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131 lines
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<title>All-Stop Mode - Debugging with GDB</title>
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<a name="All-Stop-Mode"></a>
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<a name="All_002dStop-Mode"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Non_002dStop-Mode.html#Non_002dStop-Mode">Non-Stop Mode</a>,
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Up: <a rel="up" accesskey="u" href="Thread-Stops.html#Thread-Stops">Thread Stops</a>
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<h4 class="subsection">5.5.1 All-Stop Mode</h4>
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<p><a name="index-all_002dstop-mode-402"></a>
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In all-stop mode, whenever your program stops under <span class="sc">gdb</span> for any reason,
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<em>all</em> threads of execution stop, not just the current thread. This
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allows you to examine the overall state of the program, including
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switching between threads, without worrying that things may change
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underfoot.
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<p>Conversely, whenever you restart the program, <em>all</em> threads start
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executing. <em>This is true even when single-stepping</em> with commands
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like <code>step</code> or <code>next</code>.
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<p>In particular, <span class="sc">gdb</span> cannot single-step all threads in lockstep.
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Since thread scheduling is up to your debugging target's operating
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system (not controlled by <span class="sc">gdb</span>), other threads may
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execute more than one statement while the current thread completes a
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single step. Moreover, in general other threads stop in the middle of a
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statement, rather than at a clean statement boundary, when the program
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stops.
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<p>You might even find your program stopped in another thread after
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continuing or even single-stepping. This happens whenever some other
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thread runs into a breakpoint, a signal, or an exception before the
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first thread completes whatever you requested.
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<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
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signal, it automatically selects the thread where that breakpoint or
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signal happened. <span class="sc">gdb</span> alerts you to the context switch with a
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message such as ‘<samp><span class="samp">[Switching to Thread </span><var>n</var><span class="samp">]</span></samp>’ to identify the
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thread.
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<p>On some OSes, you can modify <span class="sc">gdb</span>'s default behavior by
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locking the OS scheduler to allow only a single thread to run.
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<dl>
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<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,
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record mode, and replay mode. If it is <code>off</code>, then there is no
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locking and any thread may run at any time. If <code>on</code>, then only
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the current thread may run when the inferior is resumed. The
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<code>step</code> mode optimizes for single-stepping; it prevents other
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threads from preempting the current thread while you are stepping, so
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that the focus of debugging does not change unexpectedly. Other
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threads never get a chance to run when you step, and they are
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completely free to run when you use commands like ‘<samp><span class="samp">continue</span></samp>’,
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‘<samp><span class="samp">until</span></samp>’, or ‘<samp><span class="samp">finish</span></samp>’. However, unless another thread hits a
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breakpoint during its timeslice, <span class="sc">gdb</span> does not change the
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current thread away from the thread that you are debugging. The
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<code>replay</code> mode behaves like <code>off</code> in record mode and like
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<code>on</code> in replay mode.
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<br><dt><code>show scheduler-locking</code><dd>Display the current scheduler locking mode.
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</dl>
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<p><a name="index-resume-threads-of-multiple-processes-simultaneously-408"></a>By default, when you issue one of the execution commands such as
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<code>continue</code>, <code>next</code> or <code>step</code>, <span class="sc">gdb</span> allows only
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threads of the current inferior to run. For example, if <span class="sc">gdb</span>
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is attached to two inferiors, each with two threads, the
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<code>continue</code> command resumes only the two threads of the current
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inferior. This is useful, for example, when you debug a program that
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forks and you want to hold the parent stopped (so that, for instance,
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it doesn't run to exit), while you debug the child. In other
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situations, you may not be interested in inspecting the current state
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of any of the processes <span class="sc">gdb</span> is attached to, and you may want
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to resume them all until some breakpoint is hit. In the latter case,
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you can instruct <span class="sc">gdb</span> to allow all threads of all the
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inferiors to run with the <code>set schedule-multiple</code><!-- /@w --> command.
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<a name="index-set-schedule_002dmultiple-409"></a>
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<dl><dt><code>set schedule-multiple</code><dd>Set the mode for allowing threads of multiple processes to be resumed
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when an execution command is issued. When <code>on</code>, all threads of
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all processes are allowed to run. When <code>off</code>, only the threads
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of the current process are resumed. The default is <code>off</code>. The
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<code>scheduler-locking</code> mode takes precedence when set to <code>on</code>,
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or while you are stepping and set to <code>step</code>.
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<br><dt><code>show schedule-multiple</code><dd>Display the current mode for resuming the execution of threads of
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multiple processes.
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</dl>
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</body></html>
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