node: Method Names in Commands, Next: The Print Command with Objective-C, Up: Objective-C
12.4.2.1 Method Names in Commands
.................................
The following commands have been extended to accept Objective-C method
names as line specifications:
* `clear'
* `break'
* `info line'
* `jump'
* `list'
A fully qualified Objective-C method name is specified as
-[CLASS METHODNAME]
where the minus sign is used to indicate an instance method and a
plus sign (not shown) is used to indicate a class method. The class
name CLASS and method name METHODNAME are enclosed in brackets, similar
to the way messages are specified in Objective-C source code. For
example, to set a breakpoint at the `create' instance method of class
`Fruit' in the program currently being debugged, enter:
break -[Fruit create]
To list ten program lines around the `initialize' class method,
enter:
list +[NSText initialize]
In the current version of GDB, the plus or minus sign is required.
In future versions of GDB, the plus or minus sign will be optional, but
you can use it to narrow the search. It is also possible to specify
just a method name:
break create
You must specify the complete method name, including any colons. If
your program's source files contain more than one `create' method,
you'll be presented with a numbered list of classes that implement that
method. Indicate your choice by number, or type `0' to exit if none
apply.
As another example, to clear a breakpoint established at the
`makeKeyAndOrderFront:' method of the `NSWindow' class, enter:
clear -[NSWindow makeKeyAndOrderFront:]
File: gdb.info, Node: The Print Command with Objective-C, Prev: Method Names in Commands, Up: Objective-C
12.4.2.2 The Print Command With Objective-C
...........................................
The print command has also been extended to accept methods. For
example:
print -[OBJECT hash]
will tell GDB to send the `hash' message to OBJECT and print the
result. Also, an additional command has been added, `print-object' or
`po' for short, which is meant to print the description of an object.
However, this command may only work with certain Objective-C libraries
that have a particular hook function, `_NSPrintForDebugger', defined.
File: gdb.info, Node: Fortran, Next: Pascal, Prev: Objective-C, Up: Supported Languages
12.4.3 Fortran
--------------
GDB can be used to debug programs written in Fortran, but it currently
supports only the features of Fortran 77 language.
Some Fortran compilers (GNU Fortran 77 and Fortran 95 compilers
among them) append an underscore to the names of variables and
functions. When you debug programs compiled by those compilers, you
will need to refer to variables and functions with a trailing
underscore.
* Menu:
* Fortran Operators:: Fortran operators and expressions
* Fortran Defaults:: Default settings for Fortran
* Special Fortran Commands:: Special GDB commands for Fortran
File: gdb.info, Node: Fortran Operators, Next: Fortran Defaults, Up: Fortran
12.4.3.1 Fortran Operators and Expressions
..........................................
Operators must be defined on values of specific types. For instance,
`+' is defined on numbers, but not on characters or other non-
arithmetic types. Operators are often defined on groups of types.
`**'
The exponentiation operator. It raises the first operand to the
power of the second one.
`:'
The range operator. Normally used in the form of array(low:high)
to represent a section of array.
File: gdb.info, Node: Fortran Defaults, Next: Special Fortran Commands, Prev: Fortran Operators, Up: Fortran
12.4.3.2 Fortran Defaults
.........................
Fortran symbols are usually case-insensitive, so GDB by default uses
case-insensitive matches for Fortran symbols. You can change that with
the `set case-insensitive' command, see *Note Symbols::, for the
details.
File: gdb.info, Node: Special Fortran Commands, Prev: Fortran Defaults, Up: Fortran
12.4.3.3 Special Fortran Commands
.................................
GDB has some commands to support Fortran-specific features, such as
displaying common blocks.
`info common [COMMON-NAME]'
This command prints the values contained in the Fortran `COMMON'
block whose name is COMMON-NAME. With no argument, the names of
all `COMMON' blocks visible at the current program location are
printed.
File: gdb.info, Node: Pascal, Next: Modula-2, Prev: Fortran, Up: Supported Languages
12.4.4 Pascal
-------------
Debugging Pascal programs which use sets, subranges, file variables, or
nested functions does not currently work. GDB does not support
entering expressions, printing values, or similar features using Pascal
syntax.
The Pascal-specific command `set print pascal_static-members'
controls whether static members of Pascal objects are displayed. *Note
pascal_static-members: Print Settings.
File: gdb.info, Node: Modula-2, Next: Ada, Prev: Pascal, Up: Supported Languages
12.4.5 Modula-2
---------------
The extensions made to GDB to support Modula-2 only support output from
the GNU Modula-2 compiler (which is currently being developed). Other
Modula-2 compilers are not currently supported, and attempting to debug
executables produced by them is most likely to give an error as GDB
reads in the executable's symbol table.
* Menu:
* M2 Operators:: Built-in operators
* Built-In Func/Proc:: Built-in functions and procedures
* M2 Constants:: Modula-2 constants
* M2 Types:: Modula-2 types
* M2 Defaults:: Default settings for Modula-2
* Deviations:: Deviations from standard Modula-2
* M2 Checks:: Modula-2 type and range checks
* M2 Scope:: The scope operators `::' and `.'
* GDB/M2:: GDB and Modula-2
File: gdb.info, Node: M2 Operators, Next: Built-In Func/Proc, Up: Modula-2
12.4.5.1 Operators
..................
Operators must be defined on values of specific types. For instance,
`+' is defined on numbers, but not on structures. Operators are often
defined on groups of types. For the purposes of Modula-2, the
following definitions hold:
* _Integral types_ consist of `INTEGER', `CARDINAL', and their
subranges.
* _Character types_ consist of `CHAR' and its subranges.
* _Floating-point types_ consist of `REAL'.
* _Pointer types_ consist of anything declared as `POINTER TO TYPE'.
* _Scalar types_ consist of all of the above.
* _Set types_ consist of `SET' and `BITSET' types.
* _Boolean types_ consist of `BOOLEAN'.
The following operators are supported, and appear in order of
increasing precedence:
`,'
Function argument or array index separator.
`:='
Assignment. The value of VAR `:=' VALUE is VALUE.
`<, >'
Less than, greater than on integral, floating-point, or enumerated
types.
`<=, >='
Less than or equal to, greater than or equal to on integral,
floating-point and enumerated types, or set inclusion on set
types. Same precedence as `<'.
`=, <>, #'
Equality and two ways of expressing inequality, valid on scalar
types. Same precedence as `<'. In GDB scripts, only `<>' is
available for inequality, since `#' conflicts with the script
comment character.
`IN'
Set membership. Defined on set types and the types of their
members. Same precedence as `<'.
`OR'
Boolean disjunction. Defined on boolean types.
`AND, &'
Boolean conjunction. Defined on boolean types.
`@'
The GDB "artificial array" operator (*note Expressions:
Expressions.).
`+, -'
Addition and subtraction on integral and floating-point types, or
union and difference on set types.
`*'
Multiplication on integral and floating-point types, or set
intersection on set types.
`/'
Division on floating-point types, or symmetric set difference on
set types. Same precedence as `*'.
`DIV, MOD'
Integer division and remainder. Defined on integral types. Same
precedence as `*'.
`-'
Negative. Defined on `INTEGER' and `REAL' data.
`^'
Pointer dereferencing. Defined on pointer types.
`NOT'
Boolean negation. Defined on boolean types. Same precedence as
`^'.
`.'
`RECORD' field selector. Defined on `RECORD' data. Same
precedence as `^'.
`[]'
Array indexing. Defined on `ARRAY' data. Same precedence as `^'.
`()'
Procedure argument list. Defined on `PROCEDURE' objects. Same
precedence as `^'.
`::, .'
GDB and Modula-2 scope operators.
_Warning:_ Set expressions and their operations are not yet
supported, so GDB treats the use of the operator `IN', or the use
of operators `+', `-', `*', `/', `=', , `<>', `#', `<=', and `>='
on sets as an error.
File: gdb.info, Node: Built-In Func/Proc, Next: M2 Constants, Prev: M2 Operators, Up: Modula-2
12.4.5.2 Built-in Functions and Procedures
..........................................
Modula-2 also makes available several built-in procedures and functions.
In describing these, the following metavariables are used:
A
represents an `ARRAY' variable.
C
represents a `CHAR' constant or variable.
I
represents a variable or constant of integral type.
M
represents an identifier that belongs to a set. Generally used in
the same function with the metavariable S. The type of S should
be `SET OF MTYPE' (where MTYPE is the type of M).
N
represents a variable or constant of integral or floating-point
type.
R
represents a variable or constant of floating-point type.
T
represents a type.
V
represents a variable.
X
represents a variable or constant of one of many types. See the
explanation of the function for details.
All Modula-2 built-in procedures also return a result, described
below.
`ABS(N)'
Returns the absolute value of N.
`CAP(C)'
If C is a lower case letter, it returns its upper case equivalent,
otherwise it returns its argument.
`CHR(I)'
Returns the character whose ordinal value is I.
`DEC(V)'
Decrements the value in the variable V by one. Returns the new
value.
`DEC(V,I)'
Decrements the value in the variable V by I. Returns the new
value.
`EXCL(M,S)'
Removes the element M from the set S. Returns the new set.
`FLOAT(I)'
Returns the floating point equivalent of the integer I.
`HIGH(A)'
Returns the index of the last member of A.
`INC(V)'
Increments the value in the variable V by one. Returns the new
value.
`INC(V,I)'
Increments the value in the variable V by I. Returns the new
value.
`INCL(M,S)'
Adds the element M to the set S if it is not already there.
Returns the new set.
`MAX(T)'
Returns the maximum value of the type T.
`MIN(T)'
Returns the minimum value of the type T.
`ODD(I)'
Returns boolean TRUE if I is an odd number.
`ORD(X)'
Returns the ordinal value of its argument. For example, the
ordinal value of a character is its ASCII value (on machines
supporting the ASCII character set). X must be of an ordered
type, which include integral, character and enumerated types.
`SIZE(X)'
Returns the size of its argument. X can be a variable or a type.
`TRUNC(R)'
Returns the integral part of R.
`TSIZE(X)'
Returns the size of its argument. X can be a variable or a type.
`VAL(T,I)'
Returns the member of the type T whose ordinal value is I.
_Warning:_ Sets and their operations are not yet supported, so
GDB treats the use of procedures `INCL' and `EXCL' as an error.
File: gdb.info, Node: M2 Constants, Next: M2 Types, Prev: Built-In Func/Proc, Up: Modula-2
12.4.5.3 Constants
..................
GDB allows you to express the constants of Modula-2 in the following
ways:
* Integer constants are simply a sequence of digits. When used in an
expression, a constant is interpreted to be type-compatible with
the rest of the expression. Hexadecimal integers are specified by
a trailing `H', and octal integers by a trailing `B'.
* Floating point constants appear as a sequence of digits, followed
by a decimal point and another sequence of digits. An optional
exponent can then be specified, in the form `E[+|-]NNN', where
`[+|-]NNN' is the desired exponent. All of the digits of the
floating point constant must be valid decimal (base 10) digits.
* Character constants consist of a single character enclosed by a
pair of like quotes, either single (`'') or double (`"'). They may
also be expressed by their ordinal value (their ASCII value,
usually) followed by a `C'.
* String constants consist of a sequence of characters enclosed by a
pair of like quotes, either single (`'') or double (`"'). Escape
sequences in the style of C are also allowed. *Note C and C++
Constants: C Constants, for a brief explanation of escape
sequences.
* Enumerated constants consist of an enumerated identifier.
* Boolean constants consist of the identifiers `TRUE' and `FALSE'.
* Pointer constants consist of integral values only.
* Set constants are not yet supported.
File: gdb.info, Node: M2 Types, Next: M2 Defaults, Prev: M2 Constants, Up: Modula-2
12.4.5.4 Modula-2 Types
.......................
Currently GDB can print the following data types in Modula-2 syntax:
array types, record types, set types, pointer types, procedure types,
enumerated types, subrange types and base types. You can also print
the contents of variables declared using these type. This section
gives a number of simple source code examples together with sample GDB
sessions.
The first example contains the following section of code:
VAR
s: SET OF CHAR ;
r: [20..40] ;
and you can request GDB to interrogate the type and value of `r' and
`s'.
(gdb) print s
{'A'..'C', 'Z'}
(gdb) ptype s
SET OF CHAR
(gdb) print r
21
(gdb) ptype r
[20..40]
Likewise if your source code declares `s' as:
VAR
s: SET ['A'..'Z'] ;
then you may query the type of `s' by:
(gdb) ptype s
type = SET ['A'..'Z']
Note that at present you cannot interactively manipulate set
expressions using the debugger.
The following example shows how you might declare an array in
Modula-2 and how you can interact with GDB to print its type and
contents:
VAR
s: ARRAY [-10..10] OF CHAR ;
(gdb) ptype s
ARRAY [-10..10] OF CHAR
Note that the array handling is not yet complete and although the
type is printed correctly, expression handling still assumes that all
arrays have a lower bound of zero and not `-10' as in the example above.
Here are some more type related Modula-2 examples:
TYPE
colour = (blue, red, yellow, green) ;
t = [blue..yellow] ;
VAR
s: t ;
BEGIN
s := blue ;
The GDB interaction shows how you can query the data type and value of
a variable.
(gdb) print s
$1 = blue
(gdb) ptype t
type = [blue..yellow]
In this example a Modula-2 array is declared and its contents
displayed. Observe that the contents are written in the same way as
their `C' counterparts.
VAR
s: ARRAY [1..5] OF CARDINAL ;
BEGIN
s[1] := 1 ;
(gdb) print s
$1 = {1, 0, 0, 0, 0}
(gdb) ptype s
type = ARRAY [1..5] OF CARDINAL
The Modula-2 language interface to GDB also understands pointer
types as shown in this example:
VAR
s: POINTER TO ARRAY [1..5] OF CARDINAL ;
BEGIN
NEW(s) ;
s^[1] := 1 ;
and you can request that GDB describes the type of `s'.
(gdb) ptype s
type = POINTER TO ARRAY [1..5] OF CARDINAL
GDB handles compound types as we can see in this example. Here we
combine array types, record types, pointer types and subrange types:
TYPE
foo = RECORD
f1: CARDINAL ;
f2: CHAR ;
f3: myarray ;
END ;
myarray = ARRAY myrange OF CARDINAL ;
myrange = [-2..2] ;
VAR
s: POINTER TO ARRAY myrange OF foo ;
and you can ask GDB to describe the type of `s' as shown below.
(gdb) ptype s
type = POINTER TO ARRAY [-2..2] OF foo = RECORD
f1 : CARDINAL;
f2 : CHAR;
f3 : ARRAY [-2..2] OF CARDINAL;
END
File: gdb.info, Node: M2 Defaults, Next: Deviations, Prev: M2 Types, Up: Modula-2
12.4.5.5 Modula-2 Defaults
..........................
If type and range checking are set automatically by GDB, they both
default to `on' whenever the working language changes to Modula-2.
This happens regardless of whether you or GDB selected the working
language.
If you allow GDB to set the language automatically, then entering
code compiled from a file whose name ends with `.mod' sets the working
language to Modula-2. *Note Having GDB Infer the Source Language:
Automatically, for further details.
File: gdb.info, Node: Deviations, Next: M2 Checks, Prev: M2 Defaults, Up: Modula-2
12.4.5.6 Deviations from Standard Modula-2
..........................................
A few changes have been made to make Modula-2 programs easier to debug.
This is done primarily via loosening its type strictness:
* Unlike in standard Modula-2, pointer constants can be formed by
integers. This allows you to modify pointer variables during
debugging. (In standard Modula-2, the actual address contained in
a pointer variable is hidden from you; it can only be modified
through direct assignment to another pointer variable or
expression that returned a pointer.)
* C escape sequences can be used in strings and characters to
represent non-printable characters. GDB prints out strings with
these escape sequences embedded. Single non-printable characters
are printed using the `CHR(NNN)' format.
* The assignment operator (`:=') returns the value of its right-hand
argument.
* All built-in procedures both modify _and_ return their argument.
File: gdb.info, Node: M2 Checks, Next: M2 Scope, Prev: Deviations, Up: Modula-2
12.4.5.7 Modula-2 Type and Range Checks
.......................................
_Warning:_ in this release, GDB does not yet perform type or range
checking.
GDB considers two Modula-2 variables type equivalent if:
* They are of types that have been declared equivalent via a `TYPE
T1 = T2' statement
* They have been declared on the same line. (Note: This is true of
the GNU Modula-2 compiler, but it may not be true of other
compilers.)
As long as type checking is enabled, any attempt to combine variables
whose types are not equivalent is an error.
Range checking is done on all mathematical operations, assignment,
array index bounds, and all built-in functions and procedures.
File: gdb.info, Node: M2 Scope, Next: GDB/M2, Prev: M2 Checks, Up: Modula-2
12.4.5.8 The Scope Operators `::' and `.'
.........................................
There are a few subtle differences between the Modula-2 scope operator
(`.') and the GDB scope operator (`::'). The two have similar syntax:
MODULE . ID
SCOPE :: ID
where SCOPE is the name of a module or a procedure, MODULE the name of
a module, and ID is any declared identifier within your program, except
another module.
Using the `::' operator makes GDB search the scope specified by
SCOPE for the identifier ID. If it is not found in the specified
scope, then GDB searches all scopes enclosing the one specified by
SCOPE.
Using the `.' operator makes GDB search the current scope for the
identifier specified by ID that was imported from the definition module
specified by MODULE. With this operator, it is an error if the
identifier ID was not imported from definition module MODULE, or if ID
is not an identifier in MODULE.
File: gdb.info, Node: GDB/M2, Prev: M2 Scope, Up: Modula-2
12.4.5.9 GDB and Modula-2
.........................
Some GDB commands have little use when debugging Modula-2 programs.
Five subcommands of `set print' and `show print' apply specifically to
C and C++: `vtbl', `demangle', `asm-demangle', `object', and `union'.
The first four apply to C++, and the last to the C `union' type, which
has no direct analogue in Modula-2.
The `@' operator (*note Expressions: Expressions.), while available
with any language, is not useful with Modula-2. Its intent is to aid
the debugging of "dynamic arrays", which cannot be created in Modula-2
as they can in C or C++. However, because an address can be specified
by an integral constant, the construct `{TYPE}ADREXP' is still useful.
In GDB scripts, the Modula-2 inequality operator `#' is interpreted
as the beginning of a comment. Use `<>' instead.
