NAME

       sd_event_add_defer, sd_event_add_post, sd_event_add_exit, sd_event_handler_t - Add static
       event sources to an event loop

SYNOPSIS

       #include <systemd/sd-event.h>

       typedef struct sd_event_source sd_event_source;

       typedef int (*sd_event_handler_t)(sd_event_source *s, void *userdata);

       int sd_event_add_defer(sd_event *event, sd_event_source **source,
                              sd_event_handler_t handler, void *userdata);

       int sd_event_add_post(sd_event *event, sd_event_source **source,
                             sd_event_handler_t handler, void *userdata);

       int sd_event_add_exit(sd_event *event, sd_event_source **source,
                             sd_event_handler_t handler, void *userdata);

DESCRIPTION

       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 is a function to call or NULL. The handler function will be passed the
       userdata pointer, which may be chosen freely by the caller. The handler may return
       negative to signal an error (see below), other return values are ignored. If handler is
       NULL, a default handler that calls sd_event_exit(3) will be used.

       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 either be disabled after
       the invocation, even if the SD_EVENT_ON mode was requested before, or it will cause the
       loop to terminate, see sd_event_source_set_exit_on_failure(3).

       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.

       If the handler parameter to sd_event_add_defer() or sd_event_add_post() 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 passed as the exit code parameter to
       sd_event_exit(3). Similar functionality is not available for sd_event_add_exit(), as these
       types of event sources are only dispatched when exiting anyway.

RETURN VALUE

       On success, these functions return 0 or a positive integer. On failure, they return a
       negative errno-style error code.

   Errors
       Returned errors may indicate the following problems:

       -ENOMEM
           Not enough memory to allocate an object.

       -EINVAL
           An invalid argument has been passed.

       -ESTALE
           The event loop is already terminated.

       -ECHILD
           The event loop has been created in a different process.

NOTES

       These APIs are implemented as a shared library, which can be compiled and linked to with
       the libsystemd pkg-config(1) file.

SEE ALSO

       systemd(1), sd-event(3), sd_event_new(3), sd_event_now(3), sd_event_add_io(3),
       sd_event_add_time(3), sd_event_add_signal(3), sd_event_add_child(3),
       sd_event_add_inotify(3), sd_event_source_set_enabled(3), sd_event_source_set_priority(3),
       sd_event_source_set_userdata(3), sd_event_source_set_description(3),
       sd_event_source_set_floating(3), sd_event_exit(3)