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ΓòÉΓòÉΓòÉ 1. Title page ΓòÉΓòÉΓòÉ

                                 Debugging with

                          The gnu Source-Level Debugger

                                       ()

                        Second GNAT Edition, for  version

                                  November 1998

                     Richard M. Stallman and Roland H. Pesch

Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998
Free Software Foundation, Inc.

Published by the Free Software Foundation
59 Temple Place - Suite 330,
Boston, MA 02111-1307 USA
Printed copies are available for $20 each.
ISBN 1-882114-11-6

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

Permission is granted to copy and distribute modified versions of this manual
under the conditions for verbatim copying, provided also that the entire
resulting derived work is distributed under the terms of a permission notice
identical to this one.

Permission is granted to copy and distribute translations of this manual into
another language, under the above conditions for modified versions.


ΓòÉΓòÉΓòÉ 2. Top node: "Debugging with " ΓòÉΓòÉΓòÉ

This file describes , the gnu symbolic debugger.

This is the Second GNAT Edition, November 1998, for  Version .

 Summary                                 Summary of GDBN

 Sample Session                          A sample GDBN session

 Invocation                              Getting in and out of GDBN
 Commands                                GDBN commands
 Running                                 Running programs under GDBN
 Stopping                                Stopping and continuing
 Stack                                   Examining the stack
 Source                                  Examining source files
 Data                                    Examining data

 Languages                               Using GDBN with different languages

 C                                       C language support

 Symbols                                 Examining the symbol table
 Altering                                Altering execution
 GDB Files                               GDBN files
 Targets                                 Specifying a debugging target
 Controlling GDB                         Controlling GDBN
 Sequences                               Canned sequences of commands

 Emacs                                   Using GDBN under gnu Emacs

 GDB Bugs                                Reporting bugs in GDBN
 Command Line Editing                    Facilities of the readline library
 Using History Interactively

 Formatting Documentation                How to format and print GDBN
                                         documentation
 Installing GDB                          Installing GDB

 Index                                   Index


ΓòÉΓòÉΓòÉ 3. Summary of ΓòÉΓòÉΓòÉ

The purpose of a debugger such as  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.

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

     Start your program, specifying anything that might affect its behavior.

     Make your program stop on specified conditions.

     Examine what has happened, when your program has stopped.

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

 You can use  to debug programs written in C or C++. For more information, see
 Supported languages.

 Free Software                           Freely redistributable software


 Contributors                            Contributors to GDB


ΓòÉΓòÉΓòÉ 3.1. Free software ΓòÉΓòÉΓòÉ

 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 cannot take these freedoms away from anyone else.


ΓòÉΓòÉΓòÉ 3.2. Contributors to GDB ΓòÉΓòÉΓòÉ

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

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

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

So that they may not regard their long labor as thankless, we particularly
thank those who shepherded GDB through major releases: Stan Shebs (release
4.14), Fred Fish (releases 4.13, 4.12, 4.11, 4.10, and 4.9), Stu Grossman and
John Gilmore (releases 4.8, 4.7, 4.6, 4.5, and 4.4), John Gilmore (releases
4.3, 4.2, 4.1, 4.0, and 3.9); Jim Kingdon (releases 3.5, 3.4, and 3.3); and
Randy Smith (releases 3.2, 3.1, and 3.0). As major maintainer of  for some
period, each contributed significantly to the structure, stability, and
capabilities of the entire debugger.

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

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

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

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

Brent Benson of Harris Computer Systems contributed DWARF 2 support.

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

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

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

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

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

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

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

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

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

Michael Snyder added support for tracepoints.

Stu Grossman wrote gdbserver.

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

Cygnus Solutions has sponsored GDB maintenance and much of its development
since 1991.


ΓòÉΓòÉΓòÉ 4. A Sample  Session ΓòÉΓòÉΓòÉ

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

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

$ cd gnu/m4
$ ./m4
define(foo,0000)

foo
0000
define(bar,defn(`foo'))

bar
0000
changequote(<QUOTE>,<UNQUOTE>)

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

Let us use  to try to see what is going on.

$  m4
Copyright 1998 Free Software Foundation, Inc.
 is free software and you are welcome to distribute copies
 of it under certain conditions; type "show copying" to see
 the conditions.
There is absolutely no warranty for ; type "show warranty"
 for details.

()

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

() set width 70

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

() break m4_changequote
Breakpoint 1 at 0x62f4: file builtin.c, line 879.

Using the run command, we start m4 running under control; as long as control
does not reach the m4_changequote subroutine, the program runs as usual:

() run
Starting program: /work/Editorial/gdb/gnu/m4/m4
define(foo,0000)

foo
0000

To trigger the breakpoint, we call changequote. suspends execution of m4,
displaying information about the context where it stops.

changequote(<QUOTE>,<UNQUOTE>)

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

Now we use the command n (next) to advance execution to the next line of the
current function.

() n
882         set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\
 : nil,

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

() s
set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
    at input.c:530
530         if (lquote != def_lquote)

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

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

We step through a few more lines to see what happens.  The first two times, we
can use `s'; the next two times we use n to avoid falling into the xstrdup
subroutine.

() s
0x3b5c  532         if (rquote != def_rquote)
() s
0x3b80  535         lquote = (lq == nil || *lq == '\0') ?  \
def_lquote : xstrdup(lq);
() n
536         rquote = (rq == nil || *rq == '\0') ? def_rquote\
 : xstrdup(rq);
() n
538         len_lquote = strlen(rquote);

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

() p lquote
$1 = 0x35d40 "<QUOTE>"
() p rquote
$2 = 0x35d50 "<UNQUOTE>"

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

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

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

() n
539         len_rquote = strlen(lquote);
() n
540     }
() p len_lquote
$3 = 9
() p len_rquote
$4 = 7

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

() p len_lquote=strlen(lquote)
$5 = 7
() p len_rquote=strlen(rquote)
$6 = 9

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

() c
Continuing.

define(baz,defn(<QUOTE>foo<UNQUOTE>))

baz
0000

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

C-d
Program exited normally.

The message `Program exited normally.' is from ; it indicates m4 has finished
executing.  We can end our session with the  quit command.

() quit


ΓòÉΓòÉΓòÉ 5. Getting In and Out of ΓòÉΓòÉΓòÉ

This chapter discusses how to start , and how to get out of it. The essentials
are:

     type `' to start GDB.

     type quit or C-d to exit.

 Invoking GDB                            How to start GDBN
 Quitting GDB                            How to quit GDBN
 Shell Commands                          How to use shell commands inside GDBN


ΓòÉΓòÉΓòÉ 5.1. Invoking ΓòÉΓòÉΓòÉ

Invoke  by running the program .  Once started,  reads commands from the
terminal until you tell it to exit.

You can also run  with a variety of arguments and options, to specify more of
your debugging environment at the outset.

The most usual way to start  is with one argument, specifying an executable
program:

 program

You can also start with both an executable program and a core file specified:

 program core

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

 program 1234

would attach  to process 1234 (unless you also have a file named `1234';  does
check for a core file first).

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

You can run gdb without printing the front material, which describes 's
non-warranty, by specifying -silent:

 -silent

You can further control how  starts up by using command-line options.  itself
can remind you of the options available.

Type

 -help

to display all available options and briefly describe their use (` -h' is a
shorter equivalent).

All options and coE13p-unx-docs\texi\gdb.texinfo line 775 : can't open included
file remote.texinfo mmand line arguments you give are processed in sequential
order.  The order makes a difference when the `-x' option is used.

 Remote Serial                           GDBN remote serial protocol

 i960-Nindy Remote                       GDBN with a remote i960 (Nindy)

 UDI29K Remote                           The UDI protocol for AMD29K
 EB29K Remote                            The EBMON protocol for AMD29K

 VxWorks Remote                          GDBN and VxWorks

 ST2000 Remote                           GDBN with a Tandem ST2000

 Hitachi Remote                          GDBN and Hitachi Microprocessors

 MIPS Remote                             GDBN and MIPS boards

 Sparclet Remote                         GDBN and Sparclet boards

 Simulator                               Simulated CPU target

 File Options                            Choosing files
 Mode Options                            Choosing modes


ΓòÉΓòÉΓòÉ 5.1.1. Choosing files ΓòÉΓòÉΓòÉ

When  starts, it reads any arguments other than options as specifying an
executable file and core file (or process ID).  This is the same as if the
arguments were specified by the `-se' and `-c' options respectively.  ( reads
the first argument that does not have an associated option flag as equivalent
to the `-se' option followed by that argument; and the second argument that
does not have an associated option flag, if any, as equivalent to the `-c'
option followed by that argument.)

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

 -symbols file
 -s file
           Read symbol table from file file.

 -exec file
 -e file
           Use file file as the executable file to execute when appropriate,
           and for examining pure data in conjunction with a core dump.

 -se file
           Read symbol table from file file and use it as the executable file.

 -core file
 -c file
           Use file file as a core dump to examine.

 -c number
           Connect to process ID number, as with the attach command (unless
           there is a file in core-dump format named number, in which case `-c'
           specifies that file as a core dump to read).

 -command file
 -x file
           Execute  commands from file file. See Command files.

 -directory directory
 -d directory
           Add directory to the path to search for source files.

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

           The `.syms' file is specific to the host machine where is run.  It
           holds an exact image of the internal  symbol table.  It cannot be
           shared across multiple host platforms.

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

 The -mapped and -readnow options are typically combined in order to build a
 `.syms' file that contains complete symbol information.  (See Commands to
 specify files, for information

 a `.syms' file for future use is:

          gdb -batch -nx -mapped -readnow programname


ΓòÉΓòÉΓòÉ 5.1.2. Choosing modes ΓòÉΓòÉΓòÉ

You can run  in various alternative modes---for example, in batch mode or quiet
mode.

 -nx
 -n
           Do not execute commands from any initialization files (normally
           called `').  Normally, the commands in these files are executed
           after all the command options and arguments have been processed.
           See Command files.

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

 -batch
           Run in batch mode.  Exit with status 0 after processing all the
           command files specified with `-x' (and all commands from
           initialization files, if not inhibited with `-n').  Exit with
           nonzero status if an error occurs in executing the  commands in the
           command files.

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

                      Program exited normally.

           (which is ordinarily issued whenever a program running under
           control terminates) is not issued when running in batch mode.

 -cd directory
           Run  using directory as its working directory, instead of the
           current directory.

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


ΓòÉΓòÉΓòÉ 5.2. Quitting ΓòÉΓòÉΓòÉ

 quit
           To exit , use the quit command (abbreviated q), or type an
           end-of-file character (usually C-d).  If you do not supply
           expression,  will terminate normally; otherwise it will terminate
           using the result of expression as the error code.

 An interrupt (often C-c) does not exit from , but rather terminates the action
 of any  command that is in progress and returns to  command level.  It is safe
 to type the interrupt character at any time because  does not allow it to take
 effect until a time when it is safe.

 If you have been using  to control an attached process or device, you can
 release it with the detach command (see Debugging an already-running process).


ΓòÉΓòÉΓòÉ 5.3. Shell commands ΓòÉΓòÉΓòÉ

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

 shell command string
           Invoke a the standard shell to execute command string. If it exists,
           the environment variable SHELL determines which shell to run.
           Otherwise  uses /bin/sh.

 The utility make is often needed in development environments. You do not have
 to use the shell command for this purpose in :

 make make-args
           Execute the make program with the specified arguments.  This is
           equivalent to `shell make make-args'.


ΓòÉΓòÉΓòÉ 6. Commands ΓòÉΓòÉΓòÉ

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

 Command Syntax                          How to give commands to GDBN
 Completion                              Command completion
 Help                                    How to ask GDBN for help


ΓòÉΓòÉΓòÉ 6.1. Command syntax ΓòÉΓòÉΓòÉ

A  command is a single line of input.  There is no limit on how long it can be.
It starts with a command name, which is followed by arguments whose meaning
depends on the command name.  For example, the command step accepts an argument
which is the number of times to step, as in `step 5'.  You can also use the
step command with no arguments.  Some command names do not allow any arguments.

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

A blank line as input to  (typing just RET) means to repeat the previous
command. Certain commands (for example, run) will not repeat this way; these
are commands whose unintentional repetition might cause trouble and which you
are unlikely to want to repeat.

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

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

Any text from a # to the end of the line is a comment; it does nothing.  This
is useful mainly in command files ( see Command files).


ΓòÉΓòÉΓòÉ 6.2. Command completion ΓòÉΓòÉΓòÉ

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

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

() info bre TAB

 fills in the rest of the word `breakpoints', since that is the only info
subcommand beginning with `bre':

() info breakpoints

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

If there is more than one possibility for the next word when you press TAB,
sounds a bell.  You can either supply more characters and try again, or just
press TAB a second time;  displays all the possible completions for that word.
For example, you might want to set a breakpoint on a subroutine whose name
begins with `make_', but when you type b make_TAB just sounds the bell.  Typing
TAB again displays all the function names in your program that begin with those
characters, for example:

() b make_ TAB
 sounds bell; press TAB again, to see:
make_a_section_from_file     make_environ
make_abs_section             make_function_type
make_blockvector             make_pointer_type
make_cleanup                 make_reference_type
make_command                 make_symbol_completion_list
() b make_

After displaying the available possibilities,  copies your partial input (`b
make_' in the example) so you can finish the command.

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

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

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

() b 'bubble( M-?
bubble(double,double)    bubble(int,int)
() b 'bubble(

In some cases,  can tell that completing a name requires using quotes.  When
this happens,  inserts the quote for you (while completing as much as it can)
if you do not type the quote in the first place:

() b bub TAB
 alters your input line to the following, and rings a bell:
() b 'bubble(

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


ΓòÉΓòÉΓòÉ 6.3. Getting help ΓòÉΓòÉΓòÉ

You can always ask  itself for information on its commands, using the command
help.

 help
 h
           You can use help (abbreviated h) with no arguments to display a
           short list of named classes of commands:

                      () 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.
                      ()

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

                      () 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.
                      ()

 help command
           With a command name as help argument,  displays a short paragraph on
           how to use that command.

 complete args
           The complete args command lists all the possible completions for the
           beginning of a command.  Use args to specify the beginning of the
           command you want completed.  For example:

                      complete i

           results in:

                      info
                      inspect
                      ignore

           This is intended for use by gnu Emacs.

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

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

 set
           You can assign the result of an expresson to an environment variable
           with set.  For example, you can set the  prompt to a $-sign with set
           prompt $.

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

           To display all the settable parameters and their current values, you
           can use show with no arguments; you may also use info set.  Both
           commands produce the same display.

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

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

 show copying
           Display information about permission for copying .

 show warranty
           Display the gnu ``NO WARRANTY'' statement.


ΓòÉΓòÉΓòÉ 7. Running Programs Under ΓòÉΓòÉΓòÉ

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

 Compilation                             Compiling for debugging
 Starting                                Starting your program

 Arguments                               Your program's arguments
 Environment                             Your program's environment
 Working Directory                       Your program's working directory
 Input/Output                            Your program's input and output
 Attach                                  Debugging an already-running process
 Kill Process                            Killing the child process
 Process Information                     Additional process information
 Threads                                 Debugging programs with multiple
                                         threads
 Processes                               Debugging programs with multiple
                                         processes


ΓòÉΓòÉΓòÉ 7.1. Compiling for debugging ΓòÉΓòÉΓòÉ

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

To request debugging information, specify the `-g' option when you run the
compiler.

