StackOverflow2013

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2013-11-07T16:30:50.350
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I had this problem recently with OS X. The solution I found was to unlink the shared memory then recreate it again. You could also try rebooting since at least for Linux the POSIX style semaphores are kernel persistent. <blockquote> POSIX named semaphores have kernel persistence: if not removed by sem_unlink(3), a semaphore will exist until the system is shut down. </blockquote> The problem happens when you try to create the shared memory with giving improper permissions for the mode, or perhaps omitting the mode altogether. When you then correct the problem and try to open the semaphore it is still using the original semaphore that was persisted. So try doing: <pre><code>#define SHAREDSEM "/sharedSem" sem_unlink(SHAREDSEM); sem_t *sharedSem = sem_open(SHAREDSEM, O_CREAT, 0777, 1); </code></pre> If this is the only place that you desire to create the semaphore then you can optionally add in the <code>O_EXCL</code> option which makes it fail if the semaphore already exists. This may be useful at least for debugging to see if this is the problem you are facing. To do this try: <pre><code>#define SHAREDSEM "/sharedSem" sem_unlink(SHAREDSEM); sem_t *sharedSem = sem_open(SHAREDSEM, O_CREAT | O_EXCL, 0777, 1); </code></pre> Note that in these examples I set the permissions to 0777 so that it is accessible by all. This is useful for debugging. In your final implementation remember to set it back to the proper permissions that you need.
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