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<title>Debugging with GDB: Interrupted System Calls</title>

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<a name="Interrupted-System-Calls"></a>
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<a name="Interrupted-System-Calls-1"></a>
<h4 class="subsection">5.5.5 Interrupted System Calls</h4>

<a name="index-thread-breakpoints-and-system-calls"></a>
<a name="index-system-calls-and-thread-breakpoints"></a>
<a name="index-premature-return-from-system-calls"></a>
<p>There is an unfortunate side effect when using <small>GDB</small> 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 <small>GDB</small> uses to implement breakpoints and other events that
stop execution.
</p>
<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>
<p>For example, do not write code like this:
</p>
<div class="smallexample">
<pre class="smallexample">  sleep (10);
</pre></div>

<p>The call to <code>sleep</code> will return early if a different thread stops
at a breakpoint or for some other reason.
</p>
<p>Instead, write this:
</p>
<div class="smallexample">
<pre class="smallexample">  int unslept = 10;
  while (unslept &gt; 0)
    unslept = sleep (unslept);
</pre></div>

<p>A system call is allowed to return early, so the system is still
conforming to its specification.  But <small>GDB</small> does cause your
multi-threaded program to behave differently than it would without
<small>GDB</small>.
</p>
<p>Also, <small>GDB</small> 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.
</p>


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