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| <h3 class="section">4.11 Debugging Forks</h3> |
| |
| <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 |
| 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. On <span class="sc">gnu</span>/Linux platforms, this feature is supported |
| with kernel version 2.5.46 and later. |
| |
| <p>The fork debugging commands are supported in native mode and when |
| connected to <code>gdbserver</code> in either <code>target remote</code> mode or |
| <code>target extended-remote</code> mode. |
| |
| <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 follow-fork-mode</code><!-- /@w -->. |
| |
| |
| <a name="index-set-follow_002dfork_002dmode-211"></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-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. |
| </dl> |
| |
| <p><a name="index-debugging-multiple-processes-213"></a>On Linux, if you want to debug both the parent and child processes, use the |
| command <code>set detach-on-fork</code><!-- /@w -->. |
| |
| |
| <a name="index-set-detach_002don_002dfork-214"></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-215"></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 ‘<samp><span class="samp">detach-on-fork</span></samp>’ 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 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 inferiors</code><!-- /@w --> command (allowing it |
| to run independently), or kill it using the <code>kill inferiors</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 follow-exec-mode</code><!-- /@w --> |
| command. |
| |
| |
| <a name="index-set-follow_002dexec_002dmode-216"></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 <null> prog1 |
| (gdb) run |
| process 12020 is executing new program: prog2 |
| Program exited normally. |
| (gdb) info inferiors |
| Id Description Executable |
| 1 <null> prog1 |
| * 2 <null> prog2 |
| </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 <null> prog1 |
| (gdb) run |
| process 12020 is executing new program: prog2 |
| Program exited normally. |
| (gdb) info inferiors |
| Id Description Executable |
| * 1 <null> prog2 |
| </pre> |
| </dl> |
| </dl> |
| |
| <p><code>follow-exec-mode</code> is supported in native mode and |
| <code>target extended-remote</code> mode. |
| |
| <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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