pthread_cond_timedwait returning immediately

Question

I\'m having a strange problem. I have the following code:

    dbg(\"condwait: timeout = %d, %d\\n\", 
        abs_timeout->tv_sec, abs_timeout->tv_nsec);
          


        

Answer 1

Overflow in timespec is usually the culprit for weird timeouts.
Check for EINVAL:

void timespec_add(struct timespec* a, struct timespec* b, struct timespec* out)
{
    time_t sec = a->tv_sec + b->tv_sec;
    long nsec = a->tv_nsec + b->tv_nsec;

    sec += nsec / 1000000000L;
    nsec = nsec % 1000000000L;

    out->tv_sec = sec;
    out->tv_nsec = nsec;
}

Answer 2

As already in other answers mentioned you have to use the absolute time. Since C11 you can use timespec_get().

struct timespec time;
timespec_get(&time, TIME_UTC);
time.tv_sec += 5;

pthread_cond_timedwait(&cond, &mutex, &time);

Answer 3

pthread_cond_timedwait takes an absolute time, not a relative time. You need to make your wait time absolute by adding to the current time to your timeout value.

Answer 4

The condition variable can spuriously unblock. You need to check it in a loop and check the condition each time through. You'll probably need to update the timeout value too.

I found some documentation for pthread_cond_timedwait here.

When using condition variables there is always a Boolean predicate involving shared variables associated with each condition wait that is true if the thread should proceed. Spurious wakeups from the pthread_cond_timedwait() or pthread_cond_wait() functions may occur. Since the return from pthread_cond_timedwait() or pthread_cond_wait() does not imply anything about the value of this predicate, the predicate should be re-evaluated upon such return.