By default, whenever a pending event becomes active (because its file descriptor is ready to read or write, or because its timeout expires), it becomes non-pending right before its callback is executed. Thus, to make the event pending again one may call Event::add on it again from inside the callback function. If the Event::PERSIST flag is set on an event, however, the event is persistent . This means that event remains pending even when its callback is activated. Event::del method can be called to make it non-pending. The timeout on a persistent event resets whenever the event's callback runs. Thus, if one has an event with flags Event::READ | Event::PERSIST and a timeout of five seconds, the event will become active: Whenever the socket or file descriptor is ready for reading. Whenever five seconds have passed since the event last became active. See also Fast portable non-blocking network programming with Libevent, About Event Persistence