* binutils.texinfo: minor typos, phrasing, formatting fixes.

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

_dnl__						-*-Texinfo-*-
_dnl__ Copyright (c) 1988 1989 1990 1991 Free Software Foundation, Inc.
\input texinfo
@setfilename _GDBP__.info
@c $Id$
@c THIS MANUAL REQUIRES TEXINFO-2 macros and info-makers to format properly.
@c
@c NOTE: this manual is marked up for preprocessing with a collection
@c of m4 macros called "pretex.m4".  If you see <_if__> and <_fi__>
@c scattered around the source, you have the full source before
@c preprocessing; if you don't, you have the source configured for
@c _HOST__ architectures (and you can of course get the full source,
@c with all configurations, from wherever you got this).
_if__(0)

THIS IS THE SOURCE PRIOR TO PREPROCESSING.  The full source needs to
be run through m4 before either tex- or info- formatting: for example,
_0__
    m4 pretex.m4 none.m4 m680x0.m4 gdb.texinfo >gdb-680x0.texinfo
_1__
will produce (assuming your path finds either GNU m4 >= 0.84, or SysV
m4; Berkeley won't do) a file suitable for formatting.  See the text in
"pretex.m4" for a fuller explanation (and the macro definitions).

_fi__(0)
@tex
\def\$#1${{#1}}  % Kluge: collect RCS revision info without $...$
\xdef\manvers{\$Revision$}  % For use in headers, footers too
@end tex
@c
@syncodeindex ky cp
@c FOR UPDATES LEADING TO THIS DRAFT, GDB CHANGELOG CONSULTED BETWEEN:
@c Wed Jul 31 12:06:29 1991  John Gilmore  (gnu at cygint.cygnus.com)
@c Sat Dec 22 02:51:40 1990  John Gilmore  (gnu at cygint)
@ifinfo
This file documents the GNU debugger _GDBN__.

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

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

@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
section entitled ``GNU General Public License'' is included exactly as
in the original, and provided that the entire resulting derived work is
distributed under the terms of a permission notice identical to this
one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that the section entitled ``GNU General Public License'' may be
included in a translation approved by the Free Software Foundation
instead of in the original English.
@end ifinfo
@smallbook
@setchapternewpage odd
_if__(_GENERIC__)
@settitle Using _GDBN__ (v4.0)
_fi__(_GENERIC__)
_if__(!_GENERIC__)
@settitle Using _GDBN__ v4.0 (_HOST__)
_fi__(!_GENERIC__)
@iftex
@finalout
@end iftex
@titlepage
@title{Using _GDBN__}
@subtitle{A Guide to the GNU Source-Level Debugger}
_if__(!_GENERIC__)
@subtitle{On _HOST__ Systems}
_fi__(!_GENERIC__)
@sp 1
@c Maybe crank this up to "Fourth Edition" when released at FSF
@c @subtitle Third Edition---_GDBN__ version 4.0
@subtitle _GDBN__ version 4.0
@subtitle July 1991
@author{Richard M. Stallman@qquad @hfill Free Software Foundation}
@author{Roland H. Pesch@qquad @hfill Cygnus Support}
@page
@tex
{\parskip=0pt
\hfill rms\@ai.mit.edu, pesch\@cygnus.com\par
\hfill {\it Using _GDBN__}, \manvers\par
\hfill \TeX{}info \texinfoversion\par
}
@end tex

@vskip 0pt plus 1filll
Copyright @copyright{} 1988, 1989, 1990, 1991 Free Software Foundation, Inc.

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

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

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

@node Top, Summary, (dir), (dir)
@ifinfo
This file describes version 4.0 of GDB, the GNU symbolic debugger.
@end ifinfo

@menu
* Summary::			Summary of _GDBN__
* New Features::		New Features in _GDBN__ version 4.0
* Sample Session::		A Sample _GDBN__ Session
* Invocation::			Getting In and Out of _GDBN__
* Commands::			
* Running::			Running Programs Under _GDBN__
* Stopping::			Stopping and Continuing
* Stack::			Examining the Stack
* Source::			Examining Source Files
* Data::			Examining Data
* Symbols::			Examining the Symbol Table
* Altering::			Altering Execution
* _GDBN__ Files::		
* Targets::			Specifying a Debugging Target 
* Controlling _GDBN__::		Controlling _GDBN__
* Sequences::			Canned Sequences of Commands
* Emacs::			Using _GDBN__ under GNU Emacs
* _GDBN__ Bugs::			Reporting Bugs in _GDBN__
* Renamed Commands::		
* Installing _GDBN__::		Installing _GDBN__
* Copying::			GNU GENERAL PUBLIC LICENSE
* Index::			Index
 --- The Detailed Node Listing ---

Summary of _GDBN__

* Free Software::		Free Software
* Contributors::		Contributors to _GDBN__

Getting In and Out of _GDBN__

* Starting _GDBN__::		Starting _GDBN__
* Leaving _GDBN__::			Leaving _GDBN__
* Shell Commands::		Shell Commands

Starting _GDBN__

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

_GDBN__ Commands

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

Running Programs Under _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

Stopping and Continuing

* Breakpoints::			Breakpoints, Watchpoints, and Exceptions
* Continuing and Stepping::	Resuming Execution
* Signals::			Signals

Breakpoints, Watchpoints, and Exceptions

* Set Breaks::			Setting Breakpoints
* Set Watchpoints::		Setting Watchpoints
* Exception Handling::		Breakpoints and Exceptions
* Delete Breaks::		Deleting Breakpoints
* Disabling::			Disabling Breakpoints
* Conditions::			Break Conditions
* Break Commands::		Breakpoint Command Lists
* Breakpoint Menus::		Breakpoint Menus
* Error in Breakpoints::	

Examining the Stack

* Frames::			Stack Frames
* Backtrace::			Backtraces
* Selection::			Selecting a Frame
* Frame Info::			Information on a Frame

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

Altering Execution

* Assignment::			Assignment to Variables
* Jumping::			Continuing at a Different Address
* Signaling::			Giving the Program a Signal
* Returning::			Returning from a Function
* Calling::			Calling your Program's Functions

_GDBN__'s 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
* Remote::			Remote Debugging

Remote Debugging

* i960-Nindy Remote::		
* EB29K Remote::		
* VxWorks Remote::		

_GDBN__ with a Remote i960 (Nindy)

* Nindy Startup::		Startup with Nindy
* Nindy Options::		Options for Nindy
* Nindy reset::			Nindy Reset Command

_GDBN__ with a Remote EB29K

* Comms (EB29K)::		Communications Setup
* gdb-EB29K::			EB29K cross-debugging
* Remote Log::			Remote Log

_GDBN__ and VxWorks

* VxWorks connection::		Connecting to VxWorks
* VxWorks download::		VxWorks Download
* VxWorks attach::		Running Tasks

Controlling _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
* Command Files::		Command Files
* Output::			Commands for Controlled Output

Reporting Bugs in _GDBN__

* Bug Criteria::		Have You Found a Bug?
* Bug Reporting::		How to Report Bugs
@end menu

@node Summary, New Features, Top, Top
@unnumbered Summary of _GDBN__

The purpose of a debugger such as _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.  

_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

_GDBN__ can be used to debug programs written in C and C++.  Pascal support
is being implemented, and Fortran support will be added when a GNU
Fortran compiler is ready.

@menu
* Free Software::		Free Software
* Contributors::		Contributors to GDB
@end menu

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

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

@c FIXME: (passim) go through all xrefs, expanding to use text headings
For full details, @pxref{Copying}.
@node Contributors,  , Free Software, Summary
@unnumberedsec Contributors to GDB

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

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

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

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

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

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

GDB 4.0 uses the BFD subroutine library to examine multiple
object-file formats; BFD was a joint project of V. Gumby
Henkel-Wallace, Rich Pixley, Steve Chamberlain, and John Gilmore.

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

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

Rich Schaefer helped with support of SunOS shared libraries.

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

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

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

@node New Features, Sample Session, Summary, Top
@unnumbered New Features since _GDBN__ version 3.5

@table @emph
@item Targets
Using the new command @code{target}, you can select at runtime whether
you are debugging local files, local processes, standalone systems over
a serial port, realtime systems over a TCP/IP connection, etc.
Internally, _GDBN__ now uses a function vector to mediate access to
different targets; if you need to add your own support for a remote
protocol, this makes it much easier.

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

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

@item Configuration
Compile-time configuration (to select a particular architecture and
operating system) is much easier.  The script @code{configure} now
allows you to configure _GDBN__ as either a native debugger or a
cross-debugger. 

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


@item Source Language
_GDBN__ now has limited support for C++ exception handling: _GDBN__ can
break when an exception is raised, before the stack is peeled back to
the exception handler's context.  

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

@item Ports
_GDBN__ has been ported to the following new architectures: AT&T 3b1,
Acorn RISC machine, HP300 running HPUX, big- and little-endian MIPS
machines, Motorola 88k, Sun 386i, and Sun 3 running SunOS 4.  In
addition, the following are supported as targets only: AMD 29k, Intel
960, and Wind River's VxWorks.

@item Shared Libraries
_GDBN__ 4.0 supports SunOS shared libraries.

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

@end table

@node Sample Session, Invocation, New Features, Top
@chapter A Sample _GDBN__ Session

You can use this manual at your leisure to read all about _GDBN__.
However, a handful of commands are enough to get started using the
debugger.  This chapter illustrates these commands.

@iftex
In this sample session, we emphasize user input like this: @i{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.
_0__
One of the preliminary versions of GNU @code{m4} (a generic macro
processor) exhibits the following bug: sometimes, when we change its
quote strings from the default, the commands used to capture one macro's
definition in 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} builtin @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
$ @i{cd gnu/m4}
$ @i{./m4}
@i{define(foo,0000)}

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

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

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

@noindent
Let's use _GDBN__ to try to see what's going on.

@smallexample
$ @i{_GDBP__ m4}
Reading symbol data from m4...done.
(_GDBP__) 
@end smallexample

@noindent
_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
then tell _GDBN__ to use a narrower display width than usual, so
that examples will fit in this manual.

@smallexample
(_GDBP__) @i{set width 70}
@end smallexample

@noindent 
Let's see how the @code{m4} builtin @code{changequote} works.
Having looked at the source, we know the relevant subroutine is
@code{m4_changequote}, so we set a breakpoint there with _GDBN__'s
@code{break} command.

@smallexample
(_GDBP__) @i{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 _GDBN__
control; as long as control does not reach the @code{m4_changequote}
subroutine, the program runs as usual:

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

@i{foo}
0000
@end smallexample

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

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

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

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

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

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

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

@smallexample
(_GDBP__) @i{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's step past the two lines that set @code{len_lquote} and
@code{len_rquote}, and then examine the values of those variables.

@smallexample
(_GDBP__) @i{n}
539         len_rquote = strlen(lquote);
(_GDBP__) @i{n}
540     @}
(_GDBP__) @i{p len_lquote}
$3 = 9
(_GDBP__) @i{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.  Let's try setting them to better values.
We can use the @code{p} command for this, since it'll print the value of
any expression---and that expression can include subroutine calls and
assignments. 

@smallexample
(_GDBP__) p len_lquote=strlen(lquote)
$5 = 7
(_GDBP__) p len_rquote=strlen(rquote)
$6 = 9
@end smallexample

@noindent
Let's see if that fixes 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
(_GDBP__) @i{c}
Continuing.

@i{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'll let @code{m4} exit by giving it an EOF as input.

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

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

@smallexample
(_GDBP__) @i{quit}
_1__@end smallexample

@node Invocation, Commands, Sample Session, Top
@chapter Getting In and Out of _GDBN__

@menu
* Starting _GDBN__::		Starting _GDBN__
* Leaving _GDBN__::			Leaving _GDBN__
* Shell Commands::		Shell Commands
@end menu

@node Starting _GDBN__, Leaving _GDBN__, Invocation, Invocation
@section Starting _GDBN__

_GDBN__ is invoked with the shell command @code{_GDBP__}.  Once started,
it reads commands from the terminal until you tell it to exit.

You can run @code{_GDBP__} with no arguments or options; but the most
usual way to start _GDBN__ is with one argument or two, specifying an
executable program as the argument:
@example
_GDBP__ program
@end example
@noindent
You can also start with both an executable program and a core file specified:
@example
_GDBP__ program core
@end example

@noindent
You can further control how _GDBN__ starts up by using command-line
options.  _GDBN__ itself can remind you of the options available:
@example
_GDBP__ -help
@end example
@noindent
will display all available options and briefly describe their use
(@samp{_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
_if__(!_GENERIC__)
_include__(gdbinv-m.m4)_dnl__
_fi__(!_GENERIC__)
@end menu

@node File Options, Mode Options, Starting _GDBN__, Starting _GDBN__
@subsection Choosing Files

As shown above, any arguments other than options specify an executable
file and core file; that is, the first argument encountered with no
associated option flag is equivalent to a @samp{-se} option, and the
second, if any, is equivalent to a @samp{-c} option.  Many options have
both long and short forms; both are shown here.  The long forms are also
recognized 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 -command=@var{file}
@itemx -x @var{file}
Execute _GDBN__ commands from file @var{file}.  @xref{Command Files}.

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

_if__(!_GENERIC__)
@node Mode Options, i960-Nindy Remote, File Options, Starting _GDBN__
_fi__(!_GENERIC__)
_if__(_GENERIC__)
@node Mode Options,  , File Options, Starting _GDBN__
_fi__(_GENERIC__)
@subsection Choosing Modes

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

@item -quiet
@itemx -q
``Quiet''.  Do not print the introductory and copyright messages.  These
messages are also suppressed in batch mode, or if an executable file name is
specified on the _GDBN__ command line.

