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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 > 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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