The libevent API provides a mechanism to execute a callback function when
a specific event occurs on a file descriptor or after a timeout has been
reached.

libevent is meant to replace the asynchronous event loop found in event
driven network servers. An application just needs to call event_dispatch()
and can then add or remove events dynamically without having to change the
event loop.

Currently, libevent supports kqueue(2), select(2), poll(2), and epoll(4).
The internal event mechanism is completely independent of the exposed event
API, and a simple update of libevent can provide new functionality without
having to redesign the applications.  As a result, Libevent allows for
portable application development and provides the most scalable event
notification mechanism available on an operating system.

WWW: http://www.monkey.org/~provos/libevent/