4.4BSD/usr/src/contrib/gdb-4.7.lbl/gdb/gdb.info-1

This is Info file ./gdb.info, produced by Makeinfo-1.47 from the input
file gdb-all.texi.

START-INFO-DIR-ENTRY
* Gdb: (gdb). The GNU debugger.
END-INFO-DIR-ENTRY
This file documents the GNU debugger GDB.

This is Edition 4.06, October 1992, of `Debugging with GDB: the GNU
Source-Level Debugger' for GDB Version 4.7.

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the section entitled "GNU General Public License" is included
exactly as in the original, and provided that the entire resulting
derived work is distributed under the terms of a permission notice
identical to this one.

Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the section entitled "GNU General Public License"
may be included in a translation approved by the Free Software
Foundation instead of in the original English.

File: gdb.info, Node: Top, Next: Summary, Prev: (DIR), Up: (DIR)

Debugging with GDB
******************

This file describes GDB, the GNU symbolic debugger.

This is Edition 4.06, October 1992, for GDB Version 4.7.

* Menu:

* Summary:: Summary of GDB
* New Features:: New features since GDB version 3.5
* Sample Session:: A sample GDB session
* Invocation:: Getting in and out of GDB
* Commands:: GDB commands
* Running:: Running programs under GDB
* Stopping:: Stopping and continuing
* Stack:: Examining the stack
* Source:: Examining source files
* Data:: Examining data
* Languages:: Using GDB with different languages
* Symbols:: Examining the symbol table
* Altering:: Altering execution
* GDB Files:: GDB's files
* Targets:: Specifying a debugging target
* Controlling GDB:: Controlling GDB
* Sequences:: Canned sequences of commands
* Emacs:: Using GDB under GNU Emacs
* GDB Bugs:: Reporting bugs in GDB
* Renamed Commands::
* Formatting Documentation:: How to format and print GDB documentation
* Installing GDB:: Installing GDB
* Copying:: GNU GENERAL PUBLIC LICENSE
* Index:: Index

File: gdb.info, Node: Summary, Next: New Features, Prev: Top, Up: Top

Summary of GDB
**************

The purpose of a debugger such as GDB is to allow you to see what is
going on "inside" another program while it executes--or what another
program was doing at the moment it crashed.

GDB can do four main kinds of things (plus other things in support of
these) to help you catch bugs in the act:

* Start your program, specifying anything that might affect its
behavior.

* Make your program stop on specified conditions.

* Examine what has happened, when your program has stopped.

* Change things in your program, so you can experiment with
correcting the effects of one bug and go on to learn about another.

You can use GDB to debug programs written in C, C++, and Modula-2.
Fortran support will be added when a GNU Fortran compiler is ready.

* Menu:

* Free Software:: Free Software
* Contributors:: Contributors to GDB

File: gdb.info, Node: Free Software, Next: Contributors, Up: Summary

Free Software
=============

GDB is "free software", protected by the GNU General Public License
(GPL). The GPL gives you the freedom to copy or adapt a licensed
program--but every person getting a copy also gets with it the freedom
to modify that copy (which means that they must get access to the
source code), and the freedom to distribute further copies. Typical
software companies use copyrights to limit your freedoms; the Free
Software Foundation uses the GPL to preserve these freedoms.

Fundamentally, the General Public License is a license which says
that you have these freedoms and that you cannot take these freedoms
away from anyone else.

For full details, *note GNU GENERAL PUBLIC LICENSE: Copying..

File: gdb.info, Node: Contributors, Prev: Free Software, Up: Summary

Contributors to GDB
===================

Richard Stallman was the original author of GDB, and of many other
GNU programs. Many others have contributed to its development. This
section attempts to credit major contributors. One of the virtues of
free software is that everyone is free to contribute to it; with
regret, we cannot actually acknowledge everyone here. The file
`ChangeLog' in the GDB distribution approximates a blow-by-blow account.

Changes much prior to version 2.0 are lost in the mists of time.

*Plea:* Additions to this section are particularly welcome. If you
or your friends (or enemies; let's be evenhanded) have been
unfairly omitted from this list, we would like to add your names!

So that they may not regard their long labor as thankless, we
particularly thank those who shepherded GDB through major releases: Stu
Grossman and John Gilmore (releases 4.6, 4.5, 4.4), John Gilmore
(releases 4.3, 4.2, 4.1, 4.0, and 3.9); Jim Kingdon (releases 3.5, 3.4,
3.3); and Randy Smith (releases 3.2, 3.1, 3.0). As major maintainer of
GDB for some period, each contributed significantly to the structure,
stability, and capabilities of the entire debugger.

