sd_event_add_time(3) — Linux manual page


SD_EVENT_ADD_TIME(3)        sd_event_add_time       SD_EVENT_ADD_TIME(3)

NAME         top

       sd_event_add_time, sd_event_add_time_relative,
       sd_event_source_get_time, sd_event_source_set_time,
       sd_event_source_get_time_clock, sd_event_time_handler_t - Add a
       timer event source to an event loop

SYNOPSIS         top

       #include <systemd/sd-event.h>

       typedef struct sd_event_source sd_event_source;

       typedef int (*sd_event_time_handler_t)(sd_event_source *s,
                                              uint64_t usec,
                                              void *userdata);

       int sd_event_add_time(sd_event *event, sd_event_source **source,
                             clockid_t clock, uint64_t usec,
                             uint64_t accuracy,
                             sd_event_time_handler_t handler,
                             void *userdata);

       int sd_event_add_time_relative(sd_event *event,
                                      sd_event_source **source,
                                      clockid_t clock, uint64_t usec,
                                      uint64_t accuracy,
                                      sd_event_time_handler_t handler,
                                      void *userdata);

       int sd_event_source_get_time(sd_event_source *source,
                                    uint64_t *usec);

       int sd_event_source_set_time(sd_event_source *source,
                                    uint64_t usec);

       int sd_event_source_set_time_relative(sd_event_source *source,
                                             uint64_t usec);

       int sd_event_source_get_time_accuracy(sd_event_source *source,
                                             uint64_t *usec);

       int sd_event_source_set_time_accuracy(sd_event_source *source,
                                             uint64_t usec);

       int sd_event_source_get_time_clock(sd_event_source *source,
                                          clockid_t *clock);

DESCRIPTION         top

       sd_event_add_time() adds a new timer event source 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
       clock parameter takes a clock identifier, one of CLOCK_REALTIME,
       CLOCK_BOOTTIME_ALARM. See timerfd_create(2) for details regarding
       the various types of clocks. The usec parameter specifies the
       earliest time, in microseconds (μs), relative to the clock's
       epoch, when the timer shall be triggered. If a time already in
       the past is specified (including 0), this timer source "fires"
       immediately and is ready to be dispatched. If the parameter is
       specified as UINT64_MAX the timer event will never elapse, which
       may be used as an alternative to explicitly disabling a timer
       event source with sd_event_source_set_enabled(3). The accuracy
       parameter specifies an additional accuracy value in μs specifying
       how much the timer event may be delayed. Use 0 to select the
       default accuracy (250ms). Use 1μs for maximum accuracy. Consider
       specifying 60000000μs (1min) or larger for long-running events
       that may be delayed substantially. Picking higher accuracy values
       allows the system to coalesce timer events more aggressively,
       improving power efficiency.

       The handler is a function to call when the timer elapses or NULL.
       The userdata pointer will be passed to the handler function, and
       may be chosen freely by the caller. The configured trigger time
       is also passed to the handler, even if the call actually happens
       slightly later, subject to the specified accuracy value, the
       kernel timer slack (see prctl(2)), and additional scheduling
       latencies. To query the actual time the handler was called use
       sd_event_now(3). 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.

       By default, the timer will elapse once (SD_EVENT_ONESHOT), but
       this may be changed with sd_event_source_set_enabled(3). 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). Note that a timer event
       set to SD_EVENT_ON will fire continuously unless its configured
       time is updated using sd_event_source_set_time().

       sd_event_add_time_relative() is like sd_event_add_time(), but
       takes a relative time specification. It's relative to the current
       time of the event loop iteration, as returned by sd_event_now(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 if it is still
       referenced, disable the event source using
       sd_event_source_set_enabled(3) with SD_EVENT_OFF.

       If the second parameter of sd_event_add_time() is 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_time() 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

       sources that may wake up the system from suspend.

       In order to set up relative timers (that is, relative to the
       current time), retrieve the current time via sd_event_now(3), add
       the desired timespan to it, and use the result as the usec
       parameter to sd_event_add_time().

       In order to set up repetitive timers (that is, timers that are
       triggered in regular intervals), set up the timer normally, for
       the first invocation. Each time the event handler is invoked,
       update the timer's trigger time with sd_event_source_set_time(3)
       for the next timer iteration, and reenable the timer using
       sd_event_source_set_enabled(). To calculate the next point in
       time to pass to sd_event_source_set_time(), either use as base
       the usec parameter passed to the timer callback, or the timestamp
       returned by sd_event_now(). In the former case timer events will
       be regular, while in the latter case the scheduling latency will
       keep accumulating on the timer.

       sd_event_source_get_time() retrieves the configured time value of
       an event source created previously with sd_event_add_time() or
       sd_event_add_time_relative(). It takes the event source object
       and a pointer to a variable to store the time in, relative to the
       selected clock's epoch, in μs. The returned value is relative to
       the epoch, even if the event source was created with a relative
       time via sd_event_add_time_relative().

       sd_event_source_set_time() changes the time of an event source
       created previously with sd_event_add_time() or
       sd_event_add_time_relative(). It takes the event source object
       and a time relative to the selected clock's epoch, in μs.

       sd_event_source_set_time_relative() is similar to
       sd_event_source_set_time(), but takes a time relative to the
       current time of the event loop iteration, as returned by

       sd_event_source_get_time_accuracy() retrieves the configured
       accuracy value of an event source created previously with
       sd_event_add_time(). It takes the event source object and a
       pointer to a variable to store the accuracy in. The accuracy is
       specified in μs.

       sd_event_source_set_time_accuracy() changes the configured
       accuracy of a timer event source created previously with
       sd_event_add_time(). It takes the event source object and
       accuracy, in μs.

       sd_event_source_get_time_clock() retrieves the configured clock
       of an event source created previously with sd_event_add_time().
       It takes the event source object and a pointer to a variable to
       store the clock identifier in.

RETURN VALUE         top

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

       Returned values 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.

           The selected clock is not supported by the event loop

           The passed event source is not a timer event source.

           The passed relative time is outside of the allowed range for
           time values (i.e. the specified value added to the current
           time is outside the 64 bit unsigned integer range).

           Added in version 247.

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_time(), sd_event_source_get_time(),
       sd_event_source_set_time(), sd_event_source_get_time_accuracy(),
       sd_event_source_set_time_accuracy(), and
       sd_event_source_get_time_clock() were added in version 213.

       sd_event_time_handler_t() was added in version 217.

       sd_event_add_time_relative() and
       sd_event_source_set_time_relative() were added in version 247.

SEE ALSO         top

       systemd(1), sd-event(3), sd_event_new(3), sd_event_now(3),
       sd_event_add_io(3), sd_event_add_signal(3),
       sd_event_add_child(3), sd_event_add_inotify(3),
       sd_event_add_defer(3), sd_event_source_set_enabled(3),
       sd_event_source_set_priority(3), sd_event_source_set_userdata(3),
       sd_event_source_set_floating(3), clock_gettime(2),
       timerfd_create(2), prctl(2)

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 2024-06-14.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2024-06-13.)  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 257~devel                                   SD_EVENT_ADD_TIME(3)

Pages that refer to this page: sd-event(3)sd_event_add_child(3)sd_event_add_defer(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_now(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)