Debugging with GDB

Next: , Previous: (dir), Up: (dir)

Debugging with gdb

This file describes gdb, the gnu symbolic debugger.

This is the Ninth Edition, for gdb Version

Copyright (C) 1988-2005 Free Software Foundation, Inc.

Table of Contents

Next: , Previous: 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:

You can use gdb to debug programs written in C and C++. For more information, see Supported languages. For more information, see C and C++.

Support for Modula-2 is partial. For information on Modula-2, see Modula-2.

Debugging Pascal programs which use sets, subranges, file variables, or nested functions does not currently work. gdb does not support entering expressions, printing values, or similar features using Pascal syntax.

gdb can be used to debug programs written in Fortran, although it may be necessary to refer to some variables with a trailing underscore.

gdb can be used to debug programs written in Objective-C, using either the Apple/NeXT or the GNU Objective-C runtime.

Next: , 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

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

Previous: 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 6.1, 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.

Next: , Previous: 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
     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, Copyright 1999 Free Software Foundation, Inc...

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

To trigger the breakpoint, we call changequote. gdb suspends execution of m4, displaying information about the context where it stops.

     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);
     535         lquote = (lq == nil || *lq == '\0') ? def_lquote\
      : xstrdup (lq);
     536         rquote = (rq == nil || *rq == '\0') ? def_rquote\
      : xstrdup (rq);
     538         len_lquote = strlen(rquote);
     539         len_rquote = strlen(lquote);
     540     }
     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

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:

     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

Next: , Previous: 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:

Next: , 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 (see Arguments) to `-O2 -c foo.c'.

You can run gdb without printing the front material, which describes gdb's non-warranty, by specifying -silent:

     gdb -silent

You can further control how gdb starts up by using command-line options. gdb itself can remind you of the options available.


     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.

Next: , 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 ./, eg. ./12345.

If gdb has not been configured to included core file support, such as for most embedded targets, then it will complain about a second argument and ignore it.

Many options have both long and short forms; both are shown in the following list. gdb also recognizes the long forms if you truncate them, so long as enough of the option is present to be unambiguous. (If you prefer, you can flag option arguments with `--' rather than `-', though we illustrate the more usual convention.)

-symbols file
-s file
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.
-c number
-pid number
-p number
Connect to process ID number, as with the attach command. If there is no such process, gdb will attempt to open a core file named number.
-command file
-x file
Execute gdb commands from file file. See Command files.
-directory directory
-d directory
Add directory to the path to search for source files.
Warning: this option depends on operating system facilities that are not supported on all systems.
If memory-mapped files are available on your system through the mmap system call, you can use this option to have gdb write the symbols from your program into a reusable file in the current directory. If the program you are debugging is called /tmp/fred, the mapped symbol file is /tmp/fred.syms. Future gdb debugging sessions notice the presence of this file, and can quickly map in symbol information from it, rather than reading the symbol table from the executable program.

The .syms file is specific to the host machine where gdb is run. It holds an exact image of the internal gdb symbol table. It cannot be shared across multiple host platforms.

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.

You typically combine the -mapped and -readnow options in order to build a .syms file that contains complete symbol information. (See Commands to specify files, for information on .syms files.) A simple gdb invocation to do nothing but build a .syms file for future use is:

     gdb -batch -nx -mapped -readnow programname

Next: , Previous: 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.

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. See Command files.
“Quiet”. Do not print the introductory and copyright messages. These messages are also suppressed in batch mode.
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.

“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.
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.
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.
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' (see 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 superseeded by gdb/mi (see GDB/MI).

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.
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 (see gdb Text User Interface). Alternatively, the Text User Interface can be enabled by invoking the program `gdbtui'. Do not use this option if you run gdb from Emacs (see Using gdb under gnu 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. See Command Interpreters.

`--interpreter=mi' (or `--interpreter=mi2') causes gdb to use the gdb/mi interface (see The gdb/mi Interface) included since gdb version 6.0. The previous gdb/mi interface, included in gdb version 5.3 and selected with `--interpreter=mi1', is deprecated. Earlier gdb/mi interfaces are no longer supported.

Open the executable and core files for both reading and writing. This is equivalent to the `set write on' command inside gdb (see Patching).
This option causes gdb to print statistics about time and memory usage after it completes each command and returns to the prompt.
This option causes gdb to print its version number and no-warranty blurb, and exit.

Previous: 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 (see interpreter).
  2. Reads the init file (if any) in your home directory1 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. See Command Files, for more details about gdb command files.
  6. Reads the command history recorded in the history file. See 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 (see Command Files) and are processed by gdb in the same way. The init file in your home directory can set options (such as `set complaints') that affect subsequent processing of command line options and operands. Init files are not executed if you use the `-nx' option (see Choosing modes).

The gdb init files are normally called .gdbinit. On some configurations of gdb, the init file is known by a different name (these are typically environments where a specialized form of gdb may need to coexist with other forms, hence a different name for the specialized version's init file). These are the environments with special init file names:

Next: , Previous: Invoking GDB, Up: Invocation

2.2 Quitting gdb

quit [expression]
To exit gdb, use the quit command (abbreviated q), or type an end-of-file character (usually C-d). If you do not supply expression, gdb will terminate normally; otherwise it will terminate using the result of expression as the error code.

An interrupt (often C-c) does not exit from gdb, but rather terminates the action of any gdb command that is in progress and returns to gdb command level. It is safe to type the interrupt character at any time because gdb does not allow it to take effect until a time when it is safe.

If you have been using gdb to control an attached process or device, you can release it with the detach command (see Debugging an already-running process).

Next: , Previous: 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'.

Previous: 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.

Next: , Previous: 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).

Next: , 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 dont-repeat.

The list and x commands, when you repeat them with <RET>, construct new arguments rather than repeating exactly as typed. This permits easy scanning of source or memory.

gdb can also use <RET> in another way: to partition lengthy output, in a way similar to the common utility more (see Screen size). Since it is easy to press one <RET> too many in this situation, gdb disables command repetition after any command that generates this sort of display.

Any text from a # to the end of the line is a comment; it does nothing. This is useful mainly in command files (see Command files).

The C-o binding is useful for repeating a complex sequence of commands. This command accepts the current line, like RET, and then fetches the next line relative to the current line from the history for editing.

Next: , Previous: 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 C++ expressions. You can use the command set overload-resolution off to disable overload resolution; see gdb features for C++.

Previous: Completion, Up: Commands

3.3 Getting help

You can always ask gdb itself for information on its commands, using the command help.

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
          Command name abbreviations are allowed if unambiguous.

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:


This is intended for use by gnu Emacs.

In addition to help, you can use the gdb commands info and show to inquire about the state of your program, or the state of gdb itself. Each command supports many topics of inquiry; this manual introduces each of them in the appropriate context. The listings under info and under show in the Index point to all the sub-commands. See Index.

This command (abbreviated i) is for describing the state of your program. For example, you can list the arguments given to your program with info args, list the registers currently in use with info registers, or list the breakpoints you have set with info breakpoints. You can get a complete list of the info sub-commands with help info.

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 $.

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.

Next: , Previous: 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.

Next: , 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!). See Variables, for more information about debugging optimized code.

Older versions of the gnu C compiler permitted a variant option `-gg' for debugging information. gdb no longer supports this format; if your gnu C compiler has this option, do not use it.

gdb knows about preprocessor macros and can show you their expansion (see Macros). Most compilers do not include information about preprocessor macros in the debugging information if you specify the -g flag alone, because this information is rather large. Version 3.1 and later of gcc, the gnu C compiler, provides macro information if you specify the options -gdwarf-2 and -g3; the former option requests debugging information in the Dwarf 2 format, and the latter requests “extra information”. In the future, we hope to find more compact ways to represent macro information, so that it can be included with -g alone.

Next: , Previous: Compilation, Up: Running

4.2 Starting your program

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 (see Getting In and Out of gdb), or by using the file or exec-file command (see Commands to specify files).

If you are running your program in an execution environment that supports processes, run creates an inferior process and makes that process run your program. (In environments without processes, run jumps to the start of your program.)

The execution of a program is affected by certain information it receives from its superior. gdb provides ways to specify this information, which you must do before starting your program. (You can change it after starting your program, but such changes only affect your program the next time you start it.) This information may be divided into four categories:

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. See Your program's 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. See Your program's 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. See Your program's 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. See Your program's input and output.

Warning: While input and output redirection work, you cannot use pipes to pass the output of the program you are debugging to another program; if you attempt this, gdb is likely to wind up debugging the wrong program.

When you issue the run command, your program begins to execute immediately. See Stopping and continuing, for discussion of how to arrange for your program to stop. Once your program has stopped, you may call functions in your program, using the print or call commands. See Examining Data.

If the modification time of your symbol file has changed since the last time gdb read its symbols, gdb discards its symbol table, and reads it again. When it does this, gdb tries to retain your current breakpoints.

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.

Next: , Previous: 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.

Next: , Previous: 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.

Next: , Previous: 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. See Commands to specify files.

cd directory
Set the gdb working directory to directory.

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 (see SVR4 Process Information) to find out the current working directory of the debuggee.

Next: , Previous: 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.

Next: , Previous: 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 (see Specifying source directories). You can also use the file command to load the program. See Commands to Specify Files.

The first thing gdb does after arranging to debug the specified process is to stop it. You can examine and modify an attached process with all the gdb commands that are ordinarily available when you start processes with run. You can insert breakpoints; you can step and continue; you can modify storage. If you would rather the process continue running, you may use the continue command after attaching gdb to the process.

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 or use the run command while you have an attached process, you kill that process. By default, gdb asks for confirmation if you try to do either of these things; you can control whether or not you need to confirm by using the set confirm command (see Optional warnings and messages).

Next: , Previous: Attach, Up: Running

4.8 Killing the child process

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).

Next: , Previous: 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:

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 LynxOS, you might see

     [New process 35 thread 27]

when gdb notices a new thread. In contrast, on an SGI system, the systag is simply something like `process 368', with no further qualifier.

For debugging purposes, gdb associates its own thread number—always a single integer—with each thread in your program.

info 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] args
The thread apply command allows you to apply a command to one or more threads. Specify the numbers of the threads that you want affected with the command argument threadno. threadno is the internal gdb thread number, as shown in the first field of the `info threads' display. To apply a command to all threads, use thread apply all args.

Whenever gdb stops your program, due to a breakpoint or a signal, it automatically selects the thread where that breakpoint or signal happened. gdb alerts you to the context switch with a message of the form `[Switching to systag]' to identify the thread.

See Stopping and starting multi-thread programs, for more information about how gdb behaves when you stop and start programs with multiple threads.

See Setting watchpoints, for information about watchpoints in programs with multiple threads.

Previous: 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 (see Attach). From that point on you can debug the child process just like any other process which you attached to.

On some systems, gdb provides support for debugging programs that create additional processes using the fork or vfork functions. Currently, the only platforms with this feature are HP-UX (11.x and later only?) and GNU/Linux (kernel version 2.5.60 and later).

By default, when a program forks, gdb will continue to debug the parent process and the child process will run unimpeded.

If you want to follow the child process instead of the parent process, use the command set follow-fork-mode.

set follow-fork-mode 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:
The original process is debugged after a fork. The child process runs unimpeded. This is the default.
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.

If you ask to debug a child process and a vfork is followed by an exec, gdb executes the new target up to the first breakpoint in the new target. If you have a breakpoint set on main in your original program, the breakpoint will also be set on the child process's main.

When a child process is spawned by vfork, you cannot debug the child or parent until an exec call completes.

If you issue a run command to gdb after an exec call executes, the new target restarts. To restart the parent process, use the file command with the parent executable name as its argument.

You can use the catch command to make gdb stop whenever a fork, vfork, or exec call is made. See Setting catchpoints.

Next: , Previous: 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.

Next: , 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 (see Setting breakpoints), to specify the place where your program should stop by line number, function name or exact address in the program.

On some systems, you can set breakpoints in shared libraries before the executable is run. There is a minor limitation on HP-UX systems: you must wait until the executable is run in order to set breakpoints in shared library routines that are not called directly by the program (for example, routines that are arguments in a pthread_create call).

A watchpoint is a special breakpoint that stops your program when the value of an expression changes. You must use a different command to set watchpoints (see Setting watchpoints), but aside from that, you can manage a watchpoint like any other breakpoint: you enable, disable, and delete both breakpoints and watchpoints using the same commands.

You can arrange to have values from your program displayed automatically whenever gdb stops at a breakpoint. See Automatic display.

A catchpoint is another special breakpoint that stops your program when a certain kind of event occurs, such as the throwing of a C++ exception or the loading of a library. As with watchpoints, you use a different command to set a catchpoint (see Setting catchpoints), but aside from that, you can manage a catchpoint like any other breakpoint. (To stop when your program receives a signal, use the handle command; see Signals.)

gdb assigns a number to each breakpoint, watchpoint, or catchpoint when you create it; these numbers are successive integers starting with one. In many of the commands for controlling various features of breakpoints you use the breakpoint number to say which breakpoint you want to change. Each breakpoint may be enabled or disabled; if disabled, it has no effect on your program until you enable it again.

Some gdb commands accept a range of breakpoints on which to operate. A breakpoint range is either a single breakpoint number, like `5', or two such numbers, in increasing order, separated by a hyphen, like `5-7'. When a breakpoint range is given to a command, all breakpoint in that range are operated on.

Next: , 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 Convenience variables, for a discussion of what you can do with convenience variables.

You have several ways to say where the breakpoint should go.

break function
Set a breakpoint at entry to function function. When using source languages that permit overloading of symbols, such as C++, function may refer to more than one possible place to break. See Breakpoint menus, for a discussion of that situation.
break +offset
break -offset
Set a breakpoint some number of lines forward or back from the position at which execution stopped in the currently selected stack frame. (See Frames, for a description of stack frames.)
break linenum
Set a breakpoint at line linenum in the current source file. The current source file is the last file whose source text was printed. The breakpoint will stop your program just before it executes any of the code on that line.
break filename:linenum
Set a breakpoint at line linenum in source file filename.
break filename:function
Set a breakpoint at entry to function function found in file filename. Specifying a file name as well as a function name is superfluous except when multiple files contain similarly named functions.
break *address
Set a breakpoint at address address. You can use this to set breakpoints in parts of your program which do not have debugging information or source files.
When called without any arguments, break sets a breakpoint at the next instruction to be executed in the selected stack frame (see Examining the Stack). In any selected frame but the innermost, this makes your program stop as soon as control returns to that frame. This is similar to the effect of a finish command in the frame inside the selected frame—except that finish does not leave an active breakpoint. If you use break without an argument in the innermost frame, gdb stops the next time it reaches the current location; this may be useful inside loops.

gdb normally ignores breakpoints when it resumes execution, until at least one instruction has been executed. If it did not do this, you would be unable to proceed past a breakpoint without first disabling the breakpoint. This rule applies whether or not the breakpoint already existed when your program stopped.

break ... if 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. See Break 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. See Disabling breakpoints.

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 (see Disabling). See Break conditions. For remote targets, you can restrict the number of hardware breakpoints gdb will use, see set remote hardware-breakpoint-limit.

thbreak 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. See Disabling breakpoints. See also Break 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 os. There is an implicit .* leading and trailing the regular expression you supply, so to match only functions that begin with foo, use ^foo.

When debugging C++ programs, rbreak is useful for setting breakpoints on overloaded functions that are not members of any special classes.

The rbreak command can be used to set breakpoints in all the functions in a program, like this:

          (gdb) rbreak .

info breakpoints [n]
info break [n]
info watchpoints [n]
Print a table of all breakpoints, watchpoints, and catchpoints set and not deleted, with the following columns for each breakpoint:
Breakpoint Numbers
Breakpoint, watchpoint, or catchpoint.
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.
Where the breakpoint is in your program, as a memory address. If the breakpoint is pending (see below for details) on a future load of a shared library, the address will be listed as `<PENDING>'.
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 (see Examining memory).

info break displays a count of the number of times the breakpoint has been hit. This is especially useful in conjunction with the ignore command. You can ignore a large number of breakpoint hits, look at the breakpoint info to see how many times the breakpoint was hit, and then run again, ignoring one less than that number. This will get you quickly to the last hit of that breakpoint.

gdb allows you to set any number of breakpoints at the same place in your program. There is nothing silly or meaningless about this. When the breakpoints are conditional, this is even useful (see Break conditions).

If a specified breakpoint location cannot be found, it may be due to the fact that the location is in a shared library that is yet to be loaded. In such a case, you may want gdb to create a special breakpoint (known as a pending breakpoint) that attempts to resolve itself in the future when an appropriate shared library gets loaded.

Pending breakpoints are useful to set at the start of your gdb session for locations that you know will be dynamically loaded later by the program being debugged. When shared libraries are loaded, a check is made to see if the load resolves any pending breakpoint locations. If a pending breakpoint location gets resolved, a regular breakpoint is created and the original pending breakpoint is removed.

gdb provides some additional commands for controlling pending breakpoint support:

set breakpoint pending 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.

Normal breakpoint operations apply to pending breakpoints as well. You may specify a condition for a pending breakpoint and/or commands to run when the breakpoint is reached. You can also enable or disable the pending breakpoint. When you specify a condition for a pending breakpoint, the parsing of the condition will be deferred until the point where the pending breakpoint location is resolved. Disabling a pending breakpoint tells gdb to not attempt to resolve the breakpoint on any subsequent shared library load. When a pending breakpoint is re-enabled, gdb checks to see if the location is already resolved. This is done because any number of shared library loads could have occurred since the time the breakpoint was disabled and one or more of these loads could resolve the location.

gdb itself sometimes sets breakpoints in your program for special purposes, such as proper handling of longjmp (in C programs). These internal breakpoints are assigned negative numbers, starting with -1; `info breakpoints' does not display them. You can see these breakpoints with the gdb maintenance command `maint info breakpoints' (see maint info breakpoints).

Next: , Previous: 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.

Depending on your system, watchpoints may be implemented in software or hardware. gdb does software watchpointing by single-stepping your program and testing the variable's value each time, which is hundreds of times slower than normal execution. (But this may still be worth it, to catch errors where you have no clue what part of your program is the culprit.)

On some systems, such as HP-UX, gnu/Linux and most other x86-based targets, gdb includes support for hardware watchpoints, which do not slow down the running of your program.

watch expr
Set a watchpoint for an expression. gdb will break when expr is written into by the program and its value changes.

rwatch expr
Set a watchpoint that will break when the value of expr is read by the program.

awatch expr
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 (see Set Breaks).

gdb sets a hardware watchpoint if possible. Hardware watchpoints execute very quickly, and the debugger reports a change in value at the exact instruction where the change occurs. If gdb cannot set a hardware watchpoint, it sets a software watchpoint, which executes more slowly and reports the change in value at the next statement, not the instruction, after the change occurs.

You can force gdb to use only software watchpoints with the set can-use-hw-watchpoints 0 command. With this variable set to zero, gdb will never try to use hardware watchpoints, even if the underlying system supports them. (Note that hardware-assisted watchpoints that were set before setting can-use-hw-watchpoints to zero will still use the hardware mechanism of watching expressiion values.)

set can-use-hw-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 set remote hardware-breakpoint-limit.

When you issue the watch command, gdb reports

     Hardware watchpoint num: expr

if it was able to set a hardware watchpoint.

Currently, the awatch and rwatch commands can only set hardware watchpoints, because accesses to data that don't change the value of the watched expression cannot be detected without examining every instruction as it is being executed, and gdb does not do that currently. If gdb finds that it is unable to set a hardware breakpoint with the awatch or rwatch command, it will print a message like this:

     Expression cannot be implemented with read/access watchpoint.

Sometimes, gdb cannot set a hardware watchpoint because the data type of the watched expression is wider than what a hardware watchpoint on the target machine can handle. For example, some systems can only watch regions that are up to 4 bytes wide; on such systems you cannot set hardware watchpoints for an expression that yields a double-precision floating-point number (which is typically 8 bytes wide). As a work-around, it might be possible to break the large region into a series of smaller ones and watch them with separate watchpoints.

If you set too many hardware watchpoints, gdb might be unable to insert all of them when you resume the execution of your program. Since the precise number of active watchpoints is unknown until such time as the program is about to be resumed, gdb might not be able to warn you about this when you set the watchpoints, and the warning will be printed only when the program is resumed:

     Hardware watchpoint num: Could not insert watchpoint

If this happens, delete or disable some of the watchpoints.

The SPARClite DSU will generate traps when a program accesses some data or instruction address that is assigned to the debug registers. For the data addresses, DSU facilitates the watch command. However the hardware breakpoint registers can only take two data watchpoints, and both watchpoints must be the same kind. For example, you can set two watchpoints with watch commands, two with rwatch commands, or two with awatch commands, but you cannot set one watchpoint with one command and the other with a different command. gdb will reject the command if you try to mix watchpoints. Delete or disable unused watchpoint commands before setting new ones.

If you call a function interactively using print or call, any watchpoints you have set will be inactive until gdb reaches another kind of breakpoint or the call completes.

gdb automatically deletes watchpoints that watch local (automatic) variables, or expressions that involve such variables, when they go out of scope, that is, when the execution leaves the block in which these variables were defined. In particular, when the program being debugged terminates, all local variables go out of scope, and so only watchpoints that watch global variables remain set. If you rerun the program, you will need to set all such watchpoints again. One way of doing that would be to set a code breakpoint at the entry to the main function and when it breaks, set all the watchpoints.

Warning: In multi-thread programs, watchpoints have only limited usefulness. With the current watchpoint implementation, gdb can only watch the value of an expression in a single thread. If you are confident that the expression can only change due to the current thread's activity (and if you are also confident that no other thread can become current), then you can use watchpoints as usual. However, gdb may not notice when a non-current thread's activity changes the expression.

HP-UX Warning: In multi-thread programs, software watchpoints have only limited usefulness. If gdb creates a software watchpoint, it can only watch the value of an expression in a single thread. If you are confident that the expression can only change due to the current thread's activity (and if you are also confident that no other thread can become current), then you can use software watchpoints as usual. However, gdb may not notice when a non-current thread's activity changes the expression. (Hardware watchpoints, in contrast, watch an expression in all threads.)

See set remote hardware-watchpoint-limit.

Next: , Previous: 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:
The throwing of a C++ exception.
The catching of a C++ exception.
A call to exec. This is currently only available for HP-UX.
A call to fork. This is currently only available for HP-UX.
A call to vfork. This is currently only available for HP-UX.
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 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:

Sometimes catch is not the best way to debug exception handling: if you need to know exactly where an exception is raised, it is better to stop before the exception handler is called, since that way you can see the stack before any unwinding takes place. If you set a breakpoint in an exception handler instead, it may not be easy to find out where the exception was raised.

To stop just before an exception handler is called, you need some knowledge of the implementation. In the case of gnu C++, exceptions are raised by calling a library function named __raise_exception which has the following ANSI C interface:

         /* addr is where the exception identifier is stored.
            id is the exception identifier.  */
         void __raise_exception (void **addr, void *id);

To make the debugger catch all exceptions before any stack unwinding takes place, set a breakpoint on __raise_exception (see Breakpoints; watchpoints; and exceptions).

With a conditional breakpoint (see Break conditions) that depends on the value of id, you can stop your program when a specific exception is raised. You can use multiple conditional breakpoints to stop your program when any of a number of exceptions are raised.

Next: , Previous: 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.

Delete any breakpoints at the next instruction to be executed in the selected stack frame (see Selecting a frame). When the innermost frame is selected, this is a good way to delete a breakpoint where your program just stopped.
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.

Next: , Previous: 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.

A breakpoint, watchpoint, or catchpoint can have any of four different states of enablement:

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 (see Setting breakpoints), breakpoints that you set are initially enabled; subsequently, they become disabled or enabled only when you use one of the commands above. (The command until can set and delete a breakpoint of its own, but it does not change the state of your other breakpoints; see Continuing and stepping.)

Next: , Previous: 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 (see Expressions). A breakpoint with a condition evaluates the expression each time your program reaches it, and your program stops only if the condition is true.

This is the converse of using assertions for program validation; in that situation, you want to stop when the assertion is violated—that is, when the condition is false. In C, if you want to test an assertion expressed by the condition assert, you should set the condition `! assert' on the appropriate breakpoint.

Conditions are also accepted for watchpoints; you may not need them, since a watchpoint is inspecting the value of an expression anyhow—but it might be simpler, say, to just set a watchpoint on a variable name, and specify a condition that tests whether the new value is an interesting one.

