SD_EVENT_ADD_DEFER

Section: sd_event_add_defer (3)
Updated:
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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.

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.