pthread_cond_timedwait returning immediately

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.

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;}

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.