Break conditions can have side effects, and may even call functions in your program. This can be useful, for example, to activate functions that log program progress, or to use your own print functions to format special data structures. The effects are completely predictable unless there is another enabled breakpoint at the same address. (In that case, gdb might see the other breakpoint first and stop your program without checking the condition of this one.) Note that breakpoint commands are usually more convenient and flexible than break conditions for the purpose of performing side effects when a breakpoint is reached (see Breakpoint command lists).

Break conditions can be specified when a breakpoint is set, by using `if' in the arguments to the break command. See Setting breakpoints. They can also be changed at any time with the condition command.

You can also use the if keyword with the watch command. The catch command does not recognize the if keyword; condition is the only way to impose a further condition on a catchpoint.

condition bnum 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. See 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. See Continuing and stepping.

If a breakpoint has a positive ignore count and a condition, the condition is not checked. Once the ignore count reaches zero, gdb resumes checking the condition.

You could achieve the effect of the ignore count with a condition such as `$foo-- <= 0' using a debugger convenience variable that is decremented each time. See Convenience variables.

Ignore counts apply to breakpoints, watchpoints, and catchpoints.

Next: , Previous: 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 ...
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. See Commands for controlled output.

For example, here is how you could use breakpoint commands to print the value of x at entry to foo whenever x is positive.

     break foo if x>0
     printf "x is %d\n",x

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
     set x = y + 4

Next: , Previous: 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. If you realize this is a problem, you can use something like `break function(types)' to specify which particular version of the function you want. Otherwise, gdb offers you a menu of numbered choices for different possible breakpoints, and waits for your selection with the prompt `>'. The first two options are always `[0] cancel' and `[1] all'. Typing 1 sets a breakpoint at each definition of function, and typing 0 aborts the break command without setting any new breakpoints.

For example, the following session excerpt shows an attempt to set a breakpoint at the overloaded symbol String::after. We choose three particular definitions of that function name:

     (gdb) b String::after
     [0] cancel
     [1] all
     [2]; line number:867
     [3]; line number:860
     [4]; line number:875
     [5]; line number:853
     [6]; line number:846
     [7]; line number:735
     > 2 4 6
     Breakpoint 1 at 0xb26c: file, line 867.
     Breakpoint 2 at 0xb344: file, line 875.
     Breakpoint 3 at 0xafcc: file, line 846.
     Multiple breakpoints were set.
     Use the "delete" command to delete unwanted

Next: , Previous: 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.

Previous: 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.

Next: , Previous: 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. See 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 (see Break conditions).

The argument ignore-count is meaningful only when your program stopped due to a breakpoint. At other times, the argument to continue is ignored.

The synonyms c and fg (for foreground, as the debugged program is deemed to be the foreground program) are provided purely for convenience, and have exactly the same behavior as continue.

To resume execution at a different place, you can use return (see Returning from a function) to go back to the calling function; or jump (see Continuing at a different address) to go to an arbitrary location in your program.

A typical technique for using stepping is to set a breakpoint (see Breakpoints; watchpoints; and catchpoints) at the beginning of the function or the section of your program where a problem is believed to lie, run your program until it stops at that breakpoint, and then step through the suspect area, examining the variables that are interesting, until you see the problem happen.

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.

Continue running until just after function in the selected stack frame returns. Print the returned value (if any).

Contrast this with the return command (see Returning from a function).

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 of argument acceptable to break (see Setting breakpoints). This form of the command uses breakpoints, and hence is quicker than until without an argument. The specified location is actually reached only if it is in the current frame. This implies that until can be used to skip over recursive function invocations. For instance in the code below, if the current location is line 96, issuing until 99 will execute the program up to line 99 in the same invocation of factorial, i.e. after the inner invocations have returned.
          94	int factorial (int value)
          95	{
          96	    if (value > 1) {
          97            value *= factorial (value - 1);
          98	    }
          99	    return (value);
          100     }

advance location
Continue running the program up to the given location. An argument is required, which should be of the same form as arguments for the break command. Execution will also stop upon exit from the current stack frame. This command is similar to until, but advance will not skip over recursive function calls, and the target location doesn't have to be in the same frame as the current one.

stepi arg
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. See Automatic display.

An argument is a repeat count, as in step.

nexti arg
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.

Next: , Previous: 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 C-c); SIGSEGV is the signal a program gets from referencing a place in memory far away from all the areas in use; SIGALRM occurs when the alarm clock timer goes off (which happens only if your program has requested an alarm).

Some signals, including SIGALRM, are a normal part of the functioning of your program. Others, such as SIGSEGV, indicate errors; these signals are fatal (they kill your program immediately) if the program has not specified in advance some other way to handle the signal. SIGINT does not indicate an error in your program, but it is normally fatal so it can carry out the purpose of the interrupt: to kill the program.

gdb has the ability to detect any occurrence of a signal in your program. You can tell gdb in advance what to do for each kind of signal.

Normally, gdb is set up to let the non-erroneous signals like SIGALRM be silently passed to your program (so as not to interfere with their role in the program's functioning) but to stop your program immediately whenever an error signal happens. You can change these settings with the handle command.

info 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 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. The keywords say what change to make.

The keywords allowed by the handle command can be abbreviated. Their full names are:

gdb should not stop your program when this signal happens. It may still print a message telling you that the signal has come in.
gdb should stop your program when this signal happens. This implies the print keyword as well.
gdb should print a message when this signal happens.
gdb should not mention the occurrence of the signal at all. This implies the nostop keyword as well.
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.
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'. See Giving your program a signal.

Previous: Signals, Up: Stopping

5.4 Stopping and starting multi-thread programs

When your program has multiple threads (see Debugging programs with multiple threads), you can choose whether to set breakpoints on all threads, or on a particular thread.

break linespec thread threadno
break linespec thread threadno if ...
linespec specifies source lines; there are several ways of writing them, 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.

Next: , Previous: 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. See Selecting a frame.

When your program stops, gdb automatically selects the currently executing frame and describes it briefly, similar to the frame command (see Information about a frame).

Next: , 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 (see $fp) while execution is going on in that frame.

gdb assigns numbers to all existing stack frames, starting with zero for the innermost frame, one for the frame that called it, and so on upward. These numbers do not really exist in your program; they are assigned by gdb to give you a way of designating stack frames in gdb commands.

Some compilers provide a way to compile functions so that they operate without stack frames. (For example, the gcc option


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.

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.

Next: , Previous: 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.

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 C-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
Print the values of the local variables also.
bt full

The names where and info stack (abbreviated info s) are additional aliases for backtrace.

Each line in the backtrace shows the frame number and the function name. The program counter value is also shown—unless you use set print address off. The backtrace also shows the source file name and line number, as well as the arguments to the function. The program counter value is omitted if it is at the beginning of the code for that line number.

Here is an example of a backtrace. It was made with the command `bt 3', so it shows the innermost three frames.

     #0  m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8)
         at builtin.c:993
     #1  0x6e38 in expand_macro (sym=0x2b600) at macro.c:242
     #2  0x6840 in expand_token (obs=0x0, t=177664, td=0xf7fffb08)
         at macro.c:71
     (More stack frames follow...)

The display for frame zero does not begin with a program counter value, indicating that your program has stopped at the beginning of the code for line 993 of builtin.c.

If your program was compiled with optimizations, some compilers will optimize away arguments passed to functions if those arguments are never used after the call. Such optimizations generate code that passes arguments through registers, but doesn't store those arguments in the stack frame. gdb has no way of displaying such arguments in stack frames other than the innermost one. Here's what such a backtrace might look like:

     #0  m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8)
         at builtin.c:993
     #1  0x6e38 in expand_macro (sym=<value optimized out>) at macro.c:242
     #2  0x6840 in expand_token (obs=0x0, t=<value optimized out>, td=0xf7fffb08)
         at macro.c:71
     (More stack frames follow...)

The values of arguments that were not saved in their stack frames are shown as `<value optimized out>'.

If you need to display the values of such optimized-out arguments, either deduce that from other variables whose values depend on the one you are interested in, or recompile without optimizations.

Most programs have a standard user entry point—a place where system libraries and startup code transition into user code. For C this is main2. When gdb finds the entry function in a backtrace it will terminate the backtrace, to avoid tracing into highly system-specific (and generally uninteresting) code.

If you need to examine the startup code, or limit the number of levels in a backtrace, you can change this behavior:

set backtrace past-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 encouter 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.

Next: , Previous: 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. See Printing source lines, 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.

Previous: Selection, Up: Stack

6.4 Information about a frame

There are several other commands to print information about the selected stack frame.

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. See Selecting a frame.

info frame
info f
This command prints a verbose description of the selected stack frame, including:

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. See Selecting a frame.

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. See Setting catchpoints.

Next: , Previous: 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 (see Selecting a frame), gdb prints the line where execution in that frame has stopped. You can print other portions of source files by explicit command.

If you use gdb through its gnu Emacs interface, you may prefer to use Emacs facilities to view source; see Using gdb under gnu Emacs.

Next: , 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.

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.
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 (see Examining the 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, 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.
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.
As described in the preceding table.

Here are the ways of specifying a single source line—all the kinds of linespec.

Specifies line number of the current source file. When a list command has two linespecs, this refers to the same source file as the first linespec.
Specifies the line offset lines after the last line printed. When used as the second linespec in a list command that has two, this specifies the line offset lines down from the first linespec.
Specifies the line offset lines before the last line printed.
Specifies line number in the source file filename.
Specifies the line that begins the body of the function function. For example: in C, this is the line with the open brace.
Specifies the line of the open-brace 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.
Specifies the line containing the program address address. address may be any expression.

Next: , Previous: List, Up: Source

7.2 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.

Here are the forms of the edit command most commonly used:

Edit the current source file at the active line number in the program.
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.
edit filename:number
Specifies line number in the source file filename.
edit 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.
edit *address
Specifies the line containing the program address address. address may be any expression.

7.2.1 Choosing your editor

You can customize gdb to use any editor you want 3. By default, it is /bin/ex, but you can change this by setting the environment variable EDITOR before using gdb. For example, to configure gdb to use the vi editor, you could use these commands with the sh shell:

     export EDITOR
     gdb ...

or in the csh shell,

     setenv EDITOR /usr/bin/vi
     gdb ...

Next: , Previous: Edit, Up: Source

7.3 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.

Next: , Previous: Search, Up: Source

7.4 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. Neither is the current working directory, unless it happens to be in the source path.

Whenever you reset or rearrange the source path, gdb clears out any information it has cached about where source files are found and where each line is in the file.

When you start gdb, its source path includes only `cdir' and `cwd', in that order. To add other directories, use the directory command.

directory dirname ...
dir dirname ...
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.

Reset the source path to empty again. This requires confirmation.
show directories
Print the source path: show which directories it contains.

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 empty.
  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.

Previous: Source Path, Up: Source

7.5 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 understood by the list command (see Printing source lines).

For example, we can use info line to discover the location of the object code for the first line of function m4_changequote:

     (gdb) info line m4_changequote
     Line 895 of "builtin.c" starts at pc 0x634c and ends at 0x6350.

We can also inquire (using *addr as the form for linespec) what source line covers a particular address:

     (gdb) info line *0x63ff
     Line 926 of "builtin.c" starts at pc 0x63e4 and ends at 0x6404.

After info line, the default address for the x command is changed to the starting address of the line, so that `x/i' is sufficient to begin examining the machine code (see Examining memory). Also, this address is saved as the value of the convenience variable $_ (see Convenience variables).

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.

Next: , Previous: 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 (see Using gdb with Different 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 Output formats.
print /f
If you omit expr, gdb displays the last value again (from the value history; see 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. See Examining memory.

If you are interested in information about types, or about how the fields of a struct or a class are declared, use the ptype exp command rather than print. See Examining the Symbol Table.

Next: , 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 Compilation.

gdb supports array constants in expressions input by the user. The syntax is {element, element...}. For example, you can use the command print {1, 2, 3} to build up an array in memory that is malloced in the target program.

Because C is so widespread, most of the expressions shown in examples in this manual are in C. See Using gdb with Different Languages, for information on how to use expressions in other languages.

In this section, we discuss operators that you can use in gdb expressions regardless of your programming language.

Casts are supported in all languages, not just in C, because it is so useful to cast a number into a pointer in order to examine a structure at that address in memory.

gdb supports these operators, in addition to those common to programming languages:

`@' is a binary operator for treating parts of memory as arrays. See Artificial arrays, for more information.
`::' allows you to specify a variable in terms of the file or function where it is defined. See Program 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.

Next: , Previous: 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 (see Selecting a frame); they must be either:


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:


Here file or function is the name of the context for the static variable. In the case of file names, you can use quotes to make sure gdb parses the file name as a single word—for example, to print a global value of x defined in f2.c:

     (gdb) p 'f2.c'::x

This use of `::' is very rarely in conflict with the very similar use of the same notation in C++. gdb also supports use of the C++ scope resolution operator in gdb expressions.

Warning: Occasionally, a local variable may appear to have the wrong value at certain points in a function—just after entry to a new scope, and just before exit.
You may see this problem when you are stepping by machine instructions. This is because, on most machines, it takes more than one instruction to set up a stack frame (including local variable definitions); if you are stepping by machine instructions, variables may appear to have the wrong values until the stack frame is completely built. On exit, it usually also takes more than one machine instruction to destroy a stack frame; after you begin stepping through that group of instructions, local variable definitions may be gone.

This may also happen when the compiler does significant optimizations. To be sure of always seeing accurate values, turn off all optimization when compiling.

Another possible effect of compiler optimizations is to optimize unused variables out of existence, or assign variables to registers (as opposed to memory addresses). Depending on the support for such cases offered by the debug info format used by the compiler, gdb might not be able to display values for such local variables. If that happens, gdb will print a message like this:

     No symbol "foo" in current context.

To solve such problems, either recompile without optimizations, or use a different debug info format, if the compiler supports several such formats. For example, gcc, the gnu C/C++ compiler, usually supports the -gstabs+ option. -gstabs+ produces debug info in a format that is superior to formats such as COFF. You may be able to use DWARF 2 (-gdwarf-2), which is also an effective form for debug info. See Options for Debugging Your Program or gnu CC. See Debugging C++, for more info about debug info formats that are best suited to C++ programs.

Next: , Previous: 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 (see Value history), after printing one out.

Another way to create an artificial array is to use a cast. This re-interprets a value as if it were an array. The value need not be in memory:

     (gdb) p/x (short[2])0x12345678
     $1 = {0x1234, 0x5678}

As a convenience, if you leave the array length out (as in `(type[])value') gdb calculates the size to fill the value (as `sizeof(value)/sizeof(type)':

     (gdb) p/x (short[])0x12345678
     $2 = {0x1234, 0x5678}

Sometimes the artificial array mechanism is not quite enough; in moderately complex data structures, the elements of interest may not actually be adjacent—for example, if you are interested in the values of pointers in an array. One useful work-around in this situation is to use a convenience variable (see Convenience variables) as a counter in an expression that prints the first interesting value, and then repeat that expression via <RET>. For instance, suppose you have an array dtab of pointers to structures, and you are interested in the values of a field fv in each structure. Here is an example of what you might type:

     set $i = 0
     p dtab[$i++]->fv

Next: , Previous: 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:

Regard the bits of the value as an integer, and print the integer in hexadecimal.
Print as integer in signed decimal.
Print as integer in unsigned decimal.
Print as integer in octal.
Print as integer in binary. The letter `t' stands for “two”. 4
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. See info symbol.

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.
Regard the bits of the value as a floating point number and print using typical floating point syntax.

For example, to print the program counter in hex (see Registers), type

     p/x $pc

Note that no space is required before the slash; this is because command names in gdb cannot contain a slash.

To reprint the last value in the value history with a different format, you can use the print command with just a format and no expression. For example, `p/x' reprints the last value in hex.

Next: , Previous: 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
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'), and in addition `s' (for null-terminated strings) and `i' (for machine instructions). The default is `x' (hexadecimal) initially. The default changes each time you use either x or print.
u, the unit size
The unit size is any of
Halfwords (two bytes).
Words (four bytes). This is the initial default.
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. See 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'; see Registers) in hexadecimal (`x').

Since the letters indicating unit sizes are all distinct from the letters specifying output formats, you do not have to remember whether unit size or format comes first; either order works. The output specifications `4xw' and `4wx' mean exactly the same thing. (However, the count n must come first; `wx4' does not work.)

Even though the unit size u is ignored for the formats `s' and `i', you might still want to use a count n; for example, `3i' specifies that you want to see three machine instructions, including any operands. The command disassemble gives an alternative way of inspecting machine instructions; see Source and machine code.

All the defaults for the arguments to x are designed to make it easy to continue scanning memory with minimal specifications each time you use x. For example, after you have inspected three machine instructions with `x/3i addr', you can inspect the next seven with just `x/7'. If you use <RET> to repeat the x command, the repeat count n is used again; the other arguments default as for successive uses of x.

The addresses and contents printed by the x command are not saved in the value history because there is often too much of them and they would get in the way. Instead, gdb makes these values available for subsequent use in expressions as values of the convenience variables $_ and $__. After an x command, the last address examined is available for use in expressions in the convenience variable $_. The contents of that address, as examined, are available in the convenience variable $__.

If the x command has a repeat count, the address and contents saved are from the last memory unit printed; this is not the same as the last address printed if several units were printed on the last line of output.

When you are debugging a program running on a remote target machine (see Remote), you may wish to verify the program's image in the remote machine's memory against the executable file you downloaded to the target. The compare-sections command is provided for such situations.

compare-sections [section-name]
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.

Next: , Previous: 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 on how elaborate your format specification is—it uses x if you specify a unit size, or one of the two formats (`i' and `s') that are only supported by x; otherwise it uses print.

display expr
Add the expression expr to the list of expressions to display each time your program stops. See 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. See 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'. See Examining 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; see 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 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.

Next: , Previous: 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 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 repeats
Set the threshold for suppressing display of repeated array elelments. 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}
          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:
Allow gdb to choose a decoding style by inspecting your program.
Decode based on the gnu C++ compiler (g++) encoding algorithm. This is the default.
Decode based on the HP ANSI C++ (aCC) encoding algorithm.
Decode based on the Lucid C++ compiler (lcc) encoding algorithm.
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.

Next: , Previous: 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 +'.

Next: , Previous: 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 (see Registers). (Value history references, in contrast, are numbers preceded by `$'. See Value history.)

You can save a value in a convenience variable with an assignment expression, just as you would set a variable in your program. For example:

     set $foo = *object_ptr

would save in $foo the value contained in the object pointed to by object_ptr.

Using a convenience variable for the first time creates it, but its value is void until you assign a new value. You can alter the value with another assignment at any time.

Convenience variables have no fixed types. You can assign a convenience variable any type of value, including structures and arrays, even if that variable already has a value of a different type. The convenience variable, when used as an expression, has the type of its current value.

show convenience
Print a list of convenience variables used so far, and their values. Abbreviated show conv.

One of the ways to use a convenience variable is as a counter to be incremented or a pointer to be advanced. For example, to print a field from successive elements of an array of structures:

     set $i = 0
     print bar[$i++]->contents

Repeat that command by typing <RET>.

Some convenience variables are created automatically by gdb and given values likely to be useful.

The variable $_ is automatically set by the x command to the last address examined (see Examining memory). Other commands which provide a default address for x to examine also set $_ to that address; these commands include info line and info breakpoint. The type of $_ is void * except when set by the x command, in which case it is a pointer to the type of $__.

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.
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.

Next: , Previous: 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 pointer5 with

     set $sp += 4

Whenever possible, these four standard register names are available on your machine even though the machine has different canonical mnemonics, so long as there is no conflict. The info registers command shows the canonical names. For example, on the SPARC, info registers displays the processor status register as $psr but you can also refer to it as $ps; and on x86-based machines $ps is an alias for the eflags register.

gdb always considers the contents of an ordinary register as an integer when the register is examined in this way. Some machines have special registers which can hold nothing but floating point; these registers are considered to have floating point values. There is no way to refer to the contents of an ordinary register as floating point value (although you can print it as a floating point value with `print/f $regname').

Some registers have distinct “raw” and “virtual” data formats. This means that the data format in which the register contents are saved by the operating system is not the same one that your program normally sees. For example, the registers of the 68881 floating point coprocessor are always saved in “extended” (raw) format, but all C programs expect to work with “double” (virtual) format. In such cases, gdb normally works with the virtual format only (the format that makes sense for your program), but the info registers command prints the data in both formats.

Normally, register values are relative to the selected stack frame (see Selecting a frame). This means that you get the value that the register would contain if all stack frames farther in were exited and their saved registers restored. In order to see the true contents of hardware registers, you must select the innermost frame (with `frame 0').

However, gdb must deduce where registers are saved, from the machine code generated by your compiler. If some registers are not saved, or if gdb is unable to locate the saved registers, the selected stack frame makes no difference.

Next: , Previous: 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.

Next: , Previous: 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.

Next: , Previous: 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 `qPart:auxv:read' packet, see auxiliary vector.

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.

Next: , Previous: 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.

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 the target's maximum memory address. (0xffff on 16 bit targets, 0xffffffff on 32 bit targets, etc.)

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.
The list of attributes set for this memory region.

8.14.1 Attributes 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.

Memory is read only.
Memory is write only.
Memory is read/write. This is the default. Memory Access Size

The acccess size attributes tells gdb to use specific sized accesses in the memory region. Often memory mapped device registers require specific sized accesses. If no access size attribute is specified, gdb may use accesses of any size.

Use 8 bit memory accesses.
Use 16 bit memory accesses.
Use 32 bit memory accesses.
Use 64 bit memory accesses. 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.

Enable gdb to cache target memory.
Disable gdb from caching target memory. This is the default.

Next: , Previous: 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:

Raw binary form.
Intel hex format.
Motorola S-record format.
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.

Next: , Previous: 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. See Files, for information on invoking gdb in the post-mortem debugging mode.

Occasionally, you may wish to produce a core file of the program you are debugging in order to preserve a snapshot of its state. gdb has a special command for that.

generate-core-file [file]
gcore [file]
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, 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).

Next: , Previous: 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 (see Remote Debugging) to debug a program running on an IBM mainframe, which uses the ebcdic character set, then the host character set is Latin-1, and the target character set is ebcdic. If you give gdb the command set target-charset EBCDIC-US, then gdb translates between ebcdic and Latin 1 as you print character or string values, or use character and string literals in expressions.

gdb has no way to automatically recognize which character set the inferior program uses; you must tell it, using the set target-charset command, described below.

Here are the commands for controlling gdb's character set support:

set target-charset 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:

Seven-bit U.S. ascii. gdb can use this as its host character set.
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.
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.

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'.

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'.

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 uses the target character set for character and string literals you use in expressions:

     (gdb) print '+'
     $3 = 43 '+'

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'

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'

As above, gdb uses the target character set for character and string literals you use in expressions:

     (gdb) print '+'
     $10 = 78 '+'

The ibm1047 character set uses the number 78 to encode the `+' character.

Previous: Character Sets, Up: Data

8.18 Caching Data of Remote Targets

gdb can cache data exchanged between the debugger and a remote target (see Remote). Such caching generally improves performance, because it reduces the overhead of the remote protocol by bundling memory reads and writes into large chunks. Unfortunately, gdb does not currently know anything about volatile registers, and thus data caching will produce incorrect results when volatile registers are in use.

set remotecache 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.

Next: , Previous: 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. See Compilation.

A program may define a macro at one point, remove that definition later, and then provide a different definition after that. Thus, at different points in the program, a macro may have different definitions, or have no definition at all. If there is a current stack frame, gdb uses the macros in scope at that frame's source code line. Otherwise, gdb uses the macros in scope at the current listing location; see List.

At the moment, gdb does not support the ## token-splicing operator, the # stringification operator, or variable-arity macros.

Whenever gdb evaluates an expression, it always expands any macro invocations present in the expression. gdb also provides the following commands for working with macros explicitly.

macro expand 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, ...

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
     4       #define M 42
     5       #define ADD(x) (M + x)
     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)

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");

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

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

Next: , Previous: 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. See Targets. In addition, your remote target must know how to collect trace data. This functionality is implemented in the remote stub; however, none of the stubs distributed with gdb support tracepoints as of this writing.

This chapter describes the tracepoint commands and features.

Next: , 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 (see Set Breaks), a tracepoint has a number assigned to it by gdb. Like with breakpoints, tracepoint numbers are successive integers starting from one. Many of the commands associated with tracepoints take the tracepoint number as their argument, to identify which tracepoint to work on.

