Returns: TRUE on
success, FALSE on error.
sem_acquire() blocks (if
necessary) until the semaphore can be acquired. A process
attempting to acquire a semaphore which it has already
acquired will block forever if acquiring the semaphore would
cause its max_acquire value to be exceeded.
After processing a request, any semaphores acquired by
the process but not explicitly released will be released
automatically and a warning will be generated.
See also: sem_get() and
sem_release().