* gdb.texinfo (Symbols): Add class NAME to doc for ptype.

[thirdparty/binutils-gdb.git] / gdb / doc / gdb.texinfo

\input texinfo      @c -*-texinfo-*-
@c Copyright (c) 1988 1989 1990 1991 1992 1993 Free Software Foundation, Inc.
@c
@c %**start of header 
@c makeinfo ignores cmds prev to setfilename, so its arg cannot make use
@c of @set vars.  However, you can override filename with makeinfo -o.
@setfilename gdb.info
@c
@include gdb-cfg.texi
@c
@ifset GENERIC
@settitle Debugging with @value{GDBN}
@end ifset
@ifclear GENERIC
@settitle Debugging with @value{GDBN} (@value{TARGET})
@end ifclear
@setchapternewpage odd
@c %**end of header

@iftex
@c @smallbook
@c @cropmarks
@end iftex

@finalout
@syncodeindex ky cp

@c readline appendices use @vindex
@syncodeindex vr cp

@c ===> NOTE! <==
@c Determine the edition number in *three* places by hand:
@c      1. First ifinfo section  2. title page  3. top node
@c To find the locations, search for !!set

@c GDB CHANGELOG CONSULTED BETWEEN:
@c Fri Oct 11 23:27:06 1991  John Gilmore  (gnu at cygnus.com)
@c Sat Dec 22 02:51:40 1990  John Gilmore  (gnu at cygint)

@c THIS MANUAL REQUIRES TEXINFO-2 macros and info-makers to format properly.

@ifinfo
@c This is a dir.info fragment to support semi-automated addition of
@c manuals to an info tree.  zoo@cygnus.com is developing this facility.
@format
START-INFO-DIR-ENTRY
* Gdb::                         The GNU debugger.
END-INFO-DIR-ENTRY
@end format
@end ifinfo
@c
@c
@ifinfo
This file documents the GNU debugger @value{GDBN}.

@c !!set edition, date, version
This is Edition 4.09, August 1993, 
of @cite{Debugging with @value{GDBN}: the GNU Source-Level Debugger}
for GDB Version @value{GDBVN}.

Copyright (C) 1988, '89, '90, '91, '92, '93 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.

@ignore
Permission is granted to process this file through TeX and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).

@end ignore
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also 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.
@end ifinfo

@titlepage
@title Debugging with @value{GDBN}
@subtitle The GNU Source-Level Debugger
@ifclear GENERIC
@subtitle (@value{TARGET})
@end ifclear
@sp 1
@c !!set edition, date, version
@subtitle Edition 4.09, for @value{GDBN} version @value{GDBVN}
@subtitle August 1993
@author Richard M. Stallman and Roland H. Pesch
@page
@tex
{\parskip=0pt
\hfill (Send bugs and comments on @value{GDBN} to bug-gdb\@prep.ai.mit.edu.)\par
\hfill {\it Debugging with @value{GDBN}}\par
\hfill \TeX{}info \texinfoversion\par
\hfill pesch\@cygnus.com\par
}
@end tex

@vskip 0pt plus 1filll
Copyright @copyright{} 1988, '89, '90, '91, '92, '93 Free Software
Foundation, Inc. 
@sp 2
Published by the Free Software Foundation @*
675 Massachusetts Avenue, @*
Cambridge, MA 02139 USA @*
Printed copies are available for $20 each. @*
ISBN 1-882114-11-6 @*

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
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.
@end titlepage
@page

@ifinfo
@node Top
@top Debugging with @value{GDBN}

This file describes @value{GDBN}, the GNU symbolic debugger.

@c !!set edition, date, version
This is Edition 4.09, April 1993, for GDB Version @value{GDBVN}.

@menu
* Summary::                     Summary of @value{GDBN}
@ifset NOVEL
* New Features::                New features since GDB version 3.5
@end ifset
@ifclear BARETARGET
* Sample Session::              A sample @value{GDBN} session
@end ifclear

* Invocation::                  Getting in and out of @value{GDBN}
* Commands::                    @value{GDBN} commands
* Running::                     Running programs under @value{GDBN}
* Stopping::                    Stopping and continuing
* Stack::                       Examining the stack
* Source::                      Examining source files
* Data::                        Examining data
@ifclear CONLY
* Languages::                   Using @value{GDBN} with different languages
@end ifclear
@ifset CONLY
* C::                           C language support
@end ifset
@c remnant makeinfo bug, blank line needed after two end-ifs?

* Symbols::                     Examining the symbol table
* Altering::                    Altering execution
* GDB Files::                   @value{GDBN} files
* Targets::                     Specifying a debugging target
* Controlling GDB::             Controlling @value{GDBN}
* Sequences::                   Canned sequences of commands
@ifclear DOSHOST
* Emacs::                       Using @value{GDBN} under GNU Emacs
@end ifclear

* GDB Bugs::                    Reporting bugs in @value{GDBN}
* Command Line Editing::        Facilities of the readline library
* Using History Interactively::
@ifset NOVEL
* Renamed Commands::
@end ifset
@ifclear PRECONFIGURED
* Formatting Documentation::    How to format and print GDB documentation
* Installing GDB::              Installing GDB
@end ifclear

* Index::                       Index
@end menu
@end ifinfo

@node Summary
@unnumbered Summary of @value{GDBN}

The purpose of a debugger such as @value{GDBN} is to allow you to see what is
going on ``inside'' another program while it executes---or what another
program was doing at the moment it crashed.

@value{GDBN} can do four main kinds of things (plus other things in support of
these) to help you catch bugs in the act:

@itemize @bullet
@item
Start your program, specifying anything that might affect its behavior.

@item
Make your program stop on specified conditions.

@item
Examine what has happened, when your program has stopped.

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

@ifclear CONLY
@ifclear MOD2
You can use @value{GDBN} to debug programs written in C or C++.
@end ifclear
@ifset MOD2
You can use @value{GDBN} to debug programs written in C, C++, and
Modula-2.
@end ifset
@ifset FORTRAN
@cindex Fortran
@value{GDBN} can be used to debug programs written in Fortran, although
it does not yet support entering expressions, printing values, etc.
using Fortran syntax.  It may be necessary to refer to some variables
with a trailing underscore.
@end ifset
@end ifclear

@menu
* Free Software::               Freely redistributable software
* Contributors::                Contributors to GDB
@end menu

@node Free Software
@unnumberedsec Free software

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

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

@node Contributors
@unnumberedsec Contributors to GDB

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

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

@quotation
@emph{Plea:} Additions to this section are particularly welcome.  If you
or your friends (or enemies, to be evenhanded) have been unfairly
omitted from this list, we would like to add your names!
@end quotation

So that they may not regard their long labor as thankless, we
particularly thank those who shepherded GDB through major releases: Fred
Fish (release 4.9), Stu Grossman and John Gilmore (releases 4.8, 4.7,
4.6, 4.5, 4.4), John Gilmore (releases 4.3, 4.2, 4.1, 4.0, and 3.9); Jim
Kingdon (releases 3.5, 3.4, 3.3); and Randy Smith (releases 3.2, 3.1,
3.0).  As major maintainer of GDB for some period, each contributed
significantly to the structure, stability, and capabilities of the
entire debugger.

Richard Stallman, assisted at various times by Peter TerMaat, Chris
Hanson, and Richard Mlynarik, handled releases through 2.8.

@ifclear CONLY
Michael Tiemann is the author of most of the GNU C++ support in GDB,
with significant additional contributions from Per Bothner.  James
Clark wrote the GNU C++ demangler.  Early work on C++ was by Peter
TerMaat (who also did much general update work leading to release 3.0).
@end ifclear

GDB 4 uses the BFD subroutine library to examine multiple
object-file formats; BFD was a joint project of David V.
Henkel-Wallace, Rich Pixley, Steve Chamberlain, and John Gilmore.

David Johnson wrote the original COFF support; Pace Willison did
the original support for encapsulated COFF.

Adam de Boor and Bradley Davis contributed the ISI Optimum V support.
Per Bothner, Noboyuki Hikichi, and Alessandro Forin contributed MIPS
support.  Jean-Daniel Fekete contributed Sun 386i support.  Chris
Hanson improved the HP9000 support.  Noboyuki Hikichi and Tomoyuki
Hasei contributed Sony/News OS 3 support.  David Johnson contributed
Encore Umax support.  Jyrki Kuoppala contributed Altos 3068 support.
Keith Packard contributed NS32K support.  Doug Rabson contributed
Acorn Risc Machine support.  Chris Smith contributed Convex support
(and Fortran debugging).  Jonathan Stone contributed Pyramid support.
Michael Tiemann contributed SPARC support.  Tim Tucker contributed
support for the Gould NP1 and Gould Powernode.  Pace Willison
contributed Intel 386 support.  Jay Vosburgh contributed Symmetry
support.

Rich Schaefer and Peter Schauer helped with support of SunOS shared
libraries.

Jay Fenlason and Roland McGrath ensured that GDB and GAS agree about
several machine instruction sets.

Patrick Duval, Ted Goldstein, Vikram Koka and Glenn Engel helped
develop remote debugging.  Intel Corporation and Wind River Systems
contributed remote debugging modules for their products.

Brian Fox is the author of the readline libraries providing
command-line editing and command history.

Andrew Beers of SUNY Buffalo wrote the language-switching code,
@ifset MOD2
the Modula-2 support,
@end ifset
and contributed the Languages chapter of this manual.

Fred Fish wrote most of the support for Unix System Vr4.  
@ifclear CONLY
He also enhanced the command-completion support to cover C++ overloaded
symbols.
@end ifclear

Hitachi America, Ltd. sponsored the support for Hitachi microprocessors.

@ifset NOVEL
@node New Features
@unnumbered New Features since GDB Version 3.5

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

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

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

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

@item Configuration and Ports
Compile-time configuration (to select a particular architecture and
operating system) is much easier.  The script @code{configure} now
allows you to configure GDB as either a native debugger or a
cross-debugger. @xref{Installing GDB}, for details on how to
configure.

@item Interaction
The user interface to the GDB control variables is simpler,
and is consolidated in two commands, @code{set} and @code{show}.  Output
lines are now broken at readable places, rather than overflowing onto
the next line.  You can suppress output of machine-level addresses,
displaying only source language information.

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

@ifset MOD2
@item Modula-2
GDB now has preliminary support for the GNU Modula-2 compiler, currently
under development at the State University of New York at Buffalo.
Coordinated development of both GDB and the GNU Modula-2 compiler will
continue.  Other Modula-2 compilers are currently not supported, and
attempting to debug programs compiled with them will likely result in an
error as the symbol table of the executable is read in.
@end ifset

@item Command Rationalization
Many GDB commands have been renamed to make them easier to remember
and use.  In particular, the subcommands of @code{info} and
@code{show}/@code{set} are grouped to make the former refer to the state
of your program, and the latter refer to the state of GDB itself.
@xref{Renamed Commands}, for details on what commands were renamed.

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

@item Reference Card
GDB 4 has a reference card.  @xref{Formatting Documentation,,Formatting
the Documentation}, for instructions about how to print it.
@end table
@end ifset

@ifclear BARETARGET
@node Sample Session
@chapter A Sample @value{GDBN} Session

You can use this manual at your leisure to read all about @value{GDBN}.
However, a handful of commands are enough to get started using the
debugger.  This chapter illustrates those commands.

@iftex
In this sample session, we emphasize user input like this: @b{input},
to make it easier to pick out from the surrounding output.
@end iftex

@c FIXME: this example may not be appropriate for some configs, where
@c FIXME...primary interest is in remote use.

One of the preliminary versions of GNU @code{m4} (a generic macro
processor) exhibits the following bug: sometimes, when we change its
quote strings from the default, the commands used to capture one macro
definition within another stop working.  In the following short @code{m4}
session, we define a macro @code{foo} which expands to @code{0000}; we
then use the @code{m4} built-in @code{defn} to define @code{bar} as the
same thing.  However, when we change the open quote string to
@code{<QUOTE>} and the close quote string to @code{<UNQUOTE>}, the same
procedure fails to define a new synonym @code{baz}:

@smallexample
$ @b{cd gnu/m4}
$ @b{./m4}
@b{define(foo,0000)}

@b{foo}
0000
@b{define(bar,defn(`foo'))}

@b{bar}
0000
@b{changequote(<QUOTE>,<UNQUOTE>)}

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

@noindent
Let us use @value{GDBN} to try to see what is going on.

@smallexample
$ @b{@value{GDBP} m4}
@c FIXME: this falsifies the exact text played out, to permit smallbook
@c FIXME... format to come out better.
GDB is free software and you are welcome to distribute copies
 of it under certain conditions; type "show copying" to see 
 the conditions.
There is absolutely no warranty for GDB; type "show warranty" 
 for details.
GDB @value{GDBVN}, Copyright 1993 Free Software Foundation, Inc...
(@value{GDBP})
@end smallexample

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

@smallexample
(@value{GDBP}) @b{set width 70}
@end smallexample

@noindent
We need to see how the @code{m4} built-in @code{changequote} works.
Having looked at the source, we know the relevant subroutine is
@code{m4_changequote}, so we set a breakpoint there with the @value{GDBN}
@code{break} command.

@smallexample
(@value{GDBP}) @b{break m4_changequote}
Breakpoint 1 at 0x62f4: file builtin.c, line 879.
@end smallexample

@noindent
Using the @code{run} command, we start @code{m4} running under @value{GDBN}
control; as long as control does not reach the @code{m4_changequote}
subroutine, the program runs as usual:

@smallexample
(@value{GDBP}) @b{run}
Starting program: /work/Editorial/gdb/gnu/m4/m4
@b{define(foo,0000)}

@b{foo}
0000
@end smallexample

@noindent
To trigger the breakpoint, we call @code{changequote}.  @value{GDBN}
suspends execution of @code{m4}, displaying information about the
context where it stops.

@smallexample
@b{changequote(<QUOTE>,<UNQUOTE>)}

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

@noindent
Now we use the command @code{n} (@code{next}) to advance execution to
the next line of the current function.

@smallexample
(@value{GDBP}) @b{n}
882         set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\
 : nil,
@end smallexample

@noindent
@code{set_quotes} looks like a promising subroutine.  We can go into it
by using the command @code{s} (@code{step}) instead of @code{next}.
@code{step} goes to the next line to be executed in @emph{any}
subroutine, so it steps into @code{set_quotes}.

@smallexample
(@value{GDBP}) @b{s}
set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
    at input.c:530
530         if (lquote != def_lquote)
@end smallexample

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

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

@noindent
We will step through a few more lines to see what happens.  The first two
times, we can use @samp{s}; the next two times we use @code{n} to avoid
falling into the @code{xstrdup} subroutine.

@smallexample
(@value{GDBP}) @b{s}
0x3b5c  532         if (rquote != def_rquote)
(@value{GDBP}) @b{s}
0x3b80  535         lquote = (lq == nil || *lq == '\0') ?  \
def_lquote : xstrdup(lq);
(@value{GDBP}) @b{n}
536         rquote = (rq == nil || *rq == '\0') ? def_rquote\
 : xstrdup(rq);
(@value{GDBP}) @b{n}
538         len_lquote = strlen(rquote);
@end smallexample

@noindent
The last line displayed looks a little odd; we can examine the variables
@code{lquote} and @code{rquote} to see if they are in fact the new left
and right quotes we specified.  We use the command @code{p}
(@code{print}) to see their values.

@smallexample
(@value{GDBP}) @b{p lquote}
$1 = 0x35d40 "<QUOTE>"
(@value{GDBP}) @b{p rquote}
$2 = 0x35d50 "<UNQUOTE>"
@end smallexample

@noindent
@code{lquote} and @code{rquote} are indeed the new left and right quotes.
To look at some context, we can display ten lines of source
surrounding the current line with the @code{l} (@code{list}) command.

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

@noindent
Let us step past the two lines that set @code{len_lquote} and
@code{len_rquote}, and then examine the values of those variables.

@smallexample
(@value{GDBP}) @b{n}
539         len_rquote = strlen(lquote);
(@value{GDBP}) @b{n}
540     @}
(@value{GDBP}) @b{p len_lquote}
$3 = 9
(@value{GDBP}) @b{p len_rquote}
$4 = 7
@end smallexample

@noindent
That certainly looks wrong, assuming @code{len_lquote} and
@code{len_rquote} are meant to be the lengths of @code{lquote} and
@code{rquote} respectively.  We can set them to better values using
the @code{p} command, since it can print the value of
any expression---and that expression can include subroutine calls and
assignments.

@smallexample
(@value{GDBP}) @b{p len_lquote=strlen(lquote)}
$5 = 7
(@value{GDBP}) @b{p len_rquote=strlen(rquote)}
$6 = 9
@end smallexample

@noindent
Is that enough to fix the problem of using the new quotes with the
@code{m4} built-in @code{defn}?  We can allow @code{m4} to continue
executing with the @code{c} (@code{continue}) command, and then try the
example that caused trouble initially:

@smallexample
(@value{GDBP}) @b{c}
Continuing.

@b{define(baz,defn(<QUOTE>foo<UNQUOTE>))}

baz
0000
@end smallexample

@noindent
Success!  The new quotes now work just as well as the default ones.  The
problem seems to have been just the two typos defining the wrong
lengths.  We allow @code{m4} exit by giving it an EOF as input:

@smallexample
@b{C-d}
Program exited normally.
@end smallexample

@noindent
The message @samp{Program exited normally.} is from @value{GDBN}; it
indicates @code{m4} has finished executing.  We can end our @value{GDBN}
session with the @value{GDBN} @code{quit} command.

@smallexample
(@value{GDBP}) @b{quit}
@end smallexample
@end ifclear

@node Invocation
@chapter Getting In and Out of @value{GDBN}

This chapter discusses how to start @value{GDBN}, and how to get out of it.
(The essentials: type @samp{@value{GDBP}} to start GDB, and type @kbd{quit}
or @kbd{C-d} to exit.)

@menu
* Invoking GDB::                How to start @value{GDBN}
* Quitting GDB::                 How to quit @value{GDBN}
* Shell Commands::              How to use shell commands inside @value{GDBN}
@end menu

@node Invoking GDB
@section Invoking @value{GDBN}

@ifset H8EXCLUSIVE
For details on starting up @value{GDBP} as a
remote debugger attached to a Hitachi microprocessor, see @ref{Hitachi
Remote,,@value{GDBN} and Hitachi Microprocessors}.
@end ifset

Invoke @value{GDBN} by running the program @code{@value{GDBP}}.  Once started,
@value{GDBN} reads commands from the terminal until you tell it to exit.

You can also run @code{@value{GDBP}} with a variety of arguments and options,
to specify more of your debugging environment at the outset.

@ifset GENERIC
The command-line options described here are designed
to cover a variety of situations; in some environments, some of these
options may effectively be unavailable.  
@end ifset

The most usual way to start @value{GDBN} is with one argument,
specifying an executable program:

@example
@value{GDBP} @var{program}
@end example

@ifclear BARETARGET
@noindent
You can also start with both an executable program and a core file
specified:

@example
@value{GDBP} @var{program} @var{core}
@end example

You can, instead, specify a process ID as a second argument, if you want
to debug a running process:

@example
@value{GDBP} @var{program} 1234
@end example

@noindent
would attach @value{GDBN} to process @code{1234} (unless you also have a file
named @file{1234}; @value{GDBN} does check for a core file first).

Taking advantage of the second command-line argument requires a fairly
complete operating system; when you use @value{GDBN} as a remote debugger
attached to a bare board, there may not be any notion of ``process'',
and there is often no way to get a core dump.
@end ifclear

@noindent
You can further control how @value{GDBN} starts up by using command-line
options.  @value{GDBN} itself can remind you of the options available.

@noindent
Type

@example
@value{GDBP} -help
@end example

@noindent
to display all available options and briefly describe their use
(@samp{@value{GDBP} -h} is a shorter equivalent).

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


@menu
@ifclear GENERIC
@ifset REMOTESTUB
* Remote Serial::               @value{GDBN} remote serial protocol
@end ifset
@ifset I960
* i960-Nindy Remote::           @value{GDBN} with a remote i960 (Nindy)
@end ifset
@ifset AMD29K
* UDI29K Remote::               @value{GDBN} and the UDI protocol for AMD29K
* EB29K Remote::                @value{GDBN} with a remote EB29K
@end ifset
@ifset VXWORKS
* VxWorks Remote::              @value{GDBN} and VxWorks
@end ifset
@ifset ST2000
* ST2000 Remote::               @value{GDBN} with a Tandem ST2000
@end ifset
@ifset H8
* Hitachi Remote::              @value{GDBN} and Hitachi Microprocessors
@end ifset
@ifset MIPS
* MIPS Remote::                 @value{GDBN} and MIPS boards
@end ifset
@ifset SIMS
* Simulator::                   Simulated CPU target
@end ifset
@end ifclear
@c remnant makeinfo bug requires this blank line after *two* end-ifblahs:

* File Options::                Choosing files
* Mode Options::                Choosing modes
@end menu

@ifclear GENERIC
@include remote.texi
@end ifclear

@node File Options
@subsection Choosing files

@ifclear BARETARGET
When @value{GDBN} starts, it reads any arguments other than options as
specifying an executable file and core file (or process ID).  This is
the same as if the arguments were specified by the @samp{-se} and
@samp{-c} options respectively.  (@value{GDBN} reads the first argument
that does not have an associated option flag as equivalent to the
@samp{-se} option followed by that argument; and the second argument
that does not have an associated option flag, if any, as equivalent to
the @samp{-c} option followed by that argument.)
@end ifclear
@ifset BARETARGET
When @value{GDBN} starts, it reads any argument other than options as
specifying an executable file.  This is the same as if the argument was
specified by the @samp{-se} option.
@end ifset

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

@table @code
@item -symbols @var{file}
@itemx -s @var{file}
Read symbol table from file @var{file}.

@item -exec @var{file}
@itemx -e @var{file}
Use file @var{file} as the executable file to execute when
@ifset BARETARGET
appropriate.
@end ifset
@ifclear BARETARGET
appropriate, and for examining pure data in conjunction with a core
dump.
@end ifclear

@item -se @var{file}
Read symbol table from file @var{file} and use it as the executable
file.

@ifclear BARETARGET
@item -core @var{file}
@itemx -c @var{file}
Use file @var{file} as a core dump to examine.

@item -c @var{number}
Connect to process ID @var{number}, as with the @code{attach} command
(unless there is a file in core-dump format named @var{number}, in which
case @samp{-c} specifies that file as a core dump to read).
@end ifclear

@item -command @var{file}
@itemx -x @var{file}
Execute @value{GDBN} commands from file @var{file}.  @xref{Command
Files,, Command files}.

@item -directory @var{directory}
@itemx -d @var{directory}
Add @var{directory} to the path to search for source files.

@ifclear BARETARGET
@item -m
@itemx -mapped
@emph{Warning: this option depends on operating system facilities that are not
supported on all systems.}@*
If memory-mapped files are available on your system through the @code{mmap}
system call, you can use this option 
to have @value{GDBN} write the symbols from your
program into a reusable file in the current directory.  If the program you are debugging is
called @file{/tmp/fred}, the mapped symbol file will be @file{./fred.syms}.
Future @value{GDBN} debugging sessions will notice the presence of this file,
and will quickly map in symbol information from it, rather than reading
the symbol table from the executable program.

@c FIXME! Really host, not target?
The @file{.syms} file is specific to the host machine where @value{GDBN}
is run.  It holds an exact image of the internal @value{GDBN} symbol
table.  It cannot be shared across multiple host platforms.
@end ifclear

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

@ifclear BARETARGET
The @code{-mapped} and @code{-readnow} options are typically combined in
order to build a @file{.syms} file that contains complete symbol
information.  (@xref{Files,,Commands to specify files}, for information
on @file{.syms} files.)  A simple GDB invocation to do nothing but build
a @file{.syms} file for future use is:

@example
        gdb -batch -nx -mapped -readnow programname
@end example
@end ifclear

@node Mode Options
@subsection Choosing modes

You can run @value{GDBN} in various alternative modes---for example, in
batch mode or quiet mode.

@table @code
@item -nx
@itemx -n
Do not execute commands from any initialization files (normally called
@file{@value{GDBINIT}}).  Normally, the commands in these files are
executed after all the command options and arguments have been
processed.  @xref{Command Files,,Command files}.

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

@item -batch
Run in batch mode.  Exit with status @code{0} after processing all the
command files specified with @samp{-x} (and all commands from
initialization files, if not inhibited with @samp{-n}).  Exit with
nonzero status if an error occurs in executing the @value{GDBN} commands
in the command files.

Batch mode may be useful for running @value{GDBN} as a filter, for example to
download and run a program on another computer; in order to make this
more useful, the message

@example
Program exited normally.
@end example

@noindent
(which is ordinarily issued whenever a program running under @value{GDBN} control
terminates) is not issued when running in batch mode.

@item -cd @var{directory}
Run @value{GDBN} using @var{directory} as its working directory,
instead of the current directory.

@ifset LUCID
@item -context @var{authentication}
When the Energize programming system starts up @value{GDBN}, it uses this
option to trigger an alternate mode of interaction.
@var{authentication} is a pair of numeric codes that identify @value{GDBN}
as a client in the Energize environment.  Avoid this option when you run
@value{GDBN} directly from the command line.  See @ref{Energize,,Using
@value{GDBN} with Energize} for more discussion of using @value{GDBN} with Energize.
@end ifset

@ifclear DOSHOST
@item -fullname
@itemx -f
Emacs sets this option when it runs @value{GDBN} as a subprocess.  It tells @value{GDBN}
to output the full file name and line number in a standard,
recognizable fashion each time a stack frame is displayed (which
includes each time your program stops).  This recognizable format looks
like two @samp{\032} characters, followed by the file name, line number
and character position separated by colons, and a newline.  The
Emacs-to-@value{GDBN} interface program uses the two @samp{\032} characters as
a signal to display the source code for the frame.
@end ifclear

@ifset SERIAL
@item -b @var{bps}
Set the line speed (baud rate or bits per second) of any serial
interface used by @value{GDBN} for remote debugging.

@item -tty @var{device}
Run using @var{device} for your program's standard input and output.
@c FIXME: kingdon thinks there is more to -tty.  Investigate.
@end ifset
@end table

@node Quitting GDB
@section Quitting @value{GDBN}
@cindex exiting @value{GDBN}
@cindex leaving @value{GDBN}

@table @code
@item quit
@kindex quit
@kindex q
To exit @value{GDBN}, use the @code{quit} command (abbreviated @code{q}), or type
an end-of-file character (usually @kbd{C-d}).
@end table

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

@ifclear BARETARGET
If you have been using @value{GDBN} to control an attached process or
device, you can release it with the @code{detach} command
(@pxref{Attach, ,Debugging an already-running process}).
@end ifclear

@node Shell Commands
@section Shell commands

If you need to execute occasional shell commands during your
debugging session, there is no need to leave or suspend @value{GDBN}; you can
just use the @code{shell} command.

@table @code
@item shell @var{command string}
@kindex shell
@cindex shell escape
Invoke a the standard shell to execute @var{command string}.
@ifclear DOSHOST
If it exists, the environment variable @code{SHELL} determines which
shell to run.  Otherwise @value{GDBN} uses @code{/bin/sh}.
@end ifclear
@end table

The utility @code{make} is often needed in development environments.
You do not have to use the @code{shell} command for this purpose in
@value{GDBN}:

@table @code
@item make @var{make-args}
@kindex make
@cindex calling make
Execute the @code{make} program with the specified
arguments.  This is equivalent to @samp{shell make @var{make-args}}.
@end table

@node Commands
@chapter @value{GDBN} Commands

You can abbreviate a @value{GDBN} command to the first few letters of the command
name, if that abbreviation is unambiguous; and you can repeat certain
@value{GDBN} commands by typing just @key{RET}.  You can also use the @key{TAB}
key to get @value{GDBN} to fill out the rest of a word in a command (or to
show you the alternatives available, if there is more than one possibility).

@menu
* Command Syntax::              How to give commands to @value{GDBN}
* Completion::                  Command completion
* Help::                        How to ask @value{GDBN} for help
@end menu

@node Command Syntax
@section Command syntax

A @value{GDBN} 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 @code{step} accepts an argument which is the number of times to
step, as in @samp{step 5}.  You can also use the @code{step} command
with no arguments.  Some command names do not allow any arguments.

@cindex abbreviation
@value{GDBN} command names may always be truncated if that abbreviation is
unambiguous.  Other possible command abbreviations are listed in the
documentation for individual commands.  In some cases, even ambiguous
abbreviations are allowed; for example, @code{s} is specially defined as
equivalent to @code{step} even though there are other commands whose
names start with @code{s}.  You can test abbreviations by using them as
arguments to the @code{help} command.

@cindex repeating commands
@kindex RET
A blank line as input to @value{GDBN} (typing just @key{RET}) means to
repeat the previous command. Certain commands (for example, @code{run})
will not repeat this way; these are commands for which unintentional
repetition might cause trouble and which you are unlikely to want to
repeat.

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

@value{GDBN} can also use @key{RET} in another way: to partition lengthy
output, in a way similar to the common utility @code{more}
(@pxref{Screen Size,,Screen size}).  Since it is easy to press one
@key{RET} too many in this situation, @value{GDBN} disables command
repetition after any command that generates this sort of display.

@kindex #
@cindex comment
Any text from a @kbd{#} to the end of the line is a comment; it does
nothing.  This is useful mainly in command files (@pxref{Command
Files,,Command files}).

@node Completion
@section Command completion

@cindex completion
@cindex word completion
@value{GDBN} can fill in the rest of a word in a command for you, if there is
only one possibility; it can also show you what the valid possibilities
are for the next word in a command, at any time.  This works for @value{GDBN}
commands, @value{GDBN} subcommands, and the names of symbols in your program.

Press the @key{TAB} key whenever you want @value{GDBN} to fill out the rest
of a word.  If there is only one possibility, @value{GDBN} will fill in the
word, and wait for you to finish the command (or press @key{RET} to
enter it).  For example, if you type

@c FIXME "@key" does not distinguish its argument sufficiently to permit
@c complete accuracy in these examples; space introduced for clarity.
@c If texinfo enhancements make it unnecessary, it would be nice to
@c replace " @key" by "@key" in the following...
@example
(@value{GDBP}) info bre @key{TAB}
@end example

@noindent
@value{GDBN} fills in the rest of the word @samp{breakpoints}, since that is
the only @code{info} subcommand beginning with @samp{bre}:

@example
(@value{GDBP}) info breakpoints
@end example

@noindent
You can either press @key{RET} at this point, to run the @code{info
breakpoints} command, or backspace and enter something else, if
@samp{breakpoints} does not look like the command you expected.  (If you
were sure you wanted @code{info breakpoints} in the first place, you
might as well just type @key{RET} immediately after @samp{info bre},
to exploit command abbreviations rather than command completion).

