GEMini Atari/GEMini_Atari_CD-ROM_Walnut_Creek_December_1993.iso/zip/gnu/gdb_18b.lzh/GDB.DOC

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Info file: gdb.info,    -*-Text-*-
produced by texinfo-format-buffer
from file: gdb.texinfo

This file documents the GNU debugger GDB.

Copyright (C) 1988, 1989, 1990, 1991 Free Software Foundation, Inc.

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, Prev: Top, Up: (DIR), Next: Top

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

The purpose of a debugger such as GDB is to allow you to execute another
program while examining what is going on inside it.  We call the other
program "your program" or "the program being debugged".

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

  1. Start the program, specifying anything that might affect its behavior.

  2. Make the program stop on specified conditions.

  3. Examine what has happened, when the program has stopped, so that you
     can see bugs happen.

  4. Change things in the program, so you can correct the effects of one bug
     and go on to learn about another without having to recompile first.

GDB can be used to debug programs written in C and C++.  Pascal support
is being implemented, and Fortran support will be added when a GNU
Fortran compiler is written. Currently the atariST port of GDB will be
very confused when asked to debug programs written in  C++. We hope to
fix this at a future time.

* Menu:

* License::    The GNU General Public License gives you permission
           to redistribute GDB on certain terms; and also
           explains that there is no warranty.
* User Interface::      GDB command syntax and input and output conventions.
* Files::      Specifying files for GDB to operate on.
* Options::    GDB arguments and options.
* Compilation::Compiling your program so you can debug it.
* Running::    Running your program under GDB.
* Stopping::   Making your program stop.  Why it may stop.  What to do then.
* Stack::      Examining your program's stack.
* Source::     Examining your program's source files.
* Data::       Examining data in your program.
* Symbols::    Examining the debugger's symbol table.
* Altering::   Altering things in your program.
* Sequences::  Canned command sequences for repeated use.
* Emacs::      Using GDB through GNU Emacs.
* Remote::     Remote kernel debugging across a serial line.
* Commands::   Index of GDB commands.
* Concepts::   Index of GDB concepts.


File: gdb.info  Node: License, Prev: Top, Up: Top, Next: User Interface

GNU GENERAL PUBLIC LICENSE
**************************
                        Version 1, February 1989

     Copyright (C) 1989 Free Software Foundation, Inc.
     675 Mass Ave, Cambridge, MA 02139, USA

     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.


Preamble
========

  The license agreements of most software companies try to keep users
at the mercy of those companies.  By contrast, our General Public
License is intended to guarantee your freedom to share and change free
software---to make sure the software is free for all its users.  The
General Public License applies to the Free Software Foundation's
software and to any other program whose authors commit to using it.
You can use it for your programs, too.

  When we speak of free software, we are referring to freedom, not
price.  Specifically, the General Public License is designed to make
sure that you have the freedom to give away or sell copies of free
software, that you receive source code or can get it if you want it,
that you can change the software or use pieces of it in new free
programs; and that you know you can do these things.

  To protect your rights, we need to make restrictions that forbid
anyone to deny you these rights or to ask you to surrender the rights.
These restrictions translate to certain responsibilities for you if you
distribute copies of the software, or if you modify it.

  For example, if you distribute copies of a such a program, whether
gratis or for a fee, you must give the recipients all the rights that
you have.  You must make sure that they, too, receive or can get the
source code.  And you must tell them their rights.

  We protect your rights with two steps: (1) copyright the software, and
(2) offer you this license which gives you legal permission to copy,
distribute and/or modify the software.

  Also, for each author's protection and ours, we want to make certain
that everyone understands that there is no warranty for this free
software.  If the software is modified by someone else and passed on, we
want its recipients to know that what they have is not the original, so
that any problems introduced by others will not reflect on the original
authors' reputations.

  The precise terms and conditions for copying, distribution and
modification follow.

                          TERMS AND CONDITIONS

  1. This License Agreement applies to any program or other work which
     contains a notice placed by the copyright holder saying it may be
     distributed under the terms of this General Public License.  The
     "Program", below, refers to any such program or work, and a "work based
     on the Program" means either the Program or any work containing the
     Program or a portion of it, either verbatim or with modifications.  Each
     licensee is addressed as "you".

  2. You may copy and distribute verbatim copies of the Program's source
     code as you receive it, in any medium, provided that you conspicuously and
     appropriately publish on each copy an appropriate copyright notice and
     disclaimer of warranty; keep intact all the notices that refer to this
     General Public License and to the absence of any warranty; and give any
     other recipients of the Program a copy of this General Public License
     along with the Program.  You may charge a fee for the physical act of
     transferring a copy.

  3. You may modify your copy or copies of the Program or any portion of
     it, and copy and distribute such modifications under the terms of Paragraph
     1 above, provided that you also do the following:

        * cause the modified files to carry prominent notices stating that
          you changed the files and the date of any change; and

        * cause the whole of any work that you distribute or publish, that
          in whole or in part contains the Program or any part thereof, either
          with or without modifications, to be licensed at no charge to all
          third parties under the terms of this General Public License (except
          that you may choose to grant warranty protection to some or all
          third parties, at your option).

        * If the modified program normally reads commands interactively when
          run, you must cause it, when started running for such interactive use
          in the simplest and most usual way, to print or display an
          announcement including an appropriate copyright notice and a notice
          that there is no warranty (or else, saying that you provide a
          warranty) and that users may redistribute the program under these
          conditions, and telling the user how to view a copy of this General
          Public License.

        * You may charge a fee for the physical act of transferring a
          copy, and you may at your option offer warranty protection in
          exchange for a fee.

     Mere aggregation of another independent work with the Program (or its
     derivative) on a volume of a storage or distribution medium does not bring
     the other work under the scope of these terms.

  4. You may copy and distribute the Program (or a portion or derivative of
     it, under Paragraph 2) in object code or executable form under the terms of
     Paragraphs 1 and 2 above provided that you also do one of the following:

        * accompany it with the complete corresponding machine-readable
          source code, which must be distributed under the terms of
          Paragraphs 1 and 2 above; or,

        * accompany it with a written offer, valid for at least three
          years, to give any third party free (except for a nominal charge
          for the cost of distribution) a complete machine-readable copy of the
          corresponding source code, to be distributed under the terms of
          Paragraphs 1 and 2 above; or,

        * accompany it with the information you received as to where the
          corresponding source code may be obtained.  (This alternative is
          allowed only for noncommercial distribution and only if you
          received the program in object code or executable form alone.)

     Source code for a work means the preferred form of the work for making
     modifications to it.  For an executable file, complete source code means
     all the source code for all modules it contains; but, as a special
     exception, it need not include source code for modules which are standard
     libraries that accompany the operating system on which the executable
     file runs, or for standard header files or definitions files that
     accompany that operating system.

  5. You may not copy, modify, sublicense, distribute or transfer the
     Program except as expressly provided under this General Public License.
     Any attempt otherwise to copy, modify, sublicense, distribute or transfer
     the Program is void, and will automatically terminate your rights to use
     the Program under this License.  However, parties who have received
     copies, or rights to use copies, from you under this General Public
     License will not have their licenses terminated so long as such parties
     remain in full compliance.

  6. By copying, distributing or modifying the Program (or any work based
     on the Program) you indicate your acceptance of this license to do so,
     and all its terms and conditions.

  7. Each time you redistribute the Program (or any work based on the
     Program), the recipient automatically receives a license from the original
     licensor to copy, distribute or modify the Program subject to these
     terms and conditions.  You may not impose any further restrictions on the
     recipients' exercise of the rights granted herein.

  8. The Free Software Foundation may publish revised and/or new versions
     of the General Public License from time to time.  Such new versions will
     be similar in spirit to the present version, but may differ in detail to
     address new problems or concerns.

     Each version is given a distinguishing version number.  If the Program
     specifies a version number of the license which applies to it and "any
     later version", you have the option of following the terms and conditions
     either of that version or of any later version published by the Free
     Software Foundation.  If the Program does not specify a version number of
     the license, you may choose any version ever published by the Free Software
     Foundation.

  9. If you wish to incorporate parts of the Program into other free
     programs whose distribution conditions are different, write to the author
     to ask for permission.  For software which is copyrighted by the Free
     Software Foundation, write to the Free Software Foundation; we sometimes
     make exceptions for this.  Our decision will be guided by the two goals
     of preserving the free status of all derivatives of our free software and
     of promoting the sharing and reuse of software generally.

