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<h4 class="subsection">5.4.5 Interrupted System Calls</h4>

<p><a name="index-thread-breakpoints-and-system-calls-348"></a><a name="index-system-calls-and-thread-breakpoints-349"></a><a name="index-premature-return-from-system-calls-350"></a>There is an unfortunate side effect when using <span class="sc">gdb</span> to debug
multi-threaded programs.  If one thread stops for a
breakpoint, or for some other reason, and another thread is blocked in a
system call, then the system call may return prematurely.  This is a
consequence of the interaction between multiple threads and the signals
that <span class="sc">gdb</span> uses to implement breakpoints and other events that
stop execution.

   <p>To handle this problem, your program should check the return value of
each system call and react appropriately.  This is good programming
style anyways.

   <p>For example, do not write code like this:

<pre class="smallexample">       sleep (10);
</pre>
   <p>The call to <code>sleep</code> will return early if a different thread stops
at a breakpoint or for some other reason.

   <p>Instead, write this:

<pre class="smallexample">       int unslept = 10;
       while (unslept &gt; 0)
         unslept = sleep (unslept);
</pre>
   <p>A system call is allowed to return early, so the system is still
conforming to its specification.  But <span class="sc">gdb</span> does cause your
multi-threaded program to behave differently than it would without
<span class="sc">gdb</span>.

   <p>Also, <span class="sc">gdb</span> uses internal breakpoints in the thread library to
monitor certain events such as thread creation and thread destruction. 
When such an event happens, a system call in another thread may return
prematurely, even though your program does not appear to stop.

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