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\input texinfo      @c -*-texinfo-*-
@c Copyright 1988-1999
@c 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
@settitle Debugging with @value{GDBN}
@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 !!set GDB manual's edition---not the same as GDB version!
@set EDITION Seventh

@c !!set GDB manual's revision date
@set DATE February 1999

@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: (gdb).                     The @sc{gnu} debugger.
END-INFO-DIR-ENTRY
@end format
@end ifinfo
@c
@c
@ifinfo
This file documents the @sc{gnu} debugger @value{GDBN}.


This is the @value{EDITION} Edition, @value{DATE}, 
of @cite{Debugging with @value{GDBN}: the @sc{gnu} Source-Level Debugger}
for @value{GDBN} Version @value{GDBVN}.

Copyright (C) 1988-1999 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 @sc{gnu} Source-Level Debugger
@sp 1
@ifclear HPPA
@subtitle @value{EDITION} Edition, for @value{GDBN} version @value{GDBVN}
@subtitle @value{DATE}
@author Richard M. Stallman and Roland H. Pesch
@end ifclear
@ifset HPPA
@subtitle Edition @value{EDITION}, for @value{HPVER} (based on @value{GDBN} @value{GDBVN})
@subtitle @value{DATE}
@author Richard M. Stallman and Roland H. Pesch (modified by HP)
@end ifset
@page
@ifclear HPPA
@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
}
@end tex
@end ifclear
@ifset HPPA
@tex
{\parskip=0pt
\hfill {\it Debugging with @value{GDBN}}\par
\hfill \TeX{}info \texinfoversion\par
}
@end tex
@end ifset

@vskip 0pt plus 1filll
Copyright @copyright{} 1988-1999 Free Software Foundation, Inc. 
@sp 2
@ifclear HPPA
Published by the Free Software Foundation @*
59 Temple Place - Suite 330, @*
Boston, MA 02111-1307 USA @*
Printed copies are available for $20 each. @*
ISBN 1-882114-11-6 @*
@end ifclear
                
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, Summary, (dir), (dir)
@top Debugging with @value{GDBN}

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

This is the @value{EDITION} Edition, @value{DATE}, for @value{GDBN} Version 
@value{GDBVN}.

Copyright (C) 1988-1999 Free Software Foundation, Inc.
@menu
* Summary::                     Summary of @value{GDBN}
* Sample Session::              A sample @value{GDBN} session

* 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

* Languages::                   Using @value{GDBN} with different languages
* C::                           C language support

* 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
* Emacs::                       Using @value{GDBN} under @sc{gnu} Emacs

* GDB Bugs::                    Reporting bugs in @value{GDBN}

@ifclear PRECONFIGURED
@ifclear HPPA
* Formatting Documentation::    How to format and print @value{GDBN} documentation
@end ifclear

@end ifclear

* Command Line Editing::        Command Line Editing
* Using History Interactively:: Using History Interactively
* Installing GDB::              Installing GDB
* Index::                       Index

 --- The Detailed Node Listing ---

Summary of @value{GDBN}

* Free Software::               Freely redistributable software
* Contributors::                Contributors to GDB

Getting In and Out of @value{GDBN}

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

Invoking @value{GDBN}

* File Options::                Choosing files
* Mode Options::                Choosing modes

@value{GDBN} Commands

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

Running Programs Under @value{GDBN}

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

* Threads::                     Debugging programs with multiple threads
* Processes::                   Debugging programs with multiple processes

Stopping and Continuing

* Breakpoints::                 Breakpoints, watchpoints, and catchpoints
* Continuing and Stepping::     Resuming execution
* Signals::                     Signals
* Thread Stops::                Stopping and starting multi-thread programs

Breakpoints and watchpoints

* Set Breaks::                  Setting breakpoints
* Set Watchpoints::             Setting watchpoints
* Set Catchpoints::             Setting catchpoints
* Delete Breaks::               Deleting breakpoints
* Disabling::                   Disabling breakpoints
* Conditions::                  Break conditions
* Break Commands::              Breakpoint command lists
* Breakpoint Menus::            Breakpoint menus

Examining the Stack

* Frames::                      Stack frames
* Backtrace::                   Backtraces
* Selection::                   Selecting a frame
* Frame Info::                  Information on a frame
* Alpha/MIPS Stack::            Alpha and MIPS machines and the function stack

Examining Source Files

* List::                        Printing source lines
* Search::                      Searching source files
* Source Path::                 Specifying source directories
* Machine Code::                Source and machine code

Examining Data

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

Using @value{GDBN} with Different Languages

* Setting::                     Switching between source languages
* Show::                        Displaying the language
* Checks::                      Type and range checks
* Support::                     Supported languages

Switching between source languages

* Filenames::                   Filename extensions and languages.
* Manually::                    Setting the working language manually
* Automatically::               Having @value{GDBN} infer the source language

Type and range checking

* Type Checking::               An overview of type checking
* Range Checking::              An overview of range checking

Supported languages

C Language Support

* C::                           C and C++
* C Operators::                 C and C++ operators
* C Constants::                 C and C++ constants
* C plus plus expressions::     C++ expressions
* C Defaults::                  Default settings for C and C++
* C Checks::                    C and C++ type and range checks
* Debugging C::                 @value{GDBN} and C
* Debugging C plus plus::       @value{GDBN} features for C++

Modula-2

* 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

Altering Execution

* Assignment::                  Assignment to variables
* Jumping::                     Continuing at a different address
* Signaling::                   Giving your program a signal
* Returning::                   Returning from a function
* Calling::                     Calling your program's functions
* Patching::                    Patching your program

@value{GDBN} Files

* Files::                       Commands to specify files
* Symbol Errors::               Errors reading symbol files

Specifying a Debugging Target

* Active Targets::              Active targets
* Target Commands::             Commands for managing targets
@ifclear HPPA
* Byte Order::                  Choosing target byte order
* Remote::                      Remote debugging

Remote debugging
@end ifclear

* Remote Serial::               @value{GDBN} remote serial protocol

* i960-Nindy Remote::           @value{GDBN} with a remote i960 (Nindy)

* UDI29K Remote::               The UDI protocol for AMD29K

* EB29K Remote::                The EBMON protocol for AMD29K

* VxWorks Remote::              @value{GDBN} and VxWorks

* ST2000 Remote::               @value{GDBN} with a Tandem ST2000

* Hitachi Remote::              @value{GDBN} and Hitachi Microprocessors

* MIPS Remote::                 @value{GDBN} and MIPS boards

* Simulator::                   Simulated CPU target

Controlling @value{GDBN}

* Prompt::                      Prompt
* Editing::                     Command editing
* History::                     Command history
* Screen Size::                 Screen size
* Numbers::                     Numbers
* Messages/Warnings::           Optional warnings and messages

Canned Sequences of Commands

* Define::                      User-defined commands
* Hooks::                       User-defined command hooks
* Command Files::               Command files
* Output::                      Commands for controlled output

Reporting Bugs in @value{GDBN}

* Bug Criteria::                Have you found a bug?
* Bug Reporting::               How to report bugs

Installing @value{GDBN}

* Separate Objdir::             Compiling @value{GDBN} in another directory
* Config Names::                Specifying names for hosts and targets
* Configure Options::           Summary of options for configure
@end menu

@end ifinfo

@node Summary, Sample Session, Top, Top
@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

You can use @value{GDBN} to debug programs written in C or C++.
@c "MOD2" used as a "miscellaneous languages" flag here.
@c This is acceptable while there is no real doc for Chill and Pascal.
For more information, see @ref{Support,,Supported languages}.
For more information, see @ref{C,,C and C++}.

Support for Modula-2 and Chill is partial.  For information on Modula-2,
see @ref{Modula-2,,Modula-2}.  There is no further documentation on Chill yet.

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

@cindex Fortran
@value{GDBN} can be used to debug programs written in Fortran, although
it does not yet support entering expressions, printing values, or
similar features using Fortran syntax.  It may be necessary to refer to
some variables with a trailing underscore.

@ifset HPPA
This version of the manual documents HP Wildebeest (WDB) Version 0.75,
implemented on HP 9000 systems running Release 10.20, 10.30, or 11.0 of
the HP-UX operating system.  HP WDB 0.75 can be used to debug code
generated by the HP ANSI C and HP ANSI C++ compilers as well as the
@sc{gnu} C and C++ compilers.  It does not support the debugging of
Fortran, Modula-2, or Chill programs.
@end ifset

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

@node Free Software, Contributors, Summary, Summary
@unnumberedsec Free software

@value{GDBN} is @dfn{free software}, protected by the @sc{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,  , Free Software, Summary
@unnumberedsec Contributors to GDB

Richard Stallman was the original author of GDB, and of many other
@sc{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 @value{GDBN} 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 many labors as thankless, we
particularly thank those who shepherded @value{GDBN} through major
releases:
Jim Blandy (release 4.18);
Jason Molenda (release 4.17);
Stan Shebs (release 4.14);
Fred Fish (releases 4.16, 4.15, 4.13, 4.12, 4.11, 4.10, and 4.9);
Stu Grossman and John Gilmore (releases 4.8, 4.7, 4.6, 4.5, and 4.4);
John Gilmore (releases 4.3, 4.2, 4.1, 4.0, and 3.9);
Jim Kingdon (releases 3.5, 3.4, and 3.3);
and Randy Smith (releases 3.2, 3.1, and 3.0).

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

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

@value{GDBN} 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.

Brent Benson of Harris Computer Systems contributed DWARF 2 support.

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

Andreas Schwab contributed M68K Linux support.

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

Jay Fenlason and Roland McGrath ensured that @value{GDBN} and GAS agree
about several machine instruction sets.

Patrick Duval, Ted Goldstein, Vikram Koka and Glenn Engel helped develop
remote debugging.  Intel Corporation, Wind River Systems, AMD, and ARM
contributed remote debugging modules for the i960, VxWorks, A29K UDI,
and RDI targets, respectively.

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

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

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

Hitachi America, Ltd. sponsored the support for H8/300, H8/500, and
Super-H processors.

NEC sponsored the support for the v850, Vr4xxx, and Vr5xxx processors.

Mitsubishi sponsored the support for D10V, D30V, and M32R/D processors.

Toshiba sponsored the support for the TX39 Mips processor.

Matsushita sponsored the support for the MN10200 and MN10300 processors.

Fujitsu sponsored the support for SPARClite and FR30 processors

Kung Hsu, Jeff Law, and Rick Sladkey added support for hardware
watchpoints.

Michael Snyder added support for tracepoints.

Stu Grossman wrote gdbserver.

Jim Kingdon, Peter Schauer, Ian Taylor, and Stu Grossman made
nearly innumerable bug fixes and cleanups throughout GDB.

The following people at the Hewlett-Packard Company contributed
support for the PA-RISC 2.0 architecture, HP-UX 10.20, 10.30, and 11.0
(narrow mode), HP's implementation of kernel threads, HP's aC++
compiler, and the terminal user interface: Ben Krepp, Richard Title,
John Bishop, Susan Macchia, Kathy Mann, Satish Pai, India Paul, Steve
Rehrauer, and Elena Zannoni.  Kim Haase provided HP-specific
information in this manual.

Cygnus Solutions has sponsored GDB maintenance and much of its
development since 1991.  Cygnus engineers who have worked on GDB
fulltime include Mark Alexander, Jim Blandy, Per Bothner, Edith Epstein,
Chris Faylor, Fred Fish, Martin Hunt, Jim Ingham, John Gilmore, Stu
Grossman, Kung Hsu, Jim Kingdon, John Metzler, Fernando Nasser, Geoffrey
Noer, Dawn Perchik, Rich Pixley, Zdenek Radouch, Keith Seitz, Stan
Shebs, David Taylor, and Elena Zannoni.  In addition, Dave Brolley, Ian
Carmichael, Steve Chamberlain, Nick Clifton, JT Conklin, Stan Cox, DJ
Delorie, Ulrich Drepper, Frank Eigler, Doug Evans, Sean Fagan, David
Henkel-Wallace, Richard Henderson, Jeff Holcomb, Jeff Law, Jim Lemke,
Tom Lord, Bob Manson, Michael Meissner, Jason Merrill, Catherine Moore,
Drew Moseley, Ken Raeburn, Gavin Romig-Koch, Rob Savoye, Jamie Smith,
Mike Stump, Ian Taylor, Angela Thomas, Michael Tiemann, Tom Tromey, Ron
Unrau, Jim Wilson, and David Zuhn have made contributions both large
and small.


@node Sample Session, Invocation, Summary, Top
@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 @sc{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.

@ifclear HPPA
@smallexample
$ @b{@value{GDBP} m4}
@c FIXME: this falsifies the exact text played out, to permit smallbook
@c FIXME... format to come out better.
@value{GDBN} 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 @value{GDBN}; type "show warranty" 
 for details.

@value{GDBN} @value{GDBVN}, Copyright 1999 Free Software Foundation, Inc...
(@value{GDBP})
@end smallexample
@end ifclear
@ifset HPPA
@smallexample
$ @b{@value{GDBP} m4}
Wildebeest 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 Wildebeest; type "show warranty"
for details.

Hewlett-Packard Wildebeest 0.75 (based on GDB 4.16)
(built for PA-RISC 1.1 or 2.0, HP-UX 10.20)
Copyright 1996, 1997 Free Software Foundation, Inc.
(@value{GDBP})
@end smallexample
@end ifset

@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 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 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

@node Invocation, Commands, Sample Session, Top
@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 are: 
@itemize @bullet
@item 
type @samp{@value{GDBP}} to start GDB.
@item 
type @kbd{quit} or @kbd{C-d} to exit.
@end itemize

@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, Quitting GDB, Invocation, Invocation
@section Invoking @value{GDBN}

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.

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

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

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

@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).

@ifclear HPPA
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

You can run @code{gdb} without printing the front material, which describes
@value{GDBN}'s non-warranty, by specifying @code{-silent}:

@smallexample
@value{GDBP} -silent
@end smallexample

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

@node File Options
@subsection Choosing files

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.)

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

Many options have both long and short forms; both are shown in the
following list.  @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 appropriate,
and for examining pure data in conjunction with a core dump.

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

@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).

@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 HPPA
@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 is @file{./fred.syms}.
Future @value{GDBN} debugging sessions notice the presence of this file,
and can quickly map in symbol information from it, rather than reading
the symbol table from the executable program.

The @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

@ifclear HPPA
@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 ifclear
@end table

@ifclear HPPA
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,  , File Options, Invoking GDB
@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{.gdbinit}, or @file{gdb.ini} on PCs).  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.