For each tracepoint, you can specify, in advance, some arbitrary set of data that you want the target to collect in the trace buffer when it hits that tracepoint. The collected data can include registers, local variables, or global data. Later, you can use gdb commands to examine the values these data had at the time the tracepoint was hit.

This section describes commands to set tracepoints and associated conditions and actions.

Next: , Up: Set Tracepoints

10.1.1 Create and Delete Tracepoints

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. See Set Breaks. The trace command defines a tracepoint, which is a point in the target program where the debugger will briefly stop, collect some data, and then allow the program to continue. Setting a tracepoint or changing its commands doesn't take effect until the next tstart command; thus, you cannot change the tracepoint attributes once a trace experiment is running.

Here are some examples of using the trace command:

          (gdb) trace foo.c:121    // a source file and line number
          (gdb) trace +2           // 2 lines forward
          (gdb) trace my_function  // first source line of function
          (gdb) trace *my_function // EXACT start address of function
          (gdb) trace *0x2117c4    // an address

You can abbreviate trace as tr.

The convenience variable $tpnum records the tracepoint number of the most recently set tracepoint.

delete tracepoint [num]
Permanently delete one or more tracepoints. With no argument, the default is to delete all tracepoints.


          (gdb) delete trace 1 2 3 // remove three tracepoints
          (gdb) delete trace       // remove all tracepoints

You can abbreviate this command as del tr.

Next: , Previous: 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.

Next: , Previous: 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.


          (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.

Next: , Previous: 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

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:
collect all registers
collect all function arguments
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 (see info scope) 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.

Next: , Previous: 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:
          (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

This command can be abbreviated info tp.

Previous: Listing Tracepoints, Up: Set Tracepoints

10.1.6 Starting and Stopping Trace Experiment

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.

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 (see Tracepoint Passcounts), or if the trace buffer becomes full.

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

Next: , Previous: 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.

Next: , 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'.
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

Next: , Previous: 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'

Previous: 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 (see Command Files).

Previous: 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

Next: , Previous: 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.

Next: , 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 (see A code overlay) shows a system with separate data and instruction address spaces. To map an overlay, the program copies its code from the larger address space to the instruction address space. Since the overlays shown here all use the same mapped address, only one may be mapped at a time. For a system with a single address space for data and instructions, the diagram would be similar, except that the program variables and heap would share an address space with the main program and the overlay area.

An overlay loaded into instruction memory and ready for use is called a mapped overlay; its mapped address is its address in the instruction memory. An overlay not present (or only partially present) in instruction memory is called unmapped; its load address is its address in the larger memory. The mapped address is also called the virtual memory address, or VMA; the load address is also called the load memory address, or LMA.

Unfortunately, overlays are not a completely transparent way to adapt a program to limited instruction memory. They introduce a new set of global constraints you must keep in mind as you design your program:

The overlay system described above is rather simple, and could be improved in many ways:

Next: , Previous: 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 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, 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:

Next: , Previous: 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 (see Overlay Commands), gdb looks in the inferior's memory for certain variables describing the current state of the overlays.

Here are the variables your overlay manager must define to support gdb's automatic overlay debugging:

This variable must be an array of the following structures:
            /* 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;

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.

Previous: 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 (see Overlay Description). Unfortunately, since linker scripts are specific to a particular host system, target architecture, and target memory layout, this manual cannot provide portable sample code demonstrating gdb's overlay support.

However, the gdb source distribution does contain an overlaid program, with linker scripts for a few systems, as part of its test suite. The program consists of the following files from gdb/testsuite/gdb.base:

The main program file.
A simple overlay manager, used by overlays.c.
Overlay modules, loaded and used by overlays.c.
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.

Next: , Previous: 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.

Next: , 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. See Displaying the language.

This is most commonly a problem when you use a program, such as cfront or f2c, that generates C but is written in another language. In that case, make the program use #line directives in its C output; that way gdb will know the correct language of the source code of the original program, and will display that source code, not the generated C code.

Next: , 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 source file.
C source file
C++ source file
Objective-C source file
Fortran source file
Modula-2 source file
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. See Displaying the language.

Next: , Previous: 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.

Previous: 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.

Next: , Previous: 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. See Information about a frame, to identify the other information listed here.
info source
Display the source language of this source file. See Examining the Symbol Table, 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.

Next: , Previous: 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. See Supported languages, for the default settings of supported languages.

Next: , 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

error--> 1 + 2.3

The second example fails because the CARDINAL 1 is not type-compatible with the REAL 2.3.

For the expressions you use in gdb commands, you can tell the gdb type checker to skip checking; to treat any mismatches as errors and abandon the expression; or to only issue warnings when type mismatches occur, but evaluate the expression anyway. When you choose the last of these, gdb evaluates expressions like the second example above, but also issues a warning.

Even if you turn type checking off, there may be other reasons related to type that prevent gdb from evaluating an expression. For instance, gdb does not know how to add an int and a struct foo. These particular type errors have nothing to do with the language in use, and usually arise from expressions, such as the one described above, which make little sense to evaluate anyway.

Each language defines to what degree it is strict about type. For instance, both Modula-2 and C require the arguments to arithmetical operators to be numbers. In C, enumerated types and pointers can be represented as numbers, so that they are valid arguments to mathematical operators. See Supported languages, for further details on specific languages.

gdb provides some additional commands for controlling the type checker:

set check type auto
Set type checking on or off based on the current working language. See 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.

Previous: 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. See 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. See 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.

Next: , Previous: 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 (see Expressions) can be used with the constructs of any supported language.

The following sections detail to what degree each source language is supported by gdb. These sections are not meant to be language tutorials or references, but serve only as a reference guide to what the gdb expression parser accepts, and what input and output formats should look like for different languages. There are many good books written on each of these languages; please look to these for a language reference or tutorial.

Next: , 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+. See Options for Debugging Your Program or gnu CC.

Next: , Up: C 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:

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.
Used in an expression of the form a opb, 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 (see 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)' (or, if you prefer, simply `&&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 (see 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.

Next: , Previous: C Operators, Up: C C and C++ constants

gdb allows you to express the constants of C and C++ in the following ways:

Next: , Previous: C Constants, Up: C 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. See gdb features for C++.

    You must specify set overload-resolution off in order to use an explicit function signature to call an overloaded function, as in

              p 'foo(char,int)'('x', 13)

    The gdb command-completion facility can simplify this; see Command completion.

  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 (see Program variables).

In addition, when used with HP's C++ compiler, gdb supports calling virtual functions correctly, printing out virtual bases of objects, calling functions in a base subobject, casting objects, and invoking user-defined operators.

Next: , Previous: C plus plus expressions, Up: C 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++. See Having gdb infer the source language, for further details.

Next: , Previous: C Defaults, Up: C 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:

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.

Next: , Previous: C Checks, Up: C 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. See Expressions.

Previous: Debugging C, Up: C 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. See 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. See Setting breakpoints.

catch throw
catch catch
Debug C++ exception handling using these commands. See Setting catchpoints.

ptype typename
Print inheritance relationships as well as other information for type typename. See Examining the Symbol Table.

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. See Print settings.
set print object
show print object
Choose whether to print derived (actual) or declared types of objects. See Print settings.
set print vtbl
show print vtbl
Control the format for printing virtual function tables. See 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 C++ 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. See Command completion, for details on how to do this.

Next: , Previous: 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 info classes, and info selectors, for a few more commands specific to Objective-C support.

Next: , Previous: Objective-C, Up: Objective-C Method Names in Commands

The following commands have been extended to accept Objective-C method names as line specifications:

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:]

Previous: Method Names in Commands, Up: Objective-C 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.

Next: , Previous: Objective-C, Up: Supported languages

12.4.3 Fortran

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 current program location are printed.

Fortran symbols are usually case-insensitive, so gdb by default uses case-insensitive matches for Fortran symbols. You can change that with the `set case-insensitive' command, see Symbols, for the details.

Next: , Previous: 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. See pascal_static-members.

Next: , Previous: 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.

Next: , Up: Modula-2 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:

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.
Set membership. Defined on set types and the types of their members. Same precedence as <.
Boolean disjunction. Defined on boolean types.
AND, &
Boolean conjunction. Defined on boolean types.
The gdb “artificial array” operator (see 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 *.
Integer division and remainder. Defined on integral types. Same precedence as *.
Negative. Defined on INTEGER and REAL data.
Pointer dereferencing. Defined on pointer types.
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: Sets 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.

Next: , Previous: M2 Operators, Up: Modula-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:

represents an ARRAY variable.
represents a CHAR constant or variable.
represents a variable or constant of integral type.
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).
represents a variable or constant of integral or floating-point type.
represents a variable or constant of floating-point type.
represents a type.
represents a variable.
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.

Returns the absolute value of n.
If c is a lower case letter, it returns its upper case equivalent, otherwise it returns its argument.
Returns the character whose ordinal value is i.
Decrements the value in the variable v by one. Returns the new value.
Decrements the value in the variable v by i. Returns the new value.
Removes the element m from the set s. Returns the new set.
Returns the floating point equivalent of the integer i.
Returns the index of the last member of a.
Increments the value in the variable v by one. Returns the new value.
Increments the value in the variable v by i. Returns the new value.
Adds the element m to the set s if it is not already there. Returns the new set.
Returns the maximum value of the type t.
Returns the minimum value of the type t.
Returns boolean TRUE if i is an odd number.
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.
Returns the size of its argument. x can be a variable or a type.
Returns the integral part of r.
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.

Next: , Previous: Built-In Func/Proc, Up: Modula-2 Constants

gdb allows you to express the constants of Modula-2 in the following ways:

Next: , Previous: M2 Constants, Up: Modula-2 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. See Having gdb set the language automatically, for further details.

Next: , Previous: M2 Defaults, Up: Modula-2 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:

Next: , Previous: Deviations, Up: Modula-2 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:

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.

Next: , Previous: M2 Checks, Up: Modula-2 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.

Previous: M2 Scope, Up: Modula-2 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 (see Expressions), while available with any language, is not useful with Modula-2. Its intent is to aid the debugging of dynamic arrays, which cannot be created in Modula-2 as they can in C or C++. However, because an address can be specified by an integral constant, the construct `{type}adrexp' is still useful.

In gdb scripts, the Modula-2 inequality operator # is interpreted as the beginning of a comment. Use <> instead.

Previous: 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.

Next: , Up: Ada 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

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.

Next: , Previous: Ada Mode Intro, Up: Ada Omissions from Ada

Here are the notable omissions from the subset:

Next: , Previous: Omissions from Ada, Up: Ada Additions to Ada

As it does for other languages, gdb makes certain generic extensions to Ada (see Expressions):

In addition, gdb provides a few other shortcuts and outright additions specific to Ada:

Next: , Previous: Additions to Ada, Up: Ada 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.

Previous: Stopping Before Main Program, Up: Ada Known Peculiarities of Ada Mode

Besides the omissions listed previously (see Omissions from Ada), we know of several problems with and limitations of Ada mode in gdb, some of which will be fixed with planned future releases of the debugger and the GNU Ada compiler.

Previous: 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.

Next: , Previous: 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 (see Choosing files), or by one of the file-management commands (see Commands to specify files).

Occasionally, you may need to refer to symbols that contain unusual characters, which gdb ordinarily treats as word delimiters. The most frequent case is in referring to static variables in other source files (see Program variables). File names are recorded in object files as debugging symbols, but gdb would ordinarily parse a typical file name, like foo.c, as the three words `foo' `.' `c'. To allow gdb to recognize `foo.c' as a single symbol, enclose it in single quotes; for example,

     p 'foo.c'::x

looks up the value of x in the scope of the file foo.c.

set case-sensitive 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 expr
Print the data type of expression expr. expr is not actually evaluated, and any side-effecting operations (such as assignments or function calls) inside it do not take place. See Expressions.
Print the data type of $, the last value in the value history.

ptype typename
Print a description of data type typename. typename may be the name of a type, or for C code it may have the form `class class-name', `struct struct-tag', `union union-tag' or `enum enum-tag'.
ptype expr
Print a description of the type of expression expr. ptype differs from whatis by printing a detailed description, instead of just the name of the type.

For example, for this variable declaration:

          struct complex {double real; double imag;} v;

the two commands give this output:

          (gdb) whatis v
          type = struct complex
          (gdb) ptype v
          type = struct complex {
              double real;
              double imag;

As with whatis, using ptype without an argument refers to the type of $, the last value in the value history.

info types 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. 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 collect.

info source
Show information about the current source file—that is, the source file for the function containing the current point of execution:

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 (eg. `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. See Commands to specify files, for a discussion of how gdb reads symbols (in the description of symbol-file).

maint info symtabs [ regexp ]
maint info psymtabs [ regexp ]
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

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)
              debugformat DWARF 2

Next: , Previous: 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.

Next: , Up: Altering

14.1 Assignment to variables

To alter the value of a variable, evaluate an assignment expression. See 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). See Using gdb with Different 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 (see 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 (see 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.

Next: , Previous: 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
Resume execution at line linespec. Execution stops again immediately if there is a breakpoint there. See Printing source lines, for a description of the different forms of linespec. It is common practice to use the tbreak command in conjunction with jump. See Setting breakpoints.

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.

jump *address
Resume execution at the instruction at address address.

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. See 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.

Next: , Previous: 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 (see Signals). The signal command passes the signal directly to your program.

Next: , Previous: Signaling, Up: Altering

14.4 Returning from a function

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 (see Selecting a frame), 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 (see Continuing and stepping) resumes execution until the selected stack frame returns naturally.

Next: , Previous: Returning, Up: Altering

14.5 Calling program functions

print expr
Evaluate the expression expr and display the resuling 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.

Previous: 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.

Next: , Previous: 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.

Next: , 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 (see Getting In and Out of gdb).

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 (see file). 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.

On systems with memory-mapped files, an auxiliary file named filename.syms may hold symbol table information for filename. If so, gdb maps in the symbol table from filename.syms, starting up more quickly. See the descriptions of the file options `-mapped' and `-readnow' (available on the command line, see -readnow, and with the commands file, symbol-file, or add-symbol-file, described below), for more information.

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 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 its convenience variables, the value history, and all 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. See Optional warnings and messages.)

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 ] [ -mapped ]
file filename [ -readnow ] [ -mapped ]
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.

If memory-mapped files are available on your system through the mmap system call, you can use another option, `-mapped', to cause gdb to write the symbols for your program into a reusable file. Future gdb debugging sessions map in symbol information from this auxiliary symbol file (if the program has not changed), rather than spending time reading the symbol table from the executable program. Using the `-mapped' option has the same effect as starting gdb with the `-mapped' command-line option.

You can use both options together, to make sure the auxiliary symbol file has all the symbol information for your program.

The auxiliary symbol file for a program called myprog is called `myprog.syms'. Once this file exists (so long as it is newer than the corresponding executable), gdb always attempts to use it when you debug myprog; no special options or commands are needed.

The .syms file is specific to the host machine where you run gdb. It holds an exact image of the internal gdb symbol table. It cannot be shared across multiple host platforms.

core-file [filename]
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 (see Killing the child process).

add-symbol-file filename address
add-symbol-file filename address [ -readnow ] [ -mapped ]
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:

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.

You can use the `-mapped' and `-readnow' options just as with the symbol-file command, to change how gdb manages the symbol table information for filename.

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 (see 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 (see Specifying a Debugging Target), 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):
Display sections for all loaded object files, including shared libraries.
Display info only for named sections.
Display info only for sections for which section-flags are true. The section flags that gdb currently knows about are:
Section will have space allocated in the process when loaded. Set for all sections except those containing debug information.
Section will be loaded from the file into the child process memory. Set for pre-initialized code and data, clear for .bss sections.
Section needs to be relocated before loading.
Section cannot be modified by the child process.
Section contains executable code only.
Section contains data only (no executable code).
Section will reside in ROM.
Section contains data for constructor/destructor lists.
Section is not empty.
An instruction to the linker to not output the section.
A notification to the linker that the section contains COFF shared library information.
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.

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 expresion 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.
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 solib-absolute-prefix path
If this variable is set, path will be used as a prefix for any absolute shared library paths; many runtime loaders store the absolute paths to the shared library in the target program's memory. If you use `solib-absolute-prefix' to find shared libraries, they need to be laid out in the same way that they are on the target, with e.g. a /usr/lib hierarchy under path.

You can set the default value of `solib-absolute-prefix' by using the configure-time `--with-sysroot' option.

show solib-absolute-prefix
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 `solib-absolute-prefix' 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 `solib-absolute-prefix', be sure to set `solib-absolute-prefix' to a nonexistant directory to prevent gdb from finding your host's libraries.

show solib-search-path
Display the current shared library search path.

Next: , Previous: 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.

If an executable's debugging information has been extracted to a separate file, the executable should contain a debug link giving the name of the debugging information file (with no directory components), and a checksum of its contents. (The exact form of a debug link is described below.) If the full name of the directory containing the executable is execdir, and the executable has a debug link that specifies the name debugfile, then gdb will automatically search for the debugging information file in three places:

gdb checks under each of these names for a debugging information file whose checksum matches that given in the link, and reads the debugging information from the first one it finds.

So, for example, if you ask gdb to debug /usr/bin/ls, which has a link containing the name ls.debug, and the global debug directory is /usr/lib/debug, then gdb will look for debug information in /usr/bin/ls.debug, /usr/bin/.debug/ls.debug, and /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:

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 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.

As of December 2002, there is no standard GNU utility to produce separated executable / debugging information file pairs. 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 removes the debugging information from the executable file foo, places it in the file foo.debug, and leaves behind a debug link in foo.

Since there are many different ways to compute CRC's (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,
       unsigned char *end;
       crc = ~crc & 0xffffffff;
       for (end = buf + len; buf < end; ++buf)
         crc = crc32_table[(crc ^ *buf) & 0xff] ^ (crc >> 8);
       return ~crc & 0xffffffff;

Previous: 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 (see Optional warnings and messages).

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. See Optional warnings and messages.)

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.

Next: , Previous: 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 (see Commands for managing targets).

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
These are alias commands for, respectively, set architecture and show architecture.

Next: , 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 (see Commands to specify files). To specify as a target a process that is already running, use the attach command (see Debugging an already-running process).

Next: , Previous: 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 (see Commands to specify 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.

See Commands to specify 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 dev
Remote serial target in GDB-specific protocol. The argument dev specifies what serial device to use for the connection (e.g. /dev/ttya). See Remote debugging. 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

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 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, such as the size of the data chunks used by gdb to download program parts to the remote target.

set download-write-size size
Set the write size used when downloading a program. Only used when downloading a program onto a remote target. Specify zero or a negative value to disable blocked writes. The actual size of each transfer is also limited by the size of the target packet and the memory cache.

show download-write-size
Show the current value of the write size.
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.

load does not repeat if you press <RET> again after using it.

Next: , Previous: 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.

Next: , Previous: Byte Order, Up: Targets

16.4 Remote debugging

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.

Once you've connected to the remote target, gdb allows you to send arbitrary commands to the remote monitor:

remote command
Send an arbitrary command string to the remote monitor.

Previous: Remote, Up: Targets

16.5 Kernel Object Display

Some targets support kernel object display. Using this facility, gdb communicates specially with the underlying operating system and can display information about operating system-level objects such as mutexes and other synchronization objects. Exactly which objects can be displayed is determined on a per-OS basis.

Use the set os command to set the operating system. This tells gdb which kernel object display module to initialize:

     (gdb) set os cisco

The associated command show os displays the operating system set with the set os command; if no operating system has been set, show os will display an empty string `""'.

If set os succeeds, gdb will display some information about the operating system, and will create a new info command which can be used to query the target. The info command is named after the operating system:

     (gdb) info cisco
     List of Cisco Kernel Objects
     Object     Description
     any        Any and all objects

Further subcommands can be used to query about particular objects known by the kernel.

There is currently no way to determine whether a given operating system is supported other than to try setting it with set os name, where name is the name of the operating system you want to try.

Next: , Previous: Targets, Up: Top

17 Debugging remote programs

Next: , 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 symobl and debugging information. Start up gdb as usual, using the name of the local copy of your program as the first argument.

If you're using a serial line, you may want to give gdb the `--baud' option, or use the set remotebaud command (see set remotebaud) before the target command.

After that, use target remote to establish communications with the target machine. Its argument specifies how to communicate—either via a devicename attached to a direct serial line, or a TCP or UDP port (possibly to a terminal server which in turn has a serial line to the target). For example, to use a serial line connected to the device named /dev/ttyb:

     target remote /dev/ttyb

To use a TCP connection, use an argument of the form host:port or tcp:host:port. 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 of 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.

To use a UDP connection, use an argument of the form udp:host:port. 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.

Now 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 <C-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.

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.

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 commands directly to the remote monitor.

Next: , Previous: Connecting, Up: Remote Debugging

17.2 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.

On the target machine,
you need to have a copy of the program you want to debug. 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).6 You must use the same port number with the host gdb target remote command.

On some targets, gdbserver can also attach to running programs. This is accomplished via the --attach argument. The syntax is:

          target> gdbserver comm --attach 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 comm --attach `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.

On the host machine,
connect to your target (see Connecting to a remote target). 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'. You don't need to use the load command in gdb when using gdbserver, since the program is already on the target. However, if you want to load the symbols (as you normally would), do that with the file command, and issue it before connecting to the server; otherwise, you will get an error message saying "Program is already running", since the program is considered running after the connection.

Next: , Previous: Server, Up: Remote Debugging

17.3 Using the gdbserve.nlm program

gdbserve.nlm is a control program for NetWare systems, which allows you to connect your program with a remote gdb via target remote.

gdb and gdbserve.nlm communicate via a serial line, using the standard gdb remote serial protocol.

On the target machine,
you need to have a copy of the program you want to debug. gdbserve.nlm 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 syntax is:

          load gdbserve [ BOARD=board ] [ PORT=port ]
                        [ BAUD=baud ] program [ args ... ]

board and port specify the serial line; baud specifies the baud rate used by the connection. port and node default to 0, baud defaults to 9600bps.

For example, to debug Emacs with the argument `foo.txt'and communicate with gdb over serial port number 2 or board 1 using a 19200bps connection:

          load gdbserve BOARD=1 PORT=2 BAUD=19200 emacs foo.txt

On the gdb host machine, connect to your target (see Connecting to a remote target).

Next: , Previous: NetWare, 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 system-call-allowed.

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 press the <Ctrl-C> key to interrupt the program running on the remote. If set to off, gdb sends the `Strl-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 remotedebug
Control the debugging of the remote protocol. When enabled, each packet sent to or received from the remote target is displayed. The defaults is off.
show remotedebug
Show the current setting of the remote protocol debugging.
set remotedevice device
Set the name of the serial port through which to communicate to the remote target to device. This is the device used by gdb to open the serial communications line to the remote target. There's no default, so you must set a valid port name for the remote serial communications to work. (Some varieties of the target command accept the port name as part of their arguments.)
show remotedevice
Show the current name of the serial port.
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 fetch-register-packet
set remote set-register-packet
set remote P-packet
set remote p-packet
Determine whether gdb can set and fetch registers from the remote target using the `P' packets. The default depends on the remote stub's support of the `P' packets (gdb queries the stub when this packet is first required).
show remote fetch-register-packet
show remote set-register-packet
show remote P-packet
show remote p-packet
Show the current setting of using the `P' packets for setting and fetching registers from the remote target.

set remote binary-download-packet
set remote X-packet
Determine whether gdb sends downloads in binary mode using the `X' packets. The default is on.
show remote binary-download-packet
show remote X-packet
Show the current setting of using the `X' packets for binary downloads.
set remote read-aux-vector-packet
Set the use of the remote protocol's `qPart:auxv:read' (target auxiliary vector read) request. This request is used to fetch the remote target's auxiliary vector, see Auxiliary Vector. The default setting depends on the remote stub's support of this request (gdb queries the stub when this request is first required). See qPart, for more information about this request.
show remote read-aux-vector-packet
Show the current setting of use of the `qPart:auxv:read' request.
set remote symbol-lookup-packet
Set the use of the remote protocol's `qSymbol' (target symbol lookup) request. This request is used to communicate symbol information to the remote target, e.g., whenever a new shared library is loaded by the remote (see shared libraries). The default setting depends on the remote stub's support of this request (gdb queries the stub when this request is first required). See qSymbol, for more information about this request.
show remote symbol-lookup-packet
Show the current setting of use of the `qSymbol' request.
set remote verbose-resume-packet
Set the use of the remote protocol's `vCont' (descriptive resume) request. This request is used to resume specific threads in the remote target, and to single-step or signal them. The default setting depends on the remote stub's support of this request (gdb queries the stub when this request is first required). This setting affects debugging of multithreaded programs: if `vCont' cannot be used, gdb might be unable to single-step a specific thread, especially under set scheduler-locking off; it is also impossible to pause a specific thread. See vCont, for more details.
show remote verbose-resume-packet
Show the current setting of use of the `vCont' request
set remote software-breakpoint-packet
set remote hardware-breakpoint-packet
set remote write-watchpoint-packet
set remote read-watchpoint-packet
set remote access-watchpoint-packet
set remote Z-packet
These commands enable or disable the use of `Z' packets for setting breakpoints and watchpoints in the remote target. The default depends on the remote stub's support of the `Z' packets (gdb queries the stub when each packet is first required). The command set remote Z-packet, kept for back-compatibility, turns on or off all the features that require the use of `Z' packets.
show remote software-breakpoint-packet
show remote hardware-breakpoint-packet
show remote write-watchpoint-packet
show remote read-watchpoint-packet
show remote access-watchpoint-packet
show remote Z-packet
Show the current setting of `Z' packets usage.
set remote get-thread-local-storage-address
This command enables or disables the use of the `qGetTLSAddr' (Get Thread Local Storage Address) request packet. The default depends on whether the remote stub supports this request. See qGetTLSAddr, for more details about this packet.
show remote get-thread-local-storage-address
Show the current setting of `qGetTLSAddr' packet usage.

