sem_wait will lock the unlocked semaphore and decrement the semaphore value by one. sem_wait: This test case verifies that the semaphore shall remain locked until the sem_post is executed and return successfully. sem_wait hangs when it tries to lock a locked semaphore, and then killed by sending a kill signal (SIGKILL). errno return EAGAIN: The semaphore can't be immediately locked by sem_trywait when its already locked. errno return EINVAL: the semaphore argument doesn't not refer to a valid semaphore errno return EINTR: a signal interrupted the waiting semaphore. errno return EDEADLK: sem_wait/trywait will fail if a deadlock condition is detected. sem_wait/trywait return 0 on success and -1 of failure. sem_trywait shall try to lock the unlocked semaphore and decrement the semaphore value by one. sem_trywait shall fail to lock a locked semaphore and try to decrement the semaphore value by one. If the semaphore count is 0, the call blocks until the semphore can be locked or the operation is interrupted by a signal.