@item -fullname
@itemx -f
@sc{gnu} 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.

@ifclear HPPA
@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.
@end ifclear

@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.

@ifset HPPA
@item -tui
Use a Terminal User Interface.  For information, use your Web browser to
read the file @file{TUI.html}, which is usually installed in the
directory @code{/opt/langtools/wdb/doc} on HP-UX systems.  Do not use
this option if you run @value{GDBN} from Emacs (see @pxref{Emacs, ,Using
@value{GDBN} under @sc{gnu} Emacs}).

@item -xdb
Run in XDB compatibility mode, allowing the use of certain XDB commands.
For information, see the file @file{xdb_trans.html}, which is usually
installed in the directory @code{/opt/langtools/wdb/doc} on HP-UX
systems.
@end ifset
@end table

@node Quitting GDB, Shell Commands, Invoking GDB, Invocation
@section Quitting @value{GDBN}
@cindex exiting @value{GDBN}
@cindex leaving @value{GDBN}

@table @code
@kindex quit @r{[}@var{expression}@r{]}
@kindex q
@item quit
To exit @value{GDBN}, use the @code{quit} command (abbreviated @code{q}), or 
type an end-of-file character (usually @kbd{C-d}).  If you do not supply 
@var{expression}, @value{GDBN} will terminate normally; otherwise it will 
terminate using the result of @var{expression} as the error code.
@end table

@cindex interrupt
An interrupt (often @kbd{C-c}) does not exit from @value{GDBN}, but rather
terminates the action of any @value{GDBN} command that is in progress and
returns 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.

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}).

@node Shell Commands,  , Quitting GDB, Invocation
@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
@kindex shell
@cindex shell escape
@item shell @var{command string}
Invoke a standard shell to execute @var{command string}.
If it exists, the environment variable @code{SHELL} determines which
shell to run.  Otherwise @value{GDBN} uses @code{/bin/sh}.
@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
@kindex make
@cindex calling make
@item make @var{make-args}
Execute the @code{make} program with the specified
arguments.  This is equivalent to @samp{shell make @var{make-args}}.
@end table

@node Commands, Running, Invocation, Top
@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, Completion, Commands, Commands
@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 whose 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, Help, Command Syntax, Commands
@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} fills in the
word, and waits 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} sounds a bell.  You can either supply more
characters and try again, or just press @key{TAB} a second time;
@value{GDBN} displays 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 displays 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 either by holding down a
key designated as the @key{META} shift on your keyboard (if there is
one) while typing @kbd{?}, or 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.

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 requires using
quotes.  When this happens, @value{GDBN} inserts 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.

For more information about overloaded functions, @pxref{C plus plus
expressions, ,C++ expressions}.  You can use the command @code{set
overload-resolution off} to disable overload resolution;
@pxref{Debugging C plus plus, ,@value{GDBN} features for C++}.


@node Help,  , Completion, Commands
@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
@kindex h
@item help
@itemx 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} displays a
short paragraph on how to use that command.

@kindex complete
@item complete @var{args}
The @code{complete @var{args}} command lists all the possible completions
for the beginning of a command.  Use @var{args} to specify the beginning of the
command you want completed.  For example:

@smallexample
complete i
@end smallexample

@noindent results in:

@smallexample
@group
info
inspect
ignore
@end group
@end smallexample

@noindent This is intended for use by @sc{gnu} Emacs.
@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
@kindex info
@kindex i
@item info
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 set
@item set
You can assign the result of an expression to an environment variable with 
@code{set}.  For example, you can set the @value{GDBN} prompt to a $-sign with
@code{set prompt $}.

@kindex show
@item show
In contrast to @code{info}, @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 @sc{gnu} ``NO WARRANTY'' statement.
@end table

@node Running, Stopping, Commands, Top
@chapter Running Programs Under @value{GDBN}

When you run a program under @value{GDBN}, you must first generate
debugging information when you compile it.

You may start @value{GDBN} with its arguments, if any, in an environment
of your choice.  If you are doing native debugging, you may redirect
your program's input and output, debug an already running process, or
kill a child process.

@menu
* Compilation::                 Compiling for debugging
* Starting::                    Starting your program
* Arguments::                   Your program's arguments
* Environment::                 Your program's environment

* Working Directory::           Your program's working directory
* Input/Output::                Your program's input and output
* Attach::                      Debugging an already-running process
* Kill Process::                Killing the child process
* Process Information::         Additional process information

* Threads::                     Debugging programs with multiple threads
* Processes::                   Debugging programs with multiple processes
@end menu

@node Compilation, Starting, Running, Running
@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.

@ifclear HPPA
@value{NGCC}, the @sc{gnu} C compiler, supports @samp{-g} with or without
@end ifclear
@ifset HPPA
The HP ANSI C and C++ compilers, as well as @value{NGCC}, the @sc{gnu} C
compiler, support @samp{-g} with or without
@end ifset
@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 shows you what is
really there.  Do not be too surprised when the execution path does not
exactly match your source file!  An extreme example: if you define a
variable, but never use it, @value{GDBN} never sees 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 to us as a bug (including a test case!).

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

@need 2000
@node Starting, Arguments, Compilation, Running
@section Starting your program
@cindex starting
@cindex running

@table @code
@kindex run
@item run
@itemx r
Use the @code{run} command to start your program under @value{GDBN}.
You must first specify the program name (except on VxWorks) 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

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 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 affect
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

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} discards its symbol
table, and reads it again.  When it does this, @value{GDBN} tries to retain
your current breakpoints.

@node Arguments, Environment, Starting, Running
@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} uses.  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} executes 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.

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

@node Environment, Working Directory, Arguments, Running
@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
@kindex path
@item path @var{directory}
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 is 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.

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

@kindex show environment
@item show environment @r{[}@var{varname}@r{]}
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}.

@kindex set environment
@item set environment @var{varname} @r{[}=@r{]} @var{value}
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.)

@kindex unset environment
@item unset environment @var{varname}
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 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, Input/Output, Environment, Running
@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
@kindex cd
@item cd @var{directory}
Set the @value{GDBN} working directory to @var{directory}.

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

@node Input/Output, Attach, Working Directory, Running
@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
@kindex info terminal
@item 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, Kill Process, Input/Output, Running
@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} shows your active
targets.)  The command takes as argument a process ID.  The usual way to
find out the process-id of a Unix process is with the @code{ps} utility,
or with the @samp{jobs -l} shell command.

@code{attach} does 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 you use @code{attach}, the debugger finds the program running in
the process first by looking in the current working directory, then (if
the program is not found) by using the source file search path
(@pxref{Source Path, ,Specifying source directories}).  You can also use
the @code{file} command to load the program.  @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
@ifclear HPPA
processes with @code{run}.  You can insert breakpoints; you can step and
@end ifclear
@ifset HPPA
processes with @code{run}.  You can insert breakpoints (except in shared
libraries); you can step and
@end ifset
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
@kindex detach
@item 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} does 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, @value{GDBN} asks
for confirmation if you try to do either of these things; you can
control whether or not you need to confirm by using the @code{set
confirm} command (@pxref{Messages/Warnings, ,Optional warnings and
messages}).

@node Kill Process, Process Information, Attach, Running
@section Killing the child process

@table @code
@kindex kill
@item 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.

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} notices that the file has changed, and
reads the symbol table again (while trying to preserve your current
breakpoint settings).

@node Process Information, Threads, Kill Process, Running
@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.  
@code{info proc} works only on SVR4 systems that support @code{procfs}.
This includes OSF/1 (Digital Unix), Solaris, Irix, and Unixware,
but not HP-UX or Linux, for example.

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

@kindex info proc mappings
@item 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.

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

@kindex info proc id
@item 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.

@kindex info proc status
@item 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

@node Threads, Processes, Process Information, Running
@section Debugging programs with multiple threads

@cindex threads of execution
@cindex multiple threads
@cindex switching threads
In some operating systems, such as HP-UX and Solaris, a single program
may have more than one @dfn{thread} of execution.  The precise semantics
of threads differ from one operating system to another, but in general
the threads of a single program are akin to multiple processes---except
that they share one address space (that is, they can all examine and
modify the same variables).  On the other hand, each thread has its own
registers and execution stack, and perhaps private memory.

@value{GDBN} provides these facilities for debugging multi-thread
programs:

@itemize @bullet
@item automatic notification of new threads
@item @samp{thread @var{threadno}}, a command to switch among threads
@item @samp{info threads}, a command to inquire about existing threads
@item @samp{thread apply [@var{threadno}] [@var{all}] @var{args}}, 
a command to apply a command to a list of threads
@item thread-specific breakpoints
@end itemize

@ifclear HPPA
@quotation
@emph{Warning:} These facilities are not yet available on every
@value{GDBN} configuration where the operating system supports threads.
If your @value{GDBN} does not support threads, these commands have no
effect.  For example, a system without thread support shows no output
from @samp{info threads}, and always rejects the @code{thread} command,
like this:

@smallexample
(@value{GDBP}) info threads
(@value{GDBP}) thread 1
Thread ID 1 not known.  Use the "info threads" command to
see the IDs of currently known threads.
@end smallexample
@c FIXME to implementors: how hard would it be to say "sorry, this GDB
@c                        doesn't support threads"?
@end quotation
@end ifclear

@cindex focus of debugging
@cindex current thread
The @value{GDBN} thread debugging facility allows you to observe all
threads while your program runs---but whenever @value{GDBN} takes
control, one thread in particular is always the focus of debugging.
This thread is called the @dfn{current thread}.  Debugging commands show
program information from the perspective of the current thread.

@ifclear HPPA
@kindex New @var{systag}
@cindex thread identifier (system)
@c FIXME-implementors!! It would be more helpful if the [New...] message
@c included GDB's numeric thread handle, so you could just go to that
@c thread without first checking `info threads'.
Whenever @value{GDBN} detects a new thread in your program, it displays
the target system's identification for the thread with a message in the
form @samp{[New @var{systag}]}.  @var{systag} is a thread identifier
whose form varies depending on the particular system.  For example, on
LynxOS, you might see

@example
[New process 35 thread 27]
@end example

@noindent
when @value{GDBN} notices a new thread.  In contrast, on an SGI system,
the @var{systag} is simply something like @samp{process 368}, with no
further qualifier.

@c FIXME!! (1) Does the [New...] message appear even for the very first
@c         thread of a program, or does it only appear for the
@c         second---i.e., when it becomes obvious we have a multithread
@c         program?
@c         (2) *Is* there necessarily a first thread always?  Or do some
@c         multithread systems permit starting a program with multiple
@c         threads ab initio?      

@cindex thread number
@cindex thread identifier (GDB)
For debugging purposes, @value{GDBN} associates its own thread
number---always a single integer---with each thread in your program.

@table @code
@kindex info threads
@item info threads
Display a summary of all threads currently in your
program.  @value{GDBN} displays for each thread (in this order):

@enumerate
@item the thread number assigned by @value{GDBN}

@item the target system's thread identifier (@var{systag})

@item the current stack frame summary for that thread
@end enumerate

@noindent
An asterisk @samp{*} to the left of the @value{GDBN} thread number
indicates the current thread.

For example, 
@end table
@c end table here to get a little more width for example

@smallexample
(@value{GDBP}) info threads
  3 process 35 thread 27  0x34e5 in sigpause ()
  2 process 35 thread 23  0x34e5 in sigpause ()
* 1 process 35 thread 13  main (argc=1, argv=0x7ffffff8)
    at threadtest.c:68
@end smallexample
@end ifclear
@ifset HPPA

@cindex thread number
@cindex thread identifier (GDB)
For debugging purposes, @value{GDBN} associates its own thread
number---a small integer assigned in thread-creation order---with each
thread in your program.

@kindex New @var{systag}
@cindex thread identifier (system)
@c FIXME-implementors!! It would be more helpful if the [New...] message
@c included GDB's numeric thread handle, so you could just go to that
@c thread without first checking `info threads'.
Whenever @value{GDBN} detects a new thread in your program, it displays
both @value{GDBN}'s thread number and the target system's identification for the thread with a message in the
form @samp{[New @var{systag}]}.  @var{systag} is a thread identifier
whose form varies depending on the particular system.  For example, on
HP-UX, you see

@example
[New thread 2 (system thread 26594)]
@end example

@noindent
when @value{GDBN} notices a new thread. 

@table @code
@kindex info threads
@item info threads
Display a summary of all threads currently in your
program.  @value{GDBN} displays for each thread (in this order):

@enumerate
@item the thread number assigned by @value{GDBN}

@item the target system's thread identifier (@var{systag})

@item the current stack frame summary for that thread
@end enumerate

@noindent
An asterisk @samp{*} to the left of the @value{GDBN} thread number
indicates the current thread.

For example, 
@end table
@c end table here to get a little more width for example

@example
(@value{GDBP}) info threads
    * 3 system thread 26607  worker (wptr=0x7b09c318 "@@") at quicksort.c:137
      2 system thread 26606  0x7b0030d8 in __ksleep () from /usr/lib/libc.2
      1 system thread 27905  0x7b003498 in _brk () from /usr/lib/libc.2
@end example
@end ifset

@table @code
@kindex thread @var{threadno}
@item thread @var{threadno}
Make thread number @var{threadno} the current thread.  The command
argument @var{threadno} is the internal @value{GDBN} thread number, as
shown in the first field of the @samp{info threads} display.
@value{GDBN} responds by displaying the system identifier of the thread
you selected, and its current stack frame summary:

@smallexample
@c FIXME!! This example made up; find a @value{GDBN} w/threads and get real one
(@value{GDBP}) thread 2
@ifclear HPPA
[Switching to process 35 thread 23]
@end ifclear
@ifset HPPA
[Switching to thread 2 (system thread 26594)]
@end ifset
0x34e5 in sigpause ()
@end smallexample

@noindent
As with the @samp{[New @dots{}]} message, the form of the text after
@samp{Switching to} depends on your system's conventions for identifying
threads.  

@kindex thread apply
@item thread apply [@var{threadno}] [@var{all}]  @var{args}
The @code{thread apply} command allows you to apply a command to one or
more threads.  Specify the numbers of the threads that you want affected
with the command argument @var{threadno}.  @var{threadno} is the internal
@value{GDBN} thread number, as shown in the first field of the @samp{info
threads} display.  To apply a command to all threads, use 
@code{thread apply all} @var{args}. 
@end table

@cindex automatic thread selection
@cindex switching threads automatically
@cindex threads, automatic switching
Whenever @value{GDBN} stops your program, due to a breakpoint or a
signal, it automatically selects the thread where that breakpoint or
signal happened.  @value{GDBN} alerts you to the context switch with a
message of the form @samp{[Switching to @var{systag}]} to identify the
thread.

