Question

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Interprocess semaphores sometimes not working as expected

In my application, I notice that a semaphore of type sem_t sometimes become 1 from 0 without executing sem_post. How come? What can cause this? The semaphore is used for inter-process communication and you can look at the code here.

Answer 1

The code that you are linking to doesn’t capture the return values from the sem_t calls. If you look in the manual you can see that e.g sem_wait can return prematurely, so-called spurious wakeups.

Always check the return codes of these functions. If the return is -1 check errno for the corresponding error and decide if it is a transient error. If so, iterate.