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

Batch mode may be useful for running _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 _GDBN__ control
terminates) is not issued when running in batch mode.

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

@item -fullname
@itemx -f
Emacs sets this option when it runs _GDBN__ as a subprocess.  It tells _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 the 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-_GDBN__ interface program uses the two @samp{\032} characters as
a signal to display the source code for the frame.

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

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

_if__(!_GENERIC__)
_include__(gdbinv-s.m4)
_fi__(!_GENERIC__)

@node Leaving _GDBN__, Shell Commands, Starting _GDBN__, Invocation
@section Leaving _GDBN__
@cindex exiting _GDBN__
@table @code
@item quit
@kindex quit
@kindex q
To exit _GDBN__, use the @code{quit} command (abbreviated @code{q}), or type
an end-of-file character (usually @kbd{C-d}).  
@end table

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

If you've been using _GDBN__ to control an attached process or device,
you can release it with the @code{detach} command; @pxref{Attach}.

@node Shell Commands,  , Leaving _GDBN__, Invocation
@section Shell Commands
If you need to execute occasional shell commands during your
debugging session, there's no need to leave or suspend _GDBN__; you can
just use the @code{shell} command.

@table @code
@item shell @var{command string}
@kindex shell
@cindex shell escape
Directs _GDBN__ to invoke an inferior shell to execute @var{command
string}.  If it exists, the environment variable @code{SHELL} is used
for the name of the shell to run.  Otherwise _GDBN__ uses
@code{/bin/sh}.
@end table

The utility @code{make} is often needed in development environments.
You don't have to use the @code{shell} command for this purpose in _GDBN__:

@table @code
@item make @var{make-args}
@kindex make
@cindex calling make
Causes _GDBN__ to execute an inferior @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 _GDBN__ Commands

@menu
* Command Syntax::		Command Syntax
* Help::			Getting Help
@end menu

@node Command Syntax, Help, Commands, Commands
@section Command Syntax
A _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
_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 _GDBN__ (typing just @key{RET}) means to
repeat the previous command. Certain commands (for example, @code{run})
will not repeat this way; these are commands for which unintentional
repetition might cause trouble and which you are unlikely to want to
repeat.

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

@kindex #
@cindex comment
A line of input starting with @kbd{#} is a comment; it does nothing.
This is useful mainly in command files (@xref{Command Files}).

@node Help,  , Command Syntax, Commands
@section Getting Help
@cindex online documentation
@kindex help
You can always ask _GDBN__ itself for information on its commands, using the
command @code{help}.  

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

List of commands:

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

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

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

In addition to @code{help}, you can use the _GDBN__ commands @code{info}
and @code{show} to inquire about the state of your program, or the state
of _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.
@c FIXME: @pxref{Index} used to be here, but even though it shows up in
@c FIXME...the 'aux' file with a pageno the xref can't find it.  

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

@kindex show
@item show
In contrast, @code{show} is for describing the state of _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 _GDBN__ is running.  You should include this
information in _GDBN__ bug-reports.  If multiple versions of _GDBN__ are
in use at your site, you may occasionally want to make sure what version
of _GDBN__ you're running; as _GDBN__ evolves, new commands are
introduced, and old ones may wither away.  The version number is also
announced when you start _GDBN__ with no arguments.

@kindex show copying
@item show copying
Display information about permission for copying _GDBN__.

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

@node Running, Stopping, Commands, Top
@chapter Running Programs Under _GDBN__

@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
@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.

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

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

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

@ignore
@comment As far as I know, there are no cases in which _GDBN__ will
@comment produce strange output in this case.  (but no promises).
If your program includes archives made with the @code{ar} program, and
if the object files used as input to @code{ar} were compiled without the
@samp{-g} option and have names longer than 15 characters, _GDBN__ will get
confused reading the program's symbol table.  No error message will be
given, but _GDBN__ may behave strangely.  The reason for this problem is a
deficiency in the Unix archive file format, which cannot represent file
names longer than 15 characters.

To avoid this problem, compile the archive members with the @samp{-g}
option or use shorter file names.  Alternatively, use a version of GNU
@code{ar} dated more recently than August 1989.
@end ignore


@node Starting, Arguments, Compilation, Running
@section Starting your Program
@cindex starting
@cindex running
@table @code
@item run
@itemx r
@kindex run
Use the @code{run} command to start your program under _GDBN__.  You
must first specify the program name
_if__(_VXWORKS__) 
(except on VxWorks)
_fi__(_VXWORKS__)
with an argument to _GDBN__
(@pxref{Invocation}), or using the @code{file} or @code{exec-file}
command (@pxref{Files}).  
@refill
@end table

On targets that support processes, @code{run} creates an inferior
process and makes that process run your program.  On other targets,
@code{run} jumps to the start of the program.

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

@table @asis
@item The @i{arguments.}
You 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}.@refill

@item The @i{environment.}
Your program normally inherits its environment from _GDBN__, but you can
use the _GDBN__ commands @code{set environment} and @code{unset
environment} to change parts of the environment that will be given to
the program.  @xref{Environment}.@refill

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

@item The @i{standard input and output.}
Your program normally uses the same device for standard input and
standard output as _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}.

@cindex pipes
@emph{Warning:} While input and output redirection work, you can't use
pipes to pass the output of the program you're debugging to another
program; if you attempt this, _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}, for discussion of how to arrange for your
program to stop.  Once your program has been started by the @code{run}
command (and then stopped), you may evaluate expressions that involve
calls to functions in the inferior, using the @code{print} or
@code{call} commands.  @xref{Data}.

If the modification time of your symbol file has changed since the last
time _GDBN__ read its symbols, _GDBN__ will discard its symbol table and re-read
it.  In this process, it 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 the program.
_GDBN__ uses the shell indicated by your environment variable
@code{SHELL} if it exists; otherwise, _GDBN__ uses @code{/bin/sh}.

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

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

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

@node Environment, 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 the program with a modified
environment without having to start _GDBN__ over again.

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

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

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

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

@item set environment @var{varname} @r{[}=@r{]} @var{value}
@kindex set environment
Sets environment variable @var{varname} to @var{value}.  The value
changes for your program only, not for _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 doesn't include leading, trailing
@c blanks. Gnu asks: does anyone care?

For example, this command:

@example
set env USER = foo
@end example

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

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

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

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

@table @code
@item cd @var{directory}
@kindex cd
Set _GDBN__'s working directory to @var{directory}.

@item pwd
@kindex pwd
Print _GDBN__'s 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 _GDBN__ does input and output to
the same terminal that _GDBN__ uses.  _GDBN__ switches the terminal to
its own terminal modes to interact with you, but it records the terminal
modes your program was using and switches back to them when you continue
running your program.

@table @code
@item info terminal
@kindex info terminal
Displays _GDBN__'s recorded information about the terminal modes your
program is using.
@end table

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

_0__@example
run > outfile
_1__@end example

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

@kindex tty
@cindex controlling terminal
Another way to specify where the 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 _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 _GDBN__.
(@code{info files} will show your active targets.)  The command takes as
argument a process ID.  The usual way to find out the process-id of
a Unix process is with the @code{ps} utility, or with the @samp{jobs -l}
shell command.   

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

To use @code{attach}, you must be debugging in an environment which
supports processes.  You must also have permission to send the process a
signal, and it must have the same effective user ID as the _GDBN__
process.

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

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

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

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

@node Kill Process,  , Attach, Running
@c @group
@section Killing the Child Process

@table @code
@item kill
@kindex kill
Kill the child process in which your program is running under _GDBN__.
@end table

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

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

The @code{kill} command is also useful if you wish to recompile and
relink the 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}, _GDBN__ will notice that the file has changed, and
will re-read the symbol table (while trying to preserve your current
breakpoint settings).

@node Stopping, Stack, Running, Top
@chapter Stopping and Continuing

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

Inside _GDBN__, your program may stop for any of several reasons, such
as a signal, a breakpoint, or reaching a new line after a _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 _GDBN__ provide ample
explanation of the status of your program---but you can also explicitly
request this information at any time.

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

@menu
* Breakpoints::			Breakpoints, Watchpoints, and Exceptions
* Continuing and Stepping::	Resuming Execution
* Signals::			Signals
@end menu

@node Breakpoints, Continuing and Stepping, Stopping, Stopping
@section Breakpoints, Watchpoints, and Exceptions

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

@cindex watchpoints
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}), 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.

Each breakpoint or watchpoint is assigned a number when it is created;
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 the program until you enable it again.

@menu
* Set Breaks::			Setting Breakpoints
* Set Watchpoints::		Setting Watchpoints
* Exception Handling::		Breakpoints and Exceptions
* Delete Breaks::		Deleting Breakpoints
* Disabling::			Disabling Breakpoints
* Conditions::			Break Conditions
* Break Commands::		Breakpoint Command Lists
* Breakpoint Menus::		Breakpoint Menus
* Error in Breakpoints::	
@end menu

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

@kindex break
@kindex b
Breakpoints are set with the @code{break} command (abbreviated @code{b}).

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}, 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 will stop the 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 the 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}).  In any selected frame but the innermost, this will
cause the program to stop as soon as control returns to that frame.
This is similar to the effect of a @code{finish} command in the frame
inside the selected frame---except that @code{finish} doesn't leave an
active breakpoint.  If you use @code{break} without an argument in the
innermost frame, _GDBN__ will stop the next time it reaches the current
location; this may be useful inside loops.

_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 the 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}, for more
information on breakpoint conditions.

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

@item rbreak @var{regex}
@kindex rbreak
@cindex regular expression
@c FIXME: 2nd sentence below C++ only?
Set breakpoints on all functions matching the regular expression
@var{regex}.  This is useful for setting breakpoints on overloaded
functions that are not members of any special classes.  This command
sets an unconditional breakpoint on all matches, printing a list of all
breakpoints it set. Once these breakpoints are set, they are treated
just like the breakpoints set with the @code{break} command.  They can
be deleted, disabled, made conditional, etc., in the standard ways.

@kindex info breakpoints
@kindex $_
@item info breakpoints @r{[}@var{n}@r{]}
@item info break @r{[}@var{n}@r{]}
Print a list of all breakpoints (but not watchpoints) set and not
deleted, showing their numbers, where in the program they are, and any
special features in use for them. Disabled breakpoints are included in
the list, but marked as disabled. @code{info break} with a breakpoint
number @var{n} as argument lists only that breakpoint.  The convenience
variable @code{$_} and the default examining-address for the @code{x}
command are set to the address of the last breakpoint listed
(@pxref{Memory}).  The equivalent command for watchpoints is @code{info
watch}. @end table

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

@node Set Watchpoints, Exception Handling, Set Breaks, Breakpoints
@subsection Setting Watchpoints
@cindex setting watchpoints
You can use a watchpoint to stop execution whenever the value of an
expression changes, without having to predict a particular place 
where this may happen.

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

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

@kindex info watchpoints
@item info watchpoints
This command prints a list of watchpoints; it is otherwise similar to
@code{info break}.
@end table

@node Exception Handling, Delete Breaks, Set Watchpoints, Breakpoints
@subsection Breakpoints and Exceptions
@cindex exception handlers

Some languages, such as GNU C++, implement exception handling.  _GDBN__
can be used to examine what caused the program to raise an exception
and to list the exceptions the program is prepared to handle at a
given point in time.

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

You can use @code{info catch} to list active exception handlers;
@pxref{Frame Info}.

There are currently some limitations to exception handling in _GDBN__.
These will be corrected in a future release.

@itemize @bullet
@item
If you call a function interactively, _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 the user and cause the program to simply continue
running until it hits a breakpoint, catches a signal that _GDBN__ is
listening for, or exits.
@item
You cannot raise an exception interactively.
@item
You cannot interactively install an exception handler.
@end itemize

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

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

@example
    /* 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}).  

With a conditional breakpoint (@xref{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 the
program when any of a number of exceptions are raised.

@node Delete Breaks, Disabling, Exception Handling, Breakpoints
@subsection Deleting Breakpoints

@cindex clearing breakpoints, watchpoints
@cindex deleting breakpoints, watchpoints
It is often necessary to eliminate a breakpoint or watchpoint once it
has done its job and you no longer want the 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 the program.  With the @code{delete} command you can
delete individual breakpoints or watchpoints by specifying their
breakpoint numbers.

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

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

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

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

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

@node Disabling, Conditions, Delete Breaks, Breakpoints
@subsection Disabling Breakpoints

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

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

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

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

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

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

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

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

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

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

@node Conditions, Break Commands, Disabling, Breakpoints
@subsection Break Conditions
@cindex conditional breakpoints
@cindex breakpoint conditions

The simplest sort of breakpoint breaks every time the 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.  (@xref{Expressions}).  A breakpoint with a
condition evaluates the expression each time the program reaches it, and
the 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 ca 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, _GDBN__ might see the other breakpoint first and stop the program
without checking the condition of this one.)  Note that breakpoint
commands are usually more convenient and flexible for the purpose of
performing side effects when a breakpoint is reached (@pxref{Break
Commands}).

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}.
They can also be changed at any time with the @code{condition} command.
The @code{watch} command doesn't recognize the @code{if} keyword;
@code{condition} is the only way to impose a further condition on a
watchpoint. 

@table @code 
@item condition @var{bnum} @var{expression} 
@kindex condition 
Specify @var{expression} as the break condition for breakpoint or
watchpoint number @var{bnum}.  From now on, this breakpoint will stop
the program only if the value of @var{expression} is true (nonzero, in
C).  When you use @code{condition}, _GDBN__ checks @var{expression}
immediately for syntactic correctness, and to determine whether symbols
in it have referents in the context of your breakpoint.  _GDBN__ does
not actually evaluate @var{expression} at the time the @code{condition}
command is given, however.  @xref{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 the program reaches a breakpoint whose
ignore count is positive, then instead of stopping, it just decrements
the ignore count by one and continues.  As a result, if the ignore count
value is @var{n}, the breakpoint will not stop the next @var{n} times it
is reached.

