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INFO-DIR-SECTION Programming & development tools.
START-INFO-DIR-ENTRY
* Gdb: (gdb).                     The GNU debugger.
END-INFO-DIR-ENTRY

   This file documents the GNU debugger GDB.

   This is the Eighth Edition, March 2000, of `Debugging with GDB: the
GNU Source-Level Debugger' for GDB Version 5.0.

   Copyright (C) 1988-2000 Free Software Foundation, Inc.

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

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


File: gdb.info,  Node: Environment,  Next: Working Directory,  Prev: Arguments,  Up: Running

Your program's environment
==========================

   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 GDB over
again.

`path DIRECTORY'
     Add DIRECTORY to the front of the `PATH' environment variable (the
     search path for executables), for both GDB and your program.  You
     may specify several directory names, separated by whitespace or by
     a system-dependent separator character (`:' on Unix, `;' on MS-DOS
     and MS-Windows).  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 GDB searches the path.  If you use
     `.' instead, it refers to the directory where you executed the
     `path' command.  GDB 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 GDB 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 the debugged 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:_ On Unix systems, GDB 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'.


File: gdb.info,  Node: Working Directory,  Next: Input/Output,  Prev: Environment,  Up: Running

Your program's working directory
================================

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

   The GDB working directory also serves as a default for the commands
that specify files for GDB to operate on.  *Note Commands to specify
files: Files.

`cd DIRECTORY'
     Set the GDB working directory to DIRECTORY.

`pwd'
     Print the GDB working directory.


File: gdb.info,  Node: Input/Output,  Next: Attach,  Prev: Working Directory,  Up: Running

Your program's input and output
===============================

   By default, the program you run under GDB does input and output to
the same terminal that GDB uses.  GDB 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 GDB 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
GDB still comes from your terminal.


File: gdb.info,  Node: Attach,  Next: Kill Process,  Prev: Input/Output,  Up: Running

Debugging an already-running process
====================================

`attach PROCESS-ID'
     This command attaches to a running process--one that was started
     outside GDB.  (`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 you use `attach', the debugger finds the program running in the
process first by looking in the current working directory, then (if the
program is not found) by using the source file search path (*note
Specifying source directories: Source Path.).  You can also use the
`file' command to load the program.  *Note Commands to Specify Files:
Files.

   The first thing GDB does after arranging to debug the specified
process is to stop it.  You can examine and modify an attached process
with all the GDB 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
GDB to the process.

`detach'
     When you have finished debugging the attached process, you can use
     the `detach' command to release it from GDB control.  Detaching
     the process continues its execution.  After the `detach' command,
     that process and GDB 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 GDB or use the `run' command while you have an attached
process, you kill that process.  By default, GDB 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 (*note Optional
warnings and messages: Messages/Warnings.).


File: gdb.info,  Node: Kill Process,  Next: Threads,  Prev: Attach,  Up: Running

Killing the child process
=========================

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

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

   On some operating systems, a program cannot be executed outside GDB
while you have breakpoints set on it inside GDB.  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', GDB notices that the file has changed, and reads
the symbol table again (while trying to preserve your current
breakpoint settings).


File: gdb.info,  Node: Threads,  Next: Processes,  Prev: Kill Process,  Up: Running

Debugging programs with multiple threads
========================================

   In some operating systems, such as HP-UX and Solaris, 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.

   GDB 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 GDB
     configuration where the operating system supports threads.  If
     your GDB 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:

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

   The GDB thread debugging facility allows you to observe all threads
while your program runs--but whenever GDB 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 GDB 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 GDB 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, GDB 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.  GDB
     displays for each thread (in this order):

       1. the thread number assigned by GDB

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

       3. the current stack frame summary for that thread

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

     For example,

     (gdb) 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

   On HP-UX systems:

   For debugging purposes, GDB associates its own thread number--a
small integer assigned in thread-creation order--with each thread in
your program.

   Whenever GDB detects a new thread in your program, it displays both
GDB's thread number and 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 HP-UX, you see

     [New thread 2 (system thread 26594)]

when GDB notices a new thread.

