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, library or module instance.

NOTES

       Functions described here are available as a shared library, which can be compiled against and linked to
       with the libsystemd pkg-config(1) file.

       The code described here uses getenv(3), which is declared to be not multi-thread-safe. This means that
       the code calling the functions described here must not call setenv(3) from a parallel thread. It is
       recommended to only do calls to setenv() from an early phase of the program when no other threads have
       been started.

HISTORY

       sd_event_add_defer(), sd_event_add_post(), sd_event_add_exit(), and sd_event_handler_t() were added in
       version 217.

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)