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sem_wait(3)

NAME

     sem_wait, sem_trywait -- decrement (lock) a semaphore

LIBRARY

     Standard C Library (libc, -lc)

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

     Upon successful completion, the value 0 is returned; otherwise the
     value -1 is returned and the global variable errno is set to indicate the
     error.

ERRORS

     The sem_wait() and sem_trywait() functions will fail if:

     [EINVAL]		The sem argument 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), sem(4)

STANDARDS

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

FreeBSD 5.4		       February 15, 2000		   FreeBSD 5.4

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