Richard Stallman, assisted at various times by Pete 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. 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 4 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.

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.
Keith Packard contributed NS32K support. Doug Rabson contributed Acorn
Risc Machine 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.

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 and Wind River Systems
contributed remote debugging modules for their products.

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 and
the Modula-2 support, and contributed the Languages chapter of this
manual.

Fred Fish wrote most of the support for Unix System Vr4, and enhanced
the command-completion support to cover C++ overloaded symbols.

File: gdb.info, Node: New Features, Next: Sample Session, Prev: Summary, Up: Top

New Features since GDB version 3.5
**********************************

*Targets*
Using the new command `target', you can select at runtime whether
you are debugging local files, local processes, standalone systems
over a serial port, realtime systems over a TCP/IP connection,
etc. The command `load' can download programs into a remote
system. Serial stubs are available for Motorola 680x0, Intel
80386, and Sparc remote systems; GDB also supports debugging
realtime processes running under VxWorks, using SunRPC Remote
Procedure Calls over TCP/IP to talk to a debugger stub on the
target system. Internally, GDB now uses a function vector to
mediate access to different targets; if you need to add your own
support for a remote protocol, this makes it much easier.

*Watchpoints*
GDB now sports watchpoints as well as breakpoints. You can use a
watchpoint to stop execution whenever the value of an expression
changes, without having to predict a particular place in your
program where this may happen.

*Wide Output*
Commands that issue wide output now insert newlines at places
designed to make the output more readable.

*Object Code Formats*
GDB uses a new library called the Binary File Descriptor (BFD)
Library to permit it to switch dynamically, without
reconfiguration or recompilation, between different object-file
formats. Formats currently supported are COFF, a.out, and the
Intel 960 b.out; files may be read as .o's, archive libraries, or
core dumps. BFD is available as a subroutine library so that
other programs may take advantage of it, and the other GNU binary
utilities are being converted to use it.

*Configuration and Ports*
Compile-time configuration (to select a particular architecture and
operating system) is much easier. The script `configure' now
allows you to configure GDB as either a native debugger or a
cross-debugger. *Note Installing GDB::, for details on how to
configure.

*Interaction*
The user interface to GDB's control variables has been simplified
and consolidated in two commands, `set' and `show'. Output lines
are now broken at readable places, rather than overflowing onto
the next line. You can suppress output of machine-level addresses,
displaying only source language information.

*C++*
GDB now supports C++ multiple inheritance (if used with a GCC
version 2 compiler), and also has limited support for C++ exception
handling, with the commands `catch' and `info catch': GDB can
break when an exception is raised, before the stack is peeled back
to the exception handler's context.

*Modula-2*
GDB now has preliminary support for the GNU Modula-2 compiler,
currently under development at the State University of New York at
Buffalo. Coordinated development of both GDB and the GNU Modula-2
compiler will continue into 1992. Other Modula-2 compilers are
currently not supported, and attempting to debug programs compiled
with them will likely result in an error as the symbol table of the
executable is read in.

*Command Rationalization*
Many GDB commands have been renamed to make them easier to remember
and use. In particular, the subcommands of `info' and
`show'/`set' are grouped to make the former refer to the state of
your program, and the latter refer to the state of GDB itself.
*Note Renamed Commands::, for details on what commands were
renamed.

*Shared Libraries*
GDB 4 can debug programs and core files that use SunOS, SVR4, or
IBM RS/6000 shared libraries.

*Reference Card*
GDB 4 has a reference card. *Note Formatting the Documentation:
Formatting Documentation, for instructions to print it.

*Work in Progress*
Kernel debugging for BSD and Mach systems; Tahoe and HPPA
architecture support.

File: gdb.info, Node: Sample Session, Next: Invocation, Prev: New Features, Up: Top

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 these 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's
definition in another stop working. In the following short `m4'
session, we define a macro `foo' which expands to `0000'; we then use
the `m4' built-in `defn' to define `bar' as the same thing. However,
when we change the open quote string to `<QUOTE>' and the close quote
string to `<UNQUOTE>', the same procedure fails to define a new synonym
`baz':

$ cd gnu/m4
$ ./m4
define(foo,0000)

foo
0000
define(bar,defn(`foo'))

bar
0000
changequote(<QUOTE>,<UNQUOTE>)

define(baz,defn(<QUOTE>foo<UNQUOTE>))
baz
C-d
m4: End of input: 0: fatal error: EOF in string

Let's use GDB to try to see what's 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 4.7, Copyright 1992 Free Software Foundation, Inc...
(gdb)

GDB reads only enough symbol data to know where to find the rest when
needed; as a result, the first prompt comes up very quickly. We now
tell GDB to use a narrower display width than usual, so that examples
will fit in this manual.

