RENAMES: "demangle" and "asm-demangle" now "set print" subcmds.

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

\input texinfo
@setfilename _GDBP__.info
@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 FULL SOURCE.  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 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).
    To permit maximum flexibility, the full source also does not contain
any "info" markup that can be generated automatically; you should first
preprocess it as above, then run it through C-u texinfo-master-menu,
before actually info-formatting it.
_fi__(0)
@c
@syncodeindex ky cp
@c FOR UPDATES LEADING TO THIS DRAFT, GDB CHANGELOG CONSULTED BETWEEN:
@c Tue Feb 26 01:47:07 1991  Cygnus John Gilmore  (cygnus at yuba)
@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 author 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
@c @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 April 1991
@author{Richard M. Stallman}
@author{Roland H. Pesch --- Cygnus Support}
@page

@tex
\def\$#1${{#1}}  % Kluge: collect RCS revision info without $...$
\xdef\manvers{\$Revision$}  % For use in headers, footers too
{\parskip=0pt
\hfill Cygnus Support\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 author instead of in the
original English.
@end titlepage
@page

@node 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:

@enumerate
@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 enumerate

_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.

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

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

For full details, @pxref{License}.

@node New Features,,,
@unnumbered New Features in _GDBN__ version 4.0

@itemize @bullet
@item
TARGETS: Using the new command @samp{target}, you can select at runtime
whether you are debugging local files, local processes, standalone
systems over the serial port, realtime systems over a TCP/IP
connection, etc.  _GDBN__ now uses a function vector to mediate access to
all the different possible targets, making it much easier to add
support for new remote protocols.

@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 the inferior
process where this may happen. 

@item
OBJECT CODE FORMATS: _GDBN__ uses a new scheme called Binary File
Descriptors (BFD) to permit it to switch dynamically, without
reconfiguration or recompilation, between different object-file
formats.  Formats currently supported are COFF, a.out, and the new
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: You must still choose a particular machine architecture
and operating system for _GDBN__'s host and target systems when _GDBN__ is built.
The script @samp{config.gdb} now handles specification of separate host
and target configurations.

@item
INTERACTION: _GDBN__ now uses the GNU readline interface to read its
input; this provides inline editing of commands, using the familiar
Emacs or @code{vi} keymaps, and command-history support. The user interface
to _GDBN__'s control variables has been simplified and consolidated in two
commands, @samp{set} and @samp{show}.  Output lines are now broken at
readable places, rather than overflowing onto the next line.

@item
SOURCE LANGUAGE: _GDBN__ now understands C++ source as well as C. Multiple
inheritance is supported when used with G++ 2.0. There is also 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.  You can suppress output of machine-level addresses,
displaying only source language information.

@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 itemize

@node Sample Session,,,
@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.

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

@c FIXME: this example may not be appropriate for some configs, where
@c FIXME...primary interest is in remote use.
We'll be using _GDBN__ to inspect GNU @code{m4} (a generic macro
processor).

_0__@smallexample
$ @var{cd gm4/common}

$ @var{_GDBP__ m4}
Reading symbol data from m4...done.
(_GDBP__) set width 70
@end smallexample

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

@smallexample
(_GDBP__) @var{break m4_changequote}
Breakpoint 1 at 0x59d4: file builtin.c, line 812.
@end smallexample

@noindent 
We've chosen to see how the @code{m4} builtin @samp{changequote} works.
Having looked at the source, we knew the relevant subroutine is
@samp{m4_changequote}.  We've set a breakpoint there with _GDBN__'s
@code{break} command.

@smallexample
(_GDBP__) @var{run}
Starting program: /s1/gnu/src/gm4/common/m4 
@var{`usual' quotes <not these>}
usual quotes <not these>
@end smallexample

@noindent
Using the @samp{run} command, we've started @code{m4} running under
_GDBN__ control; while we don't touch the @samp{m4_changequote}
subroutine, the program runs as usual---it filters standard input. 

@smallexample
@var{changequote(<,>)}

Breakpoint 1, m4_changequote (argc=3, argv=0x2b958) at builtin.c:812
812     if (bad_argc(TOKEN_DATA_TEXT(argv[0]), argc, 1, 3))
@end smallexample
@noindent
To trigger the breakpoint, we called @code{changequote}.  _GDBN__
suspended execution of @code{m4}, displaying information about the
context where it stopped.

@group
@smallexample
(_GDBP__) @var{s}
bad_argc (name=0xf851cfb4<Address 0xf851cfb4 out of bounds>, argc=3,
    min=1, max=3) at builtin.c:230
230     if (min > 0 && argc < min) @{
@end smallexample
@noindent
We've used the command @samp{s} (@code{step}) to advance execution to
the next source line; since there was a subroutine call, we've stopped
in the first line of that subroutine, not in the next line of
@code{m4_changequote}.
@end group

The command @samp{next} would have taken us to the next line of
@code{m4_changequote}.  To see where we are in the stack, we can use the
@samp{backtrace} command (which can also be spelled @samp{bt}).

@smallexample
(_GDBP__) @var{bt}
#0  bad_argc (name=0xf851cfb4<Address 0xf851cfb4 out of bounds>, 
    argc=3, min=1, max=3) at builtin.c:230
#1  0x59ec in m4_changequote (argc=3, argv=0x2b958) at builtin.c:812
#2  0x6e38 in expand_macro (sym=0x2b060) at macro.c:242
#3  0x6840 in expand_token (obs=0x0, t=176224, td=0xf7fffb08)
    at macro.c:71
#4  0x6794 in expand_input () at macro.c:40
#5  0x28dc in main (argc=0, argv=0xf7fffbf8) at m4.c:174
@end smallexample

@noindent
We'll tell _GDBN__ to finish execution of this subroutine, to get back
to @code{m4_changequote}.

@smallexample
(_GDBP__) @var{finish}
Run till exit from #0  bad_argc (name=0xf851cfb4<Address 0xf851cfb4 out
 of bounds>, 
    argc=3, min=1, max=3) at builtin.c:230
0x59ec in m4_changequote (argc=3, argv=0x2b958) at builtin.c:812
812     if (bad_argc(TOKEN_DATA_TEXT(argv[0]), argc, 1, 3))
Value returned is $1 = false
@end smallexample

@noindent
We're still in the middle of the first line of @code{m4_changequote};
@samp{finish} just allowed the subroutine call to complete.
The display beginning ``@code{0x59ec in}@dots'', preceding the
display of line @code{812}, is a reminder of that situation from
_GDBN__.

Now that we're past the subroutine call, using the @code{step} command
again will get us to the next line of @code{m4_changequote}.

@smallexample
(_GDBP__) @var{s}
815     lquote = (argc >= 2) ? TOKEN_DATA_TEXT(argv[1])[0] : DEF_LQUOTE;
(_GDBP__) @var{whatis lquote}
type = char
(_GDBP__) @var{p lquote}
$2 = 96 '`'
@end smallexample

@noindent
When we stepped to another line, @code{m4} was about to set a variable
@samp{lquote}; we inspected its type with @samp{whatis} and its value
with @samp{p} (the @samp{print} command).  We can see some context by
displaying the surrounding source code, with the @samp{l} (@code{list})
command. 

