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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