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211 lines
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<title>Forks - Debugging with GDB</title>
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<div class="node">
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<a name="Forks"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Checkpoint_002fRestart.html#Checkpoint_002fRestart">Checkpoint/Restart</a>,
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Previous: <a rel="previous" accesskey="p" href="Threads.html#Threads">Threads</a>,
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Up: <a rel="up" accesskey="u" href="Running.html#Running">Running</a>
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<hr>
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</div>
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<h3 class="section">4.11 Debugging Forks</h3>
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<p><a name="index-fork_002c-debugging-programs-which-call-208"></a><a name="index-multiple-processes-209"></a><a name="index-processes_002c-multiple-210"></a>On most systems, <span class="sc">gdb</span> has no special support for debugging
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programs which create additional processes using the <code>fork</code>
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function. When a program forks, <span class="sc">gdb</span> will continue to debug the
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parent process and the child process will run unimpeded. If you have
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set a breakpoint in any code which the child then executes, the child
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will get a <code>SIGTRAP</code> signal which (unless it catches the signal)
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will cause it to terminate.
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<p>However, if you want to debug the child process there is a workaround
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which isn't too painful. Put a call to <code>sleep</code> in the code which
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the child process executes after the fork. It may be useful to sleep
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only if a certain environment variable is set, or a certain file exists,
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so that the delay need not occur when you don't want to run <span class="sc">gdb</span>
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on the child. While the child is sleeping, use the <code>ps</code> program to
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get its process ID. Then tell <span class="sc">gdb</span> (a new invocation of
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<span class="sc">gdb</span> if you are also debugging the parent process) to attach to
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the child process (see <a href="Attach.html#Attach">Attach</a>). From that point on you can debug
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the child process just like any other process which you attached to.
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<p>On some systems, <span class="sc">gdb</span> provides support for debugging programs
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that create additional processes using the <code>fork</code> or <code>vfork</code>
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functions. On <span class="sc">gnu</span>/Linux platforms, this feature is supported
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with kernel version 2.5.46 and later.
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<p>The fork debugging commands are supported in native mode and when
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connected to <code>gdbserver</code> in either <code>target remote</code> mode or
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<code>target extended-remote</code> mode.
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<p>By default, when a program forks, <span class="sc">gdb</span> will continue to debug
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the parent process and the child process will run unimpeded.
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<p>If you want to follow the child process instead of the parent process,
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use the command <code>set follow-fork-mode</code><!-- /@w -->.
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<a name="index-set-follow_002dfork_002dmode-211"></a>
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<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
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<code>vfork</code>. A call to <code>fork</code> or <code>vfork</code> creates a new
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process. The <var>mode</var> argument can be:
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<dl>
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<dt><code>parent</code><dd>The original process is debugged after a fork. The child process runs
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unimpeded. This is the default.
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<br><dt><code>child</code><dd>The new process is debugged after a fork. The parent process runs
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unimpeded.
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</dl>
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<p><a name="index-show-follow_002dfork_002dmode-212"></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.
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</dl>
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<p><a name="index-debugging-multiple-processes-213"></a>On Linux, if you want to debug both the parent and child processes, use the
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command <code>set detach-on-fork</code><!-- /@w -->.
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<a name="index-set-detach_002don_002dfork-214"></a>
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<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
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retain debugger control over them both.
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<dl>
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<dt><code>on</code><dd>The child process (or parent process, depending on the value of
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<code>follow-fork-mode</code>) will be detached and allowed to run
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independently. This is the default.
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<br><dt><code>off</code><dd>Both processes will be held under the control of <span class="sc">gdb</span>.
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One process (child or parent, depending on the value of
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<code>follow-fork-mode</code>) is debugged as usual, while the other
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is held suspended.
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</dl>
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<p><a name="index-show-detach_002don_002dfork-215"></a><br><dt><code>show detach-on-fork</code><dd>Show whether detach-on-fork mode is on/off.
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</dl>
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<p>If you choose to set ‘<samp><span class="samp">detach-on-fork</span></samp>’ mode off, then <span class="sc">gdb</span>
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will retain control of all forked processes (including nested forks).
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You can list the forked processes under the control of <span class="sc">gdb</span> by
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using the <code>info inferiors</code><!-- /@w --> command, and switch from one fork
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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>).
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<p>To quit debugging one of the forked processes, you can either detach
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from it by using the <code>detach inferiors</code><!-- /@w --> command (allowing it
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to run independently), or kill it using the <code>kill inferiors</code><!-- /@w -->
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command. See <a href="Inferiors-and-Programs.html#Inferiors-and-Programs">Debugging Multiple Inferiors and Programs</a>.
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<p>If you ask to debug a child process and a <code>vfork</code> is followed by an
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<code>exec</code>, <span class="sc">gdb</span> executes the new target up to the first
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breakpoint in the new target. If you have a breakpoint set on
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<code>main</code> in your original program, the breakpoint will also be set on
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the child process's <code>main</code>.
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<p>On some systems, when a child process is spawned by <code>vfork</code>, you
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cannot debug the child or parent until an <code>exec</code> call completes.
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<p>If you issue a <code>run</code> command to <span class="sc">gdb</span> after an <code>exec</code>
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call executes, the new target restarts. To restart the parent
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process, use the <code>file</code> command with the parent executable name
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as its argument. By default, after an <code>exec</code> call executes,
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<span class="sc">gdb</span> discards the symbols of the previous executable image.
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You can change this behaviour with the <code>set follow-exec-mode</code><!-- /@w -->
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command.
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<a name="index-set-follow_002dexec_002dmode-216"></a>
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<dl><dt><code>set follow-exec-mode </code><var>mode</var><dd>
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Set debugger response to a program call of <code>exec</code>. An
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<code>exec</code> call replaces the program image of a process.
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<p><code>follow-exec-mode</code> can be:
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<dl>
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<dt><code>new</code><dd><span class="sc">gdb</span> creates a new inferior and rebinds the process to this
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new inferior. The program the process was running before the
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<code>exec</code> call can be restarted afterwards by restarting the
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original inferior.
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<p>For example:
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<pre class="smallexample"> (gdb) info inferiors
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(gdb) info inferior
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Id Description Executable
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* 1 <null> prog1
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(gdb) run
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process 12020 is executing new program: prog2
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Program exited normally.
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(gdb) info inferiors
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Id Description Executable
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1 <null> prog1
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* 2 <null> prog2
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</pre>
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<br><dt><code>same</code><dd><span class="sc">gdb</span> keeps the process bound to the same inferior. The new
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executable image replaces the previous executable loaded in the
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inferior. Restarting the inferior after the <code>exec</code> call, with
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e.g., the <code>run</code> command, restarts the executable the process was
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running after the <code>exec</code> call. This is the default mode.
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<p>For example:
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<pre class="smallexample"> (gdb) info inferiors
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Id Description Executable
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* 1 <null> prog1
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(gdb) run
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process 12020 is executing new program: prog2
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Program exited normally.
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(gdb) info inferiors
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Id Description Executable
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* 1 <null> prog2
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</pre>
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</dl>
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</dl>
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<p><code>follow-exec-mode</code> is supported in native mode and
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<code>target extended-remote</code> mode.
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<p>You can use the <code>catch</code> command to make <span class="sc">gdb</span> stop whenever
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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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</body></html>
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