sd_event_add_defer(3) — Linux manual page


SD_EVENT_ADD_DEFER(3)      sd_event_add_defer      SD_EVENT_ADD_DEFER(3)

NAME         top

       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         top

       #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         top

       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

       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         top

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

       Returned errors may indicate the following problems:

           Not enough memory to allocate an object.

           An invalid argument has been passed.

           The event loop is already terminated.

           The event loop has been created in a different process,
           library or module instance.

NOTES         top

       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

HISTORY         top

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

SEE ALSO         top

       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_floating(3), sd_event_exit(3)

COLOPHON         top

       This page is part of the systemd (systemd system and service
       manager) project.  Information about the project can be found at
       ⟨⟩.  If you have
       a bug report for this manual page, see
       This page was obtained from the project's upstream Git repository
       ⟨⟩ on 2023-12-22.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2023-12-22.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
       is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

systemd 255                                        SD_EVENT_ADD_DEFER(3)

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