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

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

When you debug a program compiled with `-g -O', remember that the optimizer is
rearranging your code; the debugger shows you what is really there.  Do not be
too surprised when the execution path does not exactly match your source file!
An extreme example: if you define a variable, but never use it,  never sees
that variable---because the compiler optimizes it out of existence.

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

Older versions of the gnu C compiler permitted a variant option `-gg' for
debugging information.  no longer supports this format; if your gnu C compiler
has this option, do not use it.


ΓòÉΓòÉΓòÉ 7.2. Starting your program ΓòÉΓòÉΓòÉ

 run
 r
           Use the run command to start your program under .  You must first
           specify the program name with an argument to  ( see Getting In and
           Out of), or by using the file or exec-file command (see Commands to
           specify files).

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

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

 The arguments.
           Specify the arguments to give your program as the arguments of the
           run command.  If a shell is available on your target, the shell is
           used to pass the arguments, so that you may use normal conventions
           (such as wildcard expansion or variable substitution) in describing
           the arguments.  In Unix systems, you can control which shell is used
           with the SHELL environment variable. See Your program_s arguments.

 The environment.
           Your program normally inherits its environment from , but you can
           use the  commands set environment and unset environment to change
           parts of the environment that affect your program.  See Your
           program_s environment.

 The working directory.
           Your program inherits its working directory from .  You can set the
           working directory with the cd command in . See Your program_s
           working directory.

 The standard input and output.
           Your program normally uses the same device for standard input and
           standard output as  is using.  You can redirect input and output in
           the run command line, or you can use the tty command to set a
           different device for your program. See Your program_s input and
           output.

           Warning: While input and output redirection work, you cannot use
           pipes to pass the output of the program you are debugging to another
           program; if you attempt this,  is likely to wind up debugging the
           wrong program.

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

 If the modification time of your symbol file has changed since the last time
 read its symbols,  discards its symbol table, and reads it again.  When it
 does this,  tries to retain your current breakpoints.


ΓòÉΓòÉΓòÉ 7.3. Your program's arguments ΓòÉΓòÉΓòÉ

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

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

 set args
           Specify the arguments to be used the next time your program is run.
           If set args has no arguments, run executes your program with no
           arguments.  Once you have run your program with arguments, using set
           args before the next run is the only way to run it again without
           arguments.

 show args
           Show the arguments to give your program when it is started.


ΓòÉΓòÉΓòÉ 7.4. Your program's environment ΓòÉΓòÉΓòÉ

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

 path directory
           Add directory to the front of the PATH environment variable (the
           search path for executables), for both  and your program. You may
           specify several directory names, separated by `:' or whitespace.  If
           directory is already in the path, it is moved to the front, so it is
           searched sooner.

           You can use the string `$cwd' to refer to whatever is the current
           working directory at the time  searches the path.  If you use `.'
           instead, it refers to the directory where you executed the path
           command.  replaces `.' in the directory argument (with the current
           path) before adding directory to the search path.

 show paths
           Display the list of search paths for executables (the PATH
           environment variable).

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

 set environment varname [=] value
           Set environment variable varname to value.  The value changes for
           your program only, not for  itself.  value may be any string; the
           values of environment variables are just strings, and any
           interpretation is supplied by your program itself.  The value
           parameter is optional; if it is eliminated, the variable is set to a
           null value.

           For example, this command:

                      set env USER = foo

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

 unset environment varname
           Remove variable varname from the environment to be passed to your
           program.  This is different from `set env varname ='; unset
           environment removes the variable from the environment, rather than
           assigning it an empty value.

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


ΓòÉΓòÉΓòÉ 7.5. Your program's working directory ΓòÉΓòÉΓòÉ

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

The  working directory also serves as a default for the commands that specify
files for  to operate on. See Commands to specify files.

 cd directory
           Set the  working directory to directory.

 pwd
           Print the  working directory.


ΓòÉΓòÉΓòÉ 7.6. Your program's input and output ΓòÉΓòÉΓòÉ

By default, the program you run under  does input and output to the same
terminal that  uses.  switches the terminal to its own terminal modes to
interact with you, but it records the terminal modes your program was using and
switches back to them when you continue running your program.

 info terminal
           Displays information recorded by  about the terminal modes your
           program is using.

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

  run > outfile

 starts your program, diverting its output to the file `outfile'.

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

  tty /dev/ttyb

 directs that processes started with subsequent run commands default to do
 input and output on the terminal `/dev/ttyb' and have that as their
 controlling terminal.

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

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


ΓòÉΓòÉΓòÉ 7.7. Debugging an already-running process ΓòÉΓòÉΓòÉ

 attach process-id
           This command attaches to a running process---one that was started
           outside .  (info files shows your active targets.)  The command
           takes as argument a process ID.  The usual way to find out the
           process-id of a Unix process is with the ps utility, or with the
           `jobs -l' shell command.

           attach does not repeat if you press RET a second time after
           executing the command.

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

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

 The first thing  does after arranging to debug the specified process is to
 stop it.  You can examine and modify an attached process with all the
 commands that are ordinarily available when you start processes with 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 continue
 command after attaching  to the process.

 detach
           When you have finished debugging the attached process, you can use
           the detach command to release it from  control.  Detaching the
           process continues its execution.  After the detach command, that
           process and  become completely independent once more, and you are
           ready to attach another process or start one with run. detach does
           not repeat if you press RET again after executing the command.

 If you exit  or use the run command while you have an attached process, you
 kill that process.  By default,  asks for confirmation if you try to do either
 of these things; you can control whether or not you need to confirm by using
 the set confirm command ( see Optional warnings and messages).


ΓòÉΓòÉΓòÉ 7.8. Killing the child process ΓòÉΓòÉΓòÉ

 kill
           Kill the child process in which your program is running under .

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

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

 The kill command is also useful if you wish to recompile and relink your
 program, since on many systems it is impossible to modify an executable file
 while it is running in a process.  In this case, when you next type run,
 notices that the file has changed, and reads the symbol table again (while
 trying to preserve your current breakpoint settings).


ΓòÉΓòÉΓòÉ 7.9. Additional process information ΓòÉΓòÉΓòÉ

Some operating systems provide a facility called `/proc' that can be used to
examine the image of a running process using file-system subroutines.  If  is
configured for an operating system with this facility, the command info proc is
available to report on several kinds of information about the process running
your program. info proc works only on SVR4 systems that support procfs.

 info proc
           Summarize available information about the process.

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

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

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

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

 info proc all
           Show all the above information about the process.


ΓòÉΓòÉΓòÉ 7.10. Debugging programs with multiple threads ΓòÉΓòÉΓòÉ

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

 provides these facilities for debugging multi-thread programs:

     automatic notification of new threads

     `thread threadno', a command to switch among threads

     `info threads', a command to inquire about existing threads

     `thread apply [threadno] [all] args', a command to apply a command to a
      list of threads

     thread-specific breakpoints

 Warning: These facilities are not yet available on every  configuration where
 the operating system supports threads. If your  does not support threads,
 these commands have no effect.  For example, a system without thread support
 shows no output from `info threads', and always rejects the thread command,
 like this:

  () info threads
  () thread 1
  Thread ID 1 not known.  Use the "info threads" command to
  see the IDs of currently known threads.

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

 Whenever  detects a new thread in your program, it displays the target
 system's identification for the thread with a message in the form `[New
 systag]'.  systag is a thread identifier whose form varies depending on the
 particular system.  For example, on LynxOS, you might see

  [New process 35 thread 27]

 when  notices a new thread.  In contrast, on an SGI system, the systag is
 simply something like `process 368', with no further qualifier.

 For debugging purposes,  associates its own thread number---always a single
 integer---with each thread in your program.

 info threads
           Display a summary of all threads currently in your program.
           displays for each thread (in this order):

             1. the thread number assigned by

             2. the target system's thread identifier (systag)

             3. the current stack frame summary for that thread

           An asterisk `*' to the left of the  thread number indicates the
           current thread.

           For example,

  () info threads
    3 process 35 thread 27  0x34e5 in sigpause ()
    2 process 35 thread 23  0x34e5 in sigpause ()
  * 1 process 35 thread 13  main (argc=1, argv=0x7ffffff8)
      at threadtest.c:68

 thread threadno
           Make thread number threadno the current thread.  The command
           argument threadno is the internal  thread number, as shown in the
           first field of the `info threads' display.  responds by displaying
           the system identifier of the thread you selected, and its current
           stack frame summary:

                      () thread 2
                      [Switching to process 35 thread 23]
                      0x34e5 in sigpause ()

           As with the `[New ...]' message, the form of the text after
           `Switching to' depends on your system's conventions for identifying
           threads.

 thread apply [threadno] [all]  args
           The thread apply command allows you to apply a command to one or
           more threads.  Specify the numbers of the threads that you want
           affected with the command argument threadno.  threadno is the
           internal  thread number, as shown in the first field of the `info
           threads' display.  To apply a command to all threads, use thread
           apply all args.

 Whenever  stops your program, due to a breakpoint or a signal, it
 automatically selects the thread where that breakpoint or signal happened.
 alerts you to the context switch with a message of the form `[Switching to
 systag]' to identify the thread.

 See Stopping and starting multi-thread programs, for more information about
 how  behaves when you stop and start programs with multiple threads.

 See Setting watchpoints, for information about watchpoints in programs with
 multiple threads.


ΓòÉΓòÉΓòÉ 7.11. Debugging programs with multiple processes ΓòÉΓòÉΓòÉ

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

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


ΓòÉΓòÉΓòÉ 8. Stopping and Continuing ΓòÉΓòÉΓòÉ

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

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

 info program
           Display information about the status of your program: whether it is
           running or not, what process it is, and why it stopped.

 Breakpoints                             Breakpoints, watchpoints, and
                                         exceptions

 Breakpoints                             Breakpoints and watchpoints

 Continuing and Stepping                 Resuming execution

 Signals                                 Signals

 Thread Stops                            Stopping and starting multi-thread
                                         programs


ΓòÉΓòÉΓòÉ 8.1. Breakpoints, watchpoints, and exceptions ΓòÉΓòÉΓòÉ

A breakpoint makes your program stop whenever a certain point in the program is
reached.  For each breakpoint, you can add conditions to control in finer
detail whether your program stops. You can set breakpoints with the break
command and its variants (see Setting breakpoints), to specify the place where
your program should stop by line number, function name or exact address in the
program. In languages with exception handling (such as gnu C++), you can also
set breakpoints where an exception is raised ( see Breakpoints and exceptions).

In SunOS 4.x, SVR4, and Alpha OSF/1 configurations, you can now set breakpoints
in shared libraries before the executable is run.

A watchpoint is a special breakpoint that stops your program when the value of
an expression changes.  You must use a different command to set watchpoints (
see Setting watchpoints), but aside from that, you can manage a watchpoint like
any other breakpoint: you enable, disable, and delete both breakpoints and
watchpoints using the same commands.

You can arrange to have values from your program displayed automatically
whenever  stops at a breakpoint. See Automatic display.

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

 Set Breaks                              Setting breakpoints
 Set Watchpoints                         Setting watchpoints

 Exception Handling                      Breakpoints and exceptions

 Delete Breaks                           Deleting breakpoints
 Disabling                               Disabling breakpoints
 Conditions                              Break conditions
 Break Commands                          Breakpoint command lists

 Breakpoint Menus                        Breakpoint menus


ΓòÉΓòÉΓòÉ 8.1.1. Setting breakpoints ΓòÉΓòÉΓòÉ

Breakpoints are set with the break command (abbreviated b).  The debugger
convenience variable `$bpnum' records the number of the breakpoints you've set
most recently; see Convenience variables, for a discussion of what you can do
with convenience variables.

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

 break function
           Set a breakpoint at entry to function function. When using source
           languages that permit overloading of symbols, such as C++, function
           may refer to more than one possible place to break. See Breakpoint
           menus, for a discussion of that situation.

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

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

 break filename:linenum
           Set a breakpoint at line linenum in source file filename.

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

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

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

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

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

 tbreak args
           Set a breakpoint enabled only for one stop.  args are the same as
           for the break command, and the breakpoint is set in the same way,
           but the breakpoint is automatically deleted after the first time
           your program stops there.  See Disabling breakpoints.

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

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

 rbreak regex
           Set breakpoints on all functions matching the regular expression
           regex.  This command sets an unconditional breakpoint on all
           matches, printing a list of all breakpoints it set.  Once these
           breakpoints are set, they are treated just like the breakpoints set
           with the break command.  You can delete them, disable them, or make
           them conditional the same way as any other breakpoint.

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

 info breakpoints [n]
 info break [n]
 info watchpoints [n]
           Print a table of all breakpoints and watchpoints set and not
           deleted, with the following columns for each breakpoint:

           Breakpoint Numbers

           Type
                          Breakpoint or watchpoint.

           Disposition
                          Whether the breakpoint is marked to be disabled or
                          deleted when hit.

           Enabled or Disabled
                          Enabled breakpoints are marked with `y'.  `n' marks
                          breakpoints that are not enabled.

           Address
                          Where the breakpoint is in your program, as a memory
                          address

           What
                          Where the breakpoint is in the source for your
                          program, as a file and line number.

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

           info break with a breakpoint number n as argument lists only that
           breakpoint.  The convenience variable $_ and the default
           examining-address for the x command are set to the address of the
           last breakpoint listed (see Examining memory).

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

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

  itself sometimes sets breakpoints in your program for special purposes, such
 as proper handling of longjmp (in C programs). These internal breakpoints are
 assigned negative numbers, starting with -1; `info breakpoints' does not
 display them.

 You can see these breakpoints with the  maintenance command `maint info
 breakpoints'.

 maint info breakpoints
           Using the same format as `info breakpoints', display both the
           breakpoints you've set explicitly, and those  is using for internal
           purposes.  Internal breakpoints are shown with negative breakpoint
           numbers.  The type column identifies what kind of breakpoint is
           shown:

           breakpoint
                          Normal, explicitly set breakpoint.

           watchpoint
                          Normal, explicitly set watchpoint.

           longjmp
                          Internal breakpoint, used to handle correctly
                          stepping through longjmp calls.

           longjmp resume
                          Internal breakpoint at the target of a longjmp.

           until
                          Temporary internal breakpoint used by the  until
                          command.

           finish
                          Temporary internal breakpoint used by the  finish
                          command.


ΓòÉΓòÉΓòÉ 8.1.2. Setting watchpoints ΓòÉΓòÉΓòÉ

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

Watchpoints currently execute two orders of magnitude more slowly than other
breakpoints, but this can be well worth it to catch errors where you have no
clue what part of your program is the culprit.

 watch expr
           Set a watchpoint for an expression.  will break when expr is written
           into by the program and its value changes. This can be used with the
           new trap-generation provided by SPARClite DSU.  DSU will generate
           traps when a program accesses some date or instruction address that
           is assigned to the debug registers. For the data addresses, DSU
           facilitates the watch command. However the hardware breakpoint
           registers can only take two data watchpoints, and both watchpoints
           must be the same kind.  For example, you can set two watchpoints
           with watch commands, two with rwatch commands, *or* two with awatch
           commands, but you cannot set one watchpoint with one command and the
           other with a different command.  will reject the command if you try
           to mix watchpoints. Delete or disable unused watchpoint commands
           before setting new ones.

 rwatch expr
           Set a watchpoint that will break when watch args is read by the
           program. If you use both watchpoints, both must be set with the
           rwatch command.

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

 info watchpoints
           This command prints a list of watchpoints and breakpoints; it is the
           same as info break.

