All watchers can be active(waiting for events) or inactive(paused). Only active watchers will have their callbacks invoked. All callbacks will be called with at least two arguments: watcher - the watcher, and revents a bitmask of received events. Watcher callbacks are passed to the watcher contructors(the classes derived from EvWatcher - EvCheck::__construct , EvChild::__construct etc.). A watcher callback should match the following prototype: void callback object watcher NULL int revents NULL watcher The watcher instance(of a class extending EvWatcher ). revents Watcher received events . Each watcher type has its associated bit in revents , so one can use the same callback for multiple watchers. The event mask is named after the type, i.e. EvChild (or EvLoop::child ) sets EV::CHILD , EvPrepare (or EvLoop::prepare ) sets Ev::PREPARE , EvPeriodic (or EvLoop::periodic ) sets Ev::PERIODIC and so on, with the exception of I/O events (which can set both Ev::READ and Ev::WRITE bits).