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

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

@item continue @var{count}
@itemx c @var{count}
@itemx fg @var{count}
@kindex continue @var{count}
Continue execution of the program, setting the ignore count of the
breakpoint that the program stopped at to @var{count} minus one.
Thus, the program will not stop at this breakpoint until the
@var{count}'th time it is reached.

An argument to this command is meaningful only when the program stopped
due to a breakpoint.  At other times, the argument to @code{continue} is
ignored.

The synonym @code{fg} is provided purely for convenience, and has
exactly the same behavior as other forms of the command.
@end table

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

You could achieve the effect of the ignore count with a
condition such as _0__@w{@samp{$foo-- <= 0}}_1__ using a debugger convenience
variable that is decremented each time.  @xref{Convenience Vars}.

@node Break Commands, Breakpoint Menus, Conditions, Breakpoints
@subsection Breakpoint Command Lists

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

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

To remove all commands from a breakpoint, type @code{commands} followed
immediately by @code{end}; that is, give no commands.

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

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

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

@kindex silent
If the first command specified 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 the remaining commands too print nothing, you will see no sign that
the breakpoint was reached at all.  @code{silent} is meaningful only 
at the beginning of a breakpoint command list.

The commands @code{echo} and @code{output} that allow you to print precisely
controlled output are often useful in silent breakpoints.  @xref{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.

_0__@example
break foo if x>0
commands
silent
echo x is\040
output x
echo \n
cont
end
_1__@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 the 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

@cindex lost output
One deficiency in the operation of automatically continuing breakpoints
under Unix appears when your program uses raw mode for the terminal.
_GDBN__ switches back to its own terminal modes (not raw) before executing
commands, and then must switch back to raw mode when your program is
continued.  This causes any pending terminal input to be lost. 
@c FIXME: revisit below when GNU sys avail.
@c In the GNU system, this will be fixed by changing the behavior of
@c terminal modes.

Under Unix, you can get around this problem by writing actions into
the breakpoint condition rather than in commands.  For example

@example
condition 5  (x = y + 4), 0
@end example

@noindent
specifies a condition expression (@xref{Expressions}) that will change
@code{x} as needed, then always have the value zero so the program will
not stop.  No input is lost here, because _GDBN__ evaluates break
conditions  without changing the terminal modes.  When you want to have
nontrivial conditions for performing the side effects, the operators
@samp{&&}, @samp{||} and @samp{?@dots{}:} may be useful.

@node Breakpoint Menus, Error in Breakpoints, Break Commands, Breakpoints
@subsection Breakpoint Menus
@cindex C++ 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 _GDBN__ where you
want a breakpoint.  _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:

@example
(_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.
(_GDBP__) 
@end example


@node Error in Breakpoints,  , Breakpoint Menus, Breakpoints
@subsection ``Cannot Insert Breakpoints''

@c FIXME: "cannot insert breakpoints" error, v unclear.  
@c        Q in pending mail to Gilmore. [email protected], 26mar91
Under some operating systems, breakpoints cannot be used in a program if
any other process is running that program.  In this situation,
attempting to run or continue a program with a breakpoint causes _GDBN__
to stop the other process.

When this happens, you have three ways to proceed:

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

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

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

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

@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 resuming program
execution for a very limited time: one line of source code, or one
machine instruction.  Either when continuing or when stepping, the
program may stop even sooner, due to a breakpoint or to a signal.  (If
due to a signal, you may want to use @code{handle}, or use @samp{signal
0} to resume execution; @pxref{Signals}.)

@table @code
@item continue @r{[}@var{ignore-count}@r{]}
@kindex continue
Resume program execution, at the address where the 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}).

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

@end table

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

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

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

@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.

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

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

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

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

Contrast this with the @code{return} command (@pxref{Returning}).

@item until
@kindex until
@item u
@kindex u
Continue running until a source line past the current line, in the
current stack frame, is reached.  This command is used to avoid single
stepping through a loop more than once.  It is like the @code{next}
command, except that when @code{until} encounters a jump, it
automatically continues execution until the program counter is greater
than the address of the jump.

This means that when you reach the end of a loop after single stepping
though it, @code{until} will cause the program to continue execution
until the loop is exited.  In contrast, a @code{next} command at the end
of a loop will simply step back to the beginning of the loop, which
would force you to step through the next iteration.

@code{until} always stops the 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
(_GDBP__) f
#0  main (argc=4, argv=0xf7fffae8) at m4.c:206
206		    expand_input();
(_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}
@item u @var{location}
Continue running the 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}).  This form of the command uses breakpoints, and hence is
quicker than @code{until} without an argument.

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

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

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

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

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


@node Signals,  , 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 the program has
requested an alarm).

@cindex fatal signals
Some signals, including @code{SIGALRM}, are a normal part of the
functioning of the program.  Others, such as @code{SIGSEGV}, indicate
errors; these signals are @dfn{fatal} (kill the 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 the program, but it is normally
fatal so it can carry out the purpose of the interrupt: to kill the program.

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

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

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

@item handle @var{signal} @var{keywords}@dots{}
@kindex handle
Change the way _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
_GDBN__ should not stop the program when this signal happens.  It may
still print a message telling you that the signal has come in.

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

@item print
_GDBN__ should print a message when this signal happens.

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

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

@item nopass
_GDBN__ should not allow the program to see this signal.
@end table
@c @end group

When a signal has been set to stop the program, the program cannot see the
signal until you continue.  It will see the signal then, if @code{pass} is
in effect for the signal in question @i{at that time}.  In other words,
after _GDBN__ reports a signal, you can use the @code{handle} command with
@code{pass} or @code{nopass} to control whether that signal will be seen by
the program when you later continue it.

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

@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, the information about
where in the program the call was made from is saved in a block of data
called a @dfn{stack frame}.  The frame also contains the arguments of the
call and the local variables of the function that was called.  All the
stack frames are allocated in a region of memory called the @dfn{call
stack}.

When your program stops, the _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 _GDBN__ and many _GDBN__ commands
refer implicitly to the selected frame.  In particular, whenever you ask
_GDBN__ for the value of a variable in the program, the value is found in the
selected frame.  There are special _GDBN__ commands to select whichever frame
you are interested in.

When the program stops, _GDBN__ automatically selects the currently executing
frame and describes it briefly as the @code{frame} command does
(@pxref{Frame Info, Info}).

@menu
* Frames::			Stack Frames
* Backtrace::			Backtraces
* Selection::			Selecting a Frame
* Frame Info::			Information on a Frame
@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 of those bytes whose
address serves as the address of the frame.  Usually this address is kept
in a register called the @dfn{frame pointer register} while execution is
going on in that frame.

@cindex frame number
_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 _GDBN__ to give you a way of designating stack
frames in _GDBN__ commands.

@cindex frameless execution
Some compilers allow functions to be compiled so that they operate
without stack frames.  (For example, the @code{_GCC__} option
@samp{-fomit-frame-pointer} will generate functions without a frame.)
This is occasionally done with heavily used library functions to save
the frame setup time.  _GDBN__ has limited facilities for dealing with
these function invocations.  If the innermost function invocation has no
stack frame, _GDBN__ will nevertheless regard it as though it had a
separate frame, which is numbered zero as usual, allowing correct
tracing of the function call chain.  However, _GDBN__ has no provision
for frameless functions elsewhere in the stack.

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

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

@table @code
@item backtrace
@itemx bt
@kindex backtrace
@kindex bt
Print a backtrace of the entire stack: one line per frame for all
frames in the stack.

You can stop the backtrace at any time by typing the system interrupt
character, normally @kbd{C-c}.

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

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

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

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

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

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

@noindent
The display for frame zero doesn't begin with a program counter
value, indicating that the 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 the program work on
whichever stack frame is selected at the moment.  Here are the commands for
selecting a stack frame; all of them finish by printing a brief description
of the stack frame just selected.

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

@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 _GDBN__ to assign numbers properly to all frames.  In
addition, this can be useful when the program has multiple stacks and
switches between them.

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

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

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

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

@smallexample
(_GDBP__) up
#1  0x22f0 in main (argc=1, argv=0xf7fffbf4, env=0xf7fffbfc) at env.c:10
10              read_input_file (argv[i]);
@end smallexample

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

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

@end table

@node Frame Info,  , 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 (@pxref{Selection}).

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

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

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

@item info locals
@kindex info locals
Print the local variables of the selected frame, each on a separate
line.  These are all variables declared static or automatic within all
program blocks that execution in this frame is currently inside of.

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

@node Source, Data, Stack, Top
@chapter Examining Source Files

_GDBN__ can print parts of your program's source, since the debugging
information recorded in your program tells _GDBN__ what source files
were used to built it.  When your program stops, _GDBN__ spontaneously
prints the line where it stopped.  Likewise, when you select a stack
frame (@pxref{Selection}), _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 _GDBN__ through its GNU Emacs interface, you may prefer to
use Emacs facilities to view source; @pxref{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}).  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 ten lines centered around line number @var{linenum} in the
current source file.

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

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

@item list -
Print ten lines just before the lines last printed.
@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 ten 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 ten lines ending with @var{last}.

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

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

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

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

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

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

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

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

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

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

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

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

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

@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.  _GDBN__ has a list of directories to search for source files;
this is called the @dfn{source path}.  Each time _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 _GDBN__ can't find a source file in the source path, and the object
program records a directory, _GDBN__ tries that directory too.  If the
source path is empty, and there is no record of the compilation
directory, _GDBN__ will, as a last resort, look in the current
directory.

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

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

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

You can use the string @samp{$cdir} to refer to the compilation
directory (if one is recorded), and @samp{$cwd} to refer to the current
working directory.  @samp{$cwd} is not the same as @samp{.}---the former
tracks the current working directory as it changes during your _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 don't 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, _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 viceversa), and the command @code{disassemble} to display
a range of addresses as machine instructions.

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

For example, we can use @code{info line} to inquire on where the object
code for the first line of function @code{m4_changequote} lies:
@smallexample
(_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
(_GDBP__) info line *0x63ff
Line 926 of "builtin.c" starts at pc 0x63e4 and ends at 0x6404.
@end smallexample

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

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

We can use @code{disassemble} to inspect the object code
range shown in the last @code{info line} example:

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

@end smallexample

@node Data, Symbols, Source, Top
@chapter Examining Data

@cindex printing data
@cindex examining data
@kindex print
@kindex inspect
@c "inspect" isn't quite a synonym if you're using Epoch, which we don't
@c document because it's 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 any valid expression of the language
the program is written in (for now, C or C++).  You type

@example
print @var{exp}
@end example

@noindent
where @var{exp} is any valid expression (in the source language), and
the value of @var{exp} is printed in a format appropriate to its data
type.

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}.

@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 _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 legal in an expression in
_GDBN__.  This includes conditional expressions, function calls, casts
and string constants.  It unfortunately does not include symbols defined
by preprocessor @code{#define} commands, or C++ expressions involving
@samp{::}, the name resolution operator.
@c FIXME: actually C++ a::b works except in obscure circumstances where it
@c FIXME...can conflict with GDB's own name scope resolution.

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

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

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

@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}.@refill
@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}); they must either be global (or static) or be visible
according to the scope rules of the programming language from the point of
execution in that frame.  This means that in the function

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

@noindent
the variable @code{a} is usable whenever the program is executing
within the function @code{foo}, but the variable @code{b} is visible
only while the program is executing inside the block in which @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 variable in a particular file, using the colon-colon notation:

@cindex colon-colon
@kindex ::
@example
@var{file}::@var{variable}
@end example

@noindent
Here @var{file} is the name of the source file whose variable you want.

@cindex C++ name resolution
This use of @samp{::} is very rarely in conflict with the very similar
use of the same notation in C++.  _GDBN__ also supports use of the C++
name resolution operator in _GDBN__ expressions.

@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 the
function, and just before exit.  You may see this problem when you're
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're stepping by machine instructions,
variables may appear to have the wrong values until the stack frame is
completely built.  On function 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.
@end quotation

@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.

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

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

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

@example
p *array@@len
@end example

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

Sometimes the artificial array mechanism isn't quite enough; in
moderately complex data structures, the elements of interest may not
actually be adjacent---for example, if you're 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}) 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're interested in the
values of a field @code{fv} in each structure.  Here's 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, _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''.

@item a
Print as an address, both absolute in hex and as an offset from the
nearest preceding symbol.  This format can be used to discover where (in
what function) an unknown address is located:
@example
(_GDBP__) p/a 0x54320
_0__$3 = 0x54320 <_initialize_vx+396>_1__
@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 _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

@cindex examining memory
@table @code
@kindex x
@item x/@var{nfu} @var{expr}
The command @code{x} (for `examine') can be used to examine memory
without being constrained by your program's data types.  You can specify
the unit size @var{u} of memory to inspect, and a repeat count @var{n} of how
many of those units to display.  @code{x} understands the formats
@var{f} used by @code{print}; two additional formats, @samp{s} (string)
and @samp{i} (machine instruction) can be used without specifying a unit
size.
@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 don't have to remember whether
unit size or format comes first; either order will work.  The output
specifications @samp{4xw} and @samp{4wx} mean exactly the same thing.

After the format specification, you supply an expression for the address
where _GDBN__ is to begin reading from 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} for more
information on expressions.

These are the memory units @var{u} you can specify with the @code{x}
command:

@table @code
@item b
Examine individual bytes.

@item h
Examine halfwords (two bytes each).

@item w
Examine words (four bytes each).

