libevent-core-2.1-7 2.1.11-stable-1 (amd64 binary) in ubuntu focal

 Libevent is an asynchronous event notification library that provides a
 mechanism to execute a callback function when a specific event occurs
 on a file descriptor or after a timeout has been reached.
 .
 It 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).
 .
 The libevent_core library includes event loops, timers, buffer code,
 and various small compatibility functions.
 .
 If you're writing software that only uses libevent's event loop, you
 should link against only the libevent_core library.