By default, the timer will elapse once (SD_EVENT_ONESHOT), but this may be changed with sd_event_source_set_enabled(3). 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. Note that a timer event set to SD_EVENT_ON will fire continuously unless its configured time is updated using sd_event_source_set_time().

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 if it is still referenced, disable the event source using sd_event_source_set_enabled(3) with SD_EVENT_OFF.

If the second parameter of sd_event_add_time() is 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.

If the handler to sd_event_add_time() is NULL, and the event source fires, this will be considered a request to exit the event loop. In this case, the userdata parameter, cast to an integer, is used for the exit code passed to sd_event_exit(3).

Use CLOCK_BOOTTIME_ALARM and CLOCK_REALTIME_ALARM to define event sources that may wake up the system from suspend.

In order to set up relative timers (that is, relative to the current time), retrieve the current time via sd_event_now(3), add the desired timespan to it, and use the result as the usec parameter to sd_event_add_time().

In order to set up repetitive timers (that is, timers that are triggered in regular intervals), set up the timer normally, for the first invocation. Each time the event handler is invoked, update the timer's trigger time with sd_event_source_set_time(3) for the next timer iteration, and reenable the timer using sd_event_source_set_enabled(). To calculate the next point in time to pass to sd_event_source_set_time(), either use as base the usec parameter passed to the timer callback, or the timestamp returned by sd_event_now(). In the former case timer events will be regular, while in the latter case the scheduling latency will keep accumulating on the timer.

sd_event_source_get_time() retrieves the configured time value of an event source created previously with sd_event_add_time(). It takes the event source object and a pointer to a variable to store the time in, relative to the selected clock's epoch, in µs.

sd_event_source_set_time() changes the time of an event source created previously with sd_event_add_time(). It takes the event source object and a time relative to the selected clock's epoch, in µs.

sd_event_source_get_time_accuracy() retrieves the configured accuracy value of a event source created previously with sd_event_add_time(). It takes the event source object and a pointer to a variable to store the accuracy in. The accuracy is specified in µs.

sd_event_source_set_time_accuracy() changes the configured accuracy of a timer event source created previously with sd_event_add_time(). It takes the event source object and accuracy, in µs.

sd_event_source_get_time_clock() retrieves the configured clock of a event source created previously with sd_event_add_time(). It takes the event source object and a pointer to a variable to store the clock identifier in.