@xref{Thread Stops,,Stopping and starting multi-thread programs}, for
more information about how @value{GDBN} behaves when you stop and start
programs with multiple threads.

@xref{Set Watchpoints,,Setting watchpoints}, for information about
watchpoints in programs with multiple threads.

@ifclear HPPA
@node Processes,  , Threads, Running
@section Debugging programs with multiple processes

@cindex fork, debugging programs which call
@cindex multiple processes
@cindex processes, multiple
@value{GDBN} has no special support for debugging programs which create
additional processes using the @code{fork} function.  When a program
forks, @value{GDBN} will continue to debug the parent process and the
child process will run unimpeded.  If you have set a breakpoint in any
code which the child then executes, the child will get a @code{SIGTRAP}
signal which (unless it catches the signal) will cause it to terminate.

However, if you want to debug the child process there is a workaround
which isn't too painful.  Put a call to @code{sleep} in the code which
the child process executes after the fork.  It may be useful to sleep
only if a certain environment variable is set, or a certain file exists,
so that the delay need not occur when you don't want to run @value{GDBN}
on the child.  While the child is sleeping, use the @code{ps} program to
get its process ID.  Then tell @value{GDBN} (a new invocation of
@value{GDBN} if you are also debugging the parent process) to attach to
the child process (see @ref{Attach}).  From that point on you can debug
the child process just like any other process which you attached to.
@end ifclear
@ifset HPPA
@node Processes,  , Threads, Running
@section Debugging programs with multiple processes

@cindex fork, debugging programs which call
@cindex multiple processes
@cindex processes, multiple

@value{GDBN} provides support for debugging programs that create
additional processes using the @code{fork} or @code{vfork} function.

By default, when a program forks, @value{GDBN} will continue to debug
the parent process and the child process will run unimpeded.

If you want to follow the child process instead of the parent process,
use the command @w{@code{set follow-fork-mode}}.

@table @code
@kindex set follow-fork-mode
@item set follow-fork-mode @var{mode}
Set the debugger response to a program call of @code{fork} or
@code{vfork}.  A call to @code{fork} or @code{vfork} creates a new
process.  The @var{mode} can be:

@table @code
@item parent
The original process is debugged after a fork.  The child process runs
unimpeded.

@item child
The new process is debugged after a fork.  The parent process runs
unimpeded.

@item ask
The debugger will ask for one of the above choices.
@end table

@item show follow-fork-mode
Display the current debugger response to a fork or vfork call.
@end table

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

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

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

You can use the @code{catch} command to make @value{GDBN} stop whenever
a @code{fork}, @code{vfork}, or @code{exec} call is made.  @xref{Set
Catchpoints, ,Setting catchpoints}.
@end ifset

@node Stopping, Stack, Running, Top
@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 a signal, 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
@kindex info program
@item info program
Display information about the status of your program: whether it is
running or not, what process it is, and why it stopped.
@end table

@menu
* Breakpoints::                 Breakpoints, watchpoints, and catchpoints
* Continuing and Stepping::     Resuming execution
* Signals::                     Signals
* Thread Stops::                Stopping and starting multi-thread programs
@end menu

@node Breakpoints, Continuing and Stepping, Stopping, Stopping
@section Breakpoints, watchpoints, and catchpoints

@cindex breakpoints
A @dfn{breakpoint} makes your program stop whenever a certain point in
the program is reached.  For each breakpoint, you can add conditions to
control in finer detail whether your program stops.  You can set
breakpoints with the @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.

In HP-UX, SunOS 4.x, SVR4, and Alpha OSF/1 configurations, you can set
breakpoints in shared libraries before the executable is run.  There is
a minor limitation on HP-UX systems: you must wait until the executable
is run in order to set breakpoints in shared library routines that are
not called directly by the program (for example, routines that are
arguments in a @code{pthread_create} call).

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

@cindex breakpoint numbers
@cindex numbers for breakpoints
@value{GDBN} assigns a number to each breakpoint, watchpoint, or
catchpoint when you create it; these numbers are successive integers
starting with one.  In many of the commands for controlling various
features of breakpoints you use the breakpoint number to say which
breakpoint you want to change.  Each breakpoint may be @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
* Set Catchpoints::             Setting catchpoints
* Delete Breaks::               Deleting breakpoints
* Disabling::                   Disabling breakpoints
* Conditions::                  Break conditions
* Break Commands::              Breakpoint command lists
* Breakpoint Menus::            Breakpoint menus

@c  * Error in Breakpoints::        ``Cannot insert breakpoints''
@end menu

@node Set Breaks, Set Watchpoints, Breakpoints, Breakpoints
@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 breakpoints 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}.  
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.

@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 stops 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 makes your program 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} stops
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.

@kindex tbreak
@item tbreak @var{args}
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 deleted after the first time your
program stops there.  @xref{Disabling, ,Disabling breakpoints}.

@ifclear HPPA
@kindex hbreak
@item hbreak @var{args}
Set a hardware-assisted breakpoint.  @var{args} are the same as for the 
@code{break} command and the breakpoint is set in the same way, but the 
breakpoint requires hardware support and some target hardware may not
have this support.  The main purpose of this is EPROM/ROM code
debugging, so you can set a breakpoint at an instruction without 
changing the instruction.  This can be used with the new trap-generation 
provided by SPARClite DSU.  DSU will generate traps when a program accesses
some data or instruction address that is assigned to the debug registers.  
However the hardware breakpoint registers can only take two data breakpoints,
and @value{GDBN} will reject this command if more than two are used.  
Delete or disable unused hardware breakpoints before setting
new ones.  @xref{Conditions, ,Break conditions}.

@kindex thbreak
@item thbreak @var{args}
Set a hardware-assisted breakpoint enabled only for one stop.  @var{args}
are the same as for the @code{hbreak} command and the breakpoint is set in
the same way.  However, like the @code{tbreak} command,  
the breakpoint is automatically deleted after the
first time your program stops there.  Also, like the @code{hbreak}
command, the breakpoint requires hardware support and some target hardware 
may not have this support.  @xref{Disabling, ,Disabling breakpoints}.  
Also @xref{Conditions, ,Break conditions}.
@end ifclear

@kindex rbreak
@cindex regular expression
@item rbreak @var{regex}
@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.  You can
delete them, disable them, or make them conditional the same way as any
other breakpoint.

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

@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, watchpoints, and catchpoints set and
not deleted, with the following columns for each breakpoint:

@table @emph
@item Breakpoint Numbers
@item Type
Breakpoint, watchpoint, or catchpoint.
@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}).  

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

@ifset HPPA
@item shlib events
Shared library events.
@end ifset
@end table
@end table


@node Set Watchpoints, Set Catchpoints, Set Breaks, Breakpoints
@subsection Setting watchpoints

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

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

On some systems, such as HP-UX and Linux, GDB includes support for
hardware watchpoints, which do not slow down the running of your
program.

@table @code
@kindex watch
@item watch @var{expr}
Set a watchpoint for an expression.  @value{GDBN} will break when @var{expr}
is written into by the program and its value changes.

@kindex rwatch
@item rwatch @var{expr}
Set a watchpoint that will break when watch @var{expr} is read by the program.
If you use both watchpoints, both must be set with the @code{rwatch}
command.

@kindex awatch
@item awatch @var{expr}
Set a watchpoint that will break when @var{args} is read and written into
by the program.  If you use both watchpoints, both must be set with the
@code{awatch} command.

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

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

When you issue the @code{watch} command, @value{GDBN} reports

@example
Hardware watchpoint @var{num}: @var{expr}
@end example

@noindent
if it was able to set a hardware watchpoint.

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

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

@quotation
@cindex watchpoints and threads
@cindex threads and watchpoints
@ifclear HPPA
@emph{Warning:} In multi-thread programs, watchpoints have only limited
usefulness.  With the current watchpoint implementation, @value{GDBN}
can only watch the value of an expression @emph{in a single thread}.  If
you are confident that the expression can only change due to the current
thread's activity (and if you are also confident that no other thread
can become current), then you can use watchpoints as usual.  However,
@value{GDBN} may not notice when a non-current thread's activity changes
the expression.
@end ifclear
@ifset HPPA
@emph{Warning:} In multi-thread programs, software watchpoints have only
limited usefulness.  If @value{GDBN} creates a software watchpoint, it
can only watch the value of an expression @emph{in a single thread}.  If
you are confident that the expression can only change due to the current
thread's activity (and if you are also confident that no other thread
can become current), then you can use software watchpoints as usual.
However, @value{GDBN} may not notice when a non-current thread's
activity changes the expression.  (Hardware watchpoints, in contrast,
watch an expression in all threads.)
@end ifset
@end quotation

@node Set Catchpoints, Delete Breaks, Set Watchpoints, Breakpoints
@subsection Setting catchpoints
@cindex catchpoints
@cindex exception handlers
@cindex event handling

You can use @dfn{catchpoints} to cause the debugger to stop for certain
kinds of program events, such as C++ exceptions or the loading of a
shared library.  Use the @code{catch} command to set a catchpoint.

@table @code
@kindex catch
@item catch @var{event}
Stop when @var{event} occurs.  @var{event} can be any of the following:
@table @code
@item throw
@kindex catch throw
The throwing of a C++ exception.

@item catch
@kindex catch catch
The catching of a C++ exception.

@item exec
@kindex catch exec
A call to @code{exec}.  This is currently only available for HP-UX.

@item fork
@kindex catch fork
A call to @code{fork}.  This is currently only available for HP-UX.

@item vfork
@kindex catch vfork
A call to @code{vfork}.  This is currently only available for HP-UX.

@item load
@itemx load @var{libname}
@kindex catch load
The dynamic loading of any shared library, or the loading of the library
@var{libname}.  This is currently only available for HP-UX.

@item unload
@itemx unload @var{libname}
@kindex catch unload
The unloading of any dynamically loaded shared library, or the unloading
of the library @var{libname}.  This is currently only available for HP-UX.
@end table

@item tcatch @var{event}
Set a catchpoint that is enabled only for one stop.  The catchpoint is
automatically deleted after the first time the event is caught.

@end table

Use the @code{info break} command to list the current catchpoints.

There are currently some limitations to C++ exception handling
(@code{catch throw} and @code{catch catch}) in @value{GDBN}:

@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 either to abort or to
simply continue running until it hits a breakpoint, catches a signal
that @value{GDBN} is listening for, or exits.  This is the case even if
you set a catchpoint for the exception; catchpoints on exceptions are
disabled within interactive calls.

@item
You cannot raise an exception interactively.

@item
You cannot install an exception handler interactively.
@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 @sc{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.


@node Delete Breaks, Disabling, Set Catchpoints, Breakpoints
@subsection Deleting breakpoints

@cindex clearing breakpoints, watchpoints, catchpoints
@cindex deleting breakpoints, watchpoints, catchpoints
It is often necessary to eliminate a breakpoint, watchpoint, or
catchpoint once it has done its job and you no longer want your program
to stop there.  This is called @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, watchpoints, or catchpoints 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
@kindex clear
@item 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.

@cindex delete breakpoints
@kindex delete
@kindex d
@item delete @r{[}breakpoints@r{]} @r{[}@var{bnums}@dots{}@r{]}
Delete the breakpoints, watchpoints, or catchpoints 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, Conditions, Delete Breaks, Breakpoints
@subsection Disabling breakpoints

@kindex disable breakpoints
@kindex enable breakpoints
Rather than deleting a breakpoint, watchpoint, or catchpoint, 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, watchpoints, and catchpoints 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, watchpoints, and
catchpoints if you do not know which numbers to use.

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

@itemize @bullet
@item
Enabled.  The breakpoint stops 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 stops your program, but then becomes
disabled.  A breakpoint set with the @code{tbreak} command starts out in
this state.
@item
Enabled for deletion.  The breakpoint stops your program, but
immediately after it does so it is deleted permanently.
@end itemize

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

@table @code
@kindex disable breakpoints
@kindex disable
@kindex dis
@item disable @r{[}breakpoints@r{]} @r{[}@var{bnums}@dots{}@r{]}
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}.

@kindex enable breakpoints
@kindex enable
@item enable @r{[}breakpoints@r{]} @r{[}@var{bnums}@dots{}@r{]}
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.  @value{GDBN} disables any
of these breakpoints immediately after stopping your program.

@item enable @r{[}breakpoints@r{]} delete @var{bnums}@dots{}
Enable the specified breakpoints to work once, then die.  @value{GDBN}
deletes any of these breakpoints as soon as your program stops there.
@end table

Except 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 does not change the state of your other
breakpoints; see @ref{Continuing and Stepping, ,Continuing and
stepping}.)

@node Conditions, Break Commands, Disabling, Breakpoints
@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.
@ifclear HPPA
@c The watch command now seems to recognize the if keyword.
@c catch doesn't, though.
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.
@end ifclear
@ifset HPPA
You can also use the @code{if} keyword with the @code{watch} command.
The @code{catch} command does not recognize the @code{if} keyword;
@code{condition} is the only way to impose a further condition on a
catchpoint.
@end ifset

@table @code
@kindex condition
@item condition @var{bnum} @var{expression}
Specify @var{expression} as the break condition for breakpoint,
watchpoint, or catchpoint number @var{bnum}.  After you set a condition,
breakpoint @var{bnum} stops 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 does not stop the next @var{n} times
your program reaches it.

@table @code
@kindex ignore
@item ignore @var{bnum} @var{count}
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 does 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,
@value{GDBN} resumes checking the condition.

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

Ignore counts apply to breakpoints, watchpoints, and catchpoints.


