The function int pthread_mutex_unlock(pthread_mutex_t *mutex); releases the mutex object 'mutex'. If there are threads blocked on the mutex object referenced by 'mutex' when pthread_mutex_unlock() is called, resulting in the mutex becoming available, the scheduling policy shall determine which thread shall acquire the mutex. Upon success, it returns 0. Upon failure, it may return: -[EINVAL] 'mutex' does not refer to an initialized mutex object -[EPERM] The current thread does not own 'mutex' It shall not return [EINTR]. If the mutex type is PTHREAD_MUTEX_RECURSIVE: 1. If a thread attempts to unlock a mutex that it does not own 2. or an unlocked mutex, an error is returned.