@smallexample
(_GDBP__) @var{l}
810     token_data **argv;
811     @{
812     if (bad_argc(TOKEN_DATA_TEXT(argv[0]), argc, 1, 3))
813     return;
814     
815     lquote = (argc >= 2) ? TOKEN_DATA_TEXT(argv[1])[0] : DEF_LQUOTE;
816     rquote = (argc >= 3) ? TOKEN_DATA_TEXT(argv[2])[0] : DEF_RQUOTE;
817     @}
818     
819     /* 
(_GDBP__) @var{s}
816     rquote = (argc >= 3) ? TOKEN_DATA_TEXT(argv[2])[0] : DEF_RQUOTE;
(_GDBP__) @var{s}
817     @}
(_GDBP__) @var{p lquote}
$3 = 60 '<'
(_GDBP__) @var{p rquote}
$4 = 62 '>'
@end smallexample

@noindent
We proceeded past another line with @samp{s}, and inspected the new
values of @code{m4}'s internal variables @code{rquote} and
@code{lquote}.

Since we're done with our inspection of this subroutine, we'll tell
_GDBN__ to allow @code{m4} to continue running, with the @samp{c}
(@code{continue}) command:

@smallexample
(_GDBP__) @var{c}
Continuing.

@var{`usual' quotes <not these>}
`usual' quotes not these

Program exited normally.
(_GDBP__) @var{quit}

$ 
_1__@end smallexample

@noindent 
Finally, when we ended the @code{m4} run, _GDBN__ told us
``@code{Program exited normally.}'' We ended our _GDBN__ session with
the _GDBN__ @samp{quit} command.


@node Invocation,,,
@chapter Getting In and Out of _GDBN__

@node Starting _GDBN__,,,
@section Starting _GDBN__

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

You can start by just calling @samp{_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

You can further control how _GDBN__ starts up by using command-line
options.

_GDBN__ itself can remind you of the options available:
@example
gdb -help
@end example
@noindent
will display all available options and briefly describe their use
(@samp{gdb -h} is a shorter equivalent).

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

@node File Options,,,
@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.

@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

@node Mode Options,,, 
@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 code @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
This option is used when Emacs 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 bps) of any serial interface used by
_GDBN__ for remote debugging.

@item -tty @var{device}
Run using @code{device} for your program's standard input and output.
@end table

_if__(_I960__)
@node i960-Nindy Remote,,,
@subsection _GDBN__ with a Remote i960 (Nindy)

``Nindy'' is the name of a ROM Monitor program for Intel 960 target
systems.  When _GDBN__ is configured to control a remote Intel 960 using
Nindy, you can tell _GDBN__ how to connect to the 960 in several ways:

@itemize @bullet
@item
Through command line options specifying device, baud rate, and protocol;

@item
By responding to a prompt on startup;

@item
By using the @samp{target} command at any point during your _GDBN__ session.
@end itemize

@node Nindy Startup,,,
@subsubsection Startup with Nindy

The command-line options for Nindy are detailed below.  If you simply
start @code{_GDBP__} without using options to specify a serial port, you are
prompted for it, @emph{before} you reach the ordinary _GDBN__ prompt:
@example
Attach /dev/ttyNN -- specify NN, or "quit" to quit:  
@end example
@noindent
You can, if you choose, simply start up with no Nindy connection by
responding to the prompt with an empty line.  If you do this, and later
wish to attach to Nindy, use @samp{target} (@pxref{Target Commands}). 

@node Nindy Options,,,
@subsubsection Options for Nindy

These are the startup options for beginning your _GDBN__ session with a
Nindy-960 board attached:

@table @code
@item -r @var{port}
Specify the serial port name of a serial interface to be used to connect
to the target system.  This option is only available when _GDBN__ is
configured for the Intel 960 target architecture.  You may specify
@var{port} as any of: a full pathname (e.g. @samp{-r /dev/ttya}), a
device name in @samp{/dev} (e.g. @samp{-r ttya}), or simply the unique
suffix for a specific @code{tty} (e.g. @samp{-r a}).

@item -O
(An uppercase letter ``O'', not a zero.)  Specify that _GDBN__ should use
the ``old'' Nindy monitor protocol to connect to the target system.
This option is only available when _GDBN__ is configured for the Intel 960
target architecture.

@quotation
@emph{Warning:} if you specify @samp{-O}, but are actually trying to
connect to a target system that expects the newer protocol, the connection
will fail, appearing to be a speed mismatch.  _GDBN__ will repeatedly
attempt to reconnect at several different line speeds.  You can abort
this process with an interrupt.
@end quotation

@item -brk
Specify that _GDBN__ should first send a @samp{BREAK} signal to the target
system, in an attempt to reset it, before connecting to a Nindy target.

@quotation
@emph{Warning:} Many target systems do not have the hardware that this
requires; it only works with a few boards.
@end quotation

@end table

The standard @samp{-b} option controls the line speed used on the serial
port. 

@group
@node Nindy reset,,,
@subsubsection Nindy Reset Command
@table @code
@item reset
@kindex reset
For a Nindy target, this command sends a ``break'' to the remote target
system; this is only useful if the target has been equipped with a
circuit to perform a hard reset (or some other interesting action) when
a break is detected.
@end table
@end group
_fi__(_I960__)

_if__(_AMD29K__)
@node EB29K Remote,,,
@subsection _GDBN__ with a Remote EB29K

@cindex EB29K board
@cindex running 29K programs
@cindex 29K

To use _GDBN__ from a Unix system to run programs on AMD's EB29K
board in a PC, you must first connect a serial cable between the PC
and a serial port on the Unix system.  In the following, we assume
you've hooked the cable between the PC's @samp{COM1} port and
@samp{/dev/ttya} on the Unix system.

@node Comms (EB29K),,,
@subsubsection Communications Setup
The next step is to set up the PC's port, by doing something like the
following in DOS on the PC:
_0__@example
C:\> MODE com1:9600,n,8,1,none
_1__@end example
@noindent
This example---run on an MS DOS 4.0 system---sets the PC port to 9600
bps, no parity, eight data bits, one stop bit, and no ``retry'' action;
you must match the communications parameters when establishing the Unix
end of the connection as well.
@c FIXME: Who knows what this "no retry action" crud from the DOS manual may
@c       mean?  It's optional; leave it out? ---pesch@cygnus.com, 25feb91 

To give control of the PC to the Unix side of the serial line, type
the following at the DOS console:
_0__@example
C:\> CTTY com1
_1__@end example
@noindent
(Later, if you wish to return control to the DOS console, you can use
the command @samp{CTTY con}---but you must send it over the device that
had control, in our example over the @samp{COM1} serial line).

From the Unix host, use a communications program such as @code{tip} or
@code{cu} to communicate with the PC; for example,
@example
cu -s 9600 -l /dev/ttya
@end example
@noindent
The @code{cu} options shown specify, respectively, the linespeed and the
serial port to use.  If you use @code{tip} instead, your command line
may look something like the following instead:
@example
tip -9600 /dev/ttya
@end example
@noindent
Your system may define a different name where our example uses
@samp{/dev/ttya} as the argument to @code{tip}.  The communications
parameters, including what port to use, are associated with the
@code{tip} argument in the ``remote'' descriptions file---normally the
system table @file{/etc/remote}.
@c FIXME: What if anything needs doing to match the "n,8,1,none" part of
@c the DOS side's comms setup?  cu can support -o (odd
@c parity), -e (even parity)---apparently no settings for no parity or
@c for character size.  Taken from stty maybe...?  John points out tip
@c can set these as internal variables, eg ~s parity=none; man stty
@c suggests that it *might* work to stty these options with stdin or
@c stdout redirected... ---pesch@cygnus.com, 25feb91

@kindex EBMON
Using the @samp{tip} or @samp{cu} connection, change the DOS working
directory to the directory containing a copy of your 29K program, then
start the PC program @samp{EBMON} (an EB29K control program supplied
with your board by AMD).  You should see an initial display from
@code{EBMON} similar to the one that follows, ending with the
@code{EBMON} prompt @samp{#}---
_0__@example
C:\> G:

G:\> CD \usr\joe\work29k

G:\USR\JOE\WORK29K> EBMON
Am29000 PC Coprocessor Board Monitor, version 3.0-18
Copyright 1990 Advanced Micro Devices, Inc.
Written by Gibbons and Associates, Inc.

