io_wait() checks the descriptors that the program is interested in to
see whether any of them are ready. If none of them are ready, io_wait()
tries to pause until one of them is ready, so that it does not take time
away from other programs running on the same computer.
io_wait pays attention to timeouts: if a descriptor reaches its timeout,
and the program is interested in reading or writing that descriptor,
io_wait will return promptly.
Under some circumstances, io_wait will return even though no interesting
descriptors are ready. Do not assume that a descriptor is ready merely
because io_wait has returned.
io_wait is not interrupted by the delivery of a signal. Programs that
expect interruption are unreliable: they will block if the same signal
is delivered a moment before io_wait. The correct way to handle signals
is with the self-pipe trick.