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.Dd February 29, 2012
.Dt SEM_WAIT 3
.Os
.Sh NAME
.Nm sem_wait ,
.Nm sem_timedwait ,
.Nm sem_trywait
.Nd decrement (lock) a semaphore
.Sh LIBRARY
.Lb librt
.Sh SYNOPSIS
.In semaphore.h
.Ft int
.Fn sem_wait "sem_t *sem"
.Ft int
.Fn sem_timedwait "sem_t *sem" "const struct timespec * restrict abstime"
.Ft int
.Fn sem_trywait "sem_t *sem"
.Sh DESCRIPTION
The
.Fn sem_wait
function decrements (locks) the semaphore pointed to by
.Fa sem ,
but blocks if the value of
.Fa sem
is zero, until the value is non-zero and the value can be decremented.
.Pp
The
.Fn sem_timedwait
function performs the same action, but will not wait beyond
.Fa abstime
for the value of
.Fa sem
to be non-zero.
.Pp
The
.Fn sem_trywait
function decrements (locks) the semaphore pointed to by
.Fa sem
only if the value is non-zero.
Otherwise, the semaphore is not decremented and an error is returned.
.Sh RETURN VALUES
.Rv -std sem_wait
.Sh ERRORS
.Fn sem_wait ,
.Fn sem_timedwait ,
and
.Fn sem_trywait
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa sem
points to an invalid semaphore.
.El
.Pp
Additionally,
.Fn sem_trywait
will fail if:
.Bl -tag -width Er
.It Bq Er EAGAIN
The semaphore value was zero, and thus could not be decremented.
.El
.Pp
.Fn sem_timedwait
will also fail if:
.Bl -tag -width Er
.It Bq Er ETIMEDOUT
The semaphore value was zero and
.Fa abstime
was reached.
.El
.Sh SEE ALSO
.Xr sem_post 3
.Sh STANDARDS
.Fn sem_wait
and
.Fn sem_trywait
conform to
.St -p1003.1-96 .