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) and select(2). Support for poll(2)
and /dev/poll is planned. The internal event mechanism is completely
independent of the exposed event API, and a simple update of libevent can
provide this new functionality without having to redesign the server
applications.