@node Break Commands, Breakpoint Menus, Conditions, Breakpoints
@subsection Breakpoint command lists

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

@table @code
@kindex commands
@kindex end
@item commands @r{[}@var{bnum}@r{]}
@itemx @dots{} @var{command-list} @dots{}
@itemx 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, watchpoint, or catchpoint 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
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

@node Breakpoint Menus,  , Break Commands, Breakpoints
@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 is 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
@group
(@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 group
@end smallexample

@c  @ifclear BARETARGET
@c  @node Error in Breakpoints
@c  @subsection ``Cannot insert breakpoints''
@c
@c  FIXME!! 14/6/95  Is there a real example of this?  Let's use it.
@c
@c  Under some operating systems, breakpoints cannot be used in a program if
@c  any other process is running that program.  In this situation,
@c  attempting to run or continue a program with a breakpoint causes 
@c  @value{GDBN} to stop the other process.
@c
@c  When this happens, you have three ways to proceed:
@c
@c  @enumerate
@c  @item
@c  Remove or disable the breakpoints, then continue.
@c
@c  @item
@c  Suspend @value{GDBN}, and copy the file containing your program to a new 
@c  name.  Resume @value{GDBN} and use the @code{exec-file} command to specify
@c  that @value{GDBN} should run your program under that name.  
@c  Then start your program again.
@c
@c  @item
@c  Relink your program so that the text segment is nonsharable, using the
@c  linker option @samp{-N}.  The operating system limitation may not apply
@c  to nonsharable executables.
@c  @end enumerate
@c  @end ifclear

@node Continuing and Stepping, Signals, Breakpoints, Stopping
@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 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}.)

@table @code
@kindex continue
@kindex c
@kindex fg
@item continue @r{[}@var{ignore-count}@r{]}
@itemx c @r{[}@var{ignore-count}@r{]}
@itemx fg @r{[}@var{ignore-count}@r{]}
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
(@pxref{Breakpoints, ,Breakpoints; watchpoints; and catchpoints}) at the
beginning of the function or the section of your program where a problem
is believed to lie, run your program until it stops at that breakpoint,
and then step through the suspect area, examining the variables that are
interesting, until you see the problem happen.

@table @code
@kindex step
@kindex s
@item step
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
@c "without debugging information" is imprecise; actually "without line
@c numbers in the debugging information".  (gcc -g1 has debugging info but
@c not line numbers).  But it seems complex to try to make that
@c distinction here.
@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.  Likewise, it will not step into a function which
is compiled without debugging information.  To step through functions
without debugging information, use the @code{stepi} command, described
below.
@end quotation

The @code{step} command now only stops at the first instruction of a
source line.  This prevents the multiple stops that used to occur in
switch statements, for loops, etc.  @code{step} continues to stop if a
function that has debugging information is called within the line.

Also, the @code{step} command now only enters a subroutine if there is line
number information for the subroutine.  Otherwise it acts like the
@code{next} command.  This avoids problems when using @code{cc -gl} 
on MIPS machines.  Previously, @code{step} entered subroutines if there
was any debugging information about the routine.  

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

@kindex next
@kindex n
@item next @r{[}@var{count}@r{]}
Continue to the next source line in the current (innermost) stack frame.
This is similar to @code{step}, but function calls that appear within
the line of code are executed without stopping.  Execution stops when
control reaches a different line of code at the original stack level
that was executing when you gave the @code{next} command.  This command
is abbreviated @code{n}.

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


@c  FIX ME!!  Do we delete this, or is there a way it fits in with
@c  the following paragraph?   ---  Vctoria
@c
@c  @code{next} within a function that lacks debugging information acts like
@c  @code{step}, but any function calls appearing within the code of the
@c  function are executed without stopping.

The @code{next} command now only stops at the first instruction of a
source line.  This prevents the multiple stops that used to occur in
switch statements, for loops, etc. 

@kindex finish
@item 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}).

@kindex until
@kindex u
@item until
@itemx 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} makes your program continue execution until it
exits the loop.  In contrast, a @code{next} command at the end of a loop
simply steps back to the beginning of the loop, which forces you to step
through the next iteration.

@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.

@kindex stepi
@kindex si
@item stepi
@itemx 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 makes @value{GDBN} automatically display the next
instruction to be executed, each time your program stops.  @xref{Auto
Display,, Automatic display}.

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

@need 750
@kindex nexti
@kindex ni
@item nexti
@itemx 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

@node Signals, Thread Stops, Continuing and Stepping, Stopping
@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
@kindex info signals
@item 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.

@code{info handle} is the new alias for @code{info signals}.

@kindex handle
@item handle @var{signal} @var{keywords}@dots{}
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
can handle the signal, or else it may terminate 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 sees 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 your
program sees that signal when you continue.

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}. 

@node Thread Stops,  , Signals, Stopping
@section Stopping and starting multi-thread programs

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

@table @code
@cindex breakpoints and threads
@cindex thread breakpoints
@kindex break @dots{} thread @var{threadno}
@item break @var{linespec} thread @var{threadno}
@itemx break @var{linespec} thread @var{threadno} if @dots{}
@var{linespec} specifies source lines; there are several ways of
writing them, but the effect is always to specify some source line.

Use the qualifier @samp{thread @var{threadno}} with a breakpoint command
to specify that you only want @value{GDBN} to stop the program when a
particular thread reaches this breakpoint.  @var{threadno} is one of the
numeric thread identifiers assigned by @value{GDBN}, shown in the first
column of the @samp{info threads} display.

If you do not specify @samp{thread @var{threadno}} when you set a
breakpoint, the breakpoint applies to @emph{all} threads of your
program.

You can use the @code{thread} qualifier on conditional breakpoints as
well; in this case, place @samp{thread @var{threadno}} before the
breakpoint condition, like this:

@smallexample
(gdb) break frik.c:13 thread 28 if bartab > lim
@end smallexample

@end table

@cindex stopped threads
@cindex threads, stopped
Whenever your program stops under @value{GDBN} for any reason,
@emph{all} threads of execution stop, not just the current thread.  This
allows you to examine the overall state of the program, including
switching between threads, without worrying that things may change
underfoot.

@cindex continuing threads
@cindex threads, continuing
Conversely, whenever you restart the program, @emph{all} threads start
executing.  @emph{This is true even when single-stepping} with commands
like @code{step} or @code{next}.  

In particular, @value{GDBN} cannot single-step all threads in lockstep.
Since thread scheduling is up to your debugging target's operating
system (not controlled by @value{GDBN}), other threads may
execute more than one statement while the current thread completes a
single step.  Moreover, in general other threads stop in the middle of a
statement, rather than at a clean statement boundary, when the program
stops.

You might even find your program stopped in another thread after
continuing or even single-stepping.  This happens whenever some other
thread runs into a breakpoint, a signal, or an exception before the
first thread completes whatever you requested.

On some OSes, you can lock the OS scheduler and thus allow only a single
thread to run.

@table @code
@item set scheduler-locking @var{mode}
Set the scheduler locking mode.  If it is @code{off}, then there is no
locking and any thread may run at any time.  If @code{on}, then only the
current thread may run when the inferior is resumed.  The @code{step}
mode optimizes for single-stepping.  It stops other threads from
``seizing the prompt'' by preempting the current thread while you are
stepping.  Other threads will only rarely (or never) get a chance to run
when you step.  They are more likely to run when you ``next'' over a
function call, and they are completely free to run when you use commands
like ``continue'', ``until'', or ``finish''.  However, unless another
thread hits a breakpoint during its timeslice, they will never steal the
GDB prompt away from the thread that you are debugging.

@item show scheduler-locking
Display the current scheduler locking mode.
@end table


@node Stack, Source, Stopping, Top
@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, information about the call 
is generated. 
That information includes the location of the call in your program, 
the arguments of the call, 
and the local variables of the function being called.
The information is saved in a block of data called a @dfn{stack frame}.  
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. @xref{Selection, ,Selecting a frame}.

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

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

@end menu

@node Frames, Backtrace, Stack, Stack
@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 byte 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} generates 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} nevertheless regards it as though
it had a separate frame, which is numbered zero as usual, allowing
correct tracing of the function call chain.  However, @value{GDBN} has
no provision for frameless functions elsewhere in the stack.

@table @code
@kindex frame
@item frame @var{args}
The @code{frame} command allows you to move from one stack frame to another, 
and to print the stack frame you select.  @var{args} may be either the
address of the frame or the stack frame number.  Without an argument, 
@code{frame} prints the current stack frame.  

@kindex select-frame
@item select-frame
The @code{select-frame} command allows you to move from one stack frame
to another without printing the frame.  This is the silent version of
@code{frame}.
@end table

@node Backtrace, Selection, Frames, Stack
@section Backtraces

@cindex backtraces
@cindex tracebacks
@cindex stack traces
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
@kindex backtrace
@kindex bt
@item backtrace
@itemx 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, Frame Info, Backtrace, Stack
@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
@kindex frame
@kindex f
@item frame @var{n}
@itemx f @var{n}
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.

@ifclear HPPA
On the SPARC architecture, @code{frame} needs two addresses to
select an arbitrary frame: a frame pointer and a stack pointer.

On the MIPS and Alpha architecture, it needs two addresses: a stack
pointer and a program counter.

On the 29k architecture, it needs three addresses: a register stack
pointer, a program counter, and a memory stack pointer.
@c note to future updaters: this is conditioned on a flag
@c SETUP_ARBITRARY_FRAME in the tm-*.h files.  The above is up to date
@c as of 27 Jan 1994.
@end ifclear

@kindex up
@item up @var{n}
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.

@kindex down
@kindex do
@item down @var{n}
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.  

@need 1000
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
prints ten lines centered on the point of execution in the frame.
@xref{List, ,Printing source lines}.

@table @code
@kindex down-silently
@kindex up-silently
@item up-silently @var{n}
@itemx down-silently @var{n}
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, Alpha/MIPS Stack, Selection, Stack
@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}.

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

@itemize @bullet
@item 
the address of the frame 
@item
the address of the next frame down (called by this frame)
@item
the address of the next frame up (caller of this frame)
@item
the language in which the source code corresponding to this frame is written
@item
the address of the frame's arguments
@item
the program counter saved in it (the address of execution in the caller frame)
@item
which registers were saved in the frame
@end itemize

@noindent 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.  This requires the same kind of address (more than one for some
architectures) that you specify in the @code{frame} command.
@xref{Selection, ,Selecting a frame}.

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

@node Alpha/MIPS Stack,  , Frame Info, Stack
@section MIPS/Alpha machines and the function stack

@cindex stack on Alpha
@cindex stack on MIPS
@cindex Alpha stack
@cindex MIPS stack
Alpha- and 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:

@table @code
@cindex @code{heuristic-fence-post} (Alpha,MIPS)
@item set heuristic-fence-post @var{limit}
Restrict @value{GDBN} to examining at most @var{limit} bytes in its search
for the beginning of a function.  A value of @var{0} (the default)
means there is no limit.  However, except for @var{0}, the larger the
limit the more bytes @code{heuristic-fence-post} must search and 
therefore the longer it takes to run.

@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 Alpha or MIPS processors.


@node Source, Data, Stack, Top
@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.

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

@menu
* List::                        Printing source lines
* Search::                      Searching source files
* Source Path::                 Specifying source directories
* Machine Code::                Source and machine code
@end menu

@node List, Search, Source, Source
@section Printing source lines

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

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

@kindex show listsize
@item show listsize
Display the number of lines that @code{list} prints.
@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}
Specifies the line that begins the body of the function @var{function}.
For example: in C, this is the line with the open brace.

@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

@node Search, Source Path, List, Source
@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
@kindex search
@kindex forward-search
@item forward-search @var{regexp}
@itemx search @var{regexp}
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 the 
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

@node Source Path, Machine Code, Search, Source
@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} looks in the current directory as a
last resort.

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

@kindex directory
@kindex dir
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{}
@item dir @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 @value{GDBN} searches it sooner.

@kindex cdir
@kindex cwd
@kindex $cdir
@kindex $cwd
@cindex compilation directory
@cindex current directory
@cindex working directory
@cindex directory, current
@cindex directory, compilation
You can use the string @samp{$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,  , Source Path, Source
@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.  When run under @sc{gnu} Emacs
mode, the @code{info line} command now causes the arrow to point to the
line specified.  Also, @code{info line} prints addresses in symbolic form as 
well as hex.

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

The following example shows the disassembly of a range of addresses of
HP PA-RISC 2.0 code:

@smallexample
(@value{GDBP}) disas 0x32c4 0x32e4
Dump of assembler code from 0x32c4 to 0x32e4:
0x32c4 <main+204>:      addil 0,dp
0x32c8 <main+208>:      ldw 0x22c(sr0,r1),r26
0x32cc <main+212>:      ldil 0x3000,r31
0x32d0 <main+216>:      ble 0x3f8(sr4,r31)
0x32d4 <main+220>:      ldo 0(r31),rp
0x32d8 <main+224>:      addil -0x800,dp
0x32dc <main+228>:      ldo 0x588(r1),r26
0x32e0 <main+232>:      ldil 0x3000,r31
End of assembler dump.
@end smallexample

Some architectures have more than one commonly-used set of instruction
mnemonics or other syntax.

@table @code
@kindex set assembly-language
@cindex assembly instructions
@cindex instructions, assembly
@cindex machine instructions
@cindex listing machine instructions
@item set assembly-language @var{instruction-set}
Select the instruction set to use when disassembling the
program via the @code{disassemble} or @code{x/i} commands.

Currently this command is only defined for the Intel x86 family.  You
can set @var{instruction-set} to either @code{i386} or @code{i8086}.
The default is @code{i386}.
@end table


@node Data, Languages, Source, Top
@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}.  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}).

@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 or class 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
* Floating Point Hardware::     Floating point hardware
@end menu

@node Expressions, Variables, Data, Data
@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.

@value{GDBN} now supports array constants in expressions input by
the user.  The syntax is @var{@{element, element@dots{}@}}.  For example,
you can now use the command @code{print @{1, 2, 3@}} to build up an array in 
memory that is malloc'd in the target program.

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 in order to examine a structure
at that address in memory.
@c FIXME: casts supported---Mod2 true?

@value{GDBN} supports these operators, in addition to those common
to 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}.

@cindex @{@var{type}@}
@cindex type casting memory
@cindex memory, viewing as typed object
@cindex casts, to view memory
@item @{@var{type}@} @var{addr}
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, Arrays, Expressions, Data
@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 be either:

@itemize @bullet
@item
global (or file-static)
@end itemize

@noindent or 

@itemize @bullet
@item
visible according to the scope rules of the
programming language from the point of execution in that frame
@end itemize  

@noindent 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

@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

@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.

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

@node Arrays, Output Formats, Variables, Data
@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
and be an individual object.  The right operand should be the desired length
of the array.  The result is an array value whose elements are all of
the type of the left argument.  The first element is actually the left
argument; the second element comes from bytes of memory immediately
following those that hold the first element, and so on.  Here is an
example.  If a program says

@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.

