sd_event_add_defer

systemd
sd_event_add_defer 3
sd_event_add_defer sd_event_add_post sd_event_add_exit sd_event_handler_t Add static event sources to an event loop

    #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.

## 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