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.Dd April 16, 2013
.Dt SEM_WAIT 3
.Os
.Sh NAME
.Nm sem_wait ,
.Nm sem_trywait
.Nd decrement (lock) a semaphore
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In semaphore.h
.Ft int
.Fn sem_wait "sem_t *sem"
.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_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
.Sh ERRORS
The
.Fn sem_wait
and
.Fn sem_trywait
functions will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa sem
argument
points to an invalid semaphore.
.El
.Pp
Additionally,
.Fn sem_wait
will fail if:
.Bl -tag -width Er
.It Bq Er EINTR
A signal interrupted this function.
.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_getvalue 3 ,
.Xr sem_post 3 ,
.Xr sem_timedwait 3 ,
.Xr sem 4
.Sh STANDARDS
The
.Fn sem_wait
and
.Fn sem_trywait
functions conform to
.St -p1003.1-96 .