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POthread synchronization: making sure function gets called in order
 

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    <p>I'm writing a program in which I need to make sure a particular function is called is not being executed in more than one thread at a time.</p> <p>Here I've written some simplified pseudocode that does exactly what is done in my real program.</p> <pre><code>mutex _enqueue_mutex; mutex _action_mutex; queue _queue; bool _executing_queue; // called in multiple threads, possibly simultaneously do_action() { _enqueue_mutex.lock() object o; _queue.enqueue(o); _enqueue_mutex.unlock(); execute_queue(); } execute_queue() { if (!executing_queue) { _executing_queue = true; enqueue_mutex.lock(); bool is_empty = _queue.isEmpty(); _enqueue_mutex.lock(); while (!is_empty) { _action_mutex.lock(); _enqueue_mutex.lock(); object o = _queue.dequeue(); is_empty = _queue.isEmpty(); _enqueue_mutex.unlock(); // callback is called when "o" is done being used by "do_stuff_to_object_with_callback" also, this function doesn't block, it is executed on its own thread (hence the need for the callback to know when it's done) do_stuff_to_object_with_callback(o, &amp;some_callback); } _executing_queue = false; } } some_callback() { _action_mutex.unlock(); } </code></pre> <p>Essentially, the idea is that <code>_action_mutex</code> is locked in the while loop (I should say that <code>lock</code> is assumed to be blocking until it can be locked again), and expected to be unlocked when the completion callback is called (<code>some_callback</code> in the above code).</p> <p>This, does not seem to be working though. What happens is if the <code>do_action</code> is called more than once at the same time, the program locks up. I think it might be related to the while loop executing more than once simultaneously, but I just cant see how that could be the case. Is there something wrong with my approach? Is there a better approach?</p> <p>Thanks</p>
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