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

@item g
Examine giant words (8 bytes).
@end table

You can combine these unit specifications with any of the formats
described for @code{print}.  @xref{Output formats}.

@code{x} has two additional output specifications which derive the unit
size from the data inspected:

@table @code
@item s
Print a null-terminated string of characters.  Any explicitly specified
unit size is ignored; instead, the unit is however many bytes it takes
to reach a null character (including the null character).

@item i
Print a machine instruction in assembler syntax (or nearly).  Any
specified unit size is ignored; the number of bytes in an instruction
varies depending on the type of machine, the opcode and the addressing
modes used.  The command @code{disassemble} gives an alternative way of
inspecting machine instructions.  @xref{Machine Code}.
@end table

If you omit either the format @var{f} or the unit size @var{u}, @code{x}
will use the same one that was used last.  If you don't use any letters
or digits after the slash, you can omit the slash as well.

You can also omit the address to examine.  Then the address used is just
after the last unit examined.  This is why string and instruction
formats actually compute a unit-size based on the data: so that the next
string or instruction examined will start in the right place.  

When the @code{print} command shows a value that resides in memory,
@code{print} also sets the default address for the @code{x} command.
@code{info line} also sets the default for @code{x}, to the address of
the start of the machine code for the specified line (@pxref{Machine
Code}), and @code{info breakpoints} sets it to the address of the last
breakpoint listed (@pxref{Set Breaks}).

When you use @key{RET} to repeat an @code{x} command, the address
specified previously (if any) is ignored, so that the repeated command
examines the successive locations in memory rather than the same ones.

You can examine several consecutive units of memory with one command by
writing a repeat-count after the slash (before the format letters, if
any).  Omitting the repeat count @var{n} displays one unit of the
appropriate size.  The repeat count must be a decimal integer.  It has
the same effect as repeating the @code{x} command @var{n} times except
that the output may be more compact, with several units per line.  For
example,

@example
x/10i $pc
@end example

@noindent
prints ten instructions starting with the one to be executed next in the
selected frame.  After doing this, you could print a further seven
instructions with

@example
x/7
@end example

@noindent
---where the format and address are allowed to default.

@kindex $_
@kindex $__
The addresses and contents printed by the @code{x} command are not put
in the value history because there is often too much of them and they
would get in the way.  Instead, _GDBN__ makes these values available for
subsequent use in expressions as values of the convenience variables
@code{$_} 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 _GDBN__ will print its value each time the program stops.
Each expression added to the list is given a number to identify it;
to remove an expression from the list, you specify that number.
The automatic display looks like this:

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

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

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

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

@item display/@var{fmt} @var{exp}
For @var{fmt} specifying only a display format and not a size or
count, add the expression @var{exp} to the auto-display list but
arranges to display it each time in the specified format @var{fmt}.
@xref{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 the program stops.  Examining means in effect
doing @samp{x/@var{fmt} @var{addr}}.  @xref{Memory}.
@end table

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

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

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

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

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

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

@item info display
@kindex info display
Print the list of expressions previously set up to display
automatically, each one with its item number, but without showing the
values.  This includes disabled expressions, which are marked as such.
It also includes expressions which would not be displayed right now
because they refer to automatic variables not currently available.
@end table

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

@node Print Settings, Value History, Auto Display, Data
@section Print Settings

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

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

@table @code
@item set print address
@item set print address on
@kindex set print address
_GDBN__ will print memory addresses showing the location of stack
traces, structure values, pointer values, breakpoints, and so forth,
even when it also displays the contents of those addresses.  The default
is on.  For example, this is what a stack frame display looks like, with
@code{set print address on}:
@smallexample
(_GDBP__) f
#0  set_quotes (lq=0x34c78 "<<", rq=0x34c88 ">>") 
    at input.c:530
530	    if (lquote != def_lquote)
@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}:
@example
(_GDBP__) set print addr off
(_GDBP__) f
#0  set_quotes (lq="<<", rq=">>") at input.c:530
530	    if (lquote != def_lquote)
@end example

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

@item set print array
@itemx set print array on
@kindex set print array
_GDBN__ will pretty print arrays.  This format is more convenient to read,
but uses more space.  The default is off.

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

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

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

@item show print elements
@kindex show print elements
Display the number of elements of a large array that _GDBN__ will print
before losing patience.

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

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

@item set print pretty off
Cause _GDBN__ to print structures in a compact format, like this:

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

@noindent
This is the default format.

@item show print pretty
@kindex show print pretty
Show which format _GDBN__ will use to print structures.

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

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

@item show print sevenbit-strings
Show whether or not _GDBN__ will print only seven-bit characters.

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

@item set print union off
Tell _GDBN__ not to print unions which are contained in structures.

@item show print union
@kindex show print union
Ask _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

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

@table @code
@item set print demangle 
@itemx set print demangle on 
@kindex set print demangle
Print C++ names in their source form rather than in the mangled form
in which they are passed to the assembler and linker for type-safe linkage.
The default is on.

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

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

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

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

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

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

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

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

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

@end table

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

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

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

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

For example, suppose you have just printed a pointer to a structure and
want to see the contents of the structure.  It suffices to type

@example
p *$
@end example

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

@example
p *$.next
@end example

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

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

@example
print x
set x=5
@end example

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

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

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

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

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

@node Convenience Vars, Registers, Value History, Data
@section Convenience Variables

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

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

@example
set $foo = *object_ptr
@end example

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

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

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

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

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

_0__@example
set $i = 0
print bar[$i++]->contents
@i{@dots{} repeat that command by typing @key{RET}.}
_1__@end example

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

@table @code
@item $_
The variable @code{$_} is automatically set by the @code{x} command to
the last address examined (@pxref{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}.

@item $__
The variable @code{$__} is automatically set by the @code{x} command
to the value found in the last address examined.
@end table

@node Registers, Floating Point Hardware, Convenience Vars, Data
@section Registers

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

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

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

The register names @code{$pc} and @code{$sp} are used on most machines
for the program counter register and the stack pointer.  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 with
@example
set $sp += 4
@end example

@noindent
The last is a way of removing one word from the stack, on machines where
stacks grow downward in memory (most machines, nowadays).  This assumes
that the innermost stack frame is selected; setting @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}.)

Often @code{$fp} is used for a register that contains a pointer to the
current stack frame, and @code{$ps} is sometimes used for a register
that contains the processor status.  These standard register names may
be available on your machine even though the @code{info registers}
command shows other 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}.

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

@node Floating Point Hardware,  , Registers, Data
@section Floating Point Hardware
@cindex floating point
Depending on the host machine architecture, _GDBN__ may be able to give
you more information about the status of the floating point hardware.

@table @code
@item info float
@kindex info float
If available, provides hardware-dependent information about the floating
point unit.  The exact contents and layout vary depending on the
floating point chip.
@end table
@c FIXME: this is a cop-out.  Try to get examples, explanations.  Only
@c FIXME...supported currently on arm's and 386's.  Mark properly with 
@c FIXME... m4 macros to isolate general statements from hardware-dep, 
@c FIXME... at that point.

@node Symbols, Altering, Data, 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 the program executes.  _GDBN__ finds it in your
program's symbol table, in the file indicated when you started _GDBN__ 
(@pxref{File Options}), or by one of the file-management commands
(@pxref{Files}).

@table @code
@item info address @var{symbol}
@kindex info address
Describe where the data for @var{symbol} is stored.  For a register
variable, this says which register it is kept in.  For a non-register
local variable, this prints the stack-frame offset at which the variable
is always stored.

Note the contrast with @samp{print &@var{symbol}}, which does not work
at all for a register variables, and for a stack local variable prints
the exact address of the current instantiation of the variable.

@item whatis @var{exp}
@kindex whatis
Print the data type of expression @var{exp}.  @var{exp} is not
actually evaluated, and any side-effecting operations (such as
assignments or function calls) inside it do not take place.
@xref{Expressions}.

@item whatis
Print the data type of @code{$}, the last value in the value history.

@item ptype @var{typename}
@kindex ptype
Print a description of data type @var{typename}.  @var{typename} may be
the name of a type, or for C code it may have the form
@samp{struct @var{struct-tag}}, @samp{union @var{union-tag}} or
@samp{enum @var{enum-tag}}.@refill

@item ptype @var{exp}
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, if your program declares a variable
as
@example
struct complex @{double real; double imag;@} v;
@end example
@noindent
compare the output of the two commands:
@example
(_GDBP__) whatis v
type = struct complex
(_GDBP__) ptype v
type = struct complex @{
    double real;
    double imag;
@}
@end example

@item info types @var{regexp}
@itemx info types
@kindex info types 
Print a brief description of all types whose name matches @var{regexp}
(or all types in your program, if you supply no argument).  Each
complete typename is matched as though it were a complete line; thus,
@samp{i type value} gives information on all types in your program whose
name includes the string @code{value}, but @samp{i type ^value$} gives
information only on types whose complete name is @code{value}.

This command differs from @code{ptype} in two ways: first, like
@code{whatis}, it does not print a detailed description; second, it
lists all source files where a type is defined.

@item info source
@kindex info source
Show the name of the current source file---that is, the source file for
the function containing the current point of execution.

@item info sources
@kindex info sources
Print the names of all source files in the program for which there is
debugging information, organized into two lists: those for which symbols
have been read in, and those for which symbols will be read in on
demand.
@c FIXME: above passive AND awkward!

@item info functions
@kindex info functions
Print the names and data types of all defined functions.

@item info functions @var{regexp}
Print the names and data types of all defined functions
whose names contain a match for regular expression @var{regexp}.
Thus, @samp{info fun step} finds all functions whose names
include @code{step}; @samp{info fun ^step} finds those whose names
start with @code{step}.

@item info variables
@kindex info variables
Print the names and data types of all variables that are declared
outside of functions (i.e., excluding local variables).

@item info variables @var{regexp}
Print the names and data types of all variables (except for local
variables) whose names contain a match for regular expression
@var{regexp}.


@ignore
This was never implemented.
@item info methods
@itemx info methods @var{regexp}
@kindex info methods
The @code{info methods} command permits the user to examine all defined
methods within C++ program, or (with the @var{regexp} argument) a
specific set of methods found in the various C++ classes.  Many
C++ classes provide a large number of methods.  Thus, the output
from the @code{ptype} command can be overwhelming and hard to use.  The
@code{info-methods} command filters the methods, printing only those
which match the regular-expression @var{regexp}.
@end ignore

@item printsyms @var{filename}
@kindex printsyms
Write a complete dump of the debugger's symbol data into the
file @var{filename}.
@end table

@node Altering, _GDBN__ Files, Symbols, Top
@chapter Altering Execution

Once you think you have found an error in the program, you might want to
find out for certain whether correcting the apparent error would lead to
correct results in the rest of the run.  You can find the answer by
experiment, using the _GDBN__ features for altering execution of the
program.

For example, you can store new values into variables or memory
locations, give the program a signal, restart it at a different address,
or even return prematurely from a function to its caller.

@menu
* Assignment::			Assignment to Variables
* Jumping::			Continuing at a Different Address
* Signaling::			Giving the Program a Signal
* Returning::			Returning from a Function
* Calling::			Calling your Program's Functions
@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}.  For example,

@example
print x=4
@end example

@noindent
would store the value 4 into the variable @code{x}, and then print the
value of the assignment expression (which is 4).  All the assignment
operators of C are supported, including the increment operators
@samp{++} and @samp{--}, and combining assignments such as @samp{+=} and
_0__@samp{<<=}_1__.

@kindex set
@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}).  The
expression is evaluated only for its effects.

If the beginning of the argument string of the @code{set} command
appears identical to a @code{set} subcommand, use the @code{set
variable} command instead of just @code{set}.  This command is identical
to @code{set} except for its lack of subcommands.  For example, a
program might well have a variable @code{width}---which leads to
an error if we try to set a new value with just @samp{set width=13}, as
we might if @code{set width} didn't happen to be a _GDBN__ command:
@example
(_GDBP__) whatis width
type = double
(_GDBP__) p width
$4 = 13
(_GDBP__) set width=47
Invalid syntax in expression.
@end example
@noindent
The invalid expression, of course, is @samp{=47}.  What we can do in
order to actually set our program's variable @code{width} is 
@example
(_GDBP__) set var width=47
@end example

_GDBN__ allows more implicit conversions in assignments than C does; you can
freely store an integer value into a pointer variable or vice versa, and
any structure can be converted to any other structure that is the same
length or shorter.
@comment FIXME: how do structs align/pad in these conversions? 
@comment        /[email protected] 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}).  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 the program, you do so at the place where
it stopped, with the @code{continue} command.  You can instead continue at
an address of your own choosing, with the following commands:

@table @code
@item jump @var{linespec}
@kindex jump
Resume execution at line @var{linespec}.  Execution will stop
immediately if there is a breakpoint there.  @xref{List} for a
description of the different forms of @var{linespec}.

The @code{jump} command does not change the current stack frame, or
the stack pointer, or the contents of any memory location or any
register other than the program counter.  If line @var{linespec} is in
a different function from the one currently executing, the results may
be bizarre if the two functions expect different patterns of arguments or
of local variables.  For this reason, the @code{jump} command requests
confirmation if the specified line is not in the function currently
executing.  However, even bizarre results are predictable if you are
well acquainted with the machine-language code of the program.

@item jump *@var{address}
Resume execution at the instruction at address @var{address}.
@end table

You can get much the same effect as the @code{jump} command by storing a
new value into the register @code{$pc}.  The difference is that this
does not start the program running; it only changes the address where it
@emph{will} run when it is continued.  For example,

@example
set $pc = 0x485
@end example

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

The most common occasion to use the @code{jump} command is to back up,
perhaps with more breakpoints set, over a portion of a program that has
already executed, in order to examine its execution in more detail.