 Warning: in multi-thread programs, watchpoints have only limited usefulness.
 With the current watchpoint implementation, can only watch the value of an
 expression in a single thread.  If you are confident that the expression can
 only change due to the current thread's activity (and if you are also
 confident that no other thread can become current), then you can use
 watchpoints as usual.  However,  may not notice when a non-current thread's
 activity changes the expression.


ΓòÉΓòÉΓòÉ 8.1.3. Breakpoints and exceptions ΓòÉΓòÉΓòÉ

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

 catch exceptions
           You can set breakpoints at active exception handlers by using the
           catch command.  exceptions is a list of names of exceptions to
           catch.

 You can use info catch to list active exception handlers. See Information
 about a frame.

 There are currently some limitations to exception handling in :

     If you call a function interactively,  normally returns control to you
      when the function has finished executing.  If the call raises an
      exception, however, the call may bypass the mechanism that returns
      control to you and cause your program to simply continue running until it
      hits a breakpoint, catches a signal that  is listening for, or exits.

     You cannot raise an exception interactively.

     You cannot install an exception handler interactively.

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

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

      /* addr is where the exception identifier is stored.
         ID is the exception identifier.  */
      void __raise_exception (void **addr, void *id);

 To make the debugger catch all exceptions before any stack unwinding takes
 place, set a breakpoint on __raise_exception (see Breakpoints; watchpoints;
 and exceptions).

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


ΓòÉΓòÉΓòÉ 8.1.4. Deleting breakpoints ΓòÉΓòÉΓòÉ

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

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

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

 clear
           Delete any breakpoints at the next instruction to be executed in the
           selected stack frame (see Selecting a frame).  When the innermost
           frame is selected, this is a good way to delete a breakpoint where
           your program just stopped.

 clear function
 clear filename:function
           Delete any breakpoints set at entry to the function function.

 clear linenum
 clear filename:linenum
           Delete any breakpoints set at or within the code of the specified
           line.

 delete [breakpoints] [bnums...]
           Delete the breakpoints or watchpoints of the numbers specified as
           arguments.  If no argument is specified, delete all breakpoints (
           asks confirmation, unless you have set confirm off).  You can
           abbreviate this command as d.


ΓòÉΓòÉΓòÉ 8.1.5. Disabling breakpoints ΓòÉΓòÉΓòÉ

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

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

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

     Enabled.  The breakpoint stops your program.  A breakpoint set with the
      break command starts out in this state.

     Disabled.  The breakpoint has no effect on your program.

     Enabled once.  The breakpoint stops your program, but then becomes
      disabled.  A breakpoint set with the tbreak command starts out in this
      state.

     Enabled for deletion.  The breakpoint stops your program, but immediately
      after it does so it is deleted permanently.

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

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

 enable [breakpoints] [bnums...]
           Enable the specified breakpoints (or all defined breakpoints).  They
           become effective once again in stopping your program.

 enable [breakpoints] once bnums...
           Enable the specified breakpoints temporarily.  disables any of these
           breakpoints immediately after stopping your program.

 enable [breakpoints] delete bnums...
           Enable the specified breakpoints to work once, then die. deletes any
           of these breakpoints as soon as your program stops there.

 Except for a breakpoint set with tbreak ( see Setting breakpoints),
 breakpoints that you set are initially enabled; subsequently, they become
 disabled or enabled only when you use one of the commands above.  (The command
 until can set and delete a breakpoint of its own, but it does not change the
 state of your other breakpoints; see Continuing and stepping.)


ΓòÉΓòÉΓòÉ 8.1.6. Break conditions ΓòÉΓòÉΓòÉ

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

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

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

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

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

 condition bnum expression
           Specify expression as the break condition for breakpoint or
           watchpoint number bnum.  After you set a condition, breakpoint bnum
           stops your program only if the value of expression is true (nonzero,
           in C).  When you use condition, checks expression immediately for
           syntactic correctness, and to determine whether symbols in it have
           referents in the context of your breakpoint.  does not actually
           evaluate expression at the time the condition command is given,
           however.  See Expressions.

 condition bnum
           Remove the condition from breakpoint number bnum.  It becomes an
           ordinary unconditional breakpoint.

 A special case of a breakpoint condition is to stop only when the breakpoint
 has been reached a certain number of times.  This is so useful that there is a
 special way to do it, using the ignore count of the breakpoint.  Every
 breakpoint has an ignore count, which is an integer.  Most of the time, the
 ignore count is zero, and therefore has no effect.  But if your program
 reaches a breakpoint whose ignore count is positive, then instead of stopping,
 it just decrements the ignore count by one and continues.  As a result, if the
 ignore count value is n, the breakpoint does not stop the next n times your
 program reaches it.

 ignore bnum count
           Set the ignore count of breakpoint number bnum to count. The next
           count times the breakpoint is reached, your program's execution does
           not stop; other than to decrement the ignore count, takes no action.

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

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

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

           You could achieve the effect of the ignore count with a condition
           such as `$foo-- <= 0' using a debugger convenience variable that is
           decremented each time. See Convenience variables.


ΓòÉΓòÉΓòÉ 8.1.7. Breakpoint command lists ΓòÉΓòÉΓòÉ

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

 commands [bnum]
 ... command-list ...
 end
           Specify a list of commands for breakpoint number bnum.  The commands
           themselves appear on the following lines.  Type a line containing
           just end to terminate the commands.

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

           With no bnum argument, commands refers to the last breakpoint or
           watchpoint set (not to the breakpoint most recently encountered).

 Pressing RET as a means of repeating the last  command is disabled within a
 command-list.

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

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

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

 The commands echo, output, and printf allow you to print precisely controlled
 output, and are often useful in silent breakpoints.  See Commands for
 controlled output.

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

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

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

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


ΓòÉΓòÉΓòÉ 8.1.8. Breakpoint menus ΓòÉΓòÉΓòÉ

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

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

() b String::after
[0] cancel
[1] all
[2] file:String.cc; line number:867
[3] file:String.cc; line number:860
[4] file:String.cc; line number:875
[5] file:String.cc; line number:853
[6] file:String.cc; line number:846
[7] file:String.cc; line number:735
> 2 4 6
Breakpoint 1 at 0xb26c: file String.cc, line 867.
Breakpoint 2 at 0xb344: file String.cc, line 875.
Breakpoint 3 at 0xafcc: file String.cc, line 846.
Multiple breakpoints were set.
Use the "delete" command to delete unwanted
 breakpoints.
()


ΓòÉΓòÉΓòÉ 8.2. Continuing and stepping ΓòÉΓòÉΓòÉ

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

 continue [ignore-count]
 c [ignore-count]
 fg [ignore-count]
           Resume program execution, at the address where your program last
           stopped; any breakpoints set at that address are bypassed.  The
           optional argument ignore-count allows you to specify a further
           number of times to ignore a breakpoint at this location; its effect
           is like that of ignore (see Break conditions).

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

           The synonyms c and fg are provided purely for convenience, and have
           exactly the same behavior as continue.

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

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

 step
           Continue running your program until control reaches a different
           source line, then stop it and return control to .  This command is
           abbreviated s.

           Warning: If you use the step command while control is within a
           function that was compiled without debugging information, execution
           proceeds until control reaches a function that does have debugging
           information.  Likewise, it will not step into a function which is
           compiled without debugging information.  To step through functions
           without debugging information, use the stepi command, described
           below.

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

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

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

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

           An argument count is a repeat count, as for step.

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

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

           Contrast this with the return command ( see Returning from a
           function).
           u

 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 next
           command, except that when until encounters a jump, it automatically
           continues execution until the program counter is greater than the
           address of the jump.

           This means that when you reach the end of a loop after single
           stepping though it, until makes your program continue execution
           until it exits the loop.  In contrast, a next command at the end of
           a loop simply steps back to the beginning of the loop, which forces
           you to step through the next iteration.

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

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

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

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

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

 until location
 u location
           Continue running your program until either the specified location is
           reached, or the current stack frame returns.  location is any of the
           forms of argument acceptable to break ( see Setting breakpoints).
           This form of the command uses breakpoints, and hence is quicker than
           until without an argument.

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

           It is often useful to do `display/i $pc' when stepping by machine
           instructions.  This makes  automatically display the next
           instruction to be executed, each time your program stops. See
           Automatic display.

           An argument is a repeat count, as in step.

 nexti
 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 next.


ΓòÉΓòÉΓòÉ 8.3. Stopping and starting multi-thread programs ΓòÉΓòÉΓòÉ

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

 break linespec thread threadno
 break linespec thread threadno if ...
           linespec specifies source lines; there are several ways of writing
           them, but the effect is always to specify some source line.

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

           If you do not specify `thread threadno' when you set a breakpoint,
           the breakpoint applies to all threads of your program.

           You can use the thread qualifier on conditional breakpoints as well;
           in this case, place `thread threadno' before the breakpoint
           condition, like this:

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

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

 Conversely, whenever you restart the program, all threads start executing.
 This is true even when single-stepping with commands like step or next.

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

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


ΓòÉΓòÉΓòÉ 9. Examining the Stack ΓòÉΓòÉΓòÉ

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

Each time your program performs a function call, information about the call is
generated. That information includes the location of the call in your program,
the arguments of the call, and the local variables of the function being
called. The information is saved in a block of data called a stack frame. The
stack frames are allocated in a region of memory called the call stack.

When your program stops, the  commands for examining the stack allow you to see
all of this information.

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

When your program stops,  automatically selects the currently executing frame
and describes it briefly, similar to the frame command (see Information about a
frame).

 Frames                                  Stack frames
 Backtrace                               Backtraces
 Selection                               Selecting a frame
 Frame Info                              Information on a frame

 MIPS Stack                              MIPS machines and the function stack


ΓòÉΓòÉΓòÉ 9.1. Stack frames ΓòÉΓòÉΓòÉ

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

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

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

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

Some compilers provide a way to compile functions so that they operate without
stack frames.  (For example, the  option `-fomit-frame-pointer' generates
functions without a frame.) This is occasionally done with heavily used library
functions to save the frame setup time.  has limited facilities for dealing
with these function invocations.  If the innermost function invocation has no
stack frame,  nevertheless regards it as though it had a separate frame, which
is numbered zero as usual, allowing correct tracing of the function call chain.
However,  has no provision for frameless functions elsewhere in the stack.

 frame args
           The frame command allows you to move from one stack frame to
           another, and to print the stack frame you select.  args may be
           either the address of the frame or the stack frame number.  Without
           an argument, frame prints the current stack frame.

 select-frame
           The select-frame command allows you to move from one stack frame to
           another without printing the frame.  This is the silent version of
           frame.


ΓòÉΓòÉΓòÉ 9.2. Backtraces ΓòÉΓòÉΓòÉ

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

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

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

 backtrace n
 bt n
           Similar, but print only the innermost n frames.

 backtrace -n
 bt -n
           Similar, but print only the outermost n frames.

 The names where and info stack (abbreviated info s) are additional aliases for
 backtrace.

 Each line in the backtrace shows the frame number and the function name. The
 program counter value is also shown---unless you use 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 `bt 3', so it
 shows the innermost three frames.

  #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...)

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


ΓòÉΓòÉΓòÉ 9.3. Selecting a frame ΓòÉΓòÉΓòÉ

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

 frame n
 f n
           Select frame number n.  Recall that frame zero is the innermost
           (currently executing) frame, frame one is the frame that called the
           innermost one, and so on.  The highest-numbered frame is the one for
           main.

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

           On the SPARC architecture, frame needs two addresses to select an
           arbitrary frame: a frame pointer and a stack pointer.

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

           On the 29k architecture, it needs three addresses: a register stack
           pointer, a program counter, and a memory stack pointer.

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

 down n
           Move n frames down the stack.  For positive numbers n, this advances
           toward the innermost frame, to lower frame numbers, to frames that
           were created more recently.  n defaults to one.  You may abbreviate
           down as do.

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

 For example:

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

 After such a printout, the list command with no arguments prints ten lines
 centered on the point of execution in the frame. See Printing source lines.

 up-silently n
 down-silently n
           These two commands are variants of up and 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  command
           scripts, where the output might be unnecessary and distracting.


ΓòÉΓòÉΓòÉ 9.4. Information about a frame ΓòÉΓòÉΓòÉ

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

 frame
 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 f.  With an argument,
           this command is used to select a stack frame. See Selecting a frame.

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

               the address of the frame

               the address of the next frame down (called by this frame)

               the address of the next frame up (caller of this frame)

               the language in which the source code corresponding to this
                frame is written

               the address of the frame's arguments

               the program counter saved in it (the address of execution in
                the caller frame)

               which registers were saved in the frame

           The verbose description is useful when something has gone wrong that
           has made the stack format fail to fit the usual conventions.

 info frame addr
 info f addr
           Print a verbose description of the frame at address addr, without
           selecting that frame.  The selected frame remains unchanged by this
           command.  This requires the same kind of address (more than one for
           some architectures) that you specify in the frame command. See
           Selecting a frame.

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

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

 info catch
           Print a list of all the exception handlers that are active in the
           current stack frame at the current point of execution.  To see other
           exception handlers, visit the associated frame (using the up, down,
           or frame commands); then type info catch. See Breakpoints and
           exceptions.


ΓòÉΓòÉΓòÉ 10. Examining Source Files ΓòÉΓòÉΓòÉ

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

If you use  through its gnu Emacs interface, you may prefer to use Emacs
facilities to view source; see Using  under gnu Emacs.

 List                                    Printing source lines

 Search                                  Searching source files

 Source Path                             Specifying source directories
 Machine Code                            Source and machine code


ΓòÉΓòÉΓòÉ 10.1. Printing source lines ΓòÉΓòÉΓòÉ

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

Here are the forms of the list command most commonly used:

 list linenum
           Print lines centered around line number linenum in the current
           source file.

 list function
           Print lines centered around the beginning of function function.

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

 list -
           Print lines just before the lines last printed.

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

 set listsize count
           Make the list command display count source lines (unless the list
           argument explicitly specifies some other number).

 show listsize
           Display the number of lines that list prints.

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

 In general, the list command expects you to supply zero, one or two linespecs.
 Linespecs specify source lines; there are several ways of writing them but the
 effect is always to specify some source line. Here is a complete description
 of the possible arguments for list:

 list linespec
           Print lines centered around the line specified by linespec.

 list first,last
           Print lines from first to last.  Both arguments are linespecs.

 list ,last
           Print lines ending with last.

 list first,
           Print lines starting with first.

 list +
           Print lines just after the lines last printed.

 list -
           Print lines just before the lines last printed.

 list
           As described in the preceding table.

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

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

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

 -offset
           Specifies the line offset lines before the last line printed.

 filename:number
           Specifies line number in the source file filename.

 function
           Specifies the line that begins the body of the function function.
           For example: in C, this is the line with the open brace.

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

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


ΓòÉΓòÉΓòÉ 10.2. Searching source files ΓòÉΓòÉΓòÉ

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

 forward-search regexp
 search regexp
           The command `forward-search regexp' checks each line, starting with
           the one following the last line listed, for a match for regexp.  It
           lists the line that is found.  You can use the synonym `search
           regexp' or abbreviate the command name as fo.

 reverse-search regexp
           The command `reverse-search regexp' checks each line, starting with
           the one before the last line listed and going backward, for a match
           for regexp.  It lists the line that is found.  You can abbreviate
           this command as rev.