If there is more than one possibility for the next word when you press
@key{TAB}, @value{GDBN} will sound a bell.  You can either supply more
characters and try again, or just press @key{TAB} a second time, and
@value{GDBN} will display all the possible completions for that word.  For
example, you might want to set a breakpoint on a subroutine whose name
begins with @samp{make_}, but when you type @kbd{b make_@key{TAB}} @value{GDBN}
just sounds the bell.  Typing @key{TAB} again will display all the
function names in your program that begin with those characters, for
example:

@example
(@value{GDBP}) b make_ @key{TAB}
@exdent @value{GDBN} sounds bell; press @key{TAB} again, to see:
make_a_section_from_file     make_environ               
make_abs_section             make_function_type         
make_blockvector             make_pointer_type          
make_cleanup                 make_reference_type        
make_command                 make_symbol_completion_list
(@value{GDBP}) b make_
@end example

@noindent
After displaying the available possibilities, @value{GDBN} copies your
partial input (@samp{b make_} in the example) so you can finish the
command.

If you just want to see the list of alternatives in the first place, you
can press @kbd{M-?} rather than pressing @key{TAB} twice. @kbd{M-?}
means @kbd{@key{META} ?}.  You can type this
@ifclear DOSHOST
either by holding down a
key designated as the @key{META} shift on your keyboard (if there is
one) while typing @kbd{?}, or
@end ifclear
as @key{ESC} followed by @kbd{?}.

@cindex quotes in commands
@cindex completion of quoted strings
Sometimes the string you need, while logically a ``word'', may contain
parentheses or other characters that @value{GDBN} normally excludes from its
notion of a word.  To permit word completion to work in this situation,
you may enclose words in @code{'} (single quote marks) in @value{GDBN} commands.

@ifclear CONLY
The most likely situation where you might need this is in typing the
name of a C++ function.  This is because C++ allows function overloading
(multiple definitions of the same function, distinguished by argument
type).  For example, when you want to set a breakpoint you may need to
distinguish whether you mean the version of @code{name} that takes an
@code{int} parameter, @code{name(int)}, or the version that takes a
@code{float} parameter, @code{name(float)}.  To use the word-completion
facilities in this situation, type a single quote @code{'} at the
beginning of the function name.  This alerts @value{GDBN} that it may need to
consider more information than usual when you press @key{TAB} or
@kbd{M-?} to request word completion:

@example
(@value{GDBP}) b 'bubble( @key{M-?}
bubble(double,double)    bubble(int,int)
(@value{GDBP}) b 'bubble(
@end example

In some cases, @value{GDBN} can tell that completing a name will require
quotes.  When this happens, @value{GDBN} will insert the quote for you (while
completing as much as it can) if you do not type the quote in the first
place:

@example
(@value{GDBP}) b bub @key{TAB}
@exdent @value{GDBN} alters your input line to the following, and rings a bell:
(@value{GDBP}) b 'bubble(
@end example

@noindent
In general, @value{GDBN} can tell that a quote is needed (and inserts it) if
you have not yet started typing the argument list when you ask for
completion on an overloaded symbol.
@end ifclear


@node Help
@section Getting help
@cindex online documentation
@kindex help

You can always ask @value{GDBN} itself for information on its commands, using the
command @code{help}.

@table @code
@item help
@itemx h
@kindex h
You can use @code{help} (abbreviated @code{h}) with no arguments to
display a short list of named classes of commands:

@smallexample
(@value{GDBP}) help
List of classes of commands:

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

Type "help" followed by a class name for a list of 
commands in that class.
Type "help" followed by command name for full 
documentation.
Command name abbreviations are allowed if unambiguous.
(@value{GDBP})
@end smallexample

@item help @var{class}
Using one of the general help classes as an argument, you can get a
list of the individual commands in that class.  For example, here is the
help display for the class @code{status}:

@smallexample
(@value{GDBP}) help status
Status inquiries.

List of commands:

@c Line break in "show" line falsifies real output, but needed
@c to fit in smallbook page size.
show -- Generic command for showing things set
 with "set"
info -- Generic command for printing status

Type "help" followed by command name for full 
documentation.
Command name abbreviations are allowed if unambiguous.
(@value{GDBP})
@end smallexample

@item help @var{command}
With a command name as @code{help} argument, @value{GDBN} will display a
short paragraph on how to use that command.
@end table

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

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

@kindex show
@item show
In contrast, @code{show} is for describing the state of @value{GDBN} itself.
You can change most of the things you can @code{show}, by using the
related command @code{set}; for example, you can control what number
system is used for displays with @code{set radix}, or simply inquire
which is currently in use with @code{show radix}.

@kindex info set
To display all the settable parameters and their current
values, you can use @code{show} with no arguments; you may also use
@code{info set}.  Both commands produce the same display.
@c FIXME: "info set" violates the rule that "info" is for state of
@c FIXME...program.  Ck w/ GNU: "info set" to be called something else,
@c FIXME...or change desc of rule---eg "state of prog and debugging session"?
@end table
@c @end group

Here are three miscellaneous @code{show} subcommands, all of which are
exceptional in lacking corresponding @code{set} commands:

@table @code
@kindex show version
@cindex version number
@item show version
Show what version of @value{GDBN} is running.  You should include this
information in @value{GDBN} bug-reports.  If multiple versions of @value{GDBN} are in
use at your site, you may occasionally want to determine which version
of @value{GDBN} you are running; as @value{GDBN} evolves, new commands are introduced,
and old ones may wither away.  The version number is also announced
when you start @value{GDBN}.

@kindex show copying
@item show copying
Display information about permission for copying @value{GDBN}.

@kindex show warranty
@item show warranty
Display the GNU ``NO WARRANTY'' statement.
@end table

@node Running
@chapter Running Programs Under @value{GDBN}

When you run a program under @value{GDBN}, you must first generate
debugging information when you compile it.
@ifclear BARETARGET
You may start it with its arguments, if any, in an environment of your
choice.  You may redirect your program's input and output, debug an
already running process, or kill a child process.
@end ifclear

@menu
* Compilation::                 Compiling for debugging
* Starting::                    Starting your program
@ifclear BARETARGET
* Arguments::                   Your program's arguments
* Environment::                 Your program's environment
* Working Directory::           Your program's working directory
* Input/Output::                Your program's input and output
* Attach::                      Debugging an already-running process
* Kill Process::                Killing the child process
* Process Information::         Additional process information
@end ifclear
@end menu

@node Compilation
@section Compiling for debugging

In order to debug a program effectively, you need to generate
debugging information when you compile it.  This debugging information
is stored in the object file; it describes the data type of each
variable or function and the correspondence between source line numbers
and addresses in the executable code.

To request debugging information, specify the @samp{-g} option when you run
the compiler.

Many C compilers are unable to handle the @samp{-g} and @samp{-O}
options together.  Using those compilers, you cannot generate optimized
executables containing debugging information.

@value{NGCC}, the GNU C compiler, supports @samp{-g} with or without
@samp{-O}, making it possible to debug optimized code.  We recommend
that you @emph{always} use @samp{-g} whenever you compile a program.
You may think your program is correct, but there is no sense in pushing
your luck.

@cindex optimized code, debugging
@cindex debugging optimized code
When you debug a program compiled with @samp{-g -O}, remember that the
optimizer is rearranging your code; the debugger will show you what is
really there.  Do not be too surprised when the execution path does not
exactly match your source file!  An extreme example: if you define a
variable, but never use it, @value{GDBN} will never see that
variable---because the compiler optimizes it out of existence.

Some things do not work as well with @samp{-g -O} as with just
@samp{-g}, particularly on machines with instruction scheduling.  If in
doubt, recompile with @samp{-g} alone, and if this fixes the problem,
please report it as a bug (including a test case!).

Older versions of the GNU C compiler permitted a variant option
@w{@samp{-gg}} for debugging information.  @value{GDBN} no longer supports this
format; if your GNU C compiler has this option, do not use it.

@need 2000
@node Starting
@section Starting your program
@cindex starting
@cindex running

@table @code
@item run
@itemx r
@kindex run
Use the @code{run} command to start your program under @value{GDBN}.  You must
first specify the program name
@ifset VXWORKS
(except on VxWorks)
@end ifset
with an argument to @value{GDBN} (@pxref{Invocation, ,Getting In and
Out of @value{GDBN}}), or by using the @code{file} or @code{exec-file}
command (@pxref{Files, ,Commands to specify files}).

@end table

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

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

@table @asis
@item The @emph{arguments.}
Specify the arguments to give your program as the arguments of the
@code{run} command.  If a shell is available on your target, the shell
is used to pass the arguments, so that you may use normal conventions
(such as wildcard expansion or variable substitution) in describing
the arguments.  In Unix systems, you can control which shell is used
with the @code{SHELL} environment variable. @xref{Arguments, ,Your
program's arguments}.

@item The @emph{environment.}
Your program normally inherits its environment from @value{GDBN}, but you can
use the @value{GDBN} commands @code{set environment} and @code{unset
environment} to change parts of the environment that will be given to
your program.  @xref{Environment, ,Your program's environment}.

@item The @emph{working directory.}
Your program inherits its working directory from @value{GDBN}.  You can set
the @value{GDBN} working directory with the @code{cd} command in @value{GDBN}.
@xref{Working Directory, ,Your program's working directory}.

@item The @emph{standard input and output.}
Your program normally uses the same device for standard input and
standard output as @value{GDBN} is using.  You can redirect input and output
in the @code{run} command line, or you can use the @code{tty} command to
set a different device for your program.
@xref{Input/Output, ,Your program's input and output}.

@cindex pipes
@emph{Warning:} While input and output redirection work, you cannot use
pipes to pass the output of the program you are debugging to another
program; if you attempt this, @value{GDBN} is likely to wind up debugging the
wrong program.
@end table
@end ifclear

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

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

@ifclear BARETARGET
@node Arguments
@section Your program's arguments

@cindex arguments (to your program)
The arguments to your program can be specified by the arguments of the
@code{run} command.  They are passed to a shell, which expands wildcard
characters and performs redirection of I/O, and thence to your program.
Your @code{SHELL} environment variable (if it exists) specifies what
shell @value{GDBN} if you do not define @code{SHELL}, @value{GDBN} uses
@code{/bin/sh}.

@code{run} with no arguments uses the same arguments used by the previous
@code{run}, or those set by the @code{set args} command.

@kindex set args
@table @code
@item set args
Specify the arguments to be used the next time your program is run.  If
@code{set args} has no arguments, @code{run} will execute your program
with no arguments.  Once you have run your program with arguments,
using @code{set args} before the next @code{run} is the only way to run
it again without arguments.

@item show args
@kindex show args
Show the arguments to give your program when it is started.
@end table

@node Environment
@section Your program's environment

@cindex environment (of your program)
The @dfn{environment} consists of a set of environment variables and
their values.  Environment variables conventionally record such things as
your user name, your home directory, your terminal type, and your search
path for programs to run.  Usually you set up environment variables with
the shell and they are inherited by all the other programs you run.  When
debugging, it can be useful to try running your program with a modified
environment without having to start @value{GDBN} over again.

@table @code
@item path @var{directory}
@kindex path
Add @var{directory} to the front of the @code{PATH} environment variable
(the search path for executables), for both @value{GDBN} and your program.
You may specify several directory names, separated by @samp{:} or
whitespace.  If @var{directory} is already in the path, it is moved to
the front, so it will be searched sooner.

You can use the string @samp{$cwd} to refer to whatever is the current
working directory at the time @value{GDBN} searches the path.  If you
use @samp{.} instead, it refers to the directory where you executed the
@code{path} command.  @value{GDBN} replaces @samp{.} in the
@var{directory} argument (with the current path) before adding
@var{directory} to the search path.
@c 'path' is explicitly nonrepeatable, but RMS points out it is silly to
@c document that, since repeating it would be a no-op.

@item show paths
@kindex show paths
Display the list of search paths for executables (the @code{PATH}
environment variable).

@item show environment @r{[}@var{varname}@r{]}
@kindex show environment
Print the value of environment variable @var{varname} to be given to
your program when it starts.  If you do not supply @var{varname},
print the names and values of all environment variables to be given to
your program.  You can abbreviate @code{environment} as @code{env}.

@item set environment @var{varname} @r{[}=@r{]} @var{value}
@kindex set environment
Set environment variable @var{varname} to @var{value}.  The value
changes for your program only, not for @value{GDBN} itself.  @var{value} may
be any string; the values of environment variables are just strings, and
any interpretation is supplied by your program itself.  The @var{value}
parameter is optional; if it is eliminated, the variable is set to a
null value.
@c "any string" here does not include leading, trailing
@c blanks. Gnu asks: does anyone care?

For example, this command:

@example
set env USER = foo
@end example

@noindent
tells a Unix program, when subsequently run, that its user is named
@samp{foo}.  (The spaces around @samp{=} are used for clarity here; they
are not actually required.)

@item unset environment @var{varname}
@kindex unset environment
Remove variable @var{varname} from the environment to be passed to your
program.  This is different from @samp{set env @var{varname} =};
@code{unset environment} removes the variable from the environment,
rather than assigning it an empty value.
@end table

@emph{Warning:} @value{GDBN} runs your program using the shell indicated
by your @code{SHELL} environment variable if it exists (or
@code{/bin/sh} if not).  If your @code{SHELL} variable names a shell
that runs an initialization file---such as @file{.cshrc} for C-shell, or
@file{.bashrc} for BASH---any variables you set in that file will affect
your program.  You may wish to move setting of environment variables to
files that are only run when you sign on, such as @file{.login} or
@file{.profile}.

@node Working Directory
@section Your program's working directory

@cindex working directory (of your program)
Each time you start your program with @code{run}, it inherits its
working directory from the current working directory of @value{GDBN}.
The @value{GDBN} working directory is initially whatever it inherited
from its parent process (typically the shell), but you can specify a new
working directory in @value{GDBN} with the @code{cd} command.

The @value{GDBN} working directory also serves as a default for the commands
that specify files for @value{GDBN} to operate on.  @xref{Files, ,Commands to
specify files}.

@table @code
@item cd @var{directory}
@kindex cd
Set the @value{GDBN} working directory to @var{directory}.

@item pwd
@kindex pwd
Print the @value{GDBN} working directory.
@end table

@node Input/Output
@section Your program's input and output

@cindex redirection
@cindex i/o
@cindex terminal
By default, the program you run under @value{GDBN} does input and output to
the same terminal that @value{GDBN} uses.  @value{GDBN} switches the terminal to
its own terminal modes to interact with you, but it records the terminal
modes your program was using and switches back to them when you continue
running your program.

@table @code
@item info terminal
@kindex info terminal
Displays information recorded by @value{GDBN} about the terminal modes your
program is using.
@end table

You can redirect your program's input and/or output using shell
redirection with the @code{run} command.  For example,

@example
run > outfile
@end example

@noindent
starts your program, diverting its output to the file @file{outfile}.

@kindex tty
@cindex controlling terminal
Another way to specify where your program should do input and output is
with the @code{tty} command.  This command accepts a file name as
argument, and causes this file to be the default for future @code{run}
commands.  It also resets the controlling terminal for the child
process, for future @code{run} commands.  For example,

@example
tty /dev/ttyb
@end example

@noindent
directs that processes started with subsequent @code{run} commands
default to do input and output on the terminal @file{/dev/ttyb} and have
that as their controlling terminal.

An explicit redirection in @code{run} overrides the @code{tty} command's
effect on the input/output device, but not its effect on the controlling
terminal.

When you use the @code{tty} command or redirect input in the @code{run}
command, only the input @emph{for your program} is affected.  The input
for @value{GDBN} still comes from your terminal.

@node Attach
@section Debugging an already-running process
@kindex attach
@cindex attach

@table @code
@item attach @var{process-id}
This command attaches to a running process---one that was started
outside @value{GDBN}.  (@code{info files} will show your active
targets.)  The command takes as argument a process ID.  The usual way to
find out the process-id of a Unix process is with the @code{ps} utility,
or with the @samp{jobs -l} shell command.

@code{attach} will not repeat if you press @key{RET} a second time after
executing the command.
@end table

To use @code{attach}, your program must be running in an environment
which supports processes; for example, @code{attach} does not work for
programs on bare-board targets that lack an operating system.  You must
also have permission to send the process a signal.

When using @code{attach}, you should first use the @code{file} command
to specify the program running in the process and load its symbol table.
@xref{Files, ,Commands to Specify Files}.

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

@table @code
@item detach
@kindex detach
When you have finished debugging the attached process, you can use the
@code{detach} command to release it from @value{GDBN} control.  Detaching
the process continues its execution.  After the @code{detach} command,
that process and @value{GDBN} become completely independent once more, and you
are ready to @code{attach} another process or start one with @code{run}.
@code{detach} will not repeat if you press @key{RET} again after
executing the command.
@end table

If you exit @value{GDBN} or use the @code{run} command while you have an attached
process, you kill that process.  By default, you will be asked for
confirmation if you try to do either of these things; you can control
whether or not you need to confirm by using the @code{set confirm} command
(@pxref{Messages/Warnings, ,Optional warnings and messages}).

@node Kill Process
@c @group
@section Killing the child process

@table @code
@item kill
@kindex kill
Kill the child process in which your program is running under @value{GDBN}.
@end table

This command is useful if you wish to debug a core dump instead of a
running process.  @value{GDBN} ignores any core dump file while your program
is running.
@c @end group

On some operating systems, a program cannot be executed outside @value{GDBN}
while you have breakpoints set on it inside @value{GDBN}.  You can use the
@code{kill} command in this situation to permit running your program
outside the debugger.

The @code{kill} command is also useful if you wish to recompile and
relink your program, since on many systems it is impossible to modify an
executable file while it is running in a process.  In this case, when you
next type @code{run}, @value{GDBN} will notice that the file has changed, and
will re-read the symbol table (while trying to preserve your current
breakpoint settings).

@node Process Information
@section Additional process information

@kindex /proc
@cindex process image
Some operating systems provide a facility called @samp{/proc} that can
be used to examine the image of a running process using file-system
subroutines.  If @value{GDBN} is configured for an operating system with this
facility, the command @code{info proc} is available to report on several
kinds of information about the process running your program.

@table @code
@item info proc
@kindex info proc
Summarize available information about the process.

@item info proc mappings
@kindex info proc mappings
Report on the address ranges accessible in the program, with information
on whether your program may read, write, or execute each range.

@item info proc times
@kindex info proc times
Starting time, user CPU time, and system CPU time for your program and
its children.

@item info proc id
@kindex info proc id
Report on the process IDs related to your program: its own process ID,
the ID of its parent, the process group ID, and the session ID.

@item info proc status
@kindex info proc status
General information on the state of the process.  If the process is
stopped, this report includes the reason for stopping, and any signal
received.

@item info proc all
Show all the above information about the process.
@end table
@end ifclear

@node Stopping
@chapter Stopping and Continuing

The principal purposes of using a debugger are so that you can stop your
program before it terminates; or so that, if your program runs into
trouble, you can investigate and find out why.

Inside @value{GDBN}, your program may stop for any of several reasons, such
as
@ifclear BARETARGET
a signal,
@end ifclear
a breakpoint, or reaching a new line after a @value{GDBN}
command such as @code{step}.  You may then examine and change
variables, set new breakpoints or remove old ones, and then continue
execution.  Usually, the messages shown by @value{GDBN} provide ample
explanation of the status of your program---but you can also explicitly
request this information at any time.

@table @code
@item info program
@kindex info program
Display information about the status of your program: whether it is
running or not,
@ifclear BARETARGET
what process it is,
@end ifclear
and why it stopped.
@end table

@menu
@ifclear CONLY
* Breakpoints::                 Breakpoints, watchpoints, and exceptions
@end ifclear
@ifset CONLY
* Breakpoints::                 Breakpoints and watchpoints
@end ifset
@c Remnant makeinfo bug requires blank line after *successful* end-if in menu:

* Continuing and Stepping::     Resuming execution
@ifset POSIX
* Signals::                     Signals
@end ifset
@end menu

@c makeinfo node-defaulting requires adjacency of @node and sectioning cmds
@c ...hence distribute @node Breakpoints over two possible @if expansions.
@c
@ifclear CONLY
@node Breakpoints
@section Breakpoints, watchpoints, and exceptions
@end ifclear
@ifset CONLY
@node Breakpoints
@section Breakpoints and watchpoints
@end ifset

@cindex breakpoints
A @dfn{breakpoint} makes your program stop whenever a certain point in
the program is reached.  For each breakpoint, you can add various
conditions to control in finer detail whether your program will stop.
You can set breakpoints with the @code{break} command and its variants
(@pxref{Set Breaks, ,Setting breakpoints}), to specify the place where
your program should stop by line number, function name or exact address
in the program.  
@ifclear CONLY
In languages with exception handling (such as GNU C++), you can also set
breakpoints where an exception is raised (@pxref{Exception Handling,
,Breakpoints and exceptions}).
@end ifclear

@cindex watchpoints
@cindex memory tracing
@cindex breakpoint on memory address
@cindex breakpoint on variable modification
A @dfn{watchpoint} is a special breakpoint that stops your program
when the value of an expression changes.  You must use a different
command to set watchpoints (@pxref{Set Watchpoints, ,Setting
watchpoints}), but aside from that, you can manage a watchpoint like
any other breakpoint: you enable, disable, and delete both breakpoints
and watchpoints using the same commands.

You can arrange to have values from your program displayed automatically
whenever @value{GDBN} stops at a breakpoint.  @xref{Auto Display,
,Automatic display}.

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

@menu
* Set Breaks::                  Setting breakpoints
* Set Watchpoints::             Setting watchpoints
@ifclear CONLY
* Exception Handling::          Breakpoints and exceptions
@end ifclear

* Delete Breaks::               Deleting breakpoints
* Disabling::                   Disabling breakpoints
* Conditions::                  Break conditions
* Break Commands::              Breakpoint command lists
@ifclear CONLY
* Breakpoint Menus::            Breakpoint menus
@end ifclear
@ifclear BARETARGET
* Error in Breakpoints::        ``Cannot insert breakpoints''
@end ifclear
@end menu

@node Set Breaks
@subsection Setting breakpoints

@c FIXME LMB what does GDB do if no code on line of breakpt?  
@c       consider in particular declaration with/without initialization.
@c
@c FIXME 2 is there stuff on this already? break at fun start, already init?

@kindex break
@kindex b
@kindex $bpnum
@cindex latest breakpoint
Breakpoints are set with the @code{break} command (abbreviated
@code{b}).  The debugger convenience variable @samp{$bpnum} records the 
number of the beakpoint you've set most recently; see @ref{Convenience
Vars,, Convenience variables}, for a discussion of what you can do with
convenience variables.

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

@table @code
@item break @var{function}
Set a breakpoint at entry to function @var{function}.  
@ifclear CONLY
When using source languages that permit overloading of symbols, such as
C++, @var{function} may refer to more than one possible place to break.
@xref{Breakpoint Menus,,Breakpoint menus}, for a discussion of that situation.
@end ifclear

@item break +@var{offset}
@itemx break -@var{offset}
Set a breakpoint some number of lines forward or back from the position
at which execution stopped in the currently selected frame.

@item break @var{linenum}
Set a breakpoint at line @var{linenum} in the current source file.
That file is the last file whose source text was printed.  This
breakpoint will stop your program just before it executes any of the
code on that line.

@item break @var{filename}:@var{linenum}
Set a breakpoint at line @var{linenum} in source file @var{filename}.

@item break @var{filename}:@var{function}
Set a breakpoint at entry to function @var{function} found in file
@var{filename}.  Specifying a file name as well as a function name is
superfluous except when multiple files contain similarly named
functions.

@item break *@var{address}
Set a breakpoint at address @var{address}.  You can use this to set
breakpoints in parts of your program which do not have debugging
information or source files.

@item break
When called without any arguments, @code{break} sets a breakpoint at
the next instruction to be executed in the selected stack frame
(@pxref{Stack, ,Examining the Stack}).  In any selected frame but the
innermost, this will cause your program to stop as soon as control
returns to that frame.  This is similar to the effect of a
@code{finish} command in the frame inside the selected frame---except
that @code{finish} does not leave an active breakpoint.  If you use
@code{break} without an argument in the innermost frame, @value{GDBN} will stop
the next time it reaches the current location; this may be useful
inside loops.

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

@item break @dots{} if @var{cond}
Set a breakpoint with condition @var{cond}; evaluate the expression
@var{cond} each time the breakpoint is reached, and stop only if the
value is nonzero---that is, if @var{cond} evaluates as true.
@samp{@dots{}} stands for one of the possible arguments described
above (or no argument) specifying where to break.  @xref{Conditions,
,Break conditions}, for more information on breakpoint conditions.

@item tbreak @var{args}
@kindex tbreak
Set a breakpoint enabled only for one stop.  @var{args} are the
same as for the @code{break} command, and the breakpoint is set in the same
way, but the breakpoint is automatically disabled after the first time your
program stops there.  @xref{Disabling, ,Disabling breakpoints}.

@item rbreak @var{regex}
@kindex rbreak
@cindex regular expression
@c FIXME what kind of regexp?
Set breakpoints on all functions matching the regular expression
@var{regex}.  This command
sets an unconditional breakpoint on all matches, printing a list of all
breakpoints it set. Once these breakpoints are set, they are treated
just like the breakpoints set with the @code{break} command.  They can
be deleted, disabled, made conditional, etc., in the standard ways.

@ifclear CONLY
When debugging C++ programs, @code{rbreak} is useful for setting
breakpoints on overloaded functions that are not members of any special
classes.
@end ifclear

@kindex info breakpoints
@cindex @code{$_} and @code{info breakpoints}
@item info breakpoints @r{[}@var{n}@r{]}
@itemx info break @r{[}@var{n}@r{]}
@itemx info watchpoints @r{[}@var{n}@r{]}
Print a table of all breakpoints and watchpoints set and not
deleted, with the following columns for each breakpoint:

@table @emph
@item Breakpoint Numbers
@item Type
Breakpoint or watchpoint.
@item Disposition
Whether the breakpoint is marked to be disabled or deleted when hit.
@item Enabled or Disabled
Enabled breakpoints are marked with @samp{y}.  @samp{n} marks breakpoints
that are not enabled.
@item Address
Where the breakpoint is in your program, as a memory address
@item What
Where the breakpoint is in the source for your program, as a file and
line number.
@end table

@noindent
If a breakpoint is conditional, @code{info break} shows the condition on
the line following the affected breakpoint; breakpoint commands, if any,
are listed after that.

@noindent
@code{info break} with a breakpoint
number @var{n} as argument lists only that breakpoint.  The
convenience variable @code{$_} and the default examining-address for
the @code{x} command are set to the address of the last breakpoint
listed (@pxref{Memory, ,Examining memory}).  
@end table

@value{GDBN} allows you to set any number of breakpoints at the same place in
your program.  There is nothing silly or meaningless about this.  When
the breakpoints are conditional, this is even useful
(@pxref{Conditions, ,Break conditions}).

@cindex negative breakpoint numbers
@cindex internal @value{GDBN} breakpoints
@value{GDBN} itself sometimes sets breakpoints in your program for special
purposes, such as proper handling of @code{longjmp} (in C programs).
These internal breakpoints are assigned negative numbers, starting with
@code{-1}; @samp{info breakpoints} does not display them.

You can see these breakpoints with the @value{GDBN} maintenance command
@samp{maint info breakpoints}.

@table @code
@kindex maint info breakpoints
@item maint info breakpoints
Using the same format as @samp{info breakpoints}, display both the
breakpoints you've set explicitly, and those @value{GDBN} is using for
internal purposes.  Internal breakpoints are shown with negative
breakpoint numbers.  The type column identifies what kind of breakpoint
is shown:

@table @code
@item breakpoint
Normal, explicitly set breakpoint.

@item watchpoint
Normal, explicitly set watchpoint.

@item longjmp
Internal breakpoint, used to handle correctly stepping through
@code{longjmp} calls.

@item longjmp resume
Internal breakpoint at the target of a @code{longjmp}.

@item until
Temporary internal breakpoint used by the @value{GDBN} @code{until} command.

@item finish
Temporary internal breakpoint used by the @value{GDBN} @code{finish} command.
@end table

@end table


@node Set Watchpoints
@subsection Setting watchpoints
@cindex setting watchpoints

You can use a watchpoint to stop execution whenever the value of an
expression changes, without having to predict a particular place
where this may happen.

Watchpoints currently execute two orders of magnitude more slowly than
other breakpoints, but this can be well worth it to catch errors where
you have no clue what part of your program is the culprit.  Some
processors provide special hardware to support watchpoint evaluation; future
releases of @value{GDBN} will use such hardware if it is available.

@table @code
@kindex watch
@item watch @var{expr}
Set a watchpoint for an expression.

@kindex info watchpoints
@item info watchpoints
This command prints a list of watchpoints and breakpoints; it is the
same as @code{info break}.
@end table

@ifclear CONLY
@node Exception Handling
@subsection Breakpoints and exceptions
@cindex exception handlers

Some languages, such as GNU C++, implement exception handling.  You can
use @value{GDBN} to examine what caused your program to raise an exception,
and to list the exceptions your program is prepared to handle at a
given point in time.

@table @code
@item catch @var{exceptions}
@kindex catch
You can set breakpoints at active exception handlers by using the
@code{catch} command.  @var{exceptions} is a list of names of exceptions
to catch.
@end table

You can use @code{info catch} to list active exception handlers.
@xref{Frame Info, ,Information about a frame}.

There are currently some limitations to exception handling in @value{GDBN}.
These will be corrected in a future release.

@itemize @bullet
@item
If you call a function interactively, @value{GDBN} normally returns
control to you when the function has finished executing.  If the call
raises an exception, however, the call may bypass the mechanism that
returns control to you and cause your program to simply continue
running until it hits a breakpoint, catches a signal that @value{GDBN} is
listening for, or exits.
@item
You cannot raise an exception interactively.
@item
You cannot interactively install an exception handler.
@end itemize

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

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

@example
    /* @var{addr} is where the exception identifier is stored.
       ID is the exception identifier.  */
    void __raise_exception (void **@var{addr}, void *@var{id});
@end example

@noindent
To make the debugger catch all exceptions before any stack
unwinding takes place, set a breakpoint on @code{__raise_exception}
(@pxref{Breakpoints, ,Breakpoints; watchpoints; and exceptions}).