Another way to create an artificial array is to use a cast.
This re-interprets a value as if it were an array.
The value need not be in memory:
@example
(@value{GDBP}) p/x (short[2])0x12345678
$1 = @{0x1234, 0x5678@}
@end example

As a convenience, if you leave the array length out (as in
@samp{(@var{type})[])@var{value}}) gdb calculates the size to fill
the value (as @samp{sizeof(@var{value})/sizeof(@var{type})}:
@example
(@value{GDBP}) p/x (short[])0x12345678
$2 = @{0x1234, 0x5678@}
@end example

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, Memory, Arrays, Data
@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
@cindex unknown address, locating
Print as an address, both absolute in hexadecimal and as an offset from
the nearest preceding symbol.  You can use this format used to discover
where (in what function) an unknown address is located:

@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, Auto Display, Output Formats, Data
@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},
@samp{s} (null-terminated string), or @samp{i} (machine instruction).
The default is @samp{x} (hexadecimal) initially.
The default changes each time you use 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 works.  The output
specifications @samp{4xw} and @samp{4wx} mean exactly the same thing.
(However, the count @var{n} must come first; @samp{wx4} does 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, Print Settings, Memory, Data
@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} prints its value each time your program stops.
Each expression added to the list is given a number to identify it;
to remove an expression from the list, you specify that number.
The automatic display looks like this:

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

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

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

@kindex disable display
@item disable display @var{dnums}@dots{}
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.

@kindex enable display
@item enable display @var{dnums}@dots{}
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.

@kindex info display
@item 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}, @value{GDBN} displays this argument while your program
continues to stop inside that function.  When it stops elsewhere---where
there is no variable @code{last_char}---the display is disabled
automatically.  The next time your program stops where @code{last_char}
is meaningful, you can enable the display expression once again.

@node Print Settings, Value History, Auto Display, Data
@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
@kindex set print address
@item set print address
@itemx set print address on
@value{GDBN} prints memory addresses showing the location of stack
traces, structure values, pointer values, breakpoints, and so forth,
even when it also displays the contents of those addresses.  The default
is @code{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.

@kindex show print address
@item 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 clarify.  One way to do this is with
@code{info line}, for example @samp{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
@kindex set print symbol-filename
@item set print symbol-filename on
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.

@kindex show print symbol-filename
@item 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

Another situation where it is helpful to show symbol filenames and line
numbers is when disassembling code; @value{GDBN} shows you the line
number and source file that corresponds to each instruction.

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

@table @code
@kindex set print max-symbolic-offset
@item set print max-symbolic-offset @var{max-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 tells @value{GDBN} 
to always print the symbolic form of an address if any symbol precedes it.

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

@cindex wild pointer, interpreting
@cindex pointer, finding referent
If you have a pointer and you are not sure where it points, try
@samp{set print symbol-filename on}.  Then you can determine the name
and source file location of the variable where it points, using
@samp{p/a @var{pointer}}.  This interprets the address in symbolic form.
For example, here @value{GDBN} shows that a variable @code{ptt} points
at another variable @code{t}, defined in @file{hi2.c}:

@example
(@value{GDBP}) set print symbol-filename on
(@value{GDBP}) p/a ptt
$4 = 0xe008 <t in hi2.c>
@end example

@quotation
@emph{Warning:} For pointers that point to a local variable, @samp{p/a}
does not show the symbol name and filename of the referent, even with
the appropriate @code{set print} options turned on.
@end quotation

Other settings control how different kinds of objects are printed:

@table @code
@kindex set print array
@item set print array
@itemx set print array on
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.

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

@kindex set print elements
@item set print elements @var{number-of-elements}
Set a limit on how many elements of an array @value{GDBN} will print.
If @value{GDBN} is printing a large array, it stops 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  @var{number-of-elements} to zero means that the printing is unlimited.

@kindex show print elements
@item show print elements
Display the number of elements of a large array that @value{GDBN} will print.
If the number is 0, then the printing is unlimited.

@kindex set print null-stop
@item set print null-stop
Cause @value{GDBN} to stop printing the characters of an array when the first
@sc{NULL} is encountered.  This is useful when large arrays actually
contain only short strings.

@kindex set print pretty
@item set print pretty on
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.

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

@kindex set print sevenbit-strings
@item set print sevenbit-strings on
Print using only seven-bit characters; if this option is set,
@value{GDBN} displays any eight-bit characters (in strings or
character values) using the notation @code{\}@var{nnn}.  This setting is
best if you are working in English (@sc{ascii}) and you use the
high-order bit of characters as a marker or ``meta'' bit.

@item set print sevenbit-strings off
Print full eight-bit characters.  This allows the use of more
international character sets, and is the default.

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

@kindex set print union
@item set print union on
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.

@kindex show print union
@item 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

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

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

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

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

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

@kindex set demangle-style
@cindex C++ symbol decoding style
@cindex symbol decoding style, C++
@item set demangle-style @var{style}
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 @sc{gnu} C++ compiler (@code{g++}) encoding algorithm.  
@ifclear HPPA
This is the default.
@end ifclear

@item hp
Decode based on the HP ANSI C++ (@code{aCC}) 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
If you omit @var{style}, you will see a list of possible formats.

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

@kindex set print object
@item set print object
@itemx set print object on
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.

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

@kindex set print static-members
@item set print static-members
@itemx set print static-members on
Print static members when displaying a C++ object.  The default is on.

@item set print static-members off
Do not print static members when displaying a C++ object.

@kindex show print static-members
@item show print static-members
Show whether C++ static members are printed, or not.

@c These don't work with HP ANSI C++ yet.
@kindex set print vtbl
@item set print vtbl
@itemx set print vtbl on
Pretty print C++ virtual function tables.  The default is off.
@ifset HPPA
(The @code{vtbl} commands do not work on programs compiled with the HP
ANSI C++ compiler (@code{aCC}).)
@end ifset

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

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

@node Value History, Convenience Vars, Print Settings, Data
@section Value history

@cindex value history
Values printed by the @code{print} command are saved in the @value{GDBN} 
@dfn{value history}.  This allows you to refer to them in other expressions.  
Values are kept until the symbol table is re-read or discarded 
(for example with the @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, @code{show values +} 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, Registers, Value History, Data
@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
@kindex show convenience
@item 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
@end example

@noindent Repeat that command by typing @key{RET}.

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

@table @code
@kindex $_
@item $_
The variable @code{$_} 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{$__}.

@kindex $__
@item $__
The variable @code{$__} 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.

@item $_exitcode
@kindex $_exitcode
The variable @code{$_exitcode} is automatically set to the exit code when
the program being debugged terminates.
@end table

@ifset HPPA
If you refer to a function or variable name that begins with a dollar
sign, @value{GDBN} searches for a user or system name first, before it
searches for a convenience variable.
@end ifset

@node Registers, Floating Point Hardware, Convenience Vars, Data
@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
@kindex info registers
@item info registers
Print the names and values of all registers except floating-point
registers (in the selected stack frame).

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

@item info registers @var{regname} @dots{}
Print the @dfn{relativized} value of each specified register @var{regname}.
As discussed in detail below, register values are normally relative to 
the selected stack frame.  @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 makes no difference.

@table @code
@kindex set rstack_high_address
@cindex AMD 29K register stack
@cindex register stack, AMD29K
@item set rstack_high_address @var{address}
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 probably want to precede with @samp{0x} to specify in
hexadecimal.

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

@node Floating Point Hardware,  , Registers, Data
@section Floating point hardware
@cindex floating point

Depending on the configuration, @value{GDBN} may be able to give
you more information about the status of the floating point hardware.

@table @code
@kindex info float
@item info float
Display hardware-dependent information about the floating
point unit.  The exact contents and layout vary depending on the
floating point chip.  Currently, @samp{info float} is supported on
the ARM and x86 machines.
@end table

@node Languages, Symbols, Data, Top
@chapter Using @value{GDBN} with Different Languages
@cindex languages

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 appear as 
@samp{0x1ae}, while in Modula-2 they appear as @samp{1AEH}.

@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 is called the @dfn{working
language}.

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

@node Setting, Show, Languages, Languages
@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.  The working language is
used to determine how expressions you type are interpreted, how values
are printed, etc.

In addition to the working language, every source file that
@value{GDBN} knows about has its own working language.  For some object
file formats, the compiler might indicate which language a particular
source file is in.  However, most of the time @value{GDBN} infers the
language from the name of the file.  The language of a source file
controls whether C++ names are demangled---this way @code{backtrace} can
show each frame appropriately for its own language.  There is no way to
set the language of a source file from within @value{GDBN}.  

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

@menu
* Filenames::                   Filename extensions and languages.
* Manually::                    Setting the working language manually
* Automatically::               Having @value{GDBN} infer the source language
@end menu

@node Filenames, Manually, Setting, Setting
@subsection List of filename extensions and languages

If a source file name ends in one of the following extensions, then
@value{GDBN} infers that its language is the one indicated.

@table @file

@item .c
C source file

@item .C
@itemx .cc
@itemx .cp
@itemx .cpp
@itemx .cxx
@itemx .c++
C++ source file

@item .f
@itemx .F
Fortran source file

@ifclear HPPA
@item .ch
@itemx .c186
@itemx .c286
CHILL source file.
@end ifclear

@item .mod
Modula-2 source file

@item .s
@itemx .S
Assembler source file.  This actually behaves almost like C, but
@value{GDBN} does not skip over function prologues when stepping.
@end table

In addition, you may set the language associated with a filename
extension.  @xref{Show, , Displaying the language}.

@node Manually, Automatically, Filenames, Setting
@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 
@code{c} or @code{modula-2}.
For a list of the supported languages, type @samp{set language}.

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.

@node Automatically,  , Manually, Setting
@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 working language.  That is, 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, Checks, Setting, Languages
@section Displaying the language

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

@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
Display the source language for this frame.  This language becomes the 
working language if you use an identifier from this frame.
@xref{Frame Info, ,Information about a frame}, to identify the other 
information listed here.

@item info source
Display the source language of this source file.
@xref{Symbols, ,Examining the Symbol Table}, to identify the other 
information listed here.
@end table

In unusual circumstances, you may have source files with extensions
not in the standard list.  You can then set the extension associated
with a language explicitly:

@kindex set extension-language
@kindex info extensions
@table @code
@item set extension-language @var{.ext} @var{language}
Set source files with extension @var{.ext} to be assumed to be in
the source language @var{language}.

@item info extensions
List all the filename extensions and the associated languages.
@end table

@node Checks, Support, Show, Languages
@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} does 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, Range Checking, Checks, Checks
@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,

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

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

For the 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 to 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 if you turn type checking off, there may be other reasons 
related to type that 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,  , Type Checking, Checks
@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

@node Support,  , Checks, Languages
@section Supported languages

@value{GDBN} supports C, C++, Fortran, Chill, assembly, and Modula-2.
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 accepts, and what input and output
formats should look like for different languages.  There are many good
books written on each of these languages; please look to these for a
language reference or tutorial.

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

@node C, Modula-2, , Support
@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 those languages
together.

@ifclear HPPA
@cindex C++
@kindex g++
@cindex @sc{gnu} C++
The C++ debugging facilities are jointly implemented by the C++
compiler and @value{GDBN}.  Therefore, to debug your C++ code
effectively, you must compile your C++ programs with a supported
C++ compiler, such as @sc{gnu} @code{g++}, or the HP ANSI C++
compiler (@code{aCC}).

For best results when using @sc{gnu} C++, use the stabs debugging
format.  You can select that format explicitly with the @code{g++}
command-line options @samp{-gstabs} or @samp{-gstabs+}.  See
@ref{Debugging Options,,Options for Debugging Your Program or @sc{gnu}
CC, gcc.info, Using @sc{gnu} CC}, for more information.
@end ifclear
@ifset HPPA
@cindex C++
@kindex g++
@cindex @sc{gnu} C++
You can use @value{GDBN} to debug C programs compiled with either the HP
C compiler (@code{cc}) or the GNU C compiler (@code{gcc}), and to debug
programs compiled with either the HP ANSI C++ compiler (@code{aCC}) or
the @sc{gnu} C++ compiler (@code{g++}).

If you compile with the @sc{gnu} C++ compiler, use the stabs debugging
format for best results when debugging.  You can select that format
explicitly with the @code{g++} command-line options @samp{-gstabs} or
@samp{-gstabs+}.  See @ref{Debugging Options,,Options for Debugging Your
Program or @sc{gnu} CC, gcc.info, Using @sc{gnu} CC}, for more
information.
@end ifset

@menu
* C Operators::                 C and C++ operators
* C Constants::                 C and C++ constants
* C plus plus expressions::     C++ expressions
* C Defaults::                  Default settings for C and C++
* C Checks::                    C and C++ type and range checks
* Debugging C::                 @value{GDBN} and C
* Debugging C plus plus::       @value{GDBN} features for C++
@end menu

@node C Operators, C Constants, , C
@subsubsection C and C++ operators

@cindex C and C++ 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 C and C++, the following definitions hold:

@itemize @bullet
@item
@ifclear HPPA
@emph{Integral types} include @code{int} with any of its storage-class
specifiers; @code{char}; and @code{enum}.
@end ifclear
@ifset HPPA
@emph{Integral types} include @code{int} with any of its storage-class
specifiers; @code{char}; @code{enum}; and, for C++, @code{bool}.
@end ifset

@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{++}.

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.

@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.

@ifset HPPA
@item .*@r{, }->*
Dereferences of pointers to members.
@end ifset

@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{->}.

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

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

@ifset HPPA
If an operator is redefined in the user code, @value{GDBN} usually
attempts to invoke the redefined version instead of using the operator's
predefined meaning.
@end ifset

@menu
* C Constants::             
@end menu

@node C Constants, C plus plus expressions, C Operators, C
@subsubsection C and C++ constants

@cindex C and C++ constants

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

@itemize @bullet
@item
Integer constants are a sequence of digits.  Octal constants are
specified by a leading @samp{0} (i.e. 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

@menu
* C plus plus expressions::           
* C Defaults::                  
* C Checks::                    

* Debugging C::                 
@end menu

@node C plus plus expressions, C Defaults, C Constants, C
@subsubsection C++ expressions

@cindex expressions in C++
@value{GDBN} expression handling can interpret most C++ expressions.

