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<h4 class="subsection">5.4.1 All-Stop Mode</h4>

<p><a name="index-all_002dstop-mode-318"></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-319"></a><a name="index-switching-threads-automatically-320"></a><a name="index-threads_002c-automatic-switching-321"></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 &lsquo;<samp><span class="samp">[Switching to Thread </span><var>n</var><span class="samp">]</span></samp>&rsquo; 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-322"></a><a name="index-lock-scheduler-323"></a>Set the scheduler locking 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 only rarely (or never) get a chance to run
when you step.  They are more likely to run when you &lsquo;<samp><span class="samp">next</span></samp>&rsquo; over a
function call, and they are completely free to run when you use commands
like &lsquo;<samp><span class="samp">continue</span></samp>&rsquo;, &lsquo;<samp><span class="samp">until</span></samp>&rsquo;, or &lsquo;<samp><span class="samp">finish</span></samp>&rsquo;.  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.

     <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-324"></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&nbsp;schedule-multiple</code><!-- /@w --> command.

     
<a name="index-set-schedule_002dmultiple-325"></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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