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SEM_WAIT(3) Library Functions Manual SEM_WAIT(3) NAME sem_wait, sem_timedwait, 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_timedwait(sem_t *sem, const struct timespec * restrict abstime); 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_timedwait() function performs the same action, but will not wait beyond abstime for the value of sem to be non-zero. 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 indicate the error. ERRORS sem_wait(), sem_timedwait(), 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. sem_timedwait() will also fail if: [ETIMEDOUT] The semaphore value was zero and abstime was reached. SEE ALSO sem_post(3) STANDARDS sem_wait() and sem_trywait() conform to ISO/IEC 9945-1:1996 (``POSIX.1''). NetBSD 7.1.2 February 29, 2012 NetBSD 7.1.2