@ifclear HPPA
@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++
@c FIXME!! GDB may eventually be able to debug C++ using DWARF; check
@c periodically whether this has happened...
@quotation
@emph{Warning:} @value{GDBN} can only debug C++ code if you use the
proper compiler.  Typically, C++ debugging depends on the use of
additional debugging information in the symbol table, and thus requires
special support.  In particular, if your compiler generates a.out, MIPS
@sc{ecoff}, RS/6000 @sc{xcoff}, or @sc{elf} with stabs extensions to the
symbol table, these facilities are all available.  (With @sc{gnu} CC,
you can use the @samp{-gstabs} option to request stabs debugging
extensions explicitly.)  Where the object code format is standard
@sc{coff} or @sc{dwarf} in @sc{elf}, on the other hand, most of the C++
support in @value{GDBN} does @emph{not} work.
@end quotation
@end ifclear

@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++.

@ifclear HPPA
@cindex call overloaded functions
@cindex type conversions in C++
@item
You can call overloaded functions; @value{GDBN} resolves 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} does not perform conversions requiring constructors or
user-defined type operators.
@end ifclear
@ifset HPPA
@cindex call overloaded functions
@cindex overloaded functions
@cindex type conversions in C++
@item
You can call overloaded functions; @value{GDBN} resolves the function
call to the right definition, with some restrictions.  GDB does not
perform overload resolution involving user-defined type conversions,
calls to constructors, or instantiations of templates that do not exist
in the program.  It also cannot handle ellipsis argument lists or
default arguments.

It does perform integral conversions and promotions, floating-point
promotions, arithmetic conversions, pointer conversions, conversions of
class objects to base classes, and standard conversions such as those of
functions or arrays to pointers; it requires an exact match on the
number of function arguments.

Overload resolution is always performed, unless you have specified
@code{set overload-resolution off}.  @xref{Debugging C plus plus,
,@value{GDBN} features for C++}.

You must specify@code{set overload-resolution off} in order to use an
explicit function signature to call an overloaded function, as in
@smallexample
p 'foo(char,int)'('x', 13)
@end smallexample
The @value{GDBN} command-completion facility can simplify this;
@pxref{Completion, ,Command completion}.

@end ifset

@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

@ifset HPPA
In addition, @value{GDBN} supports calling virtual functions correctly,
printing out virtual bases of objects, calling functions in a base
subobject, casting objects, and invoking user-defined operators.
@end ifset

@node C Defaults, C Checks, C plus plus expressions, C
@subsubsection C and C++ defaults

@cindex C and C++ defaults

@ifclear HPPA
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}
selects the working language.
@end ifclear

If you allow @value{GDBN} to set the language automatically, it
recognizes source files whose names end with @file{.c}, @file{.C}, or
@file{.cc}, etc, and when @value{GDBN} enters code compiled from one of
these files, it sets the working language to C or C++.
@xref{Automatically, ,Having @value{GDBN} infer the source language},
for further details.

@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. roland 16jul93.

@node C Checks, Debugging C, C Defaults, C Constants
@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}
considers two variables type equivalent if:

@itemize @bullet
@item
The two variables are structured and have the same structure, union, or
enumerated tag.

@item
The 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.

@node Debugging C, Debugging C plus plus, C Checks, C
@subsubsection @value{GDBN} and C

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} or @code{class} is also printed.  Otherwise, it
appears as @samp{@{...@}}.

The @code{@@} operator aids in the debugging of dynamic arrays, formed
with pointers and a memory allocation function.  @xref{Expressions,
,Expressions}.

@menu
* Debugging C plus plus::       
@end menu

@node Debugging C plus plus,  , Debugging C, C
@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 throw
@itemx catch catch
Debug C++ exception handling using these commands.  @xref{Set
Catchpoints, , Setting catchpoints}.

@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}.
@ifset HPPA
(The @code{vtbl} commands do not work on programs compiled with the HP
ANSI C++ compiler (@code{aCC}).)

@kindex set overload-resolution
@cindex overloaded functions
@item set overload-resolution on
Enable overload resolution for C++ expression evaluation.  The default
is on.  For overloaded functions, @value{GDBN} evaluates the arguments
and searches for a function whose signature matches the argument types,
using the standard C++ conversion rules (@pxref{C plus plus expressions, ,C++
expressions} for details).  If it cannot find a match, it emits a
message.

@item set overload-resolution off
Disable overload resolution for C++ expression evaluation.  For
overloaded functions that are not class member functions, @value{GDBN}
chooses the first function of the specified name that it finds in the
symbol table, whether or not its arguments are of the correct type.  For
overloaded functions that are class member functions, @value{GDBN}
searches for a function whose signature @emph{exactly} matches the
argument types.
@end ifset

@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

@node Modula-2,  , C, Support
@subsection Modula-2

@cindex Modula-2

The extensions made to @value{GDBN} to support Modula-2 only support
output from the @sc{gnu} Modula-2 compiler (which is currently being
developed).  Other Modula-2 compilers are not currently supported, and
attempting to debug executables produced by them is most likely
to give an error as @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, Built-In Func/Proc, Modula-2, Modula-2
@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}
treats 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, M2 Constants, M2 Operators, Modula-2
@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} treats the use of procedures @code{INCL} and @code{EXCL} as
an error.
@end quotation

@cindex Modula-2 constants
@node M2 Constants, M2 Defaults, Built-In Func/Proc, Modula-2
@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, Deviations, M2 Constants, Modula-2
@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} sets the
working language to Modula-2. @xref{Automatically, ,Having @value{GDBN} set
the language automatically}, for further details.

@node Deviations, M2 Checks, M2 Defaults, Modula-2
@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} prints 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, M2 Scope, Deviations, Modula-2
@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
@sc{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, GDB/M2, M2 Checks, Modula-2
@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} searches 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,  , M2 Scope, Modula-2
@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.

@node Symbols, Altering, Languages, Top
@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}).

@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
@kindex info address
@item info address @var{symbol}
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.

@kindex whatis
@item whatis @var{exp}
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.

@kindex ptype
@item ptype @var{typename}
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 @samp{class
@var{class-name}}, @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.

@kindex info types
@item info types @var{regexp}
@itemx 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.

@kindex info source
@item 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.

@kindex info sources
@item 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.

@kindex info functions
@item 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}.

@kindex info variables
@item 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.
@kindex info methods
@item info methods
@itemx info methods @var{regexp}
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

@ifclear HPPA
@cindex reloading symbols
Some systems allow individual object files that make up your program to
be replaced without stopping and restarting your program.  For example,
in VxWorks you can simply recompile a defective object file and keep on
running.  If you are running on one of these systems, you can allow
@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.

@kindex show symbol-reloading
@item show symbol-reloading
Show the current @code{on} or @code{off} setting.
@end table
@end ifclear

@ifset HPPA
@kindex set opaque-type-resolution
@item set opaque-type-resolution on
Tell @value{GDBN} to resolve opaque types.  An opaque type is a type
declared as a pointer to a @code{struct}, @code{class}, or
@code{union}---for example, @code{struct MyType *}---that is used in one
source file although the full declaration of @code{struct MyType} is in
another source file.  The default is on.

A change in the setting of this subcommand will not take effect until
the next time symbols for a file are loaded.

@item set opaque-type-resolution off
Tell @value{GDBN} not to resolve opaque types.  In this case, the type
is printed as follows:
@smallexample
@{<no data fields>@}
@end smallexample

@kindex show opaque-type-resolution
@item show opaque-type-resolution
Show whether opaque types are resolved or not.
@end ifset

@kindex maint print symbols
@cindex symbol dump
@kindex maint print psymbols
@cindex partial symbol dump
@item maint print symbols @var{filename}
@itemx maint print psymbols @var{filename}
@itemx maint print msymbols @var{filename}
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, GDB Files, Symbols, Top
@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, give your program a signal, restart it at a different
address, or even return prematurely from a function.

@menu
* Assignment::                  Assignment to variables
* Jumping::                     Continuing at a different address
* Signaling::                   Giving your program a signal
* Returning::                   Returning from a function
* Calling::                     Calling your program's functions
* Patching::                    Patching your program
@end menu

@node Assignment, Jumping, Altering, Altering
@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).  
@xref{Languages, ,Using @value{GDBN} with Different Languages}, for more
information on operators in supported languages.

@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.

@ifclear HPPA
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
@end ifclear
@ifset HPPA
Because the @code{set} command has many subcommands that can conflict
with the names of program variables, it is a good idea to use the
@code{set variable} command instead of just @code{set}.  For example, if
your program has a variable @code{g}, you run into problems if you try
to set a new value with just @samp{set g=4}, because @value{GDBN} has
the command @code{set gnutarget}, abbreviated @code{set g}:

@example
@group
(@value{GDBP}) whatis g
type = double
(@value{GDBP}) p g
$1 = 1
(@value{GDBP}) set g=4
(gdb) p g
$2 = 1
(@value{GDBP}) r
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/smith/cc_progs/a.out
"/home/smith/cc_progs/a.out": can't open to read symbols: Invalid bfd target.
(@value{GDBP}) show g
The current BFD target is "=4".
@end group
@end example

@noindent
The program variable @code{g} did not change, and you silently set the
@code{gnutarget} to an invalid value.  In order to set the variable
@code{g}, use

@example
(@value{GDBP}) set var g=4
@end example
@end ifset

@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        /doc@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, Signaling, Assignment, Altering
@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
@kindex jump
@item jump @var{linespec}
Resume execution at line @var{linespec}.  Execution stops again
immediately if there is a breakpoint there.  @xref{List, ,Printing
source lines}, for a description of the different forms of
@var{linespec}.  It is common practice to use the @code{tbreak} command
in conjunction with @code{jump}.  @xref{Set Breaks, ,Setting
breakpoints}.

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

@ifclear HPPA
@c Doesn't work on HP-UX; have to set $pcoqh and $pcoqt.
You can get much the same effect as the @code{jump} command by storing a
new value into the register @code{$pc}.  The difference is that this
does not start your program running; it only changes the address of where it
@emph{will} run when you continue.  For example,

@example
set $pc = 0x485
@end example

@noindent
makes the next @code{continue} command or stepping command execute at
address @code{0x485}, rather than at the address where your program stopped.
@xref{Continuing and Stepping, ,Continuing and stepping}.
@end ifclear

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.

@c @group
@node Signaling, Returning, Jumping, Altering
@section Giving your program a signal

@table @code
@kindex signal
@item signal @var{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.


@node Returning, Calling, Signaling, Altering
@section Returning from a function

@table @code
@cindex returning from a function
@kindex return
@item return
@itemx return @var{expression}
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, Patching, Returning, Altering
@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.  If the result is not void, it 
is printed and saved in the value history. 

@ifclear HPPA
For the A29K, a user-controlled variable @code{call_scratch_address},
specifies the location of a scratch area to be used when @value{GDBN}
calls a function in the target.  This is necessary because the usual
method of putting the scratch area on the stack does not work in systems
that have separate instruction and data spaces.
@end ifclear

@node Patching,  , Calling, Altering
@section Patching programs

@cindex patching binaries
@cindex writing into executables
@cindex writing into corefiles

By default, @value{GDBN} opens the file containing your program's
executable code (or the corefile) read-only.  This prevents accidental
alterations to machine code; but it also prevents you from intentionally
patching your program's binary.

If you'd like to be able to patch the binary, you can specify that
explicitly with the @code{set write} command.  For example, you might
want to turn on internal debugging flags, or even to make emergency
repairs.

@table @code
@kindex set write
@item set write on
@itemx set write off
If you specify @samp{set write on}, @value{GDBN} opens executable and
core files for both reading and writing; if you specify @samp{set write
off} (the default), @value{GDBN} opens them read-only.

If you have already loaded a file, you must load it again (using the
@code{exec-file} or @code{core-file} command) after changing @code{set
write}, for your new setting to take effect.

@item show write
@kindex show write
Display whether executable files and core files are opened for writing
as well as reading.
@end table

@node GDB Files, Targets, Altering, Top
@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.  To debug a core dump of a previous run, you must also tell
@value{GDBN} the name of the core dump file.

@menu
* Files::                       Commands to specify files
* Symbol Errors::               Errors reading symbol files
@end menu

@node Files, Symbol Errors, GDB Files, GDB Files
@section Commands to specify files

@cindex symbol table
@cindex core dump file

You may want to specify executable and core dump file names.  The usual
way to do this is at start-up time, using the arguments to
@value{GDBN}'s start-up commands (@pxref{Invocation, , Getting In and
Out of @value{GDBN}}).

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
@cindex executable file
@kindex file
@item file @var{filename}
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.

@ifclear HPPA
On systems with memory-mapped files, an auxiliary file
@file{@var{filename}.syms} may hold symbol table information for
@var{filename}.  If so, @value{GDBN} maps in the symbol table from
@file{@var{filename}.syms}, starting up more quickly.  See the
descriptions of the file 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}, described below), 
for more information.
@end ifclear

@item file
@code{file} with no argument makes @value{GDBN} discard any information it
has on both executable file and the symbol table.

@kindex exec-file
@item exec-file @r{[} @var{filename} @r{]}
Specify that the program to be run (but not the symbol table) is found
in @var{filename}.  @value{GDBN} searches the environment variable @code{PATH}
if necessary to locate your program.  Omitting @var{filename} means to
discard information on the executable file.

@kindex symbol-file
@item symbol-file @r{[} @var{filename} @r{]}
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} does not repeat if you press @key{RET} again after
executing it once.

When @value{GDBN} is configured for a particular environment, it
understands debugging information in whatever format is the standard
generated for that environment; you may use either a @sc{gnu} compiler, or
other compilers that adhere to the local conventions.
@ifclear HPPA
Best results are usually obtained from @sc{gnu} compilers; for example,
using @code{@value{GCC}} you can generate debugging information for
optimized code.
@end ifclear

For most kinds of object files, with the exception of old SVR3 systems
using COFF, 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}.)

@ifclear HPPA
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.  Note that ``stabs-in-COFF''
still does the two-stage strategy, since the debug info is actually
in stabs format.

@kindex readnow
@cindex reading symbols immediately
@cindex symbols, reading immediately
@kindex mapped
@cindex memory-mapped symbol file
@cindex saving symbol table
@item symbol-file @var{filename} @r{[} -readnow @r{]} @r{[} -mapped @r{]}
@itemx file @var{filename} @r{[} -readnow @r{]} @r{[} -mapped @r{]}
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.  
@end ifclear

@ifclear HPPA
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 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} always attempts 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.

@kindex core
@kindex core-file
@item core-file @r{[} @var{filename} @r{]}
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

@ifclear HPPA
@kindex add-symbol-file
@cindex dynamic linking
@item add-symbol-file @var{filename} @var{address}
@itemx add-symbol-file @var{filename} @var{address} @r{[} -readnow @r{]} @r{[} -mapped @r{]}
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} does 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}.