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

@node Delete Breaks
@subsection Deleting breakpoints

@cindex clearing breakpoints, watchpoints
@cindex deleting breakpoints, watchpoints
It is often necessary to eliminate a breakpoint or watchpoint once it
has done its job and you no longer want your program to stop there.  This
is called @dfn{deleting} the breakpoint.  A breakpoint that has been
deleted no longer exists; it is forgotten.

With the @code{clear} command you can delete breakpoints according to
where they are in your program.  With the @code{delete} command you can
delete individual breakpoints or watchpoints by specifying their
breakpoint numbers.

It is not necessary to delete a breakpoint to proceed past it.  @value{GDBN}
automatically ignores breakpoints on the first instruction to be executed
when you continue execution without changing the execution address.

@table @code
@item clear
@kindex clear
Delete any breakpoints at the next instruction to be executed in the
selected stack frame (@pxref{Selection, ,Selecting a frame}).  When
the innermost frame is selected, this is a good way to delete a
breakpoint where your program just stopped.

@item clear @var{function}
@itemx clear @var{filename}:@var{function}
Delete any breakpoints set at entry to the function @var{function}.

@item clear @var{linenum}
@itemx clear @var{filename}:@var{linenum}
Delete any breakpoints set at or within the code of the specified line.

@item delete @r{[}breakpoints@r{]} @r{[}@var{bnums}@dots{}@r{]}
@cindex delete breakpoints
@kindex delete
@kindex d
Delete the breakpoints or watchpoints of the numbers specified as
arguments.  If no argument is specified, delete all breakpoints (@value{GDBN}
asks confirmation, unless you have @code{set confirm off}).  You
can abbreviate this command as @code{d}.
@end table

@node Disabling
@subsection Disabling breakpoints

@cindex disabled breakpoints
@cindex enabled breakpoints
Rather than deleting a breakpoint or watchpoint, you might prefer to
@dfn{disable} it.  This makes the breakpoint inoperative as if it had
been deleted, but remembers the information on the breakpoint so that
you can @dfn{enable} it again later.

You disable and enable breakpoints and watchpoints with the
@code{enable} and @code{disable} commands, optionally specifying one or
more breakpoint numbers as arguments.  Use @code{info break} or
@code{info watch} to print a list of breakpoints or watchpoints if you
do not know which numbers to use.

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

@itemize @bullet
@item
Enabled.  The breakpoint will stop your program.  A breakpoint set
with the @code{break} command starts out in this state.
@item
Disabled.  The breakpoint has no effect on your program.
@item
Enabled once.  The breakpoint will stop your program, but
when it does so it will become disabled.  A breakpoint set
with the @code{tbreak} command starts out in this state.
@item
Enabled for deletion.  The breakpoint will stop your program, but
immediately after it does so it will be deleted permanently.
@end itemize

You can use the following commands to enable or disable breakpoints and
watchpoints:

@table @code
@item disable @r{[}breakpoints@r{]} @r{[}@var{bnums}@dots{}@r{]}
@kindex disable breakpoints
@kindex disable
@kindex dis
Disable the specified breakpoints---or all breakpoints, if none are
listed.  A disabled breakpoint has no effect but is not forgotten.  All
options such as ignore-counts, conditions and commands are remembered in
case the breakpoint is enabled again later.  You may abbreviate
@code{disable} as @code{dis}.

@item enable @r{[}breakpoints@r{]} @r{[}@var{bnums}@dots{}@r{]}
@kindex enable breakpoints
@kindex enable
Enable the specified breakpoints (or all defined breakpoints).  They
become effective once again in stopping your program.

@item enable @r{[}breakpoints@r{]} once @var{bnums}@dots{}
Enable the specified breakpoints temporarily.  Each will be disabled
again the next time it stops your program.

@item enable @r{[}breakpoints@r{]} delete @var{bnums}@dots{}
Enable the specified breakpoints to work once and then die.  Each of
the breakpoints will be deleted the next time it stops your program.
@end table

Save for a breakpoint set with @code{tbreak} (@pxref{Set Breaks,
,Setting breakpoints}), breakpoints that you set are initially enabled;
subsequently, they become disabled or enabled only when you use one of
the commands above.  (The command @code{until} can set and delete a
breakpoint of its own, but it will not change the state of your other
breakpoints; see @ref{Continuing and Stepping, ,Continuing and
stepping}.)

@node Conditions
@subsection Break conditions
@cindex conditional breakpoints
@cindex breakpoint conditions

@c FIXME what is scope of break condition expr?  Context where wanted?
@c      in particular for a watchpoint?  
The simplest sort of breakpoint breaks every time your program reaches a
specified place.  You can also specify a @dfn{condition} for a
breakpoint.  A condition is just a Boolean expression in your
programming language (@pxref{Expressions, ,Expressions}).  A breakpoint with
a condition evaluates the expression each time your program reaches it,
and your program stops only if the condition is @emph{true}.

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

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

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

Break conditions can be specified when a breakpoint is set, by using
@samp{if} in the arguments to the @code{break} command.  @xref{Set
Breaks, ,Setting breakpoints}.  They can also be changed at any time
with the @code{condition} command.  The @code{watch} command does not
recognize the @code{if} keyword; @code{condition} is the only way to
impose a further condition on a watchpoint.

@table @code
@item condition @var{bnum} @var{expression}
@kindex condition
Specify @var{expression} as the break condition for breakpoint or
watchpoint number @var{bnum}.  From now on, this breakpoint will stop
your program only if the value of @var{expression} is true (nonzero, in
C).  When you use @code{condition}, @value{GDBN} checks @var{expression}
immediately for syntactic correctness, and to determine whether symbols
in it have referents in the context of your breakpoint.
@c FIXME so what does GDB do if there is no referent?  Moreover, what
@c about watchpoints?
@value{GDBN} does
not actually evaluate @var{expression} at the time the @code{condition}
command is given, however.  @xref{Expressions, ,Expressions}.

@item condition @var{bnum}
Remove the condition from breakpoint number @var{bnum}.  It becomes
an ordinary unconditional breakpoint.
@end table

@cindex ignore count (of 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 @dfn{ignore
count} of the breakpoint.  Every breakpoint has an ignore count, which
is an integer.  Most of the time, the ignore count is zero, and
therefore has no effect.  But if your program reaches a breakpoint whose
ignore count is positive, then instead of stopping, it just decrements
the ignore count by one and continues.  As a result, if the ignore count
value is @var{n}, the breakpoint will not stop the next @var{n} times it
is reached.

@table @code
@item ignore @var{bnum} @var{count}
@kindex ignore
Set the ignore count of breakpoint number @var{bnum} to @var{count}.
The next @var{count} times the breakpoint is reached, your program's
execution will not stop; other than to decrement the ignore count, @value{GDBN}
takes no action.

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

When you use @code{continue} to resume execution of your program from a
breakpoint, you can specify an ignore count directly as an argument to
@code{continue}, rather than using @code{ignore}.  @xref{Continuing and
Stepping,,Continuing and stepping}.

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

You could achieve the effect of the ignore count with a condition such
as @w{@samp{$foo-- <= 0}} using a debugger convenience variable that
is decremented each time.  @xref{Convenience Vars, ,Convenience
variables}.
@end table

@node Break Commands
@subsection Breakpoint command lists

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

@table @code
@item commands @r{[}@var{bnum}@r{]}
@itemx @dots{} @var{command-list} @dots{}
@itemx end
@kindex commands
@kindex end
Specify a list of commands for breakpoint number @var{bnum}.  The commands
themselves appear on the following lines.  Type a line containing just
@code{end} to terminate the commands.

To remove all commands from a breakpoint, type @code{commands} and
follow it immediately with @code{end}; that is, give no commands.

With no @var{bnum} argument, @code{commands} refers to the last
breakpoint or watchpoint set (not to the breakpoint most recently
encountered).
@end table

Pressing @key{RET} as a means of repeating the last @value{GDBN} command is
disabled within a @var{command-list}.

You can use breakpoint commands to start your program up again.  Simply
use the @code{continue} command, or @code{step}, or any other command
that resumes execution.

Any other commands in the command list, after a command that resumes
execution, are ignored.  This is because any time you resume execution
(even with a simple @code{next} or @code{step}), you may encounter
another breakpoint---which could have its own command list, leading to
ambiguities about which list to execute.

@kindex silent
If the first command you specify in a command list is @code{silent}, the
usual message about stopping at a breakpoint is not printed.  This may
be desirable for breakpoints that are to print a specific message and
then continue.  If none of the remaining commands print anything, you
will see no sign that the breakpoint was reached.  @code{silent} is
meaningful only at the beginning of a breakpoint command list.

The commands @code{echo}, @code{output}, and @code{printf} allow you to
print precisely controlled output, and are often useful in silent
breakpoints.  @xref{Output, ,Commands for controlled output}.

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

@example
break foo if x>0
commands
silent
printf "x is %d\n",x
cont
end
@end example

One application for breakpoint commands is to compensate for one bug so
you can test for another.  Put a breakpoint just after the erroneous line
of code, give it a condition to detect the case in which something
erroneous has been done, and give it commands to assign correct values
to any variables that need them.  End with the @code{continue} command
so that your program does not stop, and start with the @code{silent}
command so that no output is produced.  Here is an example:

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

@ifclear CONLY
@node Breakpoint Menus
@subsection Breakpoint menus
@cindex overloading
@cindex symbol overloading

Some programming languages (notably C++) permit a single function name
to be defined several times, for application in different contexts.
This is called @dfn{overloading}.  When a function name is overloaded,
@samp{break @var{function}} is not enough to tell @value{GDBN} where you want
a breakpoint.  If you realize this will be a problem, you can use
something like @samp{break @var{function}(@var{types})} to specify which
particular version of the function you want.  Otherwise, @value{GDBN} offers
you a menu of numbered choices for different possible breakpoints, and
waits for your selection with the prompt @samp{>}.  The first two
options are always @samp{[0] cancel} and @samp{[1] all}.  Typing @kbd{1}
sets a breakpoint at each definition of @var{function}, and typing
@kbd{0} aborts the @code{break} command without setting any new
breakpoints.

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

@c FIXME! This is likely to change to show arg type lists, at least
@smallexample
(@value{GDBP}) b String::after
[0] cancel
[1] all
[2] file:String.cc; line number:867
[3] file:String.cc; line number:860
[4] file:String.cc; line number:875
[5] file:String.cc; line number:853
[6] file:String.cc; line number:846
[7] file:String.cc; line number:735
> 2 4 6
Breakpoint 1 at 0xb26c: file String.cc, line 867.
Breakpoint 2 at 0xb344: file String.cc, line 875.
Breakpoint 3 at 0xafcc: file String.cc, line 846.
Multiple breakpoints were set.
Use the "delete" command to delete unwanted
 breakpoints.
(@value{GDBP})
@end smallexample
@end ifclear

@ifclear BARETARGET
@node Error in Breakpoints
@subsection ``Cannot insert breakpoints''

@c FIXME: "cannot insert breakpoints" error, v unclear.
@c        Q in pending mail to Gilmore. ---pesch@cygnus.com, 26mar91
@c        some light may be shed by looking at instances of
@c        ONE_PROCESS_WRITETEXT.  But error message seems possible otherwise
@c        too.  pesch, 20sep91
Under some operating systems, breakpoints cannot be used in a program if
any other process is running that program.  In this situation,
attempting to run or continue a program with a breakpoint causes @value{GDBN}
to stop the other process.

When this happens, you have three ways to proceed:

@enumerate
@item
Remove or disable the breakpoints, then continue.

@item
Suspend @value{GDBN}, and copy the file containing your program to a new name.
Resume @value{GDBN} and use the @code{exec-file} command to specify that @value{GDBN}
should run your program under that name.  Then start your program again.

@c FIXME: RMS commented here "Show example".  Maybe when someone
@c explains the first FIXME: in this section...

@item
Relink your program so that the text segment is nonsharable, using the
linker option @samp{-N}.  The operating system limitation may not apply
to nonsharable executables.
@end enumerate
@end ifclear

@node Continuing and Stepping
@section Continuing and stepping

@cindex stepping
@cindex continuing
@cindex resuming execution
@dfn{Continuing} means resuming program execution until your program
completes normally.  In contrast, @dfn{stepping} means executing just
one more ``step'' of your program, where ``step'' may mean either one
line of source code, or one machine instruction (depending on what
particular command you use).  Either when continuing
or when stepping, your program may stop even sooner, due to 
@ifset BARETARGET
a breakpoint.
@end ifset
@ifclear BARETARGET
a breakpoint or a signal.  (If due to a signal, you may want to use
@code{handle}, or use @samp{signal 0} to resume execution.
@xref{Signals, ,Signals}.)
@end ifclear

@table @code
@item continue @r{[}@var{ignore-count}@r{]}
@itemx c @r{[}@var{ignore-count}@r{]}
@itemx fg @r{[}@var{ignore-count}@r{]}
@kindex continue
@kindex c
@kindex fg
Resume program execution, at the address where your program last stopped;
any breakpoints set at that address are bypassed.  The optional argument
@var{ignore-count} allows you to specify a further number of times to
ignore a breakpoint at this location; its effect is like that of
@code{ignore} (@pxref{Conditions, ,Break conditions}).

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

The synonyms @code{c} and @code{fg} are provided purely for convenience,
and have exactly the same behavior as @code{continue}.
@end table

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

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

@table @code
@item step
@kindex step
@kindex s
Continue running your program until control reaches a different source
line, then stop it and return control to @value{GDBN}.  This command is
abbreviated @code{s}.

@quotation
@emph{Warning:} If you use the @code{step} command while control is
within a function that was compiled without debugging information,
execution proceeds until control reaches a function that does have
debugging information.
@end quotation

@item step @var{count}
Continue running as in @code{step}, but do so @var{count} times.  If a
breakpoint is reached,
@ifclear BARETARGET
or a signal not related to stepping occurs before @var{count} steps,
@end ifclear
stepping stops right away.

@item next @r{[}@var{count}@r{]}
@kindex next
@kindex n
Continue to the next source line in the current (innermost) stack frame.
Similar to @code{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 @code{next} command was given.  This command is abbreviated
@code{n}.

An argument @var{count} is a repeat count, as for @code{step}.

@code{next} within a function that lacks debugging information acts like
@code{step}, but any function calls appearing within the code of the
function are executed without stopping.

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

Contrast this with the @code{return} command (@pxref{Returning,
,Returning from a function}).

@item until
@kindex until
@itemx u
@kindex u
Continue running until a source line past the current line, in the
current stack frame, is reached.  This command is used to avoid single
stepping through a loop more than once.  It is like the @code{next}
command, except that when @code{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, @code{until} will cause your program to continue execution
until the loop is exited.  In contrast, a @code{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.

@code{until} always stops your program if it attempts to exit the current
stack frame.

@code{until} may produce somewhat counterintuitive results if the order
of machine code does not match the order of the source lines.  For
example, in the following excerpt from a debugging session, the @code{f}
(@code{frame}) command shows that execution is stopped at line
@code{206}; yet when we use @code{until}, we get to line @code{195}:

@example
(@value{GDBP}) f
#0  main (argc=4, argv=0xf7fffae8) at m4.c:206
206                 expand_input();
(@value{GDBP}) until
195             for ( ; argc > 0; NEXTARG) @{
@end example

This happened because, for execution efficiency, the compiler had
generated code for the loop closure test at the end, rather than the
start, of the loop---even though the test in a C @code{for}-loop is
written before the body of the loop.  The @code{until} command appeared
to step back to the beginning of the loop when it advanced to this
expression; however, it has not really gone to an earlier
statement---not in terms of the actual machine code.

@code{until} with no argument works by means of single
instruction stepping, and hence is slower than @code{until} with an
argument.

@item until @var{location}
@itemx u @var{location}
Continue running your program until either the specified location is
reached, or the current stack frame returns.  @var{location} is any of
the forms of argument acceptable to @code{break} (@pxref{Set Breaks,
,Setting breakpoints}).  This form of the command uses breakpoints,
and hence is quicker than @code{until} without an argument.

@item stepi
@itemx si
@kindex stepi
@kindex si
Execute one machine instruction, then stop and return to the debugger.

It is often useful to do @samp{display/i $pc} when stepping by machine
instructions.  This will cause the next instruction to be executed to
be displayed automatically at each stop.  @xref{Auto Display,
,Automatic display}.

An argument is a repeat count, as in @code{step}.

@need 750
@item nexti
@itemx ni
@kindex nexti
@kindex ni
Execute one machine instruction, but if it is a function call,
proceed until the function returns.

An argument is a repeat count, as in @code{next}.
@end table

@ifset POSIX
@node Signals
@section Signals
@cindex signals

A signal is an asynchronous event that can happen in a program.  The
operating system defines the possible kinds of signals, and gives each
kind a name and a number.  For example, in Unix @code{SIGINT} is the
signal a program gets when you type an interrupt (often @kbd{C-c});
@code{SIGSEGV} is the signal a program gets from referencing a place in
memory far away from all the areas in use; @code{SIGALRM} occurs when
the alarm clock timer goes off (which happens only if your program has
requested an alarm).

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

@value{GDBN} has the ability to detect any occurrence of a signal in your
program.  You can tell @value{GDBN} in advance what to do for each kind of
signal.

@cindex handling signals
Normally, @value{GDBN} is set up to ignore non-erroneous signals like @code{SIGALRM}
(so as not to interfere with their role in the functioning of your program)
but to stop your program immediately whenever an error signal happens.
You can change these settings with the @code{handle} command.

@table @code
@item info signals
@kindex info signals
Print a table of all the kinds of signals and how @value{GDBN} has been told to
handle each one.  You can use this to see the signal numbers of all
the defined types of signals.

@item handle @var{signal} @var{keywords}@dots{}
@kindex handle
Change the way @value{GDBN} handles signal @var{signal}.  @var{signal} can be the
number of a signal or its name (with or without the @samp{SIG} at the
beginning).  The @var{keywords} say what change to make.
@end table

@c @group
The keywords allowed by the @code{handle} command can be abbreviated.
Their full names are:

@table @code
@item nostop
@value{GDBN} should not stop your program when this signal happens.  It may
still print a message telling you that the signal has come in.

@item stop
@value{GDBN} should stop your program when this signal happens.  This implies
the @code{print} keyword as well.

@item print
@value{GDBN} should print a message when this signal happens.

@item noprint
@value{GDBN} should not mention the occurrence of the signal at all.  This
implies the @code{nostop} keyword as well.

@item pass
@value{GDBN} should allow your program to see this signal; your program will be
able to handle the signal, or may be terminated if the signal is fatal
and not handled.

@item nopass
@value{GDBN} should not allow your program to see this signal.
@end table
@c @end group

When a signal stops your program, the signal is not visible until you
continue.  Your program will see the signal then, if @code{pass} is in
effect for the signal in question @emph{at that time}.  In other words,
after @value{GDBN} reports a signal, you can use the @code{handle}
command with @code{pass} or @code{nopass} to control whether that
signal will be seen by your program when you later continue it.

You can also use the @code{signal} command to prevent your program from
seeing a signal, or cause it to see a signal it normally would not see,
or to give it any signal at any time.  For example, if your program stopped
due to some sort of memory reference error, you might store correct
values into the erroneous variables and continue, hoping to see more
execution; but your program would probably terminate immediately as
a result of the fatal signal once it saw the signal.  To prevent this,
you can continue with @samp{signal 0}.  @xref{Signaling, ,Giving your
program a signal}. 
@end ifset

@node Stack
@chapter Examining the Stack

When your program has stopped, the first thing you need to know is where it
stopped and how it got there.

@cindex call stack
Each time your program performs a function call, the information about
where in your program the call was made from is saved in a block of data
called a @dfn{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 @dfn{call
stack}.

When your program stops, the @value{GDBN} commands for examining the
stack allow you to see all of this information.

@cindex selected frame
One of the stack frames is @dfn{selected} by @value{GDBN} and many
@value{GDBN} commands refer implicitly to the selected frame.  In
particular, whenever you ask @value{GDBN} for the value of a variable in
your program, the value is found in the selected frame.  There are
special @value{GDBN} commands to select whichever frame you are
interested in.

When your program stops, @value{GDBN} automatically selects the
currently executing frame and describes it briefly as the @code{frame}
command does (@pxref{Frame Info, ,Information about a frame}).

@menu
* Frames::                      Stack frames
* Backtrace::                   Backtraces
* Selection::                   Selecting a frame
* Frame Info::                  Information on a frame
@ifset MIPS
* MIPS Stack::                  MIPS machines and the function stack
@end ifset
@end menu

@node Frames
@section Stack frames

@cindex frame
@cindex stack frame
The call stack is divided up into contiguous pieces called @dfn{stack
frames}, or @dfn{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.

@cindex initial frame
@cindex outermost frame
@cindex innermost frame
When your program is started, the stack has only one frame, that of the
function @code{main}.  This is called the @dfn{initial} frame or the
@dfn{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 @dfn{innermost} frame.  This is the most
recently created of all the stack frames that still exist.

@cindex frame pointer
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 @dfn{frame pointer register} while execution is
going on in that frame.

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

@c below produces an acceptable overful hbox. --mew 13aug1993
@cindex frameless execution
Some compilers provide a way to compile functions so that they operate
without stack frames.  (For example, the @code{@value{GCC}} option
@samp{-fomit-frame-pointer} will generate functions without a frame.)
This is occasionally done with heavily used library functions to save
the frame setup time.  @value{GDBN} has limited facilities for dealing
with these function invocations.  If the innermost function invocation
has no stack frame, @value{GDBN} will nevertheless regard it as though
it had a separate frame, which is numbered zero as usual, allowing
correct tracing of the function call chain.  However, @value{GDBN} has
no provision for frameless functions elsewhere in the stack.

@node Backtrace
@section Backtraces

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

@table @code
@item backtrace
@itemx bt
@kindex backtrace
@kindex 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 @kbd{C-c}.

@item backtrace @var{n}
@itemx bt @var{n}
Similar, but print only the innermost @var{n} frames.

@item backtrace -@var{n}
@itemx bt -@var{n}
Similar, but print only the outermost @var{n} frames.
@end table

@kindex where
@kindex info stack
@kindex info s
The names @code{where} and @code{info stack} (abbreviated @code{info s})
are additional aliases for @code{backtrace}.

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

Here is an example of a backtrace.  It was made with the command
@samp{bt 3}, so it shows the innermost three frames.

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

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

@node Selection
@section Selecting a frame

Most commands for examining the stack and other data in your program work on
whichever stack frame is selected at the moment.  Here are the commands for
selecting a stack frame; all of them finish by printing a brief description
of the stack frame just selected.

@table @code
@item frame @var{n}
@itemx f @var{n}
@kindex frame
@kindex f
Select frame number @var{n}.  Recall that frame zero is the innermost
(currently executing) frame, frame one is the frame that called the
innermost one, and so on.  The highest-numbered frame is the one for
@code{main}.

@item frame @var{addr}
@itemx f @var{addr}
Select the frame at address @var{addr}.  This is useful mainly if the
chaining of stack frames has been damaged by a bug, making it
impossible for @value{GDBN} to assign numbers properly to all frames.  In
addition, this can be useful when your program has multiple stacks and
switches between them.

@ifset SPARC
On the SPARC architecture, @code{frame} needs two addresses to
select an arbitrary frame: a frame pointer and a stack pointer.
@c note to future updaters: this is conditioned on a flag
@c FRAME_SPECIFICATION_DYADIC in the tm-*.h files, currently only used
@c by SPARC, hence the specific attribution.  Generalize or list all
@c possibilities if more supported machines start doing this.
@end ifset

@item up @var{n}
@kindex up
Move @var{n} frames up the stack.  For positive numbers @var{n}, this
advances toward the outermost frame, to higher frame numbers, to frames
that have existed longer.  @var{n} defaults to one.

@item down @var{n}
@kindex down
@kindex do
Move @var{n} frames down the stack.  For positive numbers @var{n}, this
advances toward the innermost frame, to lower frame numbers, to frames
that were created more recently.  @var{n} defaults to one.  You may
abbreviate @code{down} as @code{do}.
@end table

All of these commands end by printing two lines of output describing the
frame.  The first line shows the frame number, the function name, the
arguments, and the source file and line number of execution in that
frame.  The second line shows the text of that source line.  

For example:
@smallexample
@group
(@value{GDBP}) up
#1  0x22f0 in main (argc=1, argv=0xf7fffbf4, env=0xf7fffbfc)
    at env.c:10
10              read_input_file (argv[i]);
@end group
@end smallexample

After such a printout, the @code{list} command with no arguments will
print ten lines centered on the point of execution in the frame.
@xref{List, ,Printing source lines}.

@table @code
@item up-silently @var{n}
@itemx down-silently @var{n}
@kindex down-silently
@kindex up-silently
These two commands are variants of @code{up} and @code{down},
respectively; they differ in that they do their work silently, without
causing display of the new frame.  They are intended primarily for use
in @value{GDBN} command scripts, where the output might be unnecessary and
distracting.
@end table

@node Frame Info
@section Information about a frame

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

@table @code
@item frame
@itemx f
When used without any argument, this command does not change which
frame is selected, but prints a brief description of the currently
selected stack frame.  It can be abbreviated @code{f}.  With an
argument, this command is used to select a stack frame.
@xref{Selection, ,Selecting a frame}.

@item info frame
@itemx info f
@kindex info frame
@kindex info f
This command prints a verbose description of the selected stack frame,
including the address of the frame, the addresses of the next frame down
(called by this frame) and the next frame up (caller of this frame), the
language that the source code corresponding to this frame was written in,
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.

@item info frame @var{addr}
@itemx info f @var{addr}
Print a verbose description of the frame at address @var{addr},
without selecting that frame.  The selected frame remains unchanged by
this command.

@item info args
@kindex info args
Print the arguments of the selected frame, each on a separate line.

@item info locals
@kindex info locals
Print the local variables of the selected frame, each on a separate
line.  These are all variables (declared either static or automatic)
accessible at the point of execution of the selected frame.

@ifclear CONLY
@item info catch
@kindex info catch
@cindex catch exceptions
@cindex exception handlers
Print a list of all the exception handlers that are active in the
current stack frame at the current point of execution.  To see other
exception handlers, visit the associated frame (using the @code{up},
@code{down}, or @code{frame} commands); then type @code{info catch}.
@xref{Exception Handling, ,Breakpoints and exceptions}.
@end ifclear
@end table

@ifset MIPS
@node MIPS Stack
@section MIPS machines and the function stack

@cindex stack on MIPS
@cindex MIPS stack
MIPS based computers use an unusual stack frame, which sometimes
requires @value{GDBN} to search backward in the object code to find the
beginning of a function.

@cindex response time, MIPS debugging
To improve response time (especially for embedded applications, where
@value{GDBN} may be restricted to a slow serial line for this search)
you may want to limit the size of this search, using one of these
commands:
@c FIXME! So what happens when GDB does *not* find the beginning of a
@c function?

@cindex @code{heuristic-fence-post} (MIPS)
@table @code
@item set heuristic-fence-post @var{limit}
Restrict @var{GDBN} to examining at most @var{limit} bytes in its search
for the beginning of a function.  A value of @code{0} (the default)
means there is no limit.

@item show heuristic-fence-post
Display the current limit.
@end table

@noindent
These commands are available @emph{only} when @value{GDBN} is configured
for debugging programs on MIPS processors.
@end ifset

@node Source
@chapter Examining Source Files

@value{GDBN} can print parts of your program's source, since the debugging
information recorded in the program tells @value{GDBN} what source files were
used to build it.  When your program stops, @value{GDBN} spontaneously prints
the line where it stopped.  Likewise, when you select a stack frame
(@pxref{Selection, ,Selecting a frame}), @value{GDBN} prints the line where
execution in that frame has stopped.  You can print other portions of
source files by explicit command.

@ifclear DOSHOST
If you use @value{GDBN} through its GNU Emacs interface, you may prefer to use
Emacs facilities to view source; @pxref{Emacs, ,Using @value{GDBN} under GNU
Emacs}.
@end ifclear

@menu
* List::                        Printing source lines
@ifclear DOSHOST
* Search::                      Searching source files
@end ifclear

* Source Path::                 Specifying source directories
* Machine Code::                Source and machine code
@end menu

@node List
@section Printing source lines

@kindex list
@kindex l
To print lines from a source file, use the @code{list} command
(abbreviated @code{l}).  There are several ways to specify what part
of the file you want to print.

Here are the forms of the @code{list} command most commonly used:

@table @code
@item list @var{linenum}
Print lines centered around line number @var{linenum} in the
current source file.

@item list @var{function}
Print lines centered around the beginning of function
@var{function}.

@item list
Print more lines.  If the last lines printed were printed with a
@code{list} command, this prints lines following the last lines
printed; however, if the last line printed was a solitary line printed
as part of displaying a stack frame (@pxref{Stack, ,Examining the
Stack}), this prints lines centered around that line.

@item list -
Print lines just before the lines last printed.
@end table

By default, @value{GDBN} prints ten source lines with any of these forms of
the @code{list} command.  You can change this using @code{set listsize}:

@table @code
@item set listsize @var{count}
@kindex set listsize
Make the @code{list} command display @var{count} source lines (unless
the @code{list} argument explicitly specifies some other number).

@item show listsize
@kindex show listsize
Display the number of lines that @code{list} will currently display by
default.
@end table

Repeating a @code{list} command with @key{RET} discards the argument,
so it is equivalent to typing just @code{list}.  This is more useful
than listing the same lines again.  An exception is made for an
argument of @samp{-}; that argument is preserved in repetition so that
each repetition moves up in the source file.

@cindex linespec
In general, the @code{list} command expects you to supply zero, one or two
@dfn{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 @code{list}:

@table @code
@item list @var{linespec}
Print lines centered around the line specified by @var{linespec}.

@item list @var{first},@var{last}
Print lines from @var{first} to @var{last}.  Both arguments are
linespecs.

@item list ,@var{last}
Print lines ending with @var{last}.

@item list @var{first},
Print lines starting with @var{first}.

@item list +
Print lines just after the lines last printed.

