.\" Copyright (c) 2008, David Xu <davidxu@FreeBSD.org>
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.\" Portions of this text are reprinted and reproduced in electronic form
.\" from IEEE Std 1003.1, 2004 Edition, Standard for Information Technology --
.\" Portable Operating System Interface (POSIX), The Open Group Base
.\" Specifications Issue 6, Copyright (C) 2001-2004 by the Institute of
.\" Electrical and Electronics Engineers, Inc and The Open Group.  In the
.\" event of any discrepancy between this version and the original IEEE and
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.Dd May 24, 2017
.Dt SEM_TIMEDWAIT 3
.Os
.Sh NAME
.Nm sem_timedwait ,
.Nm sem_clockwait_np
.Nd "lock a semaphore"
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In semaphore.h
.In time.h
.Ft int
.Fn sem_timedwait "sem_t * restrict sem" "const struct timespec * restrict abs_timeout"
.Ft int
.Fn sem_clockwait_np "sem_t * restrict sem" "clockid_t clock_id" "int flags" "const struct timespec * rqtp" "struct timespec * rmtp"
.Sh DESCRIPTION
The
.Fn sem_timedwait
function locks the semaphore referenced by
.Fa sem ,
as in the
.Xr sem_wait 3
function.
However, if the semaphore cannot be locked without waiting for
another process or thread to unlock the semaphore by performing
a
.Xr sem_post 3
function, this wait will be terminated when the specified timeout expires.
.Pp
The timeout will expire when the absolute time specified by
.Fa abs_timeout
passes, as measured by the clock on which timeouts are based (that is,
when the value of that clock equals or exceeds
.Fa abs_timeout ) ,
or if the
absolute time specified by
.Fa abs_timeout
has already been passed at the time of the call.
.Pp
Note that the timeout is based on the
.Dv CLOCK_REALTIME
clock.
.Pp
The validity of the
.Fa abs_timeout
is not checked if the semaphore can be locked immediately.
.Pp
The
.Fn sem_clockwait_np
function is a more flexible variant of
.Fn sem_timedwait .
The
.Fa clock_id
parameter specifies the reference clock.
If the
.Fa flags
parameter contains
.Dv TIMER_ABSTIME ,
then the requested timeout
.Pq Fa rqtp
is an absolute timeout; otherwise,
the timeout is relative.
If this function fails with
.Er EINTR
and the timeout is relative,
a non-NULL
.Fa rmtp
will be updated to contain the amount of time remaining in the interval
.Po
the requested time minus the time actually slept
.Pc .
An absolute timeout has no effect on
.Fa rmtp .
A single structure can be used for both
.Fa rqtp
and
.Fa rmtp .
.Sh RETURN VALUES
These
functions return zero if the calling process successfully performed the
semaphore lock operation on the semaphore designated by
.Fa sem .
If the call was unsuccessful, the state of the semaphore is unchanged,
and the function returns a value of \-1 and sets the global variable
.Va errno
to indicate the error.
.Sh ERRORS
These functions will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa sem
argument does not refer to a valid semaphore, or the process or thread would
have blocked, and the
.Fa abs_timeout
parameter specified a nanoseconds field value less than zero or greater than
or equal to 1000 million.
.It Bq Er ETIMEDOUT
The semaphore could not be locked before the specified timeout expired.
.It Bq Er EINTR
A signal interrupted this function.
.El
.Sh SEE ALSO
.Xr sem_post 3 ,
.Xr sem_trywait 3 ,
.Xr sem_wait 3
.Sh STANDARDS
The
.Fn sem_timedwait
function conforms to
.St -p1003.1-2004 .
The
.Fn sem_clockwait_np
function is not specified by any standard;
it exists only on
.Fx
at the time of this writing.
.Sh HISTORY
The
.Fn sem_timedwait
function first appeared in
.Fx 5.0 .
The
.Fn sem_clockwait_np
function first appeared in
.Fx 11.1 .