(gdb) set width 70

Let's 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 GDB's `break' command.

(gdb) break m4_changequote
Breakpoint 1 at 0x62f4: file builtin.c, line 879.

Using the `run' command, we start `m4' running under GDB control; as
long as control does not reach the `m4_changequote' subroutine, the
program runs as usual:

(gdb) run
Starting program: /work/Editorial/gdb/gnu/m4/m4
define(foo,0000)

foo
0000

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

changequote(<QUOTE>,<UNQUOTE>)

Breakpoint 1, m4_changequote (argc=3, argv=0x33c70)
at builtin.c:879
879 if (bad_argc(TOKEN_DATA_TEXT(argv[0]),argc,1,3))

Now we use the command `n' (`next') to advance execution to the next
line of the current function.

(gdb) n
882 set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\
: nil,

`set_quotes' looks like a promising subroutine. We can go into it by
using the command `s' (`step') instead of `next'. `step' goes to the
next line to be executed in *any* subroutine, so it steps into
`set_quotes'.

(gdb) s
set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
at input.c:530
530 if (lquote != def_lquote)

The display that shows the subroutine where `m4' is now suspended (and
its arguments) is called a stack frame display. It shows a summary of
the stack. We can use the `backtrace' command (which can also be
spelled `bt'), to see where we are in the stack as a whole: the
`backtrace' command displays a stack frame for each active subroutine.

(gdb) bt
#0 set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
at input.c:530
#1 0x6344 in m4_changequote (argc=3, argv=0x33c70)
at builtin.c:882
#2 0x8174 in expand_macro (sym=0x33320) at macro.c:242
#3 0x7a88 in expand_token (obs=0x0, t=209696, td=0xf7fffa30)
at macro.c:71
#4 0x79dc in expand_input () at macro.c:40
#5 0x2930 in main (argc=0, argv=0xf7fffb20) at m4.c:195

Let's 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; let's examine the variables
`lquote' and `rquote' to see if they are in fact the new left and right
quotes we specified. We can 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. Let's
look at some context; we can display ten lines of source surrounding
the current line, with the `l' (`list') command.

(gdb) l
533 xfree(rquote);
534
535 lquote = (lq == nil || *lq == '\0') ? def_lquote\
: xstrdup (lq);
536 rquote = (rq == nil || *rq == '\0') ? def_rquote\
: xstrdup (rq);
537
538 len_lquote = strlen(rquote);
539 len_rquote = strlen(lquote);
540 }
541
542 void

Let's 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. Let's
try setting them to better values. We can use the `p' command for this,
since it'll 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

Let's see if that fixes the problem of using the new quotes with the
`m4' built-in `defn'. We can allow `m4' to continue executing with the
`c' (`continue') command, and then try the example that caused trouble
initially:

(gdb) c
Continuing.

define(baz,defn(<QUOTE>foo<UNQUOTE>))

baz
0000

Success! The new quotes now work just as well as the default ones. The
problem seems to have been just the two typos defining the wrong
lengths. We'll let `m4' exit by giving it an EOF as input.

C-d
Program exited normally.

The message `Program exited normally.' is from GDB; it indicates `m4'
has finished executing. We can end our GDB session with the GDB `quit'
command.

(gdb) quit

File: gdb.info, Node: Invocation, Next: Commands, Prev: Sample Session, Up: Top

Getting In and Out of GDB
*************************

This chapter discusses how to start GDB, and how to get out of it.
(The essentials: type `gdb' to start GDB, and type `quit' or `C-d' to
exit.)

* Menu:

* Invoking GDB:: Starting GDB
* Leaving GDB:: Leaving GDB
* Shell Commands:: Shell Commands

File: gdb.info, Node: Invoking GDB, Next: Leaving GDB, Up: Invocation

Starting GDB
============

For details on starting up gdb as a remote debugger attached to a
Hitachi H8/300 board, see *Note GDB and the Hitachi H8/300: Hitachi
H8/300 Remote.

Start GDB by running the program `gdb'. Once it's running, 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.

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

Type

gdb -help

to display all available options and briefly describe their use (`gdb
-h' is a shorter equivalent).

All options and command line arguments you give are processed in
sequential order. The order makes a difference when the `-x' option is
used.