@item list -
Print lines just before the lines last printed.

@item list
As described in the preceding table.
@end table

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

@table @code
@item @var{number}
Specifies line @var{number} of the current source file.
When a @code{list} command has two linespecs, this refers to
the same source file as the first linespec.

@item +@var{offset}
Specifies the line @var{offset} lines after the last line printed.
When used as the second linespec in a @code{list} command that has
two, this specifies the line @var{offset} lines down from the
first linespec.

@item -@var{offset}
Specifies the line @var{offset} lines before the last line printed.

@item @var{filename}:@var{number}
Specifies line @var{number} in the source file @var{filename}.

@item @var{function}
@c FIXME: "of the open-brace" is C-centric.  When we add other langs...
Specifies the line of the open-brace that begins the body of the
function @var{function}.

@item @var{filename}:@var{function}
Specifies the line of the open-brace that begins the body of the
function @var{function} in the file @var{filename}.  You only need the
file name with a function name to avoid ambiguity when there are
identically named functions in different source files.

@item *@var{address}
Specifies the line containing the program address @var{address}.
@var{address} may be any expression.
@end table

@ifclear DOSHOST
@node Search
@section Searching source files
@cindex searching
@kindex reverse-search

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

@table @code
@item forward-search @var{regexp}
@itemx search @var{regexp}
@kindex search
@kindex forward-search
The command @samp{forward-search @var{regexp}} checks each line,
starting with the one following the last line listed, for a match for
@var{regexp}.  It lists the line that is found.  You can use
synonym @samp{search @var{regexp}} or abbreviate the command name as
@code{fo}.

@item reverse-search @var{regexp}
The command @samp{reverse-search @var{regexp}} checks each line, starting
with the one before the last line listed and going backward, for a match
for @var{regexp}.  It lists the line that is found.  You can abbreviate
this command as @code{rev}.
@end table
@end ifclear

@node Source Path
@section Specifying source directories

@cindex source path
@cindex directories for source files
Executable programs sometimes do not record the directories of the source
files from which they were compiled, just the names.  Even when they do,
the directories could be moved between the compilation and your debugging
session.  @value{GDBN} has a list of directories to search for source files;
this is called the @dfn{source path}.  Each time @value{GDBN} 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 @emph{not} used for this purpose.  Neither is
the current working directory, unless it happens to be in the source
path.

If @value{GDBN} cannot find a source file in the source path, and the object
program records a directory, @value{GDBN} tries that directory too.  If the
source path is empty, and there is no record of the compilation
directory, @value{GDBN} will, as a last resort, look in the current
directory.

Whenever you reset or rearrange the source path, @value{GDBN} will clear out
any information it has cached about where source files are found, where
each line is in the file, etc.

@kindex directory
When you start @value{GDBN}, its source path is empty.
To add other directories, use the @code{directory} command.

@table @code
@item directory @var{dirname} @dots{}
Add directory @var{dirname} to the front of the source path.  Several
directory names may be given to this command, separated by @samp{:} or
whitespace.  You may specify a directory that is already in the source
path; this moves it forward, so it will be searched sooner.

You can use the string @samp{$cdir} to refer to the compilation
directory (if one is recorded), and @samp{$cwd} to refer to the current
working directory.  @samp{$cwd} is not the same as @samp{.}---the former
tracks the current working directory as it changes during your @value{GDBN}
session, while the latter is immediately expanded to the current
directory at the time you add an entry to the source path.

@item directory
Reset the source path to empty again.  This requires confirmation.

@c RET-repeat for @code{directory} is explicitly disabled, but since
@c repeating it would be a no-op we do not say that.  (thanks to RMS)

@item show directories
@kindex show directories
Print the source path: show which directories it contains.
@end table

If your source path is cluttered with directories that are no longer of
interest, @value{GDBN} may sometimes cause confusion by finding the wrong
versions of source.  You can correct the situation as follows:

@enumerate
@item
Use @code{directory} with no argument to reset the source path to empty.

@item
Use @code{directory} with suitable arguments to reinstall the
directories you want in the source path.  You can add all the
directories in one command.
@end enumerate

@node Machine Code
@section Source and machine code

You can use the command @code{info line} to map source lines to program
addresses (and vice versa), and the command @code{disassemble} to display
a range of addresses as machine instructions.

@table @code
@item info line @var{linespec}
@kindex info line
Print the starting and ending addresses of the compiled code for
source line @var{linespec}.  You can specify source lines in any of
the ways understood by the @code{list} command (@pxref{List, ,Printing
source lines}).
@end table

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

@smallexample
(@value{GDBP}) info line m4_changecom
Line 895 of "builtin.c" starts at pc 0x634c and ends at 0x6350.
@end smallexample

@noindent
We can also inquire (using @code{*@var{addr}} as the form for
@var{linespec}) what source line covers a particular address:
@smallexample
(@value{GDBP}) info line *0x63ff
Line 926 of "builtin.c" starts at pc 0x63e4 and ends at 0x6404.
@end smallexample

@cindex @code{$_} and @code{info line}
After @code{info line}, the default address for the @code{x} command
is changed to the starting address of the line, so that @samp{x/i} is
sufficient to begin examining the machine code (@pxref{Memory,
,Examining memory}).  Also, this address is saved as the value of the
convenience variable @code{$_} (@pxref{Convenience Vars, ,Convenience
variables}).

@table @code
@kindex disassemble
@item disassemble
@cindex assembly instructions
@cindex instructions, assembly
@cindex machine instructions
@cindex listing machine instructions
This specialized command dumps a range of memory as machine
instructions.  The default memory range is the function surrounding the
program counter of the selected frame.  A single argument to this
command is a program counter value; the function surrounding this value
will be dumped.  Two arguments specify a range of addresses (first
inclusive, second exclusive) to dump.
@end table

@ifclear H8EXCLUSIVE
We can use @code{disassemble} to inspect the object code
range shown in the last @code{info line} example (the example
shows SPARC machine instructions):


@smallexample
(@value{GDBP}) disas 0x63e4 0x6404
Dump of assembler code from 0x63e4 to 0x6404:
0x63e4 <builtin_init+5340>:     ble 0x63f8 <builtin_init+5360>
0x63e8 <builtin_init+5344>:     sethi %hi(0x4c00), %o0
0x63ec <builtin_init+5348>:     ld [%i1+4], %o0
0x63f0 <builtin_init+5352>:     b 0x63fc <builtin_init+5364>
0x63f4 <builtin_init+5356>:     ld [%o0+4], %o0
0x63f8 <builtin_init+5360>:     or %o0, 0x1a4, %o0
0x63fc <builtin_init+5364>:     call 0x9288 <path_search>
0x6400 <builtin_init+5368>:     nop
End of assembler dump.
@end smallexample
@end ifclear

@ifset H8EXCLUSIVE
For example, here is the beginning of the output for the
disassembly of a function @code{fact}:


@smallexample
(@value{GDBP}) disas fact
Dump of assembler code for function fact:
to 0x808c:
0x802c <fact>: 6d f2            mov.w r2,@@-r7
0x802e <fact+2>:  6d f3         mov.w r3,@@-r7
0x8030 <fact+4>:  6d f6         mov.w r6,@@-r7
0x8032 <fact+6>:  0d 76         mov.w r7,r6
0x8034 <fact+8>:  6f 70 00 08   mov.w @@(0x8,r7),r0
0x8038 <fact+12>  19 11         sub.w   r1,r1
 .
 .
 . 
@end smallexample
@end ifset

@node Data
@chapter Examining Data

@cindex printing data
@cindex examining data
@kindex print
@kindex inspect
@c "inspect" is not quite a synonym if you are using Epoch, which we do not
@c document because it is nonstandard...  Under Epoch it displays in a
@c different window or something like that.
The usual way to examine data in your program is with the @code{print}
command (abbreviated @code{p}), or its synonym @code{inspect}.  
@ifclear CONLY
It evaluates and prints the value of an expression of the language your
program is written in (@pxref{Languages, ,Using @value{GDBN} with Different
Languages}).
@end ifclear

@table @code
@item print @var{exp}
@itemx print /@var{f} @var{exp}
@var{exp} is an expression (in the source language).  By default the
value of @var{exp} is printed in a format appropriate to its data type;
you can choose a different format by specifying @samp{/@var{f}}, where
@var{f} is a letter specifying the format; @pxref{Output Formats,,Output
formats}.

@item print
@itemx print /@var{f}
If you omit @var{exp}, @value{GDBN} displays the last value again (from the
@dfn{value history}; @pxref{Value History, ,Value history}).  This allows you to
conveniently inspect the same value in an alternative format.
@end table

A more low-level way of examining data is with the @code{x} command.
It examines data in memory at a specified address and prints it in a
specified format.  @xref{Memory, ,Examining memory}.

If you are interested in information about types, or about how the fields
of a struct
@ifclear CONLY
or class
@end ifclear
are declared, use the @code{ptype @var{exp}}
command rather than @code{print}. @xref{Symbols, ,Examining the Symbol Table}.

@menu
* Expressions::                 Expressions
* Variables::                   Program variables
* Arrays::                      Artificial arrays
* Output Formats::              Output formats
* Memory::                      Examining memory
* Auto Display::                Automatic display
* Print Settings::              Print settings
* Value History::               Value history
* Convenience Vars::            Convenience variables
* Registers::                   Registers
@ifclear HAVE-FLOAT
* Floating Point Hardware::     Floating point hardware
@end ifclear
@end menu

@node Expressions
@section Expressions

@cindex expressions
@code{print} and many other @value{GDBN} commands accept an expression and
compute its value.  Any kind of constant, variable or operator defined
by the programming language you are using is valid in an expression in
@value{GDBN}.  This includes conditional expressions, function calls, casts
and string constants.  It unfortunately does not include symbols defined
by preprocessor @code{#define} commands.

@ifclear CONLY
Because C is so widespread, most of the expressions shown in examples in
this manual are in C.  @xref{Languages, , Using @value{GDBN} with Different
Languages}, for information on how to use expressions in other
languages.

In this section, we discuss operators that you can use in @value{GDBN}
expressions regardless of your programming language.

Casts are supported in all languages, not just in C, because it is so
useful to cast a number into a pointer so as to examine a structure
at that address in memory.
@c FIXME: casts supported---Mod2 true?
@end ifclear

@value{GDBN} supports these operators in addition to those of programming
languages:

@table @code
@item @@
@samp{@@} is a binary operator for treating parts of memory as arrays.
@xref{Arrays, ,Artificial arrays}, for more information.

@item ::
@samp{::} allows you to specify a variable in terms of the file or
function where it is defined.  @xref{Variables, ,Program variables}.

@item @{@var{type}@} @var{addr}
@cindex @{@var{type}@}
@cindex type casting memory
@cindex memory, viewing as typed object
@cindex casts, to view memory
Refers to an object of type @var{type} stored at address @var{addr} in
memory.  @var{addr} may be any expression whose value is an integer or
pointer (but parentheses are required around binary operators, just as in
a cast).  This construct is allowed regardless of what kind of data is
normally supposed to reside at @var{addr}.
@end table

@node Variables
@section 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
(@pxref{Selection, ,Selecting a frame}); 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

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

@noindent
you can examine and use the variable @code{a} whenever your program is
executing within the function @code{foo}, but you can only use or
examine the variable @code{b} while your program is executing inside
the block where @code{b} is declared.

@cindex variable name conflict
There is an exception: you can refer to a variable or function whose
scope is a single source file even if the current execution point is not
in this file.  But it is possible to have more than one such variable or
function with the same name (in different source files).  If that
happens, referring to that name has unpredictable effects.  If you wish,
you can specify a static variable in a particular function or file,
using the colon-colon notation:

@cindex colon-colon
@iftex
@c info cannot cope with a :: index entry, but why deprive hard copy readers?
@kindex ::
@end iftex
@example
@var{file}::@var{variable}
@var{function}::@var{variable}
@end example

@noindent
Here @var{file} or @var{function} is the name of the context for the
static @var{variable}.  In the case of file names, you can use quotes to
make sure @value{GDBN} parses the file name as a single word---for example,
to print a global value of @code{x} defined in @file{f2.c}:

@example
(@value{GDBP}) p 'f2.c'::x
@end example

@ifclear CONLY
@cindex C++ scope resolution
This use of @samp{::} is very rarely in conflict with the very similar
use of the same notation in C++.  @value{GDBN} also supports use of the C++
scope resolution operator in @value{GDBN} expressions.
@c FIXME: Um, so what happens in one of those rare cases where it's in
@c conflict??  --mew
@end ifclear

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

@node Arrays
@section Artificial arrays

@cindex artificial array
@kindex @@
It is often useful to print out several successive objects of the
same type in memory; a section of an array, or an array of
dynamically determined size for which only a pointer exists in the
program.

You can do this by referring to a contiguous span of memory as an
@dfn{artificial array}, using the binary operator @samp{@@}.  The left
operand of @samp{@@} should be the first element of the desired array,
as an individual object.  The right operand should be the desired length
of the array.  The result is an array value whose elements are all of
the type of the left argument.  The first element is actually the left
argument; the second element comes from bytes of memory immediately
following those that hold the first element, and so on.  Here is an
example.  If a program says

@example
int *array = (int *) malloc (len * sizeof (int));
@end example

@noindent
you can print the contents of @code{array} with

@example
p *array@@len
@end example

The left operand of @samp{@@} must reside in memory.  Array values made
with @samp{@@} in this way behave just like other arrays in terms of
subscripting, and are coerced to pointers when used in expressions.
Artificial arrays most often appear in expressions via the value history
(@pxref{Value History, ,Value history}), after printing one out.

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

@example
set $i = 0
p dtab[$i++]->fv
@key{RET}
@key{RET}
@dots{}
@end example

@node Output Formats
@section Output formats

@cindex formatted output
@cindex output formats
By default, @value{GDBN} prints a value according to its data type.  Sometimes
this is not what you want.  For example, you might want to print a number
in hex, or a pointer in decimal.  Or you might want to view data in memory
at a certain address as a character string or as an instruction.  To do
these things, specify an @dfn{output format} when you print a value.

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

@table @code
@item x
Regard the bits of the value as an integer, and print the integer in
hexadecimal.

@item d
Print as integer in signed decimal.

@item u
Print as integer in unsigned decimal.

@item o
Print as integer in octal.

@item t
Print as integer in binary.  The letter @samp{t} stands for ``two''.
@footnote{@samp{b} cannot be used because these format letters are also
used with the @code{x} command, where @samp{b} stands for ``byte'';
@pxref{Memory,,Examining memory}.}

@item a
Print as an address, both absolute in hex and as an offset from the
nearest preceding symbol.  This format can be used to discover where (in
what function) an unknown address is located:

@example
(@value{GDBP}) p/a 0x54320
$3 = 0x54320 <_initialize_vx+396>
@end example

@item c
Regard as an integer and print it as a character constant.

@item f
Regard the bits of the value as a floating point number and print
using typical floating point syntax.
@end table

For example, to print the program counter in hex (@pxref{Registers}), type

@example
p/x $pc
@end example

@noindent
Note that no space is required before the slash; this is because command
names in @value{GDBN} cannot contain a slash.

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

@node Memory
@section Examining memory

You can use the command @code{x} (for ``examine'') to examine memory in
any of several formats, independently of your program's data types.

@cindex examining memory
@table @code
@kindex x
@item x/@var{nfu} @var{addr}
@itemx x @var{addr}
@itemx x
Use the @code{x} command to examine memory.
@end table

@var{n}, @var{f}, and @var{u} are all optional parameters that specify how
much memory to display and how to format it; @var{addr} is an
expression giving the address where you want to start displaying memory.
If you use defaults for @var{nfu}, you need not type the slash @samp{/}.
Several commands set convenient defaults for @var{addr}.

@table @r
@item @var{n}, the repeat count
The repeat count is a decimal integer; the default is 1.  It specifies
how much memory (counting by units @var{u}) to display.
@c This really is **decimal**; unaffected by 'set radix' as of GDB
@c 4.1.2.

@item @var{f}, the display format
The display format is one of the formats used by @code{print},
or @samp{s} (null-terminated string) or @samp{i} (machine instruction).
The default is @samp{x} (hexadecimal) initially, or the format from the
last time you used either @code{x} or @code{print}.

@item @var{u}, the unit size
The unit size is any of

@table @code
@item b
Bytes.
@item h
Halfwords (two bytes).
@item w
Words (four bytes).  This is the initial default.
@item g
Giant words (eight bytes).
@end table

Each time you specify a unit size with @code{x}, that size becomes the
default unit the next time you use @code{x}.  (For the @samp{s} and
@samp{i} formats, the unit size is ignored and is normally not written.)

@item @var{addr}, starting display address
@var{addr} is the address where you want @value{GDBN} to begin displaying
memory.  The expression need not have a pointer value (though it may);
it is always interpreted as an integer address of a byte of memory.
@xref{Expressions, ,Expressions}, for more information on expressions.  The default for
@var{addr} is usually just after the last address examined---but several
other commands also set the default address: @code{info breakpoints} (to
the address of the last breakpoint listed), @code{info line} (to the
starting address of a line), and @code{print} (if you use it to display
a value from memory).
@end table

For example, @samp{x/3uh 0x54320} is a request to display three halfwords
(@code{h}) of memory, formatted as unsigned decimal integers (@samp{u}),
starting at address @code{0x54320}.  @samp{x/4xw $sp} prints the four
words (@samp{w}) of memory above the stack pointer (here, @samp{$sp};
@pxref{Registers}) in hexadecimal (@samp{x}).

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

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

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

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

If the @code{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.

@node Auto Display
@section Automatic display
@cindex automatic display
@cindex display of expressions

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 @dfn{automatic
display list} so that @value{GDBN} will print its value each time your program stops.
Each expression added to the list is given a number to identify it;
to remove an expression from the list, you specify that number.
The automatic display looks like this:

@example
2: foo = 38
3: bar[5] = (struct hack *) 0x3804
@end example

@noindent
This display shows item numbers, expressions and their current values.  As with
displays you request manually using @code{x} or @code{print}, you can
specify the output format you prefer; in fact, @code{display} decides
whether to use @code{print} or @code{x} depending on how elaborate your
format specification is---it uses @code{x} if you specify a unit size,
or one of the two formats (@samp{i} and @samp{s}) that are only
supported by @code{x}; otherwise it uses @code{print}.

@table @code
@item display @var{exp}
@kindex display
Add the expression @var{exp} to the list of expressions to display
each time your program stops.  @xref{Expressions, ,Expressions}.

@code{display} will not repeat if you press @key{RET} again after using it.

@item display/@var{fmt} @var{exp}
For @var{fmt} specifying only a display format and not a size or
count, add the expression @var{exp} to the auto-display list but
arrange to display it each time in the specified format @var{fmt}.
@xref{Output Formats,,Output formats}.

@item display/@var{fmt} @var{addr}
For @var{fmt} @samp{i} or @samp{s}, or including a unit-size or a
number of units, add the expression @var{addr} as a memory address to
be examined each time your program stops.  Examining means in effect
doing @samp{x/@var{fmt} @var{addr}}.  @xref{Memory, ,Examining memory}.
@end table

For example, @samp{display/i $pc} can be helpful, to see the machine
instruction about to be executed each time execution stops (@samp{$pc}
is a common name for the program counter; @pxref{Registers}).

@table @code
@item undisplay @var{dnums}@dots{}
@itemx delete display @var{dnums}@dots{}
@kindex delete display
@kindex undisplay
Remove item numbers @var{dnums} from the list of expressions to display.

@code{undisplay} will not repeat if you press @key{RET} after using it.
(Otherwise you would just get the error @samp{No display number @dots{}}.)

@item disable display @var{dnums}@dots{}
@kindex disable display
Disable the display of item numbers @var{dnums}.  A disabled display
item is not printed automatically, but is not forgotten.  It may be
enabled again later.

@item enable display @var{dnums}@dots{}
@kindex enable display
Enable display of item numbers @var{dnums}.  It becomes effective once
again in auto display of its expression, until you specify otherwise.

@item display
Display the current values of the expressions on the list, just as is
done when your program stops.

@item info display
@kindex 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.
@end table

If a display expression refers to local variables, then it does not make
sense outside the lexical context for which it was set up.  Such an
expression is disabled when execution enters a context where one of its
variables is not defined.  For example, if you give the command
@code{display last_char} while inside a function with an argument
@code{last_char}, then this argument will be displayed while your program
continues to stop inside that function.  When it stops elsewhere---where
there is no variable @code{last_char}---display is disabled.  The next time
your program stops where @code{last_char} is meaningful, you can enable the
display expression once again.

@node Print Settings
@section Print settings

@cindex format options
@cindex print settings
@value{GDBN} provides the following ways to control how arrays, structures,
and symbols are printed.

@noindent
These settings are useful for debugging programs in any language:

@table @code
@item set print address
@itemx set print address on
@kindex set print address
@value{GDBN} will print memory addresses showing the location of stack
traces, structure values, pointer values, breakpoints, and so forth,
even when it also displays the contents of those addresses.  The default
is on.  For example, this is what a stack frame display looks like, with
@code{set print address on}:

@smallexample
@group
(@value{GDBP}) f
#0  set_quotes (lq=0x34c78 "<<", rq=0x34c88 ">>")
    at input.c:530
530         if (lquote != def_lquote)
@end group
@end smallexample

@item set print address off
Do not print addresses when displaying their contents.  For example,
this is the same stack frame displayed with @code{set print address off}:

@smallexample
@group
(@value{GDBP}) set print addr off
(@value{GDBP}) f
#0  set_quotes (lq="<<", rq=">>") at input.c:530
530         if (lquote != def_lquote)
@end group
@end smallexample

You can use @samp{set print address off} to eliminate all machine
dependent displays from the @value{GDBN} interface.  For example, with
@code{print address off}, you should get the same text for backtraces on
all machines---whether or not they involve pointer arguments.

@item show print address
@kindex show print address
Show whether or not addresses are to be printed.
@end table

When @value{GDBN} prints a symbolic address, it normally prints the
closest earlier symbol plus an offset.  If that symbol does not uniquely
identify the address (for example, it is a name whose scope is a single
source file), you may need to disambiguate.  One way to do this is with
@code{info line}, for example @code{info line *0x4537}.  Alternately,
you can set @value{GDBN} to print the source file and line number when
it prints a symbolic address:

@table @code
@item set print symbol-filename on
@kindex set print symbol-filename
Tell @value{GDBN} to print the source file name and line number of a
symbol in the symbolic form of an address.

@item set print symbol-filename off
Do not print source file name and line number of a symbol.  This is the
default.

@item show print symbol-filename
@kindex show print symbol-filename
Show whether or not @value{GDBN} will print the source file name and
line number of a symbol in the symbolic form of an address.
@end table

Also, you may wish to see the symbolic form only if the address being
printed is reasonably close to the closest earlier symbol:

@table @code
@item set print max-symbolic-offset @var{max-offset}
@kindex set print max-symbolic-offset
Tell @value{GDBN} to only display the symbolic form of an address if the
offset between the closest earlier symbol and the address is less than
@var{max-offset}.  The default is 0, which means to always print the
symbolic form of an address, if any symbol precedes it.

@item show print max-symbolic-offset
@kindex show print max-symbolic-offset
Ask how large the maximum offset is that @value{GDBN} will print in a
symbolic address.
@end table

@table @code
@item set print array
@itemx set print array on
@kindex set print array
@value{GDBN} will pretty-print arrays.  This format is more convenient to read,
but uses more space.  The default is off.

@item set print array off
Return to compressed format for arrays.

@item show print array
@kindex show print array
Show whether compressed or pretty format is selected for displaying
arrays.

@item set print elements @var{number-of-elements}
@kindex set print elements
If @value{GDBN} is printing a large array, it will stop printing after it has
printed the number of elements set by the @code{set print elements} command.
This limit also applies to the display of strings.
Setting the number of elements to zero means that the printing is unlimited.

@item show print elements
@kindex show print elements
Display the number of elements of a large array that @value{GDBN} will print
before losing patience.

@item set print pretty on
@kindex set print pretty
Cause @value{GDBN} to print structures in an indented format with one member per
line, like this:

@smallexample
@group
$1 = @{
  next = 0x0,
  flags = @{
    sweet = 1,
    sour = 1
  @},
  meat = 0x54 "Pork"
@}
@end group
@end smallexample

@item set print pretty off
Cause @value{GDBN} to print structures in a compact format, like this:

@smallexample
@group
$1 = @{next = 0x0, flags = @{sweet = 1, sour = 1@}, \
meat = 0x54 "Pork"@}
@end group
@end smallexample

@noindent
This is the default format.

@item show print pretty
@kindex show print pretty
Show which format @value{GDBN} will use to print structures.

@item set print sevenbit-strings on
@kindex set print sevenbit-strings
Print using only seven-bit characters; if this option is set,
@value{GDBN} will display any eight-bit characters (in strings or character
values) using the notation @code{\}@var{nnn}.  For example, @kbd{M-a} is
displayed as @code{\341}.

@item set print sevenbit-strings off
Print using either seven-bit or eight-bit characters, as required.  This
is the default.

@item show print sevenbit-strings
@kindex show print sevenbit-strings
Show whether or not @value{GDBN} will print only seven-bit characters.

@item set print union on
@kindex set print union
Tell @value{GDBN} to print unions which are contained in structures.  This is the
default setting.

@item set print union off
Tell @value{GDBN} not to print unions which are contained in structures.

@item show print union
@kindex show print union
Ask @value{GDBN} whether or not it will print unions which are contained in
structures.

For example, given the declarations

@smallexample
typedef enum @{Tree, Bug@} Species;
typedef enum @{Big_tree, Acorn, Seedling@} Tree_forms;
typedef enum @{Caterpillar, Cocoon, Butterfly@} 
              Bug_forms;

struct thing @{
  Species it;
  union @{
    Tree_forms tree;
    Bug_forms bug;
  @} form;
@};

struct thing foo = @{Tree, @{Acorn@}@};
@end smallexample

@noindent
with @code{set print union on} in effect @samp{p foo} would print

@smallexample
$1 = @{it = Tree, form = @{tree = Acorn, bug = Cocoon@}@}
@end smallexample

@noindent
and with @code{set print union off} in effect it would print

@smallexample
$1 = @{it = Tree, form = @{...@}@}
@end smallexample
@end table

@ifclear CONLY
@need 1000
@noindent
These settings are of interest when debugging C++ programs:

@table @code
@item set print demangle
@itemx set print demangle on
@kindex set print demangle
Print C++ names in their source form rather than in the encoded
(``mangled'') form passed to the assembler and linker for type-safe
linkage.  The default is @samp{on}.

@item show print demangle
@kindex show print demangle
Show whether C++ names will be printed in mangled or demangled form.

@item set print asm-demangle
@itemx set print asm-demangle on
@kindex set print asm-demangle
Print C++ names in their source form rather than their mangled form, even
in assembler code printouts such as instruction disassemblies.
The default is off.

@item show print asm-demangle
@kindex show print asm-demangle
Show whether C++ names in assembly listings will be printed in mangled
or demangled form.

@item set demangle-style @var{style}
@kindex set demangle-style
@cindex C++ symbol decoding style
@cindex symbol decoding style, C++
Choose among several encoding schemes used by different compilers to
represent C++ names.  The choices for @var{style} are currently:

@table @code
@item auto
Allow @value{GDBN} to choose a decoding style by inspecting your program.

@item gnu
Decode based on the GNU C++ compiler (@code{g++}) encoding algorithm.

@item lucid
Decode based on the Lucid C++ compiler (@code{lcc}) encoding algorithm.

@item arm
Decode using the algorithm in the @cite{C++ Annotated Reference Manual}.
@strong{Warning:} this setting alone is not sufficient to allow
debugging @code{cfront}-generated executables.  @value{GDBN} would
require further enhancement to permit that.
@end table

@item show demangle-style
@kindex show demangle-style
Display the encoding style currently in use for decoding C++ symbols.

@item set print object
@itemx set print object on
@kindex set print object
When displaying a pointer to an object, identify the @emph{actual}
(derived) type of the object rather than the @emph{declared} type, using
the virtual function table.

@item set print object off
Display only the declared type of objects, without reference to the
virtual function table.  This is the default setting.

@item show print object
@kindex show print object
Show whether actual, or declared, object types will be displayed.

@item set print vtbl
@itemx set print vtbl on
@kindex set print vtbl
Pretty print C++ virtual function tables.  The default is off.

@item set print vtbl off
Do not pretty print C++ virtual function tables.

@item show print vtbl
@kindex show print vtbl
Show whether C++ virtual function tables are pretty printed, or not.
@end table
@end ifclear

@node Value History
@section Value history

@cindex value history
Values printed by the @code{print} command are saved in the @value{GDBN} @dfn{value
history} so that you can refer to them in other expressions.  Values are
kept until the symbol table is re-read or discarded (for example with
the @code{file} or @code{symbol-file} commands).  When the symbol table
changes, the value history is discarded, since the values may contain
pointers back to the types defined in the symbol table.

@cindex @code{$}
@cindex @code{$$}
@cindex history number
The values printed are given @dfn{history numbers} by which you can
refer to them.  These are successive integers starting with one.
@code{print} shows you the history number assigned to a value by
printing @samp{$@var{num} = } before the value; here @var{num} is the
history number.

To refer to any previous value, use @samp{$} followed by the value's
history number.  The way @code{print} labels its output is designed to
remind you of this.  Just @code{$} refers to the most recent value in
the history, and @code{$$} refers to the value before that.
@code{$$@var{n}} refers to the @var{n}th value from the end; @code{$$2}
is the value just prior to @code{$$}, @code{$$1} is equivalent to
@code{$$}, and @code{$$0} is equivalent to @code{$}.

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

@example
p *$
@end example

If you have a chain of structures where the component @code{next} points
to the next one, you can print the contents of the next one with this:

@example
p *$.next
@end example

@noindent
You can print successive links in the chain by repeating this
command---which you can do by just typing @key{RET}.

Note that the history records values, not expressions.  If the value of
@code{x} is 4 and you type these commands:

@example
print x
set x=5
@end example

@noindent
then the value recorded in the value history by the @code{print} command
remains 4 even though the value of @code{x} has changed.

@table @code
@kindex show values
@item show values
Print the last ten values in the value history, with their item numbers.
This is like @samp{p@ $$9} repeated ten times, except that @code{show
values} does not change the history.

@item show values @var{n}
Print ten history values centered on history item number @var{n}.