ΓòÉΓòÉΓòÉ 10.3. Specifying source directories ΓòÉΓòÉΓòÉ

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.
has a list of directories to search for source files; this is called the source
path.  Each time  wants a source file, it tries all the directories in the
list, in the order they are present in the list, until it finds a file with the
desired name.  Note that the executable search path is not used for this
purpose.  Neither is the current working directory, unless it happens to be in
the source path.

If  cannot find a source file in the source path, and the object program
records a directory,  tries that directory too.  If the source path is empty,
and there is no record of the compilation directory,  looks in the current
directory as a last resort.

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

When you start , its source path is empty. To add other directories, use the
directory command.

 directory dirname ...

 dir dirname ...
           Add directory dirname to the front of the source path.  Several
           directory names may be given to this command, separated by `:' or
           whitespace.  You may specify a directory that is already in the
           source path; this moves it forward, so  searches it sooner.

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

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

 show directories
           Print the source path: show which directories it contains.

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

   1. Use directory with no argument to reset the source path to empty.

   2. Use directory with suitable arguments to reinstall the directories you
      want in the source path.  You can add all the directories in one command.


ΓòÉΓòÉΓòÉ 10.4. Source and machine code ΓòÉΓòÉΓòÉ

You can use the command info line to map source lines to program addresses (and
vice versa), and the command disassemble to display a range of addresses as
machine instructions.  When run under gnu Emacs mode, the info line command now
causes the arrow to point to the line specified.  Also, info line prints
addresses in symbolic form as well as hex.

 info line linespec
           Print the starting and ending addresses of the compiled code for
           source line linespec.  You can specify source lines in any of the
           ways understood by the list command ( see Printing source lines).

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

  () info line m4_changecom
  Line 895 of "builtin.c" starts at pc 0x634c and ends at 0x6350.

 We can also inquire (using *addr as the form for linespec) what source line
 covers a particular address:

  () info line *0x63ff
  Line 926 of "builtin.c" starts at pc 0x63e4 and ends at 0x6404.

 After info line, the default address for the x command is changed to the
 starting address of the line, so that `x/i' is sufficient to begin examining
 the machine code ( see Examining memory).  Also, this address is saved as the
 value of the convenience variable $_ ( see Convenience variables).

 disassemble
           This specialized command dumps a range of memory as machine
           instructions.  The default memory range is the function surrounding
           the program counter of the selected frame.  A single argument to
           this command is a program counter value;  dumps the function
           surrounding this value.  Two arguments specify a range of addresses
           (first inclusive, second exclusive) to dump.

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

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

 set assembly-language instruction-set
           This command selects the instruction set to use when disassembling
           the program via the disassemble or x/i commands.  It is useful for
           architectures that have more than one native instruction set.

           Currently it is only defined for the Intel x86 family.  You can set
           instruction-set to either i386 or i8086.  The default is i386.


ΓòÉΓòÉΓòÉ 11. Examining Data ΓòÉΓòÉΓòÉ

The usual way to examine data in your program is with the print command
(abbreviated p), or its synonym inspect. It evaluates and prints the value of
an expression of the language your program is written in ( see Using  with
Different Languages).

 print exp
 print /f exp
           exp is an expression (in the source language).  By default the value
           of exp is printed in a format appropriate to its data type; you can
           choose a different format by specifying `/f', where f is a letter
           specifying the format; see Output formats.

 print
 print /f
           If you omit exp,  displays the last value again (from the value
           history; see Value history).  This allows you to conveniently
           inspect the same value in an alternative format.

 A more low-level way of examining data is with the x command. It examines data
 in memory at a specified address and prints it in a specified format.  See
 Examining memory.

 If you are interested in information about types, or about how the fields of a
 struct or class are declared, use the ptype exp command rather than print. See
 Examining the Symbol Table.

 Expressions                             Expressions
 Variables                               Program variables
 Arrays                                  Artificial arrays
 Output Formats                          Output formats
 Memory                                  Examining memory
 Auto Display                            Automatic display
 Print Settings                          Print settings
 Value History                           Value history
 Convenience Vars                        Convenience variables
 Registers                               Registers

 Floating Point Hardware                 Floating point hardware


ΓòÉΓòÉΓòÉ 11.1. Expressions ΓòÉΓòÉΓòÉ

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

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

Because C is so widespread, most of the expressions shown in examples in this
manual are in C. See Using  with Different Languages, for information on how to
use expressions in other languages.

In this section, we discuss operators that you can use in expressions
regardless of your programming language.

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

 supports these operators, in addition to those common to programming
languages:

 @
           `@' is a binary operator for treating parts of memory as arrays. See
           Artificial arrays, for more information.

 ::
           `::' allows you to specify a variable in terms of the file or
           function where it is defined.  See Program variables.

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


ΓòÉΓòÉΓòÉ 11.2. Program variables ΓòÉΓòÉΓòÉ

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 (see
Selecting a frame); they must be either:

     global (or static)

 or

     visible according to the scope rules of the programming language from the
      point of execution in that frame

 This means that in the function

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

 you can examine and use the variable a whenever your program is executing
 within the function foo, but you can only use or examine the variable b while
 your program is executing inside the block where 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 static
 variable in a particular function or file, using the colon-colon notation:

  file::variable
  function::variable

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

  () p 'f2.c'::x

 This use of `::' is very rarely in conflict with the very similar use of the
 same notation in C++.  also supports use of the C++ scope resolution operator
 in  expressions.

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


ΓòÉΓòÉΓòÉ 11.3. Artificial arrays ΓòÉΓòÉΓòÉ

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

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

int *array = (int *) malloc (len * sizeof (int));

you can print the contents of array with

p *array@len

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

Another way to create an artificial array is to use a cast. This re-interprets
a value as if it were an array. The value need not be in memory:

() p/x (short[2])0x12345678
$1 = {0x1234, 0x5678}

As a convenience, if you leave the array length out (as in `(type)[])value')
gdb calculates the size to fill the value (as `sizeof(value)/sizeof(type)':

() p/x (short[])0x12345678
$2 = {0x1234, 0x5678}

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

set $i = 0
p dtab[$i++]->fv
RET
RET
...


ΓòÉΓòÉΓòÉ 11.4. Output formats ΓòÉΓòÉΓòÉ

By default,  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 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 print command with a
slash and a format letter.  The format letters supported are:

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

 d
           Print as integer in signed decimal.

 u
           Print as integer in unsigned decimal.

 o
           Print as integer in octal.

 t
           Print as integer in binary.  The letter `t' stands for ``two''. (1)

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

                      () p/a 0x54320
                      $3 = 0x54320 <_initialize_vx+396>

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

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

 For example, to print the program counter in hex (see Registers), type

  p/x $pc

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

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


ΓòÉΓòÉΓòÉ 11.5. Examining memory ΓòÉΓòÉΓòÉ

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

 x/nfu addr
 x addr
 x
           Use the x command to examine memory.

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

 n, the repeat count
           The repeat count is a decimal integer; the default is 1.  It
           specifies how much memory (counting by units u) to display.

 f, the display format
           The display format is one of the formats used by print, `s'
           (null-terminated string), or `i' (machine instruction). The default
           is `x' (hexadecimal) initially. The default changes each time you
           use either x or print.

 u, the unit size
           The unit size is any of

           b
                          Bytes.

           h
                          Halfwords (two bytes).

           w
                          Words (four bytes).  This is the initial default.

           g
                          Giant words (eight bytes).

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

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

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

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

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

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

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

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


ΓòÉΓòÉΓòÉ 11.6. Automatic display ΓòÉΓòÉΓòÉ

If you find that you want to print the value of an expression frequently (to
see how it changes), you might want to add it to the automatic display list so
that  prints its value each time your program stops. Each expression added to
the list is given a number to identify it; to remove an expression from the
list, you specify that number. The automatic display looks like this:

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

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

 display exp
           Add the expression exp to the list of expressions to display each
           time your program stops.  See Expressions.

           display does not repeat if you press RET again after using it.

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

 display/fmt addr
           For fmt `i' or `s', or including a unit-size or a number of units,
           add the expression addr as a memory address to be examined each time
           your program stops.  Examining means in effect doing `x/fmt addr'.
           See Examining memory.

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

 undisplay dnums...
 delete display dnums...
           Remove item numbers dnums from the list of expressions to display.

           undisplay does not repeat if you press RET after using it.
           (Otherwise you would just get the error `No display number ...'.)

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

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

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

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

 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 display last_char while inside
 a function with an argument last_char,  displays this argument while your
 program continues to stop inside that function.  When it stops
 elsewhere---where there is no variable last_char---the display is disabled
 automatically.  The next time your program stops where last_char is
 meaningful, you can enable the display expression once again.


ΓòÉΓòÉΓòÉ 11.7. Print settings ΓòÉΓòÉΓòÉ

 provides the following ways to control how arrays, structures, and symbols are
printed.

These settings are useful for debugging programs in any language:

 set print address
 set print address on
            prints memory addresses showing the location of stack traces,
           structure values, pointer values, breakpoints, and so forth, even
           when it also displays the contents of those addresses.  The default
           is on.  For example, this is what a stack frame display looks like
           with set print address on:

                      () f
                      #0  set_quotes (lq=0x34c78 "<<", rq=0x34c88 ">>")
                          at input.c:530
                      530         if (lquote != def_lquote)

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

                      () set print addr off
                      () f
                      #0  set_quotes (lq="<<", rq=">>") at input.c:530
                      530         if (lquote != def_lquote)

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

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

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

 set print symbol-filename on
           Tell  to print the source file name and line number of a symbol in
           the symbolic form of an address.

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

 show print symbol-filename
           Show whether or not  will print the source file name and line number
           of a symbol in the symbolic form of an address.

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

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

 set print max-symbolic-offset max-offset
           Tell  to only display the symbolic form of an address if the offset
           between the closest earlier symbol and the address is less than
           max-offset.  The default is 0, which tells to always print the
           symbolic form of an address if any symbol precedes it.

 show print max-symbolic-offset
           Ask how large the maximum offset is that  prints in a symbolic
           address.

 If you have a pointer and you are not sure where it points, try `set print
 symbol-filename on'.  Then you can determine the name and source file location
 of the variable where it points, using `p/a pointer'.  This interprets the
 address in symbolic form. For example, here  shows that a variable ptt points
 at another variable t, defined in `hi2.c':

  () set print symbol-filename on
  () p/a ptt
  $4 = 0xe008 <t in hi2.c>

 Warning: For pointers that point to a local variable, `p/a' does not show the
 symbol name and filename of the referent, even with the appropriate set print
 options turned on.

 Other settings control how different kinds of objects are printed:

 set print array
 set print array on
           Pretty print arrays.  This format is more convenient to read, but
           uses more space.  The default is off.

 set print array off
           Return to compressed format for arrays.

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

 set print elements number-of-elements
           Set a limit on how many elements of an array  will print. If  is
           printing a large array, it stops printing after it has printed the
           number of elements set by the set print elements command. This limit
           also applies to the display of strings. Setting  number-of-elements
           to zero means that the printing is unlimited.

 show print elements
           Display the number of elements of a large array that  will print. If
           the number is 0, then the printing is unlimited.

 set print null-stop
           Cause  to stop printing the characters of an array when the first
           NULL is encountered.  This is useful when large arrays actually
           contain only short strings.

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

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

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

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

           This is the default format.

 show print pretty
           Show which format  is using to print structures.

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

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

 show print sevenbit-strings
           Show whether or not  is printing only seven-bit characters.

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

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

 show print union
           Ask  whether or not it will print unions which are contained in
           structures.

           For example, given the declarations

                      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}};

           with set print union on in effect `p foo' would print

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

           and with set print union off in effect it would print

                      $1 = {it = Tree, form = {...}}

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

 set print demangle
 set print demangle on
           Print C++ names in their source form rather than in the encoded
           (``mangled'') form passed to the assembler and linker for type-safe
           linkage.  The default is `on'.

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

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

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

 set demangle-style style
           Choose among several encoding schemes used by different compilers to
           represent C++ names.  The choices for style are currently:

           auto
                          Allow  to choose a decoding style by inspecting your
                          program.

           gnu
                          Decode based on the gnu C++ compiler (g++) encoding
                          algorithm. This is the default.

           lucid
                          Decode based on the Lucid C++ compiler (lcc) encoding
                          algorithm.

           arm
                          Decode using the algorithm in the C++ Annotated
                          Reference Manual. *Warning:* this setting alone is
                          not sufficient to allow debugging cfront-generated
                          executables.  would require further enhancement to
                          permit that.

           foo
                          Show the list of formats.

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

 set print object
 set print object on
           When displaying a pointer to an object, identify the actual
           (derived) type of the object rather than the declared type, using
           the virtual function table.

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

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

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

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

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

 set print vtbl
 set print vtbl on
           Pretty print C++ virtual function tables.  The default is off.

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

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


ΓòÉΓòÉΓòÉ 11.8. Value history ΓòÉΓòÉΓòÉ

Values printed by the print command are saved in the value history.  This
allows you to refer to them in other expressions. Values are kept until the
symbol table is re-read or discarded (for example with the file or 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.

The values printed are given history numbers by which you can refer to them.
These are successive integers starting with one. print shows you the history
number assigned to a value by printing `$num = ' before the value; here num is
the history number.

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

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

p *$

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

p *$.next

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

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

print x
set x=5

then the value recorded in the value history by the print command remains 4
even though the value of x has changed.

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

 show values n
           Print ten history values centered on history item number n.

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

 Pressing RET to repeat show values n has exactly the same effect as `show
 values +'.


ΓòÉΓòÉΓòÉ 11.9. Convenience variables ΓòÉΓòÉΓòÉ

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

Convenience variables are prefixed with `$'.  Any name preceded by `$' can be
used for a convenience variable, unless it is one of the predefined
machine-specific register names (see Registers). (Value history references, in
contrast, are numbers preceded by `$'.  See Value history.)

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

set $foo = *object_ptr

would save in $foo the value contained in the object pointed to by object_ptr.

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

Convenience variables have no fixed types.  You can assign a convenience
variable any type of value, 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.

 show convenience
           Print a list of convenience variables used so far, and their values.
           Abbreviated show con.

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

  set $i = 0
  print bar[$i++]->contents

 Repeat that command by typing RET.

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

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

 $__
           The variable $__ is automatically set by the x command to the value
           found in the last address examined.  Its type is chosen to match the
           format in which the data was printed.

 $_exitcode
           The variable $_exitcode is automatically set to the exit code when
           the program being debugged terminates.


ΓòÉΓòÉΓòÉ 11.10. Registers ΓòÉΓòÉΓòÉ

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

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

 info all-registers
           Print the names and values of all registers, including
           floating-point registers.

 info registers regname ...
           Print the relativized value of each specified register regname. As
           discussed in detail below, register values are normally relative to
           the selected stack frame.  regname may be any register name valid on
           the machine you are using, with or without the initial `$'.

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

  p/x $pc

 or print the instruction to be executed next with

  x/i $pc

 or add four to the stack pointer(2) with

  set $sp += 4

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

  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 print it as a
 floating point value with `print/f $regname').

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

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

 However,  must deduce where registers are saved, from the machine code
 generated by your compiler.  If some registers are not saved, or if  is unable
 to locate the saved registers, the selected stack frame makes no difference.


ΓòÉΓòÉΓòÉ 11.11. Floating point hardware ΓòÉΓòÉΓòÉ

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

 info float
           Display hardware-dependent information about the floating point
           unit.  The exact contents and layout vary depending on the floating
           point chip.  Currently, `info float' is supported on the ARM and x86
           machines.


