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 maximum number
of semaphore to be exceeded.
sem_identifier is a semaphore resource,
obtained from sem_get().
Returns TRUE on success or FALSE on failure.
After processing a request, any semaphores acquired by the process but not explicitly released will be released automatically and a warning will be generated.