@item show values +
Print ten history values just after the values last printed.  If no more
values are available, produces no display.
@end table

Pressing @key{RET} to repeat @code{show values @var{n}} has exactly the
same effect as @samp{show values +}.

@node Convenience Vars
@section Convenience variables

@cindex convenience variables
@value{GDBN} provides @dfn{convenience variables} that you can use within
@value{GDBN} to hold on to a value and refer to it later.  These variables
exist entirely within @value{GDBN}; they are not part of your program, and
setting a convenience variable has no direct effect on further execution
of your program.  That is why you can use them freely.

Convenience variables are prefixed with @samp{$}.  Any name preceded by
@samp{$} can be used for a convenience variable, unless it is one of
the predefined machine-specific register names (@pxref{Registers}).
(Value history references, in contrast, are @emph{numbers} preceded
by @samp{$}.  @xref{Value History, ,Value history}.)

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

@example
set $foo = *object_ptr
@end example

@noindent
would save in @code{$foo} the value contained in the object pointed to by
@code{object_ptr}.

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

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

@table @code
@item show convenience
@kindex show convenience
Print a list of convenience variables used so far, and their values.
Abbreviated @code{show con}.
@end table

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

@example
set $i = 0
print bar[$i++]->contents
@i{@dots{} repeat that command by typing @key{RET}.}
@end example

Some convenience variables are created automatically by @value{GDBN} and given
values likely to be useful.

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

@item $__
@kindex $__
The variable @code{$__} is automatically set by the @code{x} command
to the value found in the last address examined.  Its type is chosen
to match the format in which the data was printed.
@end table

@node Registers
@section Registers

@cindex registers
You can refer to machine register contents, in expressions, as variables
with names starting with @samp{$}.  The names of registers are different
for each machine; use @code{info registers} to see the names used on
your machine.

@table @code
@item info registers
@kindex info registers
Print the names and values of all registers except floating-point
registers (in the selected stack frame).

@item info all-registers
@kindex info all-registers
@cindex floating point registers
Print the names and values of all registers, including floating-point
registers.

@item info registers @var{regname} @dots{}
Print the relativized value of each specified register @var{regname}.  
@var{regname} may be any register name valid on the machine you are using, with
or without the initial @samp{$}.
@end table

@value{GDBN} has four ``standard'' register names that are available (in
expressions) on most machines---whenever they do not conflict with an
architecture's canonical mnemonics for registers.  The register names
@code{$pc} and @code{$sp} are used for the program counter register and
the stack pointer.  @code{$fp} is used for a register that contains a
pointer to the current stack frame, and @code{$ps} is used for a
register that contains the processor status.  For example,
you could print the program counter in hex with

@example
p/x $pc
@end example

@noindent
or print the instruction to be executed next with

@example
x/i $pc
@end example

@noindent
or add four to the stack pointer@footnote{This is a way of removing
one word from the stack, on machines where stacks grow downward in
memory (most machines, nowadays).  This assumes that the innermost
stack frame is selected; setting @code{$sp} is not allowed when other
stack frames are selected.  To pop entire frames off the stack,
regardless of machine architecture, use @code{return};
@pxref{Returning, ,Returning from a function}.} with

@example
set $sp += 4
@end example

Whenever possible, these four standard register names are available on
your machine even though the machine has different canonical mnemonics,
so long as there is no conflict.  The @code{info registers} command
shows the canonical names.  For example, on the SPARC, @code{info
registers} displays the processor status register as @code{$psr} but you
can also refer to it as @code{$ps}.

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

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

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

However, @value{GDBN} must deduce where registers are saved, from the machine
code generated by your compiler.  If some registers are not saved, or if
@value{GDBN} is unable to locate the saved registers, the selected stack
frame will make no difference.

@ifset AMD29K
@table @code
@item set rstack_high_address @var{address}
@kindex set rstack_high_address
@cindex AMD 29K register stack
@cindex register stack, AMD29K
On AMD 29000 family processors, registers are saved in a separate
``register stack''.  There is no way for @value{GDBN} to determine the extent
of this stack.  Normally, @value{GDBN} just assumes that the stack is ``large
enough''.  This may result in @value{GDBN} referencing memory locations that
do not exist.  If necessary, you can get around this problem by
specifying the ending address of the register stack with the @code{set
rstack_high_address} command.  The argument should be an address, which
you will probably want to precede with @samp{0x} to specify in
hexadecimal.

@item show rstack_high_address
@kindex show rstack_high_address
Display the current limit of the register stack, on AMD 29000 family
processors.
@end table
@end ifset

@ifclear HAVE-FLOAT
@node Floating Point Hardware
@section Floating point hardware
@cindex floating point

@c FIXME! Really host, not target?
Depending on the host machine architecture, @value{GDBN} may be able to give
you more information about the status of the floating point hardware.

@table @code
@item info float
@kindex info float
Display hardware-dependent information about the floating
point unit.  The exact contents and layout vary depending on the
floating point chip; on some platforms, @samp{info float} is not
available at all.
@end table
@c FIXME: this is a cop-out.  Try to get examples, explanations.  Only
@c FIXME...supported currently on arm's and 386's.  Mark properly with
@c FIXME... m4 macros to isolate general statements from hardware-dep,
@c FIXME... at that point.
@end ifclear

@ifclear CONLY
@node Languages
@chapter Using @value{GDBN} with Different Languages
@cindex languages

@ifset MOD2
Although programming languages generally have common aspects, they are
rarely expressed in the same manner.  For instance, in ANSI C,
dereferencing a pointer @code{p} is accomplished by @code{*p}, but in
Modula-2, it is accomplished by @code{p^}.  Values can also be
represented (and displayed) differently.  Hex numbers in C are written
like @samp{0x1ae}, while in Modula-2 they appear as @samp{1AEH}.
@end ifset

@cindex working language
Language-specific information is built into @value{GDBN} for some languages,
allowing you to express operations like the above in your program's
native language, and allowing @value{GDBN} to output values in a manner
consistent with the syntax of your program's native language.  The
language you use to build expressions, called the @dfn{working
language}, can be selected manually, or @value{GDBN} can set it
automatically.

@menu
* Setting::                     Switching between source languages
* Show::                        Displaying the language
@ifset MOD2
* Checks::                      Type and range checks
@end ifset

* Support::                     Supported languages
@end menu

@node Setting
@section Switching between source languages

There are two ways to control the working language---either have @value{GDBN}
set it automatically, or select it manually yourself.  You can use the
@code{set language} command for either purpose.  On startup, @value{GDBN}
defaults to setting the language automatically.

@menu
* Manually::                    Setting the working language manually
* Automatically::               Having @value{GDBN} infer the source language
@end menu

@node Manually
@subsection Setting the working language

If you allow @value{GDBN} to set the language automatically,
expressions are interpreted the same way in your debugging session and
your program.

@kindex set language
If you wish, you may set the language manually.  To do this, issue the
command @samp{set language @var{lang}}, where @var{lang} is the name of
a language, such as 
@ifclear MOD2
@code{c}.
@end ifclear
@ifset MOD2
@code{c} or @code{modula-2}.
@end ifset
For a list of the supported languages, type @samp{set language}.
@c FIXME: rms: eventually this command should be "help set language".

@ifset MOD2
Setting the language manually prevents @value{GDBN} from updating the working
language automatically.  This can lead to confusion if you try
to debug a program when the working language is not the same as the
source language, when an expression is acceptable to both
languages---but means different things.  For instance, if the current
source file were written in C, and @value{GDBN} was parsing Modula-2, a
command such as:

@example
print a = b + c
@end example

@noindent
might not have the effect you intended.  In C, this means to add
@code{b} and @code{c} and place the result in @code{a}.  The result
printed would be the value of @code{a}.  In Modula-2, this means to compare
@code{a} to the result of @code{b+c}, yielding a @code{BOOLEAN} value.
@end ifset

@node Automatically
@subsection Having @value{GDBN} infer the source language

To have @value{GDBN} set the working language automatically, use @samp{set
language local} or @samp{set language auto}.  @value{GDBN} then infers the
language that a program was written in by looking at the name of its
source files, and examining their extensions:

@table @file
@ifset MOD2
@item *.mod
Modula-2 source file
@end ifset

@item *.c
C source file

@item *.C
@itemx *.cc
C++ source file
@end table

This information is recorded for each function or procedure in a source
file.  When your program stops in a frame (usually by encountering a
breakpoint), @value{GDBN} sets the working language to the language recorded
for the function in that frame.  If the language for a frame is unknown
(that is, if the function or block corresponding to the frame was
defined in a source file that does not have a recognized extension), the
current working language is not changed, and @value{GDBN} issues a warning.

This may not seem necessary for most programs, which are written
entirely in one source language.  However, program modules and libraries
written in one source language can be used by a main program written in
a different source language.  Using @samp{set language auto} in this
case frees you from having to set the working language manually.

@node Show
@section Displaying the language

The following commands will help you find out which language is the
working language, and also what language source files were written in.

@kindex show language
@kindex info frame
@kindex info source
@table @code
@item show language
Display the current working language.  This is the
language you can use with commands such as @code{print} to
build and compute expressions that may involve variables in your program.

@item info frame
Among the other information listed here (@pxref{Frame Info, ,Information
about a frame}) is the source language for this frame.  This is the
language that will become the working language if you ever use an
identifier that is in this frame.

@item info source
Among the other information listed here (@pxref{Symbols, ,Examining the
Symbol Table}) is the source language of this source file.
@end table

@ifset MOD2
@node Checks
@section Type and range checking

@quotation
@emph{Warning:} In this release, the @value{GDBN} commands for type and range
checking are included, but they do not yet have any effect.  This
section documents the intended facilities.
@end quotation
@c FIXME remove warning when type/range code added

Some languages are designed to guard you against making seemingly common
errors through a series of compile- and run-time checks.  These include
checking the type of arguments to functions and operators, and making
sure mathematical overflows are caught at run time.  Checks such as
these help to ensure a program's correctness once it has been compiled
by eliminating type mismatches, and providing active checks for range
errors when your program is running.

@value{GDBN} can check for conditions like the above if you wish.
Although @value{GDBN} will not check the statements in your program, it
can check expressions entered directly into @value{GDBN} for evaluation via
the @code{print} command, for example.  As with the working language,
@value{GDBN} can also decide whether or not to check automatically based on
your program's source language.  @xref{Support, ,Supported languages},
for the default settings of supported languages.

@menu
* Type Checking::               An overview of type checking
* Range Checking::              An overview of range checking
@end menu

@cindex type checking
@cindex checks, type
@node Type Checking
@subsection An overview of type checking

Some languages, such as Modula-2, are strongly typed, meaning that the
arguments to operators and functions have to be of the correct type,
otherwise an error occurs.  These checks prevent type mismatch
errors from ever causing any run-time problems.  For example,

@example
1 + 2 @result{} 3
@exdent but
@error{} 1 + 2.3
@end example

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

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

Even though you may turn type checking off, other type-based reasons may
prevent @value{GDBN} from evaluating an expression.  For instance, @value{GDBN} does not
know how to add an @code{int} and a @code{struct foo}.  These particular
type errors have nothing to do with the language in use, and usually
arise from expressions, such as the one described above, which make
little sense to evaluate anyway.

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

@value{GDBN} provides some additional commands for controlling the type checker:

@kindex set check
@kindex set check type
@kindex show check type
@table @code
@item set check type auto
Set type checking on or off based on the current working language.
@xref{Support, ,Supported languages}, for the default settings for
each language.

@item set check type on
@itemx set check type off
Set type checking on or off, overriding the default setting for the
current working language.  Issue a warning if the setting does not
match the language default.  If any type mismatches occur in
evaluating an expression while typechecking is on, @value{GDBN} prints a
message and aborts evaluation of the expression.

@item set check type warn
Cause the type checker to issue warnings, but to always attempt to
evaluate the expression.  Evaluating the expression may still
be impossible for other reasons.  For example, @value{GDBN} cannot add
numbers and structures.

@item show type
Show the current setting of the type checker, and whether or not @value{GDBN} is
setting it automatically.
@end table

@cindex range checking
@cindex checks, range
@node Range Checking
@subsection An overview of range checking

In some languages (such as Modula-2), it is an error to exceed the
bounds of a type; this is enforced with run-time checks.  Such range
checking is meant to ensure program correctness by making sure
computations do not overflow, or indices on an array element access do
not exceed the bounds of the array.

For expressions you use in @value{GDBN} commands, you can tell
@value{GDBN} to treat range errors in one of three ways: ignore them,
always treat them as errors and abandon the expression, or issue
warnings but evaluate the expression anyway.

A range error can result from numerical overflow, from exceeding an
array index bound, or when you type a constant that is not a member
of any type.  Some languages, however, do not treat overflows as an
error.  In many implementations of C, mathematical overflow causes the
result to ``wrap around'' to lower values---for example, if @var{m} is
the largest integer value, and @var{s} is the smallest, then

@example
@var{m} + 1 @result{} @var{s}
@end example

This, too, is specific to individual languages, and in some cases
specific to individual compilers or machines.  @xref{Support, ,
Supported languages}, for further details on specific languages.

@value{GDBN} provides some additional commands for controlling the range checker:

@kindex set check
@kindex set check range
@kindex show check range
@table @code
@item set check range auto
Set range checking on or off based on the current working language.
@xref{Support, ,Supported languages}, for the default settings for
each language.

@item set check range on
@itemx set check range off
Set range checking on or off, overriding the default setting for the
current working language.  A warning is issued if the setting does not
match the language default.  If a range error occurs, then a message
is printed and evaluation of the expression is aborted.

@item set check range warn
Output messages when the @value{GDBN} range checker detects a range error,
but attempt to evaluate the expression anyway.  Evaluating the
expression may still be impossible for other reasons, such as accessing
memory that the process does not own (a typical example from many Unix
systems).

@item show range
Show the current setting of the range checker, and whether or not it is
being set automatically by @value{GDBN}.
@end table
@end ifset

@node Support
@section Supported languages

@ifset MOD2
@value{GDBN} 4 supports C, C++, and Modula-2.
@end ifset
@ifclear MOD2
@value{GDBN} 4 supports C, and C++.
@end ifclear
Some @value{GDBN} features may be used in expressions regardless of the
language you use: the @value{GDBN} @code{@@} and @code{::} operators,
and the @samp{@{type@}addr} construct (@pxref{Expressions,
,Expressions}) can be used with the constructs of any supported
language.

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

@ifset MOD2
@menu
* C::                           C and C++
* Modula-2::                    Modula-2
@end menu

@node C
@subsection C and C++
@cindex C and C++
@cindex expressions in C or C++

Since C and C++ are so closely related, many features of @value{GDBN} apply
to both languages.  Whenever this is the case, we discuss both languages
together.
@end ifset
@ifclear MOD2
@c Cancel this below, under same condition, at end of this chapter!
@raisesections
@end ifclear

@cindex C++
@kindex g++
@cindex GNU C++
The C++ debugging facilities are jointly implemented by the GNU C++
compiler and @value{GDBN}.  Therefore, to debug your C++ code effectively,
you must compile your C++ programs with the GNU C++ compiler,
@code{g++}.
@end ifclear
@ifset CONLY
@node C
@chapter C Language Support
@cindex C language
@cindex expressions in C

Information specific to the C language is built into @value{GDBN} so that you
can use C expressions while degugging.  This also permits @value{GDBN} to
output values in a manner consistent with C conventions.

@menu
* C Operators::                 C operators
* C Constants::                 C constants
* Debugging C::                 @value{GDBN} and C
@end menu
@end ifset
@ifclear CONLY
@menu
* C Operators::                 C and C++ operators
* C Constants::                 C and C++ constants
* Cplus expressions::           C++ expressions
* C Defaults::                  Default settings for C and C++
@ifset MOD2
* C Checks::                    C and C++ type and range checks
@end ifset

* Debugging C::                 @value{GDBN} and C
* Debugging C plus plus::       Special features for C++
@end menu
@end ifclear

@ifclear CONLY
@cindex C and C++ operators
@node C Operators
@subsubsection C and C++ operators
@end ifclear
@ifset CONLY
@cindex C operators
@node C Operators
@section C operators
@end ifset

Operators must be defined on values of specific types.  For instance,
@code{+} is defined on numbers, but not on structures.  Operators are
often defined on groups of types.  

@ifclear CONLY
For the purposes of C and C++, the following definitions hold:
@end ifclear

@itemize @bullet
@item
@emph{Integral types} include @code{int} with any of its storage-class
specifiers; @code{char}; and @code{enum}.

@item
@emph{Floating-point types} include @code{float} and @code{double}.

@item
@emph{Pointer types} include all types defined as @code{(@var{type}
*)}.

@item
@emph{Scalar types} include all of the above.
@end itemize

@noindent
The following operators are supported.  They are listed here
in order of increasing precedence:

@table @code
@item ,
The comma or sequencing operator.  Expressions in a comma-separated list
are evaluated from left to right, with the result of the entire
expression being the last expression evaluated.

@item =
Assignment.  The value of an assignment expression is the value
assigned.  Defined on scalar types.

@item @var{op}=
Used in an expression of the form @w{@code{@var{a} @var{op}= @var{b}}},
and translated to @w{@code{@var{a} = @var{a op b}}}.
@w{@code{@var{op}=}} and @code{=} have the same precendence.
@var{op} is any one of the operators @code{|}, @code{^}, @code{&},
@code{<<}, @code{>>}, @code{+}, @code{-}, @code{*}, @code{/}, @code{%}.

@item ?:
The ternary operator.  @code{@var{a} ? @var{b} : @var{c}} can be thought
of as:  if @var{a} then @var{b} else @var{c}.  @var{a} should be of an
integral type.

@item ||
Logical @sc{or}.  Defined on integral types.

@item &&
Logical @sc{and}.  Defined on integral types.

@item |
Bitwise @sc{or}.  Defined on integral types.

@item ^
Bitwise exclusive-@sc{or}.  Defined on integral types.

@item &
Bitwise @sc{and}.  Defined on integral types.

@item ==@r{, }!=
Equality and inequality.  Defined on scalar types.  The value of these
expressions is 0 for false and non-zero for true.

@item <@r{, }>@r{, }<=@r{, }>=
Less than, greater than, less than or equal, greater than or equal.
Defined on scalar types.  The value of these expressions is 0 for false
and non-zero for true.

@item <<@r{, }>>
left shift, and right shift.  Defined on integral types.

@item @@
The @value{GDBN} ``artificial array'' operator (@pxref{Expressions, ,Expressions}).

@item +@r{, }-
Addition and subtraction.  Defined on integral types, floating-point types and
pointer types.

@item *@r{, }/@r{, }%
Multiplication, division, and modulus.  Multiplication and division are
defined on integral and floating-point types.  Modulus is defined on
integral types.

@item ++@r{, }--
Increment and decrement.  When appearing before a variable, the
operation is performed before the variable is used in an expression;
when appearing after it, the variable's value is used before the
operation takes place.

@item *
Pointer dereferencing.  Defined on pointer types.  Same precedence as
@code{++}.

@item &
Address operator.  Defined on variables.  Same precedence as @code{++}.

@ifclear CONLY
For debugging C++, @value{GDBN} implements a use of @samp{&} beyond what is
allowed in the C++ language itself: you can use @samp{&(&@var{ref})}
(or, if you prefer, simply @samp{&&@var{ref}}) to examine the address
where a C++ reference variable (declared with @samp{&@var{ref}}) is
stored.
@end ifclear

@item -
Negative.  Defined on integral and floating-point types.  Same
precedence as @code{++}.

@item !
Logical negation.  Defined on integral types.  Same precedence as
@code{++}.

@item ~
Bitwise complement operator.  Defined on integral types.  Same precedence as
@code{++}.


@item .@r{, }->
Structure member, and pointer-to-structure member.  For convenience,
@value{GDBN} regards the two as equivalent, choosing whether to dereference a
pointer based on the stored type information.
Defined on @code{struct} and @code{union} data.

@item []
Array indexing.  @code{@var{a}[@var{i}]} is defined as
@code{*(@var{a}+@var{i})}.  Same precedence as @code{->}.

@item ()
Function parameter list.  Same precedence as @code{->}.

@ifclear CONLY
@item ::
C++ scope resolution operator.  Defined on
@code{struct}, @code{union}, and @code{class} types.
@end ifclear

@item ::
Doubled colons
@ifclear CONLY
also
@end ifclear
represent the @value{GDBN} scope operator (@pxref{Expressions,
,Expressions}).
@ifclear CONLY
Same precedence as @code{::}, above.
@end ifclear
@end table

@ifclear CONLY
@cindex C and C++ constants
@node C Constants
@subsubsection C and C++ constants

@value{GDBN} allows you to express the constants of C and C++ in the
following ways:
@end ifclear
@ifset CONLY
@cindex C constants
@node C Constants
@section C constants

@value{GDBN} allows you to express the constants of C in the
following ways:
@end ifset

@itemize @bullet
@item
Integer constants are a sequence of digits.  Octal constants are
specified by a leading @samp{0} (ie. zero), and hexadecimal constants by
a leading @samp{0x} or @samp{0X}.  Constants may also end with a letter
@samp{l}, specifying that the constant should be treated as a
@code{long} value.

@item
Floating point constants are a sequence of digits, followed by a decimal
point, followed by a sequence of digits, and optionally followed by an
exponent.  An exponent is of the form:
@samp{@w{e@r{[[}+@r{]|}-@r{]}@var{nnn}}}, where @var{nnn} is another
sequence of digits.  The @samp{+} is optional for positive exponents.

@item
Enumerated constants consist of enumerated identifiers, or their
integral equivalents.

@item
Character constants are a single character surrounded by single quotes
(@code{'}), or a number---the ordinal value of the corresponding character
(usually its @sc{ASCII} value).  Within quotes, the single character may
be represented by a letter or by @dfn{escape sequences}, which are of
the form @samp{\@var{nnn}}, where @var{nnn} is the octal representation
of the character's ordinal value; or of the form @samp{\@var{x}}, where
@samp{@var{x}} is a predefined special character---for example,
@samp{\n} for newline.

@item
String constants are a sequence of character constants surrounded
by double quotes (@code{"}).

@item
Pointer constants are an integral value.  You can also write pointers
to constants using the C operator @samp{&}.

@item
Array constants are comma-separated lists surrounded by braces @samp{@{}
and @samp{@}}; for example, @samp{@{1,2,3@}} is a three-element array of
integers, @samp{@{@{1,2@}, @{3,4@}, @{5,6@}@}} is a three-by-two array,
and @samp{@{&"hi", &"there", &"fred"@}} is a three-element array of pointers.
@end itemize

@ifclear CONLY
@node Cplus expressions
@subsubsection C++ expressions

@cindex expressions in C++
@value{GDBN} expression handling has a number of extensions to
interpret a significant subset of C++ expressions.

@cindex C++ support, not in @sc{coff}
@cindex @sc{coff} versus C++
@cindex C++ and object formats
@cindex object formats and C++
@cindex a.out and C++
@cindex @sc{ecoff} and C++
@cindex @sc{xcoff} and C++
@cindex @sc{elf}/stabs and C++
@cindex @sc{elf}/@sc{dwarf} and C++
@quotation
@emph{Warning:} Most of these extensions depend on the use of additional
debugging information in the symbol table, and thus require a rich,
extendable object code format.  In particular, if your system uses
a.out, MIPS @sc{ecoff}, RS/6000 @sc{xcoff}, or Sun @sc{elf} with stabs
extensions to the symbol table, these facilities are all available.
Where the object code format is standard @sc{coff}, on the other hand,
most of the C++ support in @value{GDBN} will @emph{not} work, nor can it.
For the standard SVr4 debugging format, @sc{dwarf} in @sc{elf}, the
standard is still evolving, so the C++ support in @value{GDBN} is still
fragile; when this debugging format stabilizes, however, C++ support
will also be available on systems that use it.
@end quotation

@enumerate

@cindex member functions
@item
Member function calls are allowed; you can use expressions like

@example
count = aml->GetOriginal(x, y)
@end example

@kindex this
@cindex namespace in C++
@item
While a member function is active (in the selected stack frame), your
expressions have the same namespace available as the member function;
that is, @value{GDBN} allows implicit references to the class instance
pointer @code{this} following the same rules as C++.

@cindex call overloaded functions
@cindex type conversions in C++
@item
You can call overloaded functions; @value{GDBN} will resolve the function
call to the right definition, with one restriction---you must use
arguments of the type required by the function that you want to call.
@value{GDBN} will not perform conversions requiring constructors or
user-defined type operators.

@cindex reference declarations
@item
@value{GDBN} understands variables declared as C++ references; you can use them in
expressions just as you do in C++ source---they are automatically
dereferenced.

In the parameter list shown when @value{GDBN} displays a frame, the values of
reference variables are not displayed (unlike other variables); this
avoids clutter, since references are often used for large structures.
The @emph{address} of a reference variable is always shown, unless
you have specified @samp{set print address off}.

@item
@value{GDBN} supports the C++ name resolution operator @code{::}---your
expressions can use it just as expressions in your program do.  Since
one scope may be defined in another, you can use @code{::} repeatedly if
necessary, for example in an expression like
@samp{@var{scope1}::@var{scope2}::@var{name}}.  @value{GDBN} also allows
resolving name scope by reference to source files, in both C and C++
debugging (@pxref{Variables, ,Program variables}).
@end enumerate

@node C Defaults
@subsubsection C and C++ defaults
@cindex C and C++ defaults

If you allow @value{GDBN} to set type and range checking automatically, they
both default to @code{off} whenever the working language changes to
C or C++.  This happens regardless of whether you, or @value{GDBN},
selected the working language.

If you allow @value{GDBN} to set the language automatically, it sets the
working language to C or C++ on entering code compiled from a source file
whose name ends with @file{.c}, @file{.C}, or @file{.cc}.
@xref{Automatically, ,Having @value{GDBN} infer the source language}, for
further details.

@ifset MOD2
@c Type checking is (a) primarily motivated by Modula-2, and (b)
@c unimplemented.  If (b) changes, it might make sense to let this node
@c appear even if Mod-2 does not, but meanwhile ignore it.  pesch 16jul93.
@node C Checks
@subsubsection C and C++ type and range checks
@cindex C and C++ checks

By default, when @value{GDBN} parses C or C++ expressions, type checking
is not used.  However, if you turn type checking on, @value{GDBN} will
consider two variables type equivalent if:

@itemize @bullet
@item
The two variables are structured and have the same structure, union, or
enumerated tag.

@item
Two two variables have the same type name, or types that have been
declared equivalent through @code{typedef}.

@ignore
@c leaving this out because neither J Gilmore nor R Pesch understand it.
@c FIXME--beers?
@item
The two @code{struct}, @code{union}, or @code{enum} variables are
declared in the same declaration.  (Note: this may not be true for all C
compilers.)
@end ignore
@end itemize

Range checking, if turned on, is done on mathematical operations.  Array
indices are not checked, since they are often used to index a pointer
that is not itself an array.
@end ifset
@end ifclear

@ifclear CONLY
@node Debugging C
@subsubsection @value{GDBN} and C
@end ifclear
@ifset CONLY
@node Debugging C
@section @value{GDBN} and C
@end ifset

The @code{set print union} and @code{show print union} commands apply to
the @code{union} type.  When set to @samp{on}, any @code{union} that is
inside a @code{struct}
@ifclear CONLY
or @code{class}
@end ifclear
will also be printed.
Otherwise, it will appear as @samp{@{...@}}.

The @code{@@} operator aids in the debugging of dynamic arrays, formed
with pointers and a memory allocation function.  @xref{Expressions,
,Expressions}.

@ifclear CONLY
@node Debugging C plus plus
@subsubsection @value{GDBN} features for C++

@cindex commands for C++
Some @value{GDBN} commands are particularly useful with C++, and some are
designed specifically for use with C++.  Here is a summary:

@table @code
@cindex break in overloaded functions
@item @r{breakpoint menus}
When you want a breakpoint in a function whose name is overloaded,
@value{GDBN} breakpoint menus help you specify which function definition
you want.  @xref{Breakpoint Menus,,Breakpoint menus}.

@cindex overloading in C++
@item rbreak @var{regex}
Setting breakpoints using regular expressions is helpful for setting
breakpoints on overloaded functions that are not members of any special
classes.
@xref{Set Breaks, ,Setting breakpoints}.

@cindex C++ exception handling
@item catch @var{exceptions}
@itemx info catch
Debug C++ exception handling using these commands.  @xref{Exception
Handling, ,Breakpoints and exceptions}.

@cindex inheritance
@item ptype @var{typename}
Print inheritance relationships as well as other information for type
@var{typename}.
@xref{Symbols, ,Examining the Symbol Table}.

@cindex C++ symbol display
@item set print demangle
@itemx show print demangle
@itemx set print asm-demangle
@itemx show print asm-demangle
Control whether C++ symbols display in their source form, both when
displaying code as C++ source and when displaying disassemblies.
@xref{Print Settings, ,Print settings}.

@item set print object
@itemx show print object
Choose whether to print derived (actual) or declared types of objects.
@xref{Print Settings, ,Print settings}.

@item set print vtbl
@itemx show print vtbl
Control the format for printing virtual function tables.
@xref{Print Settings, ,Print settings}.

@item @r{Overloaded symbol names}
You can specify a particular definition of an overloaded symbol, using
the same notation that is used to declare such symbols in C++: type
@code{@var{symbol}(@var{types})} rather than just @var{symbol}.  You can
also use the @value{GDBN} command-line word completion facilities to list the
available choices, or to finish the type list for you.
@xref{Completion,, Command completion}, for details on how to do this.
@end table
@ifclear MOD2
@c cancels "raisesections" under same conditions near bgn of chapter
@lowersections
@end ifclear

@ifset MOD2
@node Modula-2
@subsection Modula-2
@cindex Modula-2

The extensions made to @value{GDBN} to support Modula-2 only support
output from the GNU Modula-2 compiler (which is currently being
developed).  Other Modula-2 compilers are not currently supported, and
attempting to debug executables produced by them will most likely
result in an error as @value{GDBN} reads in the executable's symbol
table.