Previous: 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 (see Specifying a Debugging Target).
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. See Using the gdbserver program, 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:

For Intel 386 and compatible architectures.
For Motorola 680x0 architectures.
For Renesas SH architectures.
For sparc architectures.
For Fujitsu sparclite architectures.

The README file in the gdb distribution may list other recently added stubs.

Next: , Up: remote stub

17.5.1 What the stub can do for you

The debugging stub for your architecture supplies these three subroutines:

This routine arranges for handle_exception to run when your program stops. You must call this subroutine explicitly near the beginning of your program.
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.

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.

Next: , Previous: 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.

Previous: 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 (see What you must do for the stub):
              getDebugChar, putDebugChar,
              flush_i_cache, memset, exceptionHandler.
  2. Insert these lines near the top of your program:
  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 (see Connecting to a remote target).

Next: , Previous: 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.

Next: , Up: Configurations

18.1 Native

This section describes details specific to particular native configurations.

Next: , 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.

Next: , Previous: 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.

Next: , Previous: 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 filesystem 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.
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.

Next: , Previous: 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.

Next: , Previous: 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 subsection. The subsubsection see Non-debug DLL symbols describes working with DLLs that have no debugging 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 the six segment registers.

info dll
This is a Cygwin specific alias of info shared.

This command loads symbols from a dll similarly to add-sym command but without the need to specify a base address.

set new-console mode
If mode is on the debuggee will be started in a new console on next start. If mode is offi, 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 events seen by the debugger.

set debugexec
This boolean value adds debug output concerning execute events seen by the debugger.

set debugexceptions
This boolean value adds debug ouptut concerning exception events seen by the debugger.

set debugmemory
This boolean value adds debug ouptut concerning memory events seen 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.

Up: Cygwin Native 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 subsubsection 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 see Files or the dll-symbols command in see 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. 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 (see see 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...] 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.

Next: , Previous: 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 comamnd 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.

Previous: 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.

Next: , Previous: 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.

gdb includes the ability to debug programs running on various real-time operating systems.

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:


Next: , Up: VxWorks 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 (see Your program's 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.

Next: , Previous: VxWorks Connection, Up: VxWorks 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.)

Previous: VxWorks Download, Up: VxWorks 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.

Next: , Previous: 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.

Next: , 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:
Determine the FPU type by querying the OS ABI.
Software FPU, with mixed-endian doubles on little-endian ARM processors.
GCC-compiled FPA co-processor.
Software FPU with pure-endian doubles.
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.

Next: , Previous: ARM, Up: Embedded Processors

18.3.2 Renesas H8/300

target hms dev
A Renesas SH, H8/300, or H8/500 board, attached via serial line to your host. Use special commands device and speed to control the serial line and the communications speed used.

target e7000 dev
E7000 emulator for Renesas H8 and SH.

target sh3 dev
target sh3e dev
Renesas SH-3 and SH-3E target systems.

When you select remote debugging to a Renesas SH, H8/300, or H8/500 board, the load command downloads your program to the Renesas board and also opens it as the current executable target for gdb on your host (like the file command).

gdb needs to know these things to talk to your Renesas SH, H8/300, or H8/500:

  1. that you want to use `target hms', the remote debugging interface for Renesas microprocessors, or `target e7000', the in-circuit emulator for the Renesas SH and the Renesas 300H. (`target hms' is the default when gdb is configured specifically for the Renesas SH, H8/300, or H8/500.)
  2. what serial device connects your host to your Renesas board (the first serial device available on your host is the default).
  3. what speed to use over the serial device.

Next: , Up: H8/300 Connecting to Renesas boards

Use the special gdb command `device port' if you need to explicitly set the serial device. The default port is the first available port on your host. This is only necessary on Unix hosts, where it is typically something like /dev/ttya.

gdb has another special command to set the communications speed: `speed bps'. This command also is only used from Unix hosts; on DOS hosts, set the line speed as usual from outside gdb with the DOS mode command (for instance, mode com2:9600,n,8,1,p for a 9600bps connection).

The `device' and `speed' commands are available only when you use a Unix host to debug your Renesas microprocessor programs. If you use a DOS host, gdb depends on an auxiliary terminate-and-stay-resident program called asynctsr to communicate with the development board through a PC serial port. You must also use the DOS mode command to set up the serial port on the DOS side.

The following sample session illustrates the steps needed to start a program under gdb control on an H8/300. The example uses a sample H8/300 program called t.x. The procedure is the same for the Renesas SH and the H8/500.

First hook up your development board. In this example, we use a board attached to serial port COM2; if you use a different serial port, substitute its name in the argument of the mode command. When you call asynctsr, the auxiliary comms program used by the debugger, you give it just the numeric part of the serial port's name; for example, `asyncstr 2' below runs asyncstr on COM2.

     C:\H8300\TEST> asynctsr 2
     C:\H8300\TEST> mode com2:9600,n,8,1,p
     Resident portion of MODE loaded
     COM2: 9600, n, 8, 1, p
Warning: We have noticed a bug in PC-NFS that conflicts with asynctsr. If you also run PC-NFS on your DOS host, you may need to disable it, or even boot without it, to use asynctsr to control your development board.

Now that serial communications are set up, and the development board is connected, you can start up gdb. Call gdb with the name of your program as the argument. gdb prompts you, as usual, with the prompt `(gdb)'. Use two special commands to begin your debugging session: `target hms' to specify cross-debugging to the Renesas board, and the load command to download your program to the board. load displays the names of the program's sections, and a `*' for each 2K of data downloaded. (If you want to refresh gdb data on symbols or on the executable file without downloading, use the gdb commands file or symbol-file. These commands, and load itself, are described in Commands to specify files.)

     (eg-C:\H8300\TEST) gdb t.x
     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, Copyright 1992 Free Software Foundation, Inc...
     (gdb) target hms
     Connected to remote H8/300 HMS system.
     (gdb) load t.x
     .text   : 0x8000 .. 0xabde ***********
     .data   : 0xabde .. 0xad30 *
     .stack  : 0xf000 .. 0xf014 *

At this point, you're ready to run or debug your program. From here on, you can use all the usual gdb commands. The break command sets breakpoints; the run command starts your program; print or x display data; the continue command resumes execution after stopping at a breakpoint. You can use the help command at any time to find out more about gdb commands.

Remember, however, that operating system facilities aren't available on your development board; for example, if your program hangs, you can't send an interrupt—but you can press the reset switch!

Use the reset button on the development board

In either case, gdb sees the effect of a reset on the development board as a “normal exit” of your program.

Next: , Previous: Renesas Boards, Up: H8/300 Using the E7000 in-circuit emulator

You can use the E7000 in-circuit emulator to develop code for either the Renesas SH or the H8/300H. Use one of these forms of the `target e7000' command to connect gdb to your E7000:

target e7000 port speed
Use this form if your E7000 is connected to a serial port. The port argument identifies what serial port to use (for example, `com2'). The third argument is the line speed in bits per second (for example, `9600').
target e7000 hostname
If your E7000 is installed as a host on a TCP/IP network, you can just specify its hostname; gdb uses telnet to connect.

The following special commands are available when debugging with the Renesas E7000 ICE:

e7000 command
This sends the specified command to the E7000 monitor.
ftplogin machine username password dir
This command records information for subsequent interface with the E7000 monitor via the FTP protocol: gdb will log into the named machine using specified username and password, and then chdir to the named directory dir.
ftpload file
This command uses credentials recorded by ftplogin to fetch and load the named file from the E7000 monitor.
This command drains any pending text buffers stored on the E7000.
set usehardbreakpoints
show usehardbreakpoints
These commands set and show the use of hardware breakpoints for all breakpoints. See hardware-assisted breakpoint, for more information about using hardware breakpoints selectively.

Previous: Renesas ICE, Up: H8/300 Special gdb commands for Renesas micros

Some gdb commands are available only for the H8/300:

set machine h8300
set machine h8300h
Condition gdb for one of the two variants of the H8/300 architecture with `set machine'. You can use `show machine' to check which variant is currently in effect.

Next: , Previous: H8/300, Up: Embedded Processors

18.3.3 H8/500

set memory mod
show memory
Specify which H8/500 memory model (mod) you are using with `set memory'; check which memory model is in effect with `show memory'. The accepted values for mod are small, big, medium, and compact.

Next: , Previous: H8/500, Up: Embedded Processors

18.3.4 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 donwloadable 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:

This command resets the SDI connection.
This command shows the SDI connection status.
Instructs the remote that M32R/Chaos debugging is to be used.
Instructs the remote to use the DEBUG_DMA method of accessing memory.
Instructs the remote to use the MON_CODE method of accessing memory.
Instructs the remote to set breakpoints by IB break.
Instructs the remote to set breakpoints by DBT.

Next: , Previous: M32R/D, Up: Embedded Processors

18.3.5 M68k

The Motorola m68k configuration includes ColdFire support, and target command for the following ROM monitors.

target abug dev
ABug ROM monitor for M68K.

target cpu32bug dev
CPU32BUG monitor, running on a CPU32 (M68K) board.

target dbug dev
dBUG ROM monitor for Motorola ColdFire.

target est dev
EST-300 ICE monitor, running on a CPU32 (M68K) board.

target rom68k dev
ROM 68K monitor, running on an M68K IDP board.
target rombug dev
ROMBUG ROM monitor for OS/9000.

Next: , Previous: M68K, Up: Embedded Processors

18.3.6 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
ddb target
lsi target

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.

Next: , Previous: MIPS Embedded, Up: Embedded Processors

18.3.7 OpenRISC 1000

See OR1k Architecture document ( 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:

Load effective address/data
Store effective address/data
Access effective address ($SEA or $LEA) or data ($SDATA/$LDATA)
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 Effecive 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.

Next: , Previous: PA, Up: Embedded Processors

18.3.8 PowerPC

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 specifi to the SDS protocol are supported bygdb:

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.

Next: , Previous: OpenRISC 1000, Up: Embedded Processors

18.3.9 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.

Next: , Previous: PowerPC, Up: Embedded Processors

18.3.10 Renesas SH

target hms dev
A Renesas SH board attached via serial line to your host. Use special commands device and speed to control the serial line and the communications speed used.

target e7000 dev
E7000 emulator for Renesas SH.

target sh3 dev
target sh3e dev
Renesas SH-3 and SH-3E target systems.

Next: , Previous: SH, Up: Embedded Processors

18.3.11 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:


Next: , Up: Sparclet 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 (see Your program's 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.

Next: , Previous: Sparclet File, Up: Sparclet 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.

Next: , Previous: Sparclet Connection, Up: Sparclet 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.

Previous: Sparclet Download, Up: Sparclet 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!";

Next: , Previous: Sparclet, Up: Embedded Processors

18.3.12 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.

Next: , Previous: Sparclite, Up: Embedded Processors

18.3.13 Tandem ST2000

gdb may be used with a Tandem ST2000 phone switch, running Tandem's STDBUG protocol.

To connect your ST2000 to the host system, see the manufacturer's manual. Once the ST2000 is physically attached, you can run:

     target st2000 dev speed

to establish it as your debugging environment. dev is normally the name of a serial device, such as /dev/ttya, connected to the ST2000 via a serial line. You can instead specify dev as a TCP connection (for example, to a serial line attached via a terminal concentrator) using the syntax hostname:portnumber.

The load and attach commands are not defined for this target; you must load your program into the ST2000 as you normally would for standalone operation. gdb reads debugging information (such as symbols) from a separate, debugging version of the program available on your host computer.

These auxiliary gdb commands are available to help you with the ST2000 environment:

st2000 command
Send a command to the STDBUG monitor. See the manufacturer's manual for available commands.
Connect the controlling terminal to the STDBUG command monitor. When you are done interacting with STDBUG, typing either of two character sequences gets you back to the gdb command prompt: <RET>~. (Return, followed by tilde and period) or <RET>~<C-d> (Return, followed by tilde and control-D).

Next: , Previous: ST2000, Up: Embedded Processors

18.3.14 Zilog Z8000

When configured for debugging Zilog Z8000 targets, gdb includes a Z8000 simulator.

For the Z8000 family, `target sim' simulates either the Z8002 (the unsegmented variant of the Z8000 architecture) or the Z8001 (the segmented variant). The simulator recognizes which architecture is appropriate by inspecting the object code.

target sim args
Debug programs on a simulated CPU. If the simulator supports setup options, specify them via args.

After specifying this target, you can debug programs for the simulated CPU in the same style as programs for your host computer; use the file command to load a new program image, the run command to run your program, and so on.

As well as making available all the usual machine registers (see Registers), the Z8000 simulator provides three additional items of information as specially named registers:

Counts clock-ticks in the simulator.
Counts instructions run in the simulator.
Execution time in 60ths of a second.

You can refer to these values in gdb expressions with the usual conventions; for example, `b fputc if $cycles>5000' sets a conditional breakpoint that suspends only after at least 5000 simulated clock ticks.

Next: , Previous: Z8000, Up: Embedded Processors

18.3.15 Atmel AVR

When configured for debugging the Atmel AVR, gdb supports the following AVR-specific commands:

info io_registers
This command displays information about the AVR I/O registers. For each register, gdb prints its number and value.

Next: , Previous: AVR, Up: Embedded Processors

18.3.16 CRIS

When configured for debugging CRIS, gdb provides the following CRIS-specific commands:

set cris-version ver
Set the current CRIS version to ver, either `10' or `32'. The CRIS version affects register names and sizes. This command is useful in case autodetection of the CRIS version fails.
show cris-version
Show the current CRIS version.
set cris-dwarf2-cfi
Set the usage of DWARF-2 CFI for CRIS debugging. The default is `on'. Change to `off' when using gcc-cris whose version is below R59.
show cris-dwarf2-cfi
Show the current state of using DWARF-2 CFI.
set cris-mode mode
Set the current CRIS mode to mode. It should only be changed when debugging in guru mode, in which case it should be set to `guru' (the default is `normal').
show cris-mode
Show the current CRIS mode.

Next: , Previous: CRIS, Up: Embedded Processors

18.3.17 Renesas Super-H

For the Renesas Super-H processor, gdb provides these commands:

Show the values of all Super-H registers.

Previous: Super-H, Up: Embedded Processors

18.3.18 Windows CE

The following commands are available for Windows CE:

set remotedirectory dir
Tell gdb to upload files from the named directory dir. The default is /gdb, i.e. the root directory on the current drive.
show remotedirectory
Show the current value of the upload directory.
set remoteupload method
Set the method used to upload files to remote device. Valid values for method are `always', `newer', and `never'. The default is `newer'.
show remoteupload
Show the current setting of the upload method.
set remoteaddhost
Tell gdb whether to add this host to the remote stub's arguments when you debug over a network.
show remoteaddhost
Show whether to add this host to remote stub's arguments when debugging over a network.

Previous: Embedded Processors, Up: Configurations

18.4 Architectures

This section describes characteristics of architectures that affect all uses of gdb with the architecture, both native and cross.

Next: , Up: Architectures

18.4.1 x86 Architecture-specific issues.

set struct-convention mode
Set the convention used by the inferior to return structs and unions from functions to mode. Possible values of mode are "pcc", "reg", and "default" (the default). "default" or "pcc" means that structs are returned on the stack, while "reg" means that a struct or a union whose size is 1, 2, 4, or 8 bytes will be returned in a register.
show struct-convention
Show the current setting of the convention to return structs from functions.

Next: , Previous: i386, Up: Architectures

18.4.2 A29K

set rstack_high_address address
On AMD 29000 family processors, registers are saved in a separate register stack. There is no way for gdb to determine the extent of this stack. Normally, gdb just assumes that the stack is “large enough”. This may result in gdb referencing memory locations that do not exist. If necessary, you can get around this problem by specifying the ending address of the register stack with the set rstack_high_address command. The argument should be an address, which you probably want to precede with `0x' to specify in hexadecimal.

show rstack_high_address
Display the current limit of the register stack, on AMD 29000 family processors.

Next: , Previous: A29K, Up: Architectures

18.4.3 Alpha

See the following section.

Next: , Previous: Alpha, Up: Architectures

18.4.4 MIPS

Alpha- and MIPS-based computers use an unusual stack frame, which sometimes requires gdb to search backward in the object code to find the beginning of a function.

To improve response time (especially for embedded applications, where gdb may be restricted to a slow serial line for this search) you may want to limit the size of this search, using one of these commands:

set heuristic-fence-post limit
Restrict gdb to examining at most limit bytes in its search for the beginning of a function. A value of 0 (the default) means there is no limit. However, except for 0, the larger the limit the more bytes heuristic-fence-post must search and therefore the longer it takes to run. You should only need to use this command when debugging a stripped executable.
show heuristic-fence-post
Display the current limit.

These commands are available only when gdb is configured for debugging programs on Alpha or MIPS processors.

Several MIPS-specific commands are available when debugging MIPS programs:

set mips saved-gpreg-size size
Set the size of MIPS general-purpose registers saved on the stack. The argument size can be one of the following:
32-bit GP registers
64-bit GP registers
Use the target's default setting or autodetect the saved size from the information contained in the executable. This is the default

show mips saved-gpreg-size
Show the current size of MIPS GP registers on the stack.
set mips stack-arg-size size
Set the amount of stack space reserved for arguments to functions. The argument can be one of "32", "64" or "auto" (the default).
set mips abi arg
Tell gdb which MIPS ABI is used by the inferior. Possible values of arg are:
The default ABI associated with the current binary (this is the default).

show mips abi
Show the MIPS ABI used by gdb to debug the inferior.
set mipsfpu
show mipsfpu
See set mipsfpu.
set mips mask-address arg
This command determines whether the most-significant 32 bits of 64-bit MIPS addresses are masked off. The argument arg can be `on', `off', or `auto'. The latter is the default setting, which lets gdb determine the correct value.
show mips mask-address
Show whether the upper 32 bits of MIPS addresses are masked off or not.
set remote-mips64-transfers-32bit-regs
This command controls compatibility with 64-bit MIPS targets that transfer data in 32-bit quantities. If you have an old MIPS 64 target that transfers 32 bits for some registers, like sr and fsr, and 64 bits for other registers, set this option to `on'.
show remote-mips64-transfers-32bit-regs
Show the current setting of compatibility with older MIPS 64 targets.
set debug mips
This command turns on and off debugging messages for the MIPS-specific target code in gdb.
show debug mips
Show the current setting of MIPS debugging messages.

Previous: MIPS, Up: Architectures

18.4.5 HPPA

When gdb is debugging te HP PA architecture, it provides the following special commands:

set debug hppa
THis command determines whether HPPA architecture specific debugging messages are to be displayed.
show debug hppa
Show whether HPPA debugging messages are displayed.
maint print unwind address
This command displays the contents of the unwind table entry at the given address.

Next: , Previous: Configurations, Up: Top

19 Controlling gdb

You can alter the way gdb interacts with you by using the set command. For commands controlling how gdb displays data, see Print settings. Other settings are described here.

Next: , Up: Controlling GDB

19.1 Prompt

gdb indicates its readiness to read a command by printing a string called the prompt. This string is normally `(gdb)'. You can change the prompt string with the set prompt command. For instance, when debugging gdb with gdb, it is useful to change the prompt in one of the gdb sessions so that you can always tell which one you are talking to.

Note: set prompt does not add a space for you after the prompt you set. This allows you to set a prompt which ends in a space or a prompt that does not.

set prompt newprompt
Directs gdb to use newprompt as its prompt string henceforth.

show prompt
Prints a line of the form: `Gdb's prompt is: your-prompt'

Next: , Previous: Prompt, Up: Controlling GDB

19.2 Command editing

gdb reads its input commands via the Readline interface. This gnu library provides consistent behavior for programs which provide a command line interface to the user. Advantages are gnu Emacs-style or vi-style inline editing of commands, csh-like history substitution, and a storage and recall of command history across debugging sessions.

You may control the behavior of command line editing in gdb with the command set.

set editing
set editing on
Enable command line editing (enabled by default).
set editing off
Disable command line editing.

show editing
Show whether command line editing is enabled.

See Command Line Editing, for more details about the Readline interface. Users unfamiliar with gnu Emacs or vi are encouraged to read that chapter.

Next: , Previous: Editing, Up: Controlling GDB

19.3 Command history

gdb can keep track of the commands you type during your debugging sessions, so that you can be certain of precisely what happened. Use these commands to manage the gdb command history facility.

gdb uses the gnu History library, a part of the Readline package, to provide the history facility. See Using History Interactively, for the detailed description of the History library.

To issue a command to gdb without affecting certain aspects of the state which is seen by users, prefix it with `server '. This means that this command will not affect the command history, nor will it affect gdb's notion of which command to repeat if <RET> is pressed on a line by itself.

The server prefix does not affect the recording of values into the value history; to print a value without recording it into the value history, use the output command instead of the print command.

Here is the description of gdb commands related to command history.

set history filename fname
Set the name of the gdb command history file to fname. This is the file where gdb reads an initial command history list, and where it writes the command history from this session when it exits. You can access this list through history expansion or through the history command editing characters listed below. This file defaults to the value of the environment variable GDBHISTFILE, or to ./.gdb_history (./_gdb_history on MS-DOS) if this variable is not set.

set history save
set history save on
Record command history in a file, whose name may be specified with the set history filename command. By default, this option is disabled.
set history save off
Stop recording command history in a file.

set history size size
Set the number of commands which gdb keeps in its history list. This defaults to the value of the environment variable HISTSIZE, or to 256 if this variable is not set.

History expansion assigns special meaning to the character !. See Event Designators, for more details.

Since ! is also the logical not operator in C, history expansion is off by default. If you decide to enable history expansion with the set history expansion on command, you may sometimes need to follow ! (when it is used as logical not, in an expression) with a space or a tab to prevent it from being expanded. The readline history facilities do not attempt substitution on the strings != and !(, even when history expansion is enabled.

The commands to control history expansion are:

set history expansion on
set history expansion
Enable history expansion. History expansion is off by default.
set history expansion off
Disable history expansion.

show history
show history filename
show history save
show history size
show history expansion
These commands display the state of the gdb history parameters. show history by itself displays all four states.
show commands
Display the last ten commands in the command history.
show commands n
Print ten commands centered on command number n.
show commands +
Print ten commands just after the commands last printed.

Next: , Previous: Command History, Up: Controlling GDB

19.4 Screen size

Certain commands to gdb may produce large amounts of information output to the screen. To help you read all of it, gdb pauses and asks you for input at the end of each page of output. Type <RET> when you want to continue the output, or q to discard the remaining output. Also, the screen width setting determines when to wrap lines of output. Depending on what is being printed, gdb tries to break the line at a readable place, rather than simply letting it overflow onto the following line.

Normally gdb knows the size of the screen from the terminal driver software. For example, on Unix gdb uses the termcap data base together with the value of the TERM environment variable and the stty rows and stty cols settings. If this is not correct, you can override it with the set height and set width commands:

set height lpp
show height
set width cpl
show width
These set commands specify a screen height of lpp lines and a screen width of cpl characters. The associated show commands display the current settings.