ΓòÉΓòÉΓòÉ 12. Using  with Different Languages ΓòÉΓòÉΓòÉ

Language-specific information is built into  for some languages, allowing you
to express operations like the above in your program's native language, and
allowing  to output values in a manner consistent with the syntax of your
program's native language.  The language you use to build expressions is called
the working language.

 Setting                                 Switching between source languages
 Show                                    Displaying the language

 Checks                                  Type and range checks

 Support                                 Supported languages


ΓòÉΓòÉΓòÉ 12.1. Switching between source languages ΓòÉΓòÉΓòÉ

There are two ways to control the working language---either have set it
automatically, or select it manually yourself.  You can use the set language
command for either purpose.  On startup, defaults to setting the language
automatically.  The working language is used to determine how expressions you
type are interpreted, how values are printed, etc.

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

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

 Filenames                               Filename extensions and languages.
 Manually                                Setting the working language manually
 Automatically                           Having GDBN infer the source language


ΓòÉΓòÉΓòÉ 12.1.1. List of filename extensions and languages ΓòÉΓòÉΓòÉ

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

 `.ada'
 `.ads'
 `.adb'
 `.a'
           Ada source file.

 `.c'
           C source file

 `.C'
 `.cc'
 `.cxx'
 `.cpp'
 `.cp'
 `.c++'
           C++ source file

 `.ch'
 `.c186'
 `.c286'
           CHILL source file.

 `.s'
 `.S'
           Assembler source file.  This actually behaves almost like C, but
           does not skip over function prologues when stepping.


ΓòÉΓòÉΓòÉ 12.1.2. Setting the working language ΓòÉΓòÉΓòÉ

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

If you wish, you may set the language manually.  To do this, issue the command
`set language lang', where lang is the name of a language, such as c. For a
list of the supported languages, type `set language'.


ΓòÉΓòÉΓòÉ 12.1.3. Having  infer the source language ΓòÉΓòÉΓòÉ

To have  set the working language automatically, use `set language local' or
`set language auto'. then infers the working language.  That is, when your
program stops in a frame (usually by encountering a breakpoint),  sets the
working language to the language recorded for the function in that frame.  If
the language for a frame is unknown (that is, if the function or block
corresponding to the frame was defined in a source file that does not have a
recognized extension), the current working language is not changed, and  issues
a warning.

This may not seem necessary for most programs, which are written entirely in
one source language.  However, program modules and libraries written in one
source language can be used by a main program written in a different source
language.  Using `set language auto' in this case frees you from having to set
the worE13p-unx-docs\texi\gdb.texinfo line 5577 : unrecognized Texinfo command
@raisesections king language manually.


ΓòÉΓòÉΓòÉ 12.2. Displaying the language ΓòÉΓòÉΓòÉ

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

 show language
           Display the current working language.  This is the language you can
           use with commands such as print to build and compute expressions
           that may involve variables in your program.

 info frame
           Display the source language for this frame.  This language becomes
           the working language if you use an identifier from this frame. See
           Information about a frame, to identify the other information listed
           here.

 info source
           Display the source language of this source file. See Examining the
           Symbol Table, to identify the other information listed here.

 Type Checking                           An overview of type checking


 Range Checking                          An overview of range checking


ΓòÉΓòÉΓòÉ 12.3. Supported languages ΓòÉΓòÉΓòÉ

 4 supports C, and C++. Some  features may be used in expressions regardless of
the language you use: the  @ and :: operators, and the `{type}addr' construct (
see Expressions) can be used with the constructs of any supported language.

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

 C                                       C and C++

 Modula-2                                Modula-2

 Ada                                     Ada


ΓòÉΓòÉΓòÉ 12.3.1. C and C++ ΓòÉΓòÉΓòÉ

Since C and C++ are so closely related, many features of  apply to both
languages.  Whenever this is the case, we discuss those languages together.

The C++ debugging facilities are jointly implemented by the gnu C++ compiler
and .  Therefore, to debug your C++ code effectively, you must compile your C++
programs with the gnu C++ compiler, g++.

For best results when debugging C++ programs, use the stabs debugging format.
You can select that format explicitly with the g++ command-line options
`-gstabs' or `-gstabs+'.  See Options for Debugging Your Program or gnu CC, for
more information.

 C Operators                             C operators


 C Constants                             C constants
 Debugging C                             GDBN and C

 C Operators                             C and C++ operators
 C Constants                             C and C++ constants
 Cplus expressions                       C++ expressions
 C Defaults                              Default settings for C and C++

 C Checks                                C and C++ type and range checks

 Debugging C                             GDBN and C
 Debugging C plus plus                   Special features for C++


ΓòÉΓòÉΓòÉ 12.3.1.1. C and C++ operators ΓòÉΓòÉΓòÉ

Operators must be defined on values of specific types.  For instance,

+ is defined on numbers, but not on structures.  Operators are often defined on
groups of types.

For the purposes of C and C++, the following definitions hold:

     Integral types include int with any of its storage-class specifiers;
      char; and enum.

     Floating-point types include float and double.

     Pointer types include all types defined as (type *).

     Scalar types include all of the above.

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

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

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

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

 ?:
           The ternary operator.  a ? b : c can be thought of as:  if a then b
           else c.  a should be of an integral type.

 ||
           Logical or.  Defined on integral types.

 &&
           Logical and.  Defined on integral types.

 |
           Bitwise or.  Defined on integral types.

 ^
           Bitwise exclusive-or.  Defined on integral types.

 &
           Bitwise and.  Defined on integral types.

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

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

 <<, >>
           left shift, and right shift.  Defined on integral types.

 @
           The  ``artificial array'' operator (see Expressions).

 +, -
           Addition and subtraction.  Defined on integral types, floating-point
           types and pointer types.

 *, /, %
           Multiplication, division, and modulus.  Multiplication and division
           are defined on integral and floating-point types.  Modulus is
           defined on integral types.

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

 *
           Pointer dereferencing.  Defined on pointer types.  Same precedence
           as ++.

 &
           Address operator.  Defined on variables.  Same precedence as ++.

           For debugging C++,  implements a use of `&' beyond what is allowed
           in the C++ language itself: you can use `&(&ref)' (or, if you
           prefer, simply `&&ref') to examine the address where a C++ reference
           variable (declared with `&ref') is stored.

 -
           Negative.  Defined on integral and floating-point types.  Same
           precedence as ++.

 !
           Logical negation.  Defined on integral types.  Same precedence as
           ++.

 ~
           Bitwise complement operator.  Defined on integral types.  Same
           precedence as ++.

 ., ->
           Structure member, and pointer-to-structure member.  For convenience,
           regards the two as equivalent, choosing whether to dereference a
           pointer based on the stored type information. Defined on struct and
           union data.

 []
           Array indexing.  a[i] is defined as *(a+i).  Same precedence as ->.

 ()
           Function parameter list.  Same precedence as ->.

 ::
           C++ scope resolution operator.  Defined on struct, union, and class
           types.

 ::
           Doubled colons also represent the  scope operator ( see
           Expressions). Same precedence as ::, above.


ΓòÉΓòÉΓòÉ 12.3.1.2. C and C++ constants ΓòÉΓòÉΓòÉ

 allows you to express the constants of C and C++ in the following ways:

     Integer constants are a sequence of digits.  Octal constants are
      specified by a leading `0' (i.e. zero), and hexadecimal constants by a
      leading `0x' or `0X'.  Constants may also end with a letter `l',
      specifying that the constant should be treated as a long value.

     Floating point constants are a sequence of digits, followed by a decimal
      point, followed by a sequence of digits, and optionally followed by an
      exponent.  An exponent is of the form: `e[[+]|-]nnn', where nnn is
      another sequence of digits.  The `+' is optional for positive exponents.

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

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

     String constants are a sequence of character constants surrounded by
      double quotes (").

     Pointer constants are an integral value.  You can also write pointers to
      constants using the C operator `&'.

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


ΓòÉΓòÉΓòÉ 12.3.1.3. C++ expressions ΓòÉΓòÉΓòÉ

 expression handling has a number of extensions to interpret a significant
subset of C++ expressions.

Warning:  can only debug C++ code if you compile with the gnu C++ compiler.
Moreover, C++ debugging depends on the use of additional debugging information
in the symbol table, and thus requires special support.  has this support only
with the stabs debug format.  In particular, if your compiler generates a.out,
MIPS ecoff, RS/6000 xcoff, or elf with stabs extensions to the symbol table,
these facilities are all available.  (With gnu CC, you can use the `-gstabs'
option to request stabs debugging extensions explicitly.)  Where the object
code format is standard coff or dwarf in elf, on the other hand, most of the
C++ support in  does not work.

   1. Member function calls are allowed; you can use expressions like

            count = aml->GetOriginal(x, y)

   2. While a member function is active (in the selected stack frame), your
      expressions have the same namespace available as the member function;
      that is,  allows implicit references to the class instance pointer this
      following the same rules as C++.

   3. You can call overloaded functions;  resolves the function call to the
      right definition, with one restriction---you must use arguments of the
      type required by the function that you want to call.  does not perform
      conversions requiring constructors or user-defined type operators.

   4. understands variables declared as C++ references; you can use them in
      expressions just as you do in C++ source---they are automatically
      dereferenced.

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

   5. supports the C++ name resolution operator ::---your expressions can use
      it just as expressions in your program do.  Since one scope may be
      defined in another, you can use :: repeatedly if necessary, for example
      in an expression like `scope1::scope2::name'.  also allows resolving name
      scope by reference to source files, in both C and C++ debugging (see
      Program variables).


ΓòÉΓòÉΓòÉ 12.3.1.4. C and C++ defaults ΓòÉΓòÉΓòÉ

If you allow  to set type and range checking automatically, they both default
to off whenever the working language changes to C or C++.  This happens
regardless of whether you or selects the working language.

If you allow  to set the language automatically, it recognizes source files
whose names end with `.c', `.C', or `.cc', and when  enters code compiled from
one of these files, it sets the working language to C or C++. See Having  infer
the source language, for further details.


ΓòÉΓòÉΓòÉ 12.3.1.5. and C ΓòÉΓòÉΓòÉ

The set print union and show print union commands apply to

the union type.  When set to `on', any union that is inside a struct or class
is also printed. Otherwise, it appears as `{...}'.

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


ΓòÉΓòÉΓòÉ 12.3.1.6. features for C++ ΓòÉΓòÉΓòÉ

Some  commands are particularly useful with C++, and some are designed
specifically for use with C++.  Here is a summary:

 breakpoint menus
           When you want a breakpoint in a function whose name is overloaded,
           breakpoint menus help you specify which function definition you
           want.  See Breakpoint menus.

 rbreak regex
           Setting breakpoints using regular expressions is helpful for setting
           breakpoints on overloaded functions that are not members of any
           special classes. See Setting breakpoints.

 catch exceptions
 info catch
           Debug C++ exception handling using these commands. See Breakpoints
           and exceptions.

 ptype typename13p-unx-docs\texi\gdb.texinfo line 6071 : unrecognized Texinfo
 command @lowersections facename=default size=0x0.
           Print inheritance relationships as well as other information for
           type typename. See Examining the Symbol Table.

 set print demangle
 show print demangle
 set print asm-demangle
 show print asm-demangle
           Control whether C++ symbols display in their source form, both when
           displaying code as C++ source and when displaying disassemblies. See
           Print settings.

 set print object
 show print object
           Choose whether to print derived (actual) or declared types of
           objects. See Print settings.

 set print vtbl
 show print vtbl
           Control the format for printing virtual function tables. See Print
           settings.

 Overloaded symbol names
           You can specify a particular definition of an overloaded symbol,
           using the same notation that is used to declare such symbols in C++:
           type symbol(types) rather than just symbol.  You can also use the
           command-line word completion facilities to list the available
           choices, or to finish the type list for you. See Command completion,
           for details on how to do this.

 M2 Operators                            Built-in operators


 Built-In Func/Proc                      Built-in functions and procedures
 M2 Constants                            Modula-2 constants
 M2 Defaults                             Default settings for Modula-2
 Deviations                              Deviations from standard Modula-2
 M2 Checks                               Modula-2 type and range checks
 M2 Scope                                The scope operators :: and .
 GDB/M2                                  GDBN and Modula-2

 Ada Mode Intro                          General remarks on the Ada syntax

                                         and semantics supported by Ada mode in
                                         GDBN
 Omissions from Ada                      Restrictions on the Ada expression
                                         syntax.
 Additions to Ada                        Extensions of the Ada expression
                                         syntax.
 Stopping at Main Program                Executing to the beginning of the main
                                         program.
 Ada Exceptions                          Setting breakpoints on exception
                                         handlers.
 Ada Tasks                               Listing and setting breakpoints in
                                         tasks.
 Ada Generic Units                       Dealing with generic instantiations.
 Ada Set Commands                        New settable GDB parameters for Ada.
 Ada Glitches                            Known peculiarities of Ada mode.


ΓòÉΓòÉΓòÉ 13. Examining the Symbol Table ΓòÉΓòÉΓòÉ

The commands described in this section allow you to inquire about the symbols
(names of variables, functions and types) defined in your program.  This
information is inherent in the text of your program and does not change as your
program executes.  finds it in your program's symbol table, in the file
indicated when you started (see Choosing files), or by one of the
file-management commands (see Commands to specify files).

Occasionally, you may need to refer to symbols that contain unusual characters,
which  ordinarily treats as word delimiters.  The most frequent case is in
referring to static variables in other source files (see Program variables).
File names are recorded in object files as debugging symbols, but  would
ordinarily parse a typical file name, like `foo.c', as the three words `foo'
`.' `c'.  To allow  to recognize `foo.c' as a single symbol, enclose it in
single quotes; for example,

p 'foo.c'::x

looks up the value of x in the scope of the file `foo.c'.

 info address symbol
           Describe where the data for symbol is stored.  For a register
           variable, this says which register it is kept in.  For a
           non-register local variable, this prints the stack-frame offset at
           which the variable is always stored.

           Note the contrast with `print &symbol', which does not work at all
           for a register variable, and for a stack local variable prints the
           exact address of the current instantiation of the variable.

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

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

 ptype typename
           Print a description of data type typename.  typename may be the name
           of a type, or for C code it may have the form `class class-name',
           `struct struct-tag', `union union-tag' or `enum enum-tag'.

 ptype exp
 ptype
           Print a description of the type of expression exp.  ptype differs
           from whatis by printing a detailed description, instead of just the
           name of the type.

           For example, for this variable declaration:

                      struct complex {double real; double imag;} v;

           the two commands give this output:

                      () whatis v
                      type = struct complex
                      () ptype v
                      type = struct complex {
                          double real;
                          double imag;
                      }

           As with whatis, using ptype without an argument refers to the type
           of $, the last value in the value history.

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

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

 info source
           Show the name of the current source file---that is, the source file
           for the function containing the current point of execution---and the
           language it was written in.

 info sources
           Print the names of all source files in your program for which there
           is debugging information, organized into two lists: files whose
           symbols have already been read, and files whose symbols will be read
           when needed.

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

 info functions regexp
           Print the names and data types of all defined functions whose names
           contain a match for regular expression regexp. Thus, `info fun step'
           finds all functions whose names include step; `info fun ^step' finds
           those whose names start with step.

