libuv
"spurious wakeups" with uv_cond_wait?
quick question; what exactly does the documentation mean by expecting "spurious wakeups" with uv_cond_wait? Am I to not rely on a conditional wait only proceeding on a condition signalling from another thread, or does this mean something else?
Originally posted by @brandonpoc in https://github.com/libuv/libuv/pull/1639#issuecomment-605776019
@brandonpoc A spurious wakeup is where the "wait on condition variable" function returns without the condition variable having actually been signaled, e.g., because of the delivery of a (UNIX) signal.
That's why a condition variable is always coupled with some predicate to test for. Here's an example from libuv's semaphore emulation code:
static void uv__custom_sem_wait(uv_sem_t* sem_) {
uv_semaphore_t* sem;
sem = *(uv_semaphore_t**)sem_;
uv_mutex_lock(&sem->mutex);
while (sem->value == 0)
uv_cond_wait(&sem->cond, &sem->mutex);
sem->value--;
uv_mutex_unlock(&sem->mutex);
}
The predicate here is the while (sem->value == 0) check.