@node Signaling, Returning, Jumping, Altering
@c @group
@section Giving the Program a Signal

@table @code
@item signal @var{signalnum}
@kindex signal
Resume execution where the program stopped, but give it immediately the
signal number @var{signalnum}.

Alternatively, if @var{signalnum} is zero, continue execution without
giving a signal.  This is useful when the program stopped on account of
a signal and would ordinary see the signal when resumed with the
@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

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

@table @code
@item return
@itemx return @var{expression}
@cindex returning from a function
@kindex return
You can cancel execution of a function call with the @code{return}
command.  If you give an
@var{expression} argument, its value is used as the function's return
value. 
@end table

When you use @code{return}, _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}), 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}) resumes execution until the selected stack frame returns
naturally.@refill

@node     Calling,  , Returning, Altering
@section Calling your Program's Functions

@cindex calling functions
@kindex call
@table @code
@item call @var{expr}
Evaluate the expression @var{expr} without displaying @code{void}
returned values.
@end table

You can use this variant of the @code{print} command if you want to
execute a function from your program, but without cluttering the output
with @code{void} returned values.  The result is printed and saved in
the value history, if it is not void.

@node _GDBN__ Files, Targets, Altering, Top
@chapter _GDBN__'s Files

@menu
* Files::			Commands to Specify Files
* Symbol Errors::		Errors Reading Symbol Files
@end menu

@node Files, Symbol Errors, _GDBN__ Files, _GDBN__ Files
@section Commands to Specify Files
@cindex core dump file
@cindex symbol table
_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 the program.  To
debug a core dump of a previous run, _GDBN__ must be told the file name of
the core dump.

The usual way to specify the executable and core dump file names is with
the command arguments given when you start _GDBN__, as discussed in
@pxref{Invocation}.

Occasionally it is necessary to change to a different file during a
_GDBN__ session.  Or you may run _GDBN__ and forget to specify the files you
want to use.  In these situations the _GDBN__ commands to specify new files
are useful.

@table @code
@item file @var{filename}
@cindex executable file
@kindex file
Use @var{filename} as the program to be debugged.  It is read for its
symbols and for the contents of pure memory.  It is also the program
executed when you use the @code{run} command.  If you do not specify a
directory and the file is not found in _GDBN__'s working directory,

_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 _GDBN__ and
your program, using the @code{path} command.

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

@item exec-file @var{filename}
@kindex exec-file
Specify that the program to be run (but not the symbol table) is found
in @var{filename}.  _GDBN__ will search the environment variable @code{PATH}
if necessary to locate the program.  

@item symbol-file @var{filename}
@kindex symbol-file
Read symbol table information from file @var{filename}.  @code{PATH} is
searched when necessary.  Use the @code{file} command to get both symbol
table and program to run from the same file.

@code{symbol-file} with no argument clears out _GDBN__'s information on your
program's symbol table.

The @code{symbol-file} command causes _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 _GDBN__.

@code{symbol-file} will not repeat if you press @key{RET} again after
executing it once.

On some kinds of object files, the @code{symbol-file} command does not
actually 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,
when they are needed.

The purpose of this two-stage reading strategy is to make _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}).

When the symbol table is stored in COFF format, @code{symbol-file} does
read the symbol table data in full right away.  We haven't implemented
the two-stage strategy for COFF yet.

When _GDBN__ is configured for a particular environment, it will
understand debugging information in whatever format is the standard
generated for that environment; you may use either a GNU compiler, or
other compilers that adhere to the local conventions.  Best results are
usually obtained from GNU compilers; for example, using @code{_GCC__}
you can generate debugging information for optimized code.

@item core-file @var{filename}
@itemx core @var{filename}
@kindex core
@kindex core-file
Specify the whereabouts of a core dump file to be used as the ``contents
of memory''.  Traditionally, core files contain only some parts of the
address space of the process that generated them; _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 _GDBN__.  So, if you have been running the 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}).

@item load @var{filename}
@kindex load
_if__(_GENERIC__)
Depending on what remote debugging facilities are configured into
_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 @var{filename}'s symbol table in _GDBN__, like
the @code{add-symbol-file} command.

If @code{load} is not available on your _GDBN__, attempting to execute
it gets the error message ``@code{You can't do that when your target is
@dots{}}'' 
_fi__(_GENERIC__)

_if__(_VXWORKS__) 
On VxWorks, @code{load} will dynamically link @var{filename} on the
current target system as well as adding its symbols in _GDBN__.
_fi__(_VXWORKS__)

_if__(_I960__)
@cindex download to Nindy-960
With the Nindy interface to an Intel 960 board, @code{load} will
download @var{filename} to the 960 as well as adding its symbols in
_GDBN__. 
_fi__(_I960__)

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

@item add-symbol-file @var{filename} @var{address}
@kindex add-symbol-file
@cindex dynamic linking
The @code{add-symbol-file} command reads additional symbol table information
from the file @var{filename}.  You would use this command when that file
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; _GDBN__ cannot figure this out for itself.

The symbol table of the file @var{filename} is added to the symbol table
originally read with the @code{symbol-file} command.  You can use the
@code{add-symbol-file} command any number of times; the new symbol data thus
read keeps adding to the old.  To discard all old symbol data instead,
use the @code{symbol-file} command. 

@code{add-symbol-file} will not repeat if you press @key{RET} after using it.

@item info files
@itemx info target
@kindex info files
@kindex info target
@code{info files} and @code{info target} are synonymous; both print the
current targets (@pxref{Targets}), including the names of the executable
and core dump files currently in use by _GDBN__, and the files from
which symbols were loaded.  The command @code{help targets} lists all
possible targets rather than current ones.

@end table

All file-specifying commands allow both absolute and relative file names
as arguments.  _GDBN__ always converts the file name to an absolute path
name and remembers it that way.

@kindex sharedlibrary
@kindex share
@cindex shared libraries

_GDBN__ supports the SunOS shared library format.  Symbols from a shared
library cannot be referenced before the shared library has been linked
with the program.  (That is to say, until after you type @code{run} and 
the function @code{main} has been entered; or when examining core
files.)  Once the shared library has been linked in, you can use the
following commands:

@table @code
@item sharedlibrary @var{regex}
@itemx share @var{regex}
Load shared object library symbols for files matching a UNIX regular
expression.  

@item share
@itemx sharedlibrary
Load symbols for all shared libraries.

@item info share
@itemx info sharedlibrary
@kindex info sharedlibrary
@kindex info share
Print the names of the shared libraries which you have loaded with the
@code{sharedlibrary} command.
@end table

@code{sharedlibrary} does not repeat automatically when you press
@key{RET} after using it once.

@node Symbol Errors,  , Files, _GDBN__ Files
@section Errors Reading Symbol Files
While a symbol file is being read, _GDBN__ will occasionally encounter
problems, such as symbol types it does not recognize, or known bugs in
compiler output.  By default, it prints one message about each such
type of problem, no matter how many times the problem occurs.  You can
ask it to print more messages, to see how many times the problems occur,
or can shut the messages off entirely, with the @code{set 
complaints} command (@xref{Messages/Warnings}).

The messages currently printed, and their meanings, are:

@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.  

_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.  

_GDBN__ does not circumvent this problem, and will have trouble locating
symbols in the source file whose symbols being read.  (You can often
determine what source file is affected by specifying @code{set verbose
on}.  @xref{Messages/Warnings}.)

@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.  

_GDBN__ circumvents the problem by treating the symbol scope block as
starting on the previous source line.

@c @item{encountered DBX-style class variable debugging information.
@c You seem to have compiled your program with "g++ -g0" instead of "g++ -g".
@c Therefore _GDBN__ will not know about your class variables}
@c 
@c This error indicates that the symbol information produced for a C++
@c program includes zero-size fields, which indicated static fields in
@c a previous release of the G++ compiler.  This message is probably
@c obsolete.
@c
@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.  

_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 _GDBN__ does not yet
know how to read.  @code{0x@var{nn}} is the symbol type of the misunderstood
information, in hexadecimal.  

_GDBN__ circumvents the error by ignoring this symbol information.  This
will usually allow the program to be debugged, though certain symbols
will not be accessible.  If you encounter such a problem and feel like
debugging it, you can debug @code{_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
_GDBN__ could not find the full definition for a struct or class. 

@ignore
@c this is #if 0'd in dbxread.c as of (at least!) 17 may 1991
@item const/volatile indicator missing, got '@var{X}'

The symbol information for a C++ member function is missing some
information that the compiler should have output for it.
@end ignore

@item C++ type mismatch between compiler and debugger

_GDBN__ could not parse a type specification output by the compiler
for some C++ object.

@end table

@node     Targets, Controlling _GDBN__, _GDBN__ Files, Top
@chapter Specifying a Debugging Target 
@cindex debugging target
@kindex target
A @dfn{target} is an interface between the debugger and a particular 
kind of file or process.  

Often, you will be able to run _GDBN__ in the same host environment as the
program you are debugging; in that case, the debugging target can just be
specified as a side effect of the @code{file} or @code{core} commands.
When you need more flexibility---for example, running _GDBN__ on a
physically separate host, controlling standalone systems over a
serial port, or realtime systems over a TCP/IP connection---you can use
the @code{target} command.

@menu
* Active Targets::		Active Targets
* Target Commands::		Commands for Managing Targets
* Remote::			Remote Debugging
@end menu

@node Active Targets, Target Commands, Targets, Targets
@section Active Targets
@cindex stacking targets
@cindex active targets
@cindex multiple targets

Targets are managed in three @dfn{strata} that correspond to different
classes of target: processes, core files, and executable files.  This
allows you to (for example) start a process and inspect its activity
without abandoning your work on a core file.

More than one target can potentially respond to a request.  In
particular, when you access memory _GDBN__ will examine the three strata of
targets until it finds a target that can handle that particular address.
Strata are always examined in a fixed order: first a process if there is
one, then a core file if there is one, and finally an executable file if
there is one of those.

When you specify a new target in a given stratum, it replaces any target
previously in that stratum.

To get rid of a target without replacing it, use the @code{detach}
command.  The related command @code{attach} provides you with a way of
choosing a particular running process as a new target. @xref{Attach}.

@node Target Commands, Remote, Active Targets, Targets
@section Commands for Managing Targets

@table @code
@item target @var{type} @var{parameters}
Connects the _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 will not repeat if you press @key{RET} again
after executing the command.

@item help target
@kindex help target
Displays the names of all targets available.  To display targets
currently selected, use either @code{info target} or @code{info files}
(@pxref{Files}).

@item help target @var{name}
Describe a particular target, including any parameters necessary to
select it.
@end table

Here are some common targets (available, or not, depending on the _GDBN__
configuration):

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

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

@item target remote @var{dev}
@kindex target remote
Remote serial target in _GDBN__-specific protocol.  The argument @var{dev}
specifies what serial device to use for the connection (e.g.
@file{/dev/ttya}). @xref{Remote}.

_if__(_AMD29K__)
@item target amd-eb @var{dev} @var{speed} @var{PROG}
@kindex target amd-eb
@cindex AMD EB29K
Remote PC-resident AMD EB29K board, attached over serial lines.
@var{dev} is the serial device, as for @code{target remote};
@var{speed} allows you to specify the linespeed; and @var{PROG} is the
name of the program to be debugged, as it appears to DOS on the PC.
@xref{EB29K Remote}.

_fi__(_AMD29K__)
_if__(_I960__)
@item target nindy @var{devicename}
@kindex target nindy
An Intel 960 board controlled by a Nindy Monitor.  @var{devicename} is
the name of the serial device to use for the connection, e.g.
@file{/dev/ttya}.  @xref{i960-Nindy Remote}.

_fi__(_I960__)
_if__(_VXWORKS__)
@item target vxworks @var{machinename}
@kindex target vxworks
A VxWorks system, attached via TCP/IP.  The argument @var{machinename}
is the target system's machine name or IP address.
@xref{VxWorks Remote}.
_fi__(_VXWORKS__)
@end table

_if__(_GENERIC__)
Different targets are available on different configurations of _GDBN__; your
configuration may have more or fewer targets.
_fi__(_GENERIC__)

@node Remote,  , Target Commands, Targets
@section Remote Debugging
@cindex remote debugging

_if__(_GENERIC__)
@menu
_include__(gdbinv-m.m4)<>_dnl__
@end menu
_fi__(_GENERIC__)

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

To use the _GDBN__ remote serial protocol, the program to be debugged on
the remote machine needs to contain a debugging stub which talks to
_GDBN__ over the serial line.  Several working remote stubs are
distributed with _GDBN__; see the @file{README} file in the _GDBN__
distribution for more information.

For details of this communication protocol, see the comments in the
_GDBN__ source file @file{remote.c}.

To start remote debugging, first run _GDBN__ and specify as an executable file
the program that is running in the remote machine.  This tells _GDBN__ how
to find the program's symbols and the contents of its pure text.  Then
establish communication using the @code{target remote} command with a device
name as an argument.  For example:

@example
target remote /dev/ttyb
@end example

@noindent
if the serial line is connected to the device named @file{/dev/ttyb}.  This
will stop the remote machine if it is not already stopped.

Now you can use all the usual commands to examine and change data and to
step and continue the remote program.

To resume the remote program and stop debugging it, use the @code{detach}
command.

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

_if__(_GENERIC__)
@c Text on starting up GDB in various specific cases; it goes up front
@c in manuals configured for any of those particular situations, here
@c otherwise. 
_include__(gdbinv-s.m4)
_fi__(_GENERIC__)

@node Controlling _GDBN__, Sequences, Targets, Top
@chapter Controlling _GDBN__

You can alter many aspects of _GDBN__'s interaction with you by using
the @code{set} command.  For commands controlling how _GDBN__ displays
data, @pxref{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 _GDBN__, Controlling _GDBN__
@section Prompt
@cindex prompt
_GDBN__ indicates its readiness to read a command by printing a string
called the @dfn{prompt}.  This string is normally @samp{(_GDBP__)}.  You
can change the prompt string with the @code{set prompt} command.  For
instance, when debugging _GDBN__ with _GDBN__, it is useful to change
the prompt in one of the _GDBN__<>s so that you can always tell which
one you are talking to.

