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POmutex attribute PTHREAD_PROCESS_SHARED inverts logic?
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copied!<p>This is a follow-on to my previous question (<a href="https://stackoverflow.com/questions/17720048/pthread-mutex-unlocking-over-different-threads">pthread mutex (un)locking over different threads</a>). I got puzzled by how to handle questions and answers here, so I give it a new try :o)</p> <p>I'm trying to handle mutex through processes and threads and using the mutex attribute PTHREAD_PROCESS_SHARED to arrange this. I'Ve added a small example (based on Paolo's example from my previoous post) which is demonstrating my problem:</p> <pre class="lang-c prettyprint-override"><code>#include <stddef.h> #include <pthread.h> #include <stdio.h> #include <string.h> #include <semaphore.h> 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(&s1); // <-t2 print("t1: unlock ", pthread_mutex_unlock(&m)); sem_post(&s2); //->t2 } void *start_t2(void *arg) { sem_wait(&s2); // <-main print("t2: lock ", pthread_mutex_lock(&m)); sem_post(&s1); // ->t1 sem_wait(&s2); // <-t1 sem_post(&s1); // ->main } void main(void) { pthread_mutexattr_t mattr; pthread_mutexattr_init(&mattr); pthread_mutexattr_settype(&mattr, PTHREAD_MUTEX_ERRORCHECK_NP); pthread_mutexattr_setpshared(&mattr, PTHREAD_PROCESS_SHARED); sem_init(&s1, 0, 0); sem_init(&s2, 0, 0); print("main init", pthread_mutex_init(&m, &mattr)); pthread_t t2, t1; pthread_create(&t1, NULL, start_t1, NULL); pthread_create(&t2, NULL, start_t2, NULL); sem_post(&s2); // ->t2 sem_wait(&s1); // <-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>T1 is not permitted to unlock the mutex initialized in main and locked by T2. Which is not what I expect! T1 should 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 using default attributes (<code>pthread_mutex_init(&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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