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<h3 class="section">4.11 Debugging Forks</h3>

<p><a name="index-fork_002c-debugging-programs-which-call-179"></a><a name="index-multiple-processes-180"></a><a name="index-processes_002c-multiple-181"></a>On most systems, <span class="sc">gdb</span> has no special support for debugging
programs which create additional processes using the <code>fork</code>
function.  When a program forks, <span class="sc">gdb</span> 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 <code>SIGTRAP</code> signal which (unless it catches the signal)
will cause it to terminate.

   <p>However, if you want to debug the child process there is a workaround
which isn't too painful.  Put a call to <code>sleep</code> 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 <span class="sc">gdb</span>
on the child.  While the child is sleeping, use the <code>ps</code> program to
get its process ID.  Then tell <span class="sc">gdb</span> (a new invocation of
<span class="sc">gdb</span> if you are also debugging the parent process) to attach to
the child process (see <a href="Attach.html#Attach">Attach</a>).  From that point on you can debug
the child process just like any other process which you attached to.

   <p>On some systems, <span class="sc">gdb</span> provides support for debugging programs that
create additional processes using the <code>fork</code> or <code>vfork</code> functions. 
Currently, the only platforms with this feature are HP-UX (11.x and later
only?) and <span class="sc">gnu</span>/Linux (kernel version 2.5.60 and later).

   <p>By default, when a program forks, <span class="sc">gdb</span> will continue to debug
the parent process and the child process will run unimpeded.

   <p>If you want to follow the child process instead of the parent process,
use the command <code>set&nbsp;follow-fork-mode</code><!-- /@w -->.

     
<a name="index-set-follow_002dfork_002dmode-182"></a>
<dl><dt><code>set follow-fork-mode </code><var>mode</var><dd>Set the debugger response to a program call of <code>fork</code> or
<code>vfork</code>.  A call to <code>fork</code> or <code>vfork</code> creates a new
process.  The <var>mode</var> argument can be:

          <dl>
<dt><code>parent</code><dd>The original process is debugged after a fork.  The child process runs
unimpeded.  This is the default.

          <br><dt><code>child</code><dd>The new process is debugged after a fork.  The parent process runs
unimpeded.

     </dl>

     <p><a name="index-show-follow_002dfork_002dmode-183"></a><br><dt><code>show follow-fork-mode</code><dd>Display the current debugger response to a <code>fork</code> or <code>vfork</code> call. 
</dl>

   <p><a name="index-debugging-multiple-processes-184"></a>On Linux, if you want to debug both the parent and child processes, use the
command <code>set&nbsp;detach-on-fork</code><!-- /@w -->.

     
<a name="index-set-detach_002don_002dfork-185"></a>
<dl><dt><code>set detach-on-fork </code><var>mode</var><dd>Tells gdb whether to detach one of the processes after a fork, or
retain debugger control over them both.

          <dl>
<dt><code>on</code><dd>The child process (or parent process, depending on the value of
<code>follow-fork-mode</code>) will be detached and allowed to run
independently.  This is the default.

          <br><dt><code>off</code><dd>Both processes will be held under the control of <span class="sc">gdb</span>. 
One process (child or parent, depending on the value of
<code>follow-fork-mode</code>) is debugged as usual, while the other
is held suspended.

     </dl>

     <p><a name="index-show-detach_002don_002dfork-186"></a><br><dt><code>show detach-on-fork</code><dd>Show whether detach-on-fork mode is on/off. 
</dl>

   <p>If you choose to set &lsquo;<samp><span class="samp">detach-on-fork</span></samp>&rsquo; mode off, then <span class="sc">gdb</span>
will retain control of all forked processes (including nested forks). 
You can list the forked processes under the control of <span class="sc">gdb</span> by
using the <code>info&nbsp;inferiors</code><!-- /@w --> command, and switch from one fork
to another by using the <code>inferior</code> command (see <a href="Inferiors-and-Programs.html#Inferiors-and-Programs">Debugging Multiple Inferiors and Programs</a>).

   <p>To quit debugging one of the forked processes, you can either detach
from it by using the <code>detach&nbsp;inferior</code><!-- /@w --> command (allowing it
to run independently), or kill it using the <code>kill&nbsp;inferior</code><!-- /@w -->
command.  See <a href="Inferiors-and-Programs.html#Inferiors-and-Programs">Debugging Multiple Inferiors and Programs</a>.

   <p>If you ask to debug a child process and a <code>vfork</code> is followed by an
<code>exec</code>, <span class="sc">gdb</span> executes the new target up to the first
breakpoint in the new target.  If you have a breakpoint set on
<code>main</code> in your original program, the breakpoint will also be set on
the child process's <code>main</code>.

   <p>On some systems, when a child process is spawned by <code>vfork</code>, you
cannot debug the child or parent until an <code>exec</code> call completes.

   <p>If you issue a <code>run</code> command to <span class="sc">gdb</span> after an <code>exec</code>
call executes, the new target restarts.  To restart the parent
process, use the <code>file</code> command with the parent executable name
as its argument.  By default, after an <code>exec</code> call executes,
<span class="sc">gdb</span> discards the symbols of the previous executable image. 
You can change this behaviour with the <code>set&nbsp;follow-exec-mode</code><!-- /@w -->
command.

     
<a name="index-set-follow_002dexec_002dmode-187"></a>
<dl><dt><code>set follow-exec-mode </code><var>mode</var><dd>
Set debugger response to a program call of <code>exec</code>.  An
<code>exec</code> call replaces the program image of a process.

     <p><code>follow-exec-mode</code> can be:

          <dl>
<dt><code>new</code><dd><span class="sc">gdb</span> creates a new inferior and rebinds the process to this
new inferior.  The program the process was running before the
<code>exec</code> call can be restarted afterwards by restarting the
original inferior.

          <p>For example:

          <pre class="smallexample">               (gdb) info inferiors
               (gdb) info inferior
                 Id   Description   Executable
               * 1    &lt;null&gt;        prog1
               (gdb) run
               process 12020 is executing new program: prog2
               Program exited normally.
               (gdb) info inferiors
                 Id   Description   Executable
               * 2    &lt;null&gt;        prog2
                 1    &lt;null&gt;        prog1
</pre>
          <br><dt><code>same</code><dd><span class="sc">gdb</span> keeps the process bound to the same inferior.  The new
executable image replaces the previous executable loaded in the
inferior.  Restarting the inferior after the <code>exec</code> call, with
e.g., the <code>run</code> command, restarts the executable the process was
running after the <code>exec</code> call.  This is the default mode.

          <p>For example:

          <pre class="smallexample">               (gdb) info inferiors
                 Id   Description   Executable
               * 1    &lt;null&gt;        prog1
               (gdb) run
               process 12020 is executing new program: prog2
               Program exited normally.
               (gdb) info inferiors
                 Id   Description   Executable
               * 1    &lt;null&gt;        prog2
</pre>
          </dl>
     </dl>

   <p>You can use the <code>catch</code> command to make <span class="sc">gdb</span> stop whenever
a <code>fork</code>, <code>vfork</code>, or <code>exec</code> call is made.  See <a href="Set-Catchpoints.html#Set-Catchpoints">Setting Catchpoints</a>.

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