@table @code
@item set prompt @var{newprompt}
@kindex set prompt
Directs _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 _GDBN__
@section Command Editing
@cindex readline
@cindex command line editing
_GDBN__ reads its input commands via the @dfn{readline} interface.  This
GNU library provides consistent behavior for programs which provide a
command line interface to the user.  Advantages are @code{emacs}-style
or @code{vi}-style inline editing of commands, @code{csh}-like history
substitution, and a storage and recall of command history across
debugging sessions.

You may control the behavior of command line editing in _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 _GDBN__
@section Command History
@table @code
@cindex history substitution
@cindex history file
@kindex set history filename
@item set history filename @var{fname}
Set the name of the _GDBN__ command history file to @var{fname}.  This is
the file from which _GDBN__ will read an initial command history
list or to which it will write this list when it exits.  This list is
accessed through history expansion or through the history
command editing characters listed below.  This file defaults to the
value of the environment variable @code{GDBHISTFILE}, or to
@file{./.gdb_history} if this variable is not set.

@cindex history save
@kindex set history save
@item set history save
@itemx set history save on
Record command history in a file, whose name may be specified with the
@code{set history filename} command.  By default, this option is disabled.

@item set history save off
Stop recording command history in a file.

@cindex history size
@kindex set history size
@item set history size @var{size}
Set the number of commands which _GDBN__ will keep in its history list.
This defaults to the value of the environment variable
@code{HISTSIZE}, or to 256 if this variable is not set.
@end table

@cindex history expansion
History expansion assigns special meaning to the character @kbd{!}.
@iftex
(@xref{Event Designators}.)
@end iftex
Since @kbd{!} is also the logical not operator in C, history expansion
is off by default. If you decide to enable history expansion with the
@code{set history expansion on} command, you may sometimes need to
follow @kbd{!} (when it is used as logical not, in an expression) with
a space or a tab to prevent it from being expanded.  The readline
history facilities will not attempt substitution on the strings
@kbd{!=} and @kbd{!(}, even when history expansion is enabled.

The commands to control history expansion are:

@table @code

@kindex set history expansion
@item set history expansion on
@itemx set history expansion
Enable history expansion.  History expansion is off by default.

@item set history expansion off
Disable history expansion.

The readline code comes with more complete documentation of
editing and history expansion features.  Users unfamiliar with @code{emacs}
or @code{vi} may wish to read it. 
@iftex
@xref{Command Line Editing}.
@end iftex

@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 _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 _GDBN__
@section Screen Size
@cindex size of screen
@cindex pauses in output
Certain commands to _GDBN__ may produce large amounts of information
output to the screen.  To help you read all of it, _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.  _GDBN__ also uses the screen
width setting to determine when to wrap lines of output.  Depending on
what is being printed, it tries to break the line at a readable place,
rather than simply letting it overflow onto the following line.

Normally _GDBN__ knows the size of the screen from the termcap data base
together with the value of the @code{TERM} environment variable and the
@code{stty rows} and @code{stty cols} settings. If this is not correct,
you can override it with the @code{set height} and @code{set
width} commands:

@table @code
@item set height @var{lpp}
@itemx show height
@itemx set width @var{cpl}
@itemx show width
@kindex set height
@kindex set width
@kindex show width
@kindex show height
These @code{set} commands specify a screen height of @var{lpp} lines and
a screen width of @var{cpl} characters.  The associated @code{show}
commands display the current settings.

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

@node Numbers, Messages/Warnings, Screen Size, Controlling _GDBN__
@section Numbers
@cindex number representation
@cindex entering numbers
You can always enter numbers in octal, decimal, or hexadecimal in _GDBN__ by
the usual conventions: octal numbers begin with @samp{0}, decimal
numbers end with @samp{.}, and hexadecimal numbers begin with @samp{0x}.
Numbers that begin with none of these are, by default, entered in base
10; likewise, the default display for numbers---when no particular
format is specified---is base 10.  You can change the default base for
both input and output with the @code{set radix} command.

@table @code
@kindex set radix
@item set radix @var{base}
Set the default base for numeric input and display.  Supported choices
for @var{base} are decimal 8, 10, 16.  @var{base} must itself be
specified either unambiguously or using the current default radix; for
example, any of

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

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

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

@end table

@node Messages/Warnings,  , Numbers, Controlling _GDBN__
@section Optional Warnings and Messages
By default, _GDBN__ is silent about its inner workings.  If you are running
on a slow machine, you may want to use the @code{set verbose} command.
It will make _GDBN__ tell you when it does a lengthy internal operation, so
you won't 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
(@pxref{Files}, in the description of the command
@code{symbol-file}).
@c The following is the right way to do it, but emacs 18.55 doesn't support
@c @ref, and neither the emacs lisp manual version of texinfmt or makeinfo
@c is released.  
@ignore
see @code{symbol-file} in @ref{Files}).
@end ignore

@table @code
@kindex set verbose
@item set verbose on
Enables _GDBN__'s output of certain informational messages.

@item set verbose off
Disables _GDBN__'s 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 _GDBN__ encounters bugs in the symbol table of an object file,
it prints a single message about each type of problem it finds, then 
shuts up (@pxref{Symbol Errors}).  You can suppress these messages, or allow more than one such
message to be printed if you want to see how frequent the problems are.

@table @code
@kindex set complaints
@item set complaints @var{limit}
Permits _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 _GDBN__ is permitted to produce.
@end table

By default, _GDBN__ is cautious, and asks what sometimes seem 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
(_GDBP__) run
The program being debugged has been started already.
Start it from the beginning? (y or n) 
@end example

If you're willing to unflinchingly face the consequences of your own
commands, you can disable this ``feature'':

@table @code
@kindex set confirm
@cindex flinching
@cindex confirmation
@cindex stupid questions
@item set confirm off
Disables confirmation requests.

@item set confirm on
Enables confirmation requests (the default).

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

@node Sequences, Emacs, Controlling _GDBN__, Top
@chapter Canned Sequences of Commands

Aside from breakpoint commands (@pxref{Break Commands}), _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
* Command Files::		Command Files
* Output::			Commands for Controlled Output
@end menu

@node Define, Command Files, Sequences, Sequences
@section User-Defined Commands

@cindex user-defined command
A @dfn{user-defined command} is a sequence of _GDBN__ commands to which you
assign a new name as a command.  This is done with the @code{define}
command.

@table @code
@item define @var{commandname}
@kindex define
Define a command named @var{commandname}.  If there is already a command
by that name, you are asked to confirm that you want to redefine it.

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

@item document @var{commandname}
@kindex document
Give documentation to the user-defined command @var{commandname}.  The
command @var{commandname} must already be defined.  This command reads
lines of documentation just as @code{define} reads the lines of the
command definition, ending with @code{end}.  After the @code{document}
command is finished, @code{help} on command @var{commandname} will print
the documentation you have specified.

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

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

@item info user
@itemx info user @var{commandname}
@kindex info user
Display the _GDBN__ commands used to define @var{commandname} (but not its
documentation).  If no @var{commandname} is given, display the
definitions for all user-defined commands.
@end table

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

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

@node Command Files, Output, Define, Sequences
@section Command Files

@cindex command files
A command file for _GDBN__ is a file of lines that are _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 _GDBN__, it automatically executes commands from its
@dfn{init files}.  These are files named @file{_GDBINIT__}.  _GDBN__
reads the init file (if any) in your home directory and then the init
file (if any) in the current working directory.  (The init files are not
executed if you use the @samp{-nx} option; @pxref{Mode Options}.)  You
can also request the execution of a command file with the @code{source}
command:

@table @code
@item source @var{filename}
@kindex source
Execute the command file @var{filename}.
@end table

The lines in a command file are executed sequentially.  They are not
printed as they are executed.  An error in any command terminates execution
of the command file.

Commands that would ask for confirmation if used interactively proceed
without asking when used in a command file.  Many _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
_GDBN__ output is suppressed; the only output that appears is what is
explicitly printed by the commands in the definition.  This section
describes three commands useful for generating exactly the output you
want.

@table @code
@item echo @var{text}
@kindex echo
@c I don't consider backslash-space a standard C escape sequence
@c because it's 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.  @b{No
newline will be printed unless you specify one.} In addition to the
standard C escape sequences, a backslash followed by a space stands for a
space.  This is useful for outputting a string with spaces at the
beginning or the end, since leading and trailing spaces are otherwise
trimmed from all arguments.  Thus, to print @samp{@ and foo =@ }, use the
command @samp{echo \@ and foo = \@ }.
@c FIXME: verify hard copy actually issues enspaces for '@ '!  Will this
@c        confuse texinfo?

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

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

produces the same output as

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

@item output @var{expression}
@kindex output
Print the value of @var{expression} and nothing but that value: no
newlines, no @samp{$@var{nn} = }.  The value is not entered in the
value history either.  @xref{Expressions} 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}; @pxref{Output formats}, for more
information.

@item printf @var{string}, @var{expressions}@dots{}
@kindex printf
Print the values of the @var{expressions} under the control of
@var{string}.  The @var{expressions} are separated by commas and may
be either numbers or pointers.  Their values are printed as specified
by @var{string}, exactly as if the program were to execute

@example
printf (@var{string}, @var{expressions}@dots{});
@end example

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

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

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, _GDBN__ Bugs, Sequences, Top
@chapter Using _GDBN__ under GNU Emacs

@cindex emacs
A special interface allows you to use GNU Emacs to view (and
edit) the source files for the program you are debugging with
_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
_GDBN__ as a subprocess of Emacs, with input and output through a newly
created Emacs buffer.

Using _GDBN__ under Emacs is just like using _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 _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
_GDBN__ displays source code through Emacs.  
@end itemize

Each time _GDBN__ displays a stack frame, Emacs automatically finds the
source file for that frame and puts an arrow (_0__@samp{=>}_1__) at the
left margin of the current line.  Emacs uses a separate buffer for
source display, and splits the window to show both your _GDBN__ session
and the source.

Explicit _GDBN__ @code{list} or search commands still produce output as
usual, but you probably will have no reason to use them.

@quotation
@emph{Warning:} If the directory where your program resides is not your
current directory, it can be easy to confuse Emacs about the location of
the source files, in which case the auxiliary display buffer will not
appear to show your source.  _GDBN__ can find programs by searching your
environment's @code{PATH} variable, so the _GDBN__ input and output
session will proceed normally; but Emacs doesn't get enough information
back from _GDBN__ to locate the source files in this situation.  To
avoid this problem, either start _GDBN__ mode from the directory where
your program resides, or specify a full path name when prompted for the
@kbd{M-x gdb} argument.

A similar confusion can result if you use the _GDBN__ @code{file} command to
switch to debugging a program in some other location, from an existing
_GDBN__ buffer in Emacs.
@end quotation

By default, @kbd{M-x gdb} calls the program called @file{gdb}.  If
you need to call _GDBN__ by a different name (for example, if you keep
several configurations around, with different names) you can set the
Emacs variable @code{gdb-command-name}; for example,
@example
(setq gdb-command-name "mygdb")
@end example
@noindent
(preceded by @kbd{ESC ESC}, or typed in the @code{*scratch*} buffer, or
in your @file{.emacs} file) will make Emacs call the program named
``@code{mygdb}'' instead.

In the _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' _GDBN__ Mode.

@item M-s
Execute to another source line, like the _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 _GDBN__ @code{next} command.  Then update the display window
to show the current file and location.

@item M-i
Execute one instruction, like the _GDBN__ @code{stepi} command; update
display window accordingly.

@item M-x gdb-nexti
Execute to next instruction, using the _GDBN__ @code{nexti} command; update
display window accordingly.

@item C-c C-f
Execute until exit from the selected stack frame, like the _GDBN__
@code{finish} command.

@item M-c
Continue execution of the program, like the _GDBN__ @code{continue}
command.  @emph{Warning:} In Emacs v19, this command is @kbd{C-c C-p}.

@item M-u
Go up the number of frames indicated by the numeric argument
(@pxref{Arguments, , Numeric Arguments, emacs, The GNU Emacs Manual}),
like the _GDBN__ @code{up} command.  @emph{Warning:} In Emacs v19, this
command is @kbd{C-c C-u}.@refill

@item M-d
Go down the number of frames indicated by the numeric argument, like the
_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 _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 on the fly by defining elements of the list
@code{gdb-print-command}; once it is defined, you can format or
otherwise process numbers picked up by @kbd{C-x &} before they are
inserted.  A numeric argument to @kbd{C-x &} will both flag that you
wish special formatting, and act as an index to pick an element of the
list.  If the list element is a string, the number to be inserted is
formatted using the Emacs function @code{format}; otherwise the number
is passed as an argument to the corresponding list element.

@end table

In any source file, the Emacs command @kbd{C-x SPC} (@code{gdb-break})
tells _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 _GDBN__ buffer, to
request a frame display; when you run under Emacs, this will recreate
the source buffer if necessary to show you the context of the current
frame.