                                 NO WARRANTY

 10. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
     FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW.  EXCEPT WHEN
     OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
     PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED
     OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
     MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  THE ENTIRE RISK AS
     TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.  SHOULD THE
     PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,
     REPAIR OR CORRECTION.

 11. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL
     ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
     REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
     INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES
     ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT
     LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES
     SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE
     WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN
     ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

                      END OF TERMS AND CONDITIONS


Appendix: How to Apply These Terms to Your New Programs
=======================================================

  If you develop a new program, and you want it to be of the greatest
possible use to humanity, the best way to achieve this is to make it
free software which everyone can redistribute and change under these
terms.

  To do so, attach the following notices to the program.  It is safest to
attach them to the start of each source file to most effectively convey
the exclusion of warranty; and each file should have at least the
"copyright" line and a pointer to where the full notice is found.

     ONE LINE TO GIVE THE PROGRAM'S NAME AND A BRIEF IDEA OF WHAT IT DOES.
     Copyright (C) 19YY  NAME OF AUTHOR

     This program is free software; you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
     the Free Software Foundation; either version 1, or (at your option)
     any later version.

     This program is distributed in the hope that it will be useful,
     but WITHOUT ANY WARRANTY; without even the implied warranty of
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
     GNU General Public License for more details.

     You should have received a copy of the GNU General Public License
     along with this program; if not, write to the Free Software
     Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.

Also add information on how to contact you by electronic and paper mail.

If the program is interactive, make it output a short notice like this
when it starts in an interactive mode:

     Gnomovision version 69, Copyright (C) 19YY NAME OF AUTHOR
     Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
     This is free software, and you are welcome to redistribute it
     under certain conditions; type `show c' for details.

The hypothetical commands `show w' and `show c' should show the
appropriate parts of the General Public License.  Of course, the
commands you use may be called something other than `show w' and `show
c'; they could even be mouse-clicks or menu items---whatever suits your
program.

You should also get your employer (if you work as a programmer) or your
school, if any, to sign a "copyright disclaimer" for the program, if
necessary.  Here a sample; alter the names:

     Yoyodyne, Inc., hereby disclaims all copyright interest in the
     program `Gnomovision' (a program to direct compilers to make passes
     at assemblers) written by James Hacker.

     SIGNATURE OF TY COON, 1 April 1989
     Ty Coon, President of Vice

That's all there is to it!


File: gdb.info  Node: User Interface, Prev: License, Up: Top, Next: Files

GDB Input and Output Conventions
********************************

GDB is invoked with the shell command `gdb'.  Once started, it reads
commands from the keyboard until you tell it to exit. It is probably not
practical to invoke gdb from the desktop, though i suspect it can be
done -- I have never tried.

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 abbreviated if the abbreviation is
unambiguous.  Sometimes 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'.
Possible command abbreviations are often stated in the documentation
of the individual commands.

A blank line as input to GDB means to repeat the previous command verbatim.
Certain commands do not allow themselves to be repeated this way; these are
commands for which unintentional repetition might cause trouble and which
you are unlikely to want to repeat.  Certain others (`list' and
`x') act differently when repeated because that is more useful.

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

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 GDBs so that you tell which one you are talking to.

`set-prompt `newprompt''
     Directs GDB to use `newprompt' as its prompt string henceforth.

To exit GDB, use the `quit' command (abbreviated `q').
`Ctrl-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 `Ctrl-c' at any time because GDB does not allow
it to take effect until a time when it is safe. Be warned that on the
atariST hitting `Ctrl-c' has artifacts.

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.


File: gdb.info  Node: Files, Prev: User Interface, Up: Top, Next: Compilation

Specifying GDB's Files
**********************

GDB needs to know both the file name of the program to be debugged,
and the name of its symbol file. Unlike on `unix' these two
files are distinct files, and GDB needs to be told the file name
of each of the files individually, in order to read its symbol table and in
order to start the program.

* Menu:

* Arguments: File Arguments.   Specifying files with arguments
                                (when you start GDB).
* Commands: File Commands.     Specifying files with GDB commands.


File: gdb.info  Node: File Arguments, Prev: Files, Up: Files, Next: File Commands

Specifying Files with Arguments
===============================

The usual way to specify the executable and symbol file names is with
two command arguments given when you start GDB. Thus,

     gdb -e progm.ttp -s progm.sym

specifies `progm.ttp' as the executable program and
`progm.sym' as a symbol file to examine. Note on the atariST
there are no core files.

*Note Options::, for full information on options and arguments for
invoking GDB.


File: gdb.info  Node: File Commands, Prev: File Arguments, Up: Files

Specifying Files with Commands
==============================

Usually you specify the files for GDB to work with by giving arguments when
you invoke GDB.  But occasionally it is necessary to change to a different
file during a GDB session.  Or you may run GDB and forget to specify the
files you want to use.  In these situations the GDB commands to specify new
files are useful.

`exec-file FILENAME.TTP'
     Specify that the program to be run is found in FILENAME.TTP.

`symbol-file FILENAME.SYM'
     Read symbol table information from file FILENAME.SYM.

     `symbol-file' with no argument clears out GDB's symbol table.

`info files'
     Print the names of the executable file currently in
     use by GDB, and the file from which symbols were loaded.

`gcc-mshort'
     When trying to debug an exec--file that has been compiled
     for 16 bit integer defaults (gcc -mshort)
     the `gcc-mshort' command informs gdb of this fact. This command
     is only useful if you issue it before you read the symbol--file. The preferred
     way to specify this fact is to specify the `-mshort' command line
     option to gdb,ttp when you invoke it.

`-L'
     The `-L' options turns on logging into the file `gdb.log'. The file
     is always appended to. All of gdb's input and output is logged to the file. The
     debugees i/o is not logged here.

While all file-specifying commands allow both absolute and relative
file names as arguments, GDB always converts the file name to an absolute
one and remembers it that way. Don't be surprized by the mix of both
forward and back slashes in the full file names. The library handles
both slash flavors.

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.


File: gdb.info  Node: Compilation, Prev: Files, Up: Top, Next: Running

Compiling Your Program for Debugging
************************************

In order to debug a program effectively, you need to ask for debugging
information when you compile it.  This information in the object file
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.

The Unix C compiler is unable to handle the `-g' and `-O' options
together.  This means that you cannot ask for optimization if you ask for
debugger information.

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 the program is
correct, but there's no sense in pushing your luck.

This version of atariST-GDB does its best to handle files compiled
with the `-mshort' option to GCC. It is conceivable that it may
fail in some pathological cases.

If you use `#include <osbind.h>' or `#include <linea.h>' with
assembler bindings for `GEMDOS' or `LINEA' calls it is recommended
that you DO NOT use the inlined versions of these assembler binding. This
is easily arranged by specifying the `-D__NO_INLINE__' option to
GCC when compiling source files containing these bindings. Inlined
C functions dont cause any problems, but the assembler functions
do when you try to step over them. Note that assembler bindings
do not preclude debugging with GDB, they just cause problems when
you try to step over them with a `next' or a `step' command.

It is also important to note that to effectively use GDB you must have
GCC version 1.39 or better, and utils Patchlevel 21 or better. If you
have older versions, all bets are off.

GDB no longer supports the debugging information produced by giving the
GNU C compiler the `-gg' option, so do not use this option.


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

Running Your Program Under GDB
******************************

To start your program under GDB, use the `run' command.  The program
must already have been specified using the `exec-file' command or with
an argument to GDB (*Note Files::); what `run' does is create an
inferior process, load the program into it, passes it command line
arguements and sets it in motion. And
then it breaks immediately to allow you enter further GDB commands.
After you have done so, you can then continue execution with a `cont'
command. The only GDB command you may enter before you issue the
`run' is the `set-args' command. Note that you can also
specify arguements for the inferior process with the `run'
command. On the atariST there is no way to re-run the program
being debugged. You will have to exit GDB and start again. This
restriction is mostly due to the single tasking nature of TOS.

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 the program.

The arguments.
     You specify the arguments to give the program as the arguments of the
     `run' command.

The environment.
     The 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 the program.
     The `info environment' with show the current environment.

The working directory.
     The program inherits its working directory from GDB.  You can set GDB's
     working directory with the `cd' command in GDB.

After the `run' command, the debugger does nothing but wait for your
program to stop.  *Note Stopping::.

Note that once your program has been started by the `run' command,
you may evaluate expressions that involve calls to functions in the
inferior.  *Note Expressions::.  If you wish to evaluate a function
simply for its side affects, you may use the `set' command.
*Note Assignment::. Calling function on the atarST is currently not fully
supported (ie. its a bug).

