int
     sem_wait(sem_t *sem);

     int
     sem_trywait(sem_t *sem);

DESCRIPTION

     The semaphore referenced by sem is locked.  When calling sem_wait(), if
     the semaphore's value is zero, the calling thread will block until the
     lock is aquired or until the call is interrupted by a signal. Alterna-
     tively, the sem_trywait() function will fail if the semaphore is already
     locked, rather than blocking on the semaphore.

     If successful (the lock was aquired), sem_wait() and sem_trywait() will
     return 0.  Otherwise, -1 is returned and errno is set, and the state of
     the semaphore is unchanged.

ERRORS

     sem_wait() and sem_trywait() succeed unless:

     [EAGAIN]           The semaphore is already locked.

     [EINVAL]           sem is not a valid semaphore descriptor.

     [EDEADLK]          A deadlock was detected.

     [EINTR]            The call was interrupted by a signal.

NOTES

     Applications may encounter a priority inversion while using semaphores.
     When a thread is waiting on a semaphore which is about to be posted by a
     lower-priority thread and the lower-priority thread is preempted by
     another thread (of medium priority), a priority inversion has occured,
     and the higher-priority thread will be blocked for an unlimited time
     period.  Programmers using the realtime functionality of the system
     should take care to avoid priority inversions.

SEE ALSO

     semctl(2), semget(2), semop(2), sem_open(2), sem_post(2)

HISTORY

     sem_wait() and sem_trywait() are specified in the POSIX Realtime Exten-
     sion (1003.1b-1993/1003.1i-1995).

Darwin                           June 8, 2000                           Darwin