File: gdb.info, Node: Ada, Prev: Modula-2, Up: Supported Languages
12.4.6 Ada
----------
The extensions made to GDB for Ada only support output from the GNU Ada
(GNAT) compiler. Other Ada compilers are not currently supported, and
attempting to debug executables produced by them is most likely to be
difficult.
* Menu:
* Ada Mode Intro:: General remarks on the Ada syntax
and semantics supported by Ada mode
in GDB.
* Omissions from Ada:: Restrictions on the Ada expression syntax.
* Additions to Ada:: Extensions of the Ada expression syntax.
* Stopping Before Main Program:: Debugging the program during elaboration.
* Ada Glitches:: Known peculiarities of Ada mode.
File: gdb.info, Node: Ada Mode Intro, Next: Omissions from Ada, Up: Ada
12.4.6.1 Introduction
.....................
The Ada mode of GDB supports a fairly large subset of Ada expression
syntax, with some extensions. The philosophy behind the design of this
subset is
* That GDB should provide basic literals and access to operations for
arithmetic, dereferencing, field selection, indexing, and
subprogram calls, leaving more sophisticated computations to
subprograms written into the program (which therefore may be
called from GDB).
* That type safety and strict adherence to Ada language restrictions
are not particularly important to the GDB user.
* That brevity is important to the GDB user.
Thus, for brevity, the debugger acts as if there were implicit
`with' and `use' clauses in effect for all user-written packages,
making it unnecessary to fully qualify most names with their packages,
regardless of context. Where this causes ambiguity, GDB asks the
user's intent.
The debugger will start in Ada mode if it detects an Ada main
program. As for other languages, it will enter Ada mode when stopped
in a program that was translated from an Ada source file.
While in Ada mode, you may use `-' for comments. This is useful
mostly for documenting command files. The standard GDB comment (`#')
still works at the beginning of a line in Ada mode, but not in the
middle (to allow based literals).
The debugger supports limited overloading. Given a subprogram call
in which the function symbol has multiple definitions, it will use the
number of actual parameters and some information about their types to
attempt to narrow the set of definitions. It also makes very limited
use of context, preferring procedures to functions in the context of
the `call' command, and functions to procedures elsewhere.
File: gdb.info, Node: Omissions from Ada, Next: Additions to Ada, Prev: Ada Mode Intro, Up: Ada
12.4.6.2 Omissions from Ada
...........................
Here are the notable omissions from the subset:
* Only a subset of the attributes are supported:
- 'First, 'Last, and 'Length on array objects (not on types
and subtypes).
- 'Min and 'Max.
- 'Pos and 'Val.
- 'Tag.
- 'Range on array objects (not subtypes), but only as the right
operand of the membership (`in') operator.
- 'Access, 'Unchecked_Access, and 'Unrestricted_Access (a GNAT
extension).
- 'Address.
* The names in `Characters.Latin_1' are not available and
concatenation is not implemented. Thus, escape characters in
strings are not currently available.
* Equality tests (`=' and `/=') on arrays test for bitwise equality
of representations. They will generally work correctly for
strings and arrays whose elements have integer or enumeration
types. They may not work correctly for arrays whose element types
have user-defined equality, for arrays of real values (in
particular, IEEE-conformant floating point, because of negative
zeroes and NaNs), and for arrays whose elements contain unused
bits with indeterminate values.
* The other component-by-component array operations (`and', `or',
`xor', `not', and relational tests other than equality) are not
implemented.
* There is limited support for array and record aggregates. They are
permitted only on the right sides of assignments, as in these
examples:
set An_Array := (1, 2, 3, 4, 5, 6)
set An_Array := (1, others => 0)
set An_Array := (0|4 => 1, 1..3 => 2, 5 => 6)
set A_2D_Array := ((1, 2, 3), (4, 5, 6), (7, 8, 9))
set A_Record := (1, "Peter", True);
set A_Record := (Name => "Peter", Id => 1, Alive => True)
Changing a discriminant's value by assigning an aggregate has an
undefined effect if that discriminant is used within the record.
However, you can first modify discriminants by directly assigning
to them (which normally would not be allowed in Ada), and then
performing an aggregate assignment. For example, given a variable
`A_Rec' declared to have a type such as:
type Rec (Len : Small_Integer := 0) is record
Id : Integer;
Vals : IntArray (1 .. Len);
end record;
you can assign a value with a different size of `Vals' with two
assignments:
set A_Rec.Len := 4
set A_Rec := (Id => 42, Vals => (1, 2, 3, 4))
As this example also illustrates, GDB is very loose about the usual
rules concerning aggregates. You may leave out some of the
components of an array or record aggregate (such as the `Len'
component in the assignment to `A_Rec' above); they will retain
their original values upon assignment. You may freely use dynamic
values as indices in component associations. You may even use
overlapping or redundant component associations, although which
component values are assigned in such cases is not defined.
* Calls to dispatching subprograms are not implemented.
* The overloading algorithm is much more limited (i.e., less
selective) than that of real Ada. It makes only limited use of
the context in which a subexpression appears to resolve its
meaning, and it is much looser in its rules for allowing type
matches. As a result, some function calls will be ambiguous, and
the user will be asked to choose the proper resolution.
* The `new' operator is not implemented.
* Entry calls are not implemented.
* Aside from printing, arithmetic operations on the native VAX
floating-point formats are not supported.
* It is not possible to slice a packed array.
File: gdb.info, Node: Additions to Ada, Next: Stopping Before Main Program, Prev: Omissions from Ada, Up: Ada
12.4.6.3 Additions to Ada
.........................
As it does for other languages, GDB makes certain generic extensions to
Ada (*note Expressions::):
* If the expression E is a variable residing in memory (typically a
local variable or array element) and N is a positive integer, then
`E@N' displays the values of E and the N-1 adjacent variables
following it in memory as an array. In Ada, this operator is
generally not necessary, since its prime use is in displaying
parts of an array, and slicing will usually do this in Ada.
However, there are occasional uses when debugging programs in
which certain debugging information has been optimized away.
* `B::VAR' means "the variable named VAR that appears in function or
file B." When B is a file name, you must typically surround it in
single quotes.
* The expression `{TYPE} ADDR' means "the variable of type TYPE that
appears at address ADDR."
* A name starting with `$' is a convenience variable (*note
Convenience Vars::) or a machine register (*note Registers::).
In addition, GDB provides a few other shortcuts and outright
additions specific to Ada:
* The assignment statement is allowed as an expression, returning
its right-hand operand as its value. Thus, you may enter
set x := y + 3
print A(tmp := y + 1)
* The semicolon is allowed as an "operator," returning as its value
the value of its right-hand operand. This allows, for example,
complex conditional breaks:
break f
condition 1 (report(i); k += 1; A(k) > 100)
* Rather than use catenation and symbolic character names to
introduce special characters into strings, one may instead use a
special bracket notation, which is also used to print strings. A
sequence of characters of the form `["XX"]' within a string or
character literal denotes the (single) character whose numeric
encoding is XX in hexadecimal. The sequence of characters `["""]'
also denotes a single quotation mark in strings. For example,
"One line.["0a"]Next line.["0a"]"
contains an ASCII newline character (`Ada.Characters.Latin_1.LF')
after each period.
* The subtype used as a prefix for the attributes 'Pos, 'Min, and
'Max is optional (and is ignored in any case). For example, it is
valid to write
print 'max(x, y)
* When printing arrays, GDB uses positional notation when the array
has a lower bound of 1, and uses a modified named notation
otherwise. For example, a one-dimensional array of three integers
with a lower bound of 3 might print as
(3 => 10, 17, 1)
That is, in contrast to valid Ada, only the first component has a
`=>' clause.
* You may abbreviate attributes in expressions with any unique,
multi-character subsequence of their names (an exact match gets
preference). For example, you may use a'len, a'gth, or a'lh in
place of a'length.
* Since Ada is case-insensitive, the debugger normally maps
identifiers you type to lower case. The GNAT compiler uses
upper-case characters for some of its internal identifiers, which
are normally of no interest to users. For the rare occasions when
you actually have to look at them, enclose them in angle brackets
to avoid the lower-case mapping. For example,
gdb print <JMPBUF_SAVE>[0]
* Printing an object of class-wide type or dereferencing an
access-to-class-wide value will display all the components of the
object's specific type (as indicated by its run-time tag).
Likewise, component selection on such a value will operate on the
specific type of the object.
File: gdb.info, Node: Stopping Before Main Program, Next: Ada Glitches, Prev: Additions to Ada, Up: Ada
12.4.6.4 Stopping at the Very Beginning
.......................................
It is sometimes necessary to debug the program during elaboration, and
before reaching the main procedure. As defined in the Ada Reference
Manual, the elaboration code is invoked from a procedure called
`adainit'. To run your program up to the beginning of elaboration,
simply use the following two commands: `tbreak adainit' and `run'.
File: gdb.info, Node: Ada Glitches, Prev: Stopping Before Main Program, Up: Ada
12.4.6.5 Known Peculiarities of Ada Mode
........................................
Besides the omissions listed previously (*note Omissions from Ada::),
we know of several problems with and limitations of Ada mode in GDB,
some of which will be fixed with planned future releases of the debugger
and the GNU Ada compiler.
* Currently, the debugger has insufficient information to determine
whether certain pointers represent pointers to objects or the
objects themselves. Thus, the user may have to tack an extra
`.all' after an expression to get it printed properly.
* Static constants that the compiler chooses not to materialize as
objects in storage are invisible to the debugger.
* Named parameter associations in function argument lists are
ignored (the argument lists are treated as positional).
* Many useful library packages are currently invisible to the
debugger.
* Fixed-point arithmetic, conversions, input, and output is carried
out using floating-point arithmetic, and may give results that
only approximate those on the host machine.
* The type of the 'Address attribute may not be `System.Address'.
* The GNAT compiler never generates the prefix `Standard' for any of
the standard symbols defined by the Ada language. GDB knows about
this: it will strip the prefix from names when you use it, and
will never look for a name you have so qualified among local
symbols, nor match against symbols in other packages or
subprograms. If you have defined entities anywhere in your
program other than parameters and local variables whose simple
names match names in `Standard', GNAT's lack of qualification here
can cause confusion. When this happens, you can usually resolve
the confusion by qualifying the problematic names with package
`Standard' explicitly.
File: gdb.info, Node: Unsupported Languages, Prev: Supported Languages, Up: Languages
12.5 Unsupported Languages
==========================
In addition to the other fully-supported programming languages, GDB
also provides a pseudo-language, called `minimal'. It does not
represent a real programming language, but provides a set of
capabilities close to what the C or assembly languages provide. This
should allow most simple operations to be performed while debugging an
application that uses a language currently not supported by GDB.
If the language is set to `auto', GDB will automatically select this
language if the current frame corresponds to an unsupported language.
File: gdb.info, Node: Symbols, Next: Altering, Prev: Languages, Up: Top
13 Examining the Symbol Table
*****************************
The commands described in this chapter allow you to inquire about the
symbols (names of variables, functions and types) defined in your
program. This information is inherent in the text of your program and
does not change as your program executes. GDB finds it in your
program's symbol table, in the file indicated when you started GDB
(*note Choosing Files: File Options.), or by one of the file-management
commands (*note Commands to Specify Files: Files.).
Occasionally, you may need to refer to symbols that contain unusual
characters, which GDB ordinarily treats as word delimiters. The most
frequent case is in referring to static variables in other source files
(*note Program Variables: Variables.). File names are recorded in
object files as debugging symbols, but GDB would ordinarily parse a
typical file name, like `foo.c', as the three words `foo' `.' `c'. To
allow GDB to recognize `foo.c' as a single symbol, enclose it in single
quotes; for example,
p 'foo.c'::x
looks up the value of `x' in the scope of the file `foo.c'.
`set case-sensitive on'
`set case-sensitive off'
`set case-sensitive auto'
Normally, when GDB looks up symbols, it matches their names with
case sensitivity determined by the current source language.
Occasionally, you may wish to control that. The command `set
case-sensitive' lets you do that by specifying `on' for
case-sensitive matches or `off' for case-insensitive ones. If you
specify `auto', case sensitivity is reset to the default suitable
for the source language. The default is case-sensitive matches
for all languages except for Fortran, for which the default is
case-insensitive matches.
`show case-sensitive'
This command shows the current setting of case sensitivity for
symbols lookups.
`info address SYMBOL'
Describe where the data for SYMBOL is stored. For a register
variable, this says which register it is kept in. For a
non-register local variable, this prints the stack-frame offset at
which the variable is always stored.
Note the contrast with `print &SYMBOL', which does not work at all
for a register variable, and for a stack local variable prints the
exact address of the current instantiation of the variable.
`info symbol ADDR'
Print the name of a symbol which is stored at the address ADDR.
If no symbol is stored exactly at ADDR, GDB prints the nearest
symbol and an offset from it:
(gdb) info symbol 0x54320
_initialize_vx + 396 in section .text
This is the opposite of the `info address' command. You can use
it to find out the name of a variable or a function given its
address.
`whatis [ARG]'
Print the data type of ARG, which can be either an expression or a
data type. With no argument, print the data type of `$', the last
value in the value history. If ARG is an expression, it is not
actually evaluated, and any side-effecting operations (such as
assignments or function calls) inside it do not take place. If
ARG is a type name, it may be the name of a type or typedef, or
for C code it may have the form `class CLASS-NAME', `struct
STRUCT-TAG', `union UNION-TAG' or `enum ENUM-TAG'. *Note
Expressions: Expressions.
`ptype [ARG]'
`ptype' accepts the same arguments as `whatis', but prints a
detailed description of the type, instead of just the name of the
type. *Note Expressions: Expressions.
For example, for this variable declaration:
struct complex {double real; double imag;} v;
the two commands give this output:
(gdb) whatis v
type = struct complex
(gdb) ptype v
type = struct complex {
double real;
double imag;
}
As with `whatis', using `ptype' without an argument refers to the
type of `$', the last value in the value history.
Sometimes, programs use opaque data types or incomplete
specifications of complex data structure. If the debug
information included in the program does not allow GDB to display
a full declaration of the data type, it will say `<incomplete
type>'. For example, given these declarations:
struct foo;
struct foo *fooptr;
but no definition for `struct foo' itself, GDB will say:
(gdb) ptype foo
$1 = <incomplete type>
"Incomplete type" is C terminology for data types that are not
completely specified.
`info types REGEXP'
`info types'
Print a brief description of all types whose names match the
regular expression REGEXP (or all types in your program, if you
supply no argument). Each complete typename is matched as though
it were a complete line; thus, `i type value' gives information on
all types in your program whose names include the string `value',
but `i type ^value$' gives information only on types whose complete
name is `value'.
This command differs from `ptype' in two ways: first, like
`whatis', it does not print a detailed description; second, it
lists all source files where a type is defined.
`info scope LOCATION'
List all the variables local to a particular scope. This command
accepts a LOCATION argument--a function name, a source line, or an
address preceded by a `*', and prints all the variables local to
the scope defined by that location. (*Note Specify Location::, for
details about supported forms of LOCATION.) For example:
(gdb) info scope command_line_handler
Scope for command_line_handler:
Symbol rl is an argument at stack/frame offset 8, length 4.
Symbol linebuffer is in static storage at address 0x150a18, length 4.
Symbol linelength is in static storage at address 0x150a1c, length 4.
Symbol p is a local variable in register $esi, length 4.
Symbol p1 is a local variable in register $ebx, length 4.
Symbol nline is a local variable in register $edx, length 4.
Symbol repeat is a local variable at frame offset -8, length 4.
This command is especially useful for determining what data to
collect during a "trace experiment", see *Note collect: Tracepoint
Actions.
`info source'
Show information about the current source file--that is, the
source file for the function containing the current point of
execution:
* the name of the source file, and the directory containing it,
* the directory it was compiled in,
* its length, in lines,
* which programming language it is written in,
* whether the executable includes debugging information for
that file, and if so, what format the information is in
(e.g., STABS, Dwarf 2, etc.), and
* whether the debugging information includes information about
preprocessor macros.
`info sources'
Print the names of all source files in your program for which
there is debugging information, organized into two lists: files
whose symbols have already been read, and files whose symbols will
be read when needed.
`info functions'
Print the names and data types of all defined functions.
`info functions REGEXP'
Print the names and data types of all defined functions whose
names contain a match for regular expression REGEXP. Thus, `info
fun step' finds all functions whose names include `step'; `info
fun ^step' finds those whose names start with `step'. If a
function name contains characters that conflict with the regular
expression language (e.g. `operator*()'), they may be quoted with
a backslash.
`info variables'
Print the names and data types of all variables that are declared
outside of functions (i.e. excluding local variables).
`info variables REGEXP'
Print the names and data types of all variables (except for local
variables) whose names contain a match for regular expression
REGEXP.
`info classes'
`info classes REGEXP'
Display all Objective-C classes in your program, or (with the
REGEXP argument) all those matching a particular regular
expression.
`info selectors'
`info selectors REGEXP'
Display all Objective-C selectors in your program, or (with the
REGEXP argument) all those matching a particular regular
expression.
Some systems allow individual object files that make up your
program to be replaced without stopping and restarting your
program. For example, in VxWorks you can simply recompile a
defective object file and keep on running. If you are running on
one of these systems, you can allow GDB to reload the symbols for
automatically relinked modules:
`set symbol-reloading on'
Replace symbol definitions for the corresponding source file
when an object file with a particular name is seen again.
`set symbol-reloading off'
Do not replace symbol definitions when encountering object
files of the same name more than once. This is the default
state; if you are not running on a system that permits
automatic relinking of modules, you should leave
`symbol-reloading' off, since otherwise GDB may discard
symbols when linking large programs, that may contain several
modules (from different directories or libraries) with the
same name.
`show symbol-reloading'
Show the current `on' or `off' setting.
`set opaque-type-resolution on'
Tell GDB to resolve opaque types. An opaque type is a type
declared as a pointer to a `struct', `class', or `union'--for
example, `struct MyType *'--that is used in one source file
although the full declaration of `struct MyType' is in another
source file. The default is on.
A change in the setting of this subcommand will not take effect
until the next time symbols for a file are loaded.
`set opaque-type-resolution off'
Tell GDB not to resolve opaque types. In this case, the type is
printed as follows:
{<no data fields>}
`show opaque-type-resolution'
Show whether opaque types are resolved or not.
`maint print symbols FILENAME'
`maint print psymbols FILENAME'
`maint print msymbols FILENAME'
Write a dump of debugging symbol data into the file FILENAME.
