_LWP_COND_WAIT(2) System Calls _LWP_COND_WAIT(2)


NAME


_lwp_cond_wait, _lwp_cond_timedwait, _lwp_cond_reltimedwait - wait on a
condition variable

SYNOPSIS


#include <sys/lwp.h>

int _lwp_cond_wait(lwp_cond_t *cvp, lwp_mutex_t *mp);


int _lwp_cond_timedwait(lwp_cond_t *cvp, lwp_mutex_t *mp,
timestruc_t *abstime);


int _lwp_cond_reltimedwait(lwp_cond_t *cvp, lwp_mutex_t *mp,
timestruc_t *reltime);


DESCRIPTION


These functions are used to wait for the occurrence of a condition
represented by an LWP condition variable. LWP condition variables must be
initialized to 0 before use.


The _lwp_cond_wait() function atomically releases the LWP mutex pointed
to by mp and causes the calling LWP to block on the LWP condition
variable pointed to by cvp. The blocked LWP may be awakened by
_lwp_cond_signal(2), _lwp_cond_broadcast(2), or when interrupted by
delivery of a signal. Any change in value of a condition associated with
the condition variable cannot be inferred by the return of
_lwp_cond_wait() and any such condition must be re-evaluated.


The _lwp_cond_timedwait() function is similar to _lwp_cond_wait(), except
that the calling LWP will not block past the time of day specified by
abstime. If the time of day becomes greater than abstime,
_lwp_cond_timedwait() returns with the error code ETIME.


The _lwp_cond_reltimedwait() function is similar to _lwp_cond_wait(),
except that the calling LWP will not block past the relative time
specified by reltime. If the time of day becomes greater than the
starting time of day plus reltime, _lwp_cond_reltimedwait() returns with
the error code ETIME.


The _lwp_cond_wait(), _lwp_cond_timedwait(), and _lwp_cond_reltimedwait()
functions always return with the mutex locked and owned by the calling
lightweight process.

RETURN VALUES


Upon successful completion, 0 is returned. A non-zero value indicates an
error.

ERRORS


If any of the following conditions are detected, _lwp_cond_wait(),
_lwp_cond_timedwait(), and _lwp_cond_reltimedwait() fail and return the
corresponding value:

EINVAL
The cvp argument points to an invalid LWP condition variable or
the mp argument points to an invalid LWP mutex.


EFAULT
The mp, cvp, or abstime argument points to an illegal address.


If any of the following conditions occur, _lwp_cond_wait(),
_lwp_cond_timedwait(), and _lwp_cond_reltimedwait() fail and return the
corresponding value:

EINTR
The call was interrupted by a signal or fork(2).


If any of the following conditions occur, _lwp_cond_timedwait() and
_lwp_cond_reltimedwait() fail and return the corresponding value:

ETIME
The time specified in abstime or reltime has passed.


EXAMPLES


Example 1: Use the _lwp_cond_wait() function in a loop testing some


condition.


The _lwp_cond_wait() function is normally used in a loop testing some
condition, as follows:


lwp_mutex_t m;
lwp_cond_t cv;
int cond;
(void) _lwp_mutex_lock(&m);
while (cond == FALSE) {
(void) _lwp_cond_wait(&cv, &m);
}
(void) _lwp_mutex_unlock(&m);


Example 2: Use the _lwp_cond_timedwait() function in a loop testing some


condition.


The _lwp_cond_timedwait() function is also normally used in a loop
testing some condition. It uses an absolute timeout value as follows:


timestruc_t to;
lwp_mutex_t m;
lwp_cond_t cv;
int cond, err;
(void) _lwp_mutex_lock(&m);
to.tv_sec = time(NULL) + TIMEOUT;
to.tv_nsec = 0;
while (cond == FALSE) {
err = _lwp_cond_timedwait(&cv, &m, &to);
if (err == ETIME) {
/* timeout, do something */
break;
}
}
(void) _lwp_mutex_unlock(&m);


This example sets a bound on the total wait time even though the
_lwp_cond_timedwait() may return several times due to the condition being
signalled or the wait being interrupted.


Example 3: Use the _lwp_cond_reltimedwait() function in a loop testing


some condition.


The _lwp_cond_reltimedwait() function is also normally used in a loop
testing some condition. It uses a relative timeout value as follows:


timestruc_t to;
lwp_mutex_t m;
lwp_cond_t cv;
int cond, err;
(void) _lwp_mutex_lock(&m);
while (cond == FALSE) {
to.tv_sec = TIMEOUT;
to.tv_nsec = 0;
err = _lwp_cond_reltimedwait(&cv, &m, &to);
if (err == ETIME) {
/* timeout, do something */
break;
}
}
(void) _lwp_mutex_unlock(&m);


SEE ALSO


_lwp_cond_broadcast(2), _lwp_cond_signal(2), _lwp_kill(2),
_lwp_mutex_lock(2), fork(2), kill(2)


September 12, 2020 _LWP_COND_WAIT(2)