Enter '?' or 'H' for help

PC Coprocessor Type   = EB29K
I/O Base              = 0x208
Memory Base           = 0xd0000

Data Memory Size      = 2048KB
Available I-RAM Range = 0x8000 to 0x1fffff
Available D-RAM Range = 0x80002000 to 0x801fffff

PageSize              = 0x400
Register Stack Size   = 0x800
Memory Stack Size     = 0x1800

CPU PRL               = 0x3
Am29027 Available     = No
Byte Write Available  = Yes

# ~.
_1__@end example

Then exit the @code{cu} or @code{tip} program (done in the example by
typing @code{~.} at the @code{EBMON} prompt).  @code{EBMON} will keep
running, ready for _GDBN__ to take over.

For this example, we've assumed what is probably the most convenient
way to make sure the same 29K program is on both the PC and the Unix
system: a PC/NFS connection that establishes ``drive @code{G:}'' on the
PC as a file system on the Unix host.  If you don't have PC/NFS or
something similar connecting the two systems, you must arrange some
other way---perhaps floppy-disk transfer---of getting the 29K program
from the Unix system to the PC; _GDBN__ will @emph{not} download it over the
serial line.

@node _GDBP__-EB29K,,,
@subsubsection EB29K cross-debugging
Finally, @code{cd} to the directory containing an image of your 29K
program on the Unix system, and start _GDBN__---specifying as argument the
name of your 29K program:
@example
cd /usr/joe/work29k
_GDBP__ myfoo
@end example
Now you can use the @code{target} command:
@example
target amd-eb /dev/ttya 9600 MYFOO
@end example
@c FIXME: test above 'target amd-eb' as spelled, with caps!  caps are meant to
@c emphasize that this is the name as seen by DOS (since I think DOS is
@c single-minded about case of letters).  ---pesch@cygnus.com, 25feb91

@noindent
In this example, we've assumed your program is in a file called
@samp{myfoo}.  Note that the filename given as the last argument to
@samp{target amd-eb} should be the name of the program as it appears to DOS.
In our example this is simply @samp{MYFOO}, but in general it can include
a DOS path, and depending on your transfer mechanism may not resemble
the name on the Unix side.

At this point, you can set any breakpoints you wish; when you're ready
to see your program run on the 29K board, use the _GDBN__ command
@example
run
@end example

To stop debugging the remote program, use the _GDBN__ @samp{detach}
command.  

To return control of the PC to its console, use @code{tip} or @code{cu}
once again, after your _GDBN__ session has concluded, to attach to
@code{EBMON}.  You can then type the command @samp{q} to shut down
@code{EBMON}, returning control to the DOS command-line interpreter.
Type @samp{CTTY con} to return command input to the main DOS console,
and type @samp{~.} to leave @code{tip} or @code{cu}.

@node Remote Log,,,
@subsubsection Remote Log
@kindex eb.log
@cindex log file for EB29K
The @samp{target amd-eb} command creates a file @file{eb.log} in the
current working directory, to help debug problems with the connection.
@file{eb.log} records all the output from @code{EBMON}, including echoes
of the commands sent to it.  Running @samp{tail -f} on this file in
another window often helps to understand trouble with @code{EBMON}, or
unexpected events on the PC side of the connection.
_fi__(_AMD29K__)

_if__(_VXWORKS__)
@node VxWorks Remote,,,
@subsection _GDBN__ and VxWorks
_GDBN__ enables developers to spawn and debug tasks running on networked
VxWorks targets from a Unix host.  Already-running tasks spawned from
the VxWorks shell can also be debugged.  _GDBN__ uses code that runs on
both the UNIX host and on the VxWorks target.  The program
@code{_GDBP__} is installed and executed on the UNIX host.  

The remote debugging interface (RDB) routines are installed and executed
on the VxWorks target.  These routines are included in the VxWorks library
@code{rdb.a} and are incorporated into the system image when source-level
debugging is enabled in the VxWorks configuration.

Defining @code{INCLUDE_RDB} in the VxWorks configuration file
@code{configAll.h} includes the RDB interface routines and spawns the
source debugging task @code{tRdbTask} when VxWorks is booted.  For more
information on configuring and remaking VxWorks, see the @cite{VxWorks
Programmer's Guide}.

Once you have included the RDB interface in your VxWorks system image
and set your Unix execution search path to find _GDBN__, you are ready
to run _GDBN__.  From your UNIX host, type:

@smallexample
% _GDBP__
@end smallexample

_GDBN__ will come up showing the prompt:

@smallexample
(_GDBP__)
@end smallexample

@node VxWorks connection,,,
@subsubsection Connecting to VxWorks

The _GDBN__ command @samp{target} lets you connect to a VxWorks target on the
network.  To connect to a target whose host name is ``@code{tt}'', type:

@smallexample
(_GDBP__) target vxworks tt
@end smallexample

_GDBN__ will display a message similar to the following:

@smallexample
Attaching remote machine across net... Success!
@end smallexample

_GDBN__ will then attempt to read the symbol tables of any object
modules loaded into the VxWorks target since it was last booted.
_GDBN__ locates these files by searching the directories listed in the
command search path (@pxref{Command Path}); if it fails to find an
object file, it will display a message such as:

@smallexample
prog.o: No such file or directory.
@end smallexample

This will cause the @samp{target} command to abort.  When this happens,
you should add the appropriate directory to the search path, with the
_GDBN__ command @samp{path}, and execute the @samp{target} command
again.

@node VxWorks download,,,
@subsubsection VxWorks Download

If you have connected to the VxWorks target and you want to debug an
object that has not yet been loaded, you can use the _GDBN__ @samp{load}
command to download a file from UNIX to VxWorks incrementally.  The
object file given as an argument to the @samp{load} command is actually
opened twice: first by the VxWorks target in order to download the code,
then by _GDBN__ in order to read the symbol table.  This can lead to
problems if the current working directories on the two systems differ.
It is simplest to set the working directory on both systems to the
directory in which the object file resides, and then to reference the
file by its name, without any path.  Thus, to load a program
@samp{prog.o}, residing in @code{wherever/vw/demo/rdb}, on VxWorks type:

@smallexample
-> cd "wherever/vw/demo/rdb"
@end smallexample

On _GDBN__ type:

@smallexample
(_GDBP__) cd wherever/vw/demo/rdb 
(_GDBP__) load prog.o
@end smallexample

_GDBN__ will display a response similar to the following:

@smallexample
Reading symbol data from wherever/vw/demo/rdb/prog.o... done.
@end smallexample

You can also use the @samp{load} command to reload an object module
after editing and recompiling the corresponding source file.  Note that
this will cause _GDBN__ to delete all currently-defined breakpoints,
auto-displays, and convenience variables, and to clear the value
history.  (This is necessary in order to preserve the integrity of
debugger data structures that reference the target system's symbol
table.)

@node VxWorks attach,,,
@subsubsection Running Tasks

You can also attach to an existing task using the @samp{attach} command as
follows:

@smallexample
(_GDBP__) attach @var{taskId}
@end smallexample

where @var{taskId} is the VxWorks hexadecimal task ID.  The task can be running
or suspended when you attach to it.  If running, it will be suspended at
the time of attachment.