If you specify a height of zero lines, gdb does not pause during output no matter how long the output is. This is useful if output is to a file or to an editor buffer.

Likewise, you can specify `set width 0' to prevent gdb from wrapping its output.

set pagination on
set pagination off
Turn the output pagination on or off; the default is on. Turning pagination off is the alternative to set height 0.
show pagination
Show the current pagination mode.

Next: , Previous: Screen Size, Up: Controlling GDB

19.5 Numbers

You can always enter numbers in octal, decimal, or hexadecimal in gdb by the usual conventions: octal numbers begin with `0', decimal numbers end with `.', and hexadecimal numbers begin with `0x'. Numbers that neither begin with `0' or `0x', nor end with a `.' are, by default, entered in base 10; likewise, the default display for numbers—when no particular format is specified—is base 10. You can change the default base for both input and output with the commands described below.

set input-radix base
Set the default base for numeric input. Supported choices for base are decimal 8, 10, or 16. base must itself be specified either unambiguously or using the current input radix; for example, any of
          set input-radix 012
          set input-radix 10.
          set input-radix 0xa

sets the input base to decimal. On the other hand, `set input-radix 10' leaves the input radix unchanged, no matter what it was, since `10', being without any leading or trailing signs of its base, is interpreted in the current radix. Thus, if the current radix is 16, `10' is interpreted in hex, i.e. as 16 decimal, which doesn't change the radix.

set output-radix base
Set the default base for numeric display. Supported choices for base are decimal 8, 10, or 16. base must itself be specified either unambiguously or using the current input radix.

show input-radix
Display the current default base for numeric input.

show output-radix
Display the current default base for numeric display.
set radix [base]
show radix
These commands set and show the default base for both input and output of numbers. set radix sets the radix of input and output to the same base; without an argument, it resets the radix back to its default value of 10.

Next: , Previous: Numbers, Up: Controlling GDB

19.6 Configuring the current ABI

gdb can determine the ABI (Application Binary Interface) of your application automatically. However, sometimes you need to override its conclusions. Use these commands to manage gdb's view of the current ABI.

One gdb configuration can debug binaries for multiple operating system targets, either via remote debugging or native emulation. gdb will autodetect the OS ABI (Operating System ABI) in use, but you can override its conclusion using the set osabi command. One example where this is useful is in debugging of binaries which use an alternate C library (e.g. uClibc for gnu/Linux) which does not have the same identifying marks that the standard C library for your platform provides.

show osabi
Show the OS ABI currently in use.
set osabi
With no argument, show the list of registered available OS ABI's.
set osabi abi
Set the current OS ABI to abi.

Generally, the way that an argument of type float is passed to a function depends on whether the function is prototyped. For a prototyped (i.e. ANSI/ISO style) function, float arguments are passed unchanged, according to the architecture's convention for float. For unprototyped (i.e. K&R style) functions, float arguments are first promoted to type double and then passed.

Unfortunately, some forms of debug information do not reliably indicate whether a function is prototyped. If gdb calls a function that is not marked as prototyped, it consults set coerce-float-to-double.

set coerce-float-to-double
set coerce-float-to-double on
Arguments of type float will be promoted to double when passed to an unprototyped function. This is the default setting.
set coerce-float-to-double off
Arguments of type float will be passed directly to unprototyped functions.

show coerce-float-to-double
Show the current setting of promoting float to double.

gdb needs to know the ABI used for your program's C++ objects. The correct C++ ABI depends on which C++ compiler was used to build your application. gdb only fully supports programs with a single C++ ABI; if your program contains code using multiple C++ ABI's or if gdb can not identify your program's ABI correctly, you can tell gdb which ABI to use. Currently supported ABI's include “gnu-v2”, for g++ versions before 3.0, “gnu-v3”, for g++ versions 3.0 and later, and “hpaCC” for the HP ANSI C++ compiler. Other C++ compilers may use the “gnu-v2” or “gnu-v3” ABI's as well. The default setting is “auto”.

show cp-abi
Show the C++ ABI currently in use.
set cp-abi
With no argument, show the list of supported C++ ABI's.
set cp-abi abi
set cp-abi auto
Set the current C++ ABI to abi, or return to automatic detection.

Next: , Previous: ABI, Up: Controlling GDB

19.7 Optional warnings and messages

By default, gdb is silent about its inner workings. If you are running on a slow machine, you may want to use the set verbose command. This makes gdb tell you when it does a lengthy internal operation, so you will not think it has crashed.

Currently, the messages controlled by set verbose are those which announce that the symbol table for a source file is being read; see symbol-file in Commands to specify files.

set verbose on
Enables gdb output of certain informational messages.
set verbose off
Disables gdb output of certain informational messages.

show verbose
Displays whether set verbose is on or off.

By default, if gdb encounters bugs in the symbol table of an object file, it is silent; but if you are debugging a compiler, you may find this information useful (see Errors reading symbol files).

set complaints limit
Permits gdb to output limit complaints about each type of unusual symbols before becoming silent about the problem. Set limit to zero to suppress all complaints; set it to a large number to prevent complaints from being suppressed.

show complaints
Displays how many symbol complaints gdb is permitted to produce.

By default, gdb is cautious, and asks what sometimes seems to be a lot of stupid questions to confirm certain commands. For example, if you try to run a program which is already running:

     (gdb) run
     The program being debugged has been started already.
     Start it from the beginning? (y or n)

If you are willing to unflinchingly face the consequences of your own commands, you can disable this “feature”:

set confirm off
Disables confirmation requests.
set confirm on
Enables confirmation requests (the default).

show confirm
Displays state of confirmation requests.

Previous: Messages/Warnings, Up: Controlling GDB

19.8 Optional messages about internal happenings

gdb has commands that enable optional debugging messages from various gdb subsystems; normally these commands are of interest to gdb maintainers, or when reporting a bug. This section documents those commands.

set exec-done-display
Turns on or off the notification of asynchronous commands' completion. When on, gdb will print a message when an asynchronous command finishes its execution. The default is off.
show exec-done-display
Displays the current setting of asynchronous command completion notification.
set debug arch
Turns on or off display of gdbarch debugging info. The default is off
show debug arch
Displays the current state of displaying gdbarch debugging info.
set debug aix-thread
Display debugging messages about inner workings of the AIX thread module.
show debug aix-thread
Show the current state of AIX thread debugging info display.
set debug event
Turns on or off display of gdb event debugging info. The default is off.
show debug event
Displays the current state of displaying gdb event debugging info.
set debug expression
Turns on or off display of debugging info about gdb expression parsing. The default is off.
show debug expression
Displays the current state of displaying debugging info about gdb expression parsing.
set debug frame
Turns on or off display of gdb frame debugging info. The default is off.
show debug frame
Displays the current state of displaying gdb frame debugging info.
set debug infrun
Turns on or off display of gdb debugging info for running the inferior. The default is off. infrun.c contains GDB's runtime state machine used for implementing operations such as single-stepping the inferior.
show debug infrun
Displays the current state of gdb inferior debugging.
set debug lin-lwp
Turns on or off debugging messages from the Linux LWP debug support.
show debug lin-lwp
Show the current state of Linux LWP debugging messages.
set debug observer
Turns on or off display of gdb observer debugging. This includes info such as the notification of observable events.
show debug observer
Displays the current state of observer debugging.
set debug overload
Turns on or off display of gdb C++ overload debugging info. This includes info such as ranking of functions, etc. The default is off.
show debug overload
Displays the current state of displaying gdb C++ overload debugging info.
set debug remote
Turns on or off display of reports on all packets sent back and forth across the serial line to the remote machine. The info is printed on the gdb standard output stream. The default is off.
show debug remote
Displays the state of display of remote packets.
set debug serial
Turns on or off display of gdb serial debugging info. The default is off.
show debug serial
Displays the current state of displaying gdb serial debugging info.
set debug solib-frv
Turns on or off debugging messages for FR-V shared-library code.
show debug solib-frv
Display the current state of FR-V shared-library code debugging messages.
set debug target
Turns on or off display of gdb target debugging info. This info includes what is going on at the target level of GDB, as it happens. The default is 0. Set it to 1 to track events, and to 2 to also track the value of large memory transfers. Changes to this flag do not take effect until the next time you connect to a target or use the run command.
show debug target
Displays the current state of displaying gdb target debugging info.
set debugvarobj
Turns on or off display of gdb variable object debugging info. The default is off.
show debugvarobj
Displays the current state of displaying gdb variable object debugging info.

Next: , Previous: Controlling GDB, Up: Top

20 Canned Sequences of Commands

Aside from breakpoint commands (see Breakpoint command lists), gdb provides two ways to store sequences of commands for execution as a unit: user-defined commands and command files.

Next: , Up: Sequences

20.1 User-defined commands

A user-defined command is a sequence of gdb commands to which you assign a new name as a command. This is done with the define command. User commands may accept up to 10 arguments separated by whitespace. Arguments are accessed within the user command via $arg0...$arg9. A trivial example:

     define adder
       print $arg0 + $arg1 + $arg2

To execute the command use:

     adder 1 2 3

This defines the command adder, which prints the sum of its three arguments. Note the arguments are text substitutions, so they may reference variables, use complex expressions, or even perform inferior functions calls.

define commandname
Define a command named commandname. If there is already a command by that name, you are asked to confirm that you want to redefine it.

The definition of the command is made up of other gdb command lines, which are given following the define command. The end of these commands is marked by a line containing end.

Takes a single argument, which is an expression to evaluate. It is followed by a series of commands that are executed only if the expression is true (nonzero). There can then optionally be a line else, followed by a series of commands that are only executed if the expression was false. The end of the list is marked by a line containing end.

The syntax is similar to if: the command takes a single argument, which is an expression to evaluate, and must be followed by the commands to execute, one per line, terminated by an end. The commands are executed repeatedly as long as the expression evaluates to true.

document commandname
Document the user-defined command commandname, so that it can be accessed by help. The command commandname must already be defined. This command reads lines of documentation just as define reads the lines of the command definition, ending with end. After the document command is finished, help on command commandname displays the documentation you have written.

You may use the document command again to change the documentation of a command. Redefining the command with define does not change the documentation.

Used inside a user-defined command, this tells gdb that this command should not be repeated when the user hits <RET> (see repeat last command).

help user-defined
List all user-defined commands, with the first line of the documentation (if any) for each.

show user
show user commandname
Display the gdb commands used to define commandname (but not its documentation). If no commandname is given, display the definitions for all user-defined commands.

show max-user-call-depth
set max-user-call-depth
The value of max-user-call-depth controls how many recursion levels are allowed in user-defined commands before GDB suspects an infinite recursion and aborts the command.

When user-defined commands are executed, the commands of the definition are not printed. An error in any command stops execution of the user-defined command.

If used interactively, commands that would ask for confirmation proceed without asking when used inside a user-defined command. Many gdb commands that normally print messages to say what they are doing omit the messages when used in a user-defined command.

Next: , Previous: Define, Up: Sequences

20.2 User-defined command hooks

You may define hooks, which are a special kind of user-defined command. Whenever you run the command `foo', if the user-defined command `hook-foo' exists, it is executed (with no arguments) before that command.

A hook may also be defined which is run after the command you executed. Whenever you run the command `foo', if the user-defined command `hookpost-foo' exists, it is executed (with no arguments) after that command. Post-execution hooks may exist simultaneously with pre-execution hooks, for the same command.

It is valid for a hook to call the command which it hooks. If this occurs, the hook is not re-executed, thereby avoiding infinite recursion.

In addition, a pseudo-command, `stop' exists. Defining (`hook-stop') makes the associated commands execute every time execution stops in your program: before breakpoint commands are run, displays are printed, or the stack frame is printed.

For example, to ignore SIGALRM signals while single-stepping, but treat them normally during normal execution, you could define:

     define hook-stop
     handle SIGALRM nopass
     define hook-run
     handle SIGALRM pass
     define hook-continue
     handle SIGLARM pass

As a further example, to hook at the begining and end of the echo command, and to add extra text to the beginning and end of the message, you could define:

     define hook-echo
     echo <<<---
     define hookpost-echo
     echo --->>>\n
     (gdb) echo Hello World
     <<<---Hello World--->>>

You can define a hook for any single-word command in gdb, but not for command aliases; you should define a hook for the basic command name, e.g. backtrace rather than bt. If an error occurs during the execution of your hook, execution of gdb commands stops and gdb issues a prompt (before the command that you actually typed had a chance to run).

If you try to define a hook which does not match any known command, you get a warning from the define command.

Next: , Previous: Hooks, Up: Sequences

20.3 Command files

A command file for gdb is a text file made of lines that are gdb commands. Comments (lines starting with #) may also be included. An empty line in a command file does nothing; it does not mean to repeat the last command, as it would from the terminal.

You can request the execution of a command file with the source command:

source filename
Execute the command file filename.

The lines in a command file are executed sequentially. They are not printed as they are executed. An error in any command terminates execution of the command file and control is returned to the console.

Commands that would ask for confirmation if used interactively proceed without asking when used in a command file. Many gdb commands that normally print messages to say what they are doing omit the messages when called from command files.

gdb also accepts command input from standard input. In this mode, normal output goes to standard output and error output goes to standard error. Errors in a command file supplied on standard input do not terminate execution of the command file—execution continues with the next command.

     gdb < cmds > log 2>&1

(The syntax above will vary depending on the shell used.) This example will execute commands from the file cmds. All output and errors would be directed to log.

Previous: Command Files, Up: Sequences

20.4 Commands for controlled output

During the execution of a command file or a user-defined command, normal gdb output is suppressed; the only output that appears is what is explicitly printed by the commands in the definition. This section describes three commands useful for generating exactly the output you want.

echo text
Print text. Nonprinting characters can be included in text using C escape sequences, such as `\n' to print a newline. No newline is printed unless you specify one. In addition to the standard C escape sequences, a backslash followed by a space stands for a space. This is useful for displaying a string with spaces at the beginning or the end, since leading and trailing spaces are otherwise trimmed from all arguments. To print ` and foo = ', use the command `echo \ and foo = \ '.

A backslash at the end of text can be used, as in C, to continue the command onto subsequent lines. For example,

          echo This is some text\n\
          which is continued\n\
          onto several lines.\n

produces the same output as

          echo This is some text\n
          echo which is continued\n
          echo onto several lines.\n

output expression
Print the value of expression and nothing but that value: no newlines, no `$nn = '. The value is not entered in the value history either. See Expressions, for more information on expressions.
output/fmt expression
Print the value of expression in format fmt. You can use the same formats as for print. See Output formats, for more information.

printf string, expressions...
Print the values of the expressions under the control of string. The expressions are separated by commas and may be either numbers or pointers. Their values are printed as specified by string, exactly as if your program were to execute the C subroutine
          printf (string, expressions...);

For example, you can print two values in hex like this:

          printf "foo, bar-foo = 0x%x, 0x%x\n", foo, bar-foo

The only backslash-escape sequences that you can use in the format string are the simple ones that consist of backslash followed by a letter.

Next: , Previous: TUI, Up: Top

21 Command Interpreters

gdb supports multiple command interpreters, and some command infrastructure to allow users or user interface writers to switch between interpreters or run commands in other interpreters.

gdb currently supports two command interpreters, the console interpreter (sometimes called the command-line interpreter or cli) and the machine interface interpreter (or gdb/mi). This manual describes both of these interfaces in great detail.

By default, gdb will start with the console interpreter. However, the user may choose to start gdb with another interpreter by specifying the -i or --interpreter startup options. Defined interpreters include:

The traditional console or command-line interpreter. This is the most often used interpreter with gdb. With no interpreter specified at runtime, gdb will use this interpreter.
The newest gdb/mi interface (currently mi2). Used primarily by programs wishing to use gdb as a backend for a debugger GUI or an IDE. For more information, see The gdb/mi Interface.
The current gdb/mi interface.
The gdb/mi interface included in gdb 5.1, 5.2, and 5.3.

The interpreter being used by gdb may not be dynamically switched at runtime. Although possible, this could lead to a very precarious situation. Consider an IDE using gdb/mi. If a user enters the command "interpreter-set console" in a console view, gdb would switch to using the console interpreter, rendering the IDE inoperable!

Although you may only choose a single interpreter at startup, you may execute commands in any interpreter from the current interpreter using the appropriate command. If you are running the console interpreter, simply use the interpreter-exec command:

     interpreter-exec mi "-data-list-register-names"

gdb/mi has a similar command, although it is only available in versions of gdb which support gdb/mi version 2 (or greater).

Next: , Previous: Sequences, Up: Top

22 gdb Text User Interface

The gdb Text User Interface, TUI in short, is a terminal interface which uses the curses library to show the source file, the assembly output, the program registers and gdb commands in separate text windows.

The TUI is enabled by invoking gdb using either `gdbtui' or `gdb -tui'.

Next: , Up: TUI

22.1 TUI overview

The TUI has two display modes that can be switched while gdb runs:

In the TUI mode, gdb can display several text window on the terminal:

This window is the gdb command window with the gdb prompt and the gdb outputs. The gdb input is still managed using readline but through the TUI. The command window is always visible.
The source window shows the source file of the program. The current line as well as active breakpoints are displayed in this window.
The assembly window shows the disassembly output of the program.
This window shows the processor registers. It detects when a register is changed and when this is the case, registers that have changed are highlighted.

The source and assembly windows show the current program position by highlighting the current line and marking them with the `>' marker. Breakpoints are also indicated with two markers. A first one indicates the breakpoint type:

Breakpoint which was hit at least once.
Breakpoint which was never hit.
Hardware breakpoint which was hit at least once.
Hardware breakpoint which was never hit.

The second marker indicates whether the breakpoint is enabled or not:

Breakpoint is enabled.
Breakpoint is disabled.

The source, assembly and register windows are attached to the thread and the frame position. They are updated when the current thread changes, when the frame changes or when the program counter changes. These three windows are arranged by the TUI according to several layouts. The layout defines which of these three windows are visible. The following layouts are available:

On top of the command window a status line gives various information concerning the current process begin debugged. The status line is updated when the information it shows changes. The following fields are displayed:

Indicates the current gdb target (see Specifying a Debugging Target).
Gives information about the current process or thread number. When no process is being debugged, this field is set to No process.
Gives the current function name for the selected frame. The name is demangled if demangling is turned on (see Print Settings). When there is no symbol corresponding to the current program counter the string ?? is displayed.
Indicates the current line number for the selected frame. When the current line number is not known the string ?? is displayed.
Indicates the current program counter address.

Next: , Previous: TUI Overview, Up: TUI

22.2 TUI Key Bindings

The TUI installs several key bindings in the readline keymaps (see Command Line Editing). They allow to leave or enter in the TUI mode or they operate directly on the TUI layout and windows. The TUI also provides a SingleKey keymap which binds several keys directly to gdb commands. The following key bindings are installed for both TUI mode and the gdb standard mode.

C-x C-a
C-x a
C-x A
Enter or leave the TUI mode. When the TUI mode is left, the curses window management is left and gdb operates using its standard mode writing on the terminal directly. When the TUI mode is entered, the control is given back to the curses windows. The screen is then refreshed.

C-x 1
Use a TUI layout with only one window. The layout will either be `source' or `assembly'. When the TUI mode is not active, it will switch to the TUI mode.

Think of this key binding as the Emacs C-x 1 binding.

C-x 2
Use a TUI layout with at least two windows. When the current layout shows already two windows, a next layout with two windows is used. When a new layout is chosen, one window will always be common to the previous layout and the new one.

Think of it as the Emacs C-x 2 binding.

C-x o
Change the active window. The TUI associates several key bindings (like scrolling and arrow keys) to the active window. This command gives the focus to the next TUI window.

Think of it as the Emacs C-x o binding.

C-x s
Use the TUI SingleKey keymap that binds single key to gdb commands (see TUI Single Key Mode).

The following key bindings are handled only by the TUI mode:

Scroll the active window one page up.

Scroll the active window one page down.

Scroll the active window one line up.

Scroll the active window one line down.

Scroll the active window one column left.

Scroll the active window one column right.

Refresh the screen.

In the TUI mode, the arrow keys are used by the active window for scrolling. This means they are available for readline when the active window is the command window. When the command window does not have the focus, it is necessary to use other readline key bindings such as <C-p>, <C-n>, <C-b> and <C-f>.

Next: , Previous: TUI Keys, Up: TUI

22.3 TUI Single Key Mode

The TUI provides a SingleKey mode in which it installs a particular key binding in the readline keymaps to connect single keys to some gdb commands.





exit the SingleKey mode.




info locals


Other keys temporarily switch to the gdb command prompt. The key that was pressed is inserted in the editing buffer so that it is possible to type most gdb commands without interaction with the TUI SingleKey mode. Once the command is entered the TUI SingleKey mode is restored. The only way to permanently leave this mode is by hitting <q> or `<C-x> <s>'.

Next: , Previous: TUI Single Key Mode, Up: TUI

22.4 TUI specific commands

The TUI has specific commands to control the text windows. These commands are always available, that is they do not depend on the current terminal mode in which gdb runs. When gdb is in the standard mode, using these commands will automatically switch in the TUI mode.

info win
List and give the size of all displayed windows.
layout next
Display the next layout.
layout prev
Display the previous layout.
layout src
Display the source window only.
layout asm
Display the assembly window only.
layout split
Display the source and assembly window.
layout regs
Display the register window together with the source or assembly window.
focus next | prev | src | asm | regs | split
Set the focus to the named window. This command allows to change the active window so that scrolling keys can be affected to another window.
Refresh the screen. This is similar to using <C-L> key.
tui reg float
Show the floating point registers in the register window.
tui reg general
Show the general registers in the register window.
tui reg next
Show the next register group. The list of register groups as well as their order is target specific. The predefined register groups are the following: general, float, system, vector, all, save, restore.
tui reg system
Show the system registers in the register window.
Update the source window and the current execution point.
winheight name +count
winheight name -count
Change the height of the window name by count lines. Positive counts increase the height, while negative counts decrease it.
Set the width of tab stops to be nchars characters.

Previous: TUI Commands, Up: TUI

22.5 TUI configuration variables

The TUI has several configuration variables that control the appearance of windows on the terminal.

set tui border-kind kind
Select the border appearance for the source, assembly and register windows. The possible values are the following:
Use a space character to draw the border.
Use ascii characters + - and | to draw the border.
Use the Alternate Character Set to draw the border. The border is drawn using character line graphics if the terminal supports them.

set tui active-border-mode mode
Select the attributes to display the border of the active window. The possible values are normal, standout, reverse, half, half-standout, bold and bold-standout.
set tui border-mode mode
Select the attributes to display the border of other windows. The mode can be one of the following:
Use normal attributes to display the border.
Use standout mode.
Use reverse video mode.
Use half bright mode.
Use half bright and standout mode.
Use extra bright or bold mode.
Use extra bright or bold and standout mode.

Next: , Previous: Interpreters, Up: Top

23 Using gdb under gnu Emacs

A special interface allows you to use gnu Emacs to view (and edit) the source files for the program you are debugging with gdb.

To use this interface, use the command M-x gdb in Emacs. Give the executable file you want to debug as an argument. This command starts gdb as a subprocess of Emacs, with input and output through a newly created Emacs buffer.

Using gdb under Emacs is just like using gdb normally except for two things:

This applies both to gdb commands and their output, and to the input and output done by the program you are debugging.

This is useful because it means that you can copy the text of previous commands and input them again; you can even use parts of the output in this way.

All the facilities of Emacs' Shell mode are available for interacting with your program. In particular, you can send signals the usual way—for example, C-c C-c for an interrupt, C-c C-z for a stop.

Each time gdb displays a stack frame, Emacs automatically finds the source file for that frame and puts an arrow (`=>') at the left margin of the current line. Emacs uses a separate buffer for source display, and splits the screen to show both your gdb session and the source.

Explicit gdb list or search commands still produce output as usual, but you probably have no reason to use them from Emacs.

If you specify an absolute file name when prompted for the M-x gdb argument, then Emacs sets your current working directory to where your program resides. If you only specify the file name, then Emacs sets your current working directory to to the directory associated with the previous buffer. In this case, gdb may find your program by searching your environment's PATH variable, but on some operating systems it might not find the source. So, although the gdb input and output session proceeds normally, the auxiliary buffer does not display the current source and line of execution.

The initial working directory of gdb is printed on the top line of the gdb I/O buffer and this serves as a default for the commands that specify files for gdb to operate on. See Commands to specify files.

