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.\" $FreeBSD: src/lib/libc_r/man/sem_wait.3,v 1.8 2001/10/01 16:09:09 ru Exp $
.Dd $Mdocdate: April 24 2018 $
.Dt SEM_WAIT 3
.Os
.Sh NAME
.Nm sem_wait ,
.Nm sem_timedwait ,
.Nm sem_trywait
.Nd decrement (lock) a semaphore
.Sh SYNOPSIS
.In semaphore.h
.Ft int
.Fn sem_wait "sem_t *sem"
.Ft int
.Fn sem_timedwait "sem_t *sem" "const struct timespec *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 decrements (locks) the semaphore pointed to by
.Fa sem ,
but blocks if the value of
.Fa sem
is zero, until either the value is non-zero and can be decremented
or the system time specified by
.Fa abstime
is reached.
.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 sem_timedwait sem_trywait
.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_wait
and
.Fn sem_timedwait
will fail if:
.Bl -tag -width Er
.It Bq Er EINTR
The call was interrupted by a signal.
.El
.Pp
Additionally,
.Fn sem_timedwait
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa abstime
was
.Dv NULL
or specified a nanosecond value less than zero or greater than 1000 million.
.It Bq Er ETIMEDOUT
The semaphore value was zero and could not be decremented before
.Fa abstime
was reached.
.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
.Sh SEE ALSO
.Xr sem_destroy 3 ,
.Xr sem_getvalue 3 ,
.Xr sem_init 3 ,
.Xr sem_open 3 ,
.Xr sem_post 3
.Sh STANDARDS
.Fn sem_wait ,
.Fn sem_timedwait ,
and
.Fn sem_trywait
conform to
.St -p1003.1-2008 .