Internal Program Version: libevent  2.1.8

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. Currently, libevent supports
/dev/poll, kqueue(2), event ports, select(2), poll(2) and epoll(4).

http://libevent.org