select and poll


select(2) and poll(2) allow waiting for events
You state what set of descriptors you are interested in 
The caller is reported what descriptors are active
You can set a timeout

Normally, main() is a loop based on select(2)
A process must sleep as much as possible
Asynchronous events are notified immediately
Timeouts have the same granularity as the system clock

   Since everything is a file, most situations can be attacked
                                            with a main loop based on select(2) or poll(2)

This predates multi-threading
It may be way more efficient than multi-threading
Unless you really need concurrency on a multi-core system