man pages section 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 inabstime 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;
        SENDwhom}
}
(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);