Messages may pile up at exchanges during periods of peak activity. This situation is not a problem. Semaphores operate in a similar manner, except that no messages are involved. A task may wait at a semaphore. Another task may signal the semaphore causing the waiting task to resume.
Semaphores provide a means for one task to jog another.
Tasking in Practice So much for theory. It's pretty straightforward. From here on, atask identifies the task. This task waits up to one second for a message to arrive at anxchg . If no message arrives, then it takes alternative action.