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.
After processing a request, any semaphores acquired by the process but not
explicitly released will be released automatically and a warning will be
generated.
Parameters
sem_identifier
sem_identifier
is a semaphore resource,
obtained from sem_get().