These commands are used to debug the GDB symbol-reading code. Only
symbols with debugging data are included. If you use `maint print
symbols', GDB includes all the symbols for which it has already
collected full details: that is, FILENAME reflects symbols for
only those files whose symbols GDB has read. You can use the
command `info sources' to find out which files these are. If you
use `maint print psymbols' instead, the dump shows information
about symbols that GDB only knows partially--that is, symbols
defined in files that GDB has skimmed, but not yet read
completely. Finally, `maint print msymbols' dumps just the
minimal symbol information required for each object file from
which GDB has read some symbols. *Note Commands to Specify Files:
Files, for a discussion of how GDB reads symbols (in the
description of `symbol-file').
`maint info symtabs [ REGEXP ]'
`maint info psymtabs [ REGEXP ]'
List the `struct symtab' or `struct partial_symtab' structures
whose names match REGEXP. If REGEXP is not given, list them all.
The output includes expressions which you can copy into a GDB
debugging this one to examine a particular structure in more
detail. For example:
(gdb) maint info psymtabs dwarf2read
{ objfile /home/gnu/build/gdb/gdb
((struct objfile *) 0x82e69d0)
{ psymtab /home/gnu/src/gdb/dwarf2read.c
((struct partial_symtab *) 0x8474b10)
readin no
fullname (null)
text addresses 0x814d3c8 -- 0x8158074
globals (* (struct partial_symbol **) 0x8507a08 @ 9)
statics (* (struct partial_symbol **) 0x40e95b78 @ 2882)
dependencies (none)
}
}
(gdb) maint info symtabs
(gdb)
We see that there is one partial symbol table whose filename
contains the string `dwarf2read', belonging to the `gdb'
executable; and we see that GDB has not read in any symtabs yet at
all. If we set a breakpoint on a function, that will cause GDB to
read the symtab for the compilation unit containing that function:
(gdb) break dwarf2_psymtab_to_symtab
Breakpoint 1 at 0x814e5da: file /home/gnu/src/gdb/dwarf2read.c,
line 1574.
(gdb) maint info symtabs
{ objfile /home/gnu/build/gdb/gdb
((struct objfile *) 0x82e69d0)
{ symtab /home/gnu/src/gdb/dwarf2read.c
((struct symtab *) 0x86c1f38)
dirname (null)
fullname (null)
blockvector ((struct blockvector *) 0x86c1bd0) (primary)
linetable ((struct linetable *) 0x8370fa0)
debugformat DWARF 2
}
}
(gdb)
File: gdb.info, Node: Altering, Next: GDB Files, Prev: Symbols, Up: Top
14 Altering Execution
*********************
Once you think you have found an error in your program, you might want
to find out for certain whether correcting the apparent error would
lead to correct results in the rest of the run. You can find the
answer by experiment, using the GDB features for altering execution of
the program.
For example, you can store new values into variables or memory
locations, give your program a signal, restart it at a different
address, or even return prematurely from a function.
* Menu:
* Assignment:: Assignment to variables
* Jumping:: Continuing at a different address
* Signaling:: Giving your program a signal
* Returning:: Returning from a function
* Calling:: Calling your program's functions
* Patching:: Patching your program
File: gdb.info, Node: Assignment, Next: Jumping, Up: Altering
14.1 Assignment to Variables
============================
To alter the value of a variable, evaluate an assignment expression.
*Note Expressions: Expressions. For example,
print x=4
stores the value 4 into the variable `x', and then prints the value of
the assignment expression (which is 4). *Note Using GDB with Different
Languages: Languages, for more information on operators in supported
languages.
If you are not interested in seeing the value of the assignment, use
the `set' command instead of the `print' command. `set' is really the
same as `print' except that the expression's value is not printed and
is not put in the value history (*note Value History: Value History.).
The expression is evaluated only for its effects.
If the beginning of the argument string of the `set' command appears
identical to a `set' subcommand, use the `set variable' command instead
of just `set'. This command is identical to `set' except for its lack
of subcommands. For example, if your program has a variable `width',
you get an error if you try to set a new value with just `set
width=13', because GDB has the command `set width':
(gdb) whatis width
type = double
(gdb) p width
$4 = 13
(gdb) set width=47
Invalid syntax in expression.
The invalid expression, of course, is `=47'. In order to actually set
the program's variable `width', use
(gdb) set var width=47
Because the `set' command has many subcommands that can conflict
with the names of program variables, it is a good idea to use the `set
variable' command instead of just `set'. For example, if your program
has a variable `g', you run into problems if you try to set a new value
with just `set g=4', because GDB has the command `set gnutarget',
abbreviated `set g':
(gdb) whatis g
type = double
(gdb) p g
$1 = 1
(gdb) set g=4
(gdb) p g
$2 = 1
(gdb) r
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/smith/cc_progs/a.out
"/home/smith/cc_progs/a.out": can't open to read symbols:
Invalid bfd target.
(gdb) show g
The current BFD target is "=4".
The program variable `g' did not change, and you silently set the
`gnutarget' to an invalid value. In order to set the variable `g', use
(gdb) set var g=4
GDB allows more implicit conversions in assignments than C; you can
freely store an integer value into a pointer variable or vice versa,
and you can convert any structure to any other structure that is the
same length or shorter.
To store values into arbitrary places in memory, use the `{...}'
construct to generate a value of specified type at a specified address
(*note Expressions: Expressions.). For example, `{int}0x83040' refers
to memory location `0x83040' as an integer (which implies a certain size
and representation in memory), and
set {int}0x83040 = 4
stores the value 4 into that memory location.
File: gdb.info, Node: Jumping, Next: Signaling, Prev: Assignment, Up: Altering
14.2 Continuing at a Different Address
======================================
Ordinarily, when you continue your program, you do so at the place where
it stopped, with the `continue' command. You can instead continue at
an address of your own choosing, with the following commands:
`jump LINESPEC'
`jump LOCATION'
Resume execution at line LINESPEC or at address given by LOCATION.
Execution stops again immediately if there is a breakpoint there.
*Note Specify Location::, for a description of the different
forms of LINESPEC and LOCATION. It is common practice to use the
`tbreak' command in conjunction with `jump'. *Note Setting
Breakpoints: Set Breaks.
The `jump' command does not change the current stack frame, or the
stack pointer, or the contents of any memory location or any
register other than the program counter. If line LINESPEC is in a
different function from the one currently executing, the results
may be bizarre if the two functions expect different patterns of
arguments or of local variables. For this reason, the `jump'
command requests confirmation if the specified line is not in the
function currently executing. However, even bizarre results are
predictable if you are well acquainted with the machine-language
code of your program.
On many systems, you can get much the same effect as the `jump'
command by storing a new value into the register `$pc'. The difference
is that this does not start your program running; it only changes the
address of where it _will_ run when you continue. For example,
set $pc = 0x485
makes the next `continue' command or stepping command execute at
address `0x485', rather than at the address where your program stopped.
*Note Continuing and Stepping: Continuing and Stepping.
The most common occasion to use the `jump' command is to back
up--perhaps with more breakpoints set--over a portion of a program that
has already executed, in order to examine its execution in more detail.
File: gdb.info, Node: Signaling, Next: Returning, Prev: Jumping, Up: Altering
14.3 Giving your Program a Signal
=================================
`signal SIGNAL'
Resume execution where your program stopped, but immediately give
it the signal SIGNAL. SIGNAL can be the name or the number of a
signal. For example, on many systems `signal 2' and `signal
SIGINT' are both ways of sending an interrupt signal.
Alternatively, if SIGNAL is zero, continue execution without
giving a signal. This is useful when your program stopped on
account of a signal and would ordinary see the signal when resumed
with the `continue' command; `signal 0' causes it to resume
without a signal.
`signal' does not repeat when you press <RET> a second time after
executing the command.
Invoking the `signal' command is not the same as invoking the `kill'
utility from the shell. Sending a signal with `kill' causes GDB to
decide what to do with the signal depending on the signal handling
tables (*note Signals::). The `signal' command passes the signal
directly to your program.
File: gdb.info, Node: Returning, Next: Calling, Prev: Signaling, Up: Altering
14.4 Returning from a Function
==============================
`return'
`return EXPRESSION'
You can cancel execution of a function call with the `return'
command. If you give an EXPRESSION argument, its value is used as
the function's return value.
When you use `return', GDB discards the selected stack frame (and
all frames within it). You can think of this as making the discarded
frame return prematurely. If you wish to specify a value to be
returned, give that value as the argument to `return'.
This pops the selected stack frame (*note Selecting a Frame:
Selection.), and any other frames inside of it, leaving its caller as
the innermost remaining frame. That frame becomes selected. The
specified value is stored in the registers used for returning values of
functions.
The `return' command does not resume execution; it leaves the
program stopped in the state that would exist if the function had just
returned. In contrast, the `finish' command (*note Continuing and
Stepping: Continuing and Stepping.) resumes execution until the
selected stack frame returns naturally.
File: gdb.info, Node: Calling, Next: Patching, Prev: Returning, Up: Altering
14.5 Calling Program Functions
==============================
`print EXPR'
Evaluate the expression EXPR and display the resulting value.
EXPR may include calls to functions in the program being debugged.
`call EXPR'
Evaluate the expression EXPR without displaying `void' returned
values.
You can use this variant of the `print' command if you want to
execute a function from your program that does not return anything
(a.k.a. "a void function"), but without cluttering the output with
`void' returned values that GDB will otherwise print. If the
result is not void, it is printed and saved in the value history.
It is possible for the function you call via the `print' or `call'
command to generate a signal (e.g., if there's a bug in the function,
or if you passed it incorrect arguments). What happens in that case is
controlled by the `set unwindonsignal' command.
`set unwindonsignal'
Set unwinding of the stack if a signal is received while in a
function that GDB called in the program being debugged. If set to
on, GDB unwinds the stack it created for the call and restores the
context to what it was before the call. If set to off (the
default), GDB stops in the frame where the signal was received.
`show unwindonsignal'
Show the current setting of stack unwinding in the functions
called by GDB.
Sometimes, a function you wish to call is actually a "weak alias"
for another function. In such case, GDB might not pick up the type
information, including the types of the function arguments, which
causes GDB to call the inferior function incorrectly. As a result, the
called function will function erroneously and may even crash. A
solution to that is to use the name of the aliased function instead.
File: gdb.info, Node: Patching, Prev: Calling, Up: Altering
14.6 Patching Programs
======================
By default, GDB opens the file containing your program's executable
code (or the corefile) read-only. This prevents accidental alterations
to machine code; but it also prevents you from intentionally patching
your program's binary.
If you'd like to be able to patch the binary, you can specify that
explicitly with the `set write' command. For example, you might want
to turn on internal debugging flags, or even to make emergency repairs.
`set write on'
`set write off'
If you specify `set write on', GDB opens executable and core files
for both reading and writing; if you specify `set write off' (the
default), GDB opens them read-only.
If you have already loaded a file, you must load it again (using
the `exec-file' or `core-file' command) after changing `set
write', for your new setting to take effect.
`show write'
Display whether executable files and core files are opened for
writing as well as reading.
File: gdb.info, Node: GDB Files, Next: Targets, Prev: Altering, Up: Top
15 GDB Files
************
GDB needs to know the file name of the program to be debugged, both in
order to read its symbol table and in order to start your program. To
debug a core dump of a previous run, you must also tell GDB the name of
the core dump file.
* Menu:
* Files:: Commands to specify files
* Separate Debug Files:: Debugging information in separate files
* Symbol Errors:: Errors reading symbol files
File: gdb.info, Node: Files, Next: Separate Debug Files, Up: GDB Files
15.1 Commands to Specify Files
==============================
You may want to specify executable and core dump file names. The usual
way to do this is at start-up time, using the arguments to GDB's
start-up commands (*note Getting In and Out of GDB: Invocation.).
Occasionally it is necessary to change to a different file during a
GDB session. Or you may run GDB and forget to specify a file you want
to use. Or you are debugging a remote target via `gdbserver' (*note
file: Server.). In these situations the GDB commands to specify new
files are useful.
`file FILENAME'
Use FILENAME as the program to be debugged. It is read for its
symbols and for the contents of pure memory. It is also the
program executed when you use the `run' command. If you do not
specify a directory and the file is not found in the GDB working
directory, GDB uses the environment variable `PATH' as a list of
directories to search, just as the shell does when looking for a
program to run. You can change the value of this variable, for
both GDB and your program, using the `path' command.
You can load unlinked object `.o' files into GDB using the `file'
command. You will not be able to "run" an object file, but you
can disassemble functions and inspect variables. Also, if the
underlying BFD functionality supports it, you could use `gdb
-write' to patch object files using this technique. Note that GDB
can neither interpret nor modify relocations in this case, so
branches and some initialized variables will appear to go to the
wrong place. But this feature is still handy from time to time.
`file'
`file' with no argument makes GDB discard any information it has
on both executable file and the symbol table.
`exec-file [ FILENAME ]'
Specify that the program to be run (but not the symbol table) is
found in FILENAME. GDB searches the environment variable `PATH'
if necessary to locate your program. Omitting FILENAME means to
discard information on the executable file.
`symbol-file [ FILENAME ]'
Read symbol table information from file FILENAME. `PATH' is
searched when necessary. Use the `file' command to get both symbol
table and program to run from the same file.
`symbol-file' with no argument clears out GDB information on your
program's symbol table.
The `symbol-file' command causes GDB to forget the contents of
some breakpoints and auto-display expressions. This is because
they may contain pointers to the internal data recording symbols
and data types, which are part of the old symbol table data being
discarded inside GDB.
`symbol-file' does not repeat if you press <RET> again after
executing it once.
When GDB is configured for a particular environment, it
understands debugging information in whatever format is the
standard generated for that environment; you may use either a GNU
compiler, or other compilers that adhere to the local conventions.
Best results are usually obtained from GNU compilers; for example,
using `GCC' you can generate debugging information for optimized
code.
For most kinds of object files, with the exception of old SVR3
systems using COFF, the `symbol-file' command does not normally
read the symbol table in full right away. Instead, it scans the
symbol table quickly to find which source files and which symbols
are present. The details are read later, one source file at a
time, as they are needed.
The purpose of this two-stage reading strategy is to make GDB
start up faster. For the most part, it is invisible except for
occasional pauses while the symbol table details for a particular
source file are being read. (The `set verbose' command can turn
these pauses into messages if desired. *Note Optional Warnings
and Messages: Messages/Warnings.)
We have not implemented the two-stage strategy for COFF yet. When
the symbol table is stored in COFF format, `symbol-file' reads the
symbol table data in full right away. Note that "stabs-in-COFF"
still does the two-stage strategy, since the debug info is actually
in stabs format.
`symbol-file FILENAME [ -readnow ]'
`file FILENAME [ -readnow ]'
You can override the GDB two-stage strategy for reading symbol
tables by using the `-readnow' option with any of the commands that
load symbol table information, if you want to be sure GDB has the
entire symbol table available.
`core-file [FILENAME]'
`core'
Specify the whereabouts of a core dump file to be used as the
"contents of memory". Traditionally, core files contain only some
parts of the address space of the process that generated them; GDB
can access the executable file itself for other parts.
`core-file' with no argument specifies that no core file is to be
used.
Note that the core file is ignored when your program is actually
running under GDB. So, if you have been running your program and
you wish to debug a core file instead, you must kill the
subprocess in which the program is running. To do this, use the
`kill' command (*note Killing the Child Process: Kill Process.).
`add-symbol-file FILENAME ADDRESS'
`add-symbol-file FILENAME ADDRESS [ -readnow ]'
`add-symbol-file FILENAME -sSECTION ADDRESS ...'
The `add-symbol-file' command reads additional symbol table
information from the file FILENAME. You would use this command
when FILENAME has been dynamically loaded (by some other means)
into the program that is running. ADDRESS should be the memory
address at which the file has been loaded; GDB cannot figure this
out for itself. You can additionally specify an arbitrary number
of `-sSECTION ADDRESS' pairs, to give an explicit section name and
base address for that section. You can specify any ADDRESS as an
expression.
The symbol table of the file FILENAME is added to the symbol table
originally read with the `symbol-file' command. You can use the
`add-symbol-file' command any number of times; the new symbol data
thus read keeps adding to the old. To discard all old symbol data
instead, use the `symbol-file' command without any arguments.
Although FILENAME is typically a shared library file, an
executable file, or some other object file which has been fully
relocated for loading into a process, you can also load symbolic
information from relocatable `.o' files, as long as:
* the file's symbolic information refers only to linker symbols
defined in that file, not to symbols defined by other object
files,
* every section the file's symbolic information refers to has
actually been loaded into the inferior, as it appears in the
file, and
* you can determine the address at which every section was
loaded, and provide these to the `add-symbol-file' command.
Some embedded operating systems, like Sun Chorus and VxWorks, can
load relocatable files into an already running program; such
systems typically make the requirements above easy to meet.
However, it's important to recognize that many native systems use
complex link procedures (`.linkonce' section factoring and C++
constructor table assembly, for example) that make the
requirements difficult to meet. In general, one cannot assume
that using `add-symbol-file' to read a relocatable object file's
symbolic information will have the same effect as linking the
relocatable object file into the program in the normal way.
`add-symbol-file' does not repeat if you press <RET> after using
it.
`add-symbol-file-from-memory ADDRESS'
Load symbols from the given ADDRESS in a dynamically loaded object
file whose image is mapped directly into the inferior's memory.
For example, the Linux kernel maps a `syscall DSO' into each
process's address space; this DSO provides kernel-specific code for
some system calls. The argument can be any expression whose
evaluation yields the address of the file's shared object file
header. For this command to work, you must have used
`symbol-file' or `exec-file' commands in advance.
`add-shared-symbol-files LIBRARY-FILE'
`assf LIBRARY-FILE'
The `add-shared-symbol-files' command can currently be used only
in the Cygwin build of GDB on MS-Windows OS, where it is an alias
for the `dll-symbols' command (*note Cygwin Native::). GDB
automatically looks for shared libraries, however if GDB does not
find yours, you can invoke `add-shared-symbol-files'. It takes
one argument: the shared library's file name. `assf' is a
shorthand alias for `add-shared-symbol-files'.
`section SECTION ADDR'
The `section' command changes the base address of the named
SECTION of the exec file to ADDR. This can be used if the exec
file does not contain section addresses, (such as in the `a.out'
format), or when the addresses specified in the file itself are
wrong. Each section must be changed separately. The `info files'
command, described below, lists all the sections and their
addresses.
