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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 Ninth Edition, April 2001, of `Debugging with GDB: the

GNU Source-Level Debugger' for GDB Version 20010707.

   Copyright (C)

1988,1989,1990,1991,1992,1993,1994,1995,1996,1998,1999,2000,2001

Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document

under the terms of the GNU Free Documentation License, Version 1.1 or

any later version published by the Free Software Foundation; with the

Invariant Sections being "A Sample GDB Session" and "Free Software",

with the Front-Cover texts being "A GNU Manual," and with the

Back-Cover Texts as in (a) below.

   (a) The FSF's Back-Cover Text is: "You have freedom to copy and

modify this GNU Manual, like GNU software.  Copies published by the Free

Software Foundation raise funds for GNU development."

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) that will be passed to your program.

     The value of `PATH' used by GDB does not change.  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  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  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

breakpoint 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.