_fi__(_VXWORKS__)

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

@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 @samp{detach} command; @pxref{Attach}.

@node Shell Commands,,,
@section Shell Commands
If you just need to execute occasional shell commands during your
debugging session, there's no need to leave or suspend _GDBN__; you can
just use the @samp{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}.
The environment variable @code{SHELL} is used if it exists, otherwise _GDBN__
uses @samp{/bin/sh}.
@end table

The utility @samp{make} is often needed in development environments.
You don't have to use the @samp{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,,,
@chapter _GDBN__ Commands

@node Command Syntax,,,
@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
@samp{step} accepts an argument which is the number of times to step,
as in @samp{step 5}.  You can also use the @samp{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.  Sometimes even ambiguous
abbreviations are allowed; for example, @samp{s} is specially defined as
equivalent to @samp{step} even though there are other commands whose
names start with @samp{s}.

@cindex repeating commands
A blank line as input to _GDBN__ means to repeat the previous command.
Certain commands (for example, @samp{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 @samp{list} and @samp{x} commands construct new arguments when
repeated, rather than repeating exactly as typed, to permit easy
scanning of source or memory.

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

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

@table @code
@item help
Used with no arguments, @samp{help} displays a short list of named
categories 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{category}
Using one of the general help categories as an argument, you can get a
list of the individual commands in a category.  For example, here is the
help display for category @samp{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 @samp{help} argument, _GDBN__ will display a
short paragraph on how to use that command.  
@end table

In addition to @samp{help}, you can use the _GDBN__ commands @samp{info}
and @samp{show} to inquire about the state of your program, or the state
of _GDBN__ itself.  Both commands support many ``sub-commands'', or
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
(@pxref{Index}).

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

@kindex show
@item show
In contrast, @samp{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 @samp{set}; for example, you can control what number
system is used for displays with @samp{set radix}, or simply inquire
which is currently in use with @samp{show radix}.  

@kindex info set
To display all the settable parameters and their current
values, you can use @samp{show} with no arguments; you may also use
@samp{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

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

@table @code
@kindex show version
@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,,,
@chapter Running Programs Under _GDBN__

@node Compilation,,,
@section Compiling for Debugging

In order to debug a program most 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,,,
@section Starting your Program
@cindex starting
@cindex running
@kindex run
To start your program under _GDBN__, use the @samp{run} command.  
_if__(_VXWORKS__)
Except on VxWorks, you
_fi__(_VXWORKS__)
_if__(!_VXWORKS__)
You
_fi__(!_VXWORKS__)
must first specify the program name with an argument to _GDBN__
(@pxref{Invocation}), or using the @samp{file} or @samp{exec-file}
command (@pxref{Files}).@refill

On targets that support processes, @samp{run} creates an inferior
process and makes that process run your program.  On other targets,
@samp{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
@samp{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
(for example regular expression expansion or variable substitution) in
describing the arguments.  In Unix systems, you can control which shell
is used with the @code{SHELL} environment variable.

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

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

@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 @samp{tty} command to
set a different device for your program.
@end table

When you issue the @samp{run} command, your program begins to execute
immediately.  @xref{Stopping}, for discussion of how to arrange for your
program to stop.

Note that once your program has been started by the @samp{run} command,
you may evaluate expressions that involve calls to functions in the
inferior, using the @samp{print} or @samp{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,,,
@section Your Program's Arguments

@cindex arguments (to your program)
The arguments to your program can be specified by the arguments of the
@samp{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 @samp{/bin/sh}.

@samp{run} with no arguments uses the same arguments used by the previous
@samp{run}, or those set by the @samp{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
@samp{set args} has no arguments, @samp{run} will execute your program
with no arguments.  Once you have run your program with arguments, this
is the only way to run it again without arguments.

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

@node Environment,,,
@section Your Program's Environment

@cindex environment (of your program)
The @dfn{environment} consists of a set of environment variables and
their values.  Environment variables conventionally record such things as
your user name, your home directory, your terminal type, and your search
path for programs to run.  Usually you set up environment variables with
the shell and they are inherited by all the other programs you run.  When
debugging, it can be useful to try running 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 a 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 the current working directory.

This command will not repeat if you press @key{RET} a second time after
using it once.

@item show environment @var{varname}
@kindex show environment
Print the value of environment variable @var{varname} to be given to
your program when it starts.

@item show environment
Print the names and values of all environment variables to be given to
your program.

@item set environment @var{varname} @var{value}
@itemx set environment @var{varname} = @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 FIXME: I think "any string" here doesn't include leading, trailing
@c FIXME... blanks. Queried J Gilmore.  ---pesch@cygnus.com, 4apr91

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}.

@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}=};
@samp{unset environment} removes the variable from the environment,
rather than assigning it an empty value.  
@end table

@node Working Directory,,,
@section Your Program's Working Directory

@cindex working directory (of your program)
Each time you start your program with @samp{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 @samp{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,,,
@section Your Program's Input and Output

@cindex redirection
@cindex i/o
@cindex terminal
@cindex controlling terminal
By default, the program you run under _GDBN__ does input and output to the same
terminal that _GDBN__ uses.

You can redirect the program's input and/or output using shell
redirection with the @samp{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
Another way to specify where the program should do input and output is
with the @samp{tty} command.  This command accepts a file name as
argument, and causes this file to be the default for future @samp{run}
commands.  It also resets the controlling terminal for the child
process, for future @samp{run} commands.  For example,

@example
tty /dev/ttyb
@end example

@noindent
directs that processes started with subsequent @samp{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 @samp{run} overrides the @samp{tty} command's
effect on input/output redirection, but not its effect on the
controlling terminal.

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

@node Attach,,,
@section Debugging an Already-Running Process
@kindex attach
@cindex attach

@table @code
@item attach @var{process-id}
If your currently selected target supports processes, this command
attaches to a running process---one that was started outside _GDBN__.
(@samp{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 @code{jobs -l}
shell command.   

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

To use @samp{attach}, you must have permission to send the process a
signal, and it must have the same effective user ID as the _GDBN__
process.

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

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 ordinarily available when you start
processes with @samp{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 @samp{continue} command after
attaching _GDBN__ to the process.

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

If you exit _GDBN__ or use the @samp{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 this happens by using the @samp{set caution} command
(@pxref{Messages/Warnings}).

@group
@node Kill Process,,,
@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.
@end group

On some operating systems, you can't execute your program in another
process while breakpoints are active inside _GDBN__.  You can use the
@samp{kill} command in this situation to permit running the program
outside the debugger.

The @samp{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 which is running in a process.  In this case, when you
next type @samp{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,,,
@chapter Stopping and Continuing

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

@node Breakpoints,,,
@section Breakpoints

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

Each breakpoint is assigned a number when it is created; these numbers are
successive integers starting with 1.  In many of the commands for controlling
various features of breakpoints you use the breakpoint number to say which
breakpoint you want to change.  Each breakpoint may be @dfn{enabled} or
@dfn{disabled}; if disabled, it has no effect on the program until you
enable it again.

@node Set Breaks,,,
@subsection Setting Breakpoints

@kindex break
@kindex watch
Breakpoints are set with the @samp{break} command (abbreviated @samp{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}.

@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
Set 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 equivalent to a @samp{finish} command in the frame
inside the selected frame.  If this is done in the innermost frame, _GDBN__
will stop the next time it reaches the current location; this may be
useful inside of 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.  @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 in the @samp{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
Set a breakpoint on all functions matching @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 @samp{break} command.  They can be deleted,
disabled, made conditional, etc., in the standard ways.

