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. Furthermore, libevent also support callbacks due to signals or
regular timeouts.

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

Currently, libevent supports /dev/poll, kqueue(2), event ports, 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.
Libevent can also be used for multi-threaded applications.

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