@kindex add-shared-symbol-file
@item add-shared-symbol-file
The @code{add-shared-symbol-file} command can be used only under Harris' CXUX
operating system for the Motorola 88k.  @value{GDBN} automatically looks for 
shared libraries, however if @value{GDBN} does not find yours, you can run 
@code{add-shared-symbol-file}.  It takes no arguments.
@end ifclear

@ifclear HPPA
@kindex section
@item section
The @code{section} command changes the base address of section SECTION of 
the exec file to ADDR.  This can be used if the exec file does not contain 
section addresses, (such as in the a.out format), or when the addresses 
specified in the file itself are wrong.  Each section must be changed 
separately.  The ``info files'' command lists all the sections and their 
addresses.
@end ifclear

@kindex info files
@kindex info target
@item info files
@itemx info target
@code{info files} and @code{info target} are synonymous; both print the
current target (@pxref{Targets, ,Specifying a Debugging Target}),
including the names of the executable and core dump files currently in
use by @value{GDBN}, and the files from which symbols were loaded.  The
command @code{help target} 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 file
name and remembers it that way.

@cindex shared libraries
@ifclear HPPA
@c added HP-UX -- Kim (HP writer)
@value{GDBN} supports HP-UX, SunOS, SVr4, Irix 5, and IBM RS/6000 shared
libraries.
@end ifclear
@ifset HPPA
@value{GDBN} supports HP-UX shared libraries.
@end ifset
@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} does not understand
references to a function in a shared library, however---unless you are
debugging a core file).
@ifset HPPA
If the program loads a library explicitly, @value{GDBN} automatically
loads the symbols at the time of the @code{shl_load} call.
@end ifset
@c FIXME: some @value{GDBN} release may permit some refs to undef
@c FIXME...symbols---eg in a break cmd---assuming they are from a shared
@c FIXME...lib; check this from time to time when updating manual

@table @code
@kindex info sharedlibrary
@kindex info share
@item info share
@itemx info sharedlibrary
Print the names of the shared libraries which are currently loaded.

@kindex sharedlibrary
@kindex share
@item sharedlibrary @var{regex}
@itemx share @var{regex}

Load shared object library symbols for files matching a
Unix regular expression.
As with files loaded automatically, it only loads shared libraries
required by your program for a core file or after typing @code{run}.  If
@var{regex} is omitted all shared libraries required by your program are
loaded.
@end table

@ifset HPPA
@value{GDBN} detects the loading of a shared library and automatically
reads in symbols from the newly loaded library, up to a threshold that
is initially set but that you can modify if you wish.

Beyond that threshold, symbols from shared libraries must be explicitly
loaded.  To load these symbols, use the command @code{sharedlibrary}
@var{filename}.  The base address of the shared library is determined
automatically by @value{GDBN} and need not be specified.

To display or set the threshold, use the commands:

@table @code
@kindex set auto-solib-add
@item set auto-solib-add @var{threshold}
Set the autoloading size threshold, in megabytes.  If @var{threshold} is
nonzero, symbols from all shared object libraries will be loaded
automatically when the inferior begins execution or when the dynamic
linker informs @value{GDBN} that a new library has been loaded, until
the symbol table of the program and libraries exceeds this threshold.
Otherwise, symbols must be loaded manually, using the
@code{sharedlibrary} command.  The default threshold is 100 megabytes.

@kindex show auto-solib-add
@item show auto-solib-add
Display the current autoloading size threshold, in megabytes.
@end table
@end ifset

@node Symbol Errors,  , Files, GDB Files
@section Errors reading symbol files

While reading a symbol file, @value{GDBN} occasionally encounters 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 has 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 usually allows you to debug your program, though certain symbols
are not accessible.  If you encounter such a problem and feel like
debugging it, you can debug @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 a struct or class.

@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.

@item info mismatch between compiler and debugger

@value{GDBN} could not parse a type specification output by the compiler.

@end table

@node Targets, Controlling GDB, GDB Files, Top
@chapter Specifying a Debugging Target

@cindex debugging target
@kindex target

A @dfn{target} is the execution environment occupied by your program.
@ifclear HPPA
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 HPPA
On HP-UX systems, @value{GDBN} has been configured to support debugging
of processes running on the PA-RISC architecture.  This means that the
only possible targets are:

@itemize @bullet
@item
An executable that has been compiled and linked to run on HP-UX

@item
A live HP-UX process, either started by @value{GDBN} (with the
@code{run} command) or started outside of @value{GDBN} and attached to
(with the @code{attach} command)

@item
A core file generated by an HP-UX process that previously aborted
execution
@end itemize

@value{GDBN} on HP-UX has not been configured to support remote
debugging, or to support programs running on other platforms. You
can use the @code{target} command to specify one of the target types
configured for @value{GDBN} (@pxref{Target Commands, ,Commands for managing
targets}).
@end ifset

@menu
* Active Targets::              Active targets
* Target Commands::             Commands for managing targets
* Byte Order::                  Choosing target byte order
* Remote::                      Remote debugging

@end menu

@node Active Targets, Target Commands, Targets, Targets
@section Active targets

@cindex stacking targets
@cindex active targets
@cindex multiple targets

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 uses them in tandem, looking
first in the corefile target, then in the executable file, to satisfy
requests for memory addresses.  (Typically, these two classes of target
are complementary, since core files contain only a program's
read-write memory---variables and so on---plus machine status, while
executable files contain only the program text and initialized data.)

When you type @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 active core file or executable file target are obscured while the
process target is active.

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}).

@node Target Commands, Byte Order, Active Targets, Targets
@section Commands for managing targets

@table @code
@item target @var{type} @var{parameters}
Connects the @value{GDBN} host environment to a target 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.

The @code{target} command does not repeat if you press @key{RET} again
after executing the command.

@kindex help target
@item 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.

@kindex set gnutarget
@item set gnutarget @var{args}
@value{GDBN} uses its own library BFD to read your files.  @value{GDBN} 
knows whether it is reading an @dfn{executable},
a @dfn{core}, or a @dfn{.o} file; however, you can specify the file format 
with the @code{set gnutarget} command.  Unlike most @code{target} commands, 
with @code{gnutarget} the @code{target} refers to a program, not a machine.

@emph{Warning:} To specify a file format with @code{set gnutarget},
you must know the actual BFD name.

@noindent @xref{Files, , Commands to specify files}.

@kindex show gnutarget  
@item show gnutarget
Use the @code{show gnutarget} command to display what file format
@code{gnutarget} is set to read.  If you have not set @code{gnutarget},
@value{GDBN} will determine the file format for each file automatically,
and @code{show gnutarget} displays @samp{The current BDF target is "auto"}.
@end table

@ifclear HPPA
Here are some common targets (available, or not, depending on the GDB
configuration):
@end ifclear
@ifset HPPA
These are the valid targets on HP-UX systems:
@end ifset

@table @code
@kindex target exec
@item target exec @var{program}
An executable file.  @samp{target exec @var{program}} is the same as
@samp{exec-file @var{program}}.

@kindex target core
@item target core @var{filename}
A core dump file.  @samp{target core @var{filename}} is the same as
@samp{core-file @var{filename}}.

@kindex target remote
@item target remote @var{dev}
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}.  @code{target remote}
now supports the @code{load} command.  This is only useful if you have
some other way of getting the stub to the target system, and you can put
it somewhere in memory where it won't get clobbered by the download.

@ifclear HPPA
@kindex target sim
@item target sim
CPU simulator.  @xref{Simulator,,Simulated CPU Target}.
@end ifclear
@end table

The following targets are all CPU-specific, and only available for
specific configurations.
@c should organize by CPU

@table @code

@kindex target abug
@item target abug @var{dev}
ABug ROM monitor for M68K.

@kindex target adapt
@item target adapt @var{dev}
Adapt monitor for A29K.

@kindex target amd-eb
@item target amd-eb @var{dev} @var{speed} @var{PROG}
@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, ,The EBMON protocol for AMD29K}.

@kindex target array
@item target array @var{dev}
Array Tech LSI33K RAID controller board.

@kindex target bug
@item target bug @var{dev}
BUG monitor, running on a MVME187 (m88k) board.

@kindex target cpu32bug
@item target cpu32bug @var{dev}
CPU32BUG monitor, running on a CPU32 (M68K) board.

@kindex target dbug
@item target dbug @var{dev}
dBUG ROM monitor for Motorola ColdFire.

@kindex target ddb
@item target ddb @var{dev}
NEC's DDB monitor for Mips Vr4300.

@kindex target dink32
@item target dink32 @var{dev}
DINK32 ROM monitor for PowerPC.

@kindex target e7000
@item target e7000 @var{dev}
E7000 emulator for Hitachi H8 and SH.

@kindex target es1800
@item target es1800 @var{dev}
ES-1800 emulator for M68K.

@kindex target est
@item target est @var{dev}
EST-300 ICE monitor, running on a CPU32 (M68K) board.

@kindex target hms
@item target hms @var{dev}
A Hitachi SH, H8/300, or H8/500 board, attached via serial line to your host.
Use special commands @code{device} and @code{speed} to control the serial
line and the communications speed used.
@xref{Hitachi Remote,,@value{GDBN} and Hitachi Microprocessors}.

@kindex target lsi
@item target lsi @var{dev}
LSI ROM monitor for Mips.

@kindex target m32r
@item target m32r @var{dev}
Mitsubishi M32R/D ROM monitor.

@kindex target mips
@item target mips @var{dev}
IDT/SIM ROM monitor for Mips.

@kindex target mon960
@item target mon960 @var{dev}
MON960 monitor for Intel i960.

@kindex target nindy
@item target nindy @var{devicename}
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)}.

@kindex target nrom
@item target nrom @var{dev}
NetROM ROM emulator.  This target only supports downloading.

@kindex target op50n
@item target op50n @var{dev}
OP50N monitor, running on an OKI HPPA board.

@kindex target pmon
@item target pmon @var{dev}
PMON ROM monitor for Mips.

@kindex target ppcbug
@item target ppcbug @var{dev}
@kindex target ppcbug1
@item target ppcbug1 @var{dev}
PPCBUG ROM monitor for PowerPC.

@kindex target r3900
@item target r3900 @var{dev}
Densan DVE-R3900 ROM monitor for Toshiba R3900 Mips.

@kindex target rdi
@item target rdi @var{dev}
ARM Angel monitor, via RDI library interface.
 
@kindex target rdp
@item target rdp @var{dev}
ARM Demon monitor.

@kindex target rom68k
@item target rom68k @var{dev}
ROM 68K monitor, running on an M68K IDP board.

@kindex target rombug
@item target rombug @var{dev}
ROMBUG ROM monitor for OS/9000.

@kindex target sds
@item target sds @var{dev}
SDS monitor, running on a PowerPC board (such as Motorola's ADS).

@kindex target sparclite
@item target sparclite @var{dev}
Fujitsu sparclite boards, used only for the purpose of loading.  
You must use an additional command to debug the program. 
For example: target remote @var{dev} using @value{GDBN} standard 
remote protocol.

@kindex target sh3
@kindex target sh3e
@item target sh3 @var{dev}
@item target sh3e @var{dev}
Hitachi SH-3 and SH-3E target systems.

@kindex target st2000
@item target st2000 @var{dev} @var{speed}
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}.

@kindex target udi
@item target udi @var{keyword}
Remote AMD29K target, using the AMD UDI protocol.  The @var{keyword}
argument specifies which 29K board or simulator to use.  @xref{UDI29K
Remote,,The UDI protocol for AMD29K}.

@kindex target vxworks
@item target vxworks @var{machinename}
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}.

@kindex target w89k
@item target w89k @var{dev}
W89K monitor, running on a Winbond HPPA board.

@end table

Different targets are available on different configurations of @value{GDBN}; 
your configuration may have more or fewer targets.

Many remote targets require you to download the executable's code
once you've successfully established a connection.

@table @code

@kindex load @var{filename}
@item load @var{filename}
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{}}''

The file is loaded at whatever address is specified in the executable.
For some object file formats, you can specify the load address when you
link the program; for other formats, like a.out, the object file format
specifies a fixed address.
@c FIXME! This would be a good place for an xref to the GNU linker doc.

On VxWorks, @code{load} links @var{filename} dynamically on the
current target system as well as adding its symbols in @value{GDBN}.

@cindex download to Nindy-960
With the Nindy interface to an Intel 960 board, @code{load}
downloads @var{filename} to the 960 as well as adding its symbols in
@value{GDBN}.

@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).

@code{load} does not repeat if you press @key{RET} again after using it.
@end table

@node Byte Order, Remote, Target Commands, Targets
@section Choosing target byte order

@cindex choosing target byte order
@cindex target byte order
@kindex set endian big
@kindex set endian little
@kindex set endian auto
@kindex show endian

Some types of processors, such as the MIPS, PowerPC, and Hitachi SH,
offer the ability to run either big-endian or little-endian byte
orders.  Usually the executable or symbol will include a bit to
designate the endian-ness, and you will not need to worry about
which to use.  However, you may still find it useful to adjust
GDB's idea of processor endian-ness manually.

@table @code
@kindex set endian big
@item set endian big
Instruct @value{GDBN} to assume the target is big-endian.

@kindex set endian little
@item set endian little
Instruct @value{GDBN} to assume the target is little-endian.

@kindex set endian auto
@item set endian auto
Instruct @value{GDBN} to use the byte order associated with the
executable.

@item show endian
Display @value{GDBN}'s current idea of the target byte order.

@end table

Note that these commands merely adjust interpretation of symbolic
data on the host, and that they have absolutely no effect on the
target system.

@node Remote,  , Byte Order, Targets
@section Remote debugging
@cindex remote debugging

If you are trying to debug a program running on a machine that cannot run
@value{GDBN} 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 @value{GDBN} have special serial or TCP/IP interfaces
to make this work with particular debugging targets.  In addition,
@value{GDBN} comes with a generic serial protocol (specific to @value{GDBN}, 
but not specific to any particular target system) which you can use if you
write the remote stubs---the code that runs on the remote system to
communicate with @value{GDBN}.

Other remote targets may be available in your
configuration of @value{GDBN}; use @code{help target} to list them.

