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sd_event_add_defer(3) — Linux manual page

SD_EVENT_ADD_DEFER(3)       sd_event_add_deferSD_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_thandler,void *userdata);int sd_event_add_post(sd_event *event, sd_event_source **source,sd_event_handler_thandler, void *userdata);int sd_event_add_exit(sd_event *event, sd_event_source **source,sd_event_handler_thandler, void *userdata);

DESCRIPTION        top

       These three functions add new static event sources to an event       loop. The event loop object is specified in theevent parameter,       the event source object is returned in thesource parameter. The       event sources are enabled statically and will "fire" when the       event loop is run and the conditions described below are met.       Thehandler is a function to call orNULL. The handler function       will be passed theuserdata 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. Ifhandler isNULL,       a default handler that callssd_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 toSD_EVENT_ONthe 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 toSD_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 withsd_event_exit(3).       Thesd_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 theSD_EVENT_ON       mode was requested before, or it will cause the loop to terminate,       seesd_event_source_set_exit_on_failure(3).       To destroy an event source object usesd_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 toSD_EVENT_OFFwithsd_event_source_set_enabled(3).       If the second parameter of these functions is passed asNULLno       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 thehandler parameter tosd_event_add_defer()orsd_event_add_post()isNULL, and the event source fires, this will       be considered a request to exit the event loop. In this case, theuserdata parameter, cast to an integer, is passed as the exit code       parameter tosd_event_exit(3). Similar functionality is not       available forsd_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.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        top

       Functions described here are available as a shared library, which       can be compiled against and linked to with thelibsystemd pkg-config(1) file.       The code described here usesgetenv(3), which is declared to be       not multi-thread-safe. This means that the code calling the       functions described here must not callsetenv(3) from a parallel       thread. It is recommended to only do calls tosetenv()from an       early phase of the program when no other threads have been       started.

HISTORY        top

sd_event_add_defer(),sd_event_add_post(),sd_event_add_exit(),       andsd_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_userdata(3),sd_event_source_set_description(3),sd_event_source_set_floating(3),sd_event_exit(3)

COLOPHON        top

       This page is part of thesystemd (systemd system and service       manager) project.  Information about the project can be found at       ⟨http://www.freedesktop.org/wiki/Software/systemd⟩.  If you have a       bug report for this manual page, see       ⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.       This page was obtained from the project's upstream Git repository       ⟨https://github.com/systemd/systemd.git⟩ on 2025-08-11.  (At that       time, the date of the most recent commit that was found in the       repository was 2025-08-11.)  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 isnot part of the original manual page), send a mail to       man-pages@man7.orgsystemd 258~rc2SD_EVENT_ADD_DEFER(3)

Pages that refer to this page:sd_bus_set_close_on_exit(3), sd-event(3), sd_event_add_child(3), sd_event_add_inotify(3), sd_event_add_io(3), sd_event_add_memory_pressure(3), sd_event_add_signal(3), sd_event_add_time(3), sd_event_exit(3), sd_event_new(3), sd_event_run(3), sd_event_set_watchdog(3), sd_event_source_get_event(3), sd_event_source_get_pending(3), sd_event_source_set_description(3), sd_event_source_set_destroy_callback(3), sd_event_source_set_enabled(3), sd_event_source_set_exit_on_failure(3), sd_event_source_set_floating(3), sd_event_source_set_prepare(3), sd_event_source_set_priority(3), sd_event_source_set_ratelimit(3), sd_event_source_set_userdata(3), sd_event_source_unref(3), sd_event_wait(3), systemd.directives(7), systemd.index(7)

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