By default, M-x gdb calls the program called gdb. If you need to call gdb by a different name (for example, if you keep several configurations around, with different names) you can customize the Emacs variable gud-gdb-command-name to run the one you want.

In the gdb I/O buffer, you can use these special Emacs commands in addition to the standard Shell mode commands:

C-h m
Describe the features of Emacs' gdb Mode.
C-c C-s
Execute to another source line, like the gdb step command; also update the display window to show the current file and location.
C-c C-n
Execute to next source line in this function, skipping all function calls, like the gdb next command. Then update the display window to show the current file and location.
C-c C-i
Execute one instruction, like the gdb stepi command; update display window accordingly.
C-c C-f
Execute until exit from the selected stack frame, like the gdb finish command.
C-c C-r
Continue execution of your program, like the gdb continue command.
C-c <
Go up the number of frames indicated by the numeric argument (see Numeric Arguments), like the gdb up command.
C-c >
Go down the number of frames indicated by the numeric argument, like the gdb down command.

In any source file, the Emacs command C-x SPC (gud-break) tells gdb to set a breakpoint on the source line point is on.

If you type M-x speedbar, then Emacs displays a separate frame which shows a backtrace when the gdb I/O buffer is current. Move point to any frame in the stack and type <RET> to make it become the current frame and display the associated source in the source buffer. Alternatively, click Mouse-2 to make the selected frame become the current one.

If you accidentally delete the source-display buffer, an easy way to get it back is to type the command f in the gdb buffer, to request a frame display; when you run under Emacs, this recreates the source buffer if necessary to show you the context of the current frame.

The source files displayed in Emacs are in ordinary Emacs buffers which are visiting the source files in the usual way. You can edit the files with these buffers if you wish; but keep in mind that gdb communicates with Emacs in terms of line numbers. If you add or delete lines from the text, the line numbers that gdb knows cease to correspond properly with the code.

The description given here is for GNU Emacs version 21.3 and a more detailed description of its interaction with gdb is given in the Emacs manual (see Debuggers).

Next: , Previous: Annotations, Up: Top

24 The gdb/mi Interface

Function and Purpose

gdb/mi is a line based machine oriented text interface to gdb and is activated by specifying using the --interpreter command line option (see Mode Options). It is specifically intended to support the development of systems which use the debugger as just one small component of a larger system.

This chapter is a specification of the gdb/mi interface. It is written in the form of a reference manual.

Note that gdb/mi is still under construction, so some of the features described below are incomplete and subject to change.

Notation and Terminology

This chapter uses the following notation:


In alphabetic order: Andrew Cagney, Fernando Nasser, Stan Shebs and Elena Zannoni.

Next: , Up: GDB/MI

24.1 gdb/mi Command Syntax

Next: , Up: GDB/MI Command Syntax

24.1.1 gdb/mi Input Syntax

command ==>
cli-command | mi-command
cli-command ==>
[ token ] cli-command nl, where cli-command is any existing gdb CLI command.
mi-command ==>
[ token ] "-" operation ( " " option )* [ " --" ] ( " " parameter )* nl
token ==>
"any sequence of digits"
option ==>
"-" parameter [ " " parameter ]
parameter ==>
non-blank-sequence | c-string
operation ==>
any of the operations described in this chapter
non-blank-sequence ==>
anything, provided it doesn't contain special characters such as "-", nl, """ and of course " "
c-string ==>
""" seven-bit-iso-c-string-content """
nl ==>



Next: , Previous: GDB/MI Input Syntax, Up: GDB/MI Command Syntax

24.1.2 gdb/mi Output Syntax

The output from gdb/mi consists of zero or more out-of-band records followed, optionally, by a single result record. This result record is for the most recent command. The sequence of output records is terminated by `(gdb)'.

If an input command was prefixed with a token then the corresponding output for that command will also be prefixed by that same token.

output ==>
( out-of-band-record )* [ result-record ] "(gdb)" nl
result-record ==>
[ token ] "^" result-class ( "," result )* nl
out-of-band-record ==>
async-record | stream-record
async-record ==>
exec-async-output | status-async-output | notify-async-output
exec-async-output ==>
[ token ] "*" async-output
status-async-output ==>
[ token ] "+" async-output
notify-async-output ==>
[ token ] "=" async-output
async-output ==>
async-class ( "," result )* nl
result-class ==>
"done" | "running" | "connected" | "error" | "exit"
async-class ==>
"stopped" | others (where others will be added depending on the needs—this is still in development).
result ==>
variable "=" value
variable ==>
value ==>
const | tuple | list
const ==>
tuple ==>
"{}" | "{" result ( "," result )* "}"
list ==>
"[]" | "[" value ( "," value )* "]" | "[" result ( "," result )* "]"
stream-record ==>
console-stream-output | target-stream-output | log-stream-output
console-stream-output ==>
"~" c-string
target-stream-output ==>
"@" c-string
log-stream-output ==>
"&" c-string
nl ==>
token ==>
any sequence of digits.


See gdb/mi Stream Records, for more details about the various output records.

Previous: GDB/MI Output Syntax, Up: GDB/MI Command Syntax

24.1.3 Simple Examples of gdb/mi Interaction

This subsection presents several simple examples of interaction using the gdb/mi interface. In these examples, `->' means that the following line is passed to gdb/mi as input, while `<-' means the output received from gdb/mi.

Target Stop

Here's an example of stopping the inferior process:

     -> -stop
     <- (gdb)

and later:

     <- *stop,reason="stop",address="0x123",source="a.c:123"
     <- (gdb)
Simple CLI Command

Here's an example of a simple CLI command being passed through gdb/mi and on to the CLI.

     -> print 1+2
     <- &"print 1+2\n"
     <- ~"$1 = 3\n"
     <- ^done
     <- (gdb)
Command With Side Effects
     -> -symbol-file xyz.exe
     <- *breakpoint,nr="3",address="0x123",source="a.c:123"
     <- (gdb)
A Bad Command

Here's what happens if you pass a non-existent command:

     -> -rubbish
     <- ^error,msg="Undefined MI command: rubbish"
     <- (gdb)

Next: , Previous: GDB/MI Command Syntax, Up: GDB/MI

24.2 gdb/mi Compatibility with CLI

To help users familiar with gdb's existing CLI interface, gdb/mi accepts existing CLI commands. As specified by the syntax, such commands can be directly entered into the gdb/mi interface and gdb will respond.

This mechanism is provided as an aid to developers of gdb/mi clients and not as a reliable interface into the CLI. Since the command is being interpreteted in an environment that assumes gdb/mi behaviour, the exact output of such commands is likely to end up being an un-supported hybrid of gdb/mi and CLI output.

Next: , Previous: GDB/MI Compatibility with CLI, Up: GDB/MI

24.3 gdb/mi Output Records

Next: , Up: GDB/MI Output Records

24.3.1 gdb/mi Result Records

In addition to a number of out-of-band notifications, the response to a gdb/mi command includes one of the following result indications:

"^done" [ "," results ]
The synchronous operation was successful, results are the return values.
The asynchronous operation was successfully started. The target is running.
"^error" "," c-string
The operation failed. The c-string contains the corresponding error message.

Next: , Previous: GDB/MI Result Records, Up: GDB/MI Output Records

24.3.2 gdb/mi Stream Records

gdb internally maintains a number of output streams: the console, the target, and the log. The output intended for each of these streams is funneled through the gdb/mi interface using stream records.

Each stream record begins with a unique prefix character which identifies its stream (see gdb/mi Output Syntax). In addition to the prefix, each stream record contains a string-output. This is either raw text (with an implicit new line) or a quoted C string (which does not contain an implicit newline).

"~" string-output
The console output stream contains text that should be displayed in the CLI console window. It contains the textual responses to CLI commands.
"@" string-output
The target output stream contains any textual output from the running target.
"&" string-output
The log stream contains debugging messages being produced by gdb's internals.

Previous: GDB/MI Stream Records, Up: GDB/MI Output Records

24.3.3 gdb/mi Out-of-band Records

Out-of-band records are used to notify the gdb/mi client of additional changes that have occurred. Those changes can either be a consequence of gdb/mi (e.g., a breakpoint modified) or a result of target activity (e.g., target stopped).

The following is a preliminary list of possible out-of-band records. In particular, the exec-async-output records.


reason can be one of the following:

A breakpoint was reached.
A watchpoint was triggered.
A read watchpoint was triggered.
An access watchpoint was triggered.
An -exec-finish or similar CLI command was accomplished.
An -exec-until or similar CLI command was accomplished.
A watchpoint has gone out of scope.
An -exec-next, -exec-next-instruction, -exec-step, -exec-step-instruction or similar CLI command was accomplished.
The inferior exited because of a signal.
The inferior exited.
The inferior exited normally.
A signal was received by the inferior.

Next: , Previous: GDB/MI Output Records, Up: GDB/MI

24.4 gdb/mi Command Description Format

The remaining sections describe blocks of commands. Each block of commands is laid out in a fashion similar to this section.

Note the the line breaks shown in the examples are here only for readability. They don't appear in the real output. Also note that the commands with a non-available example (N.A.) are not yet implemented.


The motivation for this collection of commands.


A brief introduction to this collection of commands as a whole.


For each command in the block, the following is described:

      -command args...
gdb Command

The corresponding gdb CLI command.


Next: , Previous: GDB/MI Command Description Format, Up: GDB/MI

24.5 gdb/mi Breakpoint table commands

This section documents gdb/mi commands for manipulating breakpoints.

The -break-after Command

      -break-after number count

The breakpoint number number is not in effect until it has been hit count times. To see how this is reflected in the output of the `-break-list' command, see the description of the `-break-list' command below.

gdb Command

The corresponding gdb command is `ignore'.

     -break-insert main
     -break-after 1 3

The -break-condition Command

      -break-condition number expr

Breakpoint number will stop the program only if the condition in expr is true. The condition becomes part of the `-break-list' output (see the description of the `-break-list' command below).

gdb Command

The corresponding gdb command is `condition'.

     -break-condition 1 1

The -break-delete Command

      -break-delete ( breakpoint )+

Delete the breakpoint(s) whose number(s) are specified in the argument list. This is obviously reflected in the breakpoint list.

gdb command

The corresponding gdb command is `delete'.

     -break-delete 1

The -break-disable Command

      -break-disable ( breakpoint )+

Disable the named breakpoint(s). The field `enabled' in the break list is now set to `n' for the named breakpoint(s).

gdb Command

The corresponding gdb command is `disable'.

     -break-disable 2

The -break-enable Command

      -break-enable ( breakpoint )+

Enable (previously disabled) breakpoint(s).

gdb Command

The corresponding gdb command is `enable'.

     -break-enable 2

The -break-info Command

      -break-info breakpoint

Get information about a single breakpoint.

gdb command

The corresponding gdb command is `info break breakpoint'.



The -break-insert Command

      -break-insert [ -t ] [ -h ] [ -r ]
         [ -c condition ] [ -i ignore-count ]
         [ -p thread ] [ line | addr ]

If specified, line, can be one of:

The possible optional parameters of this command are:

Insert a tempoary breakpoint.
Insert a hardware breakpoint.
`-c condition'
Make the breakpoint conditional on condition.
`-i ignore-count'
Initialize the ignore-count.
Insert a regular breakpoint in all the functions whose names match the given regular expression. Other flags are not applicable to regular expresson.

The result is in the form:


where number is the gdb number for this breakpoint, funcname is the name of the function where the breakpoint was inserted, filename is the name of the source file which contains this function, and lineno is the source line number within that file.

Note: this format is open to change.

gdb Command

The corresponding gdb commands are `break', `tbreak', `hbreak', `thbreak', and `rbreak'.

     -break-insert main
     -break-insert -t foo
     addr="0x0001072c", func="main",file="recursive2.c",line="4",times="0"},
     -break-insert -r foo.*
     ~int foo(int, int);

The -break-list Command


Displays the list of inserted breakpoints, showing the following fields:

number of the breakpoint
type of the breakpoint: `breakpoint' or `watchpoint'
should the breakpoint be deleted or disabled when it is hit: `keep' or `nokeep'
is the breakpoint enabled or no: `y' or `n'
memory location at which the breakpoint is set
logical location of the breakpoint, expressed by function name, file name, line number
number of times the breakpoint has been hit

If there are no breakpoints or watchpoints, the BreakpointTable body field is an empty list.

gdb Command

The corresponding gdb command is `info break'.


Here's an example of the result when there are no breakpoints:


The -break-watch Command

      -break-watch [ -a | -r ]

Create a watchpoint. With the `-a' option it will create an access watchpoint, i.e. a watchpoint that triggers either on a read from or on a write to the memory location. With the `-r' option, the watchpoint created is a read watchpoint, i.e. it will trigger only when the memory location is accessed for reading. Without either of the options, the watchpoint created is a regular watchpoint, i.e. it will trigger when the memory location is accessed for writing. See Setting watchpoints.

Note that `-break-list' will report a single list of watchpoints and breakpoints inserted.

gdb Command

The corresponding gdb commands are `watch', `awatch', and `rwatch'.


Setting a watchpoint on a variable in the main function:

     -break-watch x

Setting a watchpoint on a variable local to a function. gdb will stop the program execution twice: first for the variable changing value, then for the watchpoint going out of scope.

     -break-watch C
     value="0x11940 \"A string argument.\""}],

Listing breakpoints and watchpoints, at different points in the program execution. Note that once the watchpoint goes out of scope, it is deleted.

     -break-watch C
     value="0x11940 \"A string argument.\""}],

Next: , Previous: GDB/MI Breakpoint Table Commands, Up: GDB/MI

24.6 gdb/mi Data Manipulation

This section describes the gdb/mi commands that manipulate data: examine memory and registers, evaluate expressions, etc.

The -data-disassemble Command

         [ -s start-addr -e end-addr ]
       | [ -f filename -l linenum [ -n lines ] ]
       -- mode


is the beginning address (or $pc)
is the end address
is the name of the file to disassemble
is the line number to disassemble around
is the the number of disassembly lines to be produced. If it is -1, the whole function will be disassembled, in case no end-addr is specified. If end-addr is specified as a non-zero value, and lines is lower than the number of disassembly lines between start-addr and end-addr, only lines lines are displayed; if lines is higher than the number of lines between start-addr and end-addr, only the lines up to end-addr are displayed.
is either 0 (meaning only disassembly) or 1 (meaning mixed source and disassembly).

The output for each instruction is composed of four fields:

Note that whatever included in the instruction field, is not manipulated directely by gdb/mi, i.e. it is not possible to adjust its format.

gdb Command

There's no direct mapping from this command to the CLI.


Disassemble from the current value of $pc to $pc + 20:

     -data-disassemble -s $pc -e "$pc + 20" -- 0
     inst="mov  2, %o0"},
     inst="sethi  %hi(0x11800), %o2"},
     inst="or  %o2, 0x140, %o1\t! 0x11940 <_lib_version+8>"},
     inst="sethi  %hi(0x11800), %o2"},
     inst="or  %o2, 0x168, %o4\t! 0x11968 <_lib_version+48>"}]

Disassemble the whole main function. Line 32 is part of main.

     -data-disassemble -f basics.c -l 32 -- 0
     inst="save  %sp, -112, %sp"},
     inst="mov   2, %o0"},
     inst="sethi %hi(0x11800), %o2"},
     {address="0x0001081c",func-name="main",offset="96",inst="ret "},
     {address="0x00010820",func-name="main",offset="100",inst="restore "}]

Disassemble 3 instructions from the start of main:

     -data-disassemble -f basics.c -l 32 -n 3 -- 0
     inst="save  %sp, -112, %sp"},
     inst="mov  2, %o0"},
     inst="sethi  %hi(0x11800), %o2"}]

Disassemble 3 instructions from the start of main in mixed mode:

     -data-disassemble -f basics.c -l 32 -n 3 -- 1
     file="/kwikemart/marge/ezannoni/flathead-dev/devo/gdb/ \
     inst="save  %sp, -112, %sp"}]},
     file="/kwikemart/marge/ezannoni/flathead-dev/devo/gdb/ \
     inst="mov  2, %o0"},
     inst="sethi  %hi(0x11800), %o2"}]}]

The -data-evaluate-expression Command

      -data-evaluate-expression expr

Evaluate expr as an expression. The expression could contain an inferior function call. The function call will execute synchronously. If the expression contains spaces, it must be enclosed in double quotes.

gdb Command

The corresponding gdb commands are `print', `output', and `call'. In gdbtk only, there's a corresponding `gdb_eval' command.


In the following example, the numbers that precede the commands are the tokens described in gdb/mi Command Syntax. Notice how gdb/mi returns the same tokens in its output.

     211-data-evaluate-expression A
     311-data-evaluate-expression &A
     411-data-evaluate-expression A+3
     511-data-evaluate-expression "A + 3"

The -data-list-changed-registers Command


Display a list of the registers that have changed.

gdb Command

gdb doesn't have a direct analog for this command; gdbtk has the corresponding command `gdb_changed_register_list'.


On a PPC MBX board:


The -data-list-register-names Command

      -data-list-register-names [ ( regno )+ ]

Show a list of register names for the current target. If no arguments are given, it shows a list of the names of all the registers. If integer numbers are given as arguments, it will print a list of the names of the registers corresponding to the arguments. To ensure consistency between a register name and its number, the output list may include empty register names.

gdb Command

gdb does not have a command which corresponds to `-data-list-register-names'. In gdbtk there is a corresponding command `gdb_regnames'.


For the PPC MBX board:

     "", "pc","ps","cr","lr","ctr","xer"]
     -data-list-register-names 1 2 3

The -data-list-register-values Command

      -data-list-register-values fmt [ ( regno )*]

Display the registers' contents. fmt is the format according to which the registers' contents are to be returned, followed by an optional list of numbers specifying the registers to display. A missing list of numbers indicates that the contents of all the registers must be returned.

Allowed formats for fmt are:

gdb Command

The corresponding gdb commands are `info reg', `info all-reg', and (in gdbtk) `gdb_fetch_registers'.


For a PPC MBX board (note: line breaks are for readability only, they don't appear in the actual output):

     -data-list-register-values r 64 65
     -data-list-register-values x

The -data-read-memory Command

      -data-read-memory [ -o byte-offset ]
        address word-format word-size
        nr-rows nr-cols [ aschar ]


An expression specifying the address of the first memory word to be read. Complex expressions containing embedded white space should be quoted using the C convention.
The format to be used to print the memory words. The notation is the same as for gdb's print command (see Output formats).
The size of each memory word in bytes.
The number of rows in the output table.
The number of columns in the output table.
If present, indicates that each row should include an ascii dump. The value of aschar is used as a padding character when a byte is not a member of the printable ascii character set (printable ascii characters are those whose code is between 32 and 126, inclusively).
An offset to add to the address before fetching memory.

This command displays memory contents as a table of nr-rows by nr-cols words, each word being word-size bytes. In total, nr-rows * nr-cols * word-size bytes are read (returned as `total-bytes'). Should less than the requested number of bytes be returned by the target, the missing words are identified using `N/A'. The number of bytes read from the target is returned in `nr-bytes' and the starting address used to read memory in `addr'.

The address of the next/previous row or page is available in `next-row' and `prev-row', `next-page' and `prev-page'.

gdb Command

The corresponding gdb command is `x'. gdbtk has `gdb_get_mem' memory read command.


Read six bytes of memory starting at bytes+6 but then offset by -6 bytes. Format as three rows of two columns. One byte per word. Display each word in hex.

     9-data-read-memory -o -6 -- bytes+6 x 1 3 2

Read two bytes of memory starting at address shorts + 64 and display as a single word formatted in decimal.

     5-data-read-memory shorts+64 d 2 1 1

Read thirty two bytes of memory starting at bytes+16 and format as eight rows of four columns. Include a string encoding with `x' used as the non-printable character.

     4-data-read-memory bytes+16 x 1 8 4 x
     {addr="0x000013b0",data=["0x20","0x21","0x22","0x23"],ascii=" !\"#"},

The -display-delete Command

      -display-delete number

Delete the display number.

gdb Command

The corresponding gdb command is `delete display'.



The -display-disable Command

      -display-disable number

Disable display number.

gdb Command

The corresponding gdb command is `disable display'.



The -display-enable Command

      -display-enable number

Enable display number.

gdb Command

The corresponding gdb command is `enable display'.



The -display-insert Command

      -display-insert expression

Display expression every time the program stops.

gdb Command

The corresponding gdb command is `display'.



The -display-list Command


List the displays. Do not show the current values.

gdb Command

The corresponding gdb command is `info display'.



The -environment-cd Command

      -environment-cd pathdir

Set gdb's working directory.

gdb Command

The corresponding gdb command is `cd'.

     -environment-cd /kwikemart/marge/ezannoni/flathead-dev/devo/gdb

The -environment-directory Command

      -environment-directory [ -r ] [ pathdir ]+

Add directories pathdir to beginning of search path for source files. If the `-r' option is used, the search path is reset to the default search path. If directories pathdir are supplied in addition to the `-r' option, the search path is first reset and then addition occurs as normal. Multiple directories may be specified, separated by blanks. Specifying multiple directories in a single command results in the directories added to the beginning of the search path in the same order they were presented in the command. If blanks are needed as part of a directory name, double-quotes should be used around the name. In the command output, the path will show up separated by the system directory-separator character. The directory-seperator character must not be used in any directory name. If no directories are specified, the current search path is displayed.

gdb Command

The corresponding gdb command is `dir'.

     -environment-directory /kwikemart/marge/ezannoni/flathead-dev/devo/gdb
     -environment-directory ""
     -environment-directory -r /home/jjohnstn/src/gdb /usr/src
     -environment-directory -r

The -environment-path Command

      -environment-path [ -r ] [ pathdir ]+

Add directories pathdir to beginning of search path for object files. If the `-r' option is used, the search path is reset to the original search path that existed at gdb start-up. If directories pathdir are supplied in addition to the `-r' option, the search path is first reset and then addition occurs as normal. Multiple directories may be specified, separated by blanks. Specifying multiple directories in a single command results in the directories added to the beginning of the search path in the same order they were presented in the command. If blanks are needed as part of a directory name, double-quotes should be used around the name. In the command output, the path will show up separated by the system directory-separator character. The directory-seperator character must not be used in any directory name. If no directories are specified, the current path is displayed.

gdb Command

The corresponding gdb command is `path'.

     -environment-path /kwikemart/marge/ezannoni/flathead-dev/ppc-eabi/gdb /bin
     -environment-path -r /usr/local/bin

The -environment-pwd Command


Show the current working directory.

gdb command

The corresponding gdb command is `pwd'.


Next: , Previous: GDB/MI Data Manipulation, Up: GDB/MI

24.7 gdb/mi Program control

Program termination

As a result of execution, the inferior program can run to completion, if it doesn't encounter any breakpoints. In this case the output will include an exit code, if the program has exited exceptionally.


Program exited normally:

     x = 55

Program exited exceptionally:

     x = 55

Another way the program can terminate is if it receives a signal such as SIGINT. In this case, gdb/mi displays this:


The -exec-abort Command


Kill the inferior running program.

gdb Command

The corresponding gdb command is `kill'.



The -exec-arguments Command

      -exec-arguments args

Set the inferior program arguments, to be used in the next `-exec-run'.

gdb Command

The corresponding gdb command is `set args'.


Don't have one around.

The -exec-continue Command


Asynchronous command. Resumes the execution of the inferior program until a breakpoint is encountered, or until the inferior exits.

gdb Command

The corresponding gdb corresponding is `continue'.

     @Hello world

The -exec-finish Command


Asynchronous command. Resumes the execution of the inferior program until the current function is exited. Displays the results returned by the function.

gdb Command

The corresponding gdb command is `finish'.


Function returning void.

     @hello from foo

Function returning other than void. The name of the internal gdb variable storing the result is printed, together with the value itself.


The -exec-interrupt Command


Asynchronous command. Interrupts the background execution of the target. Note how the token associated with the stop message is the one for the execution command that has been interrupted. The token for the interrupt itself only appears in the `^done' output. If the user is trying to interrupt a non-running program, an error message will be printed.

gdb Command

The corresponding gdb command is `interrupt'.

     ^error,msg="mi_cmd_exec_interrupt: Inferior not executing."

The -exec-next Command


Asynchronous command. Resumes execution of the inferior program, stopping when the beginning of the next source line is reached.

gdb Command

The corresponding gdb command is `next'.


