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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | NOTES | HISTORY | SEE ALSO | COLOPHON |
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SD_EVENT_ADD_TIME(3) sd_event_add_time SD_EVENT_ADD_TIME(3)
sd_event_add_time, sd_event_add_time_relative,
sd_event_source_get_time, sd_event_source_set_time,
sd_event_source_set_time_relative,
sd_event_source_get_time_accuracy,
sd_event_source_set_time_accuracy, sd_event_source_get_time_clock,
sd_event_time_handler_t - Add a timer event source to an event
loop
#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 *ret);
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 *ret);
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 *ret);
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_MONOTONIC, CLOCK_BOOTTIME, CLOCK_REALTIME_ALARM, or
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 sd_event_exit(3).
Use CLOCK_BOOTTIME_ALARM and CLOCK_REALTIME_ALARM to define event
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_now().
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.
On success, these functions return 0 or a positive integer. On
failure, they return a negative errno-style error code.
Errors
Returned values 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.
-EOPNOTSUPP
The selected clock is not supported by the event loop
implementation.
-EDOM
The passed event source is not a timer event source.
-EOVERFLOW
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.
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
started.
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.
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_description(3),
sd_event_source_set_floating(3), clock_gettime(2),
timerfd_create(2), prctl(2)
This page is part of the systemd (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 is not part of the original manual page), send a mail to
man-pages@man7.org
systemd 258~rc2 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)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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