These three functions add new static event sources to an event loop.
The event loop object is specified in the event parameter, the event
source object is returned in the source parameter. The event sources
are enabled statically and will "fire" when the event loop is run and
the conditions described below are met. The handler function will be
passed the userdata pointer, which may be chosen freely by the
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
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
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
If the handler function returns a negative error code, it will be
disabled after the invocation, even if the SD_EVENT_ON mode was
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
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.
Returned errors may indicate the following problems:
Not enough memory to allocate an object.
An invalid argument has been passed.
The event loop is already terminated.
The event loop has been created in a different process.
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systemd 233 SD_EVENT_ADD_DEFER(3)