sem_wait(3) [netbsd man page]

SEM_WAIT(3)						   BSD Library Functions Manual 					       SEM_WAIT(3)

NAME

     sem_wait, sem_trywait -- decrement (lock) a semaphore

LIBRARY

     POSIX Real-time Library (librt, -lrt)

SYNOPSIS

     #include <semaphore.h>

     int
     sem_wait(sem_t *sem);

     int
     sem_trywait(sem_t *sem);

DESCRIPTION

     The sem_wait() function decrements (locks) the semaphore pointed to by sem, but blocks if the value of sem is zero, until the value is non-
     zero and the value can be decremented.

     The sem_trywait() function decrements (locks) the semaphore pointed to by sem only if the value is non-zero.  Otherwise, the semaphore is not
     decremented and an error is returned.

RETURN VALUES

     The sem_wait() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indi-
     cate the error.

ERRORS

     sem_wait() and sem_trywait() will fail if:

     [EINVAL]		sem points to an invalid semaphore.

     Additionally, sem_trywait() will fail if:

     [EAGAIN]		The semaphore value was zero, and thus could not be decremented.

SEE ALSO

     sem_post(3)

STANDARDS

     sem_wait() and sem_trywait() conform to ISO/IEC 9945-1:1996 (``POSIX.1'').

BSD
								 January 22, 2003							       BSD

SEM_WAIT(2)						      BSD System Calls Manual						       SEM_WAIT(2)

NAME

     sem_trywait, sem_wait -- lock a semaphore

SYNOPSIS

     #include <semaphore.h>

     int
     sem_trywait(sem_t *sem);

     int
     sem_wait(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 acquired or until the call is interrupted by a signal. Alternatively, the sem_trywait() function will fail if the semaphore is
     already locked, rather than blocking on the semaphore.

     If successful (the lock was acquired), 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.

     [EDEADLK]		A deadlock was detected.

     [EINTR]		The call was interrupted by a signal.

     [EINVAL]		sem is not a valid semaphore descriptor.

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

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

HISTORY

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

Darwin								   June 8, 2000 							    Darwin