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

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

           Some systems allow individual object files that make up your program
           to be replaced without stopping and restarting your program. If you
           are running on one of these systems, you can allow  to reload the
           symbols for automatically relinked modules:

           set symbol-reloading on
                          Replace symbol definitions for the corresponding
                          source file when an object file with a particular
                          name is seen again.

           set symbol-reloading off
                          Do not replace symbol definitions when
                          re-encountering object files of the same name.  This
                          is the default state; if you are not running on a
                          system that permits automatically relinking modules,
                          you should leave symbol-reloading off, since
                          otherwise  may discard symbols when linking large
                          programs, that may contain several modules (from
                          different directories or libraries) with the same
                          name.

           show symbol-reloading
                          Show the current on or off setting.

 maint print symbols filename
 maint print psymbols filename
 maint print msymbols filename
           Write a dump of debugging symbol data into the file filename. These
           commands are used to debug the  symbol-reading code.  Only symbols
           with debugging data are included.  If you use `maint print symbols',
           includes all the symbols for which it has already collected full
           details: that is, filename reflects symbols for only those files
           whose symbols  has read.  You can use the command info sources to
           find out which files these are.  If you use `maint print psymbols'
           instead, the dump shows information about symbols that  only knows
           partially---that is, symbols defined in files that  has skimmed, but
           not yet read completely.  Finally, `maint print msymbols' dumps just
           the minimal symbol information required for each object file from
           which  has read some symbols. See Commands to specify files, for a
           discussion of how  reads symbols (in the description of
           symbol-file).


ΓòÉΓòÉΓòÉ 14. Altering Execution ΓòÉΓòÉΓòÉ

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

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

 Assignment                              Assignment to variables
 Jumping                                 Continuing at a different address

 Signaling                               Giving your program a signal

 Returning                               Returning from a function
 Calling                                 Calling your program's functions
 Patching                                Patching your program


ΓòÉΓòÉΓòÉ 14.1. Assignment to variables ΓòÉΓòÉΓòÉ

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

print x=4

stores the value 4 into the variable x, and then prints the value of the
assignment expression (which is 4). See Using  with Different Languages, for
more information on operators in supported languages.

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

If the beginning of the argument string of the set command appears identical to
a set subcommand, use the set variable command instead of just set.  This
command is identical to set except for its lack of subcommands.  For example,
if your program has a variable width, you get an error if you try to set a new
value with just `set width=13', because  has the command set width:

() whatis width
type = double
() p width
$4 = 13
() set width=47
Invalid syntax in expression.

The invalid expression, of course, is `=47'.  In order to actually set the
program's variable width, use

() set var width=47

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

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

set {int}0x83040 = 4

stores the value 4 into that memory location.


ΓòÉΓòÉΓòÉ 14.2. Continuing at a different address ΓòÉΓòÉΓòÉ

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

 jump linespec
           Resume execution at line linespec.  Execution stops again
           immediately if there is a breakpoint there. See Printing source
           lines, for a description of the different forms of linespec.

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

 jump *address
           Resume execution at the instruction at address address.

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

  set $pc = 0x485

 makes the next continue command or stepping command execute at address 0x485,
 rather than at the address where your program stopped. See Continuing and
 stepping.

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


ΓòÉΓòÉΓòÉ 14.3. Giving your program a signal ΓòÉΓòÉΓòÉ

 signal signal
           Resume execution where your program stopped, but immediately give it
           the signal signal.  signal can be the name or the number of a
           signal.  For example, on many systems signal 2 and signal SIGINT are
           both ways of sending an interrupt signal.

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

           signal does not repeat when you press RET a second time after
           executing the command.

 Invoking the signal command is not the same as invoking the kill utility from
 the shell.  Sending a signal with kill causes  to decide what to do with the
 signal depending on the signal handling tables (see Signals).  The signal
 command passes the signal directly to your program.


ΓòÉΓòÉΓòÉ 14.4. Returning from a function ΓòÉΓòÉΓòÉ

 return
 return expression
           You can cancel execution of a function call with the return command.
           If you give an expression argument, its value is used as the
           function's return value.

 When you use return,  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 return.

 This pops the selected stack frame ( see Selecting a frame), and any other
 frames inside of it, leaving its caller as the innermost remaining frame.
 That frame becomes selected.  The specified value is stored in the registers
 used for returning values of functions.

 The 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 finish command ( see Continuing and stepping) resumes execution until the
 selected stack frame returns naturally.


ΓòÉΓòÉΓòÉ 14.5. Calling program functions ΓòÉΓòÉΓòÉ

 call expr
           Evaluate the expression expr without displaying void returned
           values.

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

 A new user-controlled variable, call_scratch_address, specifies the location
 of a scratch area to be used when  calls a function in the target.  This is
 necessary because the usual method of putting the scratch area on the stack
 does not work in systems that have separate instruction and data spaces.


ΓòÉΓòÉΓòÉ 14.6. Patching programs ΓòÉΓòÉΓòÉ

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

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

 set write on
 set write off
           If you specify `set write on',  opens executable and core files for
           both reading and writing; if you specify `set write off' (the
           default),  opens them read-only.

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

 show write
           Display whether executable files and core files are opened for
           writing as well as reading.


ΓòÉΓòÉΓòÉ 15. Files ΓòÉΓòÉΓòÉ

 needs to know the file name of the program to be debugged, both in order to
read its symbol table and in order to start your program. To debug a core dump
of a previous run, you must also tell the name of the core dump file.

 Files                                   Commands to specify files
 Symbol Errors                           Errors reading symbol files


ΓòÉΓòÉΓòÉ 15.1. Commands to specify files ΓòÉΓòÉΓòÉ

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

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

 file filename
           Use 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 run command.  If you do not specify a
           directory and the file is not found in the  working directory,  uses
           the environment variable 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 and your program, using
           the path command.

           On systems with memory-mapped files, an auxiliary file
           `filename.syms' may hold symbol table information for filename.  If
           so,  maps in the symbol table from `filename.syms', starting up more
           quickly.  See the descriptions of the file options `-mapped' and
           `-readnow' (available on the command line, and with the commands
           file, symbol-file, or add-symbol-file, described below), for more
           information.

 file
           file with no argument makes  discard any information it has on both
           executable file and the symbol table.

 exec-file [ filename ]
           Specify that the program to be run (but not the symbol table) is
           found in filename.  searches the environment variable PATH if
           necessary to locate your program.  Omitting filename means to
           discard information on the executable file.

 symbol-file [ filename ]
           Read symbol table information from file filename.  PATH is searched
           when necessary.  Use the file command to get both symbol table and
           program to run from the same file.

           symbol-file with no argument clears out  information on your
           program's symbol table.

           The symbol-file command causes  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 .

           symbol-file does not repeat if you press RET again after executing
           it once.

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

           On some kinds of object files, the symbol-file command does not
           normally read the symbol table in full right away.  Instead, it
           scans the symbol table quickly to find which source files and which
           symbols are present.  The details are read later, one source file at
           a time, as they are needed.

           The purpose of this two-stage reading strategy is to make  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 set verbose command can turn these pauses into
           messages if desired. See Optional warnings and messages.)

           We have not implemented the two-stage strategy for COFF yet.  When
           the symbol table is stored in COFF format, symbol-file reads the
           symbol table data in full right away.

 symbol-file filename [ -readnow ] [ -mapped ]
 file filename [ -readnow ] [ -mapped ]
           You can override the  two-stage strategy for reading symbol tables
           by using the `-readnow' option with any of the commands that load
           symbol table information, if you want to be sure  has the entire
           symbol table available.

           If memory-mapped files are available on your system through the mmap
           system call, you can use another option, `-mapped', to cause  to
           write the symbols for your program into a reusable file.  Future
           debugging sessions map in symbol information from this auxiliary
           symbol file (if the program has not changed), rather than spending
           time reading the symbol table from the executable program.  Using
           the `-mapped' option has the same effect as starting  with the
           `-mapped' command-line option.

           You can use both options together, to make sure the auxiliary symbol
           file has all the symbol information for your program.

           The auxiliary symbol file for a program called myprog is called
           `myprog.syms'.  Once this file exists (so long as it is newer than
           the corresponding executable),  always attempts to use it when you
           debug myprog; no special options or commands are needed.

           The `.syms' file is specific to the host machine where you run .  It
           holds an exact image of the internal symbol table.  It cannot be
           shared across multiple host platforms.

 core-file [ filename ]
           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;  can
           access the executable file itself for other parts.

           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 .  So, if you have been running your program and you
           wish to debug a core file instead, you must kill the subprocess in
           which the program is running.  To do this, use the kill command (see
           Killing the child process).

 load filename
           The file is loaded at whatever address is specified in the
           executable. For some object file formats, you can specify the load
           address when you link the program; for other formats, like a.out,
           the object file format specifies a fixed address. load does not
           repeat if you press RET again after using it.

 add-symbol-file filename address
 add-symbol-file filename address [ -readnow ] [ -mapped ]
           The add-symbol-file command reads additional symbol table
           information from the file filename.  You would use this command when
           filename has been dynamically loaded (by some other means) into the
           program that is running.  address should be the memory address at
           which the file has been loaded;  cannot figure this out for itself.
           You can specify address as an expression.

           The symbol table of the file filename is added to the symbol table
           originally read with the symbol-file command.  You can use the
           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 symbol-file command.

           add-symbol-file does not repeat if you press RET after using it.

           You can use the `-mapped' and `-readnow' options just as with the
           symbol-file command, to change how  manages the symbol table
           information for filename.

 add-shared-symbol-file
           The add-shared-symbol-file command can be used only under Harris'
           CXUX operating system for the Motorola 88k.  automatically looks for
           shared libraries, however if  does not find yours, you can run
           add-shared-symbol-file.  It takes no arguments.

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

 info files
 info target
           info files and info target are synonymous; both print the current
           target (see Specifying a Debugging Target), including the names of
           the executable and core dump files currently in use by , and the
           files from which symbols were loaded.  The command help target lists
           all possible targets rather than current ones.

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

  supports SunOS, SVr4, Irix 5, and IBM RS/6000 shared libraries.
 automatically loads symbol definitions from shared libraries when you use the
 run command, or when you examine a core file. (Before you issue the run
 command,  does not understand references to a function in a shared library,
 however---unless you are debugging a core file).

 info share
 info sharedlibrary
           Print the names of the shared libraries which are currently loaded.

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


ΓòÉΓòÉΓòÉ 15.2. Errors reading symbol files ΓòÉΓòÉΓòÉ

While reading a symbol file,  occasionally encounters problems, such as symbol
types it does not recognize, or known bugs in compiler output.  By default,
does not notify you of such problems, since they are relatively common and
primarily of interest to people debugging compilers.  If you are interested in
seeing information about ill-constructed symbol tables, you can either ask  to
print only one message about each such type of problem, no matter how many
times the problem occurs; or you can ask  to print more messages, to see how
many times the problems occur, with the set complaints command ( see Optional
warnings and messages).

The messages currently printed, and their meanings, include:

 inner block not inside outer block in 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.

            circumvents the problem by treating the inner block as if it had
           the same scope as the outer block.  In the error message, symbol may
           be shown as ``(don't know)'' if the outer block is not a function.

 block at 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.

            does not circumvent this problem, and has trouble locating symbols
           in the source file whose symbols it is reading.  (You can often
           determine what source file is affected by specifying set verbose on.
           See Optional warnings and messages.)

 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.

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

 bad string table offset in symbol n
           Symbol number n contains a pointer into the string table which is
           larger than the size of the string table.

            circumvents the problem by considering the symbol to have the name
           foo, which may cause other problems if many symbols end up with this
           name.

 unknown symbol type 0xnn
           The symbol information contains new data types that  does not yet
           know how to read.  0xnn is the symbol type of the misunderstood
           information, in hexadecimal.

            circumvents the error by ignoring this symbol information.  This
           usually allows you to debug your program, though certain symbols are
           not accessible.  If you encounter such a problem and feel like
           debugging it, you can debug  with itself, breakpoint on complain,
           then go up to the function read_dbx_symtab and examine *bufp to see
           the symbol.

 stub type has NULL name
            could not find the full definition for a struct or class.

 const/volatile indicator missing (ok if using g++ v1.x), got...
           The symbol information for a C++ member function is missing some
           information that recent versions of the compiler should have output
           for it.

 info mismatch between compiler and debugger
            could not parse a type specification output by the compiler.


ΓòÉΓòÉΓòÉ 16. Specifying a Debugging Target ΓòÉΓòÉΓòÉ

A target is the execution environment occupied by your program. Often,  runs in
the same host environment as your program; in that case, the debugging target
is specified as a side effect when you use the file or core commands.  When you
need more flexibility---for example, running  on a physically separate host, or
controlling a standalone system over a serial port or a realtime system over a
TCP/IP connection---you can use the target command to specify one of the target
types configured for  ( see Commands for managing targets).

 Active Targets                          Active targets
 Target Commands                         Commands for managing targets
 Remote                                  Remote debugging


ΓòÉΓòÉΓòÉ 16.1. Active targets ΓòÉΓòÉΓòÉ

There are three classes of targets: processes, core files, and executable
files.  can work concurrently on up to three active targets, one in each class.
This allows you to (for example) start a process and inspect its activity
without abandoning your work on a core file.

For example, if you execute `gdb a.out', then the executable file a.out is the
only active target.  If you designate a core file as well---presumably from a
prior run that crashed and coredumped---then  has two active targets and uses
them in tandem, looking first in the corefile target, then in the executable
file, to satisfy requests for memory addresses.  (Typically, these two classes
of target are complementary, since core files contain only a program's
read-write memory---variables and so on---plus machine status, while executable
files contain only the program text and initialized data.)

When you type run, your executable file becomes an active process target as
well.  When a process target is active, all  commands requesting memory
addresses refer to that target; addresses in an active core file or executable
file target are obscured while the process target is active.

Use the core-file and exec-file commands to select a new core file or
executable target ( see Commands to specify files).  To specify as a target a
process that is already running, use the attach command ( see Debugging an
already-running process).


ΓòÉΓòÉΓòÉ 16.2. Commands for managing targets ΓòÉΓòÉΓòÉ

 target type parameters
           Connects the  host environment to a target machine or process.  A
           target is typically a protocol for talking to debugging facilities.
           You use the argument type to specify the type or protocol of the
           target machine.

           Further 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 target command does not repeat if you press RET again after
           executing the command.

 help target
           Displays the names of all targets available.  To display targets
           currently selected, use either info target or info files (see
           Commands to specify files).

 help target name
           Describe a particular target, including any parameters necessary to
           select it.

 set gnutarget args
           uses its own library BFD to read your files. knows whether it is
           reading an executable, a core, or a .o file, however you can specify
           the file format with the set gnutarget command.  Unlike most target
           commands, with gnutarget the target refers to a program, not a
           machine.

           Warning: To specify a file format with set gnutarget, you must know
           the actual BFD name.

           See Commands to specify files.

 show gnutarget
           Use the show gnutarget command to display what file format gnutarget
           is set to read.  If you have not set gnutarget,  will determine the
           file format for each file automatically and show gnutarget displays
           The current BDF target is "auto".

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

 target exec program
           An executable file.  `target exec program' is the same as `exec-file
           program'.

 target core filename
           A core dump file.  `target core filename' is the same as `core-file
           filename'.

 target bug dev
           BUG monitor, running on a MVME187 (m88k) board.

 target cpu32bug dev
           CPU32BUG monitor, running on a CPU32 (M68K) board.

 target op50n dev
           OP50N monitor, running on an OKI HPPA board.

 target w89k dev
           W89K monitor, running on a Winbond HPPA board.

 target est dev
           EST-300 ICE monitor, running on a CPU32 (M68K) board.

 target rom68k dev
           ROM 68K monitor, running on an IDP board.

 target array dev
           Array Tech LSI33K RAID controller board.

 target sparclite dev
           Fujitsu sparclite boards, used only for the purpose of loading. You
           must use an additional command to debug the program. For example:
           target remote dev using  standard remote protocol.