`info files'
`info target'
`info files' and `info target' are synonymous; both print the
current target (*note Specifying a Debugging Target: Targets.),
including the names of the executable and core dump files
currently in use by GDB, and the files from which symbols were
loaded. The command `help target' lists all possible targets
rather than current ones.
`maint info sections'
Another command that can give you extra information about program
sections is `maint info sections'. In addition to the section
information displayed by `info files', this command displays the
flags and file offset of each section in the executable and core
dump files. In addition, `maint info sections' provides the
following command options (which may be arbitrarily combined):
`ALLOBJ'
Display sections for all loaded object files, including
shared libraries.
`SECTIONS'
Display info only for named SECTIONS.
`SECTION-FLAGS'
Display info only for sections for which SECTION-FLAGS are
true. The section flags that GDB currently knows about are:
`ALLOC'
Section will have space allocated in the process when
loaded. Set for all sections except those containing
debug information.
`LOAD'
Section will be loaded from the file into the child
process memory. Set for pre-initialized code and data,
clear for `.bss' sections.
`RELOC'
Section needs to be relocated before loading.
`READONLY'
Section cannot be modified by the child process.
`CODE'
Section contains executable code only.
`DATA'
Section contains data only (no executable code).
`ROM'
Section will reside in ROM.
`CONSTRUCTOR'
Section contains data for constructor/destructor lists.
`HAS_CONTENTS'
Section is not empty.
`NEVER_LOAD'
An instruction to the linker to not output the section.
`COFF_SHARED_LIBRARY'
A notification to the linker that the section contains
COFF shared library information.
`IS_COMMON'
Section contains common symbols.
`set trust-readonly-sections on'
Tell GDB that readonly sections in your object file really are
read-only (i.e. that their contents will not change). In that
case, GDB can fetch values from these sections out of the object
file, rather than from the target program. For some targets
(notably embedded ones), this can be a significant enhancement to
debugging performance.
The default is off.
`set trust-readonly-sections off'
Tell GDB not to trust readonly sections. This means that the
contents of the section might change while the program is running,
and must therefore be fetched from the target when needed.
`show trust-readonly-sections'
Show the current setting of trusting readonly sections.
All file-specifying commands allow both absolute and relative file
names as arguments. GDB always converts the file name to an absolute
file name and remembers it that way.
GDB supports GNU/Linux, MS-Windows, HP-UX, SunOS, SVr4, Irix, and
IBM RS/6000 AIX shared libraries.
On MS-Windows GDB must be linked with the Expat library to support
shared libraries. *Note Expat::.
GDB automatically loads symbol definitions from shared libraries
when you use the `run' command, or when you examine a core file.
(Before you issue the `run' command, GDB does not understand references
to a function in a shared library, however--unless you are debugging a
core file).
On HP-UX, if the program loads a library explicitly, GDB
automatically loads the symbols at the time of the `shl_load' call.
There are times, however, when you may wish to not automatically load
symbol definitions from shared libraries, such as when they are
particularly large or there are many of them.
To control the automatic loading of shared library symbols, use the
commands:
`set auto-solib-add MODE'
If MODE is `on', symbols from all shared object libraries will be
loaded automatically when the inferior begins execution, you
attach to an independently started inferior, or when the dynamic
linker informs GDB that a new library has been loaded. If MODE is
`off', symbols must be loaded manually, using the `sharedlibrary'
command. The default value is `on'.
If your program uses lots of shared libraries with debug info that
takes large amounts of memory, you can decrease the GDB memory
footprint by preventing it from automatically loading the symbols
from shared libraries. To that end, type `set auto-solib-add off'
before running the inferior, then load each library whose debug
symbols you do need with `sharedlibrary REGEXP', where REGEXP is a
regular expression that matches the libraries whose symbols you
want to be loaded.
`show auto-solib-add'
Display the current autoloading mode.
To explicitly load shared library symbols, use the `sharedlibrary'
command:
`info share'
`info sharedlibrary'
Print the names of the shared libraries which are currently loaded.
`sharedlibrary REGEX'
`share REGEX'
Load shared object library symbols for files matching a Unix
regular expression. As with files loaded automatically, it only
loads shared libraries required by your program for a core file or
after typing `run'. If REGEX is omitted all shared libraries
required by your program are loaded.
`nosharedlibrary'
Unload all shared object library symbols. This discards all
symbols that have been loaded from all shared libraries. Symbols
from shared libraries that were loaded by explicit user requests
are not discarded.
Sometimes you may wish that GDB stops and gives you control when any
of shared library events happen. Use the `set stop-on-solib-events'
command for this:
`set stop-on-solib-events'
This command controls whether GDB should give you control when the
dynamic linker notifies it about some shared library event. The
most common event of interest is loading or unloading of a new
shared library.
`show stop-on-solib-events'
Show whether GDB stops and gives you control when shared library
events happen.
Shared libraries are also supported in many cross or remote debugging
configurations. A copy of the target's libraries need to be present on
the host system; they need to be the same as the target libraries,
although the copies on the target can be stripped as long as the copies
on the host are not.
For remote debugging, you need to tell GDB where the target
libraries are, so that it can load the correct copies--otherwise, it
may try to load the host's libraries. GDB has two variables to specify
the search directories for target libraries.
`set sysroot PATH'
Use PATH as the system root for the program being debugged. Any
absolute shared library paths will be prefixed with PATH; many
runtime loaders store the absolute paths to the shared library in
the target program's memory. If you use `set sysroot' to find
shared libraries, they need to be laid out in the same way that
they are on the target, with e.g. a `/lib' and `/usr/lib' hierarchy
under PATH.
The `set solib-absolute-prefix' command is an alias for `set
sysroot'.
You can set the default system root by using the configure-time
`--with-sysroot' option. If the system root is inside GDB's
configured binary prefix (set with `--prefix' or `--exec-prefix'),
then the default system root will be updated automatically if the
installed GDB is moved to a new location.
`show sysroot'
Display the current shared library prefix.
`set solib-search-path PATH'
If this variable is set, PATH is a colon-separated list of
directories to search for shared libraries. `solib-search-path'
is used after `sysroot' fails to locate the library, or if the
path to the library is relative instead of absolute. If you want
to use `solib-search-path' instead of `sysroot', be sure to set
`sysroot' to a nonexistent directory to prevent GDB from finding
your host's libraries. `sysroot' is preferred; setting it to a
nonexistent directory may interfere with automatic loading of
shared library symbols.
`show solib-search-path'
Display the current shared library search path.
File: gdb.info, Node: Separate Debug Files, Next: Symbol Errors, Prev: Files, Up: GDB Files
15.2 Debugging Information in Separate Files
============================================
GDB allows you to put a program's debugging information in a file
separate from the executable itself, in a way that allows GDB to find
and load the debugging information automatically. Since debugging
information can be very large--sometimes larger than the executable
code itself--some systems distribute debugging information for their
executables in separate files, which users can install only when they
need to debug a problem.
GDB supports two ways of specifying the separate debug info file:
* The executable contains a "debug link" that specifies the name of
the separate debug info file. The separate debug file's name is
usually `EXECUTABLE.debug', where EXECUTABLE is the name of the
corresponding executable file without leading directories (e.g.,
`ls.debug' for `/usr/bin/ls'). In addition, the debug link
specifies a CRC32 checksum for the debug file, which GDB uses to
validate that the executable and the debug file came from the same
build.
* The executable contains a "build ID", a unique bit string that is
also present in the corresponding debug info file. (This is
supported only on some operating systems, notably those which use
the ELF format for binary files and the GNU Binutils.) For more
details about this feature, see the description of the `--build-id'
command-line option in *Note Command Line Options:
(ld.info)Options. The debug info file's name is not specified
explicitly by the build ID, but can be computed from the build ID,
see below.
Depending on the way the debug info file is specified, GDB uses two
different methods of looking for the debug file:
* For the "debug link" method, GDB looks up the named file in the
directory of the executable file, then in a subdirectory of that
directory named `.debug', and finally under the global debug
directory, in a subdirectory whose name is identical to the leading
directories of the executable's absolute file name.
* For the "build ID" method, GDB looks in the `.build-id'
subdirectory of the global debug directory for a file named
`NN/NNNNNNNN.debug', where NN are the first 2 hex characters of
the build ID bit string, and NNNNNNNN are the rest of the bit
string. (Real build ID strings are 32 or more hex characters, not
10.)
So, for example, suppose you ask GDB to debug `/usr/bin/ls', which
has a debug link that specifies the file `ls.debug', and a build ID
whose value in hex is `abcdef1234'. If the global debug directory is
`/usr/lib/debug', then GDB will look for the following debug
information files, in the indicated order:
- `/usr/lib/debug/.build-id/ab/cdef1234.debug'
- `/usr/bin/ls.debug'
- `/usr/bin/.debug/ls.debug'
- `/usr/lib/debug/usr/bin/ls.debug'.
You can set the global debugging info directory's name, and view the
name GDB is currently using.
`set debug-file-directory DIRECTORY'
Set the directory which GDB searches for separate debugging
information files to DIRECTORY.
`show debug-file-directory'
Show the directory GDB searches for separate debugging information
files.
A debug link is a special section of the executable file named
`.gnu_debuglink'. The section must contain:
* A filename, with any leading directory components removed,
followed by a zero byte,
* zero to three bytes of padding, as needed to reach the next
four-byte boundary within the section, and
* a four-byte CRC checksum, stored in the same endianness used for
the executable file itself. The checksum is computed on the
debugging information file's full contents by the function given
below, passing zero as the CRC argument.
Any executable file format can carry a debug link, as long as it can
contain a section named `.gnu_debuglink' with the contents described
above.
The build ID is a special section in the executable file (and in
other ELF binary files that GDB may consider). This section is often
named `.note.gnu.build-id', but that name is not mandatory. It
contains unique identification for the built files--the ID remains the
same across multiple builds of the same build tree. The default
algorithm SHA1 produces 160 bits (40 hexadecimal characters) of the
content for the build ID string. The same section with an identical
value is present in the original built binary with symbols, in its
stripped variant, and in the separate debugging information file.
The debugging information file itself should be an ordinary
executable, containing a full set of linker symbols, sections, and
debugging information. The sections of the debugging information file
should have the same names, addresses, and sizes as the original file,
but they need not contain any data--much like a `.bss' section in an
ordinary executable.
The GNU binary utilities (Binutils) package includes the `objcopy'
utility that can produce the separated executable / debugging
information file pairs using the following commands:
objcopy --only-keep-debug foo foo.debug
strip -g foo
These commands remove the debugging information from the executable
file `foo' and place it in the file `foo.debug'. You can use the
first, second or both methods to link the two files:
* The debug link method needs the following additional command to
also leave behind a debug link in `foo':
objcopy --add-gnu-debuglink=foo.debug foo
Ulrich Drepper's `elfutils' package, starting with version 0.53,
contains a version of the `strip' command such that the command
`strip foo -f foo.debug' has the same functionality as the two
`objcopy' commands and the `ln -s' command above, together.
* Build ID gets embedded into the main executable using `ld
--build-id' or the GCC counterpart `gcc -Wl,--build-id'. Build ID
support plus compatibility fixes for debug files separation are
present in GNU binary utilities (Binutils) package since version
2.18.
Since there are many different ways to compute CRC's for the debug link
(different polynomials, reversals, byte ordering, etc.), the simplest
way to describe the CRC used in `.gnu_debuglink' sections is to give
the complete code for a function that computes it:
unsigned long
gnu_debuglink_crc32 (unsigned long crc,
unsigned char *buf, size_t len)
{
static const unsigned long crc32_table[256] =
{
0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419,
0x706af48f, 0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4,
0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07,
0x90bf1d91, 0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7, 0x136c9856,
0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9,
0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4,
0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3,
0x45df5c75, 0xdcd60dcf, 0xabd13d59, 0x26d930ac, 0x51de003a,
0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599,
0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190,
0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f,
0x9fbfe4a5, 0xe8b8d433, 0x7807c9a2, 0x0f00f934, 0x9609a88e,
0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed,
0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950,
0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3,
0xfbd44c65, 0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a,
0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5,
0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa, 0xbe0b1010,
0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17,
0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6,
0x03b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af, 0x04db2615,
0x73dc1683, 0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1, 0xf00f9344,
0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb,
0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a,
0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1,
0xa6bc5767, 0x3fb506dd, 0x48b2364b, 0xd80d2bda, 0xaf0a1b4c,
0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55, 0x316e8eef,
0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe,
0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31,
0x2cd99e8b, 0x5bdeae1d, 0x9b64c2b0, 0xec63f226, 0x756aa39c,
0x026d930a, 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38, 0x92d28e9b,
0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242,
0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1,
0x18b74777, 0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45, 0xa00ae278,
0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7,
0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc, 0x40df0b66,
0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605,
0xcdd70693, 0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8,
0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b,
0x2d02ef8d
};
unsigned char *end;
crc = ~crc & 0xffffffff;
for (end = buf + len; buf < end; ++buf)
crc = crc32_table[(crc ^ *buf) & 0xff] ^ (crc >> 8);
return ~crc & 0xffffffff;
}
This computation does not apply to the "build ID" method.
File: gdb.info, Node: Symbol Errors, Prev: Separate Debug Files, Up: GDB Files
15.3 Errors Reading Symbol Files
================================
While reading a symbol file, GDB occasionally encounters problems, such
as symbol types it does not recognize, or known bugs in compiler
output. By default, GDB does not notify you of such problems, since
they are relatively common and primarily of interest to people
debugging compilers. If you are interested in seeing information about
ill-constructed symbol tables, you can either ask GDB to print only one
message about each such type of problem, no matter how many times the
problem occurs; or you can ask GDB to print more messages, to see how
many times the problems occur, with the `set complaints' command (*note
Optional Warnings and Messages: Messages/Warnings.).
The messages currently printed, and their meanings, include:
`inner block not inside outer block in SYMBOL'
The symbol information shows where symbol scopes begin and end
(such as at the start of a function or a block of statements).
This error indicates that an inner scope block is not fully
contained in its outer scope blocks.
GDB circumvents the problem by treating the inner block as if it
had the same scope as the outer block. In the error message,
SYMBOL may be shown as "`(don't know)'" if the outer block is not a
function.
`block at ADDRESS out of order'
The symbol information for symbol scope blocks should occur in
order of increasing addresses. This error indicates that it does
not do so.
GDB does not circumvent this problem, and has trouble locating
symbols in the source file whose symbols it is reading. (You can
often determine what source file is affected by specifying `set
verbose on'. *Note Optional Warnings and Messages:
Messages/Warnings.)
`bad block start address patched'
The symbol information for a symbol scope block has a start address
smaller than the address of the preceding source line. This is
known to occur in the SunOS 4.1.1 (and earlier) C compiler.
GDB circumvents the problem by treating the symbol scope block as
starting on the previous source line.
`bad string table offset in symbol N'
Symbol number N contains a pointer into the string table which is
larger than the size of the string table.
GDB circumvents the problem by considering the symbol to have the
name `foo', which may cause other problems if many symbols end up
with this name.
`unknown symbol type `0xNN''
The symbol information contains new data types that GDB does not
yet know how to read. `0xNN' is the symbol type of the
uncomprehended information, in hexadecimal.
GDB circumvents the error by ignoring this symbol information.
This usually allows you to debug your program, though certain
symbols are not accessible. If you encounter such a problem and
feel like debugging it, you can debug `gdb' with itself, breakpoint
on `complain', then go up to the function `read_dbx_symtab' and
examine `*bufp' to see the symbol.
`stub type has NULL name'
GDB could not find the full definition for a struct or class.
`const/volatile indicator missing (ok if using g++ v1.x), got...'
The symbol information for a C++ member function is missing some
information that recent versions of the compiler should have
output for it.
`info mismatch between compiler and debugger'
GDB could not parse a type specification output by the compiler.
File: gdb.info, Node: Targets, Next: Remote Debugging, Prev: GDB Files, Up: Top
16 Specifying a Debugging Target
********************************
A "target" is the execution environment occupied by your program.
Often, GDB runs in the same host environment as your program; in
that case, the debugging target is specified as a side effect when you
use the `file' or `core' commands. When you need more flexibility--for
example, running GDB on a physically separate host, or controlling a
standalone system over a serial port or a realtime system over a TCP/IP
connection--you can use the `target' command to specify one of the
target types configured for GDB (*note Commands for Managing Targets:
Target Commands.).
It is possible to build GDB for several different "target
architectures". When GDB is built like that, you can choose one of the
available architectures with the `set architecture' command.
`set architecture ARCH'
This command sets the current target architecture to ARCH. The
value of ARCH can be `"auto"', in addition to one of the supported
architectures.
`show architecture'
Show the current target architecture.
`set processor'
`processor'
These are alias commands for, respectively, `set architecture' and
`show architecture'.
* Menu:
* Active Targets:: Active targets
* Target Commands:: Commands for managing targets
* Byte Order:: Choosing target byte order
File: gdb.info, Node: Active Targets, Next: Target Commands, Up: Targets
16.1 Active Targets
===================
There are three classes of targets: processes, core files, and
executable files. GDB can work concurrently on up to three active
targets, one in each class. This allows you to (for example) start a
process and inspect its activity without abandoning your work on a core
file.
For example, if you execute `gdb a.out', then the executable file
`a.out' is the only active target. If you designate a core file as
well--presumably from a prior run that crashed and coredumped--then GDB
has two active targets and uses them in tandem, looking first in the
corefile target, then in the executable file, to satisfy requests for
memory addresses. (Typically, these two classes of target are
complementary, since core files contain only a program's read-write
memory--variables and so on--plus machine status, while executable
files contain only the program text and initialized data.)
When you type `run', your executable file becomes an active process
target as well. When a process target is active, all GDB commands
requesting memory addresses refer to that target; addresses in an
active core file or executable file target are obscured while the
process target is active.
Use the `core-file' and `exec-file' commands to select a new core
file or executable target (*note Commands to Specify Files: Files.).
To specify as a target a process that is already running, use the
`attach' command (*note Debugging an Already-running Process: Attach.).
File: gdb.info, Node: Target Commands, Next: Byte Order, Prev: Active Targets, Up: Targets
16.2 Commands for Managing Targets
==================================
`target TYPE PARAMETERS'
Connects the GDB host environment to a target machine or process.
