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| <h4 class="subsection">5.5.1 All-Stop Mode</h4> |
| |
| <p><a name="index-all_002dstop-mode-402"></a> |
| In all-stop mode, whenever your program stops under <span class="sc">gdb</span> for any reason, |
| <em>all</em> threads of execution stop, not just the current thread. This |
| allows you to examine the overall state of the program, including |
| switching between threads, without worrying that things may change |
| underfoot. |
| |
| <p>Conversely, whenever you restart the program, <em>all</em> threads start |
| executing. <em>This is true even when single-stepping</em> with commands |
| like <code>step</code> or <code>next</code>. |
| |
| <p>In particular, <span class="sc">gdb</span> cannot single-step all threads in lockstep. |
| Since thread scheduling is up to your debugging target's operating |
| system (not controlled by <span class="sc">gdb</span>), other threads may |
| execute more than one statement while the current thread completes a |
| single step. Moreover, in general other threads stop in the middle of a |
| statement, rather than at a clean statement boundary, when the program |
| stops. |
| |
| <p>You might even find your program stopped in another thread after |
| continuing or even single-stepping. This happens whenever some other |
| thread runs into a breakpoint, a signal, or an exception before the |
| first thread completes whatever you requested. |
| |
| <p><a name="index-automatic-thread-selection-403"></a><a name="index-switching-threads-automatically-404"></a><a name="index-threads_002c-automatic-switching-405"></a>Whenever <span class="sc">gdb</span> stops your program, due to a breakpoint or a |
| signal, it automatically selects the thread where that breakpoint or |
| signal happened. <span class="sc">gdb</span> alerts you to the context switch with a |
| message such as ‘<samp><span class="samp">[Switching to Thread </span><var>n</var><span class="samp">]</span></samp>’ to identify the |
| thread. |
| |
| <p>On some OSes, you can modify <span class="sc">gdb</span>'s default behavior by |
| locking the OS scheduler to allow only a single thread to run. |
| |
| <dl> |
| <dt><code>set scheduler-locking </code><var>mode</var><dd><a name="index-scheduler-locking-mode-406"></a><a name="index-lock-scheduler-407"></a>Set the scheduler locking mode. It applies to normal execution, |
| record mode, and replay mode. If it is <code>off</code>, then there is no |
| locking and any thread may run at any time. If <code>on</code>, then only |
| the current thread may run when the inferior is resumed. The |
| <code>step</code> mode optimizes for single-stepping; it prevents other |
| threads from preempting the current thread while you are stepping, so |
| that the focus of debugging does not change unexpectedly. Other |
| threads never get a chance to run when you step, and they are |
| completely free to run when you use commands like ‘<samp><span class="samp">continue</span></samp>’, |
| ‘<samp><span class="samp">until</span></samp>’, or ‘<samp><span class="samp">finish</span></samp>’. However, unless another thread hits a |
| breakpoint during its timeslice, <span class="sc">gdb</span> does not change the |
| current thread away from the thread that you are debugging. The |
| <code>replay</code> mode behaves like <code>off</code> in record mode and like |
| <code>on</code> in replay mode. |
| |
| <br><dt><code>show scheduler-locking</code><dd>Display the current scheduler locking mode. |
| </dl> |
| |
| <p><a name="index-resume-threads-of-multiple-processes-simultaneously-408"></a>By default, when you issue one of the execution commands such as |
| <code>continue</code>, <code>next</code> or <code>step</code>, <span class="sc">gdb</span> allows only |
| threads of the current inferior to run. For example, if <span class="sc">gdb</span> |
| is attached to two inferiors, each with two threads, the |
| <code>continue</code> command resumes only the two threads of the current |
| inferior. This is useful, for example, when you debug a program that |
| forks and you want to hold the parent stopped (so that, for instance, |
| it doesn't run to exit), while you debug the child. In other |
| situations, you may not be interested in inspecting the current state |
| of any of the processes <span class="sc">gdb</span> is attached to, and you may want |
| to resume them all until some breakpoint is hit. In the latter case, |
| you can instruct <span class="sc">gdb</span> to allow all threads of all the |
| inferiors to run with the <code>set schedule-multiple</code><!-- /@w --> command. |
| |
| |
| <a name="index-set-schedule_002dmultiple-409"></a> |
| <dl><dt><code>set schedule-multiple</code><dd>Set the mode for allowing threads of multiple processes to be resumed |
| when an execution command is issued. When <code>on</code>, all threads of |
| all processes are allowed to run. When <code>off</code>, only the threads |
| of the current process are resumed. The default is <code>off</code>. The |
| <code>scheduler-locking</code> mode takes precedence when set to <code>on</code>, |
| or while you are stepping and set to <code>step</code>. |
| |
| <br><dt><code>show schedule-multiple</code><dd>Display the current mode for resuming the execution of threads of |
| multiple processes. |
| </dl> |
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