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 function will be passed the userdata pointer, which may be chosen freely by the caller.

       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.

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_source_set_enabled(3),
       sd_event_source_set_priority(3), sd_event_source_set_userdata(3), sd_event_source_set_description(3),
       sd_event_exit(3)