* Menu:

* Arguments::          Specifying the arguments for your program.
* Environment::        Specifying the environment for your program.
* Working Directory::  Specifying the working directory for giving
                       to your program when it is run.
* Input/Output::       Specifying the program's standard input and output.
* Attach::             Debugging a process started outside GDB.
* Kill Process::       Getting rid of the child process running your program.


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

Your Program's Arguments
========================

The arguments to your program are specified by the arguments of the
`run' command. No globbing or redirections are done.

The command `set-args' can is used to specify the arguments to be passed
to the program being debugged when it is run.  If `set-args' has no
arguments, it means to use no arguments when the program is run.
On the atariST there is NO way to re-run a program without arguements or
with a new set of arguements once the initial run command has been issued.
Exit GDB and enter it again afresh.


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

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 the program with different
environments without having to start the debugger over again.

`info environment VARNAME'
     Print the value of environment variable VARNAME to be given to
     your program when it is started.  This command can be abbreviated
     `i env VARNAME'.

`info environment'
     Print the names and values of all environment variables to be given to
     your program when it is started.  This command can be abbreviated
     `i env'.

`set-environment VARNAME VALUE'
`set-environment VARNAME = VALUE'
     Sets environment variable VARNAME to VALUE, 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.


`unset-environment VARNAME'
     Remove variable VARNAME from the environment to be passed to your
     program.


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

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 Files::.

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

`pwd'
     Print GDB's working directory.


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

Stopping and Continuing
***********************

When you run a program normally, it runs until it terminates.  The
principal purpose of using a debugger is so that you can stop it before
that point; or so that if the program runs into trouble you can
investigate and find out why.

* Menu:

* Signals::      Fatal signals in your program just stop it;
                 then you can use GDB to see what is going on.
* Breakpoints::  Breakpoints let you stop your program when it
                 reaches a specified point in the code.
* Continuing::   Resuming execution until the next signal or breakpoint.
* Stepping::     Stepping runs the program a short distance and
                 then stops it wherever it has come to.


File: gdb.info  Node: Signals, Prev: Stopping, Up: Stopping, Next: Breakpoints

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, `SIGINT' is the signal a program
gets when you type `Ctrl-c'; `SIGSEGV' is the signal a program
gets from referencing a place in memory far away from all the areas in use;
`SIGALRM' occurs when the alarm clock timer goes off (which happens
only if the program has requested an alarm).

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

GDB has the ability to detect any occurrence of a signal in the program
running under GDB's control.  You can tell GDB in advance what to do for
each kind of signal.

Normally, GDB is set up to ignore non-erroneous signals like `SIGALRM'
(so as not to interfere with their role in the functioning of the program)
but to stop the program immediately whenever an error signal happens.
You can change these settings with the `handle' command.  You must
specify which signal you are talking about with its number.

`info signal'
     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.

`handle SIGNALNUM KEYWORDS...'
     Change the way GDB handles signal SIGNALNUM.  The KEYWORDS
     say what change to make.

To use the `handle' command you must know the code number of the
signal you are concerned with.  To find the code number, type `info
signal' which prints a table of signal names and numbers.

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

`stop'
     GDB should stop the program when this signal happens.  This implies
     the `print' keyword as well.

`print'
     GDB should print a message when this signal happens.

`nostop'
     GDB should not stop the program when this signal happens.  It may
     still print a message telling you that the signal has come in.

`noprint'
     GDB should not mention the occurrence of the signal at all.  This
     implies the `nostop' keyword as well.

`pass'
     GDB should allow the program to see this signal; the program will be
     able to handle the signal, or may be terminated if the signal is fatal
     and not handled.

`nopass'
     GDB should not allow the program to see this signal.

When a signal has been set to stop the program, the program cannot see the
signal until you continue.  It will see 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 that signal will be seen by
the program when you later continue it.

You can also use the `signal' command to prevent the 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.  *Note Signaling::.


File: gdb.info  Node: Breakpoints, Prev: Signals, Up: Stopping, Next: Continuing

Breakpoints
===========

A "breakpoint" makes your program stop whenever a certain point in the
program is reached.  You set breakpoints explicitly with GDB commands,
specifying the place where the program should stop by line number, function
name or exact address in the program.  You can add various other conditions
to control whether the program will stop.

Each breakpoint is assigned a number when it is created; these numbers are
successive integers starting with 1.  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 the program until you
enable it again.

The command `info break' prints a list of all breakpoints set and not
deleted, showing their numbers, where in the program they are, and any
special features in use for them.  Disabled breakpoints are included in the
list, but marked as disabled.  `info break' with a breakpoint number
as argument lists only that breakpoint.  The convenience variable `$_'
and the default examining-address for the `x' command are set to the
address of the last breakpoint listed (*Note Memory::).

* Menu:

* Set Breaks::     How to establish breakpoints.
* Delete Breaks::   How to remove breakpoints no longer needed.
* Disabling::      How to disable breakpoints (turn them off temporarily).
* Conditions::     Making extra conditions on whether to stop.
* Break Commands:: Commands to be executed at a breakpoint.
* Error in Breakpoints:: "Cannot insert breakpoints" error--why, what to do.


File: gdb.info  Node: Set Breaks, Prev: Breakpoints, Up: Breakpoints, Next: Delete Breaks

Setting Breakpoints
-------------------

Breakpoints are set with the `break' command (abbreviated `b').
You have several ways to say where the breakpoint should go.

`break FUNCTION'
     Set a breakpoint at entry to function FUNCTION.

`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 frame.

`break LINENUM'
     Set a breakpoint at line LINENUM in the current source file.
     That file is the last file whose source text was printed.  This
     breakpoint will stop the 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 the program which do not have debugging
     information or source files.

`break'
     Set a breakpoint at the next instruction to be executed in the selected
     stack frame (*Note Stack::).  In any selected frame but the innermost,
     this will cause the program to stop as soon as control returns to that
     frame.  This is equivalent to a `finish' command in the frame
     inside the selected frame.  If this is done in the innermost frame, GDB
     will stop the next time it reaches the current location; this may be
     useful inside of 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 the 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.  `...' stands for one of the possible
     arguments described above (or no argument) specifying where to break.
     *Note Conditions::, for more information on breakpoint conditions.

`tbreak ARGS'
     Set a breakpoint enabled only for one stop.  ARGS are the
     same as in the `break' command, and the breakpoint is set in the same
     way, but the breakpoint is automatically disabled the first time it
     is hit.  *Note Disabling::.

GDB allows you to set any number of breakpoints at the same place in the
program.  There is nothing silly or meaningless about this.  When the
breakpoints are conditional, this is even useful (*Note Conditions::).


File: gdb.info  Node: Delete Breaks, Prev: Set Breaks, Up: Breakpoints, Next: Disabling

Deleting Breakpoints
--------------------

It is often necessary to eliminate a breakpoint once it has done its job
and you no longer want the program to stop there.  This is called
"deleting" the breakpoint.  A breakpoint that has been deleted no
longer exists in any sense; it is forgotten.

With the `clear' command you can delete breakpoints according to where
they are in the program.  With the `delete' command you can delete
individual breakpoints by specifying their breakpoint numbers.

It is not necessary to delete a breakpoint to proceed past it.  GDB
automatically ignores breakpoints in the first instruction to be executed
when you continue execution without changing the execution address.

`clear'
     Delete any breakpoints at the next instruction to be executed in the
     selected stack frame (*Note Selection::).  When the innermost frame
     is selected, this is a good way to delete a breakpoint that the program
     just stopped at.

`clear FUNCTION'
`clear FILENAME:FUNCTION'
     Delete any breakpoints set at entry to the function FUNCTION.

`clear LINENUM'
`clear FILENAME:LINENUM'
     Delete any breakpoints set at or within the code of the specified line.

`delete BNUMS...'
     Delete the breakpoints of the numbers specified as arguments.


File: gdb.info  Node: Disabling, Prev: Delete Breaks, Up: Breakpoints, Next: Conditions

Disabling Breakpoints
---------------------

Rather than deleting a breakpoint, 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 with the `enable' and
`disable' commands, specifying one or more breakpoint numbers as
arguments.  Use `info break' to print a list of breakpoints if you
don't know which breakpoint numbers to use.