A target is typically a protocol for talking to debugging
facilities. You use the argument TYPE to specify the type or
protocol of the target machine.
Further PARAMETERS are interpreted by the target protocol, but
typically include things like device names or host names to connect
with, process numbers, and baud rates.
The `target' command does not repeat if you press <RET> again
after executing the command.
`help target'
Displays the names of all targets available. To display targets
currently selected, use either `info target' or `info files'
(*note Commands to Specify Files: Files.).
`help target NAME'
Describe a particular target, including any parameters necessary to
select it.
`set gnutarget ARGS'
GDB uses its own library BFD to read your files. GDB knows
whether it is reading an "executable", a "core", or a ".o" file;
however, you can specify the file format with the `set gnutarget'
command. Unlike most `target' commands, with `gnutarget' the
`target' refers to a program, not a machine.
_Warning:_ To specify a file format with `set gnutarget', you
must know the actual BFD name.
*Note Commands to Specify Files: Files.
`show gnutarget'
Use the `show gnutarget' command to display what file format
`gnutarget' is set to read. If you have not set `gnutarget', GDB
will determine the file format for each file automatically, and
`show gnutarget' displays `The current BDF target is "auto"'.
Here are some common targets (available, or not, depending on the GDB
configuration):
`target exec PROGRAM'
An executable file. `target exec PROGRAM' is the same as
`exec-file PROGRAM'.
`target core FILENAME'
A core dump file. `target core FILENAME' is the same as
`core-file FILENAME'.
`target remote MEDIUM'
A remote system connected to GDB via a serial line or network
connection. This command tells GDB to use its own remote protocol
over MEDIUM for debugging. *Note Remote Debugging::.
For example, if you have a board connected to `/dev/ttya' on the
machine running GDB, you could say:
target remote /dev/ttya
`target remote' supports the `load' command. This is only useful
if you have some other way of getting the stub to the target
system, and you can put it somewhere in memory where it won't get
clobbered by the download.
`target sim'
Builtin CPU simulator. GDB includes simulators for most
architectures. In general,
target sim
load
run
works; however, you cannot assume that a specific memory map,
device drivers, or even basic I/O is available, although some
simulators do provide these. For info about any
processor-specific simulator details, see the appropriate section
in *Note Embedded Processors: Embedded Processors.
Some configurations may include these targets as well:
`target nrom DEV'
NetROM ROM emulator. This target only supports downloading.
Different targets are available on different configurations of GDB;
your configuration may have more or fewer targets.
Many remote targets require you to download the executable's code
once you've successfully established a connection. You may wish to
control various aspects of this process.
`set hash'
This command controls whether a hash mark `#' is displayed while
downloading a file to the remote monitor. If on, a hash mark is
displayed after each S-record is successfully downloaded to the
monitor.
`show hash'
Show the current status of displaying the hash mark.
`set debug monitor'
Enable or disable display of communications messages between GDB
and the remote monitor.
`show debug monitor'
Show the current status of displaying communications between GDB
and the remote monitor.
`load FILENAME'
Depending on what remote debugging facilities are configured into
GDB, the `load' command may be available. Where it exists, it is
meant to make FILENAME (an executable) available for debugging on
the remote system--by downloading, or dynamic linking, for example.
`load' also records the FILENAME symbol table in GDB, like the
`add-symbol-file' command.
If your GDB does not have a `load' command, attempting to execute
it gets the error message "`You can't do that when your target is
...'"
The file is loaded at whatever address is specified in the
executable. For some object file formats, you can specify the
load address when you link the program; for other formats, like
a.out, the object file format specifies a fixed address.
Depending on the remote side capabilities, GDB may be able to load
programs into flash memory.
`load' does not repeat if you press <RET> again after using it.
File: gdb.info, Node: Byte Order, Prev: Target Commands, Up: Targets
16.3 Choosing Target Byte Order
===============================
Some types of processors, such as the MIPS, PowerPC, and Renesas SH,
offer the ability to run either big-endian or little-endian byte
orders. Usually the executable or symbol will include a bit to
designate the endian-ness, and you will not need to worry about which
to use. However, you may still find it useful to adjust GDB's idea of
processor endian-ness manually.
`set endian big'
Instruct GDB to assume the target is big-endian.
`set endian little'
Instruct GDB to assume the target is little-endian.
`set endian auto'
Instruct GDB to use the byte order associated with the executable.
`show endian'
Display GDB's current idea of the target byte order.
Note that these commands merely adjust interpretation of symbolic
data on the host, and that they have absolutely no effect on the target
system.
File: gdb.info, Node: Remote Debugging, Next: Configurations, Prev: Targets, Up: Top
17 Debugging Remote Programs
****************************
If you are trying to debug a program running on a machine that cannot
run GDB in the usual way, it is often useful to use remote debugging.
For example, you might use remote debugging on an operating system
kernel, or on a small system which does not have a general purpose
operating system powerful enough to run a full-featured debugger.
Some configurations of GDB have special serial or TCP/IP interfaces
to make this work with particular debugging targets. In addition, GDB
comes with a generic serial protocol (specific to GDB, but not specific
to any particular target system) which you can use if you write the
remote stubs--the code that runs on the remote system to communicate
with GDB.
Other remote targets may be available in your configuration of GDB;
use `help target' to list them.
* Menu:
* Connecting:: Connecting to a remote target
* File Transfer:: Sending files to a remote system
* Server:: Using the gdbserver program
* Remote Configuration:: Remote configuration
* Remote Stub:: Implementing a remote stub
File: gdb.info, Node: Connecting, Next: File Transfer, Up: Remote Debugging
17.1 Connecting to a Remote Target
==================================
On the GDB host machine, you will need an unstripped copy of your
program, since GDB needs symbol and debugging information. Start up
GDB as usual, using the name of the local copy of your program as the
first argument.
GDB can communicate with the target over a serial line, or over an
IP network using TCP or UDP. In each case, GDB uses the same protocol
for debugging your program; only the medium carrying the debugging
packets varies. The `target remote' command establishes a connection
to the target. Its arguments indicate which medium to use:
`target remote SERIAL-DEVICE'
Use SERIAL-DEVICE to communicate with the target. For example, to
use a serial line connected to the device named `/dev/ttyb':
target remote /dev/ttyb
If you're using a serial line, you may want to give GDB the
`--baud' option, or use the `set remotebaud' command (*note set
remotebaud: Remote Configuration.) before the `target' command.
`target remote `HOST:PORT''
`target remote `tcp:HOST:PORT''
Debug using a TCP connection to PORT on HOST. The HOST may be
either a host name or a numeric IP address; PORT must be a decimal
number. The HOST could be the target machine itself, if it is
directly connected to the net, or it might be a terminal server
which in turn has a serial line to the target.
For example, to connect to port 2828 on a terminal server named
`manyfarms':
target remote manyfarms:2828
If your remote target is actually running on the same machine as
your debugger session (e.g. a simulator for your target running on
the same host), you can omit the hostname. For example, to
connect to port 1234 on your local machine:
target remote :1234
Note that the colon is still required here.
`target remote `udp:HOST:PORT''
Debug using UDP packets to PORT on HOST. For example, to connect
to UDP port 2828 on a terminal server named `manyfarms':
target remote udp:manyfarms:2828
When using a UDP connection for remote debugging, you should keep
in mind that the `U' stands for "Unreliable". UDP can silently
drop packets on busy or unreliable networks, which will cause
havoc with your debugging session.
`target remote | COMMAND'
Run COMMAND in the background and communicate with it using a
pipe. The COMMAND is a shell command, to be parsed and expanded
by the system's command shell, `/bin/sh'; it should expect remote
protocol packets on its standard input, and send replies on its
standard output. You could use this to run a stand-alone simulator
that speaks the remote debugging protocol, to make net connections
using programs like `ssh', or for other similar tricks.
If COMMAND closes its standard output (perhaps by exiting), GDB
will try to send it a `SIGTERM' signal. (If the program has
already exited, this will have no effect.)
Once the connection has been established, you can use all the usual
commands to examine and change data and to step and continue the remote
program.
Whenever GDB is waiting for the remote program, if you type the
interrupt character (often `Ctrl-c'), GDB attempts to stop the program.
This may or may not succeed, depending in part on the hardware and the
serial drivers the remote system uses. If you type the interrupt
character once again, GDB displays this prompt:
Interrupted while waiting for the program.
Give up (and stop debugging it)? (y or n)
If you type `y', GDB abandons the remote debugging session. (If you
decide you want to try again later, you can use `target remote' again
to connect once more.) If you type `n', GDB goes back to waiting.
`detach'
When you have finished debugging the remote program, you can use
the `detach' command to release it from GDB control. Detaching
from the target normally resumes its execution, but the results
will depend on your particular remote stub. After the `detach'
command, GDB is free to connect to another target.
`disconnect'
The `disconnect' command behaves like `detach', except that the
target is generally not resumed. It will wait for GDB (this
instance or another one) to connect and continue debugging. After
the `disconnect' command, GDB is again free to connect to another
target.
`monitor CMD'
This command allows you to send arbitrary commands directly to the
remote monitor. Since GDB doesn't care about the commands it
sends like this, this command is the way to extend GDB--you can
add new commands that only the external monitor will understand
and implement.
File: gdb.info, Node: File Transfer, Next: Server, Prev: Connecting, Up: Remote Debugging
17.2 Sending files to a remote system
=====================================
Some remote targets offer the ability to transfer files over the same
connection used to communicate with GDB. This is convenient for
targets accessible through other means, e.g. GNU/Linux systems running
`gdbserver' over a network interface. For other targets, e.g. embedded
devices with only a single serial port, this may be the only way to
upload or download files.
Not all remote targets support these commands.
`remote put HOSTFILE TARGETFILE'
Copy file HOSTFILE from the host system (the machine running GDB)
to TARGETFILE on the target system.
`remote get TARGETFILE HOSTFILE'
Copy file TARGETFILE from the target system to HOSTFILE on the
host system.
`remote delete TARGETFILE'
Delete TARGETFILE from the target system.
File: gdb.info, Node: Server, Next: Remote Configuration, Prev: File Transfer, Up: Remote Debugging
17.3 Using the `gdbserver' Program
==================================
`gdbserver' is a control program for Unix-like systems, which allows
you to connect your program with a remote GDB via `target remote'--but
without linking in the usual debugging stub.
`gdbserver' is not a complete replacement for the debugging stubs,
because it requires essentially the same operating-system facilities
that GDB itself does. In fact, a system that can run `gdbserver' to
connect to a remote GDB could also run GDB locally! `gdbserver' is
sometimes useful nevertheless, because it is a much smaller program
than GDB itself. It is also easier to port than all of GDB, so you may
be able to get started more quickly on a new system by using
`gdbserver'. Finally, if you develop code for real-time systems, you
may find that the tradeoffs involved in real-time operation make it
more convenient to do as much development work as possible on another
system, for example by cross-compiling. You can use `gdbserver' to
make a similar choice for debugging.
GDB and `gdbserver' communicate via either a serial line or a TCP
connection, using the standard GDB remote serial protocol.
_Warning:_ `gdbserver' does not have any built-in security. Do
not run `gdbserver' connected to any public network; a GDB
connection to `gdbserver' provides access to the target system
with the same privileges as the user running `gdbserver'.
17.3.1 Running `gdbserver'
--------------------------
Run `gdbserver' on the target system. You need a copy of the program
you want to debug, including any libraries it requires. `gdbserver'
does not need your program's symbol table, so you can strip the program
if necessary to save space. GDB on the host system does all the symbol
handling.
To use the server, you must tell it how to communicate with GDB; the
name of your program; and the arguments for your program. The usual
syntax is:
target> gdbserver COMM PROGRAM [ ARGS ... ]
COMM is either a device name (to use a serial line) or a TCP
hostname and portnumber. For example, to debug Emacs with the argument
`foo.txt' and communicate with GDB over the serial port `/dev/com1':
target> gdbserver /dev/com1 emacs foo.txt
`gdbserver' waits passively for the host GDB to communicate with it.
To use a TCP connection instead of a serial line:
target> gdbserver host:2345 emacs foo.txt
The only difference from the previous example is the first argument,
specifying that you are communicating with the host GDB via TCP. The
`host:2345' argument means that `gdbserver' is to expect a TCP
connection from machine `host' to local TCP port 2345. (Currently, the
`host' part is ignored.) You can choose any number you want for the
port number as long as it does not conflict with any TCP ports already
in use on the target system (for example, `23' is reserved for
`telnet').(1) You must use the same port number with the host GDB
`target remote' command.
17.3.1.1 Attaching to a Running Program
.......................................
On some targets, `gdbserver' can also attach to running programs. This
is accomplished via the `--attach' argument. The syntax is:
target> gdbserver --attach COMM PID
PID is the process ID of a currently running process. It isn't
necessary to point `gdbserver' at a binary for the running process.
You can debug processes by name instead of process ID if your target
has the `pidof' utility:
target> gdbserver --attach COMM `pidof PROGRAM`
In case more than one copy of PROGRAM is running, or PROGRAM has
multiple threads, most versions of `pidof' support the `-s' option to
only return the first process ID.
17.3.1.2 Multi-Process Mode for `gdbserver'
...........................................
When you connect to `gdbserver' using `target remote', `gdbserver'
debugs the specified program only once. When the program exits, or you
detach from it, GDB closes the connection and `gdbserver' exits.
If you connect using `target extended-remote', `gdbserver' enters
multi-process mode. When the debugged program exits, or you detach
from it, GDB stays connected to `gdbserver' even though no program is
running. The `run' and `attach' commands instruct `gdbserver' to run
or attach to a new program. The `run' command uses `set remote
exec-file' (*note set remote exec-file::) to select the program to run.
Command line arguments are supported, except for wildcard expansion
and I/O redirection (*note Arguments::).
To start `gdbserver' without supplying an initial command to run or
process ID to attach, use the `--multi' command line option. Then you
can connect using `target extended-remote' and start the program you
want to debug.
`gdbserver' does not automatically exit in multi-process mode. You
can terminate it by using `monitor exit' (*note Monitor Commands for
gdbserver::).
17.3.1.3 Other Command-Line Arguments for `gdbserver'
.....................................................
You can include `--debug' on the `gdbserver' command line. `gdbserver'
will display extra status information about the debugging process.
This option is intended for `gdbserver' development and for bug reports
to the developers.
17.3.2 Connecting to `gdbserver'
--------------------------------
Run GDB on the host system.
First make sure you have the necessary symbol files. Load symbols
for your application using the `file' command before you connect. Use
`set sysroot' to locate target libraries (unless your GDB was compiled
with the correct sysroot using `--with-sysroot').
The symbol file and target libraries must exactly match the
executable and libraries on the target, with one exception: the files
on the host system should not be stripped, even if the files on the
target system are. Mismatched or missing files will lead to confusing
results during debugging. On GNU/Linux targets, mismatched or missing
files may also prevent `gdbserver' from debugging multi-threaded
programs.
Connect to your target (*note Connecting to a Remote Target:
Connecting.). For TCP connections, you must start up `gdbserver' prior
to using the `target remote' command. Otherwise you may get an error
whose text depends on the host system, but which usually looks
something like `Connection refused'. Don't use the `load' command in
GDB when using `gdbserver', since the program is already on the target.
17.3.3 Monitor Commands for `gdbserver'
---------------------------------------
During a GDB session using `gdbserver', you can use the `monitor'
command to send special requests to `gdbserver'. Here are the
available commands.
`monitor help'
List the available monitor commands.
`monitor set debug 0'
`monitor set debug 1'
Disable or enable general debugging messages.
`monitor set remote-debug 0'
`monitor set remote-debug 1'
Disable or enable specific debugging messages associated with the
remote protocol (*note Remote Protocol::).
`monitor exit'
Tell gdbserver to exit immediately. This command should be
followed by `disconnect' to close the debugging session.
`gdbserver' will detach from any attached processes and kill any
processes it created. Use `monitor exit' to terminate `gdbserver'
at the end of a multi-process mode debug session.
---------- Footnotes ----------
(1) If you choose a port number that conflicts with another service,
`gdbserver' prints an error message and exits.
File: gdb.info, Node: Remote Configuration, Next: Remote Stub, Prev: Server, Up: Remote Debugging
17.4 Remote Configuration
=========================
This section documents the configuration options available when
debugging remote programs. For the options related to the File I/O
extensions of the remote protocol, see *Note system-call-allowed:
system.
`set remoteaddresssize BITS'
Set the maximum size of address in a memory packet to the specified
number of bits. GDB will mask off the address bits above that
number, when it passes addresses to the remote target. The
default value is the number of bits in the target's address.
`show remoteaddresssize'
Show the current value of remote address size in bits.
`set remotebaud N'
Set the baud rate for the remote serial I/O to N baud. The value
is used to set the speed of the serial port used for debugging
remote targets.
`show remotebaud'
Show the current speed of the remote connection.
`set remotebreak'
If set to on, GDB sends a `BREAK' signal to the remote when you
type `Ctrl-c' to interrupt the program running on the remote. If
set to off, GDB sends the `Ctrl-C' character instead. The default
is off, since most remote systems expect to see `Ctrl-C' as the
interrupt signal.
`show remotebreak'
Show whether GDB sends `BREAK' or `Ctrl-C' to interrupt the remote
program.
`set remoteflow on'
`set remoteflow off'
Enable or disable hardware flow control (`RTS'/`CTS') on the
serial port used to communicate to the remote target.
`show remoteflow'
Show the current setting of hardware flow control.
`set remotelogbase BASE'
Set the base (a.k.a. radix) of logging serial protocol
communications to BASE. Supported values of BASE are: `ascii',
`octal', and `hex'. The default is `ascii'.
`show remotelogbase'
Show the current setting of the radix for logging remote serial
protocol.
`set remotelogfile FILE'
Record remote serial communications on the named FILE. The
default is not to record at all.
`show remotelogfile.'
Show the current setting of the file name on which to record the
serial communications.
`set remotetimeout NUM'
Set the timeout limit to wait for the remote target to respond to
NUM seconds. The default is 2 seconds.
`show remotetimeout'
Show the current number of seconds to wait for the remote target
responses.
`set remote hardware-watchpoint-limit LIMIT'
`set remote hardware-breakpoint-limit LIMIT'
Restrict GDB to using LIMIT remote hardware breakpoint or
watchpoints. A limit of -1, the default, is treated as unlimited.