ΓòÉΓòÉΓòÉ 16.3. Choosing target byte order ΓòÉΓòÉΓòÉ

You can now choose which byte order to use with a target system. Use the set
endian big and set endian little commands. Use the set endian auto command to
instruct  to use the byte order associated with the executable. You can see the
current setting for byte order with the show endian command.

Warning: Currently, only embedded MIPS configurations support dynamic selection
of target byte order.


ΓòÉΓòÉΓòÉ 16.4. Remote debugging ΓòÉΓòÉΓòÉ

If you are trying to debug a program running on a machine that cannot run  in
the usual way, it is often useful to use remote debugging. For example, you
might use remote debugging on an operating system kernel, or on a small system
which does not have a general purpose operating system powerful enough to run a
full-featured debugger.

Some configurations of  have special serial or TCP/IP interfaces to make this
work with particular debugging targets.  In addition,  comes with a generic
serial protocol (specific to , but not specific to any particular target
system) which you can use if you write the remote stubs---the code that runs on
the remote system to communicate with .

Other remote targets may be available in your configuration of ; use help
target to list them.

 Remote Serial                           GDBN remote serial protocol

 i960-Nindy Remote                       GDBN with a remote i960 (Nindy)

 UDI29K Remote                           The UDI protocol for AMD29K
 EB29K Remote                            The EBMON protocol for AMD29K

 VxWorks Remote                          GDBN and VxWorks

 ST2000 Remote                           GDBN with a Tandem ST2000

 Hitachi Remote                          GDBN and Hitachi Microprocessors
 13p-unx-docs\texi\gdb.texinfo line 8390 : can't open included file
 remote.texinfo compact. MIPS RemoteGDBN and MIPS boards

 Sparclet Remote                         GDBN and Sparclet boards

 Simulator                               Simulated CPU target


ΓòÉΓòÉΓòÉ 17. Controlling ΓòÉΓòÉΓòÉ

You can alter the way  interacts with you by using the set command.  For
commands controlling how  displays data, see Print settings; other settings are
described here.

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


ΓòÉΓòÉΓòÉ 17.1. Prompt ΓòÉΓòÉΓòÉ

 indicates its readiness to read a command by printing a string called the
prompt.  This string is normally `()'.  You can change the prompt string with
the set prompt command.  For instance, when debugging  with , it is useful to
change the prompt in one of the  sessions so that you can always tell which one
you are talking to.

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

 set prompt newprompt
           Directs  to use newprompt as its prompt string henceforth.

 show prompt
           Prints a line of the form: `Gdb's prompt is: your-prompt'


ΓòÉΓòÉΓòÉ 17.2. Command editing ΓòÉΓòÉΓòÉ

 reads its input commands via the readline interface.  This gnu library
provides consistent behavior for programs which provide a command line
interface to the user.  Advantages are gnu Emacs-style or vi-style inline
editing of commands, 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  with the command set.

 set editing
 set editing on
           Enable command line editing (enabled by default).

 set editing off
           Disable command line editing.

 show editing
           Show whether command line editing is enabled.


ΓòÉΓòÉΓòÉ 17.3. Command history ΓòÉΓòÉΓòÉ

 can keep track of the commands you type during your debugging sessions, so
that you can be certain of precisely what happened.  Use these commands to
manage the  command history facility.

 set history filename fname
           Set the name of the  command history file to fname. This is the file
           where  reads an initial command history list, and where it writes
           the command history from this session when it exits.  You can access
           this list through history expansion or through the history command
           editing characters listed below.  This file defaults to the value of
           the environment variable GDBHISTFILE, or to `./.gdb_history' if this
           variable is not set.

 set history save
 set history save on
           Record command history in a file, whose name may be specified with
           the set history filename command.  By default, this option is
           disabled.

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

 set history size size
           Set the number of commands which  keeps in its history list. This
           defaults to the value of the environment variable HISTSIZE, or to
           256 if this variable is not set.

 History expansion assigns special meaning to the character !.

 Since ! is also the logical not operator in C, history expansion is off by
 default. If you decide to enable history expansion with the set history
 expansion on command, you may sometimes need to follow ! (when it is used as
 logical not, in an expression) with a space or a tab to prevent it from being
 expanded.  The readline history facilities do not attempt substitution on the
 strings != and !(, even when history expansion is enabled.

 The commands to control history expansion are:

 set history expansion on
 set history expansion
           Enable history expansion.  History expansion is off by default.

 set history expansion off
           Disable history expansion.

           The readline code comes with more complete documentation of editing
           and history expansion features.  Users unfamiliar with gnu Emacs or
           vi may wish to read it.

 show history
 show history filename
 show history save
 show history size
 show history expansion
           These commands display the state of the  history parameters. show
           history by itself displays all four states.

 show commands
           Display the last ten commands in the command history.

 show commands n
           Print ten commands centered on command number n.

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


ΓòÉΓòÉΓòÉ 17.4. Screen size ΓòÉΓòÉΓòÉ

Certain commands to  may produce large amounts of information output to the
screen.  To help you read all of it,  pauses and asks you for input at the end
of each page of output.  Type RET when you want to continue the output, or q to
discard the remaining output.  Also, the screen width setting determines when
to wrap lines of output.  Depending on what is being printed,  tries to break
the line at a readable place, rather than simply letting it overflow onto the
following line.

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

 set height lpp
 show height
 set width cpl
 show width
           These set commands specify a screen height of lpp lines and a screen
           width of cpl characters.  The associated show commands display the
           current settings.

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

           Likewise, you can specify `set width 0' to prevent from wrapping its
           output.


ΓòÉΓòÉΓòÉ 17.5. Numbers ΓòÉΓòÉΓòÉ

You can always enter numbers in octal, decimal, or hexadecimal in  by the usual
conventions: octal numbers begin with `0', decimal numbers end with `.', and
hexadecimal numbers begin with `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 set radix command.

 set input-radix base
           Set the default base for numeric input.  Supported choices for base
           are decimal 8, 10, or 16.  base must itself be specified either
           unambiguously or using the current default radix; for example, any
           of

                      set radix 012
                      set radix 10.
                      set radix 0xa

           sets the base to decimal.  On the other hand, `set radix 10' leaves
           the radix unchanged no matter what it was.

 set output-radix base
           Set the default base for numeric display.  Supported choices for
           base are decimal 8, 10, or 16.  base must itself be specified either
           unambiguously or using the current default radix.

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

 show output-radix
           Display the current default base for numeric display.


ΓòÉΓòÉΓòÉ 17.6. Optional warnings and messages ΓòÉΓòÉΓòÉ

By default,  is silent about its inner workings.  If you are running on a slow
machine, you may want to use the set verbose command. This makes  tell you when
it does a lengthy internal operation, so you will not think it has crashed.

Currently, the messages controlled by set verbose are those which announce that
the symbol table for a source file is being read; see symbol-file in Commands
to specify files.

 set verbose on
           Enables  output of certain informational messages.

 set verbose off
           Disables  output of certain informational messages.

 show verbose
           Displays whether set verbose is on or off.

 By default, if  encounters bugs in the symbol table of an object file, it is
 silent; but if you are debugging a compiler, you may find this information
 useful (see Errors reading symbol files).

 set complaints limit
           Permits  to output limit complaints about each type of unusual
           symbols before becoming silent about the problem.  Set limit to zero
           to suppress all complaints; set it to a large number to prevent
           complaints from being suppressed.

 show complaints
           Displays how many symbol complaints  is permitted to produce.

 By default,  is cautious, and asks what sometimes seems to be a lot of stupid
 questions to confirm certain commands.  For example, if you try to run a
 program which is already running:

  () run
  The program being debugged has been started already.
  Start it from the beginning? (y or n)

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

 set confirm off
           Disables confirmation requests.

 set confirm on
           Enables confirmation requests (the default).

 show confirm
           Displays state of confirmation requests.


ΓòÉΓòÉΓòÉ 18. Canned Sequences of Commands ΓòÉΓòÉΓòÉ

Aside from breakpoint commands ( see Breakpoint command lists),  provides two
ways to store sequences of commands for execution as a unit: user-defined
commands and command files.

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


ΓòÉΓòÉΓòÉ 18.1. User-defined commands ΓòÉΓòÉΓòÉ

A user-defined command is a sequence of  commands to which you assign a new
name as a command.  This is done with the define command.  User commands may
accept up to 10 arguments separated by whitespace. Arguments are accessed
within the user command via $arg0...$arg9. A trivial example:

define adder
  print $arg0 + $arg1 + $arg2

To execute the command use:

adder 1 2 3

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

 define commandname
           Define a command named commandname.  If there is already a command
           by that name, you are asked to confirm that you want to redefine it.

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

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

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

 document commandname
           Document the user-defined command commandname, so that it can be
           accessed by help.  The command commandname must already be defined.
           This command reads lines of documentation just as define reads the
           lines of the command definition, ending with end. After the document
           command is finished, help on command commandname displays the
           documentation you have written.

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

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

 show user
 show user commandname
           Display the  commands used to define commandname (but not its
           documentation).  If no commandname is given, display the definitions
           for all user-defined commands.

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

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


ΓòÉΓòÉΓòÉ 18.2. User-defined command hooks ΓòÉΓòÉΓòÉ

You may define hooks, which are a special kind of user-defined command.
Whenever you run the command `foo', if the user-defined command `hook-foo'
exists, it is executed (with no arguments) before that command.

In addition, a pseudo-command, `stop' exists.  Defining (`hook-stop') makes the
associated commands execute every time execution stops in your program: before
breakpoint commands are run, displays are printed, or the stack frame is
printed.

For example, to ignore SIGALRM signals while single-stepping, but treat them
normally during normal execution, you could define:

define hook-stop
handle SIGALRM nopass
end

define hook-run
handle SIGALRM pass
end

define hook-continue
handle SIGLARM pass
end

You can define a hook for any single-word command in , but not for command
aliases; you should define a hook for the basic command name, e.g.  backtrace
rather than bt. If an error occurs during the execution of your hook, execution
of  commands stops and  issues a prompt (before the command that you actually
typed had a chance to run).

If you try to define a hook which does not match any known command, you get a
warning from the define command.


ΓòÉΓòÉΓòÉ 18.3. Command files ΓòÉΓòÉΓòÉ

A command file for  is a file of lines that are commands.  Comments (lines
starting with #) may also be included. An empty line in a command file does
nothing; it does not mean to repeat the last command, as it would from the
terminal.

When you start , it automatically executes commands from its init files.  These
are files named `'.  reads the init file (if any) in your home directory, then
processes command line options and operands, and then reads the init file (if
any) in the current working directory.  This is so the init file in your home
directory can set options (such as set complaints) which affect the processing
of the command line options and operands.  The init files are not executed if
you use the `-nx' option; see Choosing modes. You can also request the
execution of a command file with the

source command:

 source filename
           Execute the command file filename.

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

 Commands that would ask for confirmation if used interactively proceed without
 asking when used in a command file.  Many  commands that normally print
 messages to say what they are doing omit the messages when called from command
 files.


ΓòÉΓòÉΓòÉ 18.4. Commands for controlled output ΓòÉΓòÉΓòÉ

During the execution of a command file or a user-defined command, normal
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.

 echo text
           Print text.  Nonprinting characters can be included in text using C
           escape sequences, such as `\n' to print a newline.  *No newline is
           printed unless you specify one.* In addition to the standard C
           escape sequences, a backslash followed by a space stands for a
           space.  This is useful for displaying a string with spaces at the
           beginning or the end, since leading and trailing spaces are
           otherwise trimmed from all arguments. To print `and foo =', use the
           command `echo \and foo = \'.

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

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

           produces the same output as

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

 output expression
           Print the value of expression and nothing but that value: no
           newlines, no `$nn = '.  The value is not entered in the value
           history either.  See Expressions, for more information on
           expressions.

 output/fmt expression
           Print the value of expression in format fmt.  You can use the same
           formats as for print. See Output formats, for more information.

 printf string, expressions...
           Print the values of the expressions under the control of string.
           The expressions are separated by commas and may be either numbers or
           pointers.  Their values are printed as specified by string, exactly
           as if your program were to execute the C subroutine

                      printf (string, expressions...);

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

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

           The only backslash-escape sequences that you can use in the format
           string are the simple ones that consist of backslash followed by a
           letter.


ΓòÉΓòÉΓòÉ 19. Using  under gnu Emacs ΓòÉΓòÉΓòÉ

A special interface allows you to use gnu Emacs to view (and edit) the source
files for the program you are debugging with .

To use this interface, use the command M-x gdb in Emacs.  Give the executable
file you want to debug as an argument.  This command starts  as a subprocess of
Emacs, with input and output through a newly created Emacs buffer.

Using  under Emacs is just like using  normally except for two things:

     All ``terminal'' input and output goes through the Emacs buffer.

 This applies both to  commands and their output, and to the input and output
 done by the program you are debugging.

 This is useful because it means that you can copy the text of previous
 commands and input them again; you can even use parts of the output in this
 way.

 All the facilities of Emacs' Shell mode are available for interacting with
 your program.  In particular, you can send signals the usual way---for
 example, C-c C-c for an interrupt, C-c C-z for a stop.

     displays source code through Emacs.

 Each time  displays a stack frame, Emacs automatically finds the source file
 for that frame and puts an arrow (`=>') at the left margin of the current
 line.  Emacs uses a separate buffer for source display, and splits the screen
 to show both your  session and the source.

 Explicit  list or search commands still produce output as usual, but you
 probably have no reason to use them from Emacs.

 Warning: If the directory where your program resides is not your current
 directory, it can be easy to confuse Emacs about the location of the source
 files, in which case the auxiliary display buffer does not appear to show your
 source.  can find programs by searching your environment's PATH variable, so
 the  input and output session proceeds normally; but Emacs does not get enough
 information back from  to locate the source files in this situation.  To avoid
 this problem, either start  mode from the directory where your program
 resides, or specify an absolute file name when prompted for the M-x gdb
 argument.

 A similar confusion can result if you use the  file command to switch to
 debugging a program in some other location, from an existing  buffer in Emacs.

 By default, M-x gdb calls the program called `gdb'.  If you need to call  by a
 different name (for example, if you keep several configurations around, with
 different names) you can set the Emacs variable gdb-command-name; for example,

  (setq gdb-command-name "mygdb")

 (preceded by ESC ESC, or typed in the *scratch* buffer, or in your `.emacs'
 file) makes Emacs call the program named ``mygdb'' instead.

 In the  I/O buffer, you can use these special Emacs commands in addition to
 the standard Shell mode commands:

 C-h m
           Describe the features of Emacs'  Mode.

 M-s
           Execute to another source line, like the  step command; also update
           the display window to show the current file and location.

 M-n
           Execute to next source line in this function, skipping all function
           calls, like the  next command.  Then update the display window to
           show the current file and location.

 M-i
           Execute one instruction, like the  stepi command; update display
           window accordingly.

 M-x gdb-nexti
           Execute to next instruction, using the  nexti command; update
           display window accordingly.

 C-c C-f
           Execute until exit from the selected stack frame, like the finish
           command.

 M-c
           Continue execution of your program, like the  continue command.