* Menu:

* File Options:: Choosing Files
* Mode Options:: Choosing Modes

File: gdb.info, Node: File Options, Next: Mode Options, Up: Invoking GDB

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'
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' option followed by that
argument.)

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.

`-command=FILE'
`-x FILE'
Execute GDB commands from file FILE. *Note Command Files::.

`-directory=DIRECTORY'
`-d DIRECTORY'
Add DIRECTORY to the path to search for source files.

`-m'
`-mapped'
*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 will be `./fred.syms'. Future
GDB debugging sessions will notice the presence of this file, and
will 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 on which GDB is
run. It holds an exact image of GDB's internal symbol table. It
cannot be shared across multiple host platforms.

`-r'
`-readnow'
Read each symbol file's entire symbol table immediately, rather
than the default, which is to read it incrementally as it is
needed. This makes startup slower, but makes future operations
faster.

The `-mapped' and `-readnow' options are typically combined in order
to build a `.syms' file that contains complete symbol information. A
simple GDB invocation to do nothing but build a `.syms' file for future
use is:

gdb -batch -nx -mapped -readnow programname

File: gdb.info, Node: Mode Options, Prev: File Options, Up: Invoking GDB

Choosing Modes
--------------

You can run GDB in various alternative modes--for example, in batch
mode or quiet mode.

`-nx'
`-n'
Do not execute commands from any `.gdbinit' initialization files.
Normally, the commands in these files are executed after all the
command options and arguments have been processed. *Note Command
Files::.

`-quiet'
`-q'
"Quiet". Do not print the introductory and copyright messages.
These messages are also suppressed in batch mode.

`-batch'
Run in batch mode. Exit with status `0' after processing all the
command files specified with `-x' (and `.gdbinit', if not
inhibited). 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.

`-cd=DIRECTORY'
Run GDB using DIRECTORY as its working directory, instead of the
current directory.

`-fullname'
`-f'
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.

`-b BPS'
Set the line speed (baud rate or bits per second) of any serial
interface used by GDB for remote debugging.

`-tty=DEVICE'
Run using DEVICE for your program's standard input and output.

File: gdb.info, Node: Leaving GDB, Next: Shell Commands, Prev: Invoking GDB, Up: Invocation

Leaving GDB
===========

`quit'
To exit GDB, use the `quit' command (abbreviated `q'), or type an
end-of-file character (usually `C-d').

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

If you have been using GDB to control an attached process or device,
you can release it with the `detach' command; *note Debugging an
Already-Running Process: Attach...

File: gdb.info, Node: Shell Commands, Prev: Leaving GDB, Up: Invocation

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'
Directs GDB to invoke an inferior shell to execute COMMAND STRING.
If it exists, the environment variable `SHELL' is used for the
name of the shell to run. Otherwise GDB uses `/bin/sh'.

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'
Causes GDB to execute an inferior `make' program with the specified
arguments. This is equivalent to `shell make MAKE-ARGS'.

File: gdb.info, Node: Commands, Next: Running, Prev: Invocation, Up: Top

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's more than one possibility).

* Menu:

* Command Syntax:: Command Syntax
* Completion:: Command Completion
* Help:: Getting Help

File: gdb.info, Node: Command Syntax, Next: Completion, Up: Commands

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 command names 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 for which unintentional repetition might
cause trouble and which you are unlikely to want to 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' (*note 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.

A line of input starting with `#' is a comment; it does nothing.
This is useful mainly in command files (*note Command Files::.).

File: gdb.info, Node: Completion, Next: Help, Prev: Command Syntax, Up: Commands

Command Completion
==================

GDB can fill in the rest of a word in a command for you, if there's
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's only one possibility, GDB will fill in the word, and
wait 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's 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'
doesn't 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 will sound a bell. You can either supply more
characters and try again, or just press TAB a second time, and GDB will
display 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 will display 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
(GDBP__) 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 will require
quotes. When this happens, GDB will insert 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.

File: gdb.info, Node: Help, Prev: Completion, Up: Commands

Getting Help
============

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

`help'
`h'
You can use `help' (abbreviated `h') with no arguments to display
a short list of named classes of commands:

(gdb) help
List of classes of commands:

running -- Running the program
stack -- Examining the stack
data -- Examining data
breakpoints -- Making program stop at certain points
files -- Specifying and examining files
status -- Status inquiries
support -- Support facilities
user-defined -- User-defined commands
aliases -- Aliases of other commands
obscure -- Obscure features

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:

show -- Generic command for showing things set with "set"
info -- Generic command for printing status

Type "help" followed by command name for full
documentation.
Command name abbreviations are allowed if unambiguous.
(gdb)

`help COMMAND'
With a command name as `help' argument, GDB will display a short
paragraph on how to use that command.