A breakpoint can have any of four different states of enablement:

   * Enabled.  The breakpoint will stop the program.  A breakpoint made
     with the `break' command starts out in this state.
   * Disabled.  The breakpoint has no effect on the program.
   * Enabled once.  The breakpoint will stop the program, but
     when it does so it will become disabled.  A breakpoint made
     with the `tbreak' command starts out in this state.
   * Enabled for deletion.  The breakpoint will stop the program, but
     immediately after it does so it will be deleted permanently.

You change the state of enablement of a breakpoint with the following
commands:

`disable breakpoints BNUMS...'
`disable BNUMS...'
     Disable the specified breakpoints.  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.

`enable breakpoints BNUMS...'
`enable BNUMS...'
     Enable the specified breakpoints.  They become effective once again in
     stopping the program, until you specify otherwise.

`enable breakpoints once BNUMS...'
`enable once BNUMS...'
     Enable the specified breakpoints temporarily.  Each will be disabled
     again the next time it stops the program (unless you have used one of
     these commands to specify a different state before that time comes).

`enable breakpoints delete BNUMS...'
`enable delete BNUMS...'
     Enable the specified breakpoints to work once and then die.  Each of
     the breakpoints will be deleted the next time it stops the program
     (unless you have used one of these commands to specify a different
     state before that time comes).

Aside from the automatic disablement or deletion of a breakpoint when it
stops the program, which happens only in certain states, the state of
enablement of a breakpoint changes only when one of the commands above
is used.


File: gdb.info  Node: Conditions, Prev: Disabling, Up: Breakpoints, Next: Break Commands

Break Conditions
----------------

The simplest sort of breakpoint breaks every time the program reaches a
specified place.  You can also specify a "condition" for a
breakpoint.  A condition is just a boolean expression in your
programming language.  (*Note Expressions::).  A breakpoint with a
condition evaluates the expression each time the program reaches it, and
the program stops only if the condition is true.

Break conditions may have side effects, and may even call functions in your
program.  These may sound like strange things to do, but their 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 the program without checking the condition of this one.)  Note that
breakpoint commands are usually more convenient and flexible for the
purpose of performing side effects when a breakpoint is reached
(*Note Break Commands::).

Break conditions can be specified when a breakpoint is set, by using
`if' in the arguments to the `break' command.  *Note Set Breaks::.
They can also be changed at any time with the `condition' command:

`condition BNUM EXPRESSION'
     Specify EXPRESSION as the break condition for breakpoint number
     BNUM.  From now on, this breakpoint will stop the program only if
     the value of EXPRESSION is true (nonzero, in C).  EXPRESSION
     is not evaluated at the time the `condition' command is given.
     *Note 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 the 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 will not stop the next N times it
is reached.

`ignore BNUM COUNT'
     Set the ignore count of breakpoint number BNUM to COUNT.
     The next COUNT times the breakpoint is reached, it will not stop.

     To make the breakpoint stop the next time it is reached, specify
     a count of zero.

`cont COUNT'
     Continue execution of the program, setting the ignore count of the
     breakpoint that the program stopped at to COUNT minus one.
     Thus, the program will not stop at this breakpoint until the
     COUNT'th time it is reached.

     This command is allowed only when the program stopped due to a
     breakpoint.  At other times, the argument to `cont' is ignored.

If a breakpoint has a positive ignore count and a condition, the condition
is not checked.  Once the ignore count reaches zero, the condition will
start to be checked.

Note that you could achieve the effect of the ignore count with a
condition such as `$foo-- <= 0' using a debugger convenience
variable that is decremented each time.  *Note Convenience Vars::.


File: gdb.info  Node: Break Commands, Prev: Conditions, Up: Breakpoints, Next: Error in Breakpoints

Commands Executed on Breaking
-----------------------------

You can give any breakpoint a series of commands to execute when the
program stops due to that breakpoint.  For example, you might want to
print the values of certain expressions, or enable other breakpoints.

`commands BNUM'
     Specify 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, use the command
     `commands' and follow it immediately by `end'; that is, give
     no commands.


It is possible for breakpoint commands to start the program up again.
Simply use the `cont' command, or `step', or any other command
to resume execution.  However, any remaining breakpoint commands are
ignored.  When the program stops again, GDB will act according to the
cause of that stop.

If the first command specified 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 the remaining commands too print nothing, you will see no sign that
the breakpoint was reached at all.  `silent' is not really a command;
it is meaningful only at the beginning of the commands for a breakpoint.

The commands `echo' and `output' that allow you to print precisely
controlled output are often useful in silent breakpoints.  *Note Output::.

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

     break foo if x>0
     commands 1 /* where 1 is the breakpoint number of the above break */
     silent
     echo x is\040
     output x
     echo \n
     cont
     end

One application for breakpoint commands is to correct one bug so you can
test 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 `cont' command so that the program does not
stop, and start with the `silent' command so that no output is
produced.  Here is an example:

     break 403
     commands 1
     silent
     set x = y + 4
     cont
     end


File: gdb.info  Node: Error in Breakpoints, Prev: Break Commands, Up: Breakpoints

"Cannot Insert Breakpoints" Error
---------------------------------

Under some operating systems, breakpoints cannot be used in a program if
any other process is running that program.  Attempting to run or
continue the program with a breakpoint in this case will cause GDB to
stop it.

When this happens, you have one way to proceed:

  1. Remove or disable the breakpoints, then continue.



File: gdb.info  Node: Continuing, Prev: Breakpoints, Up: Stopping, Next: Stepping

Continuing
==========

After your program stops, most likely you will want it to run some more if
the bug you are looking for has not happened yet.

`cont'
     Continue running the program at the place where it stopped.

If the program stopped at a breakpoint, the place to continue running
is the address of the breakpoint.  You might expect that continuing would
just stop at the same breakpoint immediately.  In fact, `cont'
takes special care to prevent that from happening.  You do not need
to delete the breakpoint to proceed through it after stopping at it.

You can, however, specify an ignore-count for the breakpoint that the
program stopped at, by means of an argument to the `cont' command.
*Note Conditions::.

If the program stopped because of a signal other than `SIGINT' or
`SIGTRAP', continuing will cause the program to see that signal.
You may not want this to happen.  For example, if the 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 the program would probably terminate immediately as
a result of the fatal signal once it sees the signal.  To prevent this,
you can continue with `signal 0'.  *Note Signaling::.  You can
also act in advance to prevent the program from seeing certain kinds
of signals, using the `handle' command (*Note Signals::).

File: gdb.info  Node: Stepping, Prev: Continuing, Up: Stopping

Stepping
========

"Stepping" means setting your program in motion for a limited time, so
that control will return automatically to the debugger after one line of
code or one machine instruction.  Breakpoints are active during stepping
and the program will stop for them even if it has not gone as far as the
stepping command specifies.

`step'
     Continue running the program until control reaches a different line,
     then stop it and return control to the debugger.  This command is
     abbreviated `s'.

     This command may be given when control is within a function for which
     there is no debugging information.  In that case, execution will proceed
     until control reaches a different function, or is about to return from
     this function.  An argument repeats this action.

`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'
     Similar to `step', but any function calls appearing within the line of
     code are executed without stopping.  Execution stops when control reaches a
     different line of code at the stack level which was executing when the
     `next' command was given.  This command is abbreviated `n'.

     An argument is a repeat count, as in `step'.

     `next' within a function without debugging information acts as does
     `step', but any function calls appearing within the code of the
     function are executed without stopping.

`finish'
     Continue running until just after the selected stack frame returns (or
     until there is some other reason to stop, such as a fatal signal or a
     breakpoint).  Print value returned by the selected stack frame (if any).

     Contrast this with the `return' command (*Note Returning::).

`until'
     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' will cause the program to continue execution
     until the loop is exited.  In contrast, a `next' command at the end
     of a loop will simply step back to the beginning of the loop, which
     would force you to step through the next iteration.

     `until' always stops the program if it attempts to exit the current
     stack frame.

     `until' may produce somewhat counterintuitive results if the order
     of the source lines does not match the actual order of execution.  For
     example, in a typical C `for'-loop, the third expression in the
     `for'-statement (the loop-step expression) is executed after the
     statements in the body of the loop, but is written before them.
     Therefore, the `until' command would appear to step back to the
     beginning of the loop when it advances to this expression.  However, it
     has not really done so, not in terms of the actual machine code.

     Note that `until' with no argument works by means of single
     instruction stepping, and hence is slower than `until' with an
     argument.