@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
* Remote Serial::               @value{GDBN} remote serial protocol
* i960-Nindy Remote::           @value{GDBN} with a remote i960 (Nindy)
* UDI29K Remote::               The UDI protocol for AMD29K
* EB29K Remote::                The EBMON protocol for AMD29K
* VxWorks Remote::              @value{GDBN} and VxWorks
* ST2000 Remote::               @value{GDBN} with a Tandem ST2000
* Hitachi Remote::              @value{GDBN} and Hitachi Microprocessors
* MIPS Remote::                 @value{GDBN} and MIPS boards
* Sparclet Remote::             @value{GDBN} and Sparclet boards
* Simulator::                   Simulated CPU target
@end menu

@include remote.texi


@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, Editing, Controlling GDB, Controlling GDB
@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.

@emph{Note:}  @code{set prompt} no longer adds a space for you after the
prompt you set.  This allows you to set a prompt which ends in a space
or a prompt that does not.

@table @code
@kindex set prompt
@item set prompt @var{newprompt}
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, History, Prompt, Controlling GDB
@section Command editing
@cindex readline
@cindex command line editing

@value{GDBN} reads its input commands via the @dfn{readline} interface.  This
@sc{gnu} library provides consistent behavior for programs which provide a
command line interface to the user.  Advantages are @sc{gnu} Emacs-style
or @dfn{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, Screen Size, Editing, Controlling GDB
@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
@kindex GDBHISTFILE
@item set history filename @var{fname}
Set the name of the @value{GDBN} command history file to @var{fname}.
This is the file where @value{GDBN} reads an initial command history
list, and where it writes the command history from this session when it
exits.  You can access this list through history expansion or through
the history command editing characters listed below.  This file defaults
to the value of the environment variable @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} keeps 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 do 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 @sc{gnu} 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, Numbers, History, Controlling GDB
@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
@kindex set height
@kindex set width
@kindex show width
@kindex show height
@item set height @var{lpp}
@itemx show height
@itemx set width @var{cpl}
@itemx show width
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} does not pause during 
output no matter how long the output is.  This is useful if output is to a 
file or to an editor buffer.

Likewise, you can specify @samp{set width 0} to prevent @value{GDBN}
from wrapping its output.
@end table

@node Numbers, Messages/Warnings, Screen Size, Controlling GDB
@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 input-radix
@item set input-radix @var{base}
Set the default base for numeric input.  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

@smallexample
set radix 012
set radix 10.
set radix 0xa
@end smallexample

@noindent
sets the base to decimal.  On the other hand, @samp{set radix 10}
leaves the radix unchanged no matter what it was.

@kindex set output-radix
@item set output-radix @var{base}
Set the default base for numeric 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.

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

@kindex show output-radix
@item show output-radix
Display the current default base for numeric display.
@end table

@node Messages/Warnings,  , Numbers, Controlling GDB
@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.
This makes @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).

@kindex show confirm
@item show confirm
Displays state of confirmation requests.
@end table

@node Sequences, Emacs, Controlling GDB, Top
@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, Hooks, Sequences, Sequences
@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.  User commands may accept up to 10 arguments separated by whitespace.
Arguments are accessed within the user command via @var{$arg0@dots{}$arg9}.
A trivial example:

@smallexample
define adder
  print $arg0 + $arg1 + $arg2
@end smallexample

@noindent To execute the command use:

@smallexample
adder 1 2 3
@end smallexample

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

@table @code
@kindex define
@item define @var{commandname}
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}.

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

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

@kindex document
@item document @var{commandname}
Document the user-defined command @var{commandname}, so that it can be
accessed by @code{help}.  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} displays the documentation you have written.

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.

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

@kindex show user
@item show user
@itemx show user @var{commandname}
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

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

If used interactively, commands that would ask for confirmation proceed
without asking when used inside a user-defined command.  Many @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, Command Files, Define, Sequences
@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.

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

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
get a warning from the @code{define} command.

@node Command Files, Output, Hooks, Sequences
@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{.gdbinit}
When you start @value{GDBN}, it automatically executes commands from its
@dfn{init files}.  These are files named @file{.gdbinit} on Unix, or
@file{gdb.ini} on DOS/Windows.  @value{GDBN} reads the init file (if
any) in your home directory, then processes command line options and
operands, and then reads the init file (if any) in the current working
directory.  This is so the init file in your home directory can set
options (such as @code{set complaints}) which affect the processing of
the command line options and operands.  The init files are not executed
if you use the @samp{-nx} option; @pxref{Mode Options, ,Choosing modes}.

@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 @value{GDBN} 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:

@kindex .vxgdbinit
@itemize @bullet
@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

You can also request the execution of a command file with the
@code{source} command:

@table @code
@kindex source
@item source @var{filename}
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,  , Command Files, Sequences
@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
@kindex echo
@item echo @var{text}
@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 is printed unless you specify one.}
In addition to the standard C escape sequences, a backslash followed
by a space stands for a space.  This is useful for displaying a
string with spaces at the beginning or the end, since leading and
trailing spaces are otherwise trimmed from all arguments.  
To print @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

@kindex output
@item output @var{expression}
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.

@kindex printf
@item printf @var{string}, @var{expressions}@dots{}
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

@node Emacs, GDB Bugs, Sequences, Top
@chapter Using @value{GDBN} under @sc{gnu} Emacs

@cindex Emacs
@cindex @sc{gnu} Emacs
A special interface allows you to use @sc{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.
@ifset HPPA
(Do not use the @code{-tui} option to run @value{GDBN} from Emacs.)
@end ifset

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 have no reason to use them from Emacs.

@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 does 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 proceeds 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 an absolute file 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) makes 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 @sc{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 &} indicates that you
wish special formatting, and also acts 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 recreates
the source buffer if necessary to show you the context of the current
frame.

The source files displayed in Emacs are in ordinary Emacs buffers
which are visiting the source files in the usual way.  You can edit
the files with these buffers if you wish; but keep in mind that @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 cease
to correspond properly with the code.

@c The following dropped because Epoch is nonstandard.  Reactivate
@c if/when v19 does something similar. ---doc@cygnus.com 19dec1990
@ignore
@kindex Emacs Epoch environment
@kindex Epoch
@kindex inspect

Version 18 of @sc{gnu} 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

@node GDB Bugs
@c links whacked to pacify makeinfo
@c , Command Line Editing, Emacs, Top
@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, Bug Reporting, GDB Bugs, GDB Bugs
@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
@cindex fatal signal
@cindex debugger crash
@cindex crash of debugger
@item
If the debugger gets a fatal signal, for any input whatever, that is a
@value{GDBN} bug.  Reliable debuggers never crash.

@cindex error on valid input
@item
If @value{GDBN} produces an error message for valid input, that is a
bug.  (Note that if you're cross debugging, the problem may also be
somewhere in the connection to the target.)

@cindex invalid input
@item
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,  , Bug Criteria, GDB Bugs
@section How to report bugs
@cindex bug reports
@cindex @value{GDBN} bugs, reporting

@ifclear HPPA
A number of companies and individuals offer support for @sc{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 @sc{gnu} Emacs
distribution.
@c should add a web page ref...

In any event, we also recommend that you send bug reports for
@value{GDBN} to this addresses:

@example
bug-gdb@@prep.ai.mit.edu
@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
@sc{gnu} Debugger Bugs
Free Software Foundation Inc.
59 Temple Place - Suite 330
Boston, MA 02111-1307
USA
@end example
@end ifclear

@ifset HPPA
If you obtained HP GDB as part of your HP ANSI C or HP ANSI C++ compiler
kit, report problems to your HP Support Representative.

If you obtained HP GDB from the Hewlett-Packard Web site, report
problems by electronic mail to @code{wdb-www@@ch.hp.com}.
@end ifset

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.  It may be that the bug has been reported previously, but neither
you nor we can know that unless your bug report is complete and
self-contained.

Sometimes people give a few sketchy facts and ask, ``Does this ring a
bell?''  Those bug reports are useless, and we urge everyone to
@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.

@ifclear HPPA
@item
What compiler (and its version) was used to compile @value{GDBN}---e.g.
``@value{GCC}--2.8.1''.
@end ifclear

@item
What compiler (and its version) was used to compile the program you are
debugging---e.g.  ``@value{GCC}--2.8.1'', or ``HP92453-01 A.10.32.03 HP
C Compiler''.  For GCC, you can say @code{gcc --version} to get this
information; for other compilers, see the documentation for those
compilers.

@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.  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.

@ifclear HPPA
@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 ifclear
@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, and so on.

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


@ifclear PRECONFIGURED
@ifclear HPPA
@node Formatting Documentation
@c links whacked to pacify makeinfo
@c , Installing GDB, Renamed Commands, Top
@appendix Formatting Documentation

@cindex @value{GDBN} reference card
@cindex reference card
The @value{GDBN} 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 @value{GDBN} reference card is designed to print in @dfn{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 @value{GDBN} 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.

@value{GDBN} 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-@value{GDBVN}/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 @sc{gnu}
Emacs or the standalone @code{info} program, available as part of the
@sc{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
@value{GDBN} 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 either read or
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 type:

@example
make gdb.dvi
@end example

Then give @file{gdb.dvi} to your @sc{dvi} printing program.
@end ifclear

@node Installing GDB, Index, Using History Interactively, Top
@appendix Installing @value{GDBN}
@cindex configuring @value{GDBN}
@cindex installation

@ifset HPPA
If you obtain @value{GDBN} (HP WDB 0.75) as part of your HP ANSI C or
HP ANSI C++ Developer's Kit at HP-UX Release 11.0, you do not have to
take any special action to build or install @value{GDBN}.

If you obtain @value{GDBN} (HP WDB 0.75) from an HP web site, you may
download either a @code{swinstall}-able package or a source tree, or
both.

Most customers will want to install the @value{GDBN} binary that is part
of the @code{swinstall}-able package.  To do so, use a command of the
form

@smallexample
/usr/sbin/swinstall -s @var{package-name} WDB
@end smallexample

Alternatively, it is possible to build @value{GDBN} from the source
distribution.  Sophisticated customers who want to modify the debugger
sources to tailor @value{GDBN} to their their needs may wish to do this.
The source distribution consists of a @code{tar}'ed source tree rooted
at @file{gdb-4.16/...}.  The instructions that follow describe how to
build a @file{gdb} executable from this source tree.  HP believes that
these instructions apply to the WDB source tree that it distributes.
However, HP does not explicitly support building a @file{gdb} for any
non-HP platform from the WDB source tree.  It may work, but HP has not
tested it for any platforms other than those described in the WDB 0.75
Release Notes.
@end ifset

@value{GDBN} comes with a @code{configure} script that automates the process
of preparing @value{GDBN} 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 @value{GDBN} 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 @value{GDBN} distribution includes all the source code you need for 
@value{GDBN} in a single directory, whose name is usually composed by 
appending the version number to @samp{gdb}.

For example, the @value{GDBN} 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 @value{GDBN} and all its supporting libraries

@item gdb-@value{GDBVN}/gdb
the source specific to @value{GDBN} itself

@item gdb-@value{GDBVN}/bfd
source for the Binary File Descriptor library

@item gdb-@value{GDBVN}/include
@sc{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 @sc{gnu} command-line interface

@item gdb-@value{GDBVN}/glob
source for the @sc{gnu} filename pattern-matching subroutine

@item gdb-@value{GDBVN}/mmalloc
source for the @sc{gnu} memory-mapped malloc package
@end table

The simplest way to configure and build @value{GDBN} 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 @value{GDBN} 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 @value{GDBN} 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.

@need 750
@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 @value{GDBN} 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 @value{GDBN} uses the shell to start your program---some systems refuse to
let @value{GDBN} debug child processes whose programs are not readable.

@menu
* Separate Objdir::             Compiling @value{GDBN} in another directory
* Config Names::                Specifying names for hosts and targets
* Configure Options::           Summary of options for configure
@end menu

@node Separate Objdir, Config Names, Installing GDB, Installing GDB
@section Compiling @value{GDBN} in another directory

If you want to run @value{GDBN} 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 (@sc{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 is assumed.)

For example, with version @value{GDBVN}, you can build @value{GDBN} 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 @value{GDBN} itself in
@file{gdb-sun4/gdb}.

One popular reason to build several @value{GDBN} configurations in separate
directories is to configure @value{GDBN} for cross-compiling (where 
@value{GDBN} runs on one machine---the @dfn{host}---while debugging 
programs that run on another machine---the @dfn{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{dirname}/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, Configure Options, Separate Objdir, Installing GDB
@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 @value{GDBN} 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 i386-linux
i386-pc-linux-gnu
% sh config.sub alpha-linux
alpha-unknown-linux-gnu
% sh config.sub hp9k700
hppa1.1-hp-hpux
% sh config.sub sun4
sparc-sun-sunos4.1.1
% sh config.sub sun3
m68k-sun-sunos4.1.1
% sh config.sub i986v
Invalid configuration `i986v': machine `i986v' not recognized
@end smallexample

@noindent
@code{config.sub} is also distributed in the @value{GDBN} source
directory (@file{gdb-@value{GDBVN}}, for version @value{GDBVN}).

@node Configure Options,  , Config Names, Installing GDB
@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}.

@example
configure @r{[}--help@r{]}
          @r{[}--prefix=@var{dir}@r{]}
          @r{[}--exec-prefix=@var{dir}@r{]}
          @r{[}--srcdir=@var{dirname}@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 --exec-prefix=@var{dir}
Configure the source to install programs under directory
@file{@var{dir}}.

@c avoid splitting the warning from the explanation:
@need 2000
@item --srcdir=@var{dirname}
@strong{Warning: using this option requires @sc{gnu} @code{make}, or another
@code{make} that implements the @code{VPATH} feature.}@*
Use this option to make configurations in directories separate from the
@value{GDBN} 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{dirname}.  @code{configure} creates directories under
the working directory in parallel to the source directories below
@var{dirname}.

@item --norecursion
Configure only the directory level where @code{configure} is executed; do not
propagate configuration to subdirectories.

@item --target=@var{target}
Configure @value{GDBN} for cross-debugging programs running on the specified
@var{target}.  Without this option, @value{GDBN} is configured to debug
programs that run on the same machine (@var{host}) as @value{GDBN} itself.

There is no convenient way to generate a list of all available targets.

@item @var{host} @dots{}
Configure @value{GDBN} to run on the specified @var{host}.

There is no convenient way to generate a list of all available hosts.
@end table

There are many other options available as well, but they are generally
needed for special purposes only.
@end ifclear

 
@node Index,  , Installing GDB, Top
@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: doc@cygnus.com, 1991.
@end tex

@contents
@bye