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

       1. the thread number assigned by GDB

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

       3. the current stack frame summary for that thread

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

     For example,

     (gdb) info threads
         * 3 system thread 26607  worker (wptr=0x7b09c318 "@") \

     at quicksort.c:137
           2 system thread 26606  0x7b0030d8 in __ksleep () \

     from /usr/lib/libc.2
           1 system thread 27905  0x7b003498 in _brk () \

     from /usr/lib/libc.2

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

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

   *Note Stopping and starting multi-thread programs: Thread Stops, for
more information about how GDB behaves when you stop and start programs
with multiple threads.

   *Note Setting watchpoints: Set Watchpoints, for information about
watchpoints in programs with multiple threads.


File: gdb.info,  Node: Processes,  Prev: Threads,  Up: Running

Debugging programs with multiple processes
==========================================

   On most systems, GDB has no special support for debugging programs
which create additional processes using the `fork' function.  When a
program forks, GDB 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 GDB on the
child.  While the child is sleeping, use the `ps' program to get its
process ID.  Then tell GDB (a new invocation of GDB if you are also
debugging the parent process) to attach to the child process (*note
Attach::).  From that point on you can debug the child process just
like any other process which you attached to.

   On HP-UX (11.x and later only?), GDB provides support for debugging
programs that create additional processes using the `fork' or `vfork'
function.

   By default, when a program forks, GDB will continue to debug the
parent process and the child process will run unimpeded.

   If you want to follow the child process instead of the parent
process, use the command `set follow-fork-mode'.

`set follow-fork-mode MODE'
     Set the debugger response to a program call of `fork' or `vfork'.
     A call to `fork' or `vfork' creates a new process.  The MODE can
     be:

    `parent'
          The original process is debugged after a fork.  The child
          process runs unimpeded.  This is the default.

    `child'
          The new process is debugged after a fork.  The parent process
          runs unimpeded.

    `ask'
          The debugger will ask for one of the above choices.

`show follow-fork-mode'
     Display the current debugger response to a `fork' or `vfork' call.

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

   When a child process is spawned by `vfork', you cannot debug the
child or parent until an `exec' call completes.

   If you issue a `run' command to GDB after an `exec' call executes,
the new target restarts.  To restart the parent process, use the `file'
command with the parent executable name as its argument.

   You can use the `catch' command to make GDB stop whenever a `fork',
`vfork', or `exec' call is made.  *Note Setting catchpoints: Set
Catchpoints.


File: gdb.info,  Node: Stopping,  Next: Stack,  Prev: Running,  Up: Top

Stopping and Continuing
***********************

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

   Inside GDB, your program may stop for any of several reasons, such
as a signal, a breakpoint, or reaching a new line after a GDB 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 GDB 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.

* Menu:

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


File: gdb.info,  Node: Breakpoints,  Next: Continuing and Stepping,  Up: Stopping

Breakpoints, watchpoints, and catchpoints
=========================================

   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 (*note Setting
breakpoints: Set Breaks.), to specify the place where your program
should stop by line number, function name or exact address in the
program.

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

   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 (*note Setting watchpoints: Set Watchpoints.), but
aside from that, you can manage a watchpoint like any other breakpoint:
you enable, disable, and delete both breakpoints and watchpoints using
the same commands.

   You can arrange to have values from your program displayed
automatically whenever GDB stops at a breakpoint.  *Note Automatic
display: Auto Display.

   A "catchpoint" is another special breakpoint that stops your program
when a certain kind of event occurs, such as the throwing of a C++
exception or the loading of a library.  As with watchpoints, you use a
different command to set a catchpoint (*note Setting catchpoints: Set
Catchpoints.), but aside from that, you can manage a catchpoint like any
other breakpoint.  (To stop when your program receives a signal, use the
`handle' command; see *Note Signals: Signals.)

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

   Some GDB commands accept a range of breakpoints on which to operate.
A breakpoint range is either a single breakpoint number, like `5', or
two such numbers, in increasing order, separated by a hyphen, like
`5-7'.  When a breakpoint range is given to a command, all breakpoint
in that range are operated on.

* Menu:

* Set Breaks::                  Setting breakpoints
* Set Watchpoints::             Setting watchpoints
* Set Catchpoints::             Setting catchpoints
* Delete Breaks::               Deleting breakpoints
* Disabling::                   Disabling breakpoints
* Conditions::                  Break conditions
* Break Commands::              Breakpoint command lists
* Breakpoint Menus::            Breakpoint menus
* Error in Breakpoints::        ``Cannot insert breakpoints''


File: gdb.info,  Node: Set Breaks,  Next: Set Watchpoints,  Up: Breakpoints

Setting breakpoints
-------------------

   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 *Note Convenience variables:
Convenience Vars, 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.
     *Note Breakpoint menus: 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
     "stack frame".  (*Note Frames: Frames, for a description of stack
     frames.)