           Warning: In Emacs v19, this command is C-c C-p.

 M-u
           Go up the number of frames indicated by the numeric argument (see
           Numeric Arguments), like the  up command.

           Warning: In Emacs v19, this command is C-c C-u.

 M-d
           Go down the number of frames indicated by the numeric argument, like
           the  down command.

           Warning: In Emacs v19, this command is C-c C-d.

 C-x &
           Read the number where the cursor is positioned, and insert it at the
           end of the  I/O buffer.  For example, if you wish to disassemble
           code around an address that was displayed earlier, type disassemble;
           then move the cursor to the address display, and pick up the
           argument for disassemble by typing C-x &.

           You can customize this further by defining elements of the list
           gdb-print-command; once it is defined, you can format or otherwise
           process numbers picked up by C-x & before they are inserted.  A
           numeric argument to C-x & indicates that you wish special
           formatting, and also acts as an index to pick an element of the
           list.  If the list element is a string, the number to be inserted is
           formatted using the Emacs function format; otherwise the number is
           passed as an argument to the corresponding list element.

 In any source file, the Emacs command C-x SPC (gdb-break) tells  to set a
 breakpoint on the source line point is on.

 If you accidentally delete the source-display buffer, an easy way to get it
 back is to type the command f in the  buffer, to request a frame display; when
 you run under Emacs, this recreates the source buffer if necessary to show you
 the context of the current frame.

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


ΓòÉΓòÉΓòÉ 20. Reporting Bugs in ΓòÉΓòÉΓòÉ

Your bug reports play an essential role in making  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  work better.  Bug reports are
your contribution to the maintenance of .

In order for a bug report to serve its purpose, you must include the
information that enables us to fix the bug.

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


ΓòÉΓòÉΓòÉ 20.1. Have you found a bug? ΓòÉΓòÉΓòÉ

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

     If the debugger gets a fatal signal, for any input whatever, that is a
      bug.  Reliable debuggers never crash.

     If  produces an error message for valid input, that is a bug.

     If  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''.

     If you are an experienced user of debugging tools, your suggestions for
      improvement of  are welcome in any case.


ΓòÉΓòÉΓòÉ 20.2. How to report bugs ΓòÉΓòÉΓòÉ

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

You can find contact information for many support companies and individuals in
the file `etc/SERVICE' in the gnu Emacs distribution.

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

[e-mail]
{ucbvax|mit-eddie|uunet}!prep.ai.mit.edu!bug-gdb

*Do not send bug reports to `info-gdb', or to `help-gdb', or to any
newsgroups.* Most users of  do not want to receive bug reports.  Those that do
have arranged to receive `bug-gdb'.

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

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

gnu Debugger Bugs
Free Software Foundation Inc.
59 Temple Place - Suite 330
Boston, MA 02111-1307
USA

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

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

Keep in mind that the purpose of a bug report is to enable us to fix the bug if
it is new to us. 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 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:

     The version of .  announces it if you start with no arguments; you can
      also print it at any time using show version.

      Without this, we will not know whether there is any point in looking for
      the bug in the current version of .

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

     What compiler (and its version) was used to compile ---e.g. ``--2.0''.

     What compiler (and its version) was used to compile the program you are
      debugging---e.g.  ``--2.0''.

     The command arguments you gave the compiler to compile your example and
      observe the bug.  For example, did you use `-O'?  To guarantee you will
      not omit something important, list them all.  A copy of the Makefile (or
      the output from make) is sufficient.

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

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

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

      Even if the problem you experience is a fatal signal, you should still
      say so explicitly.  Suppose something strange is going on, such as, your
      copy of  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.

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

      The line numbers in our development sources will not match those in your
      sources.  Your line numbers would convey no useful information to us.

 Here are some things that are nE13p-unx-docs\texi\gdb.texinfo line 9451 :
 can't open included file rluser.texinfo E13p-unx-docs\texi\gdb.texinfo line
 9452 : can't open included file inc-hist.texinfo ot necessary:

     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 instead of the
      original one, that is a convenience for us.  Errors in the output will be
      easier to spot, running under the debugger will take less time, and so
      on.

      However, simplification is not vital; if you do not want to do this,
      report the bug anyway and send us the entire test case you used.

     A patch for the bug.

      A patch for the bug does help us if it is a good one.  But do not omit
      the necessary information, such as the test case, on the assumption that
      a patch is all we need.  We might see problems with your patch and decide
      to fix the problem another way, or we might not understand it at all.

      Sometimes with a program as complicated as  it is very hard to construct
      an example that will make the program follow a certain path through the
      code.  If you do not send us the example, we will not be able to
      construct one, so we will not be able to verify that the bug is fixed.

      And if we cannot understand what bug you are trying to fix, or why your
      patch should be an improvement, we will not install it.  A test case will
      help us to understand.

     A guess about what the bug is or what it depends on.

      Such guesses are usually wrong.  Even we cannot guess right about such
      things without first using the debugger to find the facts.


ΓòÉΓòÉΓòÉ 21. Formatting Documentation ΓòÉΓòÉΓòÉ

The  4 release includes an already-formatted reference card, ready for printing
with PostScript or Ghostscript, in the `gdb' subdirectory of the main source
directory(3).  If you can use PostScript or Ghostscript with your printer, you
can print the reference card immediately with `refcard.ps'.

The release also includes the source for the reference card.  You can format
it, using TeX, by typing:

make refcard.dvi

The  reference card is designed to print in landscape mode on US ``letter''
size paper; that is, on a sheet 11 inches wide by 8.5 inches high.  You will
need to specify this form of printing as an option to your dvi output program.

All the documentation for  comes as part of the machine-readable distribution.
The documentation is written in Texinfo format, which is a documentation system
that uses a single source file to produce both on-line information and a
printed manual.  You can use one of the Info formatting commands to create the
on-line version of the documentation and TeX (or texi2roff) to typeset the
printed version.

 includes an already formatted copy of the on-line Info version of this manual
in the `gdb' subdirectory.  The main Info file is
`gdb-version-number/gdb/gdb.info', and it refers to subordinate files matching
`gdb.info*' in the same directory.  If necessary, you can print out these
files, or read them with any editor; but they are easier to read using the info
subsystem in gnu Emacs or the standalone info program, available as part of the
gnu Texinfo distribution.

If you want to format these Info files yourself, you need one of the Info
formatting programs, such as texinfo-format-buffer or makeinfo.

If you have makeinfo installed, and are in the top level source directory
(`gdb-', in the case of version ), you can make the Info file by typing:

cd gdb
make gdb.info

If you want to typeset and print copies of this manual, you need TeX, a program
to print its dvi output files, and `texinfo.tex', the Texinfo definitions file.

TeX is a typesetting program; it does not print files directly, but produces
output files called dvi files.  To print a typeset document, you need a program
to print dvi files.  If your system has TeX installed, chances are it has such
a program.  The precise command to use depends on your system; lpr -d is
common; another (for PostScript devices) is dvips.  The dvi print command may
require a file name without any extension or a `.dvi' extension.

TeX also requires a macro definitions file called `texinfo.tex'.  This file
tells TeX how to typeset a document written in Texinfo format.  On its own, TeX
cannot either read or typeset a Texinfo file.  `texinfo.tex' is distributed
with GDB and is located in the `gdb-version-number/texinfo' directory.

If you have TeX and a dvi printer program installed, you can typeset and print
this manual.  First switch to the the `gdb' subdirectory of the main source
directory (for example, to `gdb-/gdb') and then type:

make gdb.dvi


ΓòÉΓòÉΓòÉ 22. Installing ΓòÉΓòÉΓòÉ

 comes with a configure script that automates the process of preparing  for
installation; you can then use make to build the gdb program.

The  distribution includes all the source code you need for  in a single
directory, whose name is usually composed by appending the version number to
`gdb'.

For example, the  version  distribution is in the `gdb-' directory.  That
directory contains:

 gdb-GDBVN/configure (and supporting files)
           script for configuring  and all its supporting libraries

 gdb-GDBVN/gdb
           the source specific to  itself

 gdb-GDBVN/bfd
           source for the Binary File Descriptor library

 gdb-GDBVN/include
           gnu include files

 gdb-GDBVN/libiberty
           source for the `-liberty' free software library

 gdb-GDBVN/opcodes
           source for the library of opcode tables and disassemblers

 gdb-GDBVN/readline
           source for the gnu command-line interface

 gdb-GDBVN/glob
           source for the gnu filename pattern-matching subroutine

 gdb-GDBVN/mmalloc
           source for the gnu memory-mapped malloc package

 The simplest way to configure and build  is to run configure from the
 `gdb-version-number' source directory, which in this example is the `gdb-'
 directory.

 First switch to the `gdb-version-number' source directory if you are not
 already in it; then run configure.  Pass the identifier for the platform on
 which  will run as an argument.

 For example:

  cd gdb-
  ./configure host
  make

 where host is an identifier such as `sun4' or `decstation', that identifies
 the platform where  will run. (You can often leave off host; configure tries
 to guess the correct value by examining your system.)

 Running `configure host' and then running make builds the `bfd', `readline',
 `mmalloc', and `libiberty' libraries, then gdb itself.  The configured source
 files, and the binaries, are left in the corresponding source directories.

 configure is a Bourne-shell (/bin/sh) script; if your system does not
 recognize this automatically when you run a different shell, you may need to
 run sh on it explicitly:

  sh configure host

 If you run configure from a directory that contains source directories for
 multiple libraries or programs, such as the `gdb-' source directory for
 version , configure creates configuration files for every directory level
 underneath (unless you tell it not to, with the `--norecursion' option).

 You can run the configure script from any of the subordinate directories in
 the  distribution if you only want to configure that subdirectory, but be sure
 to specify a path to it.

 For example, with version , type the following to configure only the bfd
 subdirectory:

  cd gdb-/bfd
  ../configure host

 You can install  anywhere; it has no hardwired paths. However, you should make
 sure that the shell on your path (named by the `SHELL' environment variable)
 is publicly readable.  Remember that  uses the shell to start your
 program---some systems refuse to let  debug child processes whose programs are
 not readable.

 Separate Objdir                         Compiling GDBN in another directory
 Config Names                            Specifying names for hosts and targets
 configure Options                       Summary of options for configure


ΓòÉΓòÉΓòÉ 22.1. Compiling  in another directory ΓòÉΓòÉΓòÉ

If you want to run  versions for several host or target machines, you need a
different gdb compiled for each combination of host and target.  configure is
designed to make this easy by allowing you to generate each configuration in a
separate subdirectory, rather than in the source directory.  If your make
program handles the `VPATH' feature (gnu make does), running make in each of
these directories builds the gdb program specified there.

To build gdb in a separate directory, run configure with the `--srcdir' option
to specify where to find the source. (You also need to specify a path to find
configure itself from your working directory.  If the path to configure would
be the same as the argument to `--srcdir', you can leave out the `--srcdir'
option; it is assumed.)

For example, with version , you can build  in a separate directory for a Sun 4
like this:

cd gdb-
mkdir ../gdb-sun4
cd ../gdb-sun4
../gdb-/configure sun4
make

When configure builds a configuration using a remote source directory, it
creates a tree for the binaries with the same structure (and using the same
names) as the tree under the source directory.  In the example, you'd find the
Sun 4 library `libiberty.a' in the directory `gdb-sun4/libiberty', and  itself
in `gdb-sun4/gdb'.

One popular reason to build several  configurations in separate directories is
to configure  for cross-compiling (where  runs on one machine---the
host---while debugging programs that run on another machine---the target). You
specify a cross-debugging target by giving the `--target=target' option to
configure.

When you run make to build a program or library, you must run it in a
configured directory---whatever directory you were in when you called configure
(or one of its subdirectories).

The Makefile that configure generates in each source directory also runs
recursively.  If you type make in a source directory such as `gdb-' (or in a
separate configured directory configured with `--srcdir=dirname/gdb-'), you
will build all the required libraries, and then build GDB.

When you have multiple hosts or targets configured in separate directories, you
can run make on them in parallel (for example, if they are NFS-mounted on each
of the hosts); they will not interfere with each other.


ΓòÉΓòÉΓòÉ 22.2. Specifying names for hosts and targets ΓòÉΓòÉΓòÉ

The specifications used for hosts and targets in the configure script are based
on a three-part naming scheme, but some short predefined aliases are also
supported.  The full naming scheme encodes three pieces of information in the
following pattern:

architecture-vendor-os

For example, you can use the alias sun4 as a host argument, or as the value for
target in a --target=target option.  The equivalent full name is
`sparc-sun-sunos4'.

The configure script accompanying  does not provide any query facility to list
all supported host and target names or aliases.  configure calls the Bourne
shell script config.sub to map abbreviations to full names; you can read the
script, if you wish, or you can use it to test your guesses on
abbreviations---for example:

% sh config.sub sun4
sparc-sun-sunos4.1.1
% sh config.sub sun3
m68k-sun-sunos4.1.1
% sh config.sub decstation
mips-dec-ultrix4.2
% sh config.sub hp300bsd
m68k-hp-bsd
% sh config.sub i386v
i386-unknown-sysv
% sh config.sub i786v
Invalid configuration `i786v': machine `i786v' not recognized

config.sub is also distributed in the  source directory (`gdb-', for version ).


ΓòÉΓòÉΓòÉ 22.3. configure options ΓòÉΓòÉΓòÉ

Here is a summary of the configure options and arguments that are most often
useful for building .  configure also has several other options not listed
here.  See Info file What Configure Does,,configure.info, for a full
explanation of configure.

configure [--help]
          [--prefix=dir]
          [--srcdir=dirname]
          [--norecursion] [--rm]
          [--target=target] host

You may introduce options with a single `-' rather than `--' if you prefer; but
you may abbreviate option names if you use `--'.

 --help
           Display a quick summary of how to invoke configure.

 -prefix=dir
           Configure the source to install programs and files under directory
           `dir'.

 --srcdir=dirname
           *Warning: using this option requires gnu make, or another make that
           implements the VPATH feature.*
           Use this option to make configurations in directories separate from
           the  source directories.  Among other things, you can use this to
           build (or maintain) several configurations simultaneously, in
           separate directories.  configure writes configuration specific files
           in the current directory, but arranges for them to use the source in
           the directory dirname.  configure creates directories under the
           working directory in parallel to the source directories below
           dirname.

 --norecursion
           Configure only the directory level where configure is executed; do
           not propagate configuration to subdirectories.

 --rm
           Remove files otherwise built during configuration.

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

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

 host ...
           Configure  to run on the specified host.

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

 configure accepts other options, for compatibility with configuring other gnu
 tools recursively; but these are the only options that affect  or its
 supporting libraries.


ΓòÉΓòÉΓòÉ 23. Index ΓòÉΓòÉΓòÉ

Sorry, no cp index


ΓòÉΓòÉΓòÉ <hidden>  ΓòÉΓòÉΓòÉ

`b' cannot be used because these format letters are also used with the x
command, where `b' stands for ``byte''; see Examining memory.


ΓòÉΓòÉΓòÉ <hidden>  ΓòÉΓòÉΓòÉ

This is a way of removing one word from the stack, on machines where stacks
grow downward in memory (most machines, nowadays).  This assumes that the
innermost stack frame is selected; setting $sp is not allowed when other stack
frames are selected.  To pop entire frames off the stack, regardless of machine
architecture, use return; see Returning from a function.


ΓòÉΓòÉΓòÉ <hidden>  ΓòÉΓòÉΓòÉ

In `gdb-/gdb/refcard.ps' of the version