The -exec-next-instruction Command


Asynchronous command. Executes one machine instruction. If the instruction is a function call continues until the function returns. If the program stops at an instruction in the middle of a source line, the address will be printed as well.

gdb Command

The corresponding gdb command is `nexti'.


The -exec-return Command


Makes current function return immediately. Doesn't execute the inferior. Displays the new current frame.

gdb Command

The corresponding gdb command is `return'.

     200-break-insert callee4
     value="0x11940 \"A string argument.\""}],

The -exec-run Command


Asynchronous command. Starts execution of the inferior from the beginning. The inferior executes until either a breakpoint is encountered or the program exits.

gdb Command

The corresponding gdb command is `run'.

     -break-insert main

The -exec-show-arguments Command


Print the arguments of the program.

gdb Command

The corresponding gdb command is `show args'.



The -exec-step Command


Asynchronous command. Resumes execution of the inferior program, stopping when the beginning of the next source line is reached, if the next source line is not a function call. If it is, stop at the first instruction of the called function.

gdb Command

The corresponding gdb command is `step'.


Stepping into a function:


Regular stepping:


The -exec-step-instruction Command


Asynchronous command. Resumes the inferior which executes one machine instruction. The output, once gdb has stopped, will vary depending on whether we have stopped in the middle of a source line or not. In the former case, the address at which the program stopped will be printed as well.

gdb Command

The corresponding gdb command is `stepi'.


The -exec-until Command

      -exec-until [ location ]

Asynchronous command. Executes the inferior until the location specified in the argument is reached. If there is no argument, the inferior executes until a source line greater than the current one is reached. The reason for stopping in this case will be `location-reached'.

gdb Command

The corresponding gdb command is `until'.

     -exec-until recursive2.c:6
     x = 55

The -file-exec-and-symbols Command

      -file-exec-and-symbols file

Specify the executable file to be debugged. This file is the one from which the symbol table is also read. If no file is specified, the command clears the executable and symbol information. If breakpoints are set when using this command with no arguments, gdb will produce error messages. Otherwise, no output is produced, except a completion notification.

gdb Command

The corresponding gdb command is `file'.

     -file-exec-and-symbols /kwikemart/marge/ezannoni/TRUNK/mbx/hello.mbx

The -file-exec-file Command

      -file-exec-file file

Specify the executable file to be debugged. Unlike `-file-exec-and-symbols', the symbol table is not read from this file. If used without argument, gdb clears the information about the executable file. No output is produced, except a completion notification.

gdb Command

The corresponding gdb command is `exec-file'.

     -file-exec-file /kwikemart/marge/ezannoni/TRUNK/mbx/hello.mbx

The -file-list-exec-sections Command


List the sections of the current executable file.

gdb Command

The gdb command `info file' shows, among the rest, the same information as this command. gdbtk has a corresponding command `gdb_load_info'.



The -file-list-exec-source-file Command


List the line number, the current source file, and the absolute path to the current source file for the current executable.

gdb Command

There's no gdb command which directly corresponds to this one.


The -file-list-exec-source-files Command


List the source files for the current executable.

It will always output the filename, but only when GDB can find the absolute file name of a source file, will it output the fullname.

gdb Command

There's no gdb command which directly corresponds to this one. gdbtk has an analogous command `gdb_listfiles'.


The -file-list-shared-libraries Command


List the shared libraries in the program.

gdb Command

The corresponding gdb command is `info shared'.



The -file-list-symbol-files Command


List symbol files.

gdb Command

The corresponding gdb command is `info file' (part of it).



The -file-symbol-file Command

      -file-symbol-file file

Read symbol table info from the specified file argument. When used without arguments, clears gdb's symbol table info. No output is produced, except for a completion notification.

gdb Command

The corresponding gdb command is `symbol-file'.

     -file-symbol-file /kwikemart/marge/ezannoni/TRUNK/mbx/hello.mbx

Next: , Previous: GDB/MI Program Control, Up: GDB/MI

24.8 Miscellaneous gdb commands in gdb/mi

The -gdb-exit Command


Exit gdb immediately.

gdb Command

Approximately corresponds to `quit'.


The -gdb-set Command


Set an internal gdb variable.

gdb Command

The corresponding gdb command is `set'.

     -gdb-set $foo=3

The -gdb-show Command


Show the current value of a gdb variable.

gdb command

The corresponding gdb command is `show'.

     -gdb-show annotate

The -gdb-version Command


Show version information for gdb. Used mostly in testing.

gdb Command

There's no equivalent gdb command. gdb by default shows this information when you start an interactive session.

     ~GNU gdb 5.2.1
     ~Copyright 2000 Free Software Foundation, Inc.
     ~GDB is free software, covered by the GNU General Public License, and
     ~you are welcome to change it and/or 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.
     ~This GDB was configured as
      "--host=sparc-sun-solaris2.5.1 --target=ppc-eabi".

The -interpreter-exec Command


     -interpreter-exec interpreter command

Execute the specified command in the given interpreter.

gdb Command

The corresponding gdb command is `interpreter-exec'.


     -interpreter-exec console "break main"
     &"During symbol reading, couldn't parse type; debugger out of date?.\n"
     &"During symbol reading, bad structure-type format.\n"
     ~"Breakpoint 1 at 0x8074fc6: file ../../src/gdb/main.c, line 743.\n"

The -inferior-tty-set Command


     -inferior-tty-set /dev/pts/1

Set terminal for future runs of the program being debugged.

gdb Command

The corresponding gdb command is `set inferior-tty /dev/pts/1'.


     -inferior-tty-set /dev/pts/1

The -inferior-tty-show Command



Show terminal for future runs of program being debugged.

gdb Command

The corresponding gdb command is `show inferior-tty /dev/pts/1'.


     -inferior-tty-set /dev/pts/1

Next: , Previous: GDB/MI Miscellaneous Commands, Up: GDB/MI

24.9 gdb/mi Stack Manipulation Commands

The -stack-info-frame Command


Get info on the selected frame.

gdb Command

The corresponding gdb command is `info frame' or `frame' (without arguments).


The -stack-info-depth Command

      -stack-info-depth [ max-depth ]

Return the depth of the stack. If the integer argument max-depth is specified, do not count beyond max-depth frames.

gdb Command

There's no equivalent gdb command.


For a stack with frame levels 0 through 11:

     -stack-info-depth 4
     -stack-info-depth 12
     -stack-info-depth 11
     -stack-info-depth 13

The -stack-list-arguments Command

      -stack-list-arguments show-values
         [ low-frame high-frame ]

Display a list of the arguments for the frames between low-frame and high-frame (inclusive). If low-frame and high-frame are not provided, list the arguments for the whole call stack.

The show-values argument must have a value of 0 or 1. A value of 0 means that only the names of the arguments are listed, a value of 1 means that both names and values of the arguments are printed.

gdb Command

gdb does not have an equivalent command. gdbtk has a `gdb_get_args' command which partially overlaps with the functionality of `-stack-list-arguments'.

     -stack-list-arguments 0
     -stack-list-arguments 1
      args=[{name="strarg",value="0x11940 \"A string argument.\""}]},
     {name="strarg",value="0x11940 \"A string argument.\""}]},
     {name="strarg",value="0x11940 \"A string argument.\""},
     -stack-list-arguments 0 2 2
     -stack-list-arguments 1 2 2
     {name="strarg",value="0x11940 \"A string argument.\""}]}]

The -stack-list-frames Command

      -stack-list-frames [ low-frame high-frame ]

List the frames currently on the stack. For each frame it displays the following info:

The frame number, 0 being the topmost frame, i.e. the innermost function.
The $pc value for that frame.
Function name.
File name of the source file where the function lives.
Line number corresponding to the $pc.

If invoked without arguments, this command prints a backtrace for the whole stack. If given two integer arguments, it shows the frames whose levels are between the two arguments (inclusive). If the two arguments are equal, it shows the single frame at the corresponding level.

gdb Command

The corresponding gdb commands are `backtrace' and `where'.


Full stack backtrace:


Show frames between low_frame and high_frame:

     -stack-list-frames 3 5

Show a single frame:

     -stack-list-frames 3 3

The -stack-list-locals Command

      -stack-list-locals print-values

Display the local variable names for the current frame. With an argument of 0 or --no-values, prints only the names of the variables. With argument of 1 or --all-values, prints also their values. With argument of 2 or --simple-values, prints the name, type and value for simple data types and the name and type for arrays, structures and unions. In this last case, the idea is that the user can see the value of simple data types immediately and he can create variable objects for other data types if he wishes to explore their values in more detail.

gdb Command

`info locals' in gdb, `gdb_get_locals' in gdbtk.

     -stack-list-locals 0
     -stack-list-locals --all-values
       {name="C",value="{1, 2, 3}"}]
     -stack-list-locals --simple-values
       {name="B",type="int",value="2"},{name="C",type="int [3]"}]

The -stack-select-frame Command

      -stack-select-frame framenum

Change the current frame. Select a different frame framenum on the stack.

gdb Command

The corresponding gdb commands are `frame', `up', `down', `select-frame', `up-silent', and `down-silent'.

     -stack-select-frame 2

Next: , Previous: GDB/MI Stack Manipulation, Up: GDB/MI

24.10 gdb/mi Symbol Query Commands

The -symbol-info-address Command

      -symbol-info-address symbol

Describe where symbol is stored.

gdb Command

The corresponding gdb command is `info address'.



The -symbol-info-file Command


Show the file for the symbol.

gdb Command

There's no equivalent gdb command. gdbtk has `gdb_find_file'.



The -symbol-info-function Command


Show which function the symbol lives in.

gdb Command

`gdb_get_function' in gdbtk.



The -symbol-info-line Command


Show the core addresses of the code for a source line.

gdb Command

The corresponding gdb command is `info line'. gdbtk has the `gdb_get_line' and `gdb_get_file' commands.



The -symbol-info-symbol Command

      -symbol-info-symbol addr

Describe what symbol is at location addr.

gdb Command

The corresponding gdb command is `info symbol'.



The -symbol-list-functions Command


List the functions in the executable.

gdb Command

`info functions' in gdb, `gdb_listfunc' and `gdb_search' in gdbtk.



The -symbol-list-lines Command

      -symbol-list-lines filename

Print the list of lines that contain code and their associated program addresses for the given source filename. The entries are sorted in ascending PC order.

gdb Command

There is no corresponding gdb command.

     -symbol-list-lines basics.c

The -symbol-list-types Command


List all the type names.

gdb Command

The corresponding commands are `info types' in gdb, `gdb_search' in gdbtk.



The -symbol-list-variables Command


List all the global and static variable names.

gdb Command

`info variables' in gdb, `gdb_search' in gdbtk.



The -symbol-locate Command

gdb Command

`gdb_loc' in gdbtk.



The -symbol-type Command

      -symbol-type variable

Show type of variable.

gdb Command

The corresponding gdb command is `ptype', gdbtk has `gdb_obj_variable'.



Next: , Previous: GDB/MI Symbol Query, Up: GDB/MI

24.11 gdb/mi Target Manipulation Commands

The -target-attach Command

      -target-attach pid | file

Attach to a process pid or a file file outside of gdb.

gdb command

The corresponding gdb command is `attach'.



The -target-compare-sections Command

      -target-compare-sections [ section ]

Compare data of section section on target to the exec file. Without the argument, all sections are compared.

gdb Command

The gdb equivalent is `compare-sections'.



The -target-detach Command


Disconnect from the remote target. There's no output.

gdb command

The corresponding gdb command is `detach'.


The -target-disconnect Command


Disconnect from the remote target. There's no output.

gdb command

The corresponding gdb command is `disconnect'.


The -target-download Command


Loads the executable onto the remote target. It prints out an update message every half second, which includes the fields:

The name of the section.
The size of what has been sent so far for that section.
The size of the section.
The total size of what was sent so far (the current and the previous sections).
The size of the overall executable to download.

Each message is sent as status record (see gdb/mi Output Syntax).

In addition, it prints the name and size of the sections, as they are downloaded. These messages include the following fields:

The name of the section.
The size of the section.
The size of the overall executable to download.

At the end, a summary is printed.

gdb Command

The corresponding gdb command is `load'.


Note: each status message appears on a single line. Here the messages have been broken down so that they can fit onto a page.


The -target-exec-status Command


Provide information on the state of the target (whether it is running or not, for instance).

gdb Command

There's no equivalent gdb command.



The -target-list-available-targets Command


List the possible targets to connect to.

gdb Command

The corresponding gdb command is `help target'.



The -target-list-current-targets Command


Describe the current target.

gdb Command

The corresponding information is printed by `info file' (among other things).



The -target-list-parameters Command

gdb Command

No equivalent.



The -target-select Command

      -target-select type parameters ...

Connect gdb to the remote target. This command takes two args:

The type of target, for instance `async', `remote', etc.
Device names, host names and the like. See Commands for managing targets, for more details.

The output is a connection notification, followed by the address at which the target program is, in the following form:

     ^connected,addr="address",func="function name",
       args=[arg list]
gdb Command

The corresponding gdb command is `target'.

     -target-select async /dev/ttya

Next: , Previous: GDB/MI Target Manipulation, Up: GDB/MI

24.12 gdb/mi Thread Commands

The -thread-info Command

gdb command

No equivalent.



The -thread-list-all-threads Command

gdb Command

The equivalent gdb command is `info threads'.



The -thread-list-ids Command


Produces a list of the currently known gdb thread ids. At the end of the list it also prints the total number of such threads.

gdb Command

Part of `info threads' supplies the same information.


No threads present, besides the main process:


Several threads:


The -thread-select Command

      -thread-select threadnum

Make threadnum the current thread. It prints the number of the new current thread, and the topmost frame for that thread.

gdb Command

The corresponding gdb command is `thread'.

     -thread-select 3
     args=[{name="format",value="0x8048e9c \"%*s%c %d %c\\n\""},

Next: , Previous: GDB/MI Thread Commands, Up: GDB/MI

24.13 gdb/mi Tracepoint Commands

The tracepoint commands are not yet implemented.

Previous: GDB/MI Tracepoint Commands, Up: GDB/MI

24.14 gdb/mi Variable Objects

Motivation for Variable Objects in gdb/mi

For the implementation of a variable debugger window (locals, watched expressions, etc.), we are proposing the adaptation of the existing code used by Insight.

The two main reasons for that are:

  1. It has been proven in practice (it is already on its second generation).
  2. It will shorten development time (needless to say how important it is now).

The original interface was designed to be used by Tcl code, so it was slightly changed so it could be used through gdb/mi. This section describes the gdb/mi operations that will be available and gives some hints about their use.

Note: In addition to the set of operations described here, we expect the gui implementation of a variable window to require, at least, the following operations:

Introduction to Variable Objects in gdb/mi

The basic idea behind variable objects is the creation of a named object to represent a variable, an expression, a memory location or even a CPU register. For each object created, a set of operations is available for examining or changing its properties.

Furthermore, complex data types, such as C structures, are represented in a tree format. For instance, the struct type variable is the root and the children will represent the struct members. If a child is itself of a complex type, it will also have children of its own. Appropriate language differences are handled for C, C++ and Java.

When returning the actual values of the objects, this facility allows for the individual selection of the display format used in the result creation. It can be chosen among: binary, decimal, hexadecimal, octal and natural. Natural refers to a default format automatically chosen based on the variable type (like decimal for an int, hex for pointers, etc.).

The following is the complete set of gdb/mi operations defined to access this functionality:

Operation Description

-var-create create a variable object
-var-delete delete the variable object and its children
-var-set-format set the display format of this variable
-var-show-format show the display format of this variable
-var-info-num-children tells how many children this object has
-var-list-children return a list of the object's children
-var-info-type show the type of this variable object
-var-info-expression print what this variable object represents
-var-show-attributes is this variable editable? does it exist here?
-var-evaluate-expression get the value of this variable
-var-assign set the value of this variable
-var-update update the variable and its children

In the next subsection we describe each operation in detail and suggest how it can be used.

Description And Use of Operations on Variable Objects

The -var-create Command

      -var-create {name | "-"}
         {frame-addr | "*"} expression

This operation creates a variable object, which allows the monitoring of a variable, the result of an expression, a memory cell or a CPU register.

The name parameter is the string by which the object can be referenced. It must be unique. If `-' is specified, the varobj system will generate a string “varNNNNNN” automatically. It will be unique provided that one does not specify name on that format. The command fails if a duplicate name is found.

The frame under which the expression should be evaluated can be specified by frame-addr. A `*' indicates that the current frame should be used.

expression is any expression valid on the current language set (must not begin with a `*'), or one of the following:


This operation returns the name, number of children and the type of the object created. Type is returned as a string as the ones generated by the gdb CLI:


The -var-delete Command

      -var-delete name

Deletes a previously created variable object and all of its children.

Returns an error if the object name is not found.

The -var-set-format Command

      -var-set-format name format-spec

Sets the output format for the value of the object name to be format-spec.

The syntax for the format-spec is as follows:

      format-spec ==>
      {binary | decimal | hexadecimal | octal | natural}

The -var-show-format Command

      -var-show-format name

Returns the format used to display the value of the object name.

      format ==>

The -var-info-num-children Command

      -var-info-num-children name

Returns the number of children of a variable object name:


The -var-list-children Command

      -var-list-children [print-values] name

Returns a list of the children of the specified variable object. With just the variable object name as an argument or with an optional preceding argument of 0 or --no-values, prints only the names of the variables. With an optional preceding argument of 1 or --all-values, also prints their values.

      -var-list-children n
      numchild=n,type=type},(repeats N times)]
      -var-list-children --all-values n
      numchild=n,value=value,type=type},(repeats N times)]

The -var-info-type Command

      -var-info-type name

Returns the type of the specified variable name. The type is returned as a string in the same format as it is output by the gdb CLI:


The -var-info-expression Command

      -var-info-expression name

Returns what is represented by the variable object name:


where lang-spec is {"C" | "C++" | "Java"}.

The -var-show-attributes Command

      -var-show-attributes name

List attributes of the specified variable object name:

      status=attr [ ( ,attr )* ]

where attr is { { editable | noneditable } | TBD }.

The -var-evaluate-expression Command

      -var-evaluate-expression name

Evaluates the expression that is represented by the specified variable object and returns its value as a string in the current format specified for the object:


Note that one must invoke -var-list-children for a variable before the value of a child variable can be evaluated.

The -var-assign Command

      -var-assign name expression

Assigns the value of expression to the variable object specified by name. The object must be `editable'. If the variable's value is altered by the assign, the variable will show up in any subsequent -var-update list.

     -var-assign var1 3
     -var-update *

The -var-update Command

      -var-update {name | "*"}

Update the value of the variable object name by evaluating its expression after fetching all the new values from memory or registers. A `*' causes all existing variable objects to be updated.

Next: , Previous: Emacs, Up: Top

25 gdb Annotations

This chapter describes annotations in gdb. Annotations were designed to interface gdb to graphical user interfaces or other similar programs which want to interact with gdb at a relatively high level.

The annotation mechanism has largely been superseeded by gdb/mi (see GDB/MI).

Next: , Up: Annotations

25.1 What is an Annotation?

Annotations start with a newline character, two `control-z' characters, and the name of the annotation. If there is no additional information associated with this annotation, the name of the annotation is followed immediately by a newline. If there is additional information, the name of the annotation is followed by a space, the additional information, and a newline. The additional information cannot contain newline characters.

Any output not beginning with a newline and two `control-z' characters denotes literal output from gdb. Currently there is no need for gdb to output a newline followed by two `control-z' characters, but if there was such a need, the annotations could be extended with an `escape' annotation which means those three characters as output.

The annotation level, which is specified using the --annotate command line option (see Mode Options), controls how much information gdb prints together with its prompt, values of expressions, source lines, and other types of output. Level 0 is for no anntations, 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 annotations have been made obsolete (see Limitations of the Annotation Interface).

set annotate level
The gdb command set annotate sets the level of annotations to the specified level.
show annotate
Show the current annotation level.

This chapter describes level 3 annotations.

A simple example of starting up gdb with annotations is:

     $ gdb --annotate=3
     GNU gdb 6.0
     Copyright 2003 Free Software Foundation, Inc.
     GDB is free software, covered by the GNU General Public License,
     and you are welcome to change it and/or 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.
     This GDB was configured as "i386-pc-linux-gnu"

Here `quit' is input to gdb; the rest is output from gdb. The three lines beginning `^Z^Z' (where `^Z' denotes a `control-z' character) are annotations; the rest is output from gdb.

Next: , Previous: Annotations Overview, Up: Annotations

25.2 Annotation for gdb Input

When gdb prompts for input, it annotates this fact so it is possible to know when to send output, when the output from a given command is over, etc.

Different kinds of input each have a different input type. Each input type has three annotations: a pre- annotation, which denotes the beginning of any prompt which is being output, a plain annotation, which denotes the end of the prompt, and then a post- annotation which denotes the end of any echo which may (or may not) be associated with the input. For example, the prompt input type features the following annotations:


The input types are

When gdb is prompting for a command (the main gdb prompt).

When gdb prompts for a set of commands, like in the commands command. The annotations are repeated for each command which is input.

When gdb wants the user to select between various overloaded functions.

When gdb wants the user to confirm a potentially dangerous operation.

When gdb is asking the user to press return to continue. Note: Don't expect this to work well; instead use set height 0 to disable prompting. This is because the counting of lines is buggy in the presence of annotations.

Next: , Previous: Prompting, Up: Annotations

25.3 Errors


This annotation occurs right before gdb responds to an interrupt.


This annotation occurs right before gdb responds to an error.

Quit and error annotations indicate that any annotations which gdb was in the middle of may end abruptly. For example, if a value-history-begin annotation is followed by a error, one cannot expect to receive the matching value-history-end. One cannot expect not to receive it either, however; an error annotation does not necessarily mean that gdb is immediately returning all the way to the top level.

A quit or error annotation may be preceded by


Any output between that and the quit or error annotation is the error message.

Warning messages are not yet annotated.

Next: , Previous: Errors, Up: Annotations

25.4 Invalidation Notices

The following annotations say that certain pieces of state may have changed.

The frames (for example, output from the backtrace command) may have changed.

The breakpoints may have changed. For example, the user just added or deleted a breakpoint.

Next: , Previous: Invalidation, Up: Annotations

25.5 Running the Program

When the program starts executing due to a gdb command such as step or continue,


is output. When the program stops,


is output. Before the stopped annotation, a variety of annotations describe how the program stopped.

^Z^Zexited exit-status
The program exited, and exit-status is the exit status (zero for successful exit, otherwise nonzero).

The program exited with a signal. After the ^Z^Zsignalled, the annotation continues:

where name is the name of the signal, such as SIGILL or SIGSEGV, and string is the explanation of the signal, such as Illegal Instruction or Segmentation fault. intro-text, middle-text, and end-text are for the user's benefit and have no particular format.

The syntax of this annotation is just like signalled, but gdb is just saying that the program received the signal, not that it was terminated with it.

^Z^Zbreakpoint number
The program hit breakpoint number number.

^Z^Zwatchpoint number
The program hit watchpoint number number.

Previous: Annotations for Running, Up: Annotations

25.6 Displaying Source

The following annotation is used instead of displaying source code:

     ^Z^Zsource filename:line:character:middle:addr

where filename is an absolute file name indicating which source file, line is the line number within that file (where 1 is the first line in the file), character is the character position within the file (where 0 is the first character in the file) (for most debug formats this will necessarily point to the beginning of a line), middle is `middle' if addr is in the middle of the line, or `beg' if addr is at the beginning of the line, and addr is the address in the target program associated with the source which is being displayed. addr is in the form `0x' followed by one or more lowercase hex digits (note that this does not depend on the language).

Next: , Previous: GDB/MI, Up: Top

26 Reporting Bugs in gdb

Your bug reports play an essential role in making gdb reliable.

Reporting a bug may help you by bringing a solution to your problem, or it may not. But in any case the principal function of a bug report is to help the entire community by making the next version of gdb work better. Bug reports are your contribution to the maintenance of gdb.

In order for a bug report to serve its purpose, you must include the information that enables us to fix the bug.

Next: , Up: GDB Bugs

26.1 Have you found a bug?

If you are not sure whether you have found a bug, here are some guidelines:

Previous: Bug Criteria, Up: GDB Bugs

26.2 How to report bugs

A number of companies and individuals offer support for gnu products. If you obtained gdb from a support organization, we recommend you contact that organization first.

You can find contact information for many support companies and individuals in the file etc/SERVICE in the gnu Emacs distribution.

