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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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Next: <a href="Observer-Mode.html#Observer-Mode" accesskey="n" rel="next">Observer Mode</a>, Previous: <a href="Thread_002dSpecific-Breakpoints.html#Thread_002dSpecific-Breakpoints" accesskey="p" rel="previous">Thread-Specific Breakpoints</a>, Up: <a href="Thread-Stops.html#Thread-Stops" accesskey="u" rel="up">Thread Stops</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Interrupted-System-Calls-1"></a>
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<h4 class="subsection">5.5.5 Interrupted System Calls</h4>
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<a name="index-thread-breakpoints-and-system-calls"></a>
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<a name="index-system-calls-and-thread-breakpoints"></a>
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<a name="index-premature-return-from-system-calls"></a>
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<p>There is an unfortunate side effect when using <small>GDB</small> to debug
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multi-threaded programs. If one thread stops for a
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breakpoint, or for some other reason, and another thread is blocked in a
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system call, then the system call may return prematurely. This is a
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consequence of the interaction between multiple threads and the signals
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that <small>GDB</small> uses to implement breakpoints and other events that
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stop execution.
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</p>
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<p>To handle this problem, your program should check the return value of
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each system call and react appropriately. This is good programming
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style anyways.
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</p>
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<p>For example, do not write code like this:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> sleep (10);
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</pre></div>
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<p>The call to <code>sleep</code> will return early if a different thread stops
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at a breakpoint or for some other reason.
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</p>
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<p>Instead, write this:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> int unslept = 10;
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while (unslept > 0)
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unslept = sleep (unslept);
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</pre></div>
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<p>A system call is allowed to return early, so the system is still
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conforming to its specification. But <small>GDB</small> does cause your
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multi-threaded program to behave differently than it would without
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<small>GDB</small>.
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</p>
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<p>Also, <small>GDB</small> uses internal breakpoints in the thread library to
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monitor certain events such as thread creation and thread destruction.
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When such an event happens, a system call in another thread may return
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prematurely, even though your program does not appear to stop.
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</p>
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