`until LOCATION'
     Continue running the program until either the specified location is
     reached, or the current (innermost) stack frame returns.  This form of
     the command uses breakpoints, and hence is quicker than `until'
     without an argument. `until' with arguments does not currently work on
     the atariST, only `until' without args does. `until' with args
     is a `no-op'.

`stepi'
`si'
     Execute one machine instruction, then stop and return to the debugger.

     It is often useful to do `display/i $pc' when stepping by machine
     instructions.  This will cause the next instruction to be executed to
     be displayed automatically at each stop.  *Note Auto Display::.

     An argument is a repeat count, as in `step'.

`nexti'
`ni'
     Execute one machine instruction, but if it is a subroutine call,
     proceed until the subroutine returns.

     An argument is a repeat count, as in `next'.

A typical technique for using stepping is to put a breakpoint
(*Note Breakpoints::) at the beginning of the function or the section of
the program in which a problem is believed to lie, and then step through
the suspect area, examining the variables that are interesting, until the
problem happens.

The `cont' command can be used after stepping to resume execution
until the next breakpoint or signal.


File: gdb.info  Node: Stack, Prev: Stopping, Up: Top, Next: Source

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, the information about
where in the program the call was made from is saved in a block of data
called a "stack frame".  The frame also contains the arguments of the
call and the local variables of the function that was called.  All 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 the program, the value is found in the
selected frame.  There are special GDB commands to select whichever frame
you are interested in.

When the program stops, GDB automatically selects the currently executing
frame and describes it briefly as the `frame' command does
(*Note Info: Frame Info.).

* Menu:

* Frames::          Explanation of stack frames and terminology.
* Backtrace::       Summarizing many frames at once.
* Selection::       How to select a stack frame.
* Info: Frame Info, Commands to print information on stack frames.


File: gdb.info  Node: Frames, Prev: Stack, Up: Stack, Next: Backtrace

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 of those bytes whose
address serves as the address of the frame.  Usually this address is kept
in a register called the "frame pointer register" 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 to give you a
way of talking about stack frames in GDB commands.

Many GDB commands refer implicitly to one stack frame.  GDB records a stack
frame that is called the "selected" stack frame; you can select any
frame using one set of GDB commands, and then other commands will operate
on that frame.  When your program stops, GDB automatically selects the
innermost frame.

Some functions can be compiled to run without a frame reserved for them
on the stack.  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 will give it a virtual stack frame of
0 and correctly allow tracing of the function call chain.  Results are
undefined if a function invocation besides the innermost one is
frameless. atariST GDB has virtually no support for frameless functions.


File: gdb.info  Node: Backtrace, Prev: Frames, Up: Stack, Next: Selection

Backtraces
==========

A backtrace is a summary of how the program got where it is.  It shows one
line per frame, for many frames, starting with the currently executing
frame (frame zero), followed by its caller (frame one), and on up the
stack.

`backtrace'
`bt'
     Print a backtrace of the entire stack: one line per frame for all
     frames in the stack.

     You can stop the backtrace at any time by typing the system interrupt
     character, normally `Control-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.

The names `where' and `info stack' are additional aliases
for `backtrace'.

Every line in the backtrace shows the frame number, the function name
and the program counter value.

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

     #0  rtx_equal_p (x=(rtx) 0x8e58c, y=(rtx) 0x1086c4) (/gp/rms/cc/rtlanal.c line 337)
     #1  0x246b0 in expand_call (...) (...)
     #2  0x21cfc in expand_expr (...) (...)
     (More stack frames follow...)

The functions `expand_call' and `expand_expr' are in a file
whose symbol details have not been fully read.  Full detail is available
for the function `rtx_equal_p', which is in the file
`rtlanal.c'.  Its arguments, named `x' and `y', are shown
with their typed values.


File: gdb.info  Node: Selection, Prev: Backtrace, Up: Stack, Next: Frame Info

Selecting a Frame
=================

Most commands for examining the stack and other data in the 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'
     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 `main''s
     frame.

`frame 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 the program has multiple stacks and
     switches between them.

`up N'
     Select the frame N frames up from the frame previously selected.
     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'
     Select the frame N frames down from the frame previously
     selected.  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.

All of these commands end by printing some information on the frame that
has been selected: the frame number, the function name, the arguments, the
source file and line number of execution in that frame, and the text of
that source line.  For example:

     #3  main (argc=3, argv=??, env=??) at main.c, line 67
     67        read_input_file (argv[i]);

After such a printout, the `list' command with no arguments will print
ten lines centered on the point of execution in the frame.  *Note List::.


File: gdb.info  Node: Frame Info, Prev: Selection, Up: Stack

Information on a Frame
======================

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

`frame'
     This command prints a brief description of the selected stack frame.
     It can be abbreviated `f'.  With an argument, this command is
     used to select a stack frame; with no argument, it does not change
     which frame is selected, but still prints the same information.

`info frame'
     This command prints a verbose description of the selected stack frame,
     including the address of the frame, the addresses of the next frame in
     (called by this frame) and the next frame out (caller of this frame),
     the address of the frame's arguments, the program counter saved in it
     (the address of execution in the caller frame), and which registers
     were saved in the frame.  The verbose description is useful when
     something has gone wrong that has made the stack format fail to fit
     the usual conventions.

`info frame ADDR'
     Print a verbose description of the frame at address ADDR,
     without selecting that frame.  The selected frame remains unchanged by
     this command.

`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 static or automatic within all
     program blocks that execution in this frame is currently inside of.


File: gdb.info  Node: Source, Prev: Stack, Up: Top, Next: Data

Examining Source Files
**********************

GDB knows which source files your program was compiled from, and
can print parts of their text.  When your program stops, GDB
spontaneously prints the line it stopped in.  Likewise, when you
select a stack frame (*Note Selection::), GDB prints the line
which execution in that frame has stopped in.  You can also
print parts of source files by explicit command.

* Menu:

* List::        Using the `list' command to print source files.
* Search::      Commands for searching source files.
* Source Path:: Specifying the directories to search for source files.


File: gdb.info  Node: List, Prev: Source, Up: Source, Next: Search

Printing Source Lines
=====================

To print lines from a source file, use the `list' command
(abbreviated `l').  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 ten lines centered around line number LINENUM in the
     current source file.

`list FUNCTION'
     Print ten lines centered around the beginning of function
     FUNCTION.

`list'
     Print ten more lines.  If the last lines printed were printed with a
     `list' command, this prints ten lines following the last lines
     printed; however, if the last line printed was a solitary line printed
     as part of displaying a stack frame (*Note Stack::), this prints ten
     lines centered around that line.

`list -'
     Print ten lines just before the lines last printed.

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 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 ten lines centered around the line specified by LINESPEC.

`list FIRST,LAST'
     Print lines from FIRST to LAST.  Both arguments are
     linespecs.

`list ,LAST'
     Print ten lines ending with LAST.

`list FIRST,'
     Print ten lines starting with FIRST.

`list +'
     Print ten lines just after the lines last printed.

`list -'
     Print ten lines just before the lines last printed.

`list'
     As described in the preceding table.

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

`LINENUM'
     Specifies line LINENUM of the current source file.
     When a `list' command has two linespecs, this refers to
     the same source file as the first linespec.

`+OFFSET'
     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.

`-OFFSET'
     Specifies the line OFFSET lines before the last line printed.

`FILENAME:LINENUM'
     Specifies line LINENUM in the source file FILENAME.

`FUNCTION'
     Specifies the line of the open-brace that begins the body of the
     function FUNCTION.

`FILENAME:FUNCTION'
     Specifies the line of the open-brace that begins the body of the
     function FUNCTION in the file FILENAME.  The file name is
     needed with a function name only for disambiguation of identically
     named functions in different source files.

`*ADDRESS'
     Specifies the line containing the program address ADDRESS.
     ADDRESS may be any expression.

One other command is used to map source lines to program addresses.

`info line LINENUM'
     Print the starting and ending addresses of the compiled code for
     source line LINENUM.

     The default examine address for the `x' command is changed to the
     starting address of the line, so that `x/i' is sufficient to
     begin examining the machine code (*Note Memory::).  Also, this address
     is saved as the value of the convenience variable `$_'
     (*Note Convenience Vars::).


File: gdb.info  Node: Search, Prev: List, Up: Source, Next: Source Path

Searching Source Files
======================

There are two commands for searching through the current source file for a
regular expression.

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 abbreviate the command name
as `fo'.

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 with as little as `rev'.