@cindex expressions in Modula-2
@menu
* M2 Operators::                Built-in operators
* Built-In Func/Proc::           Built-in functions and procedures
* M2 Constants::                Modula-2 constants
* M2 Defaults::                 Default settings for Modula-2
* Deviations::                  Deviations from standard Modula-2
* M2 Checks::                   Modula-2 type and range checks
* M2 Scope::                    The scope operators @code{::} and @code{.}
* GDB/M2::                      @value{GDBN} and Modula-2
@end menu

@node M2 Operators
@subsubsection Operators
@cindex Modula-2 operators

Operators must be defined on values of specific types.  For instance,
@code{+} is defined on numbers, but not on structures.  Operators are
often defined on groups of types.  For the purposes of Modula-2, the
following definitions hold:

@itemize @bullet

@item
@emph{Integral types} consist of @code{INTEGER}, @code{CARDINAL}, and
their subranges.

@item
@emph{Character types} consist of @code{CHAR} and its subranges.

@item
@emph{Floating-point types} consist of @code{REAL}.

@item
@emph{Pointer types} consist of anything declared as @code{POINTER TO
@var{type}}.

@item
@emph{Scalar types} consist of all of the above.

@item
@emph{Set types} consist of @code{SET} and @code{BITSET} types.

@item
@emph{Boolean types} consist of @code{BOOLEAN}.
@end itemize

@noindent
The following operators are supported, and appear in order of
increasing precedence:

@table @code
@item ,
Function argument or array index separator.

@item :=
Assignment.  The value of @var{var} @code{:=} @var{value} is
@var{value}.

@item <@r{, }>
Less than, greater than on integral, floating-point, or enumerated
types.

@item <=@r{, }>=
Less than, greater than, less than or equal to, greater than or equal to
on integral, floating-point and enumerated types, or set inclusion on
set types.  Same precedence as @code{<}.

@item =@r{, }<>@r{, }#
Equality and two ways of expressing inequality, valid on scalar types.
Same precedence as @code{<}.  In @value{GDBN} scripts, only @code{<>} is
available for inequality, since @code{#} conflicts with the script
comment character.

@item IN
Set membership.  Defined on set types and the types of their members.
Same precedence as @code{<}.

@item OR
Boolean disjunction.  Defined on boolean types.

@item AND@r{, }&
Boolean conjuction.  Defined on boolean types.

@item @@
The @value{GDBN} ``artificial array'' operator (@pxref{Expressions, ,Expressions}).

@item +@r{, }-
Addition and subtraction on integral and floating-point types, or union
and difference on set types.

@item *
Multiplication on integral and floating-point types, or set intersection
on set types.

@item /
Division on floating-point types, or symmetric set difference on set
types.  Same precedence as @code{*}.

@item DIV@r{, }MOD
Integer division and remainder.  Defined on integral types.  Same
precedence as @code{*}.

@item -
Negative. Defined on @code{INTEGER} and @code{REAL} data.

@item ^
Pointer dereferencing.  Defined on pointer types.

@item NOT
Boolean negation.  Defined on boolean types.  Same precedence as
@code{^}.

@item .
@code{RECORD} field selector.  Defined on @code{RECORD} data.  Same
precedence as @code{^}.

@item []
Array indexing.  Defined on @code{ARRAY} data.  Same precedence as @code{^}.

@item ()
Procedure argument list.  Defined on @code{PROCEDURE} objects.  Same precedence
as @code{^}.

@item ::@r{, }.
@value{GDBN} and Modula-2 scope operators.
@end table

@quotation
@emph{Warning:} Sets and their operations are not yet supported, so @value{GDBN}
will treat the use of the operator @code{IN}, or the use of operators
@code{+}, @code{-}, @code{*}, @code{/}, @code{=}, , @code{<>}, @code{#},
@code{<=}, and @code{>=} on sets as an error.
@end quotation

@cindex Modula-2 built-ins
@node Built-In Func/Proc
@subsubsection Built-in functions and procedures

Modula-2 also makes available several built-in procedures and functions.
In describing these, the following metavariables are used:

@table @var

@item a
represents an @code{ARRAY} variable.

@item c
represents a @code{CHAR} constant or variable.

@item i
represents a variable or constant of integral type.

@item m
represents an identifier that belongs to a set.  Generally used in the
same function with the metavariable @var{s}.  The type of @var{s} should
be @code{SET OF @var{mtype}} (where @var{mtype} is the type of @var{m}).

@item n
represents a variable or constant of integral or floating-point type.

@item r
represents a variable or constant of floating-point type.

@item t
represents a type.

@item v
represents a variable.

@item x
represents a variable or constant of one of many types.  See the
explanation of the function for details.
@end table

All Modula-2 built-in procedures also return a result, described below.

@table @code
@item ABS(@var{n})
Returns the absolute value of @var{n}.

@item CAP(@var{c})
If @var{c} is a lower case letter, it returns its upper case
equivalent, otherwise it returns its argument

@item CHR(@var{i})
Returns the character whose ordinal value is @var{i}.

@item DEC(@var{v})
Decrements the value in the variable @var{v}.  Returns the new value.

@item DEC(@var{v},@var{i})
Decrements the value in the variable @var{v} by @var{i}.  Returns the
new value.

@item EXCL(@var{m},@var{s})
Removes the element @var{m} from the set @var{s}.  Returns the new
set.

@item FLOAT(@var{i})
Returns the floating point equivalent of the integer @var{i}.

@item HIGH(@var{a})
Returns the index of the last member of @var{a}.

@item INC(@var{v})
Increments the value in the variable @var{v}.  Returns the new value.

@item INC(@var{v},@var{i})
Increments the value in the variable @var{v} by @var{i}.  Returns the
new value.

@item INCL(@var{m},@var{s})
Adds the element @var{m} to the set @var{s} if it is not already
there.  Returns the new set.

@item MAX(@var{t})
Returns the maximum value of the type @var{t}.

@item MIN(@var{t})
Returns the minimum value of the type @var{t}.

@item ODD(@var{i})
Returns boolean TRUE if @var{i} is an odd number.

@item ORD(@var{x})
Returns the ordinal value of its argument.  For example, the ordinal
value of a character is its ASCII value (on machines supporting the
ASCII character set).  @var{x} must be of an ordered type, which include
integral, character and enumerated types.

@item SIZE(@var{x})
Returns the size of its argument.  @var{x} can be a variable or a type.

@item TRUNC(@var{r})
Returns the integral part of @var{r}.

@item VAL(@var{t},@var{i})
Returns the member of the type @var{t} whose ordinal value is @var{i}.
@end table

@quotation
@emph{Warning:}  Sets and their operations are not yet supported, so
@value{GDBN} will treat the use of procedures @code{INCL} and @code{EXCL} as
an error.
@end quotation

@cindex Modula-2 constants
@node M2 Constants
@subsubsection Constants

@value{GDBN} allows you to express the constants of Modula-2 in the following
ways:

@itemize @bullet

@item
Integer constants are simply a sequence of digits.  When used in an
expression, a constant is interpreted to be type-compatible with the
rest of the expression.  Hexadecimal integers are specified by a
trailing @samp{H}, and octal integers by a trailing @samp{B}.

@item
Floating point constants appear as a sequence of digits, followed by a
decimal point and another sequence of digits.  An optional exponent can
then be specified, in the form @samp{E@r{[}+@r{|}-@r{]}@var{nnn}}, where
@samp{@r{[}+@r{|}-@r{]}@var{nnn}} is the desired exponent.  All of the
digits of the floating point constant must be valid decimal (base 10)
digits.

@item
Character constants consist of a single character enclosed by a pair of
like quotes, either single (@code{'}) or double (@code{"}).  They may
also be expressed by their ordinal value (their ASCII value, usually)
followed by a @samp{C}.

@item
String constants consist of a sequence of characters enclosed by a
pair of like quotes, either single (@code{'}) or double (@code{"}).
Escape sequences in the style of C are also allowed.  @xref{C
Constants, ,C and C++ constants}, for a brief explanation of escape
sequences.

@item
Enumerated constants consist of an enumerated identifier.

@item
Boolean constants consist of the identifiers @code{TRUE} and
@code{FALSE}.

@item
Pointer constants consist of integral values only.

@item
Set constants are not yet supported.
@end itemize

@node M2 Defaults
@subsubsection Modula-2 defaults
@cindex Modula-2 defaults

If type and range checking are set automatically by @value{GDBN}, they
both default to @code{on} whenever the working language changes to
Modula-2.  This happens regardless of whether you, or @value{GDBN},
selected the working language.

If you allow @value{GDBN} to set the language automatically, then entering
code compiled from a file whose name ends with @file{.mod} will set the
working language to Modula-2. @xref{Automatically, ,Having @value{GDBN} set
the language automatically}, for further details.

@node Deviations
@subsubsection Deviations from standard Modula-2
@cindex Modula-2, deviations from

A few changes have been made to make Modula-2 programs easier to debug.
This is done primarily via loosening its type strictness:

@itemize @bullet
@item
Unlike in standard Modula-2, pointer constants can be formed by
integers.  This allows you to modify pointer variables during
debugging.  (In standard Modula-2, the actual address contained in a
pointer variable is hidden from you; it can only be modified
through direct assignment to another pointer variable or expression that
returned a pointer.)

@item
C escape sequences can be used in strings and characters to represent
non-printable characters.  @value{GDBN} will print out strings with these
escape sequences embedded.  Single non-printable characters are
printed using the @samp{CHR(@var{nnn})} format.

@item
The assignment operator (@code{:=}) returns the value of its right-hand
argument.

@item
All built-in procedures both modify @emph{and} return their argument.
@end itemize

@node M2 Checks
@subsubsection Modula-2 type and range checks
@cindex Modula-2 checks

@quotation
@emph{Warning:} in this release, @value{GDBN} does not yet perform type or
range checking.
@end quotation
@c FIXME remove warning when type/range checks added

@value{GDBN} considers two Modula-2 variables type equivalent if:

@itemize @bullet
@item
They are of types that have been declared equivalent via a @code{TYPE
@var{t1} = @var{t2}} statement

@item
They have been declared on the same line.  (Note:  This is true of the
GNU Modula-2 compiler, but it may not be true of other compilers.)
@end itemize

As long as type checking is enabled, any attempt to combine variables
whose types are not equivalent is an error.

Range checking is done on all mathematical operations, assignment, array
index bounds, and all built-in functions and procedures.

@node M2 Scope
@subsubsection The scope operators @code{::} and @code{.}
@cindex scope
@kindex .
@cindex colon, doubled as scope operator
@ifinfo
@kindex colon-colon
@c Info cannot handle :: but TeX can.
@end ifinfo
@iftex
@kindex ::
@end iftex

There are a few subtle differences between the Modula-2 scope operator
(@code{.}) and the @value{GDBN} scope operator (@code{::}).  The two have
similar syntax:

@example

@var{module} . @var{id}
@var{scope} :: @var{id}
@end example

@noindent
where @var{scope} is the name of a module or a procedure,
@var{module} the name of a module, and @var{id} is any declared
identifier within your program, except another module.

Using the @code{::} operator makes @value{GDBN} search the scope
specified by @var{scope} for the identifier @var{id}.  If it is not
found in the specified scope, then @value{GDBN} will search all scopes
enclosing the one specified by @var{scope}.

Using the @code{.} operator makes @value{GDBN} search the current scope for
the identifier specified by @var{id} that was imported from the
definition module specified by @var{module}.  With this operator, it is
an error if the identifier @var{id} was not imported from definition
module @var{module}, or if @var{id} is not an identifier in
@var{module}.

@node GDB/M2
@subsubsection @value{GDBN} and Modula-2

Some @value{GDBN} commands have little use when debugging Modula-2 programs.
Five subcommands of @code{set print} and @code{show print} apply
specifically to C and C++: @samp{vtbl}, @samp{demangle},
@samp{asm-demangle}, @samp{object}, and @samp{union}.  The first four
apply to C++, and the last to the C @code{union} type, which has no direct
analogue in Modula-2.

The @code{@@} operator (@pxref{Expressions, ,Expressions}), while available
while using any language, is not useful with Modula-2.  Its
intent is to aid the debugging of @dfn{dynamic arrays}, which cannot be
created in Modula-2 as they can in C or C++.  However, because an
address can be specified by an integral constant, the construct
@samp{@{@var{type}@}@var{adrexp}} is still useful.  (@pxref{Expressions, ,Expressions})

@cindex @code{#} in Modula-2
In @value{GDBN} scripts, the Modula-2 inequality operator @code{#} is
interpreted as the beginning of a comment.  Use @code{<>} instead.

@end ifset
@end ifclear

@node Symbols
@chapter Examining the Symbol Table

The commands described in this section allow you to inquire about the
symbols (names of variables, functions and types) defined in your
program.  This information is inherent in the text of your program and
does not change as your program executes.  @value{GDBN} finds it in your
program's symbol table, in the file indicated when you started @value{GDBN}
(@pxref{File Options, ,Choosing files}), or by one of the
file-management commands (@pxref{Files, ,Commands to specify files}).

@c FIXME! This might be intentionally specific to C and C++; if so, move
@c to someplace in C section of lang chapter.
@cindex symbol names
@cindex names of symbols
@cindex quoting names
Occasionally, you may need to refer to symbols that contain unusual
characters, which @value{GDBN} ordinarily treats as word delimiters.  The
most frequent case is in referring to static variables in other
source files (@pxref{Variables,,Program variables}).  File names
are recorded in object files as debugging symbols, but @value{GDBN} would
ordinarily parse a typical file name, like @file{foo.c}, as the three words
@samp{foo} @samp{.} @samp{c}.  To allow @value{GDBN} to recognize
@samp{foo.c} as a single symbol, enclose it in single quotes; for example,

@example
p 'foo.c'::x
@end example

@noindent
looks up the value of @code{x} in the scope of the file @file{foo.c}.

@table @code
@item info address @var{symbol}
@kindex info address
Describe where the data for @var{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 @samp{print &@var{symbol}}, which does not work
at all for a register variable, and for a stack local variable prints
the exact address of the current instantiation of the variable.

@item whatis @var{exp}
@kindex whatis
Print the data type of expression @var{exp}.  @var{exp} is not
actually evaluated, and any side-effecting operations (such as
assignments or function calls) inside it do not take place.
@xref{Expressions, ,Expressions}.

@item whatis
Print the data type of @code{$}, the last value in the value history.

@item ptype @var{typename}
@kindex ptype
Print a description of data type @var{typename}.  @var{typename} may be
the name of a type, or for C code it may have the form
@ifclear CONLY
@samp{class @var{class-name}},
@end ifclear
@samp{struct @var{struct-tag}}, @samp{union @var{union-tag}} or
@samp{enum @var{enum-tag}}.

@item ptype @var{exp}
@itemx ptype
Print a description of the type of expression @var{exp}.  @code{ptype}
differs from @code{whatis} by printing a detailed description, instead
of just the name of the type.

For example, for this variable declaration:

@example
struct complex @{double real; double imag;@} v;
@end example

@noindent
the two commands give this output:

@example
@group
(@value{GDBP}) whatis v
type = struct complex
(@value{GDBP}) ptype v
type = struct complex @{
    double real;
    double imag;
@}
@end group
@end example

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

@item info types @var{regexp}
@itemx info types
@kindex info types
Print a brief description of all types whose name matches @var{regexp}
(or all types in your program, if you supply no argument).  Each
complete typename is matched as though it were a complete line; thus,
@samp{i type value} gives information on all types in your program whose
name includes the string @code{value}, but @samp{i type ^value$} gives
information only on types whose complete name is @code{value}.

This command differs from @code{ptype} in two ways: first, like
@code{whatis}, it does not print a detailed description; second, it
lists all source files where a type is defined.

@item info source
@kindex info source
Show the name of the current source file---that is, the source file for
the function containing the current point of execution---and the language
it was written in.

@item info sources
@kindex info sources
Print the names of all source files in your program for which there is
debugging information, organized into two lists: files whose symbols
have already been read, and files whose symbols will be read when needed.

@item info functions
@kindex info functions
Print the names and data types of all defined functions.

@item info functions @var{regexp}
Print the names and data types of all defined functions
whose names contain a match for regular expression @var{regexp}.
Thus, @samp{info fun step} finds all functions whose names
include @code{step}; @samp{info fun ^step} finds those whose names
start with @code{step}.

@item info variables
@kindex info variables
Print the names and data types of all variables that are declared
outside of functions (i.e., excluding local variables).

@item info variables @var{regexp}
Print the names and data types of all variables (except for local
variables) whose names contain a match for regular expression
@var{regexp}.

@ignore
This was never implemented.
@item info methods
@itemx info methods @var{regexp}
@kindex info methods
The @code{info methods} command permits the user to examine all defined
methods within C++ program, or (with the @var{regexp} argument) a
specific set of methods found in the various C++ classes.  Many
C++ classes provide a large number of methods.  Thus, the output
from the @code{ptype} command can be overwhelming and hard to use.  The
@code{info-methods} command filters the methods, printing only those
which match the regular-expression @var{regexp}.
@end ignore

@item maint print symbols @var{filename}
@itemx maint print psymbols @var{filename}
@itemx maint print msymbols @var{filename}
@kindex maint print symbols
@cindex symbol dump
@kindex maint print psymbols
@cindex partial symbol dump
Write a dump of debugging symbol data into the file @var{filename}.
These commands are used to debug the @value{GDBN} symbol-reading code.  Only
symbols with debugging data are included.  If you use @samp{maint print
symbols}, @value{GDBN} includes all the symbols for which it has already
collected full details: that is, @var{filename} reflects symbols for
only those files whose symbols @value{GDBN} has read.  You can use the
command @code{info sources} to find out which files these are.  If you
use @samp{maint print psymbols} instead, the dump shows information about
symbols that @value{GDBN} only knows partially---that is, symbols defined in
files that @value{GDBN} has skimmed, but not yet read completely.  Finally,
@samp{maint print msymbols} dumps just the minimal symbol information
required for each object file from which @value{GDBN} has read some symbols.
@xref{Files, ,Commands to specify files}, for a discussion of how
@value{GDBN} reads symbols (in the description of @code{symbol-file}).
@end table

@node Altering
@chapter Altering Execution

Once you think you have found an error in your program, you might want to
find out for certain whether correcting the apparent error would lead to
correct results in the rest of the run.  You can find the answer by
experiment, using the @value{GDBN} features for altering execution of the
program.

For example, you can store new values into variables or memory
locations,
@ifclear BARETARGET
give your program a signal, restart it
@end ifclear
@ifset BARETARGET
restart your program
@end ifset
at a different address, or even return prematurely from a function to
its caller.

@menu
* Assignment::                  Assignment to variables
* Jumping::                     Continuing at a different address
@ifclear BARETARGET
* Signaling::                   Giving your program a signal
@end ifclear

* Returning::                   Returning from a function
* Calling::                     Calling your program's functions
* Patching::                    Patching your program
@end menu

@node Assignment
@section Assignment to variables

@cindex assignment
@cindex setting variables
To alter the value of a variable, evaluate an assignment expression.
@xref{Expressions, ,Expressions}.  For example,

@example
print x=4
@end example

@noindent
stores the value 4 into the variable @code{x}, and then prints the
value of the assignment expression (which is 4).  
@ifclear CONLY
@xref{Languages, ,Using @value{GDBN} with Different Languages}, for more
information on operators in supported languages.
@end ifclear

@kindex set variable
@cindex variables, setting
If you are not interested in seeing the value of the assignment, use the
@code{set} command instead of the @code{print} command.  @code{set} is
really the same as @code{print} except that the expression's value is
not printed and is not put in the value history (@pxref{Value History,
,Value history}).  The expression is evaluated only for its effects.

If the beginning of the argument string of the @code{set} command
appears identical to a @code{set} subcommand, use the @code{set
variable} command instead of just @code{set}.  This command is identical
to @code{set} except for its lack of subcommands.  For example, if
your program has a variable @code{width}, you get
an error if you try to set a new value with just @samp{set width=13},
because @value{GDBN} has the command @code{set width}:

@example
(@value{GDBP}) whatis width
type = double
(@value{GDBP}) p width
$4 = 13
(@value{GDBP}) set width=47
Invalid syntax in expression.
@end example

@noindent
The invalid expression, of course, is @samp{=47}.  In
order to actually set the program's variable @code{width}, use

@example
(@value{GDBP}) set var width=47
@end example

@value{GDBN} allows more implicit conversions in assignments than C; you can
freely store an integer value into a pointer variable or vice versa,
and you can convert any structure to any other structure that is the
same length or shorter.
@comment FIXME: how do structs align/pad in these conversions?
@comment        /pesch@cygnus.com 18dec1990

To store values into arbitrary places in memory, use the @samp{@{@dots{}@}}
construct to generate a value of specified type at a specified address
(@pxref{Expressions, ,Expressions}).  For example, @code{@{int@}0x83040} refers
to memory location @code{0x83040} as an integer (which implies a certain size
and representation in memory), and

@example
set @{int@}0x83040 = 4
@end example

@noindent
stores the value 4 into that memory location.

@node Jumping
@section Continuing at a different address

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

@table @code
@item jump @var{linespec}
@kindex jump
Resume execution at line @var{linespec}.  Execution will stop
immediately if there is a breakpoint there.  @xref{List, ,Printing
source lines}, for a description of the different forms of
@var{linespec}.

The @code{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 @var{linespec} is in
a different function from the one currently executing, the results may
be bizarre if the two functions expect different patterns of arguments or
of local variables.  For this reason, the @code{jump} command requests
confirmation if the specified line is not in the function currently
executing.  However, even bizarre results are predictable if you are
well acquainted with the machine-language code of your program.

@item jump *@var{address}
Resume execution at the instruction at address @var{address}.
@end table

You can get much the same effect as the @code{jump} command by storing a
new value into the register @code{$pc}.  The difference is that this
does not start your program running; it only changes the address where it
@emph{will} run when it is continued.  For example,

@example
set $pc = 0x485
@end example

@noindent
causes the next @code{continue} command or stepping command to execute at
address @code{0x485}, rather than at the address where your program stopped.
@xref{Continuing and Stepping, ,Continuing and stepping}.

The most common occasion to use the @code{jump} command is to back up,
perhaps with more breakpoints set, over a portion of a program that has
already executed, in order to examine its execution in more detail.

@ifclear BARETARGET
@c @group
@node Signaling
@section Giving your program a signal

@table @code
@item signal @var{signal}
@kindex signal
Resume execution where your program stopped, but immediately give it the
signal @var{signal}.  @var{signal} can be the name or the number of a
signal.  For example, on many systems @code{signal 2} and @code{signal
SIGINT} are both ways of sending an interrupt signal.

Alternatively, if @var{signal} is zero, continue execution without
giving a signal.  This is useful when your program stopped on account of
a signal and would ordinary see the signal when resumed with the
@code{continue} command; @samp{signal 0} causes it to resume without a
signal.

@code{signal} does not repeat when you press @key{RET} a second time
after executing the command.
@end table
@c @end group

Invoking the @code{signal} command is not the same as invoking the
@code{kill} utility from the shell.  Sending a signal with @code{kill}
causes @value{GDBN} to decide what to do with the signal depending on
the signal handling tables (@pxref{Signals}).  The @code{signal} command
passes the signal directly to your program.

@end ifclear

@node Returning
@section Returning from a function

@table @code
@item return
@itemx return @var{expression}
@cindex returning from a function
@kindex return
You can cancel execution of a function call with the @code{return}
command.  If you give an
@var{expression} argument, its value is used as the function's return
value.
@end table

When you use @code{return}, @value{GDBN} discards the selected stack frame
(and all frames within it).  You can think of this as making the
discarded frame return prematurely.  If you wish to specify a value to
be returned, give that value as the argument to @code{return}.

This pops the selected stack frame (@pxref{Selection, ,Selecting a
frame}), and any other frames inside of it, leaving its caller as the
innermost remaining frame.  That frame becomes selected.  The
specified value is stored in the registers used for returning values
of functions.

The @code{return} command does not resume execution; it leaves the
program stopped in the state that would exist if the function had just
returned.  In contrast, the @code{finish} command (@pxref{Continuing
and Stepping, ,Continuing and stepping}) resumes execution until the
selected stack frame returns naturally.

@node Calling
@section Calling program functions

@cindex calling functions
@kindex call
@table @code
@item call @var{expr}
Evaluate the expression @var{expr} without displaying @code{void}
returned values.
@end table

You can use this variant of the @code{print} command if you want to
execute a function from your program, but without cluttering the output
with @code{void} returned values.  The result is printed and saved in
the value history, if it is not void.

@node Patching
@section Patching programs
@cindex patching binaries
@cindex writing into executables
@ifclear BARETARGET
@cindex writing into corefiles
@end ifclear

By default, @value{GDBN} opens the file containing your program's executable
code
@ifclear BARETARGET
(or the corefile)
@end ifclear
read-only.  This prevents accidental alterations
to machine code; but it also prevents you from intentionally patching
your program's binary.

If you'd like to be able to patch the binary, you can specify that
explicitly with the @code{set write} command.  For example, you might
want to turn on internal debugging flags, or even to make emergency
repairs.

@table @code
@item set write on
@itemx set write off
@kindex set write
If you specify @samp{set write on}, @value{GDBN} will open executable 
@ifclear BARETARGET
and core
@end ifclear
files for both reading and writing; if you specify @samp{set write
off} (the default), @value{GDBN} will open them read-only.

If you have already loaded a file, you must load it again (using the
@code{exec-file}
@ifclear BARETARGET
or @code{core-file}
@end ifclear
command) after changing @code{set write}, for your new setting to take
effect.

@item show write
@kindex show write
Display whether executable files 
@ifclear BARETARGET
and core files 
@end ifclear
will be opened for writing as well as reading.
@end table

@node GDB Files
@chapter @value{GDBN} Files

@value{GDBN} needs to know the file name of the program to be debugged, both in
order to read its symbol table and in order to start your program.  
@ifclear BARETARGET
To debug a core dump of a previous run, you must also tell @value{GDBN}
the name of the core dump file.
@end ifclear

@menu
* Files::                       Commands to specify files
* Symbol Errors::               Errors reading symbol files
@end menu

@node Files
@section Commands to specify files
@cindex symbol table

@ifclear BARETARGET
@cindex core dump file
The usual way to specify executable and core dump file names is with
the command arguments given when you start @value{GDBN} (@pxref{Invocation,
,Getting In and Out of @value{GDBN}}.
@end ifclear
@ifset BARETARGET
The usual way to specify an executable file name is with
the command argument given when you start @value{GDBN}, (@pxref{Invocation,
,Getting In and Out of @value{GDBN}}.
@end ifset

Occasionally it is necessary to change to a different file during a
@value{GDBN} session.  Or you may run @value{GDBN} and forget to specify
a file you want to use.  In these situations the @value{GDBN} commands
to specify new files are useful.

@table @code
@item file @var{filename}
@cindex executable file
@kindex file
Use @var{filename} as the program to be debugged.  It is read for its
symbols and for the contents of pure memory.  It is also the program
executed when you use the @code{run} command.  If you do not specify a
directory and the file is not found in the @value{GDBN} working directory, @value{GDBN}
uses the environment variable @code{PATH} as a list of directories to
search, just as the shell does when looking for a program to run.  You
can change the value of this variable, for both @value{GDBN} and your program,
using the @code{path} command.

On systems with memory-mapped files, an auxiliary symbol table file
@file{@var{filename}.syms} may be available for @var{filename}.  If it
is, @value{GDBN} will map in the symbol table from
@file{@var{filename}.syms}, starting up more quickly.  See the
descriptions of the options @samp{-mapped} and @samp{-readnow} (available
on the command line, and with the commands @code{file}, @code{symbol-file},
or @code{add-symbol-file}), for more information.

@item file
@code{file} with no argument makes @value{GDBN} discard any information it
has on both executable file and the symbol table.

@item exec-file @r{[} @var{filename} @r{]}
@kindex exec-file
Specify that the program to be run (but not the symbol table) is found
in @var{filename}.  @value{GDBN} will search the environment variable @code{PATH}
if necessary to locate your program.  Omitting @var{filename} means to
discard information on the executable file.

@item symbol-file @r{[} @var{filename} @r{]}
@kindex symbol-file
Read symbol table information from file @var{filename}.  @code{PATH} is
searched when necessary.  Use the @code{file} command to get both symbol
table and program to run from the same file.

@code{symbol-file} with no argument clears out @value{GDBN} information on your
program's symbol table.

The @code{symbol-file} command causes @value{GDBN} 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 @value{GDBN}.

@code{symbol-file} will not repeat if you press @key{RET} again after
executing it once.

When @value{GDBN} is configured for a particular environment, it will
understand debugging information in whatever format is the standard
generated for that environment; you may use either a GNU compiler, or
other compilers that adhere to the local conventions.  Best results are
usually obtained from GNU compilers; for example, using @code{@value{GCC}}
you can generate debugging information for optimized code.

On some kinds of object files, the @code{symbol-file} command does not
normally read the symbol table in full right away.  Instead, it scans
the symbol table quickly to find which source files and which symbols
are present.  The details are read later, one source file at a time,
as they are needed.

The purpose of this two-stage reading strategy is to make @value{GDBN} start up
faster.  For the most part, it is invisible except for occasional
pauses while the symbol table details for a particular source file are
being read.  (The @code{set verbose} command can turn these pauses
into messages if desired.  @xref{Messages/Warnings, ,Optional warnings
and messages}.) 

We have not implemented the two-stage strategy for COFF yet.  When the
symbol table is stored in COFF format, @code{symbol-file} reads the
symbol table data in full right away.

@item symbol-file @var{filename} @r{[} -readnow @r{]} @r{[} -mapped @r{]}
@itemx file @var{filename} @r{[} -readnow @r{]} @r{[} -mapped @r{]}
@kindex readnow
@cindex reading symbols immediately
@cindex symbols, reading immediately
@kindex mapped
@cindex memory-mapped symbol file
@cindex saving symbol table
You can override the @value{GDBN} two-stage strategy for reading symbol
tables by using the @samp{-readnow} option with any of the commands that
load symbol table information, if you want to be sure @value{GDBN} has the
entire symbol table available.  

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

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

The auxiliary symbol file for a program called @var{myprog} is called
@samp{@var{myprog}.syms}.  Once this file exists (so long as it is newer
than the corresponding executable), @value{GDBN} will always attempt to use
it when you debug @var{myprog}; no special options or commands are
needed.

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

@c FIXME: for now no mention of directories, since this seems to be in
@c flux.  13mar1992 status is that in theory GDB would look either in
@c current dir or in same dir as myprog; but issues like competing
@c GDB's, or clutter in system dirs, mean that in practice right now
@c only current dir is used.  FFish says maybe a special GDB hierarchy
@c (eg rooted in val of env var GDBSYMS) could exist for mappable symbol
@c files.