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

@c The following dropped because Epoch is nonstandard.  Reactivate
@c if/when v19 does something similar. [email protected] 19dec1990
@ignore
@kindex emacs epoch environment  
@kindex epoch
@kindex inspect

Version 18 of Emacs has a built-in window system called the @code{epoch}
environment.  Users of this environment can use a new command,
@code{inspect} which performs identically to @code{print} except that
each value is printed in its own window.
@end ignore

@node _GDBN__ Bugs, Renamed Commands, Emacs, Top
@chapter Reporting Bugs in _GDBN__
@cindex Bugs in _GDBN__
@cindex Reporting Bugs in _GDBN__

Your bug reports play an essential role in making _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 _GDBN__ work better.  Bug
reports are your contribution to the maintenance of _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, _GDBN__ Bugs, _GDBN__ 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
@item
@cindex Fatal Signal
@cindex Core Dump
If the debugger gets a fatal signal, for any input whatever, that is a
_GDBN__ bug.  Reliable debuggers never crash.

@item
@cindex error on Valid Input
If _GDBN__ produces an error message for valid input, that is a bug.

@item
@cindex Invalid Input
If _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 _GDBN__ are welcome in any case.
@end itemize

@node Bug Reporting,  , Bug Criteria, _GDBN__ Bugs
@section How to Report Bugs
@cindex Bug Reports
@cindex _GDBN__ Bugs, Reporting

A number of companies and individuals offer support for GNU products.
If you obtained _GDBN__ from a support organization, we recommend you
contact that organization first. 

Contact information for many support companies and individuals is
available in the file @file{etc/SERVICE} in the GNU Emacs distribution.

In any event, we also recommend that you send bug reports for _GDBN__ to one
of these addresses:

@example
bug-gdb@@prep.ai.mit.edu
@{ucbvax|mit-eddie|uunet@}!prep.ai.mit.edu!bug-gdb
@end example

@strong{Do not send bug reports to @samp{info-gdb}, or to
@samp{help-gdb}, or to any newsgroups.} Most users of _GDBN__ do not want to
receive bug reports.  Those that do, have arranged to receive @samp{bug-gdb}.

The mailing list @samp{bug-gdb} has a newsgroup @samp{gnu.gdb.bug} which
serves as a repeater.  The mailing list and the newsgroup carry exactly
the same messages.  Often people think of posting bug reports to the
newsgroup instead of mailing them.  This appears to work, but it has one
problem which can be crucial: a newsgroup posting often lacks a mail
path back to the sender.  Thus, if we need to ask for more information,
we may be unable to reach you.  For this reason, it is better to send
bug reports to the mailing list.

As a last resort, send bug reports on paper to:

@example
GNU Debugger Bugs
Free Software Foundation
545 Tech Square
Cambridge, MA 02139
@end example

The fundamental principle of reporting bugs usefully is this:
@strong{report all the facts}.  If you are not sure whether to state a
fact or leave it out, state it!

Often people omit facts because they think they know what causes the
problem and assume that some details don't matter.  Thus, you might
assume that the name of the variable you use in an example does not matter.
Well, probably it doesn't, but one cannot be sure.  Perhaps the bug is a
stray memory reference which happens to fetch from the location where that
name is stored in memory; perhaps, if the name were different, the contents
of that location would fool the debugger into doing the right thing despite
the bug.  Play it safe and give a specific, complete example.  That is the
easiest thing for you to do, and the most helpful.

Keep in mind that the purpose of a bug report is to enable us to fix
the bug if it is new to us.  It isn't as important what happens if
the bug is already known.  Therefore, always write your bug reports on
the assumption that the bug has not been reported previously.

Sometimes people give a few sketchy facts and ask, ``Does this ring a
bell?''  Those bug reports are useless, and we urge everyone to
@emph{refuse to respond to them} except to chide the sender to report
bugs properly.

To enable us to fix the bug, you should include all these things:

@itemize @bullet
@item
The version of _GDBN__.  _GDBN__ announces it if you start with no
arguments; you can also print it at any time using @code{show version}.

Without this, we won't know whether there is any point in looking for
the bug in the current version of _GDBN__.

@item
A complete input script, and all necessary source files, that will
reproduce the bug.  

@item
What compiler (and its version) was used to compile _GDBN__---e.g.
``_GCC__-1.37.1''.

@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 won't omit something important, list them all.

If we were to try to guess the arguments, we would probably guess wrong
and then we might not encounter the bug.

@item
The type of machine you are using, and the operating system name and
version number.

@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 _GDBN__ gets a fatal signal, then we will
certainly notice it.  But if the bug is incorrect output, we might not
notice unless it is glaringly wrong.  We are human, after all.  You
might as well not give us a chance to make a mistake.

Even if the problem you experience is a fatal signal, you should still
say so explicitly.  Suppose something strange is going on, such as,
your copy of _GDBN__ is out of synch, or you have encountered a
bug in the C library on your system.  (This has happened!)  Your copy
might crash and ours would not.  If you told us to expect a crash,
then when ours fails to crash, we would know that the bug was not
happening for us.  If you had not told us to expect a crash, then we
would not be able to draw any conclusion from our observations.

@item
If you wish to suggest changes to the _GDBN__ source, send us context
diffs.  If you even discuss something in the _GDBN__ source, refer to
it by context, not by line number.

The line numbers in our development sources won't match those in your
sources.  Your line numbers would convey no useful information to us.

@end itemize

Here are some things that are not necessary:

@itemize @bullet
@item
A description of the envelope of the bug.

Often people who encounter a bug spend a lot of time investigating
which changes to the input file will make the bug go away and which
changes will not affect it.

This is often time consuming and not very useful, because the way we
will find the bug is by running a single example under the debugger
with breakpoints, not by pure deduction from a series of examples.
We recommend that you save your time for something else.

Of course, if you can find a simpler example to report @emph{instead}
of the original one, that is a convenience for us.  Errors in the
output will be easier to spot, running under the debugger will take
less time, etc. 

However, simplification is not vital; if you don't 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 don't 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 _GDBN__ it is very hard to
construct an example that will make the program follow a certain path
through the code.  If you don't send us the example, we won't be able
to construct one, so we won't be able to verify that the bug is fixed.

And if we can't understand what bug you are trying to fix, or why your
patch should be an improvement, we won't 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 can't guess right about such
things without first using the debugger to find the facts.
@end itemize

@iftex
@include rdl-apps.texinfo
@end iftex

@node Renamed Commands, Installing _GDBN__, _GDBN__ Bugs, Top
@appendix Renamed Commands

The following commands were renamed in _GDBN__ 4.0, in order to make the
command set as a whole more consistent and easier to use and remember:

@kindex add-syms                   
@kindex delete environment         
@kindex info copying               
@kindex info convenience           
@kindex info directories           
@kindex info editing               
@kindex info history               
@kindex info targets               
@kindex info values                
@kindex info version               
@kindex info warranty              
@kindex set addressprint      
@kindex set arrayprint        
@kindex set prettyprint       
@kindex set screen-height     
@kindex set screen-width      
@kindex set unionprint        
@kindex set vtblprint         
@kindex set demangle          
@kindex set asm-demangle      
@kindex set sevenbit-strings  
@kindex set array-max         
@kindex set caution           
@kindex set history write     
@kindex show addressprint      
@kindex show arrayprint        
@kindex show prettyprint       
@kindex show screen-height     
@kindex show screen-width      
@kindex show unionprint        
@kindex show vtblprint         
@kindex show demangle          
@kindex show asm-demangle      
@kindex show sevenbit-strings  
@kindex show array-max         
@kindex show caution           
@kindex show history write     
@kindex unset                      

@ifinfo
@example
OLD COMMAND               NEW COMMAND
---------------           -------------------------------
add-syms                  add-symbol-file
delete environment        unset environment
info convenience          show convenience
info copying              show copying
info directories          show directories     
info editing              show commands
info history              show values
info targets              help target
info values               show values
info version              show version
info warranty             show warranty
set/show addressprint     set/show print address
set/show array-max        set/show print elements
set/show arrayprint       set/show print array
set/show asm-demangle     set/show print asm-demangle
set/show caution          set/show confirm
set/show demangle         set/show print demangle
set/show history write    set/show history save
set/show prettyprint      set/show print pretty
set/show screen-height    set/show height
set/show screen-width     set/show width
set/show sevenbit-strings set/show print sevenbit-strings
set/show unionprint       set/show print union
set/show vtblprint        set/show print vtbl

unset                     [No longer an alias for delete]
@end example
@end ifinfo

@tex
\vskip \parskip\vskip \baselineskip
\halign{\tt #\hfil &\qquad#&\tt #\hfil\cr
{\bf Old Command}         &&{\bf New Command}\cr
add-syms                  &&add-symbol-file\cr
delete environment        &&unset environment\cr
info convenience          &&show convenience\cr
info copying              &&show copying\cr
info directories          &&show directories     \cr
info editing              &&show commands\cr
info history              &&show values\cr
info targets              &&help target\cr
info values               &&show values\cr
info version              &&show version\cr
info warranty             &&show warranty\cr
set{\rm / }show addressprint     &&set{\rm / }show print address\cr
set{\rm / }show array-max        &&set{\rm / }show print elements\cr
set{\rm / }show arrayprint       &&set{\rm / }show print array\cr
set{\rm / }show asm-demangle     &&set{\rm / }show print asm-demangle\cr
set{\rm / }show caution          &&set{\rm / }show confirm\cr
set{\rm / }show demangle         &&set{\rm / }show print demangle\cr
set{\rm / }show history write    &&set{\rm / }show history save\cr
set{\rm / }show prettyprint      &&set{\rm / }show print pretty\cr
set{\rm / }show screen-height    &&set{\rm / }show height\cr
set{\rm / }show screen-width     &&set{\rm / }show width\cr
set{\rm / }show sevenbit-strings &&set{\rm / }show print sevenbit-strings\cr
set{\rm / }show unionprint       &&set{\rm / }show print union\cr
set{\rm / }show vtblprint        &&set{\rm / }show print vtbl\cr
\cr
unset                     &&\rm(No longer an alias for delete)\cr
}
@end tex

@node Installing _GDBN__, Copying, Renamed Commands, Top
@appendix Installing _GDBN__
@cindex configuring _GDBN__
@cindex installation

_GDBN__ is distributed with a @code{configure} script that automates the
process of preparing _GDBN__ for installation; you can then use
@code{make} to build the @code{_GDBP__} program.

The @code{configure} script that's specific to _GDBN__ is distributed in
the main _GDBN__ source directory.  However, building _GDBN__ also
requires several other directories of source common to multiple @sc{gnu}
programs.  These directories (@sc{gnu} libraries and includes) are
distributed separately, but their @code{configure} scripts and
@code{Makefile}s are designed to work together.  To ensure that
_GDBN__'s @code{Makefile} can find all the pieces, you should make a
single overall directory to hold the directories of source for @sc{gnu}
libraries and includes, and you should install the _GDBN__ source
directory there too.  In this Appendix, we refer to the directory of
@sc{gnu} source directories as @var{gnusrc}.

At a minimum, to build _GDBN__ you need the directories
@table @code
@item @var{gnusrc}/gdb
the source specific to _GDBN__ itself

@item @var{gnusrc}/bfd
source for the Binary File Descriptor Library

@item @var{gnusrc}/include
@sc{gnu} include files

@item @var{gnusrc}/libiberty
source for the @samp{-liberty} free software library

@item @var{gnusrc}/readline
source for the @sc{gnu} command-line interface
@end table
@noindent
Each of these directories has its own @code{configure} script.
@var{gnusrc} has an overall @code{configure} script, which is
distributed with the @sc{gnu} libraries and includes.

@code{configure} is designed to be called recursively, so it is most
convenient to run @code{configure} from the @var{gnusrc} directory. 
The simplest way to configure and build _GDBN__ is the following:
@example
cd @var{gnusrc}
./configure @var{host}
make
@end example
@noindent
where @var{host} is something like @samp{sun4} or @samp{vax}, that
identifies the platform where _GDBN__ will run.  This builds the three
libraries @file{bfd}, @file{readline}, and @file{libiberty}, then
@code{gdb} itself.  The configured source files, and the binaries, are
left in the corresponding source directories.

You can install @code{_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; some systems
refuse to let _GDBN__ debug child processes whose programs are not
readable, and _GDBN__ uses the shell to start your program.

@menu
* Subdirectories::		Configuration subdirectories
* configure Options::		Summary of options for configure
* Formatting Manual::		How to format and print this manual
@end menu


@node Subdirectories, configure Options, Installing _GDBN__, Installing _GDBN__
@section Configuration Subdirectories
If you build _GDBN__ for several host or target machines, and if
your @code{make} program handles the @samp{VPATH} feature
(@sc{gnu} @code{make} does), it is most convenient instead to build
the different _GDBN__ configurations in subdirectories (separate from
the source).  @code{configure} does this for you when you simultaneously
specify several configurations; but it's a good habit even for a single
configuration.  You can specify the use of subdirectories using the
@samp{+forcesubdirs} option (abbreviated @samp{+f}).  For example, you
can build _GDBN__ on a Sun 4 as follows:

@example
@group
cd @var{gnusrc}
./configure +f sun4
cd Host-sun4/Target-sun4
make
@end group
@end example

When @code{configure} uses subdirectories to build programs or
libraries, it creates nested directories
@file{Host-@var{host}/Target-@var{machine}}.  This is because _GDBN__
can be configured for cross-compiling: _GDBN__ can run on one machine
(the host) while debugging programs that run on another machine (the
target).  You specify cross-debugging targets by giving the
@samp{+target=@var{machine}} option to @code{configure}.  Specifying
only hosts still gives you two levels of subdirectory for each host,
with the same machine-name suffix on both.  On the other hand, whenever
you specify both hosts and targets on the same command line,
@code{configure} creates all combinations of the hosts and targets you
list.@refill

When you run @code{make} to build a program or library, you must run it
in a configured directory.  If you made a single configuration,
without subdirectories, run @code{make} in the source directory.
If you have @file{Host-@var{host}/Target-@var{machine}} subdirectories,
run @code{make} in those subdirectories.  