File: gdb.info  Node: Source Path, Prev: Search, Up: Source

Specifying Source Directories
=============================

Executable programs do not record the directories of the source files
from which they were compiled, just the names.  GDB remembers 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.  Note that the executable search path is
not used for this purpose.  Neither is the current working
directory, unless it happens to be in the source path.

When you start GDB, its source path contains just the current working
directory.  To add other directories, use the `directory' command.

`directory DIRNAMES...'
     Add directory DIRNAME to the end of the source path.  Several
     directory names may be given to this command, separated by whitespace or
     commas `,'.

`directory'
     Reset the source path to just the current working directory of GDB.
     This requires confirmation.

     Since this command deletes directories from the search path, it may
     change the directory in which a previously read source file will be
     discovered.  To make this work correctly, this command also clears out
     the tables GDB maintains about the source files it has already found.

`info directories'
     Print the source path: show which directories it contains.

Because the `directory' command adds to the end of the source path,
it does not affect any file that GDB has already found.  If the source
path contains directories that you do not want, and these directories
contain misleading files with names matching your source files, the
way to correct the situation is as follows:

  1. Choose the directory you want at the beginning of the source path.
     Use the `cd' command to make that the current working directory.

  2. Use `directory' with no argument to reset the source path to just
     that directory.

  3. Use `directory' with suitable arguments to add any other
     directories you want in the source path.


File: gdb.info  Node: Data, Prev: Source, Up: Top, Next: Symbols

Examining Data
**************

The usual way to examine data in your program is with the `print'
command (abbreviated `p').  It evaluates and prints the value of any
valid expression of the language the program is written in (for now, C).
You type

     print EXP

where EXP is any valid expression, and the value of EXP
is printed in a format appropriate to its data type.

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.

* Menu:

* Expressions::      Expressions that can be computed and printed.
* Variables::        Using your program's variables in expressions.
* Assignment::       Setting your program's variables.
* Arrays::           Examining part of memory as an array.
* Format Options::   Controlling how structures and arrays are printed.
* Output formats::   Specifying formats for printing values.
* Memory::           Examining memory explicitly.
* Auto Display::     Printing certain expressions whenever program stops.
* Value History::    Referring to values previously printed.
* Convenience Vars:: Giving names to values for future reference.
* Registers::        Referring to and storing in machine registers.


File: gdb.info  Node: Expressions, Prev: Data, Up: Data, Next: Variables

Expressions
===========

Many different 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 legal in an expression in GDB.  This includes
conditional expressions, function calls, casts and string constants.
It unfortunately does not include symbols defined by preprocessor
`#define' commands.

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

GDB supports three kinds of operator in addition to those of programming
languages:

`@'
     `@' is a binary operator for treating parts of memory as arrays.
     *Note Arrays::, for more information.

`::'
     `::' allows you to specify a variable in terms of the file or
     function it is defined in.  *Note 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 nonunary operators, just as in
     a cast).  This construct is allowed regardless of what kind of data
     is officially supposed to reside at ADDR.


File: gdb.info  Node: Variables, Prev: Expressions, Up: Data, Next: Arrays

Program Variables
=================

The most common kind of expression to use is the name of a variable
in your program.

Variables in expressions are understood in the selected stack frame
(*Note Selection::); they must either be global (or static) or be visible
according to the scope rules of the programming language from the point of
execution in that frame.  This means that in the function

     foo (a)
          int a;
     {
       bar (a);
       {
         int b = test ();
         bar (b);
       }
     }

the variable `a' is usable whenever the program is executing
within the function `foo', but the variable `b' is visible
only while the program is executing inside the block in which `b'
is declared.

As a special 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 (if they are in different source files).
In such a case, it is not defined which one you will get.  If you wish,
you can specify any one of them using the colon-colon construct:

     BLOCK::VARIABLE

Here BLOCK is the name of the source file whose variable you want.


File: gdb.info  Node: Arrays, Prev: Variables, Up: Data, Next: Format options

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.

This can be done by constructing an "artificial array" with the
binary operator `@'.  The left operand of `@' should be
the first element of the desired array, as an individual object.
The right operand should be the 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.
(It would probably appear in an expression via the value history,
after you had printed it out.)


File: gdb.info  Node: Output formats, Prev: Arrays, Up: Data, Next: Memory

Output formats
==============

GDB normally prints all values according to their data types.  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 an instruction.  These things
can be done with "output formats".

The simplest use of output formats is to say how to print a value
already computed.  This is done by starting the arguments of the
`print' command with a slash and a format letter.  The format
letters supported are:

`x'
     Regard the bits of the value as an integer, and print the integer in
     hexadecimal.

`d'
     Print as integer in signed decimal.

`u'
     Print as integer in unsigned decimal.

`o'
     Print as integer in octal.

`a'
     Print as an address, both absolute in hex and then relative
     to a symbol defined as an address below it.

`c'
     Regard as an integer and print it as a character constant.

`f'
     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 (*Note Registers::), type

     p/x $pc

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

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


File: gdb.info  Node: Memory, Prev: Output formats, Up: Data, Next: Auto Display

Examining Memory
----------------

The command `x' (for `examine') can be used to examine memory
without reference to the program's data types.  The format in which you
wish to examine memory is instead explicitly specified.  The allowable
formats are a superset of the formats described in the previous section.

`x' is followed by a slash and an output format specification,
followed by an expression for an address.  The expression need not have
a pointer value (though it may); it is used as an integer, as the
address of a byte of memory.  *Note Expressions:: for more information on
expressions.  For example, `x/4xw $sp' prints the four words of
memory above the stack pointer in hexadecimal.

The output format in this case specifies both how big a unit of memory
to examine and how to print the contents of that unit.  It is done
with one or two of the following letters:

These letters specify just the size of unit to examine:

`b'
     Examine individual bytes.

`h'
     Examine halfwords (two bytes each).

`w'
     Examine words (four bytes each).

     Many assemblers and cpu designers still use `word' for a 16-bit quantity,
     as a holdover from specific predecessor machines of the 1970's that really
     did use two-byte words.  But more generally the term `word' has always
     referred to the size of quantity that a machine normally operates on and
     stores in its registers.  This is 32 bits for all the machines that GDB
     runs on.

`g'
     Examine giant words (8 bytes).

These letters specify just the way to print the contents:

`x'
     Print as integers in unsigned hexadecimal.

`d'
     Print as integers in signed decimal.

`u'
     Print as integers in unsigned decimal.

`o'
     Print as integers in unsigned octal.

`a'
     Print as an address, both absolute in hex and then relative
     to a symbol defined as an address below it.

`c'
     Print as character constants.

`f'
     Print as floating point.  This works only with sizes `w' and
     `g'.

`s'
     Print a null-terminated string of characters.  The specified unit size
     is ignored; instead, the unit is however many bytes it takes to reach
     a null character (including the null character).

`i'
     Print a machine instruction in assembler syntax (or nearly).  The
     specified unit size is ignored; the number of bytes in an instruction
     varies depending on the type of machine, the opcode and the addressing
     modes used.

If either the manner of printing or the size of unit fails to be specified,
the default is to use the same one that was used last.  If you don't want
to use any letters after the slash, you can omit the slash as well.

You can also omit the address to examine.  Then the address used is
just after the last unit examined.  This is why string and instruction
formats actually compute a unit-size based on the data: so that the
next string or instruction examined will start in the right place.
The `print' command sometimes sets the default address for
the `x' command; when the value printed resides in memory, the
default is set to examine the same location.  `info line' also
sets the default for `x', to the address of the start of the
machine code for the specified line and `info breakpoints' sets
it to the address of the last breakpoint listed.

When you use RET to repeat an `x' command, it does not repeat
exactly the same: the address specified previously (if any) is ignored, so
that the repeated command examines the successive locations in memory
rather than the same ones.

You can examine several consecutive units of memory with one command by
writing a repeat-count after the slash (before the format letters, if any).
The repeat count must be a decimal integer.  It has the same effect as
repeating the `x' command that many times except that the output may
be more compact with several units per line.  For example,

     x/10i $pc

prints ten instructions starting with the one to be executed next in the
selected frame.  After doing this, you could print another ten following
instructions with

     x/10

in which the format and address are allowed to default.

     disassemble

disassembles the current function.

The addresses and contents printed by the `x' command are not put 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.

The specialized command `disassemble' is also provided to dump 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; the function
surrounding this value will be dumped.  Two arguments specify a range of
addresss (first inclusive, second exclusive) to be dumped.


File: gdb.info  Node: Auto Display, Prev: Memory, Up: Data, Next: Value History

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 will print its value each time the 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

showing item numbers, expressions and their current values.

