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POSleep while holding a boost::interprocess::scoped_lock causes it to be never released
 

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  1. POSleep while holding a boost::interprocess::scoped_lock causes it to be never released
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    <p>I'm doing IPC on <strong>Linux</strong> using <code>boost::interprocess::shared_memory_object</code> as per <a href="http://www.boost.org/doc/libs/1_38_0/doc/html/interprocess/synchronization_mechanisms.html" rel="nofollow noreferrer">the reference</a> (anonymous mutex example).</p> <p>There's a server process, which creates the <code>shared_memory_object</code> and writes to it, while holding an <code>interprocess_mutex</code> wrapped in a <code>scoped_lock</code>; and a client process which prints whatever the other one has written - in this case, it's an <code>int</code>.</p> <p>I ran into a problem: if the server sleeps while holding the mutex, the client process is never able to aquire it and waits forever.</p> <p>Buggy <b>server</b> loop:</p> <pre><code>using namespace boost::interprocess; int n = 0; while (1) { std::cerr &lt;&lt; "acquiring mutex... "; { // "data" is a struct on the shared mem. and contains a mutex and an int scoped_lock&lt;interprocess_mutex&gt; lock(data-&gt;mutex); data-&gt;a = n++; std::cerr &lt;&lt; n &lt;&lt; std::endl; sleep(1); } // if this bracket is placed before "sleep", everything works } </code></pre> <p><b>Server</b> output:</p> <pre><code>acquiring mutex... 1 acquiring mutex... 2 acquiring mutex... 3 acquiring mutex... 4 </code></pre> <p><b>Client</b> loop:</p> <pre><code>while(1) { std::cerr &lt;&lt; "acquiring mutex... "; { scoped_lock&lt;interprocess_mutex&gt; lock(data-&gt;mutex); std::cerr &lt;&lt; data-&gt;a &lt;&lt; std::endl; } sleep(1); } </code></pre> <p><b>Client</b> output (waits forever):</p> <pre><code>acquiring mutex... </code></pre> <p>The thing is, if I move the bracket to the line before the <code>sleep</code> call, everything works. Why? I didn't think sleeping with a locked mutex would cause the mutex to be eternally locked.</p> <p>The only theory I have is that when the kernel wakes up the server process, the scope ends and the mutex is released, but the waiting process isn't given a chance to run. The server then re-acquires the lock... But that doesn't seem to make a lot of sense.</p> <p>Thanks!</p>
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