`set remote exec-file FILENAME'
`show remote exec-file'
Select the file used for `run' with `target extended-remote'.
This should be set to a filename valid on the target system. If
it is not set, the target will use a default filename (e.g. the
last program run).
The GDB remote protocol autodetects the packets supported by your
debugging stub. If you need to override the autodetection, you can use
these commands to enable or disable individual packets. Each packet
can be set to `on' (the remote target supports this packet), `off' (the
remote target does not support this packet), or `auto' (detect remote
target support for this packet). They all default to `auto'. For more
information about each packet, see *Note Remote Protocol::.
During normal use, you should not have to use any of these commands.
If you do, that may be a bug in your remote debugging stub, or a bug in
GDB. You may want to report the problem to the GDB developers.
For each packet NAME, the command to enable or disable the packet is
`set remote NAME-packet'. The available settings are:
Command Name Remote Packet Related Features
`fetch-register' `p' `info registers'
`set-register' `P' `set'
`binary-download' `X' `load', `set'
`read-aux-vector' `qXfer:auxv:read' `info auxv'
`symbol-lookup' `qSymbol' Detecting
multiple threads
`attach' `vAttach' `attach'
`verbose-resume' `vCont' Stepping or
resuming multiple
threads
`run' `vRun' `run'
`software-breakpoint'`Z0' `break'
`hardware-breakpoint'`Z1' `hbreak'
`write-watchpoint' `Z2' `watch'
`read-watchpoint' `Z3' `rwatch'
`access-watchpoint' `Z4' `awatch'
`target-features' `qXfer:features:read' `set architecture'
`library-info' `qXfer:libraries:read' `info
sharedlibrary'
`memory-map' `qXfer:memory-map:read' `info mem'
`read-spu-object' `qXfer:spu:read' `info spu'
`write-spu-object' `qXfer:spu:write' `info spu'
`get-thread-local- `qGetTLSAddr' Displaying
storage-address' `__thread'
variables
`supported-packets' `qSupported' Remote
communications
parameters
`pass-signals' `QPassSignals' `handle SIGNAL'
`hostio-close-packet'`vFile:close' `remote get',
`remote put'
`hostio-open-packet' `vFile:open' `remote get',
`remote put'
`hostio-pread-packet'`vFile:pread' `remote get',
`remote put'
`hostio-pwrite-packet'`vFile:pwrite' `remote get',
`remote put'
`hostio-unlink-packet'`vFile:unlink' `remote delete'
File: gdb.info, Node: Remote Stub, Prev: Remote Configuration, Up: Remote Debugging
17.5 Implementing a Remote Stub
===============================
The stub files provided with GDB implement the target side of the
communication protocol, and the GDB side is implemented in the GDB
source file `remote.c'. Normally, you can simply allow these
subroutines to communicate, and ignore the details. (If you're
implementing your own stub file, you can still ignore the details: start
with one of the existing stub files. `sparc-stub.c' is the best
organized, and therefore the easiest to read.)
To debug a program running on another machine (the debugging
"target" machine), you must first arrange for all the usual
prerequisites for the program to run by itself. For example, for a C
program, you need:
1. A startup routine to set up the C runtime environment; these
usually have a name like `crt0'. The startup routine may be
supplied by your hardware supplier, or you may have to write your
own.
2. A C subroutine library to support your program's subroutine calls,
notably managing input and output.
3. A way of getting your program to the other machine--for example, a
download program. These are often supplied by the hardware
manufacturer, but you may have to write your own from hardware
documentation.
The next step is to arrange for your program to use a serial port to
communicate with the machine where GDB is running (the "host" machine).
In general terms, the scheme looks like this:
_On the host,_
GDB already understands how to use this protocol; when everything
else is set up, you can simply use the `target remote' command
(*note Specifying a Debugging Target: Targets.).
_On the target,_
you must link with your program a few special-purpose subroutines
that implement the GDB remote serial protocol. The file
containing these subroutines is called a "debugging stub".
On certain remote targets, you can use an auxiliary program
`gdbserver' instead of linking a stub into your program. *Note
Using the `gdbserver' Program: Server, for details.
The debugging stub is specific to the architecture of the remote
machine; for example, use `sparc-stub.c' to debug programs on SPARC
boards.
These working remote stubs are distributed with GDB:
`i386-stub.c'
For Intel 386 and compatible architectures.
`m68k-stub.c'
For Motorola 680x0 architectures.
`sh-stub.c'
For Renesas SH architectures.
`sparc-stub.c'
For SPARC architectures.
`sparcl-stub.c'
For Fujitsu SPARCLITE architectures.
The `README' file in the GDB distribution may list other recently
added stubs.
* Menu:
* Stub Contents:: What the stub can do for you
* Bootstrapping:: What you must do for the stub
* Debug Session:: Putting it all together
File: gdb.info, Node: Stub Contents, Next: Bootstrapping, Up: Remote Stub
17.5.1 What the Stub Can Do for You
-----------------------------------
The debugging stub for your architecture supplies these three
subroutines:
`set_debug_traps'
This routine arranges for `handle_exception' to run when your
program stops. You must call this subroutine explicitly near the
beginning of your program.
`handle_exception'
This is the central workhorse, but your program never calls it
explicitly--the setup code arranges for `handle_exception' to run
when a trap is triggered.
`handle_exception' takes control when your program stops during
execution (for example, on a breakpoint), and mediates
communications with GDB on the host machine. This is where the
communications protocol is implemented; `handle_exception' acts as
the GDB representative on the target machine. It begins by
sending summary information on the state of your program, then
continues to execute, retrieving and transmitting any information
GDB needs, until you execute a GDB command that makes your program
resume; at that point, `handle_exception' returns control to your
own code on the target machine.
`breakpoint'
Use this auxiliary subroutine to make your program contain a
breakpoint. Depending on the particular situation, this may be
the only way for GDB to get control. For instance, if your target
machine has some sort of interrupt button, you won't need to call
this; pressing the interrupt button transfers control to
`handle_exception'--in effect, to GDB. On some machines, simply
receiving characters on the serial port may also trigger a trap;
again, in that situation, you don't need to call `breakpoint' from
your own program--simply running `target remote' from the host GDB
session gets control.
Call `breakpoint' if none of these is true, or if you simply want
to make certain your program stops at a predetermined point for the
start of your debugging session.
File: gdb.info, Node: Bootstrapping, Next: Debug Session, Prev: Stub Contents, Up: Remote Stub
17.5.2 What You Must Do for the Stub
------------------------------------
The debugging stubs that come with GDB are set up for a particular chip
architecture, but they have no information about the rest of your
debugging target machine.
First of all you need to tell the stub how to communicate with the
serial port.
`int getDebugChar()'
Write this subroutine to read a single character from the serial
port. It may be identical to `getchar' for your target system; a
different name is used to allow you to distinguish the two if you
wish.
`void putDebugChar(int)'
Write this subroutine to write a single character to the serial
port. It may be identical to `putchar' for your target system; a
different name is used to allow you to distinguish the two if you
wish.
If you want GDB to be able to stop your program while it is running,
you need to use an interrupt-driven serial driver, and arrange for it
to stop when it receives a `^C' (`\003', the control-C character).
That is the character which GDB uses to tell the remote system to stop.
Getting the debugging target to return the proper status to GDB
probably requires changes to the standard stub; one quick and dirty way
is to just execute a breakpoint instruction (the "dirty" part is that
GDB reports a `SIGTRAP' instead of a `SIGINT').
Other routines you need to supply are:
`void exceptionHandler (int EXCEPTION_NUMBER, void *EXCEPTION_ADDRESS)'
Write this function to install EXCEPTION_ADDRESS in the exception
handling tables. You need to do this because the stub does not
have any way of knowing what the exception handling tables on your
target system are like (for example, the processor's table might
be in ROM, containing entries which point to a table in RAM).
EXCEPTION_NUMBER is the exception number which should be changed;
its meaning is architecture-dependent (for example, different
numbers might represent divide by zero, misaligned access, etc).
When this exception occurs, control should be transferred directly
to EXCEPTION_ADDRESS, and the processor state (stack, registers,
and so on) should be just as it is when a processor exception
occurs. So if you want to use a jump instruction to reach
EXCEPTION_ADDRESS, it should be a simple jump, not a jump to
subroutine.
For the 386, EXCEPTION_ADDRESS should be installed as an interrupt
gate so that interrupts are masked while the handler runs. The
gate should be at privilege level 0 (the most privileged level).
The SPARC and 68k stubs are able to mask interrupts themselves
without help from `exceptionHandler'.
`void flush_i_cache()'
On SPARC and SPARCLITE only, write this subroutine to flush the
instruction cache, if any, on your target machine. If there is no
instruction cache, this subroutine may be a no-op.
On target machines that have instruction caches, GDB requires this
function to make certain that the state of your program is stable.
You must also make sure this library routine is available:
`void *memset(void *, int, int)'
This is the standard library function `memset' that sets an area of
memory to a known value. If you have one of the free versions of
`libc.a', `memset' can be found there; otherwise, you must either
obtain it from your hardware manufacturer, or write your own.
If you do not use the GNU C compiler, you may need other standard
library subroutines as well; this varies from one stub to another, but
in general the stubs are likely to use any of the common library
subroutines which `GCC' generates as inline code.
File: gdb.info, Node: Debug Session, Prev: Bootstrapping, Up: Remote Stub
17.5.3 Putting it All Together
------------------------------
In summary, when your program is ready to debug, you must follow these
steps.
1. Make sure you have defined the supporting low-level routines
(*note What You Must Do for the Stub: Bootstrapping.):
`getDebugChar', `putDebugChar',
`flush_i_cache', `memset', `exceptionHandler'.
2. Insert these lines near the top of your program:
set_debug_traps();
breakpoint();
3. For the 680x0 stub only, you need to provide a variable called
`exceptionHook'. Normally you just use:
void (*exceptionHook)() = 0;
but if before calling `set_debug_traps', you set it to point to a
function in your program, that function is called when `GDB'
continues after stopping on a trap (for example, bus error). The
function indicated by `exceptionHook' is called with one
parameter: an `int' which is the exception number.
4. Compile and link together: your program, the GDB debugging stub for
your target architecture, and the supporting subroutines.
5. Make sure you have a serial connection between your target machine
and the GDB host, and identify the serial port on the host.
6. Download your program to your target machine (or get it there by
whatever means the manufacturer provides), and start it.
7. Start GDB on the host, and connect to the target (*note Connecting
to a Remote Target: Connecting.).
File: gdb.info, Node: Configurations, Next: Controlling GDB, Prev: Remote Debugging, Up: Top
18 Configuration-Specific Information
*************************************
While nearly all GDB commands are available for all native and cross
versions of the debugger, there are some exceptions. This chapter
describes things that are only available in certain configurations.
There are three major categories of configurations: native
configurations, where the host and target are the same, embedded
operating system configurations, which are usually the same for several
different processor architectures, and bare embedded processors, which
are quite different from each other.
* Menu:
* Native::
* Embedded OS::
* Embedded Processors::
* Architectures::
File: gdb.info, Node: Native, Next: Embedded OS, Up: Configurations
18.1 Native
===========
This section describes details specific to particular native
configurations.
* Menu:
* HP-UX:: HP-UX
* BSD libkvm Interface:: Debugging BSD kernel memory images
* SVR4 Process Information:: SVR4 process information
* DJGPP Native:: Features specific to the DJGPP port
* Cygwin Native:: Features specific to the Cygwin port
* Hurd Native:: Features specific to GNU Hurd
* Neutrino:: Features specific to QNX Neutrino
File: gdb.info, Node: HP-UX, Next: BSD libkvm Interface, Up: Native
18.1.1 HP-UX
------------
On HP-UX systems, if you refer to a function or variable name that
begins with a dollar sign, GDB searches for a user or system name
first, before it searches for a convenience variable.
File: gdb.info, Node: BSD libkvm Interface, Next: SVR4 Process Information, Prev: HP-UX, Up: Native
18.1.2 BSD libkvm Interface
---------------------------
BSD-derived systems (FreeBSD/NetBSD/OpenBSD) have a kernel memory
interface that provides a uniform interface for accessing kernel virtual
memory images, including live systems and crash dumps. GDB uses this
interface to allow you to debug live kernels and kernel crash dumps on
many native BSD configurations. This is implemented as a special `kvm'
debugging target. For debugging a live system, load the currently
running kernel into GDB and connect to the `kvm' target:
(gdb) target kvm
For debugging crash dumps, provide the file name of the crash dump
as an argument:
(gdb) target kvm /var/crash/bsd.0
Once connected to the `kvm' target, the following commands are
available:
`kvm pcb'
Set current context from the "Process Control Block" (PCB) address.
`kvm proc'
Set current context from proc address. This command isn't
available on modern FreeBSD systems.
File: gdb.info, Node: SVR4 Process Information, Next: DJGPP Native, Prev: BSD libkvm Interface, Up: Native
18.1.3 SVR4 Process Information
-------------------------------
Many versions of SVR4 and compatible systems provide a facility called
`/proc' that can be used to examine the image of a running process
using file-system subroutines. If GDB is configured for an operating
system with this facility, the command `info proc' is available to
report information about the process running your program, or about any
process running on your system. `info proc' works only on SVR4 systems
that include the `procfs' code. This includes, as of this writing,
GNU/Linux, OSF/1 (Digital Unix), Solaris, Irix, and Unixware, but not
HP-UX, for example.
`info proc'
`info proc PROCESS-ID'
Summarize available information about any running process. If a
process ID is specified by PROCESS-ID, display information about
that process; otherwise display information about the program being
debugged. The summary includes the debugged process ID, the
command line used to invoke it, its current working directory, and
its executable file's absolute file name.
On some systems, PROCESS-ID can be of the form `[PID]/TID' which
specifies a certain thread ID within a process. If the optional
PID part is missing, it means a thread from the process being
debugged (the leading `/' still needs to be present, or else GDB
will interpret the number as a process ID rather than a thread ID).
`info proc mappings'
Report the memory address space ranges accessible in the program,
with information on whether the process has read, write, or
execute access rights to each range. On GNU/Linux systems, each
memory range includes the object file which is mapped to that
range, instead of the memory access rights to that range.
`info proc stat'
`info proc status'
These subcommands are specific to GNU/Linux systems. They show
the process-related information, including the user ID and group
ID; how many threads are there in the process; its virtual memory
usage; the signals that are pending, blocked, and ignored; its
TTY; its consumption of system and user time; its stack size; its
`nice' value; etc. For more information, see the `proc' man page
(type `man 5 proc' from your shell prompt).
`info proc all'
Show all the information about the process described under all of
the above `info proc' subcommands.
`set procfs-trace'
This command enables and disables tracing of `procfs' API calls.
`show procfs-trace'
Show the current state of `procfs' API call tracing.
`set procfs-file FILE'
Tell GDB to write `procfs' API trace to the named FILE. GDB
appends the trace info to the previous contents of the file. The
default is to display the trace on the standard output.
`show procfs-file'
Show the file to which `procfs' API trace is written.
`proc-trace-entry'
`proc-trace-exit'
`proc-untrace-entry'
`proc-untrace-exit'
These commands enable and disable tracing of entries into and exits
from the `syscall' interface.
`info pidlist'
For QNX Neutrino only, this command displays the list of all the
processes and all the threads within each process.
`info meminfo'
For QNX Neutrino only, this command displays the list of all
mapinfos.
File: gdb.info, Node: DJGPP Native, Next: Cygwin Native, Prev: SVR4 Process Information, Up: Native
18.1.4 Features for Debugging DJGPP Programs
--------------------------------------------
DJGPP is a port of the GNU development tools to MS-DOS and MS-Windows.
DJGPP programs are 32-bit protected-mode programs that use the "DPMI"
(DOS Protected-Mode Interface) API to run on top of real-mode DOS
systems and their emulations.
GDB supports native debugging of DJGPP programs, and defines a few
commands specific to the DJGPP port. This subsection describes those
commands.
`info dos'
This is a prefix of DJGPP-specific commands which print
information about the target system and important OS structures.
`info dos sysinfo'
This command displays assorted information about the underlying
platform: the CPU type and features, the OS version and flavor, the
DPMI version, and the available conventional and DPMI memory.
`info dos gdt'
`info dos ldt'
`info dos idt'
These 3 commands display entries from, respectively, Global, Local,
and Interrupt Descriptor Tables (GDT, LDT, and IDT). The
descriptor tables are data structures which store a descriptor for
each segment that is currently in use. The segment's selector is
an index into a descriptor table; the table entry for that index
holds the descriptor's base address and limit, and its attributes
and access rights.
A typical DJGPP program uses 3 segments: a code segment, a data
segment (used for both data and the stack), and a DOS segment
(which allows access to DOS/BIOS data structures and absolute
addresses in conventional memory). However, the DPMI host will
usually define additional segments in order to support the DPMI
environment.
These commands allow to display entries from the descriptor tables.
Without an argument, all entries from the specified table are
displayed. An argument, which should be an integer expression,
means display a single entry whose index is given by the argument.
For example, here's a convenient way to display information about
the debugged program's data segment:
`(gdb) info dos ldt $ds'
`0x13f: base=0x11970000 limit=0x0009ffff 32-Bit Data (Read/Write, Exp-up)'
This comes in handy when you want to see whether a pointer is
outside the data segment's limit (i.e. "garbled").
`info dos pde'
`info dos pte'
These two commands display entries from, respectively, the Page
Directory and the Page Tables. Page Directories and Page Tables
are data structures which control how virtual memory addresses are
mapped into physical addresses. A Page Table includes an entry
for every page of memory that is mapped into the program's address
space; there may be several Page Tables, each one holding up to
4096 entries. A Page Directory has up to 4096 entries, one each
for every Page Table that is currently in use.
Without an argument, `info dos pde' displays the entire Page
Directory, and `info dos pte' displays all the entries in all of
the Page Tables. An argument, an integer expression, given to the
`info dos pde' command means display only that entry from the Page
Directory table. An argument given to the `info dos pte' command
means display entries from a single Page Table, the one pointed to
by the specified entry in the Page Directory.
These commands are useful when your program uses "DMA" (Direct
Memory Access), which needs physical addresses to program the DMA
controller.
These commands are supported only with some DPMI servers.