If the 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 printed only when execution is inside that lexical context.  For
example, if you give the command `display name' while inside a
function with an argument `name', then this argument will be
displayed whenever the program stops inside that function, but not when
it stops elsewhere (since this argument doesn't exist elsewhere).

`display EXP'
     Add the expression EXP to the list of expressions to display
     each time the program stops.  *Note Expressions::.

`display/FMT EXP'
     For FMT specifying only a display format and not a size or
     count, add the expression EXP to the auto-display list but
     arranges to display it each time in the specified format FMT.

`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 the program stops.  Examining means in effect
     doing `x/FMT ADDR'.  *Note Memory::.

`undisplay DNUMS...'
`delete display DNUMS...'
     Remove item numbers DNUMS from the list of expressions to display.

`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
     reenabled later.

`enable display DNUMS...'
     Enable display of item numbers DNUMS.  It becomes effective once
     again in auto display of its expression, until you specify otherwise.

`display'
     Display the current values of the expressions on the list, just as is
     done when the 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.


File: gdb.info  Node: Value History, Prev: Auto Display, Up: Data, Next: Convenience Vars

Value History
=============

Every value printed by the `print' command is saved for the entire
session in GDB's "value history" so that you can refer to it in
other expressions.

The values printed are given "history numbers" for you to refer to them
by.  These are successive integers starting with 1.  `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 `$' followed by the value's
history number.  The output printed by `print' is designed to remind
you of this.  Just `$' refers to the most recent value in the history,
and `$$' refers to the value before that.

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

It might be useful to repeat this command many times by typing RET.

Note that the history records values, not expressions.  If the value of
`x' is 4 and you type this command:

     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.

`info values'
     Print the last ten values in the value history, with their item
     numbers.  This is like `p $$9' repeated ten times, except that
     `info values' does not change the history.

`info values N'
     Print ten history values centered on history item number N.

`info values +'
     Print ten history values just after the values last printed.


File: gdb.info  Node: Convenience Vars, Prev: Value History, Up: Data, Next: Registers

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 effect on further execution of your program.  That's why
you can use them freely.

Convenience variables have names starting with `$'.  Any name starting
with `$' can be used for a convenience variable, unless it is one of
the predefined set of register names (*Note Registers::).

You can save a value in a convenience variable with an assignment
expression, just as you would set a variable in your program.  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, even if it already has a value of a different
type.  The convenience variable as an expression has whatever type its
current value has.

`info convenience'
     Print a list of convenience variables used so far, and their values.
     Abbreviated `i con'.

One of the ways to use a convenience variable is as a counter to be
incremented or a pointer to be advanced.  For example:

     set $i = 0
     print bar[$i++]->contents
     ...repeat that command by typing RET.

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

`$_'
     The variable `$_' is automatically set by the `x' command to
     the last address examined (*Note 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'.

`$__'
     The variable `$__' is automatically set by the `x' command
     to the value found in the last address examined.


File: gdb.info  Node: Registers, Prev: Convenience Vars, Up: Data

Registers
=========

Machine register contents can be referred to in expressions as variables
with names starting with `$'.  The names of registers are different
for each machine; use `info registers' to see the names used on your
machine.  The names `$pc' and `$sp' are used on all machines for
the program counter register and the stack pointer.  Often `$fp' is
used for a register that contains a pointer to the current stack frame,
and `$ps' is used for a register that contains the processor
status.  These standard register names may be available on your machine
even though the `info registers' command displays them with a
different name.  For example, on the SPARC, `info registers'
displays the processor status register as `$psr' but you can also
refer to it as `$ps'.

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 floating point.  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" format, but all C programs expect to work with
"double" 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.

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

Some registers are never saved (typically those numbered zero or one)
because they are used for returning function values; for these registers,
relativization makes no difference.

`info registers'
     Print the names and relativized values of all registers.

`info registers REGNAME'
     Print the relativized value of register REGNAME.  REGNAME
     may be any register name valid on the machine you are using, with
     or without the initial `$'.


Examples
--------

You could print the program counter in hex with

     p/x $pc

or print the instruction to be executed next with

     x/i $pc

or add four to the stack pointer with

     set $sp += 4

The last is a way of removing one word from the stack, on machines where
stacks grow downward in memory (most machines, nowadays).  This assumes
that the innermost stack frame is selected.  Setting `$sp' is
not allowed when other stack frames are selected.


File: gdb.info  Node: Symbols, Prev: Data, Up: Top, Next: Altering

Examining the Symbol Table
**************************

The commands described in this section allow you to make inquiries for
information about the symbols (names of variables, functions and types)
defined in your program.  This information is found by GDB in the symbol
table loaded by the `symbol-file' command; it is inherent in the text
of your program and does not change as the program executes.

`whatis EXP'
     Print the data type of expression EXP.  EXP is not
     actually evaluated, and any side-effecting operations (such as
     assignments or function calls) inside it do not take place.
     *Note Expressions::.

`whatis'
     Print the data type of `$', the last value in the value history.

`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 variables, and for a stack local variable prints
     the exact address of the current instantiation of the variable.

`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 `struct
     STRUCT-TAG', `union UNION-TAG' or `enum ENUM-TAG'.

`info sources'
     Print the names of all source files in the program for which there
     is debugging information.

`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'.

`info variables'
     Print the names and data types of all variables that are declared
     outside of functions (i.e., except for 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 types'
     Print all data types that are defined in the program.

`info types REGEXP'
     Print all data types that are defined in the program whose names
     contain a match for regular expression REGEXP.

`printsyms FILENAME'
     Write a complete dump of the debugger's symbol data into the
     file FILENAME.


File: gdb.info  Node: Altering, Prev: Symbols, Up: Top, Next: Sequences

Altering Execution
******************

Once you think you have find an error in the 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 the program a signal, restart it at a different address,
or even return prematurely from a function to its caller.

* Menu:

* Assignment::    Altering variable values or memory contents.
* Jumping::       Altering control flow.
* Signaling::     Making signals happen in the program.
* Returning::     Making a function return prematurely.


File: gdb.info  Node: Assignment, Prev: Altering, Up: Altering, Next: Jumping

Assignment to Variables
=======================

To alter the value of a variable, evaluate an assignment expression.
*Note Expressions::.  For example,

     print x=4

would store the value 4 into the variable `x', and then print
the value of the assignment expression (which is 4).

All the assignment operators of C are supported, including the
incrementation operators `++' and `--', and combining
assignments such as `+=' and `<<='.

If you are not interested in seeing the value of the assignment, use the
`set' command instead of the `print' command.  `set' is
really the same as `print' except that the expression's value is not
printed and is not put in the value history (*Note Value History::).  The
expression is evaluated only for side effects.

Note that if the beginning of the argument string of the `set' command
appears identical to a `set' subcommand, it may be necessary to use
the `set variable' command.  This command is identical to `set'
except for its lack of subcommands.

GDB allows more implicit conversions in assignments than C does; you can
freely store an integer value into a pointer variable or vice versa, and
any structure can be converted to any other structure that is the same
length or shorter.

To store values into arbitrary places in memory, use the `{...}'
construct to generate a value of specified type at a specified address
(*Note Expressions::).  For example, `{int}0x83040' would refer
to memory location 0x83040 as an integer (which implies a certain size
and representation in memory), and

     set {int}0x83040 = 4

would store the value 4 into that memory location.


File: gdb.info  Node: Jumping, Prev: Assignment, Up: Altering, Next: Signaling

Continuing at a Different Address
=================================

Ordinarily, when you continue the program, you do so at the place where
it stopped, with the `cont' command.  You can instead continue at
an address of your own choosing, with the following commands:

`jump LINENUM'
     Resume execution at line number LINENUM.  Execution may stop
     immediately if there is a breakpoint there.

     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 LINENUM 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 based on
     careful study of the machine-language code of the program.

`jump *ADDRESS'
     Resume execution at the instruction at address ADDRESS.

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 the program running; it only changes the address where it
*will* run when it is continued.  For example,

     set $pc = 0x485

causes the next `cont' command or stepping command to execute at
address 0x485, rather than at the address where the program stopped.
*Note Stepping::.

The most common occasion to use the `jump' command is when you have
stepped across a function call with `next', and found that the
return value is incorrect.  If all the relevant data appeared correct
before the function call, the error is probably in the function that
just returned.

