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);
    ret = pthread_cond_timedwait( &q->q_cond, &q->q_mtx, abs_timeout );
    if (ret == ETIMEDOUT)
    {
      dbg("cond timed out\n");
      return -ETIMEDOUT;
    }

dbg calls gettimeofday before every line and prepends the line with the time. It results in the following output:

    7.991151: condwait: timeout = 5, 705032704
    7.991158: cond timed out

As you can see, only 7 microseconds passed in between the two debug lines, yet pthread_cond_timedwait returned ETIMEDOUT. How can this happen? I even tried setting the clock to something else when initializing the cond variable:

int ret;
ret = pthread_condattr_init(&attributes);
if (ret != 0) printf("CONDATTR INIT FAILED: %d\n", ret);
ret = pthread_condattr_setclock(&attributes, CLOCK_REALTIME);
if (ret != 0) printf("SETCLOCK FAILED: %d\n", ret);
ret = pthread_cond_init( &q->q_cond, &attributes );
if (ret != 0) printf("COND INIT FAILED: %d\n", ret);

(none of the error messages are printed out). I tried both CLOCK_REALTIME and CLOCK_MONOTONIC.

This code is part of a blocking queue. I need functionality such that if nothing gets put on this queue in 5 seconds, something else happens. The mutex and the cond are both initialized, as the blocking queue works fine if I don't use pthread_cond_timedwait.

Answer 1

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 2

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.