NAME
sem_timedwait,
sem_clockwait_np —
lock a semaphore
LIBRARY
library “libc”
SYNOPSIS
#include
<semaphore.h>
#include <time.h>
int
sem_timedwait(sem_t
* restrict sem, const
struct timespec * restrict abs_timeout);
int
sem_clockwait_np(sem_t
* restrict sem, clockid_t
clock_id, int
flags, const struct
timespec * rqtp, struct
timespec * rmtp);
DESCRIPTION
Thesem_timedwait()
function locks the semaphore referenced by sem, as in
the 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
sem_post(3) function, this wait will be terminated when the specified
timeout expires.
The timeout will expire when the absolute time specified by abs_timeout passes, as measured by the clock on which timeouts are based (that is, when the value of that clock equals or exceeds abs_timeout), or if the absolute time specified by abs_timeout has already been passed at the time of the call.
Note that the timeout is based on the
CLOCK_REALTIME clock.
The validity of the abs_timeout is not checked if the semaphore can be locked immediately.
The
sem_clockwait_np()
function is a more flexible variant of
sem_timedwait(). The clock_id
parameter specifies the reference clock. If the flags
parameter contains TIMER_ABSTIME, then the requested
timeout (rqtp) is an absolute timeout; otherwise, the
timeout is relative. If this function fails with
EINTR and the timeout is relative, a non-NULL
rmtp will be updated to contain the amount of time
remaining in the interval (the requested time minus the time actually
slept). An absolute timeout has no effect on rmtp. A
single structure can be used for both rqtp and
rmtp.
RETURN VALUES
These functions return zero if the calling process successfully performed the semaphore lock operation on the semaphore designated by 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 errno to indicate the error.
ERRORS
These functions will fail if:
- [
EINVAL] - The sem argument does not refer to a valid semaphore, or the process or thread would have blocked, and the abs_timeout parameter specified a nanoseconds field value less than zero or greater than or equal to 1000 million.
- [
ETIMEDOUT] - The semaphore could not be locked before the specified timeout expired.
- [
EINTR] - A signal interrupted this function.
SEE ALSO
STANDARDS
The sem_timedwait() function conforms to
IEEE Std 1003.1-2004 (“POSIX.1”). The
sem_clockwait_np() function is not specified by any
standard; it exists only on FreeBSD at the time of
this writing.
HISTORY
The sem_timedwait() function first
appeared in FreeBSD 5.0. The
sem_clockwait_np() function first appeared in
FreeBSD 11.1.