In general, your next step would now be to rerun the program and execute
up to this function call, and then step into it to see where it goes
astray.  But this may be time consuming.  If the function did not have
significant side effects, you could get the same information by resuming
execution just before the function call and stepping through it.  To do this,
first put a breakpoint on that function; then, use the `jump' command
to continue on the line with the function call.

File: gdb.info  Node: Signaling, Prev: Jumping, Up: Altering, Next: Returning

Giving the Program a Signal
===========================

`signal SIGNALNUM'
     Resume execution where the program stopped, but give it immediately the
     signal number SIGNALNUM.

     Alternatively, if SIGNALNUM is zero, continue execution without
     giving a signal.  This is useful when the program stopped on account of
     a signal and would ordinary see the signal when resumed with the
     `cont' command; `signal 0' causes it to resume without a
     signal.


File: gdb.info  Node: Returning, Prev: Signaling, Up: Altering

Returning from a Function
=========================

You can cancel execution of a function call with the `return'
command.  This command has the effect of discarding the selected stack
frame (and all frames within it), so that control moves to the caller of
that function.  You can think of this as making the discarded frame
return prematurely.

First select the stack frame that you wish to return from
(*Note Selection::).  Then type the `return' command.  If you wish
to specify the value to be returned, give that as an argument.

This pops the selected stack 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.  Contrast this with the `finish' command
(*Note Stepping::), which resumes execution until the selected stack
frame returns *naturally*.


File: gdb.info  Node: Sequences, Prev: Altering, Up: Top, Next: Options

Canned Sequences of Commands
****************************

GDB provides two ways to store sequences of commands for execution as a
unit: user-defined commands and command files.

* Menu:

* Define::         User-defined commands.
* Command Files::  Command files.
* Output::         Controlled output commands useful in
                   user-defined commands and command files.


File: gdb.info  Node: Define, Prev: Sequences, Up: Sequences, Next: Command Files

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.

`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'.

`document COMMANDNAME'
     Give documentation to the user-defined command COMMANDNAME.  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 will print
     the documentation you have specified.

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

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

Commands that would ask for confirmation if used interactively 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 user-defined command.


File: gdb.info  Node: Command Files, Prev: Define, Up: Sequences, Next: Output

Command Files
=============

A command file for GDB is a file 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.

When GDB starts, it automatically executes its "init files", command
files named `.gdbinit'.  GDB reads the init file (if any) in your home
directory and then the init file (if any) in the current working
directory.  (The init files are not executed if the `-nx' option
is given.)  You can also 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.

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 used in a command file.


File: gdb.info  Node: Output, Prev: Command Files, Up: Sequences

Commands for Controlled Output
==============================

During the execution of a command file or a user-defined command, the only
output that appears is what is explicitly printed by the commands of 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 will be 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 outputting a string with spaces at the
     beginning or the end, since leading and trailing spaces are trimmed from
     all arguments.  Thus, 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.  *Note Expressions:: for more information on
     expressions.

`output/FMT EXPRESSION'
     Print the value of EXPRESSION in format FMT.
     *Note 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 the program were to execute

          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 string are
     the simple ones that consist of backslash followed by a letter.


File: gdb.info  Node: Options, Prev: Sequences, Up: Top, Next: Emacs

Options and Arguments for GDB
*****************************

When you invoke GDB, you can specify arguments telling it what files to
operate on and what other things to do.

* Menu:

* Mode Options::     Options controlling modes of operation.
* File Options::     Options to specify files (executable, coredump, commands)
* Other Arguments::  Any other arguments without options
            also specify files.


File: gdb.info  Node: Mode Options, Prev: Options, Up: Options, Next: File Options

Mode Options
============

`-mshort'
     This option must be specified when you are going to debug a program that
     has been compiled with 16 bit int (`gcc -mshort'). In earlier
     version of atariST gdb, it tried to intut this information, but now
     it must be explicitly specified by the `-mshort' option.

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

`-q'
     "Quiet".  Do not print the usual introductory messages.


File: gdb.info  Node: File Options, Prev: Mode Options, Up: Options, Next: Other Arguments

File-specifying Options
=======================

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

`-s FILE'
     Read symbol table from file 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.

`-x FILE'
     Execute GDB commands from file FILE.

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

`-D'
     Spill out debugging info about atariST GDB. Creates (or overwrites) the
     file `debug.gdb'.


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

Command Index
*************


* Menu:

* $_: Breakpoints.
* $_: List.
* $_: Memory.
* $__: Memory.
* backtrace: Backtrace.
* break: Set Breaks.
* bt: Backtrace.
* cd: Working Directory.
* clear: Delete Breaks.
* condition: Conditions.
* cont: Continuing.
* define: Define.
* delete: Delete Breaks.
* delete display: Auto Display.
* directory: Source Path.
* disable breakpoints: Disabling.
* disable: Disabling.
* disable display: Auto Display.
* disassemble: Memory.
* display: Auto Display.
* document: Define.
* down: Selection.
* echo: Output.
* enable breakpoints: Disabling.
* enable: Disabling.
* enable display: Auto Display.
* exec-file: File Commands.
* finish: Stepping.
* forward-search: Search.
* frame: Selection.
* gcc -mshort: File Commands.
* handle: Signals.
* ignore: Conditions.
* info address: Symbols.
* info args: Frame Info.
* info break: Breakpoints.
* info convenience: Convenience Vars.
* info directories: Source Path.
* info display: Auto Display.
* info environment: Environment.
* info files: File Commands.
* info frame: Frame Info.
* info functions: Symbols.
* info line: List.
* info locals: Frame Info.
* info registers: Registers.
* info signal: Signals.
* info sources: Symbols.
* info stack: Backtrace.
* info types: Symbols.
* info values: Value History.
* info variables: Symbols.
* jump: Jumping.
* list: List.
* logging: File Commands.
* next: Stepping.
* nexti: Stepping.
* ni: Stepping.
* output: Output.
* print: Data.
* printf: Output.
* printsyms: Symbols.
* ptype: Symbols.
* pwd: Working Directory.
* quit: User Interface.
* return: Returning.
* reverse-search: Search.
* run: Running.
* set: Assignment.
* set variable: Assignment.
* set-args: Arguments.
* set-environment: Environment.
* si: Stepping.
* signal: Signaling.
* silent: Break Commands.
* source: Command Files.
* step: Stepping.
* stepi: Stepping.
* symbol-file: File Commands.
* tbreak: Set Breaks.
* undisplay: Auto Display.
* unset environment: Environment.
* until: Stepping.
* up: Selection.
* whatis: Symbols.
* where: Backtrace.
* x: Memory.


File: gdb.info  Node: Concepts, Prev: Commands, Up: Top

Concept Index
*************


* Menu:

* `$': Value History.
* `$$': Value History.
* abbreviation: User Interface.
* arguments (to your program): Arguments.
* artificial array: Arrays.
* assignment: Assignment.
* automatic display: Auto Display.
* breakpoint commands: Break Commands.
* breakpoint conditions: Conditions.
* breakpoints: Breakpoints.
* call stack: Stack.
* clearing breakpoint: Delete Breaks.
* command files: Command Files.
* conditional breakpoints: Conditions.
* convenience variables: Convenience Vars.
* deleting breakpoints: Delete Breaks.
* directories for source files: Source Path.
* disabled breakpoints: Disabling.
* display of expressions: Auto Display.
* enabled breakpoints: Disabling.
* environment (of your program): Environment.
* examining data: Data.
* examining memory: Memory.
* executable file: Files.
* exiting GDB: User Interface.
* expressions: Expressions.
* fatal signals: Signals.
* formatted output: Output formats.
* frame: Frames.
* frame number: Frames.
* frame pointer: Frames.
* frameless execution: Frames.
* `.gdbinit': Command Files.
* handling signals: Signals.
* history number: Value History.
* ignore count (of breakpoint): Conditions.
* init file: Command Files.
* initial frame: Frames.
* innermost frame: Frames.
* linespec: List.
* outermost frame: Frames.
* output formats: Output formats.
* pauses in output: User Interface.
* printing data: Data.
* prompt: User Interface.
* registers: Registers.
* repeating commands: User Interface.
* returning from a function: Returning.
* running: Running.
* searching: Search.
* selected frame: Frames.
* setting variables: Assignment.
* signals: Signals.
* source path: Source Path.
* stack frame: Frames.
* stepping: Stepping.
* symbol file: Files.
* symbol table: Files.
* user-defined command: Define.
* value history: Value History.
* word: Memory.
* working directory (of your program): Working Directory.