@kindex info breakpoints
@kindex $_
@item info breakpoints
The command @samp{info breakpoints} prints a list of all breakpoints set
and not deleted, showing their numbers, where in the program they are,
and any special features in use for them.  Disabled breakpoints are
included in the list, but marked as disabled.  @samp{info break} with a
breakpoint number as argument lists only that breakpoint.  The
convenience variable @code{$_} and the default examining-address for the
@samp{x} command are set to the address of the last breakpoint listed
(@pxref{Memory}).
@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,,,
@subsection Setting Watchpoints
@cindex watchpoints
A @dfn{watchpoint} is a special breakpoint that stops your program when
the value of an expression changes.  You can use a watchpoint to stop
execution whenever the value of an expression changes, without having to
predict a particular place in the inferior process where this may
happen.  Aside from the different syntax in setting a watchpoint, it is
managed exactly like any other breakpoint and is enabled, disabled, and
deleted using exactly the same commands.

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 implement this feature; 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 watch
@item info watch
This command prints a list of watchpoints. 
@end table

@node Exception Handling,,,
@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

Breakpoints can be set at active exception handlers by using the
@samp{catch} command.  @var{exceptions} is a list of names of exceptions
to catch.
@end table

You can use @samp{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__ will normally return
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 @samp{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.  

To stop just before an exception handler is called, you need some
knowledge of the implementation.  In the case of GNU C++ exception 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 **addr, void *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}).  If you set a breakpoint in an exception handler
instead, it may not be easy to find out where the exception was raised.

With a conditional breakpoint (@xref{Conditions}) that depends on the
value of @code{id}, you can cause the debugger to stop only when a
specific exception is raised.  Multiple conditional breakpoints can be
used to stop the program when any of a number of exceptions are raised.

@node Delete Breaks,,,
@subsection Deleting Breakpoints

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

With the @samp{clear} command you can delete breakpoints according to where
they are in the program.  With the @samp{delete} command you can delete
individual breakpoints 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 breakpoints @var{bnums}@dots{}
@itemx delete @var{bnums}@dots{}
@itemx delete
@kindex delete breakpoints
@kindex delete
Delete the breakpoints of the numbers specified as arguments.  If no
argument is specified, delete all breakpoints.
@end table

@node Disabling,,,
@subsection Disabling Breakpoints

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

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

@itemize @bullet
@item
Enabled.  The breakpoint will stop the program.  A breakpoint made
with the @samp{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 made
with the @samp{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 a breakpoint:

@table @code
@item disable breakpoints @var{bnums}@dots{}
@itemx disable @var{bnums}@dots{}
@itemx disable
@kindex disable breakpoints
@kindex disable
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.

@item enable breakpoints @var{bnums}@dots{}
@itemx enable @var{bnums}@dots{}
@itemx enable
@kindex enable breakpoints
@kindex enable
Enable the specified breakpoints (or all defined breakpoints).  They
become effective once again in stopping the program, until you specify
otherwise.

@item enable breakpoints once @var{bnums}@dots{}
@itemx enable once @var{bnums}@dots{}
Enable the specified breakpoints temporarily.  Each will be disabled
again the next time it stops the program (unless you have used one of
these commands to specify a different state before that time comes).

@item enable breakpoints delete @var{bnums}@dots{}
@itemx enable 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
(unless you have used one of these commands to specify a different
state before that time comes).
@end table

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

@node Conditions,,,
@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 true.

Break conditions may have side effects, and may even call functions in your
program.  These may sound like strange things to do, but their effects are
completely predictable unless there is another enabled breakpoint at the
same address.  (In that case, _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 @samp{break} command.  @xref{Set Breaks}.
They can also be changed at any time with the @samp{condition} command:

@table @code
@item condition @var{bnum} @var{expression}
@kindex condition
Specify @var{expression} as the break condition for breakpoint number
@var{bnum}.  From now on, this breakpoint will stop the program only if
the value of @var{expression} is true (nonzero, in C).  @var{expression}
is not evaluated at the time the @samp{condition} command is given.
When you call @samp{condition}, the expression you specify is checked
immediately for syntactic correctness, and to determine whether symbols
in it have referents in the context of your breakpoint.
@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 @samp{continue} is
ignored.

The synonym @samp{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,,,
@subsection Breakpoint Command Lists

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

@table @code
@item commands @var{bnum}
@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
@samp{end} to terminate the commands.

To remove all commands from a breakpoint, use the command
@samp{commands} and follow it immediately by @samp{end}; that is, give
no commands.

With no arguments, @samp{commands} refers to the last breakpoint set
(not to the breakpoint most recently encountered).
@end table

Pressing @key{RET} as a means of repeating the last _GDBN__ command is
disabled from the time you enter @samp{commands} to just after the
corresponding @samp{end}. 

You can use breakpoint commands to start the program up again.  Simply
use the @samp{continue} command, or @samp{step}, or any other command to
resume execution.  However, if you do this, any further commands in the
same breakpoint's command list are ignored.  When the program stops
again, _GDBN__ will act according to the cause of that stop.

@kindex silent
If the first command specified is @samp{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.  @samp{silent} is not really a command;
it is meaningful only at the beginning of the commands for a breakpoint.

The commands @samp{echo} and @samp{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 correct one bug so you can
test another.  Put a breakpoint just after the erroneous line of code, give
it a condition to detect the case in which something erroneous has been
done, and give it commands to assign correct values to any variables that
need them.  End with the @samp{continue} command so that the program does not
stop, and start with the @samp{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

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. 
In the GNU system, this will be fixed by changing the behavior of
terminal modes.

Under Unix, when you have this problem, you might be able to get around
it by putting your actions into the breakpoint condition instead of
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 0 so the program will not
stop.  Loss of input is avoided here because break conditions are
evaluated 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 Error in Breakpoints,,,
@subsection ``Cannot Insert Breakpoints''

@c FIXME: "cannot insert breakpoints" error, v unclear.  
@c        Q in pending mail to Gilmore. ---pesch@cygnus.com, 26mar91
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 will cause _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 @samp{exec-file} command to specify that _GDBN__
should run the program under that name.  Then start the program again.

@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 Stepping,,,
@section Stepping

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

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

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

@item step @var{count}
Continue running as in @samp{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
@kindex next
Continue to the next source line in the current stack frame.  Similar to
@samp{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
@samp{next} command was given.  This command is abbreviated @samp{n}.

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

@samp{next} within a function that lacks debugging information acts as does
@samp{step}, but any function calls appearing within the code of the
function are executed without stopping.
@c FIXME: great; so what does *step* do within a fn that lacks debug info?

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

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

@item until
@kindex until
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 @samp{next}
command, except that when @samp{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, @samp{until} will cause the program to continue execution
until the loop is exited.  In contrast, a @samp{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.

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

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

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

@item until @var{location}
Continue running the program until either the specified location is
reached, or the current (innermost) stack frame returns.  @var{location}
is any of the forms of argument acceptable to @samp{break} (@pxref{Set
Breaks}).  This form of the command uses breakpoints, and hence is
quicker than @samp{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 @samp{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 @samp{next}.
@end table

A typical technique for using stepping is to put 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.

The @samp{continue} command can be used after stepping to resume execution
until the next breakpoint or signal.