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

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

`show'
In contrast, `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 occasionally want to make sure what
version of GDB you are running; as GDB evolves, new commands are
introduced, and old ones may wither away. The version number is
also announced when you start GDB with no arguments.

`show copying'
Display information about permission for copying GDB.

`show warranty'
Display the GNU "NO WARRANTY" statement.

File: gdb.info, Node: Running, Next: Stopping, Prev: Commands, Up: Top

Running Programs Under GDB
**************************

To debug a program, you must run it under GDB.

* Menu:

* Compilation:: Compiling for Debugging
* Starting:: Starting your Program
* Arguments:: Your Program's Arguments
* Environment:: Your Program's Environment
* Working Directory:: Your Program's Working Directory
* Input/Output:: Your Program's Input and Output
* Attach:: Debugging an Already-Running Process
* Kill Process:: Killing the Child Process
* Process Information:: Additional Process Information

File: gdb.info, Node: Compilation, Next: Starting, Up: Running

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.

Many C 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 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 will show you what's
really there. Don't be too surprised when the execution path doesn't
exactly match your source file! An extreme example: if you define a
variable, but never use it, GDB will never see 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 as a bug (including a test case!).

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.

File: gdb.info, Node: Starting, Next: Arguments, Prev: Compilation, Up: Running

Starting your Program
=====================

`run'
`r'
Use the `run' command to start your program under GDB. You must
first specify the program name (except on VxWorks) with an
argument to GDB (*note Getting In and Out of GDB: Invocation.), or
by using the `file' or `exec-file' command (*note Commands to
Specify Files: Files.).

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

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

The *arguments.*
Specify the arguments to give your program as the arguments of the
`run' command. If a shell is available on your target, the shell
is used to pass the arguments, so that you may use normal
conventions (such as wildcard expansion or variable substitution)
in describing the arguments. In Unix systems, you can control
which shell is used with the `SHELL' environment variable. *Note
Your Program's Arguments: Arguments.

The *environment.*
Your program normally inherits its environment from GDB, but you
can use the GDB commands `set environment' and `unset environment'
to change parts of the environment that will be given to your
program. *Note Your Program's Environment: Environment.

The *working directory.*
Your program inherits its working directory from GDB. You can set
GDB's working directory with the `cd' command in GDB. *Note Your
Program's Working Directory: Working Directory.

The *standard input and output.*
Your program normally uses the same device for standard input and
standard output as GDB is using. You can redirect input and output
in the `run' command line, or you can use the `tty' command to set
a different device for your program. *Note Your Program's Input
and Output: Input/Output.

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

When you issue the `run' command, your program begins to execute
immediately. *Note Stopping and Continuing: Stopping, for discussion
of how to arrange for your program to stop. Once your program has
stopped, you may calls functions in your program, using the `print' or
`call' commands. *Note Examining Data: Data.

If the modification time of your symbol file has changed since the
last time GDB read its symbols, GDB will discard its symbol table and
re-read it. When it does this, GDB tries to retain your current
breakpoints.

File: gdb.info, Node: Arguments, Next: Environment, Prev: Starting, Up: Running

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.
GDB uses the shell indicated by your environment variable `SHELL' if it
exists; otherwise, GDB uses `/bin/sh'.

`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' will execute your
program with no arguments. Once you have run your program with
arguments, using `set args' before the next `run' is the only way
to run it again without arguments.

`show args'
Show the arguments to give your program when it is started.

File: gdb.info, Node: Environment, Next: Working Directory, Prev: Arguments, Up: Running

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), for both GDB and your program. You
may specify several directory names, separated by `:' or
whitespace. If DIRECTORY is already in the path, it is moved to
the front, so it will be 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 fills in the current path where needed in the
DIRECTORY argument, before adding it 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'
Sets 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 a Unix 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.

File: gdb.info, Node: Working Directory, Next: Input/Output, Prev: Environment, Up: Running

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. GDB's working
directory is initially whatever it inherited from its parent process
(typically the shell), but you can specify a new working directory in
GDB with the `cd' command.

The GDB working directory also serves as a default for the commands
that specify files for GDB to operate on. *Note Commands to Specify
Files: Files.

`cd DIRECTORY'
Set GDB's working directory to DIRECTORY.

`pwd'
Print GDB's working directory.

File: gdb.info, Node: Input/Output, Next: Attach, Prev: Working Directory, Up: Running

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 GDB's recorded information 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.