sd_event_add_defer() adds a new event source that will be dispatched instantly, before the event loop goes to sleep again and waits for new events. By default, the handler will be called once (SD_EVENT_ONESHOT). Note that if the event source is set to SD_EVENT_ON the event loop will never go to sleep again, but continuously call the handler, possibly interleaved with other event sources.

sd_event_add_post() adds a new event source that is run before the event loop will sleep and wait for new events, but only after at least one other non-post event source was dispatched. By default, the source is enabled permanently (SD_EVENT_ON). Note that this event source type will still allow the event loop to go to sleep again, even if set to SD_EVENT_ON, as long as no other event source is ever triggered.

sd_event_add_exit() adds a new event source that will be dispatched when the event loop is terminated with sd_event_exit(3).

The sd_event_source_set_enabled(3) function may be used to enable the event source permanently (SD_EVENT_ON) or to make it fire just once (SD_EVENT_ONESHOT).

If the handler function returns a negative error code, it will be disabled after the invocation, even if the SD_EVENT_ON mode was requested before.

To destroy an event source object use sd_event_source_unref(3), but note that the event source is only removed from the event loop when all references to the event source are dropped. To make sure an event source does not fire anymore, even when there's still a reference to it kept, consider setting the event source to SD_EVENT_OFF with sd_event_source_set_enabled(3).

If the second parameter of these functions is passed as NULL no reference to the event source object is returned. In this case the event source is considered "floating", and will be destroyed implicitly when the event loop itself is destroyed.