In any event, we also recommend that you submit bug reports for gdb. The prefered method is to submit them directly using gdb's Bugs web page. Alternatively, the e-mail gateway can be used.

Do not send bug reports to `info-gdb', or to `help-gdb', or to any newsgroups. Most users of gdb do not want to receive bug reports. Those that do have arranged to receive `bug-gdb'.

The mailing list `bug-gdb' has a newsgroup `gnu.gdb.bug' which serves as a repeater. The mailing list and the newsgroup carry exactly the same messages. Often people think of posting bug reports to the newsgroup instead of mailing them. This appears to work, but it has one problem which can be crucial: a newsgroup posting often lacks a mail path back to the sender. Thus, if we need to ask for more information, we may be unable to reach you. For this reason, it is better to send bug reports to the mailing list.

The fundamental principle of reporting bugs usefully is this: report all the facts. If you are not sure whether to state a fact or leave it out, state it!

Often people omit facts because they think they know what causes the problem and assume that some details do not matter. Thus, you might assume that the name of the variable you use in an example does not matter. Well, probably it does not, but one cannot be sure. Perhaps the bug is a stray memory reference which happens to fetch from the location where that name is stored in memory; perhaps, if the name were different, the contents of that location would fool the debugger into doing the right thing despite the bug. Play it safe and give a specific, complete example. That is the easiest thing for you to do, and the most helpful.

Keep in mind that the purpose of a bug report is to enable us to fix the bug. It may be that the bug has been reported previously, but neither you nor we can know that unless your bug report is complete and self-contained.

Sometimes people give a few sketchy facts and ask, “Does this ring a bell?” Those bug reports are useless, and we urge everyone to refuse to respond to them except to chide the sender to report bugs properly.

To enable us to fix the bug, you should include all these things:

Here are some things that are not necessary:

Next: , Previous: Formatting Documentation, Up: Top

27 Command Line Editing

This chapter describes the basic features of the gnu command line editing interface.

Next: , Up: Command Line Editing

27.1 Introduction to Line Editing

The following paragraphs describe the notation used to represent keystrokes.

The text C-k is read as `Control-K' and describes the character produced when the <k> key is pressed while the Control key is depressed.

The text M-k is read as `Meta-K' and describes the character produced when the Meta key (if you have one) is depressed, and the <k> key is pressed. The Meta key is labeled <ALT> on many keyboards. On keyboards with two keys labeled <ALT> (usually to either side of the space bar), the <ALT> on the left side is generally set to work as a Meta key. The <ALT> key on the right may also be configured to work as a Meta key or may be configured as some other modifier, such as a Compose key for typing accented characters.

If you do not have a Meta or <ALT> key, or another key working as a Meta key, the identical keystroke can be generated by typing <ESC> first, and then typing <k>. Either process is known as metafying the <k> key.

The text M-C-k is read as `Meta-Control-k' and describes the character produced by metafying C-k.

In addition, several keys have their own names. Specifically, <DEL>, <ESC>, <LFD>, <SPC>, <RET>, and <TAB> all stand for themselves when seen in this text, or in an init file (see Readline Init File). If your keyboard lacks a <LFD> key, typing <C-j> will produce the desired character. The <RET> key may be labeled <Return> or <Enter> on some keyboards.

Next: , Previous: Introduction and Notation, Up: Command Line Editing

27.2 Readline Interaction

Often during an interactive session you type in a long line of text, only to notice that the first word on the line is misspelled. The Readline library gives you a set of commands for manipulating the text as you type it in, allowing you to just fix your typo, and not forcing you to retype the majority of the line. Using these editing commands, you move the cursor to the place that needs correction, and delete or insert the text of the corrections. Then, when you are satisfied with the line, you simply press <RET>. You do not have to be at the end of the line to press <RET>; the entire line is accepted regardless of the location of the cursor within the line.

Next: , Up: Readline Interaction

27.2.1 Readline Bare Essentials

In order to enter characters into the line, simply type them. The typed character appears where the cursor was, and then the cursor moves one space to the right. If you mistype a character, you can use your erase character to back up and delete the mistyped character.

Sometimes you may mistype a character, and not notice the error until you have typed several other characters. In that case, you can type C-b to move the cursor to the left, and then correct your mistake. Afterwards, you can move the cursor to the right with C-f.

When you add text in the middle of a line, you will notice that characters to the right of the cursor are `pushed over' to make room for the text that you have inserted. Likewise, when you delete text behind the cursor, characters to the right of the cursor are `pulled back' to fill in the blank space created by the removal of the text. A list of the bare essentials for editing the text of an input line follows.

Move back one character.
Move forward one character.
<DEL> or <Backspace>
Delete the character to the left of the cursor.
Delete the character underneath the cursor.
Printing characters
Insert the character into the line at the cursor.
C-_ or C-x C-u
Undo the last editing command. You can undo all the way back to an empty line.

(Depending on your configuration, the <Backspace> key be set to delete the character to the left of the cursor and the <DEL> key set to delete the character underneath the cursor, like C-d, rather than the character to the left of the cursor.)

Next: , Previous: Readline Bare Essentials, Up: Readline Interaction

27.2.2 Readline Movement Commands

The above table describes the most basic keystrokes that you need in order to do editing of the input line. For your convenience, many other commands have been added in addition to C-b, C-f, C-d, and <DEL>. Here are some commands for moving more rapidly about the line.

Move to the start of the line.
Move to the end of the line.
Move forward a word, where a word is composed of letters and digits.
Move backward a word.
Clear the screen, reprinting the current line at the top.

Notice how C-f moves forward a character, while M-f moves forward a word. It is a loose convention that control keystrokes operate on characters while meta keystrokes operate on words.

Next: , Previous: Readline Movement Commands, Up: Readline Interaction

27.2.3 Readline Killing Commands

Killing text means to delete the text from the line, but to save it away for later use, usually by yanking (re-inserting) it back into the line. (`Cut' and `paste' are more recent jargon for `kill' and `yank'.)

If the description for a command says that it `kills' text, then you can be sure that you can get the text back in a different (or the same) place later.

When you use a kill command, the text is saved in a kill-ring. Any number of consecutive kills save all of the killed text together, so that when you yank it back, you get it all. The kill ring is not line specific; the text that you killed on a previously typed line is available to be yanked back later, when you are typing another line. Here is the list of commands for killing text.

Kill the text from the current cursor position to the end of the line.
Kill from the cursor to the end of the current word, or, if between words, to the end of the next word. Word boundaries are the same as those used by M-f.
Kill from the cursor the start of the current word, or, if between words, to the start of the previous word. Word boundaries are the same as those used by M-b.
Kill from the cursor to the previous whitespace. This is different than M-<DEL> because the word boundaries differ.

Here is how to yank the text back into the line. Yanking means to copy the most-recently-killed text from the kill buffer.

Yank the most recently killed text back into the buffer at the cursor.
Rotate the kill-ring, and yank the new top. You can only do this if the prior command is C-y or M-y.

Next: , Previous: Readline Killing Commands, Up: Readline Interaction

27.2.4 Readline Arguments

You can pass numeric arguments to Readline commands. Sometimes the argument acts as a repeat count, other times it is the sign of the argument that is significant. If you pass a negative argument to a command which normally acts in a forward direction, that command will act in a backward direction. For example, to kill text back to the start of the line, you might type `M-- C-k'.

The general way to pass numeric arguments to a command is to type meta digits before the command. If the first `digit' typed is a minus sign (`-'), then the sign of the argument will be negative. Once you have typed one meta digit to get the argument started, you can type the remainder of the digits, and then the command. For example, to give the C-d command an argument of 10, you could type `M-1 0 C-d', which will delete the next ten characters on the input line.

Previous: Readline Arguments, Up: Readline Interaction

27.2.5 Searching for Commands in the History

Readline provides commands for searching through the command history for lines containing a specified string. There are two search modes: incremental and non-incremental.

Incremental searches begin before the user has finished typing the search string. As each character of the search string is typed, Readline displays the next entry from the history matching the string typed so far. An incremental search requires only as many characters as needed to find the desired history entry. To search backward in the history for a particular string, type C-r. Typing C-s searches forward through the history. The characters present in the value of the isearch-terminators variable are used to terminate an incremental search. If that variable has not been assigned a value, the <ESC> and C-J characters will terminate an incremental search. C-g will abort an incremental search and restore the original line. When the search is terminated, the history entry containing the search string becomes the current line.

To find other matching entries in the history list, type C-r or C-s as appropriate. This will search backward or forward in the history for the next entry matching the search string typed so far. Any other key sequence bound to a Readline command will terminate the search and execute that command. For instance, a <RET> will terminate the search and accept the line, thereby executing the command from the history list. A movement command will terminate the search, make the last line found the current line, and begin editing.

Readline remembers the last incremental search string. If two C-rs are typed without any intervening characters defining a new search string, any remembered search string is used.

Non-incremental searches read the entire search string before starting to search for matching history lines. The search string may be typed by the user or be part of the contents of the current line.

Next: , Previous: Readline Interaction, Up: Command Line Editing

27.3 Readline Init File

Although the Readline library comes with a set of Emacs-like keybindings installed by default, it is possible to use a different set of keybindings. Any user can customize programs that use Readline by putting commands in an inputrc file, conventionally in his home directory. The name of this file is taken from the value of the environment variable INPUTRC. If that variable is unset, the default is ~/.inputrc.

When a program which uses the Readline library starts up, the init file is read, and the key bindings are set.

In addition, the C-x C-r command re-reads this init file, thus incorporating any changes that you might have made to it.

Next: , Up: Readline Init File

27.3.1 Readline Init File Syntax

There are only a few basic constructs allowed in the Readline init file. Blank lines are ignored. Lines beginning with a `#' are comments. Lines beginning with a `$' indicate conditional constructs (see Conditional Init Constructs). Other lines denote variable settings and key bindings.

Variable Settings
You can modify the run-time behavior of Readline by altering the values of variables in Readline using the set command within the init file. The syntax is simple:
          set variable value

Here, for example, is how to change from the default Emacs-like key binding to use vi line editing commands:

          set editing-mode vi

Variable names and values, where appropriate, are recognized without regard to case.

A great deal of run-time behavior is changeable with the following variables.

Controls what happens when Readline wants to ring the terminal bell. If set to `none', Readline never rings the bell. If set to `visible', Readline uses a visible bell if one is available. If set to `audible' (the default), Readline attempts to ring the terminal's bell.
The string to insert at the beginning of the line when the insert-comment command is executed. The default value is "#".
If set to `on', Readline performs filename matching and completion in a case-insensitive fashion. The default value is `off'.
The number of possible completions that determines when the user is asked whether the list of possibilities should be displayed. If the number of possible completions is greater than this value, Readline will ask the user whether or not he wishes to view them; otherwise, they are simply listed. This variable must be set to an integer value greater than or equal to 0. The default limit is 100.
If set to `on', Readline will convert characters with the eighth bit set to an ascii key sequence by stripping the eighth bit and prefixing an <ESC> character, converting them to a meta-prefixed key sequence. The default value is `on'.
If set to `On', Readline will inhibit word completion. Completion characters will be inserted into the line as if they had been mapped to self-insert. The default is `off'.
The editing-mode variable controls which default set of key bindings is used. By default, Readline starts up in Emacs editing mode, where the keystrokes are most similar to Emacs. This variable can be set to either `emacs' or `vi'.
When set to `on', Readline will try to enable the application keypad when it is called. Some systems need this to enable the arrow keys. The default is `off'.
If set to `on', tilde expansion is performed when Readline attempts word completion. The default is `off'.

If set to `on', the history code attempts to place point at the same location on each history line retrieved with previous-history or next-history.

This variable can be set to either `on' or `off'. Setting it to `on' means that the text of the lines being edited will scroll horizontally on a single screen line when they are longer than the width of the screen, instead of wrapping onto a new screen line. By default, this variable is set to `off'.
If set to `on', Readline will enable eight-bit input (it will not clear the eighth bit in the characters it reads), regardless of what the terminal claims it can support. The default value is `off'. The name meta-flag is a synonym for this variable.
The string of characters that should terminate an incremental search without subsequently executing the character as a command (see Searching). If this variable has not been given a value, the characters <ESC> and C-J will terminate an incremental search.
Sets Readline's idea of the current keymap for key binding commands. Acceptable keymap names are emacs, emacs-standard, emacs-meta, emacs-ctlx, vi, vi-move, vi-command, and vi-insert. vi is equivalent to vi-command; emacs is equivalent to emacs-standard. The default value is emacs. The value of the editing-mode variable also affects the default keymap.
If set to `on', completed directory names have a slash appended. The default is `on'.
This variable, when set to `on', causes Readline to display an asterisk (`*') at the start of history lines which have been modified. This variable is `off' by default.
If set to `on', completed names which are symbolic links to directories have a slash appended (subject to the value of mark-directories). The default is `off'.
This variable, when set to `on', causes Readline to match files whose names begin with a `.' (hidden files) when performing filename completion, unless the leading `.' is supplied by the user in the filename to be completed. This variable is `on' by default.
If set to `on', Readline will display characters with the eighth bit set directly rather than as a meta-prefixed escape sequence. The default is `off'.
If set to `on', Readline uses an internal more-like pager to display a screenful of possible completions at a time. This variable is `on' by default.
If set to `on', Readline will display completions with matches sorted horizontally in alphabetical order, rather than down the screen. The default is `off'.
This alters the default behavior of the completion functions. If set to `on', words which have more than one possible completion cause the matches to be listed immediately instead of ringing the bell. The default value is `off'.
This alters the default behavior of the completion functions in a fashion similar to show-all-if-ambiguous. If set to `on', words which have more than one possible completion without any possible partial completion (the possible completions don't share a common prefix) cause the matches to be listed immediately instead of ringing the bell. The default value is `off'.
If set to `on', a character denoting a file's type is appended to the filename when listing possible completions. The default is `off'.

Key Bindings
The syntax for controlling key bindings in the init file is simple. First you need to find the name of the command that you want to change. The following sections contain tables of the command name, the default keybinding, if any, and a short description of what the command does.

Once you know the name of the command, simply place on a line in the init file the name of the key you wish to bind the command to, a colon, and then the name of the command. The name of the key can be expressed in different ways, depending on what you find most comfortable.

In addition to command names, readline allows keys to be bound to a string that is inserted when the key is pressed (a macro).

keynamefunction-name or macro
keyname is the name of a key spelled out in English. For example:
               Control-u: universal-argument
               Meta-Rubout: backward-kill-word
               Control-o: "> output"

In the above example, C-u is bound to the function universal-argument, M-DEL is bound to the function backward-kill-word, and C-o is bound to run the macro expressed on the right hand side (that is, to insert the text `> output' into the line).

A number of symbolic character names are recognized while processing this key binding syntax: DEL, ESC, ESCAPE, LFD, NEWLINE, RET, RETURN, RUBOUT, SPACE, SPC, and TAB.

"keyseq": function-name or macro
keyseq differs from keyname above in that strings denoting an entire key sequence can be specified, by placing the key sequence in double quotes. Some gnu Emacs style key escapes can be used, as in the following example, but the special character names are not recognized.
               "\C-u": universal-argument
               "\C-x\C-r": re-read-init-file
               "\e[11~": "Function Key 1"

In the above example, C-u is again bound to the function universal-argument (just as it was in the first example), `C-x C-r' is bound to the function re-read-init-file, and `<ESC> <[> <1> <1> <~>' is bound to insert the text `Function Key 1'.

The following gnu Emacs style escape sequences are available when specifying key sequences:

control prefix
meta prefix
an escape character
<">, a double quotation mark
<'>, a single quote or apostrophe

In addition to the gnu Emacs style escape sequences, a second set of backslash escapes is available:

alert (bell)
form feed
carriage return
horizontal tab
vertical tab
the eight-bit character whose value is the octal value nnn (one to three digits)
the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)

When entering the text of a macro, single or double quotes must be used to indicate a macro definition. Unquoted text is assumed to be a function name. In the macro body, the backslash escapes described above are expanded. Backslash will quote any other character in the macro text, including `"' and `''. For example, the following binding will make `C-x \' insert a single `\' into the line:

          "\C-x\\": "\\"

Next: , Previous: Readline Init File Syntax, Up: Readline Init File

27.3.2 Conditional Init Constructs

Readline implements a facility similar in spirit to the conditional compilation features of the C preprocessor which allows key bindings and variable settings to be performed as the result of tests. There are four parser directives used.

The $if construct allows bindings to be made based on the editing mode, the terminal being used, or the application using Readline. The text of the test extends to the end of the line; no characters are required to isolate it.
The mode= form of the $if directive is used to test whether Readline is in emacs or vi mode. This may be used in conjunction with the `set keymap' command, for instance, to set bindings in the emacs-standard and emacs-ctlx keymaps only if Readline is starting out in emacs mode.
The term= form may be used to include terminal-specific key bindings, perhaps to bind the key sequences output by the terminal's function keys. The word on the right side of the `=' is tested against both the full name of the terminal and the portion of the terminal name before the first `-'. This allows sun to match both sun and sun-cmd, for instance.
The application construct is used to include application-specific settings. Each program using the Readline library sets the application name, and you can test for a particular value. This could be used to bind key sequences to functions useful for a specific program. For instance, the following command adds a key sequence that quotes the current or previous word in Bash:
               $if Bash
               # Quote the current or previous word
               "\C-xq": "\eb\"\ef\""

This command, as seen in the previous example, terminates an $if command.
Commands in this branch of the $if directive are executed if the test fails.
This directive takes a single filename as an argument and reads commands and bindings from that file. For example, the following directive reads from /etc/inputrc:
          $include /etc/inputrc

Previous: Conditional Init Constructs, Up: Readline Init File

27.3.3 Sample Init File

Here is an example of an inputrc file. This illustrates key binding, variable assignment, and conditional syntax.

     # This file controls the behaviour of line input editing for
     # programs that use the GNU Readline library.  Existing
     # programs include FTP, Bash, and GDB.
     # You can re-read the inputrc file with C-x C-r.
     # Lines beginning with '#' are comments.
     # First, include any systemwide bindings and variable
     # assignments from /etc/Inputrc
     $include /etc/Inputrc
     # Set various bindings for emacs mode.
     set editing-mode emacs
     $if mode=emacs
     Meta-Control-h:	backward-kill-word	Text after the function name is ignored
     # Arrow keys in keypad mode
     #"\M-OD":        backward-char
     #"\M-OC":        forward-char
     #"\M-OA":        previous-history
     #"\M-OB":        next-history
     # Arrow keys in ANSI mode
     "\M-[D":        backward-char
     "\M-[C":        forward-char
     "\M-[A":        previous-history
     "\M-[B":        next-history
     # Arrow keys in 8 bit keypad mode
     #"\M-\C-OD":       backward-char
     #"\M-\C-OC":       forward-char
     #"\M-\C-OA":       previous-history
     #"\M-\C-OB":       next-history
     # Arrow keys in 8 bit ANSI mode
     #"\M-\C-[D":       backward-char
     #"\M-\C-[C":       forward-char
     #"\M-\C-[A":       previous-history
     #"\M-\C-[B":       next-history
     C-q: quoted-insert
     # An old-style binding.  This happens to be the default.
     TAB: complete
     # Macros that are convenient for shell interaction
     $if Bash
     # edit the path
     "\C-xp": "PATH=${PATH}\e\C-e\C-a\ef\C-f"
     # prepare to type a quoted word --
     # insert open and close double quotes
     # and move to just after the open quote
     "\C-x\"": "\"\"\C-b"
     # insert a backslash (testing backslash escapes
     # in sequences and macros)
     "\C-x\\": "\\"
     # Quote the current or previous word
     "\C-xq": "\eb\"\ef\""
     # Add a binding to refresh the line, which is unbound
     "\C-xr": redraw-current-line
     # Edit variable on current line.
     "\M-\C-v": "\C-a\C-k$\C-y\M-\C-e\C-a\C-y="
     # use a visible bell if one is available
     set bell-style visible
     # don't strip characters to 7 bits when reading
     set input-meta on
     # allow iso-latin1 characters to be inserted rather
     # than converted to prefix-meta sequences
     set convert-meta off
     # display characters with the eighth bit set directly
     # rather than as meta-prefixed characters
     set output-meta on
     # if there are more than 150 possible completions for
     # a word, ask the user if he wants to see all of them
     set completion-query-items 150
     # For FTP
     $if Ftp
     "\C-xg": "get \M-?"
     "\C-xt": "put \M-?"
     "\M-.": yank-last-arg

Next: , Previous: Readline Init File, Up: Command Line Editing

27.4 Bindable Readline Commands

This section describes Readline commands that may be bound to key sequences. Command names without an accompanying key sequence are unbound by default.

In the following descriptions, point refers to the current cursor position, and mark refers to a cursor position saved by the set-mark command. The text between the point and mark is referred to as the region.

Next: , Up: Bindable Readline Commands

27.4.1 Commands For Moving

beginning-of-line (C-a)
Move to the start of the current line.
end-of-line (C-e)
Move to the end of the line.
forward-char (C-f)
Move forward a character.
backward-char (C-b)
Move back a character.
forward-word (M-f)
Move forward to the end of the next word. Words are composed of letters and digits.
backward-word (M-b)
Move back to the start of the current or previous word. Words are composed of letters and digits.
clear-screen (C-l)
Clear the screen and redraw the current line, leaving the current line at the top of the screen.
redraw-current-line ()
Refresh the current line. By default, this is unbound.

Next: , Previous: Commands For Moving, Up: Bindable Readline Commands

27.4.2 Commands For Manipulating The History

accept-line (Newline or Return)
Accept the line regardless of where the cursor is. If this line is non-empty, it may be added to the history list for future recall with add_history(). If this line is a modified history line, the history line is restored to its original state.
previous-history (C-p)
Move `back' through the history list, fetching the previous command.
next-history (C-n)
Move `forward' through the history list, fetching the next command.
beginning-of-history (M-<)
Move to the first line in the history.
end-of-history (M->)
Move to the end of the input history, i.e., the line currently being entered.
reverse-search-history (C-r)
Search backward starting at the current line and moving `up' through the history as necessary. This is an incremental search.
forward-search-history (C-s)
Search forward starting at the current line and moving `down' through the the history as necessary. This is an incremental search.
non-incremental-reverse-search-history (M-p)
Search backward starting at the current line and moving `up' through the history as necessary using a non-incremental search for a string supplied by the user.
non-incremental-forward-search-history (M-n)
Search forward starting at the current line and moving `down' through the the history as necessary using a non-incremental search for a string supplied by the user.
history-search-forward ()
Search forward through the history for the string of characters between the start of the current line and the point. This is a non-incremental search. By default, this command is unbound.
history-search-backward ()
Search backward through the history for the string of characters between the start of the current line and the point. This is a non-incremental search. By default, this command is unbound.
yank-nth-arg (M-C-y)
Insert the first argument to the previous command (usually the second word on the previous line) at point. With an argument n, insert the nth word from the previous command (the words in the previous command begin with word 0). A negative argument inserts the nth word from the end of the previous command.
yank-last-arg (M-. or M-_)
Insert last argument to the previous command (the last word of the previous history entry). With an argument, behave exactly like yank-nth-arg. Successive calls to yank-last-arg move back through the history list, inserting the last argument of each line in turn.

Next: , Previous: Commands For History, Up: Bindable Readline Commands

27.4.3 Commands For Changing Text

delete-char (C-d)
Delete the character at point. If point is at the beginning of the line, there are no characters in the line, and the last character typed was not bound to delete-char, then return eof.
backward-delete-char (Rubout)
Delete the character behind the cursor. A numeric argument means to kill the characters instead of deleting them.
forward-backward-delete-char ()
Delete the character under the cursor, unless the cursor is at the end of the line, in which case the character behind the cursor is deleted. By default, this is not bound to a key.
quoted-insert (C-q or C-v)
Add the next character typed to the line verbatim. This is how to insert key sequences like C-q, for example.
tab-insert (M-<TAB>)
Insert a tab character.
self-insert (a, b, A, 1, !, ...)
Insert yourself.
transpose-chars (C-t)
Drag the character before the cursor forward over the character at the cursor, moving the cursor forward as well. If the insertion point is at the end of the line, then this transposes the last two characters of the line. Negative arguments have no effect.
transpose-words (M-t)
Drag the word before point past the word after point, moving point past that word as well. If the insertion point is at the end of the line, this transposes the last two words on the line.
upcase-word (M-u)
Uppercase the current (or following) word. With a negative argument, uppercase the previous