Each @code{configure} and @code{Makefile} under each source directory
runs recursively, so that typing @code{make} in @var{gnusrc} (or in a
@file{@var{gnusrc}/Host-@var{host}/Target-@var{machine}} subdirectory)
builds all the required libraries, then _GDBN__.@refill

If you run @code{configure} from a directory (such as @var{gnusrc}) that
contains source directories for multiple libraries or programs,
@code{configure} creates the @file{Host-@var{host}/Target-@var{machine}}
subdirectories in each library or program's source directory.  For
example, typing:
@example
cd @var{gnusrc}
configure sun4 +target=vx960
@end example
@noindent
creates the following directories:
@example
@var{gnusrc}/Host-sun4/Target-vx960
@var{gnusrc}/bfd/Host-sun4/Target-vx960
@var{gnusrc}/gdb/Host-sun4/Target-vx960
@var{gnusrc}/libiberty/Host-sun4/Target-vx960
@var{gnusrc}/readline/Host-sun4/Target-vx960
@end example
@noindent
The @code{Makefile} in @file{@var{gnusrc}/Host-sun4/Target-vx960}
will @code{cd} to the appropriate lower-level directories
(such as @file{@var{gnusrc}/bfd/Host-sun4/Target-vx960}), building each
in turn.

When you have multiple hosts or targets configured, 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 configure Options, Formatting Manual, Subdirectories, Installing _GDBN__
@section @code{configure} Options

Here is a summary of all the @code{configure} options and arguments that
you might use for building _GDBN__:

@example
configure @r{[}+destdir=@var{dir}@r{]} @r{[}+forcesubdirs@r{]} @r{[}+norecur@r{]} @r{[}+rm@r{]}
          @r{[}+target=@var{machine}@dots{}@r{]} @var{host}@dots{}
@end example
@noindent
You may introduce options with the character @samp{-} rather than
@samp{+} if you prefer; but options introduced with @samp{+} may be truncated.

@table @code
@item +destdir=@var{dir}
@var{dir} is an installation directory @emph{path prefix}.  After you
configure with this option, @code{make install} will install _GDBN__ as
@file{@var{dir}/bin/_GDBP__}, and the libraries in @file{@var{dir}/lib}.
If you specify @samp{+destdir=/usr/local}, for example, @code{make
install} creates @file{/usr/local/bin/gdb}.@refill

@item +forcesubdirs
Write configuration specific files in subdirectories of the form
@example
Host-@var{machine}/Target-@var{machine}
@end example
@noindent
(and configure the @code{Makefile} to write binaries there too).
Without this option, if you specify only one configuration for _GDBN__,
@code{configure} will use the same directory for source, configured
files, and binaries.  This option is used automatically if you specify
more than one @var{host} or more than one @samp{+target=@var{machine}}
option on the @code{configure} command line.

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

@item +rm
Remove the configuration specified by other arguments.

@item +target=@var{machine} @dots{}
Configure _GDBN__ for cross-debugging programs running on each specified
@var{machine}.  You may specify as many @samp{+target} options as you
wish.  To see a list of available targets, execute @samp{ls tconfig} in
the _GDBN__ source directory.  Without this option, _GDBN__ is
configured to debug programs that run on the same machine (@var{host})
as _GDBN__ itself.

@item @var{host} @dots{}
Configure _GDBN__ to run on each specified @var{host}.  You may specify as
many host names as you wish.  To see a list of available hosts, execute
@samp{ls xconfig} in the _GDBN__ source directory.
@end table

@noindent
@code{configure} accepts other options, for compatibility with
configuring other @sc{gnu} tools recursively; but these are the only
options that affect _GDBN__ or its supporting libraries.

@node Formatting Manual,  , configure Options, Installing _GDBN__
@section Formatting this Manual

To format the _GDBN__ manual as an Info file, you need the @sc{gnu}
@code{makeinfo} program.  Once you have it, you can type
@example
cd @var{gnusrc}/gdb
make gdb.info
@end example
@noindent
to make the Info file.

If you want to format and print copies of this manual, you need several
things: 
@itemize @bullet
@item 
@TeX{}, the public domain typesetting program written by Donald Knuth,
must be installed on your system and available through your execution
path.
@item 
@file{@var{gnusrc}/texinfo}: @TeX{} macros defining the @sc{gnu}
Documentation Format.
@item
@emph{A @sc{dvi} output program.}  @TeX{} doesn't actually make marks on
paper; it produces output files called @sc{dvi} files.  If your system
has @TeX{} installed, chances are it has a program for printing out
these files; one popular example is @code{dvips}, which can print
@sc{dvi} files on PostScript printers.
@end itemize
@noindent
Once you have these things, you can type
@example
cd @var{gnusrc}/gdb
make gdb.dvi
@end example
@noindent
to format the text of this manual, and print it with the usual output
method for @TeX{} @sc{dvi} files at your site.

@node Copying, Index, Installing _GDBN__, Top
@unnumbered GNU GENERAL PUBLIC LICENSE
@center Version 2, June 1991

@display
Copyright @copyright{} 1989, 1991 Free Software Foundation, Inc.
675 Mass Ave, Cambridge, MA 02139, USA

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

@unnumberedsec Preamble

  The licenses for most software are designed to take away your
freedom to share and change it.  By contrast, the GNU General Public
License is intended to guarantee your freedom to share and change free
software---to make sure the software is free for all its users.  This
General Public License applies to most of the Free Software
Foundation's software and to any other program whose authors commit to
using it.  (Some other Free Software Foundation software is covered by
the GNU Library General Public License instead.)  You can apply it to
your programs, too.

  When we speak of free software, we are referring to freedom, not
price.  Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
this service if you wish), that you receive source code or can get it
if you want it, that you can change the software or use pieces of it
in new free programs; and that you know you can do these things.

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

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

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

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

  Finally, any free program is threatened constantly by software
patents.  We wish to avoid the danger that redistributors of a free
program will individually obtain patent licenses, in effect making the
program proprietary.  To prevent this, we have made it clear that any
patent must be licensed for everyone's free use or not licensed at all.

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

@iftex
@unnumberedsec TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
@end iftex
@ifinfo
@center TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
@end ifinfo

@enumerate
@item
This License applies to any program or other work which contains
a notice placed by the copyright holder saying it may be distributed
under the terms of this General Public License.  The ``Program'', below,
refers to any such program or work, and a ``work based on the Program''
means either the Program or any derivative work under copyright law:
that is to say, a work containing the Program or a portion of it,
either verbatim or with modifications and/or translated into another
language.  (Hereinafter, translation is included without limitation in
the term ``modification''.)  Each licensee is addressed as ``you''.

Activities other than copying, distribution and modification are not
covered by this License; they are outside its scope.  The act of
running the Program is not restricted, and the output from the Program
is covered only if its contents constitute a work based on the
Program (independent of having been made by running the Program).
Whether that is true depends on what the Program does.

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

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

@item
You may modify your copy or copies of the Program or any portion
of it, thus forming a work based on the Program, and copy and
distribute such modifications or work under the terms of Section 1
above, provided that you also meet all of these conditions:

@alphaenumerate
@item
You must cause the modified files to carry prominent notices
stating that you changed the files and the date of any change.

@item
You must cause any work that you distribute or publish, that in
whole or in part contains or is derived from the Program or any
part thereof, to be licensed as a whole at no charge to all third
parties under the terms of this License.

@item
If the modified program normally reads commands interactively
when run, you must cause it, when started running for such
interactive use in the most ordinary way, to print or display an
announcement including an appropriate copyright notice and a
notice that there is no warranty (or else, saying that you provide
a warranty) and that users may redistribute the program under
these conditions, and telling the user how to view a copy of this
License.  (Exception: if the Program itself is interactive but
does not normally print such an announcement, your work based on
the Program is not required to print an announcement.)
@end alphaenumerate

These requirements apply to the modified work as a whole.  If
identifiable sections of that work are not derived from the Program,
and can be reasonably considered independent and separate works in
themselves, then this License, and its terms, do not apply to those
sections when you distribute them as separate works.  But when you
distribute the same sections as part of a whole which is a work based
on the Program, the distribution of the whole must be on the terms of
this License, whose permissions for other licensees extend to the
entire whole, and thus to each and every part regardless of who wrote it.

Thus, it is not the intent of this section to claim rights or contest
your rights to work written entirely by you; rather, the intent is to
exercise the right to control the distribution of derivative or
collective works based on the Program.

In addition, mere aggregation of another work not based on the Program
with the Program (or with a work based on the Program) on a volume of
a storage or distribution medium does not bring the other work under
the scope of this License.

@item
You may copy and distribute the Program (or a work based on it,
under Section 2) in object code or executable form under the terms of
Sections 1 and 2 above provided that you also do one of the following:

@alphaenumerate
@item
Accompany it with the complete corresponding machine-readable
source code, which must be distributed under the terms of Sections
1 and 2 above on a medium customarily used for software interchange; or,

@item
Accompany it with a written offer, valid for at least three
years, to give any third party, for a charge no more than your
cost of physically performing source distribution, a complete
machine-readable copy of the corresponding source code, to be
distributed under the terms of Sections 1 and 2 above on a medium
customarily used for software interchange; or,

@item
Accompany it with the information you received as to the offer
to distribute corresponding source code.  (This alternative is
allowed only for noncommercial distribution and only if you
received the program in object code or executable form with such
an offer, in accord with Subsection b above.)
@end alphaenumerate

The source code for a work means the preferred form of the work for
making modifications to it.  For an executable work, complete source
code means all the source code for all modules it contains, plus any
associated interface definition files, plus the scripts used to
control compilation and installation of the executable.  However, as a
special exception, the source code distributed need not include
anything that is normally distributed (in either source or binary
form) with the major components (compiler, kernel, and so on) of the
operating system on which the executable runs, unless that component
itself accompanies the executable.

If distribution of executable or object code is made by offering
access to copy from a designated place, then offering equivalent
access to copy the source code from the same place counts as
distribution of the source code, even though third parties are not
compelled to copy the source along with the object code.

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

@item
You are not required to accept this License, since you have not
signed it.  However, nothing else grants you permission to modify or
distribute the Program or its derivative works.  These actions are
prohibited by law if you do not accept this License.  Therefore, by
modifying or distributing the Program (or any work based on the
Program), you indicate your acceptance of this License to do so, and
all its terms and conditions for copying, distributing or modifying
the Program or works based on it.

@item
Each time you redistribute the Program (or any work based on the
Program), the recipient automatically receives a license from the
original licensor to copy, distribute or modify the Program subject to
these terms and conditions.  You may not impose any further
restrictions on the recipients' exercise of the rights granted herein.
You are not responsible for enforcing compliance by third parties to
this License.

@item
If, as a consequence of a court judgment or allegation of patent
infringement or for any other reason (not limited to patent issues),
conditions are imposed on you (whether by court order, agreement or
otherwise) that contradict the conditions of this License, they do not
excuse you from the conditions of this License.  If you cannot
distribute so as to satisfy simultaneously your obligations under this
License and any other pertinent obligations, then as a consequence you
may not distribute the Program at all.  For example, if a patent
license would not permit royalty-free redistribution of the Program by
all those who receive copies directly or indirectly through you, then
the only way you could satisfy both it and this License would be to
refrain entirely from distribution of the Program.

If any portion of this section is held invalid or unenforceable under
any particular circumstance, the balance of the section is intended to
apply and the section as a whole is intended to apply in other
circumstances.

It is not the purpose of this section to induce you to infringe any
patents or other property right claims or to contest validity of any
such claims; this section has the sole purpose of protecting the
integrity of the free software distribution system, which is
implemented by public license practices.  Many people have made
generous contributions to the wide range of software distributed
through that system in reliance on consistent application of that
system; it is up to the author/donor to decide if he or she is willing
to distribute software through any other system and a licensee cannot
impose that choice.

This section is intended to make thoroughly clear what is believed to
be a consequence of the rest of this License.

@item
If the distribution and/or use of the Program is restricted in
certain countries either by patents or by copyrighted interfaces, the
original copyright holder who places the Program under this License
may add an explicit geographical distribution limitation excluding
those countries, so that distribution is permitted only in or among
countries not thus excluded.  In such case, this License incorporates
the limitation as if written in the body of this License.

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

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

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

@iftex
@heading NO WARRANTY
@end iftex
@ifinfo
@center NO WARRANTY
@end ifinfo

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

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

@iftex
@heading END OF TERMS AND CONDITIONS
@end iftex
@ifinfo
@center END OF TERMS AND CONDITIONS
@end ifinfo

@page
@unnumberedsec Applying These Terms to Your New Programs

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

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

@smallexample
@var{one line to give the program's name and a brief idea of what it does.}
Copyright (C) 19@var{yy}  @var{name of author}

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

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

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

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

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

@smallexample
Gnomovision version 69, Copyright (C) 19@var{yy} @var{name of author}
Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
This is free software, and you are welcome to redistribute it
under certain conditions; type `show c' for details.
@end smallexample

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

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

@example
Yoyodyne, Inc., hereby disclaims all copyright interest in the program
`Gnomovision' (which makes passes at compilers) written by James Hacker.

@var{signature of Ty Coon}, 1 April 1989
Ty Coon, President of Vice
@end example

This General Public License does not permit incorporating your program into
proprietary programs.  If your program is a subroutine library, you may
consider it more useful to permit linking proprietary applications with the
library.  If this is what you want to do, use the GNU Library General
Public License instead of this License.


@node Index,  , Copying, 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\/} and}
\centerline{{\sl\fontname\tensl\/}}
\centerline{are used for emphasis.}\vfill}
\page\colophon
% Blame: [email protected], 28mar91.
@end tex

@contents
@bye