`break LINENUM'
     Set a breakpoint at line LINENUM in the current source file.  The
     current source file is the last file whose source text was printed.
     The breakpoint will stop 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
     (*note Examining the Stack: 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, GDB stops the next
     time it reaches the current location; this may be useful inside
     loops.

     GDB 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.  *Note Break conditions: 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.  *Note Disabling breakpoints:
     Disabling.

`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 and some x86-based
     targets.  These targets will generate traps when a program
     accesses some data or instruction address that is assigned to the
     debug registers.  However the hardware breakpoint registers can
     take a limited number of breakpoints.  For example, on the DSU,
     only two data breakpoints can be set at a time, and GDB will
     reject this command if more than two are used.  Delete or disable
     unused hardware breakpoints before setting new ones (*note
     Disabling: Disabling.).  *Note Break conditions: 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.  *Note Disabling breakpoints: Disabling.
     See also *Note Break conditions: 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.

     The syntax of the regular expression is the standard one used with
     tools like `grep'.  Note that this is different from the syntax
     used by shells, so for instance `foo*' matches all functions that
     include an `fo' followed by zero or more `o's.  There is an
     implicit `.*' leading and trailing the regular expression you
     supply, so to match only functions that begin with `foo', use
     `^foo'.

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

    _Breakpoint Numbers_

    _Type_
          Breakpoint, watchpoint, or catchpoint.

    _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 (*note Examining memory: Memory.).

     `info break' 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.

   GDB 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 (*note Break
conditions: Conditions.).

   GDB 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 GDB 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 GDB 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 GDB `until' command.

    `finish'
          Temporary internal breakpoint used by the GDB `finish'
          command.

    `shlib events'
          Shared library events.


File: gdb.info,  Node: Set Watchpoints,  Next: Set Catchpoints,  Prev: Set Breaks,  Up: Breakpoints

Setting watchpoints
-------------------

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

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

   On some systems, such as HP-UX, Linux and some other x86-based
targets, GDB includes support for hardware watchpoints, which do not
slow down the running of your program.

`watch EXPR'
     Set a watchpoint for an expression.  GDB will break when EXPR is
     written into by the program and its value changes.

`rwatch EXPR'
     Set a watchpoint that will break when watch EXPR is read by the
     program.

`awatch EXPR'
     Set a watchpoint that will break when EXPR is either read or
     written into by the program.

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

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

   When you issue the `watch' command, GDB reports

     Hardware watchpoint NUM: EXPR

if it was able to set a hardware watchpoint.

   Currently, the `awatch' and `rwatch' commands can only set hardware
watchpoints, because accesses to data that don't change the value of
the watched expression cannot be detected without examining every
instruction as it is being executed, and GDB does not do that
currently.  If GDB finds that it is unable to set a hardware breakpoint
with the `awatch' or `rwatch' command, it will print a message like
this:

     Expression cannot be implemented with read/access watchpoint.

   Sometimes, GDB cannot set a hardware watchpoint because the data
type of the watched expression is wider than what a hardware watchpoint
on the target machine can handle.  For example, some systems can only
watch regions that are up to 4 bytes wide; on such systems you cannot
set hardware watchpoints for an expression that yields a
double-precision floating-point number (which is typically 8 bytes
wide).  As a work-around, it might be possible to break the large region
into a series of smaller ones and watch them with separate watchpoints.

   If you set too many hardware watchpoints, GDB might be unable to
insert all of them when you resume the execution of your program.
Since the precise number of active watchpoints is unknown until such
time as the program is about to be resumed, GDB might not be able to
warn you about this when you set the watchpoints, and the warning will
be printed only when the program is resumed:

     Hardware watchpoint NUM: Could not insert watchpoint

If this happens, delete or disable some of the watchpoints.

   The SPARClite DSU will generate traps when a program accesses some
data or instruction address that is assigned to the debug registers.
For the data addresses, DSU facilitates the `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.  GDB will reject the
command if you try to mix watchpoints.  Delete or disable unused
watchpoint commands before setting new ones.

