The epoll API

Default epoll notification model is level-triggered:

"Is I/O possible?"
Like select() / poll().
File descriptor is placed in ready list if I/O is/becomes possible.

Fd may already be ready when added to the interest list, or a later I/O event may make it ready.
File descriptor remains ready until sufficient I/O is performed to make it not "ready".

epoll provides an alternative notification model: edge-triggered notification...