How do I wake up a sleeping pthread?

What you are looking for is pthread_cond_t object, pthread_cond_timedwait and pthread_cond_wait functions. You could create conditional variable isThereAnyTaskToDo and wait on it in event thread. When new event is added, you just wake event thread with pthread_cond_signal().

You have several possibilities both on *NIX platforms and on Windows. Your timer thread should wait using some kind of timed wait on event/conditional variable object. On POSIX platforms you can use pthread_cond_timedwait(). On Windows, you can either choose to compute the necessary time delta and use WaitForSingleObject() on event handle, or you could use combination of event object with CreateTimerQueueTimer() or CreateWaitableTimer(). Boost does also have some synchronization primitives that you could use to implement this with the POSIX-like primitives but portably.

UPDATE:

POSIX does have some timer functionality as well, see create_timer()

I agree with Greg and wilx - pthread_cond_timedwait() can be used to implement the behaviour you're after. I just wanted to add that you can simplify your event thread main loop:

1. try to get the next event in the event queue
2. If there is no pending event, go straight to 4
3. Get the time that the next event is supposed to occur
4. Wait on condition variable with pthread_cond_timedwait() until next event (or with pthread_cond_wait() if no scheduled events)
5. Try to get the next event in the event queue
6. If there are no events that have expired yet, go back to 4
7. Update queue (remove event, re-insert if it's a repeating event)
8. Jump back to 5

So you don't care why you woke up - whenever you wake up, you check the current time and run any events that have expired, then go back to waiting. In most cases when a new event is added you'll find that no events have expired, of course - you'll just recalculate the wait time.

You'll probably want to implement the queue as a Priority Queue, so that the next-event-to-expire is always at the front.