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

   GDB automatically deletes watchpoints that watch local (automatic)
variables, or expressions that involve such variables, when they go out
of scope, that is, when the execution leaves the block in which these
variables were defined.  In particular, when the program being debugged
terminates, _all_ local variables go out of scope, and so only
watchpoints that watch global variables remain set.  If you rerun the
program, you will need to set all such watchpoints again.  One way of
doing that would be to set a code breakpoint at the entry to the `main'
function and when it breaks, set all the watchpoints.

     _Warning:_ In multi-thread programs, watchpoints have only limited
     usefulness.  With the current watchpoint implementation, GDB 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, GDB may not notice when a non-current thread's
     activity changes the expression.

     _HP-UX Warning:_ In multi-thread programs, software watchpoints
     have only limited usefulness.  If GDB creates a software
     watchpoint, it 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 software watchpoints as usual.  However, GDB may not notice
     when a non-current thread's activity changes the expression.
     (Hardware watchpoints, in contrast, watch an expression in all
     threads.)


File: gdb.info,  Node: Set Catchpoints,  Next: Delete Breaks,  Prev: Set Watchpoints,  Up: Breakpoints

Setting catchpoints
-------------------

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

`catch EVENT'
     Stop when EVENT occurs.  EVENT can be any of the following:
    `throw'
          The throwing of a C++ exception.

    `catch'
          The catching of a C++ exception.

    `exec'
          A call to `exec'.  This is currently only available for HP-UX.

    `fork'
          A call to `fork'.  This is currently only available for HP-UX.

    `vfork'
          A call to `vfork'.  This is currently only available for
          HP-UX.

    `load'
    `load LIBNAME'
          The dynamic loading of any shared library, or the loading of
          the library LIBNAME.  This is currently only available for
          HP-UX.

    `unload'
    `unload LIBNAME'
          The unloading of any dynamically loaded shared library, or
          the unloading of the library LIBNAME.  This is currently only
          available for HP-UX.

`tcatch EVENT'
     Set a catchpoint that is enabled only for one stop.  The
     catchpoint is automatically deleted after the first time the event
     is caught.

   Use the `info break' command to list the current catchpoints.

   There are currently some limitations to C++ exception handling
(`catch throw' and `catch catch') in GDB:

   * If you call a function interactively, GDB normally returns control
     to you when the function has finished executing.  If the call
     raises an exception, however, the call may bypass the mechanism
     that returns control to you and cause your program either to abort
     or to simply continue running until it hits a breakpoint, catches
     a signal that GDB is listening for, or exits.  This is the case
     even if you set a catchpoint for the exception; catchpoints on
     exceptions are disabled within interactive calls.

   * 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' (*note
Breakpoints; watchpoints; and exceptions: Breakpoints.).

   With a conditional breakpoint (*note Break conditions: 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.


File: gdb.info,  Node: Delete Breaks,  Next: Disabling,  Prev: Set Catchpoints,  Up: Breakpoints

Deleting breakpoints
--------------------

   It is often necessary to eliminate a breakpoint, watchpoint, or
catchpoint once it has done its job and you no longer want your program
to stop there.  This is called "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, watchpoints, or catchpoints by specifying
their breakpoint numbers.

   It is not necessary to delete a breakpoint to proceed past it.  GDB
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 (*note Selecting a frame: Selection.).
     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] [RANGE...]'
     Delete the breakpoints, watchpoints, or catchpoints of the
     breakpoint ranges specified as arguments.  If no argument is
     specified, delete all breakpoints (GDB asks confirmation, unless
     you have `set confirm off').  You can abbreviate this command as
     `d'.


File: gdb.info,  Node: Disabling,  Next: Conditions,  Prev: Delete Breaks,  Up: Breakpoints

Disabling breakpoints
---------------------

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

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

   * 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.

   * Enabled for deletion.  The breakpoint stops your program, but
     immediately after it does so it is deleted permanently.  A
     breakpoint set with the `tbreak' command starts out in this state.

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

`disable [breakpoints] [RANGE...]'
     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] [RANGE...]'
     Enable the specified breakpoints (or all defined breakpoints).
     They become effective once again in stopping your program.

`enable [breakpoints] once RANGE...'
     Enable the specified breakpoints temporarily.  GDB disables any of
     these breakpoints immediately after stopping your program.

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

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