@node Continuing,,,
@section Continuing

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

@table @code
@item continue
@kindex continue
Continue running the program at the place where it stopped.
@end table

If the program stopped at a breakpoint, the place to continue running
is the address of the breakpoint.  You might expect that continuing would
just stop at the same breakpoint immediately.  In fact, @samp{continue}
takes special care to prevent that from happening.  You do not need
to delete the breakpoint to proceed through it after stopping at it.
You can, however, specify an ignore-count for the breakpoint that the
program stopped at, by means of an argument to the @samp{continue} command.
@xref{Conditions}.

If the program stopped because of a signal other than @code{SIGINT} or
@code{SIGTRAP}, continuing will cause the program to see that signal.
You may not want this to happen.  For example, if the program stopped
due to some sort of memory reference error, you might store correct
values into the erroneous variables and continue, hoping to see more
execution; but the program would probably terminate immediately as
a result of the fatal signal once it sees the signal.  To prevent this,
you can continue with @samp{signal 0}.  @xref{Signaling}.  You can
also act in advance to control what signals your program will see, using
the @samp{handle} command (@pxref{Signals}). 

@node Signals,,,
@section Signals
@cindex signals

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

@table @code
@item info signal
@kindex info signal
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

@group
The keywords allowed by the @samp{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 @samp{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 @samp{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
@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 @samp{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 @samp{handle} command with
@samp{pass} or @samp{nopass} to control whether that signal will be seen by
the program when you later continue it.

You can also use the @samp{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.  @xref{Signaling}.


@node Stack,,,
@chapter Examining the Stack

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

@cindex call stack
Each time your program performs a function call, the information about
where in 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 @samp{frame} command does
(@pxref{Frame Info, Info}).

@node Frames,,,
@section Stack Frames

@cindex frame
@cindex stack frame
The call stack is divided up into contiguous pieces called @dfn{stack
frames}, or @dfn{frames} for short; each frame is the data associated
with one call to one function.  The frame contains the arguments given
to the function, the function's local variables, and the address at
which the function is executing.

@cindex initial frame
@cindex outermost frame
@cindex innermost frame
When your program is started, the stack has only one frame, that of the
function @code{main}.  This is called the @dfn{initial} frame or the
@dfn{outermost} frame.  Each time a function is called, a new frame is
made.  Each time a function returns, the frame for that function invocation
is eliminated.  If a function is recursive, there can be many frames for
the same function.  The frame for the function in which execution is
actually occurring is called the @dfn{innermost} frame.  This is the most
recently created of all the stack frames that still exist.

@cindex frame pointer
Inside your program, stack frames are identified by their addresses.  A
stack frame consists of many bytes, each of which has its own address; each
kind of computer has a convention for choosing one of those bytes whose
address serves as the address of the frame.  Usually this address is kept
in a register called the @dfn{frame pointer register} while execution is
going on in that frame.

@cindex frame number
_GDBN__ assigns numbers to all existing stack frames, starting with
@code{0} for the innermost frame, @code{1} 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 give it a virtual stack frame of 0 and
correctly allow tracing of the function call chain.  Results are
undefined if a function invocation besides the innermost one is
frameless.

@node Backtrace,,,
@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{Control-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
The names @samp{where} and @samp{info stack} are additional aliases
for @samp{backtrace}.

Each line in the backtrace shows the frame number and the function name.
The program counter value is also shown---unless you use @samp{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
#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 smallexample

@noindent
The display for frame @code{#0} 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,,,
@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.

@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
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.
@end table

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

@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 @samp{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 @samp{up} and @samp{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,,,
@section Information on 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 @samp{f}.  With an
argument, this command is used to select a stack frame (@pxref{Selection}).

@item info frame
@kindex info frame
This command prints a verbose description of the selected stack frame,
including the address of the frame, the addresses of the next frame 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}
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 @samp{up},
@samp{down}, or @samp{frame} commands); then type @samp{info catch}.
@xref{Exception Handling}.
@end table

@node Source,,,
@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 also
print parts 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}.

@node List,,,
@section Printing Source Lines

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

Here are the forms of the @samp{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
@samp{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 @samp{list} command with @key{RET} discards the argument,
so it is equivalent to typing just @samp{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 @samp{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 @samp{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 @samp{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 @samp{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,,,
@section Searching Source Files
@cindex searching
@kindex search
@kindex forward-search
@kindex reverse-search

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

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 @samp{fo}.  The synonym @samp{search @var{regexp}} is also supported.

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 @samp{rev}.

@node Source Path,,,
@section Specifying Source Directories

@cindex source path
@cindex directories for source files
Executable programs sometimes do not record the directories of the source
files from which they were compiled, just the names.  Even when they do,
the directories could be moved between the compilation and your debugging
session.  _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 @samp{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{$cwd} to refer to the current working directory, and
@samp{$cdir} to refer to the compilation directory (if one is recorded).

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

The @samp{directory} command will not repeat if you press @key{RET} a
second time after executing it once.

@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 @samp{directory} with no argument to reset the source path to empty.

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

@node Machine Code,,,
@section Source and Machine Code
You can use the command @samp{info line} to map source lines to program
addresses, and the command @samp{disassemble} or its synonym
@samp{disasm} 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}.

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

@kindex disassemble
@kindex disasm
@item disassemble
@itemx disasm
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.  The two spellings, @samp{disasm} and
@samp{disassemble}, are equivalent.
@end table

@node Data,,,
@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 @samp{print}
command (abbreviated @samp{p}), or its synonym @samp{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 @samp{x} command.
It examines data in memory at a specified address and prints it in a
specified format.  @xref{Memory}.

@node Expressions,,,
@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
officially supposed to reside at @var{addr}.@refill
@end table

@node Variables,,,
@section Program Variables

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

Variables in expressions are understood in the selected stack frame
(@pxref{Selection}); 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.

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 construct:

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

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

@cindex name resolution (C++)
Unfortunately, this use of @samp{::} conflicts with the very similar use
of the same notation in C++; accordingly, _GDBN__ does not support use of
the C++ name resolution operator in _GDBN__ expressions.

@node Arrays,,,
@section Artificial Arrays

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

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 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.)

@node Output formats,,,
@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
@samp{print} command with a slash and a format letter.  The format
letters supported are:

@table @samp
@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 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 @samp{print} command with just a format and no
expression.  For example, @samp{p/x} reprints the last value in hex.

@node Memory,,,
@section Examining Memory

@cindex examining memory
@table @code
@kindex x
@item x/@var{Nuf} @var{expr}
The command @samp{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.  @samp{x} understands the formats
@var{f} used by @samp{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/3hu 0x54320} is a request to display three halfwords
(@code{h}) of memory, formatted as unsigned decimal integers (@code{u}),
starting at address @code{0x54320}.  @samp{x/4wx $sp} prints the four
words (@code{w}) of memory above the stack pointer (here, @samp{$sp};
@pxref{Registers}) in hexadecimal (@code{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 indicate the address where _GDBN__
is to begin reading memory for display, using an expression.  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 @samp{x}:

@table @samp
@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 @samp{print}.  @xref{Output formats}.

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

@table @samp
@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 @samp{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}, @samp{x}
will use the same one that was used last.  If you don't use any letters
after the slash, you can omit the slash as well.

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

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

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

If the @samp{x} command has a repeat count, the address and contents saved
are from the last memory unit printed; this is not the same as the last
address printed if several units were printed on the last line of output.

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

If you find that you want to print the value of an expression frequently
(to see how it changes), you might want to add it to the @dfn{automatic
display list} so that _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 @samp{x} or @samp{print}, you can
specify the output format you prefer; in fact, @dfn{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}.

@samp{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.

@samp{undisplay} will not repeat if you press @key{RET} after using it.

