NAME    [Toc]    [Back]

     sem_wait, sem_trywait - decrement (lock) a semaphore

SYNOPSIS    [Toc]    [Back]

     #include <semaphore.h>

     int
     sem_wait(sem_t *sem);

     int
     sem_trywait(sem_t *sem);

DESCRIPTION    [Toc]    [Back]

     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    [Toc]    [Back]

     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    [Toc]    [Back]

     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    [Toc]    [Back]

     sem_post(3)

STANDARDS    [Toc]    [Back]

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

OpenBSD     3.6                        February     15,      2000