`info dos address-pte ADDR'
This command displays the Page Table entry for a specified linear
address. The argument ADDR is a linear address which should
already have the appropriate segment's base address added to it,
because this command accepts addresses which may belong to _any_
segment. For example, here's how to display the Page Table entry
for the page where a variable `i' is stored:
`(gdb) info dos address-pte __djgpp_base_address + (char *)&i'
`Page Table entry for address 0x11a00d30:'
`Base=0x02698000 Dirty Acc. Not-Cached Write-Back Usr Read-Write +0xd30'
This says that `i' is stored at offset `0xd30' from the page whose
physical base address is `0x02698000', and shows all the
attributes of that page.
Note that you must cast the addresses of variables to a `char *',
since otherwise the value of `__djgpp_base_address', the base
address of all variables and functions in a DJGPP program, will be
added using the rules of C pointer arithmetics: if `i' is declared
an `int', GDB will add 4 times the value of `__djgpp_base_address'
to the address of `i'.
Here's another example, it displays the Page Table entry for the
transfer buffer:
`(gdb) info dos address-pte *((unsigned *)&_go32_info_block + 3)'
`Page Table entry for address 0x29110:'
`Base=0x00029000 Dirty Acc. Not-Cached Write-Back Usr Read-Write +0x110'
(The `+ 3' offset is because the transfer buffer's address is the
3rd member of the `_go32_info_block' structure.) The output
clearly shows that this DPMI server maps the addresses in
conventional memory 1:1, i.e. the physical (`0x00029000' +
`0x110') and linear (`0x29110') addresses are identical.
This command is supported only with some DPMI servers.
In addition to native debugging, the DJGPP port supports remote
debugging via a serial data link. The following commands are specific
to remote serial debugging in the DJGPP port of GDB.
`set com1base ADDR'
This command sets the base I/O port address of the `COM1' serial
port.
`set com1irq IRQ'
This command sets the "Interrupt Request" (`IRQ') line to use for
the `COM1' serial port.
There are similar commands `set com2base', `set com3irq', etc. for
setting the port address and the `IRQ' lines for the other 3 COM
ports.
The related commands `show com1base', `show com1irq' etc. display
the current settings of the base address and the `IRQ' lines used
by the COM ports.
`info serial'
This command prints the status of the 4 DOS serial ports. For each
port, it prints whether it's active or not, its I/O base address
and IRQ number, whether it uses a 16550-style FIFO, its baudrate,
and the counts of various errors encountered so far.
File: gdb.info, Node: Cygwin Native, Next: Hurd Native, Prev: DJGPP Native, Up: Native
18.1.5 Features for Debugging MS Windows PE Executables
-------------------------------------------------------
GDB supports native debugging of MS Windows programs, including DLLs
with and without symbolic debugging information. There are various
additional Cygwin-specific commands, described in this section.
Working with DLLs that have no debugging symbols is described in *Note
Non-debug DLL Symbols::.
`info w32'
This is a prefix of MS Windows-specific commands which print
information about the target system and important OS structures.
`info w32 selector'
This command displays information returned by the Win32 API
`GetThreadSelectorEntry' function. It takes an optional argument
that is evaluated to a long value to give the information about
this given selector. Without argument, this command displays
information about the six segment registers.
`info dll'
This is a Cygwin-specific alias of `info shared'.
`dll-symbols'
This command loads symbols from a dll similarly to add-sym command
but without the need to specify a base address.
`set cygwin-exceptions MODE'
If MODE is `on', GDB will break on exceptions that happen inside
the Cygwin DLL. If MODE is `off', GDB will delay recognition of
exceptions, and may ignore some exceptions which seem to be caused
by internal Cygwin DLL "bookkeeping". This option is meant
primarily for debugging the Cygwin DLL itself; the default value
is `off' to avoid annoying GDB users with false `SIGSEGV' signals.
`show cygwin-exceptions'
Displays whether GDB will break on exceptions that happen inside
the Cygwin DLL itself.
`set new-console MODE'
If MODE is `on' the debuggee will be started in a new console on
next start. If MODE is `off'i, the debuggee will be started in
the same console as the debugger.
`show new-console'
Displays whether a new console is used when the debuggee is
started.
`set new-group MODE'
This boolean value controls whether the debuggee should start a
new group or stay in the same group as the debugger. This affects
the way the Windows OS handles `Ctrl-C'.
`show new-group'
Displays current value of new-group boolean.
`set debugevents'
This boolean value adds debug output concerning kernel events
related to the debuggee seen by the debugger. This includes
events that signal thread and process creation and exit, DLL
loading and unloading, console interrupts, and debugging messages
produced by the Windows `OutputDebugString' API call.
`set debugexec'
This boolean value adds debug output concerning execute events
(such as resume thread) seen by the debugger.
`set debugexceptions'
This boolean value adds debug output concerning exceptions in the
debuggee seen by the debugger.
`set debugmemory'
This boolean value adds debug output concerning debuggee memory
reads and writes by the debugger.
`set shell'
This boolean values specifies whether the debuggee is called via a
shell or directly (default value is on).
`show shell'
Displays if the debuggee will be started with a shell.
* Menu:
* Non-debug DLL Symbols:: Support for DLLs without debugging symbols
File: gdb.info, Node: Non-debug DLL Symbols, Up: Cygwin Native
18.1.5.1 Support for DLLs without Debugging Symbols
...................................................
Very often on windows, some of the DLLs that your program relies on do
not include symbolic debugging information (for example,
`kernel32.dll'). When GDB doesn't recognize any debugging symbols in a
DLL, it relies on the minimal amount of symbolic information contained
in the DLL's export table. This section describes working with such
symbols, known internally to GDB as "minimal symbols".
Note that before the debugged program has started execution, no DLLs
will have been loaded. The easiest way around this problem is simply to
start the program -- either by setting a breakpoint or letting the
program run once to completion. It is also possible to force GDB to
load a particular DLL before starting the executable -- see the shared
library information in *Note Files::, or the `dll-symbols' command in
*Note Cygwin Native::. Currently, explicitly loading symbols from a
DLL with no debugging information will cause the symbol names to be
duplicated in GDB's lookup table, which may adversely affect symbol
lookup performance.
18.1.5.2 DLL Name Prefixes
..........................
In keeping with the naming conventions used by the Microsoft debugging
tools, DLL export symbols are made available with a prefix based on the
DLL name, for instance `KERNEL32!CreateFileA'. The plain name is also
entered into the symbol table, so `CreateFileA' is often sufficient. In
some cases there will be name clashes within a program (particularly if
the executable itself includes full debugging symbols) necessitating
the use of the fully qualified name when referring to the contents of
the DLL. Use single-quotes around the name to avoid the exclamation
mark ("!") being interpreted as a language operator.
Note that the internal name of the DLL may be all upper-case, even
though the file name of the DLL is lower-case, or vice-versa. Since
symbols within GDB are _case-sensitive_ this may cause some confusion.
If in doubt, try the `info functions' and `info variables' commands or
even `maint print msymbols' (*note Symbols::). Here's an example:
(gdb) info function CreateFileA
All functions matching regular expression "CreateFileA":
Non-debugging symbols:
0x77e885f4 CreateFileA
0x77e885f4 KERNEL32!CreateFileA
(gdb) info function !
All functions matching regular expression "!":
Non-debugging symbols:
0x6100114c cygwin1!__assert
0x61004034 cygwin1!_dll_crt0@0
0x61004240 cygwin1!dll_crt0(per_process *)
[etc...]
18.1.5.3 Working with Minimal Symbols
.....................................
Symbols extracted from a DLL's export table do not contain very much
type information. All that GDB can do is guess whether a symbol refers
to a function or variable depending on the linker section that contains
the symbol. Also note that the actual contents of the memory contained
in a DLL are not available unless the program is running. This means
that you cannot examine the contents of a variable or disassemble a
function within a DLL without a running program.
Variables are generally treated as pointers and dereferenced
automatically. For this reason, it is often necessary to prefix a
variable name with the address-of operator ("&") and provide explicit
type information in the command. Here's an example of the type of
problem:
(gdb) print 'cygwin1!__argv'
$1 = 268572168
(gdb) x 'cygwin1!__argv'
0x10021610: "\230y\""
And two possible solutions:
(gdb) print ((char **)'cygwin1!__argv')[0]
$2 = 0x22fd98 "/cygdrive/c/mydirectory/myprogram"
(gdb) x/2x &'cygwin1!__argv'
0x610c0aa8 <cygwin1!__argv>: 0x10021608 0x00000000
(gdb) x/x 0x10021608
0x10021608: 0x0022fd98
(gdb) x/s 0x0022fd98
0x22fd98: "/cygdrive/c/mydirectory/myprogram"
Setting a break point within a DLL is possible even before the
program starts execution. However, under these circumstances, GDB can't
examine the initial instructions of the function in order to skip the
function's frame set-up code. You can work around this by using "*&" to
set the breakpoint at a raw memory address:
(gdb) break *&'python22!PyOS_Readline'
Breakpoint 1 at 0x1e04eff0
The author of these extensions is not entirely convinced that
setting a break point within a shared DLL like `kernel32.dll' is
completely safe.
File: gdb.info, Node: Hurd Native, Next: Neutrino, Prev: Cygwin Native, Up: Native
18.1.6 Commands Specific to GNU Hurd Systems
--------------------------------------------
This subsection describes GDB commands specific to the GNU Hurd native
debugging.
`set signals'
`set sigs'
This command toggles the state of inferior signal interception by
GDB. Mach exceptions, such as breakpoint traps, are not affected
by this command. `sigs' is a shorthand alias for `signals'.
`show signals'
`show sigs'
Show the current state of intercepting inferior's signals.
`set signal-thread'
`set sigthread'
This command tells GDB which thread is the `libc' signal thread.
That thread is run when a signal is delivered to a running
process. `set sigthread' is the shorthand alias of `set
signal-thread'.
`show signal-thread'
`show sigthread'
These two commands show which thread will run when the inferior is
delivered a signal.
`set stopped'
This commands tells GDB that the inferior process is stopped, as
with the `SIGSTOP' signal. The stopped process can be continued
by delivering a signal to it.
`show stopped'
This command shows whether GDB thinks the debuggee is stopped.
`set exceptions'
Use this command to turn off trapping of exceptions in the
inferior. When exception trapping is off, neither breakpoints nor
single-stepping will work. To restore the default, set exception
trapping on.
`show exceptions'
Show the current state of trapping exceptions in the inferior.
`set task pause'
This command toggles task suspension when GDB has control.
Setting it to on takes effect immediately, and the task is
suspended whenever GDB gets control. Setting it to off will take
effect the next time the inferior is continued. If this option is
set to off, you can use `set thread default pause on' or `set
thread pause on' (see below) to pause individual threads.
`show task pause'
Show the current state of task suspension.
`set task detach-suspend-count'
This command sets the suspend count the task will be left with when
GDB detaches from it.
`show task detach-suspend-count'
Show the suspend count the task will be left with when detaching.
`set task exception-port'
`set task excp'
This command sets the task exception port to which GDB will
forward exceptions. The argument should be the value of the "send
rights" of the task. `set task excp' is a shorthand alias.
`set noninvasive'
This command switches GDB to a mode that is the least invasive as
far as interfering with the inferior is concerned. This is the
same as using `set task pause', `set exceptions', and `set
signals' to values opposite to the defaults.
`info send-rights'
`info receive-rights'
`info port-rights'
`info port-sets'
`info dead-names'
`info ports'
`info psets'
These commands display information about, respectively, send
rights, receive rights, port rights, port sets, and dead names of
a task. There are also shorthand aliases: `info ports' for `info
port-rights' and `info psets' for `info port-sets'.
`set thread pause'
This command toggles current thread suspension when GDB has
control. Setting it to on takes effect immediately, and the
current thread is suspended whenever GDB gets control. Setting it
to off will take effect the next time the inferior is continued.
Normally, this command has no effect, since when GDB has control,
the whole task is suspended. However, if you used `set task pause
off' (see above), this command comes in handy to suspend only the
current thread.
`show thread pause'
This command shows the state of current thread suspension.
`set thread run'
This command sets whether the current thread is allowed to run.
`show thread run'
Show whether the current thread is allowed to run.
`set thread detach-suspend-count'
This command sets the suspend count GDB will leave on a thread
when detaching. This number is relative to the suspend count
found by GDB when it notices the thread; use `set thread
takeover-suspend-count' to force it to an absolute value.
`show thread detach-suspend-count'
Show the suspend count GDB will leave on the thread when detaching.
`set thread exception-port'
`set thread excp'
Set the thread exception port to which to forward exceptions. This
overrides the port set by `set task exception-port' (see above).
`set thread excp' is the shorthand alias.
`set thread takeover-suspend-count'
Normally, GDB's thread suspend counts are relative to the value
GDB finds when it notices each thread. This command changes the
suspend counts to be absolute instead.
`set thread default'
`show thread default'
Each of the above `set thread' commands has a `set thread default'
counterpart (e.g., `set thread default pause', `set thread default
exception-port', etc.). The `thread default' variety of commands
sets the default thread properties for all threads; you can then
change the properties of individual threads with the non-default
commands.
File: gdb.info, Node: Neutrino, Prev: Hurd Native, Up: Native
18.1.7 QNX Neutrino
-------------------
GDB provides the following commands specific to the QNX Neutrino target:
`set debug nto-debug'
When set to on, enables debugging messages specific to the QNX
Neutrino support.
`show debug nto-debug'
Show the current state of QNX Neutrino messages.
File: gdb.info, Node: Embedded OS, Next: Embedded Processors, Prev: Native, Up: Configurations
18.2 Embedded Operating Systems
===============================
This section describes configurations involving the debugging of
embedded operating systems that are available for several different
architectures.
* Menu:
* VxWorks:: Using GDB with VxWorks
GDB includes the ability to debug programs running on various
real-time operating systems.
File: gdb.info, Node: VxWorks, Up: Embedded OS
18.2.1 Using GDB with VxWorks
-----------------------------
`target vxworks MACHINENAME'
A VxWorks system, attached via TCP/IP. The argument MACHINENAME
is the target system's machine name or IP address.
On VxWorks, `load' links FILENAME dynamically on the current target
system as well as adding its symbols in GDB.
GDB enables developers to spawn and debug tasks running on networked
VxWorks targets from a Unix host. Already-running tasks spawned from
the VxWorks shell can also be debugged. GDB uses code that runs on
both the Unix host and on the VxWorks target. The program `gdb' is
installed and executed on the Unix host. (It may be installed with the
name `vxgdb', to distinguish it from a GDB for debugging programs on
the host itself.)
`VxWorks-timeout ARGS'
All VxWorks-based targets now support the option `vxworks-timeout'.
This option is set by the user, and ARGS represents the number of
seconds GDB waits for responses to rpc's. You might use this if
your VxWorks target is a slow software simulator or is on the far
side of a thin network line.
The following information on connecting to VxWorks was current when
this manual was produced; newer releases of VxWorks may use revised
procedures.
To use GDB with VxWorks, you must rebuild your VxWorks kernel to
include the remote debugging interface routines in the VxWorks library
`rdb.a'. To do this, define `INCLUDE_RDB' in the VxWorks configuration
file `configAll.h' and rebuild your VxWorks kernel. The resulting
kernel contains `rdb.a', and spawns the source debugging task
`tRdbTask' when VxWorks is booted. For more information on configuring
and remaking VxWorks, see the manufacturer's manual.
Once you have included `rdb.a' in your VxWorks system image and set
your Unix execution search path to find GDB, you are ready to run GDB.
From your Unix host, run `gdb' (or `vxgdb', depending on your
installation).
GDB comes up showing the prompt:
(vxgdb)
* Menu:
* VxWorks Connection:: Connecting to VxWorks
* VxWorks Download:: VxWorks download
* VxWorks Attach:: Running tasks
File: gdb.info, Node: VxWorks Connection, Next: VxWorks Download, Up: VxWorks
18.2.1.1 Connecting to VxWorks
..............................
The GDB command `target' lets you connect to a VxWorks target on the
network. To connect to a target whose host name is "`tt'", type:
(vxgdb) target vxworks tt
GDB displays messages like these:
Attaching remote machine across net...
Connected to tt.
GDB then attempts to read the symbol tables of any object modules
loaded into the VxWorks target since it was last booted. GDB locates
these files by searching the directories listed in the command search
path (*note Your Program's Environment: Environment.); if it fails to
find an object file, it displays a message such as:
prog.o: No such file or directory.
When this happens, add the appropriate directory to the search path
with the GDB command `path', and execute the `target' command again.
File: gdb.info, Node: VxWorks Download, Next: VxWorks Attach, Prev: VxWorks Connection, Up: VxWorks
18.2.1.2 VxWorks Download
.........................
If you have connected to the VxWorks target and you want to debug an
object that has not yet been loaded, you can use the GDB `load' command
to download a file from Unix to VxWorks incrementally. The object file
given as an argument to the `load' command is actually opened twice:
first by the VxWorks target in order to download the code, then by GDB
in order to read the symbol table. This can lead to problems if the
current working directories on the two systems differ. If both systems
have NFS mounted the same filesystems, you can avoid these problems by
using absolute paths. Otherwise, it is simplest to set the working
directory on both systems to the directory in which the object file
resides, and then to reference the file by its name, without any path.
For instance, a program `prog.o' may reside in `VXPATH/vw/demo/rdb' in
VxWorks and in `HOSTPATH/vw/demo/rdb' on the host. To load this
program, type this on VxWorks:
-> cd "VXPATH/vw/demo/rdb"
Then, in GDB, type:
(vxgdb) cd HOSTPATH/vw/demo/rdb
(vxgdb) load prog.o
GDB displays a response similar to this:
Reading symbol data from wherever/vw/demo/rdb/prog.o... done.
You can also use the `load' command to reload an object module after
editing and recompiling the corresponding source file. Note that this
makes GDB delete all currently-defined breakpoints, auto-displays, and
convenience variables, and to clear the value history. (This is
necessary in order to preserve the integrity of debugger's data
structures that reference the target system's symbol table.)
File: gdb.info, Node: VxWorks Attach, Prev: VxWorks Download, Up: VxWorks
18.2.1.3 Running Tasks
......................
You can also attach to an existing task using the `attach' command as
follows:
(vxgdb) attach TASK
where TASK is the VxWorks hexadecimal task ID. The task can be running
or suspended when you attach to it. Running tasks are suspended at the
time of attachment.
File: gdb.info, Node: Embedded Processors, Next: Architectures, Prev: Embedded OS, Up: Configurations
18.3 Embedded Processors
========================
This section goes into details specific to particular embedded
configurations.