@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
@samp{display name} while inside a function with an argument
@code{name}, then this argument will be displayed while the program
continues to stop inside that function.  When it stops elsewhere---where
there is no variable @samp{name}---display is disabled.  The next time
your program stops where @samp{name} is meaningful, you can enable the
display expression once again.

@node Print Settings,,,
@section Print Settings

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

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

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

@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 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 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.

@item set print address
@item set print address on
@kindex set print address
_GDBN__ will print memory addresses in stack traces, structure values, pointer
values, breakpoints, etc.  The default is on.

@item set print address off
Do not print addresses.

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

@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 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 @samp{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 @samp{set print union off} in effect it would print

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

@node Value History,,,
@section Value History

@cindex value history
Values printed by the @samp{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 @samp{file} or @samp{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 1.  @samp{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{$} followed by the value's
history number.  The way @samp{print} labels its output is designed to
remind you of this.  Just @code{$} 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 @samp{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 @samp{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 @samp{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.
@end table

@node Convenience Vars,,,
@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 have names starting with @samp{$}.  Any name starting
with @samp{$} can be used for a convenience variable, unless it is one of
the predefined machine-specific register names (@pxref{Registers}).

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

@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 @samp{i 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 @samp{x} command to
the last address examined (@pxref{Memory}).  Other commands which
provide a default address for @samp{x} to examine also set @code{$_}
to that address; these commands include @samp{info line} and @samp{info
breakpoint}.

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

@node Registers,,,
@section Registers

@cindex registers
Machine register contents can be referred to in expressions as variables
with names starting with @samp{$}.  The names of registers are different
for each machine; use @samp{info registers} to see the names used on
your machine.  The names @code{$pc} and @code{$sp} are used on most
machines for the program counter register and the stack pointer.  Often
@code{$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 @samp{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 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.

@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

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 @samp{return};
@pxref{Returning}.)

@node Symbols,,,
@chapter Examining the Symbol Table

The commands described in this section allow you to inquire about the
symbols (names of variables, functions and types) defined in your
program.  This information is inherent in the text of your program and
does not change as the program executes.  _GDBN__ finds it in your
program's symbol table, as 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}.  @samp{ptype}
differs from @samp{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 sources
@kindex info sources
Print the names of all source files in the program for which there
is debugging information.

@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 @samp{step}; @samp{info fun ^step} finds those whose names
start with @samp{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 @samp{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 @samp{ptype} command can be overwhelming and hard to use.  The
@samp{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,,,
@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.

@node Assignment,,,
@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
@samp{set} command instead of the @samp{print} command.  @samp{set} is
really the same as @samp{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 @samp{set} command
appears identical to a @samp{set} subcommand, use the @samp{set
variable} command instead of just @samp{set}.  This command is identical
to @samp{set} except for its lack of subcommands.

_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        /pesch@cygnus.com 18dec1990

To store values into arbitrary places in memory, use the @samp{@{@dots{}@}}
construct to generate a value of specified type at a specified address
(@pxref{Expressions}).  For example, @code{@{int@}0x83040} refers
to memory location @code{0x83040} as an integer (which implies a certain size
and representation in memory), and

@example
set @{int@}0x83040 = 4
@end example

@noindent
stores the value 4 into that memory location.

@node Jumping,,,
@section Continuing at a Different Address

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

@table @code
@item jump @var{linenum}
@kindex jump
Resume execution at line number @var{linenum}.  Execution will stop
immediately if there is a breakpoint there.

The @samp{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{linenum} is in
a different function from the one currently executing, the results may
be bizarre if the two functions expect different patterns of arguments or
of local variables.  For this reason, the @samp{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 @samp{continue} command or stepping command to execute at
address 0x485, rather than at the address where the program stopped.
@xref{Stepping}.

The most common occasion to use the @samp{jump} command is to back up,
perhaps with more breakpoints set, over a portion of a program that has
already executed.

@group
@node Signaling,,,
@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
@samp{continue} command; @samp{signal 0} causes it to resume without a
signal.

This command does not repeat when you press @key{RET} a second time
after using it once.
@end table
@end group

@node Returning,,,
@section Returning from a Function

@table @code
@item return
@cindex returning from a function
@kindex return
You can cancel execution of a function call with the @samp{return}
command.
@end table

This command abandons execution of a function.  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 @samp{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 @samp{finish} command (@pxref{Stepping})
resumes execution until the selected stack frame returns naturally.

@node     Calling,,,
@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 @samp{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 GDB Files,,,
@chapter _GDBN__'s Files

@node 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 @samp{run} command.  If you do not specify a
directory and the file is not found in _GDBN__'s working directory,

_GDBN__ will use 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.

@samp{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 @samp{file} command to get both symbol
table and program to run from the same file.

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

The @samp{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__.

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

On some kinds of object files, the @samp{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 @samp{set verbose} command can turn these pauses into
messages if desired. @xref{Messages/Warnings}).

When the symbol table is stored in COFF format, @samp{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.

@samp{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 @samp{kill} command
(@pxref{Kill Process}).

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

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

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

@samp{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 @samp{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 @samp{symbol-file} command.  You can use the
@samp{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 @samp{symbol-file} command. 

@samp{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
@samp{info files} and @samp{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 @samp{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 @samp{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 are currently loaded.
@end table

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

@node Symbol Errors,,,
@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 @samp{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 @samp{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
@samp{complain}, then go up to the function @samp{read_dbx_symtab} and
examine @code{*bufp} to see the symbol.

@c @item stub type has NULL name
@c 
@c FIXME, Mike Tiemann needs to write about what this means.

@item const/volatile indicator missing, got 'X'

The symbol information for a C++ type is missing some information that
the compiler should have output for it.

@item C++ type mismatch between compiler and debugger

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

@end table

@node     Targets,,,
@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 @samp{file} or @samp{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 @samp{target} command.

@node Active 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 @samp{detach}
command.  The related command @samp{attach} provides you with a way of
choosing a particular running process as a new target. @xref{Attach}.

@node Target Commands,,,
@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 @samp{target} command will not repeat if you press @key{RET} again
after executing the command.

@item help targets
@kindex help targets
Displays the names of all targets available.  To display targets
currently selected, use either @samp{info target} or @samp{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.
@code{/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 @samp{target remote};
@samp{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.
@samp{/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,,,
@section Remote Debugging
@cindex remote debugging

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 be debugging an operating system kernel, or debugging
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 adapt.

To use the _GDBN__ remote serial protocol, the program to be debugged on
the remote machine needs to contain a debugging device driver which
talks to _GDBN__ over the serial line.  Several sample remote debugging
drivers 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 @samp{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 @samp{detach}
command.

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

@node Controlling _GDBN__,,,
@chapter Controlling _GDBN__

You can alter many aspects of _GDBN__'s interaction with you by using
the @samp{set} command.  For commands controlling how _GDBN__ displays
data, @pxref{Print Settings}; other settings are described here.

@node Prompt,,,
@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 @samp{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 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,,,
@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 @samp{emacs}-style
or @samp{vi}-style inline editing of commands, @samp{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 @samp{set}.  You may check the status of any of these settings
with the command @samp{show}.

@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.

@node History,,,
@section Command History
@cindex history substitution
@cindex history file
@kindex set history file
@item set history file @var{filename}
Set the name of the _GDBN__ command history file to @var{filename}.  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 write
@kindex set history write
@item set history write
@itemx set history write on
Record command history in a file, whose name may be specified with the
@samp{set history file} command.  By default, this option is disabled.

