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POpthread mutex (un)locking over different threads
 

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    <p>I have a process where main initializes a mutex calling:</p> <pre class="lang-c prettyprint-override"><code>MutexInit( pthread_mutex_t *Mutex ) { pthread_mutexattr_t mattr; pthread_mutexattr_init(&amp;mattr); pthread_mutexattr_settype(&amp;mattr, PTHREAD_MUTEX_ERRORCHECK_NP); #ifndef _POSIX_THREAD_PROCESS_SHARED #error "This platform does not support process shared mutex!" #else pthread_mutexattr_setpshared(&amp;mattr, PTHREAD_PROCESS_SHARED); #endif pthread_mutex_init( Mutex, &amp;mattr ); } </code></pre> <p><code>main</code> is initially locking mutex M1 and creates then threads T1 and T2.</p> <p>T1 is started and does some work. T2 is started and does something else and at some point doing a lock on that mutex M1. Since mutex type is PTHREAD_MUTEX_ERRORCHECK_NP T2 is not blocked, instead error EDEADLK is returned, indicating that the mutex M1 is already locked. So T2 continues trying to lock. That's fine so far.</p> <p>Then T1 comes to the point where it unlocks M1, but error EPERM is returned, saying, that T1 does not own the mutex !? So T2 never gets unlocked.</p> <p>If I remove setting the attributes from <code>MutexInit</code>:</p> <pre class="lang-c prettyprint-override"><code>pthread_mutexattr_init(&amp;mattr); pthread_mutexattr_settype(&amp;mattr, PTHREAD_MUTEX_ERRORCHECK_NP); #ifndef _POSIX_THREAD_PROCESS_SHARED #error "This platform does not support process shared mutex!" #else pthread_mutexattr_setpshared(&amp;mattr, PTHREAD_PROCESS_SHARED); #endif </code></pre> <p>and calling <code>pthread_mutex_init( Mutex, NULL );</code>, ie. default attributes, everything is working fine !</p> <p>I do need the initial <code>MutexInit</code> routine, because we're also using mutexes over processes (via shared memory).</p> <p>Does anybody have any idea ? I've read so many articles and posts, so any help will be appriciated.</p> <p><strong>EDIT</strong>: Using a modified version of Paolo's code to demonstrate my observation:</p> <p>This a the modified version of Paolo's code to fit "my sequencing":</p> <pre class="lang-c prettyprint-override"><code>#include &lt;stddef.h&gt; #include &lt;pthread.h&gt; #include &lt;stdio.h&gt; #include &lt;string.h&gt; #include &lt;semaphore.h&gt; pthread_mutex_t m; sem_t s1, s2; void print(const char *s, int err) { printf("%s %d %s\n", s, err, strerror(err)); } void *start_t1(void *arg) { sem_wait(&amp;s1); // &lt;-t2 print("t1: unlock ", pthread_mutex_unlock(&amp;m)); sem_post(&amp;s2); //-&gt;t2 } void *start_t2(void *arg) { sem_wait(&amp;s2); // &lt;-main print("t2: lock ", pthread_mutex_lock(&amp;m)); sem_post(&amp;s1); // -&gt;t1 sem_wait(&amp;s2); // &lt;-t1 sem_post(&amp;s1); // -&gt;main } void main(void) { pthread_mutexattr_t mattr; pthread_mutexattr_init(&amp;mattr); pthread_mutexattr_settype(&amp;mattr, PTHREAD_MUTEX_ERRORCHECK_NP); pthread_mutexattr_setpshared(&amp;mattr, PTHREAD_PROCESS_SHARED); sem_init(&amp;s1, 0, 0); sem_init(&amp;s2, 0, 0); print("main init", pthread_mutex_init(&amp;m, &amp;mattr)); pthread_t t2, t1; pthread_create(&amp;t1, NULL, start_t1, NULL); pthread_create(&amp;t2, NULL, start_t2, NULL); sem_post(&amp;s2); // -&gt;t2 sem_wait(&amp;s1); // &lt;-t2 pthread_join(t1, NULL); pthread_join(t2, NULL); } </code></pre> <p>The output is:</p> <pre><code>main init 0 Success t2: lock 0 Success t1: unlock 1 Operation not permitted </code></pre> <p>I would expect T1 to be allowed to unlock the mutex because of type <code>PTHREAD_PROCESS_SHARED</code>. Am I wrong ?</p> <p>If the mutex initialization is changed to defaults (<code>pthread_mutex_init(&amp;m, **NULL**)</code>), then it's working.</p> <pre><code>main init 0 Success t2: lock 0 Success t1: unlock 0 Success </code></pre> <p>Seems to be some kind of inverted logic !</p>
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    1. COThis deadlocks for me (Ubuntu 13.04 running on Virtualbox/Win7). The first `sem_wait(&s1)` never returns. This can be uglily and non-rigorously rectified by sleeping in `main()` before `sem_wait(&s1)`.
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