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.