@item set history write 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 @samp{!}.
@iftex
(@xref{Event Designators}.)
@end iftex
Since @samp{!} is also the logical not operator in C, history expansion
is off by default. If you decide to enable history expansion with the
@samp{set history expansion on} command, you may sometimes need to
follow @samp{!} (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
@samp{!=} and @samp{!(}, 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 @samp{emacs}
or @samp{vi} may wish to read it. 
@iftex
@xref{Command Line Editing}.
@end iftex

@group
@kindex show history
@item show history
@itemx show history file
@itemx show history write
@itemx show history size
@itemx show history expansion
These commands display the state of the _GDBN__ history parameters.
@samp{show history} by itself displays all four states.
@end group

@end table

@table @code
@kindex show commands
@item show commands
Display the last ten commands in the command history.

@item show commands @var{n}
Print ten commands centered on command number @var{n}.

@item show commands +
Print ten commands just after the commands last printed.

@end table

@node Screen Size,,,
@section Screen Size
@cindex size of screen
@cindex pauses in output
Certain commands to _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 @samp{set height} and @samp{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 @samp{set} commands specify a screen height of @var{lpp} lines and
a screen width of @var{cpl} characters.  The associated @samp{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,,,
@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 @samp{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,,,
@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 @samp{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 @samp{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
@samp{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 @samp{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 @samp{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.  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.
@xref{Symbol Errors}.

@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.  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 caution
@cindex flinching
@cindex stupid questions
@item set caution off
Disables cautious questions.

@item set caution on
Enables cautious questions (the default).

@item show caution
@kindex show caution
Displays state of cautious questions.
@end table

@node Sequences,,,
@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.

@node Define,,,
@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 @samp{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 @samp{define} command.  The end of these
commands is marked by a line containing @samp{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 @samp{define} reads the lines of the
command definition, ending with @samp{end}.  After the @samp{document}
command is finished, @samp{help} on command @var{commandname} will print
the documentation you have specified.

You may use the @samp{document} command again to change the
documentation of a command.  Redefining the command with @samp{define}
does not change the documentation.
@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,,,
@section Command Files

@cindex command files
A command file for _GDBN__ is a file of lines that are _GDBN__ commands.  Comments
(lines starting with @samp{#}) 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 the @samp{-nx} option is given.)  You can also request the
execution of a command file with the @samp{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,,,
@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 @samp{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,,,
@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 this purpose.

@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__ @samp{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 @samp{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__ @samp{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 ``@code{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 @samp{.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__ @samp{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__ @samp{next} command.  Then update the display window
to show the current file and location.

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

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

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

@item M-c
@c C-c C-p in emacs 19
Continue execution of the program, like the _GDBN__ @samp{continue} command.

@item M-u
@c C-c C-u in emacs 19
Go up the number of frames indicated by the numeric argument
(@pxref{Arguments, , Numeric Arguments, emacs, The GNU Emacs Manual}),
like the _GDBN__ @samp{up} command.@refill

@item M-d
@c C-c C-d in emacs 19
Go down the number of frames indicated by the numeric argument, like the
_GDBN__ @samp{down} command.

@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 @samp{disassemble} by typing @kbd{C-x &}.  

You can customize this further on the fly by defining elements of the list
@samp{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 @samp{format}; otherwise the number
is passed as an argument to the corresponding list element.

@item M-x gdb-display-frame
Explicitly request display of the source code surrounding the current
frame location, in another window.  _GDBN__ does this display automatically;
but if, for example, you accidentally kill the buffer where it is
displayed, this command is a way of getting it back.
@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.

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. ---pesch@cygnus.com 19dec1990
@ignore
@kindex emacs epoch environment  
@kindex epoch
@kindex inspect

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

@node _GDBN__ Bugs,,,
@c  node-name,  next,  previous,  up
@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.

@node Bug Criteria,,,
@section Have You Found a Bug?
@cindex Bug Criteria

If you are not sure whether you have found a bug, here are some guidelines:

@itemize @bullet
@item
@cindex Fatal Signal
@cindex 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,,,
@section How to Report Bugs
@cindex Bug Reports
@cindex Compiler 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 @samp{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 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 does not contain 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
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 @samp{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 would 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.

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 readline/inc-readline.texinfo
@include readline/inc-history.texinfo
@end iftex

@node Installing _GDBN__,,,
@appendix Installing _GDBN__
@cindex configuring _GDBN__
@cindex installation

The script @samp{config.gdb} automates the process of preparing _GDBN__ for
installation; you can then use @samp{make} to actually build it.  For
example, 

@example
config.gdb sun3os4
make
@end example

@noindent
is all that's required to install _GDBN__ on a Sun 3 running SunOS 4.  

@table @code
@kindex config.gdb
@item config.gdb @var{machine}
@itemx config.gdb -srcdir=@var{dir} @var{machine}
This is the most usual way of configuring _GDBN__; to debug programs running
on the same machine as _GDBN__ itself.  If you wish to build the _GDBN__ binaries
in a completely different directory from the sources, specify a path to
the source directory using the @samp{-srcdir} option.

@item config.gdb -host
@cindex host environments
Display a list of supported host environments for _GDBN__.

@item config.gdb @var{host} @var{target}
@itemx config.gdb -srcdir=@var{dir} @var{host} @var{target}
@cindex cross-debugging
_GDBN__ can also be used as a cross-debugger, running on a machine of one
type while debugging a program running on a machine of another type.
You configure it this way by specifying first the @var{host}, then the
@var{target} environment on the @code{config.gdb} argument list; the
@var{host} is where _GDBN__ runs, and the @var{target} is where your program
runs. @xref{Remote}.  Again, you can use @samp{-srcdir} to specify a
path to the _GDBN__ source.

@item config.gdb -target
@cindex target environments
Display a list of supported target environments for _GDBN__.
@end table

@node License,,,
@unnumbered GNU GENERAL PUBLIC LICENSE
@center Version 1, February 1989

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

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

@unnumberedsec Preamble

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

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

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

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

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

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

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

@iftex
@unnumberedsec TERMS AND CONDITIONS
@end iftex
@ifinfo
@center TERMS AND CONDITIONS
@end ifinfo

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

@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
General Public License and to the absence of any warranty; and give any
other recipients of the Program a copy of this General Public License
along with the Program.  You may charge a fee for the physical act of
transferring a copy.

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

@itemize @bullet
@item
cause the modified files to carry prominent notices stating that
you changed the files and the date of any change; and

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

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

@item
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.
@end itemize

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

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

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

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

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

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

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

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

@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.

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

@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 humanity, the best way to achieve this is to make it
free software which everyone can redistribute and change under these
terms.

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

@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 1, or (at your option)
any later version.

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

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
@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 `show w' and `show c' should show the
appropriate parts of the General Public License.  Of course, the
commands you use may be called something other than `show w' and `show
c'; they could even be mouse-clicks or menu items---whatever suits your
program.

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

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

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

That's all there is to it!

@node Index,,,
@unnumbered Index

@printindex cp

@tex
% I think something like @colophon should be in texinfo.  In the
% meantime:
\long\def\colophon{\hbox to0pt{}\vfill
\centerline{The body of this manual is set in}
\centerline{\fontname\tenrm,}
\centerline{with headings in {\bf\fontname\tenbf}}
\centerline{and examples in {\tt\fontname\tentt}.}
\centerline{{\it\fontname\tenit\/} and}
\centerline{{\sl\fontname\tensl\/}}
\centerline{are used for emphasis.}\vfill}
\page\colophon
% Blame: pesch@cygnus.com, 28mar91.
@end tex

@contents
@bye