@item core-file @r{[} @var{filename} @r{]}
@kindex core
@kindex core-file
Specify the whereabouts of a core dump file to be used as the ``contents
of memory''.  Traditionally, core files contain only some parts of the
address space of the process that generated them; @value{GDBN} can access the
executable file itself for other parts.

@code{core-file} with no argument specifies that no core file is
to be used.

Note that the core file is ignored when your program is actually running
under @value{GDBN}.  So, if you have been running your program and you wish to
debug a core file instead, you must kill the subprocess in which the
program is running.  To do this, use the @code{kill} command
(@pxref{Kill Process, ,Killing the child process}).
@end ifclear

@item load @var{filename}
@kindex load
@ifset GENERIC
Depending on what remote debugging facilities are configured into
@value{GDBN}, the @code{load} command may be available.  Where it exists, it
is meant to make @var{filename} (an executable) available for debugging
on the remote system---by downloading, or dynamic linking, for example.
@code{load} also records the @var{filename} symbol table in @value{GDBN}, like
the @code{add-symbol-file} command.

If your @value{GDBN} does not have a @code{load} command, attempting to
execute it gets the error message ``@code{You can't do that when your
target is @dots{}}''
@end ifset

The file is loaded at whatever address is specified in the executable.
For some object file formats, like a.out, the object file format fixes
the address and so it won't necessarily match the address you gave to
the linker.

@ifset VXWORKS
On VxWorks, @code{load} will dynamically link @var{filename} on the
current target system as well as adding its symbols in @value{GDBN}.
@end ifset

@ifset I960
@cindex download to Nindy-960
With the Nindy interface to an Intel 960 board, @code{load} will
download @var{filename} to the 960 as well as adding its symbols in
@value{GDBN}.
@end ifset

@ifset H8
@cindex download to H8/300 or H8/500
@cindex H8/300 or H8/500 download
@cindex download to Hitachi SH
@cindex Hitachi SH download
When you select remote debugging to a Hitachi SH, H8/300, or H8/500 board
(@pxref{Hitachi Remote,,@value{GDBN} and Hitachi Microprocessors}),
the @code{load} command downloads your program to the Hitachi board and also
opens it as the current executable target for @value{GDBN} on your host
(like the @code{file} command).
@end ifset

@code{load} will not repeat if you press @key{RET} again after using it.

@ifclear BARETARGET
@item add-symbol-file @var{filename} @var{address}
@itemx add-symbol-file @var{filename} @var{address} @r{[} -readnow @r{]} @r{[} -mapped @r{]}
@kindex add-symbol-file
@cindex dynamic linking
The @code{add-symbol-file} command reads additional symbol table information
from the file @var{filename}.  You would use this command when @var{filename}
has been dynamically loaded (by some other means) into the program that
is running.  @var{address} should be the memory address at which the
file has been loaded; @value{GDBN} cannot figure this out for itself.
You can specify @var{address} as an expression.

The symbol table of the file @var{filename} is added to the symbol table
originally read with the @code{symbol-file} command.  You can use the
@code{add-symbol-file} command any number of times; the new symbol data thus
read keeps adding to the old.  To discard all old symbol data instead,
use the @code{symbol-file} command.

@code{add-symbol-file} will not repeat if you press @key{RET} after using it.

You can use the @samp{-mapped} and @samp{-readnow} options just as with
the @code{symbol-file} command, to change how @value{GDBN} manages the symbol
table information for @var{filename}.
@end ifclear

@item info files
@itemx info target
@kindex info files
@kindex info target
@code{info files} and @code{info target} are synonymous; both print
the current target (@pxref{Targets, ,Specifying a Debugging Target}),
including the
@ifclear BARETARGET
names of the executable and core dump files
@end ifclear
@ifset BARETARGET
name of the executable file
@end ifset
currently in use by @value{GDBN}, and the files from which symbols were
loaded.  The command @code{help targets} lists all possible targets
rather than current ones.
@end table

All file-specifying commands allow both absolute and relative file names
as arguments.  @value{GDBN} always converts the file name to an absolute path
name and remembers it that way.

@ifclear BARETARGET
@cindex shared libraries
@value{GDBN} supports SunOS, SVR4, and IBM RS/6000 shared libraries.
@value{GDBN} automatically loads symbol definitions from shared libraries
when you use the @code{run} command, or when you examine a core file.
(Before you issue the @code{run} command, @value{GDBN} will not understand
references to a function in a shared library, however---unless you are
debugging a core file).
@c FIXME: next @value{GDBN} release should permit some refs to undef
@c FIXME...symbols---eg in a break cmd---assuming they are from a shared lib

@table @code
@item info share
@itemx info sharedlibrary
@kindex info sharedlibrary
@kindex info share
Print the names of the shared libraries which are currently loaded.

@item sharedlibrary @var{regex}
@itemx share @var{regex}
@kindex sharedlibrary
@kindex share
This is an obsolescent command; you can use it to explicitly load shared
object library symbols for files matching a Unix regular expression, but
as with files loaded automatically, it will only load shared libraries
required by your program for a core file or after typing @code{run}.  If
@var{regex} is omitted all shared libraries required by your program are
loaded.
@end table
@end ifclear

@node Symbol Errors
@section Errors reading symbol files

While reading a symbol file, @value{GDBN} will occasionally encounter problems,
such as symbol types it does not recognize, or known bugs in compiler
output.  By default, @value{GDBN} does not notify you of such problems, since
they are relatively common and primarily of interest to people
debugging compilers.  If you are interested in seeing information
about ill-constructed symbol tables, you can either ask @value{GDBN} to print
only one message about each such type of problem, no matter how many
times the problem occurs; or you can ask @value{GDBN} to print more messages,
to see how many times the problems occur, with the @code{set
complaints} command (@pxref{Messages/Warnings, ,Optional warnings and
messages}).

The messages currently printed, and their meanings, include:

@table @code
@item inner block not inside outer block in @var{symbol}

The symbol information shows where symbol scopes begin and end
(such as at the start of a function or a block of statements).  This
error indicates that an inner scope block is not fully contained
in its outer scope blocks.

@value{GDBN} circumvents the problem by treating the inner block as if it had
the same scope as the outer block.  In the error message, @var{symbol}
may be shown as ``@code{(don't know)}'' if the outer block is not a
function.

@item block at @var{address} out of order

The symbol information for symbol scope blocks should occur in
order of increasing addresses.  This error indicates that it does not
do so.

@value{GDBN} does not circumvent this problem, and will have trouble
locating symbols in the source file whose symbols it is reading.  (You
can often determine what source file is affected by specifying
@code{set verbose on}.  @xref{Messages/Warnings, ,Optional warnings and
messages}.)

@item bad block start address patched

The symbol information for a symbol scope block has a start address
smaller than the address of the preceding source line.  This is known
to occur in the SunOS 4.1.1 (and earlier) C compiler.

@value{GDBN} circumvents the problem by treating the symbol scope block as
starting on the previous source line.

@item bad string table offset in symbol @var{n}

@cindex foo
Symbol number @var{n} contains a pointer into the string table which is
larger than the size of the string table.

@value{GDBN} circumvents the problem by considering the symbol to have the
name @code{foo}, which may cause other problems if many symbols end up
with this name.

@item unknown symbol type @code{0x@var{nn}}

The symbol information contains new data types that @value{GDBN} does not yet
know how to read.  @code{0x@var{nn}} is the symbol type of the misunderstood
information, in hexadecimal.

@value{GDBN} circumvents the error by ignoring this symbol information.  This
will usually allow your program to be debugged, though certain symbols
will not be accessible.  If you encounter such a problem and feel like
debugging it, you can debug @code{@value{GDBP}} with itself, breakpoint on
@code{complain}, then go up to the function @code{read_dbx_symtab} and
examine @code{*bufp} to see the symbol.

@item stub type has NULL name
@value{GDBN} could not find the full definition for
@ifclear CONLY
a struct or class.
@end ifclear
@ifset CONLY
a struct.
@end ifset

@ifclear CONLY
@item const/volatile indicator missing (ok if using g++ v1.x), got@dots{}

The symbol information for a C++ member function is missing some
information that recent versions of the compiler should have output
for it.
@end ifclear

@item info mismatch between compiler and debugger

@value{GDBN} could not parse a type specification output by the compiler.
@end table

@node Targets
@chapter Specifying a Debugging Target
@cindex debugging target
@kindex target

A @dfn{target} is the execution environment occupied by your program.
@ifclear BARETARGET
Often, @value{GDBN} runs in the same host environment as your program; in
that case, the debugging target is specified as a side effect when you
use the @code{file} or @code{core} commands.  When you need more
flexibility---for example, running @value{GDBN} on a physically separate
host, or controlling a standalone system over a serial port or a
realtime system over a TCP/IP connection---you 
@end ifclear
@ifset BARETARGET
You
@end ifset
can use the @code{target} command to specify one of the target types
configured for @value{GDBN} (@pxref{Target Commands, ,Commands for managing
targets}).

@menu
* Active Targets::              Active targets
* Target Commands::             Commands for managing targets
* Remote::                      Remote debugging
@end menu

@node Active Targets
@section Active targets
@cindex stacking targets
@cindex active targets
@cindex multiple targets

@ifclear BARETARGET
There are three classes of targets: processes, core files, and
executable files.  @value{GDBN} can work concurrently on up to three active
targets, one in each class.  This allows you to (for example) start a
process and inspect its activity without abandoning your work on a core
file.

For example, if you execute @samp{gdb a.out}, then the executable file
@code{a.out} is the only active target.  If you designate a core file as
well---presumably from a prior run that crashed and coredumped---then
@value{GDBN} has two active targets and will use them in tandem, looking
first in the corefile target, then in the executable file, to satisfy
requests for memory addresses.  (Typically, these two classes of target
are complementary, since core files contain only a program's
read-write memory---variables and so on---plus machine status, while
executable files contain only the program text and initialized data.)
@end ifclear

When you type @code{run}, your executable file becomes an active process
target as well.  When a process target is active, all @value{GDBN} commands
requesting memory addresses refer to that target; addresses in an 
@ifclear BARETARGET
active core file or 
@end ifclear
executable file target are obscured while the process
target is active.

@ifset BARETARGET
Use the @code{exec-file} command to select a
new executable target (@pxref{Files, ,Commands to specify
files}).
@end ifset
@ifclear BARETARGET
Use the @code{core-file} and @code{exec-file} commands to select a
new core file or executable target (@pxref{Files, ,Commands to specify
files}).  To specify as a target a process that is already running, use
the @code{attach} command (@pxref{Attach, ,Debugging an
already-running process}).
@end ifclear

@node Target Commands
@section Commands for managing targets

@table @code
@item target @var{type} @var{parameters}
Connects the @value{GDBN} host environment to a target
@ifset BARETARGET
machine.
@end ifset
@ifclear BARETARGET
machine or process.  A target is typically a protocol for talking to
debugging facilities.  You use the argument @var{type} to specify the
type or protocol of the target machine.

Further @var{parameters} are interpreted by the target protocol, but
typically include things like device names or host names to connect
with, process numbers, and baud rates.
@end ifclear

The @code{target} command will not repeat if you press @key{RET} again
after executing the command.

@item help target
@kindex help target
Displays the names of all targets available.  To display targets
currently selected, use either @code{info target} or @code{info files}
(@pxref{Files, ,Commands to specify files}).

@item help target @var{name}
Describe a particular target, including any parameters necessary to
select it.
@end table

Here are some common targets (available, or not, depending on the GDB
configuration):

@table @code
@item target exec @var{program}
@kindex target exec
An executable file.  @samp{target exec @var{program}} is the same as
@samp{exec-file @var{program}}.

@ifclear BARETARGET
@item target core @var{filename}
@kindex target core
A core dump file.  @samp{target core @var{filename}} is the same as
@samp{core-file @var{filename}}.
@end ifclear

@ifset REMOTESTUB
@item target remote @var{dev}
@kindex target remote
Remote serial target in GDB-specific protocol.  The argument @var{dev}
specifies what serial device to use for the connection (e.g.
@file{/dev/ttya}). @xref{Remote, ,Remote debugging}.
@end ifset

@ifset SIMS
@item target sim
@kindex target sim
CPU simulator.  @xref{Simulator,,Simulated CPU Target}.
@end ifset

@ifset AMD29K
@item target udi @var{keyword}
@kindex target udi
Remote AMD29K target, using the AMD UDI protocol.  The @var{keyword}
argument specifies which 29K board or simulator to use.  @xref{UDI29K
Remote,,@value{GDBN} and the UDI protocol for AMD29K}.

@item target amd-eb @var{dev} @var{speed} @var{PROG}
@kindex target amd-eb
@cindex AMD EB29K
Remote PC-resident AMD EB29K board, attached over serial lines.
@var{dev} is the serial device, as for @code{target remote};
@var{speed} allows you to specify the linespeed; and @var{PROG} is the
name of the program to be debugged, as it appears to DOS on the PC.
@xref{EB29K Remote, ,@value{GDBN} with a remote EB29K}.

@end ifset
@ifset H8
@item target hms
@kindex target hms
A Hitachi SH, H8/300, or H8/500 board, attached via serial line to your host.
@ifclear H8EXCLUSIVE
@c Unix only, not currently of interest for H8-only manual
Use special commands @code{device} and @code{speed} to control the serial
line and the communications speed used.
@end ifclear
@xref{Hitachi Remote,,@value{GDBN} and Hitachi Microprocessors}.

@end ifset
@ifset I960
@item target nindy @var{devicename}
@kindex target nindy
An Intel 960 board controlled by a Nindy Monitor.  @var{devicename} is
the name of the serial device to use for the connection, e.g.
@file{/dev/ttya}.  @xref{i960-Nindy Remote, ,@value{GDBN} with a remote i960 (Nindy)}.

@end ifset
@ifset ST2000
@item target st2000 @var{dev} @var{speed}
@kindex target st2000
A Tandem ST2000 phone switch, running Tandem's STDBUG protocol.  @var{dev}
is the name of the device attached to the ST2000 serial line;
@var{speed} is the communication line speed.  The arguments are not used
if @value{GDBN} is configured to connect to the ST2000 using TCP or Telnet.
@xref{ST2000 Remote,,@value{GDBN} with a Tandem ST2000}.

@end ifset
@ifset VXWORKS
@item target vxworks @var{machinename}
@kindex target vxworks
A VxWorks system, attached via TCP/IP.  The argument @var{machinename}
is the target system's machine name or IP address.
@xref{VxWorks Remote, ,@value{GDBN} and VxWorks}.
@end ifset
@end table

@ifset GENERIC
Different targets are available on different configurations of @value{GDBN}; your
configuration may have more or fewer targets.
@end ifset

@node Remote
@section Remote debugging
@cindex remote debugging

If you are trying to debug a program running on a machine that cannot run
GDB in the usual way, it is often useful to use remote debugging.  For
example, you might use remote debugging on an operating system kernel, or on
a small system which does not have a general purpose operating system
powerful enough to run a full-featured debugger.

Some configurations of GDB have special serial or TCP/IP interfaces
to make this work with particular debugging targets.  In addition,
GDB comes with a generic serial protocol (specific to GDB, but
not specific to any particular target system) which you can use if you
write the remote stubs---the code that will run on the remote system to
communicate with GDB.

Other remote targets may be available in your
configuration of GDB; use @code{help targets} to list them.

@ifset GENERIC
@c Text on starting up GDB in various specific cases; it goes up front
@c in manuals configured for any of those particular situations, here
@c otherwise.
@menu
@ifset REMOTESTUB
* Remote Serial::               @value{GDBN} remote serial protocol
@end ifset
@ifset I960
* i960-Nindy Remote::           @value{GDBN} with a remote i960 (Nindy)
@end ifset
@ifset AMD29K
* UDI29K Remote::               @value{GDBN} and the UDI protocol for AMD29K
* EB29K Remote::                @value{GDBN} with a remote EB29K
@end ifset
@ifset VXWORKS
* VxWorks Remote::              @value{GDBN} and VxWorks
@end ifset
@ifset ST2000
* ST2000 Remote::               @value{GDBN} with a Tandem ST2000
@end ifset
@ifset H8
* Hitachi Remote::              @value{GDBN} and Hitachi Microprocessors
@end ifset
@ifset MIPS
* MIPS Remote::                 @value{GDBN} and MIPS boards
@end ifset
@ifset SIMS
* Simulator::                   Simulated CPU target
@end ifset
@end menu

@include remote.texi
@end ifset

@node Controlling GDB
@chapter Controlling @value{GDBN}

You can alter the way @value{GDBN} interacts with you by using
the @code{set} command.  For commands controlling how @value{GDBN} displays
data, @pxref{Print Settings, ,Print settings}; other settings are described here.

@menu
* Prompt::                      Prompt
* Editing::                     Command editing
* History::                     Command history
* Screen Size::                 Screen size
* Numbers::                     Numbers
* Messages/Warnings::           Optional warnings and messages
@end menu

@node Prompt
@section Prompt
@cindex prompt

@value{GDBN} indicates its readiness to read a command by printing a string
called the @dfn{prompt}.  This string is normally @samp{(@value{GDBP})}.  You
can change the prompt string with the @code{set prompt} command.  For
instance, when debugging @value{GDBN} with @value{GDBN}, it is useful to change
the prompt in one of the @value{GDBN} sessions so that you can always tell which
one you are talking to.

@table @code
@item set prompt @var{newprompt}
@kindex set prompt
Directs @value{GDBN} to use @var{newprompt} as its prompt string henceforth.
@kindex show prompt
@item show prompt
Prints a line of the form: @samp{Gdb's prompt is: @var{your-prompt}}
@end table

@node Editing
@section Command editing
@cindex readline
@cindex command line editing

@value{GDBN} reads its input commands via the @dfn{readline} interface.  This
GNU library provides consistent behavior for programs which provide a
command line interface to the user.  Advantages are @code{emacs}-style
or @code{vi}-style inline editing of commands, @code{csh}-like history
substitution, and a storage and recall of command history across
debugging sessions.

You may control the behavior of command line editing in @value{GDBN} with the
command @code{set}.

@table @code
@kindex set editing
@cindex editing
@item set editing
@itemx set editing on
Enable command line editing (enabled by default).

@item set editing off
Disable command line editing.

@kindex show editing
@item show editing
Show whether command line editing is enabled.
@end table

@node History
@section Command history

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

@table @code
@cindex history substitution
@cindex history file
@kindex set history filename
@item set history filename @var{fname}
Set the name of the @value{GDBN} command history file to @var{fname}.  This is
the file from which @value{GDBN} will read an initial command history
list or to which it will write this list when it exits.  This list is
accessed through history expansion or through the history
command editing characters listed below.  This file defaults to the
value of the environment variable @code{GDBHISTFILE}, or to
@file{./.gdb_history} if this variable is not set.

@cindex history save
@kindex set history save
@item set history save
@itemx set history save on
Record command history in a file, whose name may be specified with the
@code{set history filename} command.  By default, this option is disabled.

@item set history save off
Stop recording command history in a file.

@cindex history size
@kindex set history size
@item set history size @var{size}
Set the number of commands which @value{GDBN} will keep in its history list.
This defaults to the value of the environment variable
@code{HISTSIZE}, or to 256 if this variable is not set.
@end table

@cindex history expansion
History expansion assigns special meaning to the character @kbd{!}.
@ifset have-readline-appendices
@xref{Event Designators}.
@end ifset

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

The commands to control history expansion are:

@table @code

@kindex set history expansion
@item set history expansion on
@itemx set history expansion
Enable history expansion.  History expansion is off by default.

@item set history expansion off
Disable history expansion.

The readline code comes with more complete documentation of
editing and history expansion features.  Users unfamiliar with @code{emacs}
or @code{vi} may wish to read it.
@ifset have-readline-appendices
@xref{Command Line Editing}.
@end ifset

@c @group
@kindex show history
@item show history
@itemx show history filename
@itemx show history save
@itemx show history size
@itemx show history expansion
These commands display the state of the @value{GDBN} history parameters.
@code{show history} by itself displays all four states.
@c @end group
@end table

@table @code
@kindex show commands
@item show commands
Display the last ten commands in the command history.

@item show commands @var{n}
Print ten commands centered on command number @var{n}.

@item show commands +
Print ten commands just after the commands last printed.
@end table

@node Screen Size
@section Screen size
@cindex size of screen
@cindex pauses in output

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

Normally @value{GDBN} knows the size of the screen from the termcap data base
together with the value of the @code{TERM} environment variable and the
@code{stty rows} and @code{stty cols} settings. If this is not correct,
you can override it with the @code{set height} and @code{set
width} commands:

@table @code
@item set height @var{lpp}
@itemx show height
@itemx set width @var{cpl}
@itemx show width
@kindex set height
@kindex set width
@kindex show width
@kindex show height
These @code{set} commands specify a screen height of @var{lpp} lines and
a screen width of @var{cpl} characters.  The associated @code{show}
commands display the current settings.

If you specify a height of zero lines, @value{GDBN} will not pause during output
no matter how long the output is.  This is useful if output is to a file
or to an editor buffer.

Likewise, you can specify @samp{set width 0} to prevent @value{GDBN}
from wrapping its output.
@end table

@node Numbers
@section Numbers
@cindex number representation
@cindex entering numbers

You can always enter numbers in octal, decimal, or hexadecimal in @value{GDBN} by
the usual conventions: octal numbers begin with @samp{0}, decimal
numbers end with @samp{.}, and hexadecimal numbers begin with @samp{0x}.
Numbers that begin with none of these are, by default, entered in base
10; likewise, the default display for numbers---when no particular
format is specified---is base 10.  You can change the default base for
both input and output with the @code{set radix} command.

@table @code
@kindex set radix
@item set radix @var{base}
Set the default base for numeric input and display.  Supported choices
for @var{base} are decimal 8, 10, or 16.  @var{base} must itself be
specified either unambiguously or using the current default radix; for
example, any of

@example
set radix 012
set radix 10.
set radix 0xa
@end example

@noindent
will set the base to decimal.  On the other hand, @samp{set radix 10}
will leave the radix unchanged no matter what it was.

@kindex show radix
@item show radix
Display the current default base for numeric input and display.
@end table

@node Messages/Warnings
@section Optional warnings and messages

By default, @value{GDBN} is silent about its inner workings.  If you are running
on a slow machine, you may want to use the @code{set verbose} command.
It will make @value{GDBN} tell you when it does a lengthy internal operation, so
you will not think it has crashed.

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

@table @code
@kindex set verbose
@item set verbose on
Enables @value{GDBN} output of certain informational messages.

@item set verbose off
Disables @value{GDBN} output of certain informational messages.

@kindex show verbose
@item show verbose
Displays whether @code{set verbose} is on or off.
@end table

By default, if @value{GDBN} encounters bugs in the symbol table of an object
file, it is silent; but if you are debugging a compiler, you may find
this information useful (@pxref{Symbol Errors, ,Errors reading symbol files}).

@table @code
@kindex set complaints
@item set complaints @var{limit}
Permits @value{GDBN} to output @var{limit} complaints about each type of unusual
symbols before becoming silent about the problem.  Set @var{limit} to
zero to suppress all complaints; set it to a large number to prevent
complaints from being suppressed.

@kindex show complaints
@item show complaints
Displays how many symbol complaints @value{GDBN} is permitted to produce.
@end table

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

@example
(@value{GDBP}) run
The program being debugged has been started already.
Start it from the beginning? (y or n)
@end example

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

@table @code
@kindex set confirm
@cindex flinching
@cindex confirmation
@cindex stupid questions
@item set confirm off
Disables confirmation requests.

@item set confirm on
Enables confirmation requests (the default).

@item show confirm
@kindex show confirm
Displays state of confirmation requests.
@end table

@c FIXME this does not really belong here.  But where *does* it belong?
@cindex reloading symbols
Some systems allow individual object files that make up your program to
be replaced without stopping and restarting your program.
@ifset VXWORKS
For example, in VxWorks you can simply recompile a defective object file
and keep on running.
@end ifset
If you are running on one of these systems, you can allow @value{GDBN} to
reload the symbols for automatically relinked modules:

@table @code
@kindex set symbol-reloading
@item set symbol-reloading on
Replace symbol definitions for the corresponding source file when an
object file with a particular name is seen again.

@item set symbol-reloading off
Do not replace symbol definitions when re-encountering object files of
the same name.  This is the default state; if you are not running on a
system that permits automatically relinking modules, you should leave
@code{symbol-reloading} off, since otherwise @value{GDBN} may discard symbols
when linking large programs, that may contain several modules (from
different directories or libraries) with the same name.

@item show symbol-reloading
Show the current @code{on} or @code{off} setting.
@end table

@node Sequences
@chapter Canned Sequences of Commands

Aside from breakpoint commands (@pxref{Break Commands, ,Breakpoint
command lists}), @value{GDBN} provides two ways to store sequences of commands
for execution as a unit: user-defined commands and command files.

@menu
* Define::                      User-defined commands
* Hooks::                       User-defined command hooks
* Command Files::               Command files
* Output::                      Commands for controlled output
@end menu

@node Define
@section User-defined commands

@cindex user-defined command
A @dfn{user-defined command} is a sequence of @value{GDBN} commands to which you
assign a new name as a command.  This is done with the @code{define}
command.

@table @code
@item define @var{commandname}
@kindex define
Define a command named @var{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 @value{GDBN} command lines,
which are given following the @code{define} command.  The end of these
commands is marked by a line containing @code{end}.

@item document @var{commandname}
@kindex document
Give documentation to the user-defined command @var{commandname}.  The
command @var{commandname} must already be defined.  This command reads
lines of documentation just as @code{define} reads the lines of the
command definition, ending with @code{end}.  After the @code{document}
command is finished, @code{help} on command @var{commandname} will print
the documentation you have specified.

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

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

@item show user
@itemx show user @var{commandname}
@kindex show user
Display the @value{GDBN} commands used to define @var{commandname} (but not its
documentation).  If no @var{commandname} is given, display the
definitions for all user-defined commands.
@end table

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 @value{GDBN} commands
that normally print messages to say what they are doing omit the messages
when used in a user-defined command.

@node Hooks
@section User-defined command hooks
@cindex command files

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

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

@ifclear BARETARGET
For example, to ignore @code{SIGALRM} signals while
single-stepping, but treat them normally during normal execution,
you could define:

@example
define hook-stop
handle SIGALRM nopass
end

define hook-run
handle SIGALRM pass
end

define hook-continue
handle SIGLARM pass
end
@end example
@end ifclear

You can define a hook for any single-word command in @value{GDBN}, but
not for command aliases; you should define a hook for the basic command
name, e.g.  @code{backtrace} rather than @code{bt}.
@c FIXME!  So how does Joe User discover whether a command is an alias
@c or not?
If an error occurs during the execution of your hook, execution of
@value{GDBN} commands stops and @value{GDBN} issues a prompt
(before the command that you actually typed had a chance to run).

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

@node Command Files
@section Command files

@cindex command files
A command file for @value{GDBN} is a file of lines that are @value{GDBN} commands.  Comments
(lines starting with @kbd{#}) 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.

@cindex init file
@cindex @file{@value{GDBINIT}}
When you start @value{GDBN}, it automatically executes commands from its
@dfn{init files}.  These are files named @file{@value{GDBINIT}}.  @value{GDBN} 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 you use the @samp{-nx} option; @pxref{Mode Options,
,Choosing modes}.)

@ifset GENERIC
@cindex init file name
On some configurations of @value{GDBN}, the init file is known by a
different name (these are typically environments where a specialized
form of GDB may need to coexist with other forms, hence a different name
for the specialized version's init file).  These are the environments
with special init file names:

@itemize @bullet
@kindex .vxgdbinit
@item
VxWorks (Wind River Systems real-time OS): @samp{.vxgdbinit}

@kindex .os68gdbinit
@item
OS68K (Enea Data Systems real-time OS): @samp{.os68gdbinit}

@kindex .esgdbinit
@item
ES-1800 (Ericsson Telecom AB M68000 emulator): @samp{.esgdbinit}
@end itemize
@end ifset

You can also request the execution of a command file with the
@code{source} command:

@table @code
@item source @var{filename}
@kindex source
Execute the command file @var{filename}.
@end table

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 @value{GDBN} commands that
normally print messages to say what they are doing omit the messages
when called from command files.

@node Output
@section Commands for controlled output

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

@table @code
@item echo @var{text}
@kindex echo
@c I do not consider backslash-space a standard C escape sequence
@c because it is not in ANSI.
Print @var{text}.  Nonprinting characters can be included in
@var{text} using C escape sequences, such as @samp{\n} to print a
newline.  @strong{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 displaying a
string with spaces at the beginning or the end, since leading and
trailing spaces are otherwise trimmed from all arguments.  
To print @samp{@w{ }and foo =@w{ }}, use the command
@samp{echo \@w{ }and foo = \@w{ }}.

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

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

produces the same output as

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

@item output @var{expression}
@kindex output
Print the value of @var{expression} and nothing but that value: no
newlines, no @samp{$@var{nn} = }.  The value is not entered in the
value history either.  @xref{Expressions, ,Expressions}, for more information on
expressions.

@item output/@var{fmt} @var{expression}
Print the value of @var{expression} in format @var{fmt}.  You can use
the same formats as for @code{print}.  @xref{Output Formats,,Output
formats}, for more information.

@item printf @var{string}, @var{expressions}@dots{}
@kindex printf
Print the values of the @var{expressions} under the control of
@var{string}.  The @var{expressions} are separated by commas and may be
either numbers or pointers.  Their values are printed as specified by
@var{string}, exactly as if your program were to execute the C
subroutine

@example
printf (@var{string}, @var{expressions}@dots{});
@end example

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

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

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

@ifclear DOSHOST
@node Emacs
@chapter Using @value{GDBN} under GNU Emacs

@cindex emacs
A special interface allows you to use GNU Emacs to view (and
edit) the source files for the program you are debugging with
@value{GDBN}.

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

Using @value{GDBN} under Emacs is just like using @value{GDBN} normally except for two
things:

@itemize @bullet
@item
All ``terminal'' input and output goes through the Emacs buffer.
@end itemize

This applies both to @value{GDBN} commands and their output, and to the input
and output done by the program you are debugging.

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

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

@itemize @bullet
@item
@value{GDBN} displays source code through Emacs.
@end itemize

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

Explicit @value{GDBN} @code{list} or search commands still produce output as
usual, but you probably will have no reason to use them.