Whenever a specific embedded processor has a simulator, GDB allows
to send an arbitrary command to the simulator.
`sim COMMAND'
Send an arbitrary COMMAND string to the simulator. Consult the
documentation for the specific simulator in use for information
about acceptable commands.
* Menu:
* ARM:: ARM RDI
* M32R/D:: Renesas M32R/D
* M68K:: Motorola M68K
* MIPS Embedded:: MIPS Embedded
* OpenRISC 1000:: OpenRisc 1000
* PA:: HP PA Embedded
* PowerPC Embedded:: PowerPC Embedded
* Sparclet:: Tsqware Sparclet
* Sparclite:: Fujitsu Sparclite
* Z8000:: Zilog Z8000
* AVR:: Atmel AVR
* CRIS:: CRIS
* Super-H:: Renesas Super-H
File: gdb.info, Node: ARM, Next: M32R/D, Up: Embedded Processors
18.3.1 ARM
----------
`target rdi DEV'
ARM Angel monitor, via RDI library interface to ADP protocol. You
may use this target to communicate with both boards running the
Angel monitor, or with the EmbeddedICE JTAG debug device.
`target rdp DEV'
ARM Demon monitor.
GDB provides the following ARM-specific commands:
`set arm disassembler'
This commands selects from a list of disassembly styles. The
`"std"' style is the standard style.
`show arm disassembler'
Show the current disassembly style.
`set arm apcs32'
This command toggles ARM operation mode between 32-bit and 26-bit.
`show arm apcs32'
Display the current usage of the ARM 32-bit mode.
`set arm fpu FPUTYPE'
This command sets the ARM floating-point unit (FPU) type. The
argument FPUTYPE can be one of these:
`auto'
Determine the FPU type by querying the OS ABI.
`softfpa'
Software FPU, with mixed-endian doubles on little-endian ARM
processors.
`fpa'
GCC-compiled FPA co-processor.
`softvfp'
Software FPU with pure-endian doubles.
`vfp'
VFP co-processor.
`show arm fpu'
Show the current type of the FPU.
`set arm abi'
This command forces GDB to use the specified ABI.
`show arm abi'
Show the currently used ABI.
`set debug arm'
Toggle whether to display ARM-specific debugging messages from the
ARM target support subsystem.
`show debug arm'
Show whether ARM-specific debugging messages are enabled.
The following commands are available when an ARM target is debugged
using the RDI interface:
`rdilogfile [FILE]'
Set the filename for the ADP (Angel Debugger Protocol) packet log.
With an argument, sets the log file to the specified FILE. With
no argument, show the current log file name. The default log file
is `rdi.log'.
`rdilogenable [ARG]'
Control logging of ADP packets. With an argument of 1 or `"yes"'
enables logging, with an argument 0 or `"no"' disables it. With
no arguments displays the current setting. When logging is
enabled, ADP packets exchanged between GDB and the RDI target
device are logged to a file.
`set rdiromatzero'
Tell GDB whether the target has ROM at address 0. If on, vector
catching is disabled, so that zero address can be used. If off
(the default), vector catching is enabled. For this command to
take effect, it needs to be invoked prior to the `target rdi'
command.
`show rdiromatzero'
Show the current setting of ROM at zero address.
`set rdiheartbeat'
Enable or disable RDI heartbeat packets. It is not recommended to
turn on this option, since it confuses ARM and EPI JTAG interface,
as well as the Angel monitor.
`show rdiheartbeat'
Show the setting of RDI heartbeat packets.
File: gdb.info, Node: M32R/D, Next: M68K, Prev: ARM, Up: Embedded Processors
18.3.2 Renesas M32R/D and M32R/SDI
----------------------------------
`target m32r DEV'
Renesas M32R/D ROM monitor.
`target m32rsdi DEV'
Renesas M32R SDI server, connected via parallel port to the board.
The following GDB commands are specific to the M32R monitor:
`set download-path PATH'
Set the default path for finding downloadable SREC files.
`show download-path'
Show the default path for downloadable SREC files.
`set board-address ADDR'
Set the IP address for the M32R-EVA target board.
`show board-address'
Show the current IP address of the target board.
`set server-address ADDR'
Set the IP address for the download server, which is the GDB's
host machine.
`show server-address'
Display the IP address of the download server.
`upload [FILE]'
Upload the specified SREC FILE via the monitor's Ethernet upload
capability. If no FILE argument is given, the current executable
file is uploaded.
`tload [FILE]'
Test the `upload' command.
The following commands are available for M32R/SDI:
`sdireset'
This command resets the SDI connection.
`sdistatus'
This command shows the SDI connection status.
`debug_chaos'
Instructs the remote that M32R/Chaos debugging is to be used.
`use_debug_dma'
Instructs the remote to use the DEBUG_DMA method of accessing
memory.
`use_mon_code'
Instructs the remote to use the MON_CODE method of accessing
memory.
`use_ib_break'
Instructs the remote to set breakpoints by IB break.
`use_dbt_break'
Instructs the remote to set breakpoints by DBT.
File: gdb.info, Node: M68K, Next: MIPS Embedded, Prev: M32R/D, Up: Embedded Processors
18.3.3 M68k
-----------
The Motorola m68k configuration includes ColdFire support, and a target
command for the following ROM monitor.
`target dbug DEV'
dBUG ROM monitor for Motorola ColdFire.
File: gdb.info, Node: MIPS Embedded, Next: OpenRISC 1000, Prev: M68K, Up: Embedded Processors
18.3.4 MIPS Embedded
--------------------
GDB can use the MIPS remote debugging protocol to talk to a MIPS board
attached to a serial line. This is available when you configure GDB
with `--target=mips-idt-ecoff'.
Use these GDB commands to specify the connection to your target
board:
`target mips PORT'
To run a program on the board, start up `gdb' with the name of
your program as the argument. To connect to the board, use the
command `target mips PORT', where PORT is the name of the serial
port connected to the board. If the program has not already been
downloaded to the board, you may use the `load' command to
download it. You can then use all the usual GDB commands.
For example, this sequence connects to the target board through a
serial port, and loads and runs a program called PROG through the
debugger:
host$ gdb PROG
GDB is free software and ...
(gdb) target mips /dev/ttyb
(gdb) load PROG
(gdb) run
`target mips HOSTNAME:PORTNUMBER'
On some GDB host configurations, you can specify a TCP connection
(for instance, to a serial line managed by a terminal
concentrator) instead of a serial port, using the syntax
`HOSTNAME:PORTNUMBER'.
`target pmon PORT'
PMON ROM monitor.
`target ddb PORT'
NEC's DDB variant of PMON for Vr4300.
`target lsi PORT'
LSI variant of PMON.
`target r3900 DEV'
Densan DVE-R3900 ROM monitor for Toshiba R3900 Mips.
`target array DEV'
Array Tech LSI33K RAID controller board.
GDB also supports these special commands for MIPS targets:
`set mipsfpu double'
`set mipsfpu single'
`set mipsfpu none'
`set mipsfpu auto'
`show mipsfpu'
If your target board does not support the MIPS floating point
coprocessor, you should use the command `set mipsfpu none' (if you
need this, you may wish to put the command in your GDB init file).
This tells GDB how to find the return value of functions which
return floating point values. It also allows GDB to avoid saving
the floating point registers when calling functions on the board.
If you are using a floating point coprocessor with only single
precision floating point support, as on the R4650 processor, use
the command `set mipsfpu single'. The default double precision
floating point coprocessor may be selected using `set mipsfpu
double'.
In previous versions the only choices were double precision or no
floating point, so `set mipsfpu on' will select double precision
and `set mipsfpu off' will select no floating point.
As usual, you can inquire about the `mipsfpu' variable with `show
mipsfpu'.
`set timeout SECONDS'
`set retransmit-timeout SECONDS'
`show timeout'
`show retransmit-timeout'
You can control the timeout used while waiting for a packet, in
the MIPS remote protocol, with the `set timeout SECONDS' command.
The default is 5 seconds. Similarly, you can control the timeout
used while waiting for an acknowledgement of a packet with the `set
retransmit-timeout SECONDS' command. The default is 3 seconds.
You can inspect both values with `show timeout' and `show
retransmit-timeout'. (These commands are _only_ available when
GDB is configured for `--target=mips-idt-ecoff'.)
The timeout set by `set timeout' does not apply when GDB is
waiting for your program to stop. In that case, GDB waits forever
because it has no way of knowing how long the program is going to
run before stopping.
`set syn-garbage-limit NUM'
Limit the maximum number of characters GDB should ignore when it
tries to synchronize with the remote target. The default is 10
characters. Setting the limit to -1 means there's no limit.
`show syn-garbage-limit'
Show the current limit on the number of characters to ignore when
trying to synchronize with the remote system.
`set monitor-prompt PROMPT'
Tell GDB to expect the specified PROMPT string from the remote
monitor. The default depends on the target:
pmon target
`PMON'
ddb target
`NEC010'
lsi target
`PMON>'
`show monitor-prompt'
Show the current strings GDB expects as the prompt from the remote
monitor.
`set monitor-warnings'
Enable or disable monitor warnings about hardware breakpoints.
This has effect only for the `lsi' target. When on, GDB will
display warning messages whose codes are returned by the `lsi'
PMON monitor for breakpoint commands.
`show monitor-warnings'
Show the current setting of printing monitor warnings.
`pmon COMMAND'
This command allows sending an arbitrary COMMAND string to the
monitor. The monitor must be in debug mode for this to work.
File: gdb.info, Node: OpenRISC 1000, Next: PA, Prev: MIPS Embedded, Up: Embedded Processors
18.3.5 OpenRISC 1000
--------------------
See OR1k Architecture document (`www.opencores.org') for more
information about platform and commands.
`target jtag jtag://HOST:PORT'
Connects to remote JTAG server. JTAG remote server can be either
an or1ksim or JTAG server, connected via parallel port to the
board.
Example: `target jtag jtag://localhost:9999'
`or1ksim COMMAND'
If connected to `or1ksim' OpenRISC 1000 Architectural Simulator,
proprietary commands can be executed.
`info or1k spr'
Displays spr groups.
`info or1k spr GROUP'
`info or1k spr GROUPNO'
Displays register names in selected group.
`info or1k spr GROUP REGISTER'
`info or1k spr REGISTER'
`info or1k spr GROUPNO REGISTERNO'
`info or1k spr REGISTERNO'
Shows information about specified spr register.
`spr GROUP REGISTER VALUE'
`spr REGISTER VALUE'
`spr GROUPNO REGISTERNO VALUE'
`spr REGISTERNO VALUE'
Writes VALUE to specified spr register.
Some implementations of OpenRISC 1000 Architecture also have
hardware trace. It is very similar to GDB trace, except it does not
interfere with normal program execution and is thus much faster.
Hardware breakpoints/watchpoint triggers can be set using:
`$LEA/$LDATA'
Load effective address/data
`$SEA/$SDATA'
Store effective address/data
`$AEA/$ADATA'
Access effective address ($SEA or $LEA) or data ($SDATA/$LDATA)
`$FETCH'
Fetch data
When triggered, it can capture low level data, like: `PC', `LSEA',
`LDATA', `SDATA', `READSPR', `WRITESPR', `INSTR'.
`htrace' commands:
`hwatch CONDITIONAL'
Set hardware watchpoint on combination of Load/Store Effective
Address(es) or Data. For example:
`hwatch ($LEA == my_var) && ($LDATA < 50) || ($SEA == my_var) &&
($SDATA >= 50)'
`hwatch ($LEA == my_var) && ($LDATA < 50) || ($SEA == my_var) &&
($SDATA >= 50)'
`htrace info'
Display information about current HW trace configuration.
`htrace trigger CONDITIONAL'
Set starting criteria for HW trace.
`htrace qualifier CONDITIONAL'
Set acquisition qualifier for HW trace.
`htrace stop CONDITIONAL'
Set HW trace stopping criteria.
`htrace record [DATA]*'
Selects the data to be recorded, when qualifier is met and HW
trace was triggered.
`htrace enable'
`htrace disable'
Enables/disables the HW trace.
`htrace rewind [FILENAME]'
Clears currently recorded trace data.
If filename is specified, new trace file is made and any newly
collected data will be written there.
`htrace print [START [LEN]]'
Prints trace buffer, using current record configuration.
`htrace mode continuous'
Set continuous trace mode.
`htrace mode suspend'
Set suspend trace mode.
File: gdb.info, Node: PA, Next: PowerPC Embedded, Prev: OpenRISC 1000, Up: Embedded Processors
18.3.7 HP PA Embedded
---------------------
`target op50n DEV'
OP50N monitor, running on an OKI HPPA board.
`target w89k DEV'
W89K monitor, running on a Winbond HPPA board.
File: gdb.info, Node: PowerPC Embedded, Next: Sparclet, Prev: PA, Up: Embedded Processors
18.3.6 PowerPC Embedded
-----------------------
GDB provides the following PowerPC-specific commands:
`set powerpc soft-float'
`show powerpc soft-float'
Force GDB to use (or not use) a software floating point calling
convention. By default, GDB selects the calling convention based
on the selected architecture and the provided executable file.
`set powerpc vector-abi'
`show powerpc vector-abi'
Force GDB to use the specified calling convention for vector
arguments and return values. The valid options are `auto';
`generic', to avoid vector registers even if they are present;
`altivec', to use AltiVec registers; and `spe' to use SPE
registers. By default, GDB selects the calling convention based
on the selected architecture and the provided executable file.
`target dink32 DEV'
DINK32 ROM monitor.
`target ppcbug DEV'
`target ppcbug1 DEV'
PPCBUG ROM monitor for PowerPC.
`target sds DEV'
SDS monitor, running on a PowerPC board (such as Motorola's ADS).
The following commands specific to the SDS protocol are supported by
GDB:
`set sdstimeout NSEC'
Set the timeout for SDS protocol reads to be NSEC seconds. The
default is 2 seconds.
`show sdstimeout'
Show the current value of the SDS timeout.
`sds COMMAND'
Send the specified COMMAND string to the SDS monitor.
File: gdb.info, Node: Sparclet, Next: Sparclite, Prev: PowerPC Embedded, Up: Embedded Processors
18.3.8 Tsqware Sparclet
-----------------------
GDB enables developers to debug tasks running on Sparclet targets from
a Unix host. GDB uses code that runs on both the Unix host and on the
Sparclet target. The program `gdb' is installed and executed on the
Unix host.
`remotetimeout ARGS'
GDB supports the option `remotetimeout'. This option is set by
the user, and ARGS represents the number of seconds GDB waits for
responses.
When compiling for debugging, include the options `-g' to get debug
information and `-Ttext' to relocate the program to where you wish to
load it on the target. You may also want to add the options `-n' or
`-N' in order to reduce the size of the sections. Example:
sparclet-aout-gcc prog.c -Ttext 0x12010000 -g -o prog -N
You can use `objdump' to verify that the addresses are what you
intended:
sparclet-aout-objdump --headers --syms prog
Once you have set your Unix execution search path to find GDB, you
are ready to run GDB. From your Unix host, run `gdb' (or
`sparclet-aout-gdb', depending on your installation).
GDB comes up showing the prompt:
(gdbslet)
* Menu:
* Sparclet File:: Setting the file to debug
* Sparclet Connection:: Connecting to Sparclet
* Sparclet Download:: Sparclet download
* Sparclet Execution:: Running and debugging
File: gdb.info, Node: Sparclet File, Next: Sparclet Connection, Up: Sparclet
18.3.8.1 Setting File to Debug
..............................
The GDB command `file' lets you choose with program to debug.
(gdbslet) file prog
GDB then attempts to read the symbol table of `prog'. GDB locates
the file by searching the directories listed in the command search path.
If the file was compiled with debug information (option `-g'), source
files will be searched as well. GDB locates the source files by
searching the directories listed in the directory search path (*note
Your Program's Environment: Environment.). If it fails to find a file,
it displays a message such as:
prog: No such file or directory.
When this happens, add the appropriate directories to the search
paths with the GDB commands `path' and `dir', and execute the `target'
command again.
File: gdb.info, Node: Sparclet Connection, Next: Sparclet Download, Prev: Sparclet File, Up: Sparclet
18.3.8.2 Connecting to Sparclet
...............................
The GDB command `target' lets you connect to a Sparclet target. To
connect to a target on serial port "`ttya'", type:
(gdbslet) target sparclet /dev/ttya
Remote target sparclet connected to /dev/ttya
main () at ../prog.c:3
GDB displays messages like these:
Connected to ttya.
File: gdb.info, Node: Sparclet Download, Next: Sparclet Execution, Prev: Sparclet Connection, Up: Sparclet
18.3.8.3 Sparclet Download
..........................
Once connected to the Sparclet target, you can use the GDB `load'
command to download the file from the host to the target. The file
name and load offset should be given as arguments to the `load' command.
Since the file format is aout, the program must be loaded to the
starting address. You can use `objdump' to find out what this value
is. The load offset is an offset which is added to the VMA (virtual
memory address) of each of the file's sections. For instance, if the
program `prog' was linked to text address 0x1201000, with data at
0x12010160 and bss at 0x12010170, in GDB, type:
(gdbslet) load prog 0x12010000
Loading section .text, size 0xdb0 vma 0x12010000
If the code is loaded at a different address then what the program
was linked to, you may need to use the `section' and `add-symbol-file'
commands to tell GDB where to map the symbol table.
File: gdb.info, Node: Sparclet Execution, Prev: Sparclet Download, Up: Sparclet
18.3.8.4 Running and Debugging
..............................
You can now begin debugging the task using GDB's execution control
commands, `b', `step', `run', etc. See the GDB manual for the list of
commands.
(gdbslet) b main
Breakpoint 1 at 0x12010000: file prog.c, line 3.
(gdbslet) run
Starting program: prog
Breakpoint 1, main (argc=1, argv=0xeffff21c) at prog.c:3
3 char *symarg = 0;
(gdbslet) step
4 char *execarg = "hello!";
(gdbslet)
File: gdb.info, Node: Sparclite, Next: Z8000, Prev: Sparclet, Up: Embedded Processors
18.3.9 Fujitsu Sparclite
------------------------
`target sparclite DEV'
Fujitsu sparclite boards, used only for the purpose of loading.
You must use an additional command to debug the program. For
example: target remote DEV using GDB standard remote protocol.
File: gdb.info, Node: Z8000, Next: AVR, Prev: Sparclite, Up: Embedded P