@quotation
@emph{Warning:} If the directory where your program resides is not your
current directory, it can be easy to confuse Emacs about the location of
the source files, in which case the auxiliary display buffer will not
appear to show your source.  @value{GDBN} can find programs by searching your
environment's @code{PATH} variable, so the @value{GDBN} input and output
session will proceed normally; but Emacs does not get enough information
back from @value{GDBN} to locate the source files in this situation.  To
avoid this problem, either start @value{GDBN} mode from the directory where
your program resides, or specify a full path name when prompted for the
@kbd{M-x gdb} argument.

A similar confusion can result if you use the @value{GDBN} @code{file} command to
switch to debugging a program in some other location, from an existing
@value{GDBN} buffer in Emacs.
@end quotation

By default, @kbd{M-x gdb} calls the program called @file{gdb}.  If
you need to call @value{GDBN} by a different name (for example, if you keep
several configurations around, with different names) you can set the
Emacs variable @code{gdb-command-name}; for example,

@example
(setq gdb-command-name "mygdb")
@end example

@noindent
(preceded by @kbd{ESC ESC}, or typed in the @code{*scratch*} buffer, or
in your @file{.emacs} file) will make Emacs call the program named
``@code{mygdb}'' instead.

In the @value{GDBN} I/O buffer, you can use these special Emacs commands in
addition to the standard Shell mode commands:

@table @kbd
@item C-h m
Describe the features of Emacs' @value{GDBN} Mode.

@item M-s
Execute to another source line, like the @value{GDBN} @code{step} command; also
update the display window to show the current file and location.

@item M-n
Execute to next source line in this function, skipping all function
calls, like the @value{GDBN} @code{next} command.  Then update the display window
to show the current file and location.

@item M-i
Execute one instruction, like the @value{GDBN} @code{stepi} command; update
display window accordingly.

@item M-x gdb-nexti
Execute to next instruction, using the @value{GDBN} @code{nexti} command; update
display window accordingly.

@item C-c C-f
Execute until exit from the selected stack frame, like the @value{GDBN}
@code{finish} command.

@item M-c
Continue execution of your program, like the @value{GDBN} @code{continue}
command.

@emph{Warning:} In Emacs v19, this command is @kbd{C-c C-p}.

@item M-u
Go up the number of frames indicated by the numeric argument
(@pxref{Arguments, , Numeric Arguments, emacs, The GNU Emacs Manual}),
like the @value{GDBN} @code{up} command.

@emph{Warning:} In Emacs v19, this command is @kbd{C-c C-u}.

@item M-d
Go down the number of frames indicated by the numeric argument, like the
@value{GDBN} @code{down} command.

@emph{Warning:} In Emacs v19, this command is @kbd{C-c C-d}.

@item C-x &
Read the number where the cursor is positioned, and insert it at the end
of the @value{GDBN} I/O buffer.  For example, if you wish to disassemble code
around an address that was displayed earlier, type @kbd{disassemble};
then move the cursor to the address display, and pick up the
argument for @code{disassemble} by typing @kbd{C-x &}.

You can customize this further by defining elements of the list
@code{gdb-print-command}; once it is defined, you can format or
otherwise process numbers picked up by @kbd{C-x &} before they are
inserted.  A numeric argument to @kbd{C-x &} will both indicate that you
wish special formatting, and act as an index to pick an element of the
list.  If the list element is a string, the number to be inserted is
formatted using the Emacs function @code{format}; otherwise the number
is passed as an argument to the corresponding list element.
@end table

In any source file, the Emacs command @kbd{C-x SPC} (@code{gdb-break})
tells @value{GDBN} to set a breakpoint on the source line point is on.

If you accidentally delete the source-display buffer, an easy way to get
it back is to type the command @code{f} in the @value{GDBN} buffer, to
request a frame display; when you run under Emacs, this will recreate
the source buffer if necessary to show you the context of the current
frame.

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

@c The following dropped because Epoch is nonstandard.  Reactivate
@c if/when v19 does something similar. ---pesch@cygnus.com 19dec1990
@ignore
@kindex emacs epoch environment
@kindex epoch
@kindex inspect

Version 18 of Emacs has a built-in window system called the @code{epoch}
environment.  Users of this environment can use a new command,
@code{inspect} which performs identically to @code{print} except that
each value is printed in its own window.
@end ignore
@end ifclear

@ifset LUCID
@node Energize
@chapter Using @value{GDBN} with Energize

@cindex Energize
The Energize Programming System is an integrated development environment
that includes a point-and-click interface to many programming tools.
When you use @value{GDBN} in this environment, you can use the standard
Energize graphical interface to drive @value{GDBN}; you can also, if you
choose, type @value{GDBN} commands as usual in a debugging window.  Even if
you use the graphical interface, the debugging window (which uses Emacs,
and resembles the standard Emacs interface to @value{GDBN}) displays the
equivalent commands, so that the history of your debugging session is
properly reflected.

When Energize starts up a @value{GDBN} session, it uses one of the
command-line options @samp{-energize} or @samp{-cadillac} (``cadillac''
is the name of the communications protocol used by the Energize system).
This option makes @value{GDBN} run as one of the tools in the Energize Tool
Set: it sends all output to the Energize kernel, and accept input from
it as well.

See the user manual for the Energize Programming System for 
information on how to use the Energize graphical interface and the other
development tools that Energize integrates with @value{GDBN}.

@end ifset

@node GDB Bugs
@chapter Reporting Bugs in @value{GDBN}
@cindex bugs in @value{GDBN}
@cindex reporting bugs in @value{GDBN}

Your bug reports play an essential role in making @value{GDBN} reliable.

Reporting a bug may help you by bringing a solution to your problem, or it
may not.  But in any case the principal function of a bug report is to help
the entire community by making the next version of @value{GDBN} work better.  Bug
reports are your contribution to the maintenance of @value{GDBN}.

In order for a bug report to serve its purpose, you must include the
information that enables us to fix the bug.

@menu
* Bug Criteria::                Have you found a bug?
* Bug Reporting::               How to report bugs
@end menu

@node Bug Criteria
@section Have you found a bug?
@cindex bug criteria

If you are not sure whether you have found a bug, here are some guidelines:

@itemize @bullet
@item
@cindex fatal signal
@cindex debugger crash
@cindex crash of debugger
If the debugger gets a fatal signal, for any input whatever, that is a
@value{GDBN} bug.  Reliable debuggers never crash.

@item
@cindex error on valid input
If @value{GDBN} produces an error message for valid input, that is a bug.

@item
@cindex invalid input
If @value{GDBN} does not produce an error message for invalid input,
that is a bug.  However, you should note that your idea of
``invalid input'' might be our idea of ``an extension'' or ``support
for traditional practice''.

@item
If you are an experienced user of debugging tools, your suggestions
for improvement of @value{GDBN} are welcome in any case.
@end itemize

@node Bug Reporting
@section How to report bugs
@cindex bug reports
@cindex @value{GDBN} bugs, reporting

A number of companies and individuals offer support for GNU products.
If you obtained @value{GDBN} from a support organization, we recommend you
contact that organization first.

You can find contact information for many support companies and
individuals in the file @file{etc/SERVICE} in the GNU Emacs
distribution.

In any event, we also recommend that you send bug reports for @value{GDBN} to one
of these addresses:

@example
bug-gdb@@prep.ai.mit.edu
@{ucbvax|mit-eddie|uunet@}!prep.ai.mit.edu!bug-gdb
@end example

@strong{Do not send bug reports to @samp{info-gdb}, or to
@samp{help-gdb}, or to any newsgroups.} Most users of @value{GDBN} do not want to
receive bug reports.  Those that do, have arranged to receive @samp{bug-gdb}.

The mailing list @samp{bug-gdb} has a newsgroup @samp{gnu.gdb.bug} which
serves as a repeater.  The mailing list and the newsgroup carry exactly
the same messages.  Often people think of posting bug reports to the
newsgroup instead of mailing them.  This appears to work, but it has one
problem which can be crucial: a newsgroup posting often lacks a mail
path back to the sender.  Thus, if we need to ask for more information,
we may be unable to reach you.  For this reason, it is better to send
bug reports to the mailing list.

As a last resort, send bug reports on paper to:

@example
GNU Debugger Bugs
Free Software Foundation
545 Tech Square
Cambridge, MA 02139
@end example

The fundamental principle of reporting bugs usefully is this:
@strong{report all the facts}.  If you are not sure whether to state a
fact or leave it out, state it!

Often people omit facts because they think they know what causes the
problem and assume that some details do not matter.  Thus, you might
assume that the name of the variable you use in an example does not matter.
Well, probably it does not, but one cannot be sure.  Perhaps the bug is a
stray memory reference which happens to fetch from the location where that
name is stored in memory; perhaps, if the name were different, the contents
of that location would fool the debugger into doing the right thing despite
the bug.  Play it safe and give a specific, complete example.  That is the
easiest thing for you to do, and the most helpful.

Keep in mind that the purpose of a bug report is to enable us to fix
the bug if it is new to us.  It is not as important as what happens if
the bug is already known.  Therefore, always write your bug reports on
the assumption that the bug has not been reported previously.

Sometimes people give a few sketchy facts and ask, ``Does this ring a
bell?''  Those bug reports are useless, and we urge everyone to
@emph{refuse to respond to them} except to chide the sender to report
bugs properly.

To enable us to fix the bug, you should include all these things:

@itemize @bullet
@item
The version of @value{GDBN}.  @value{GDBN} announces it if you start with no
arguments; you can also print it at any time using @code{show version}.

Without this, we will not know whether there is any point in looking for
the bug in the current version of @value{GDBN}.

@item
The type of machine you are using, and the operating system name and
version number.

@item
What compiler (and its version) was used to compile @value{GDBN}---e.g.
``@value{GCC}--2.0''.

@item
What compiler (and its version) was used to compile the program you
are debugging---e.g.  ``@value{GCC}--2.0''.

@item
The command arguments you gave the compiler to compile your example and
observe the bug.  For example, did you use @samp{-O}?  To guarantee
you will not omit something important, list them all.  A copy of the
Makefile (or the output from make) is sufficient.

If we were to try to guess the arguments, we would probably guess wrong
and then we might not encounter the bug.

@item
A complete input script, and all necessary source files, that will
reproduce the bug.

@item
A description of what behavior you observe that you believe is
incorrect.  For example, ``It gets a fatal signal.''

Of course, if the bug is that @value{GDBN} gets a fatal signal, then we will
certainly notice it.  But if the bug is incorrect output, we might not
notice unless it is glaringly wrong.  We are human, after all.  You
might as well not give us a chance to make a mistake.

Even if the problem you experience is a fatal signal, you should still
say so explicitly.  Suppose something strange is going on, such as,
your copy of @value{GDBN} is out of synch, or you have encountered a
bug in the C library on your system.  (This has happened!)  Your copy
might crash and ours would not.  If you told us to expect a crash,
then when ours fails to crash, we would know that the bug was not
happening for us.  If you had not told us to expect a crash, then we
would not be able to draw any conclusion from our observations.

@item
If you wish to suggest changes to the @value{GDBN} source, send us context
diffs.  If you even discuss something in the @value{GDBN} source, refer to
it by context, not by line number.

The line numbers in our development sources will not match those in your
sources.  Your line numbers would convey no useful information to us.
@end itemize

Here are some things that are not necessary:

@itemize @bullet
@item
A description of the envelope of the bug.

Often people who encounter a bug spend a lot of time investigating
which changes to the input file will make the bug go away and which
changes will not affect it.

This is often time consuming and not very useful, because the way we
will find the bug is by running a single example under the debugger
with breakpoints, not by pure deduction from a series of examples.
We recommend that you save your time for something else.

Of course, if you can find a simpler example to report @emph{instead}
of the original one, that is a convenience for us.  Errors in the
output will be easier to spot, running under the debugger will take
less time, etc.

However, simplification is not vital; if you do not want to do this,
report the bug anyway and send us the entire test case you used.

@item
A patch for the bug.

A patch for the bug does help us if it is a good one.  But do not omit
the necessary information, such as the test case, on the assumption that
a patch is all we need.  We might see problems with your patch and decide
to fix the problem another way, or we might not understand it at all.

Sometimes with a program as complicated as @value{GDBN} it is very hard to
construct an example that will make the program follow a certain path
through the code.  If you do not send us the example, we will not be able
to construct one, so we will not be able to verify that the bug is fixed.

And if we cannot understand what bug you are trying to fix, or why your
patch should be an improvement, we will not install it.  A test case will
help us to understand.

@item
A guess about what the bug is or what it depends on.

Such guesses are usually wrong.  Even we cannot guess right about such
things without first using the debugger to find the facts.
@end itemize

@c The readline documentation is distributed with the readline code 
@c and consists of the two following files:
@c     rluser.texinfo
@c     inc-hist.texi
@c Use -I with makeinfo to point to the appropriate directory,
@c environment var TEXINPUTS with TeX.
@include rluser.texinfo
@include inc-hist.texi

@ifset NOVEL
@node Renamed Commands
@appendix Renamed Commands

The following commands were renamed in GDB 4, in order to make the
command set as a whole more consistent and easier to use and remember:

@kindex add-syms
@kindex delete environment
@kindex info copying
@kindex info convenience
@kindex info directories
@kindex info editing
@kindex info history
@kindex info targets
@kindex info values
@kindex info version
@kindex info warranty
@kindex set addressprint
@kindex set arrayprint
@kindex set prettyprint
@kindex set screen-height
@kindex set screen-width
@kindex set unionprint
@kindex set vtblprint
@kindex set demangle
@kindex set asm-demangle
@kindex set sevenbit-strings
@kindex set array-max
@kindex set caution
@kindex set history write
@kindex show addressprint
@kindex show arrayprint
@kindex show prettyprint
@kindex show screen-height
@kindex show screen-width
@kindex show unionprint
@kindex show vtblprint
@kindex show demangle
@kindex show asm-demangle
@kindex show sevenbit-strings
@kindex show array-max
@kindex show caution
@kindex show history write
@kindex unset

@c TEXI2ROFF-KILL
@ifinfo
@c END TEXI2ROFF-KILL
@example
OLD COMMAND               NEW COMMAND
@c TEXI2ROFF-KILL
---------------           -------------------------------
@c END TEXI2ROFF-KILL
add-syms                  add-symbol-file
delete environment        unset environment
info convenience          show convenience
info copying              show copying
info directories          show directories
info editing              show commands
info history              show values
info targets              help target
info values               show values
info version              show version
info warranty             show warranty
set/show addressprint     set/show print address
set/show array-max        set/show print elements
set/show arrayprint       set/show print array
set/show asm-demangle     set/show print asm-demangle
set/show caution          set/show confirm
set/show demangle         set/show print demangle
set/show history write    set/show history save
set/show prettyprint      set/show print pretty
set/show screen-height    set/show height
set/show screen-width     set/show width
set/show sevenbit-strings set/show print sevenbit-strings
set/show unionprint       set/show print union
set/show vtblprint        set/show print vtbl

unset                     [No longer an alias for delete]
@end example
@c TEXI2ROFF-KILL
@end ifinfo

@tex
\vskip \parskip\vskip \baselineskip
\halign{\tt #\hfil &\qquad#&\tt #\hfil\cr
{\bf Old Command}         &&{\bf New Command}\cr
add-syms                  &&add-symbol-file\cr
delete environment        &&unset environment\cr
info convenience          &&show convenience\cr
info copying              &&show copying\cr
info directories          &&show directories     \cr
info editing              &&show commands\cr
info history              &&show values\cr
info targets              &&help target\cr
info values               &&show values\cr
info version              &&show version\cr
info warranty             &&show warranty\cr
set{\rm / }show addressprint     &&set{\rm / }show print address\cr
set{\rm / }show array-max        &&set{\rm / }show print elements\cr
set{\rm / }show arrayprint       &&set{\rm / }show print array\cr
set{\rm / }show asm-demangle     &&set{\rm / }show print asm-demangle\cr
set{\rm / }show caution          &&set{\rm / }show confirm\cr
set{\rm / }show demangle         &&set{\rm / }show print demangle\cr
set{\rm / }show history write    &&set{\rm / }show history save\cr
set{\rm / }show prettyprint      &&set{\rm / }show print pretty\cr
set{\rm / }show screen-height    &&set{\rm / }show height\cr
set{\rm / }show screen-width     &&set{\rm / }show width\cr
set{\rm / }show sevenbit-strings &&set{\rm / }show print sevenbit-strings\cr
set{\rm / }show unionprint       &&set{\rm / }show print union\cr
set{\rm / }show vtblprint        &&set{\rm / }show print vtbl\cr
\cr
unset                     &&\rm(No longer an alias for delete)\cr
}
@end tex
@c END TEXI2ROFF-KILL
@end ifset

@ifclear PRECONFIGURED
@node Formatting Documentation
@appendix Formatting Documentation

@cindex GDB reference card
@cindex reference card
The GDB 4 release includes an already-formatted reference card, ready
for printing with PostScript or GhostScript, in the @file{gdb}
subdirectory of the main source directory@footnote{In
@file{gdb-@value{GDBVN}/gdb/refcard.ps} of the version @value{GDBVN}
release.}.  If you can use PostScript or GhostScript with your printer,
you can print the reference card immediately with @file{refcard.ps}.

The release also includes the source for the reference card.  You
can format it, using @TeX{}, by typing:

@example
make refcard.dvi
@end example

The GDB reference card is designed to print in landscape mode on US
``letter'' size paper; that is, on a sheet 11 inches wide by 8.5 inches
high.  You will need to specify this form of printing as an option to
your @sc{dvi} output program.

@cindex documentation

All the documentation for GDB comes as part of the machine-readable
distribution.  The documentation is written in Texinfo format, which is
a documentation system that uses a single source file to produce both
on-line information and a printed manual.  You can use one of the Info
formatting commands to create the on-line version of the documentation
and @TeX{} (or @code{texi2roff}) to typeset the printed version.

GDB includes an already formatted copy of the on-line Info version of
this manual in the @file{gdb} subdirectory.  The main Info file is
@file{gdb-@var{version-number}/gdb/gdb.info}, and it refers to
subordinate files matching @samp{gdb.info*} in the same directory.  If
necessary, you can print out these files, or read them with any editor;
but they are easier to read using the @code{info} subsystem in GNU Emacs
or the standalone @code{info} program, available as part of the GNU
Texinfo distribution.

If you want to format these Info files yourself, you need one of the
Info formatting programs, such as @code{texinfo-format-buffer} or
@code{makeinfo}.

If you have @code{makeinfo} installed, and are in the top level GDB
source directory (@file{gdb-@value{GDBVN}}, in the case of version @value{GDBVN}), you can
make the Info file by typing:

@example
cd gdb
make gdb.info
@end example

If you want to typeset and print copies of this manual, you need @TeX{},
a program to print its @sc{dvi} output files, and @file{texinfo.tex}, the
Texinfo definitions file.

@TeX{} is a typesetting program; it does not print files directly, but
produces output files called @sc{dvi} files.  To print a typeset
document, you need a program to print @sc{dvi} files.  If your system
has @TeX{} installed, chances are it has such a program.  The precise
command to use depends on your system; @kbd{lpr -d} is common; another
(for PostScript devices) is @kbd{dvips}.  The @sc{dvi} print command may
require a file name without any extension or a @samp{.dvi} extension.

@TeX{} also requires a macro definitions file called
@file{texinfo.tex}.  This file tells @TeX{} how to typeset a document
written in Texinfo format.  On its own, @TeX{} cannot read, much less
typeset a Texinfo file.  @file{texinfo.tex} is distributed with GDB
and is located in the @file{gdb-@var{version-number}/texinfo}
directory.

If you have @TeX{} and a @sc{dvi} printer program installed, you can
typeset and print this manual.  First switch to the the @file{gdb}
subdirectory of the main source directory (for example, to
@file{gdb-@value{GDBVN}/gdb}) and then type:

@example
make gdb.dvi
@end example

@node Installing GDB
@appendix Installing GDB
@cindex configuring GDB
@cindex installation

GDB comes with a @code{configure} script that automates the process
of preparing GDB for installation; you can then use @code{make} to
build the @code{gdb} program.
@iftex
@c irrelevant in info file; it's as current as the code it lives with.
@footnote{If you have a more recent version of GDB than @value{GDBVN},
look at the @file{README} file in the sources; we may have improved the
installation procedures since publishing this manual.}
@end iftex

The GDB distribution includes all the source code you need for GDB in
a single directory, whose name is usually composed by appending the
version number to @samp{gdb}.

For example, the GDB version @value{GDBVN} distribution is in the
@file{gdb-@value{GDBVN}} directory.  That directory contains:

@table @code
@item gdb-@value{GDBVN}/configure @r{(and supporting files)}
script for configuring GDB and all its supporting libraries.

@item gdb-@value{GDBVN}/gdb
the source specific to GDB itself

@item gdb-@value{GDBVN}/bfd
source for the Binary File Descriptor library

@item gdb-@value{GDBVN}/include
GNU include files

@item gdb-@value{GDBVN}/libiberty
source for the @samp{-liberty} free software library

@item gdb-@value{GDBVN}/opcodes
source for the library of opcode tables and disassemblers

@item gdb-@value{GDBVN}/readline
source for the GNU command-line interface

@item gdb-@value{GDBVN}/glob
source for the GNU filename pattern-matching subroutine

@item gdb-@value{GDBVN}/mmalloc
source for the GNU memory-mapped malloc package
@end table

The simplest way to configure and build GDB is to run @code{configure}
from the @file{gdb-@var{version-number}} source directory, which in
this example is the @file{gdb-@value{GDBVN}} directory.

First switch to the @file{gdb-@var{version-number}} source directory
if you are not already in it; then run @code{configure}.  Pass the
identifier for the platform on which GDB will run as an
argument.

For example:

@example
cd gdb-@value{GDBVN}
./configure @var{host}
make
@end example

@noindent
where @var{host} is an identifier such as @samp{sun4} or
@samp{decstation}, that identifies the platform where GDB will run.
(You can often leave off @var{host}; @code{configure} tries to guess the
correct value by examining your system.)

Running @samp{configure @var{host}} and then running @code{make} builds the
@file{bfd}, @file{readline}, @file{mmalloc}, and @file{libiberty}
libraries, then @code{gdb} itself.  The configured source files, and the
binaries, are left in the corresponding source directories.

@code{configure} is a Bourne-shell (@code{/bin/sh}) script; if your
system does not recognize this automatically when you run a different
shell, you may need to run @code{sh} on it explicitly:

@example
sh configure @var{host}
@end example

If you run @code{configure} from a directory that contains source
directories for multiple libraries or programs, such as the
@file{gdb-@value{GDBVN}} source directory for version @value{GDBVN}, @code{configure}
creates configuration files for every directory level underneath (unless
you tell it not to, with the @samp{--norecursion} option).

You can run the @code{configure} script from any of the
subordinate directories in the GDB distribution if you only want to
configure that subdirectory, but be sure to specify a path to it.

For example, with version @value{GDBVN}, type the following to configure only
the @code{bfd} subdirectory:

@example
@group
cd gdb-@value{GDBVN}/bfd
../configure @var{host}
@end group
@end example

You can install @code{@value{GDBP}} anywhere; it has no hardwired paths.
However, you should make sure that the shell on your path (named by
the @samp{SHELL} environment variable) is publicly readable.  Remember
that GDB uses the shell to start your program---some systems refuse to
let GDB debug child processes whose programs are not readable.

@menu
* Separate Objdir::             Compiling GDB in another directory
* Config Names::                Specifying names for hosts and targets
* configure Options::           Summary of options for configure
@end menu

@node Separate Objdir
@section Compiling GDB in another directory

If you want to run GDB versions for several host or target machines,
you need a different @code{gdb} compiled for each combination of
host and target.  @code{configure} is designed to make this easy by
allowing you to generate each configuration in a separate subdirectory,
rather than in the source directory.  If your @code{make} program
handles the @samp{VPATH} feature (GNU @code{make} does), running
@code{make} in each of these directories builds the @code{gdb}
program specified there.

To build @code{gdb} in a separate directory, run @code{configure}
with the @samp{--srcdir} option to specify where to find the source.
(You also need to specify a path to find @code{configure}
itself from your working directory.  If the path to @code{configure}
would be the same as the argument to @samp{--srcdir}, you can leave out
the @samp{--srcdir} option; it will be assumed.)

For example, with version @value{GDBVN}, you can build GDB in a separate
directory for a Sun 4 like this:

@example
@group
cd gdb-@value{GDBVN}
mkdir ../gdb-sun4
cd ../gdb-sun4
../gdb-@value{GDBVN}/configure sun4
make
@end group
@end example

When @code{configure} builds a configuration using a remote source
directory, it creates a tree for the binaries with the same structure
(and using the same names) as the tree under the source directory.  In
the example, you'd find the Sun 4 library @file{libiberty.a} in the
directory @file{gdb-sun4/libiberty}, and GDB itself in
@file{gdb-sun4/gdb}.

One popular reason to build several GDB configurations in separate
directories is to configure GDB for cross-compiling (where GDB
runs on one machine---the host---while debugging programs that run on
another machine---the target).  You specify a cross-debugging target by
giving the @samp{--target=@var{target}} option to @code{configure}.

When you run @code{make} to build a program or library, you must run
it in a configured directory---whatever directory you were in when you
called @code{configure} (or one of its subdirectories).

The @code{Makefile} that @code{configure} generates in each source
directory also runs recursively.  If you type @code{make} in a source
directory such as @file{gdb-@value{GDBVN}} (or in a separate configured
directory configured with @samp{--srcdir=@var{path}/gdb-@value{GDBVN}}), you
will build all the required libraries, and then build GDB.

When you have multiple hosts or targets configured in separate
directories, you can run @code{make} on them in parallel (for example,
if they are NFS-mounted on each of the hosts); they will not interfere
with each other.

@node Config Names
@section Specifying names for hosts and targets

The specifications used for hosts and targets in the @code{configure}
script are based on a three-part naming scheme, but some short predefined
aliases are also supported.  The full naming scheme encodes three pieces
of information in the following pattern:

@example
@var{architecture}-@var{vendor}-@var{os}
@end example

For example, you can use the alias @code{sun4} as a @var{host} argument,
or as the value for @var{target} in a @code{--target=@var{target}}
option.  The equivalent full name is @samp{sparc-sun-sunos4}.

The @code{configure} script accompanying GDB does not provide
any query facility to list all supported host and target names or
aliases.  @code{configure} calls the Bourne shell script
@code{config.sub} to map abbreviations to full names; you can read the
script, if you wish, or you can use it to test your guesses on
abbreviations---for example:

@smallexample
% sh config.sub sun4
sparc-sun-sunos4.1.1
% sh config.sub sun3
m68k-sun-sunos4.1.1
% sh config.sub decstation
mips-dec-ultrix4.2
% sh config.sub hp300bsd
m68k-hp-bsd
% sh config.sub i386v
i386-unknown-sysv
% sh config.sub i786v
Invalid configuration `i786v': machine `i786v' not recognized
@end smallexample

@noindent
@code{config.sub} is also distributed in the GDB source
directory (@file{gdb-@value{GDBVN}}, for version @value{GDBVN}).

@node configure Options
@section @code{configure} options

Here is a summary of the @code{configure} options and arguments that
are most often useful for building @value{GDBN}.  @code{configure} also has
several other options not listed here.  @inforef{What Configure
Does,,configure.info}, for a full explanation of @code{configure}.
@c FIXME: Would this be more, or less, useful as an xref (ref to printed
@c manual in the printed manual, ref to info file only from the info file)?

@example
configure @r{[}--help@r{]}
          @r{[}--prefix=@var{dir}@r{]}
          @r{[}--srcdir=@var{path}@r{]}
          @r{[}--norecursion@r{]} @r{[}--rm@r{]}
          @r{[}--target=@var{target}@r{]} @var{host}
@end example

@noindent
You may introduce options with a single @samp{-} rather than
@samp{--} if you prefer; but you may abbreviate option names if you use
@samp{--}.

@table @code
@item --help
Display a quick summary of how to invoke @code{configure}.

@item -prefix=@var{dir}
Configure the source to install programs and files under directory
@file{@var{dir}}.

@item --srcdir=@var{path}
@strong{Warning: using this option requires GNU @code{make}, or another
@code{make} that implements the @code{VPATH} feature.}@*
Use this option to make configurations in directories separate from the
GDB source directories.  Among other things, you can use this to
build (or maintain) several configurations simultaneously, in separate
directories.  @code{configure} writes configuration specific files in
the current directory, but arranges for them to use the source in the
directory @var{path}.  @code{configure} will create directories under
the working directory in parallel to the source directories below
@var{path}.

@item --norecursion
Configure only the directory level where @code{configure} is executed; do not
propagate configuration to subdirectories.

@item --rm
@emph{Remove} files otherwise built during configuration.

@c This does not work (yet if ever).  FIXME.
@c @item --parse=@var{lang} @dots{}
@c Configure the GDB expression parser to parse the listed languages.
@c @samp{all} configures GDB for all supported languages.  To get a
@c list of all supported languages, omit the argument.  Without this
@c option, GDB is configured to parse all supported languages.

@item --target=@var{target}
Configure GDB for cross-debugging programs running on the specified
@var{target}.  Without this option, GDB is configured to debug
programs that run on the same machine (@var{host}) as GDB itself.

There is no convenient way to generate a list of all available targets.

@item @var{host} @dots{}
Configure GDB to run on the specified @var{host}.

There is no convenient way to generate a list of all available hosts.
@end table

@noindent
@code{configure} accepts other options, for compatibility with
configuring other GNU tools recursively; but these are the only
options that affect GDB or its supporting libraries.
@end ifclear

@node Index
@unnumbered Index

@printindex cp

@tex
% I think something like @colophon should be in texinfo.  In the
% meantime:
\long\def\colophon{\hbox to0pt{}\vfill
\centerline{The body of this manual is set in}
\centerline{\fontname\tenrm,}
\centerline{with headings in {\bf\fontname\tenbf}}
\centerline{and examples in {\tt\fontname\tentt}.}
\centerline{{\it\fontname\tenit\/},}
\centerline{{\bf\fontname\tenbf}, and}
\centerline{{\sl\fontname\tensl\/}}
\centerline{are used for emphasis.}\vfill}
\page\colophon
% Blame: pesch@cygnus.com, 1991.
@end tex

@contents
@bye