Index · Directives systemd 231


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, 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_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_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);


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 parameter shall reference a function to call when the timer elapses. The handler function will be passed the userdata pointer, which may be chosen freely by the caller. The handler is also passed the configured trigger time, even if it is actually called 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).

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 be disabled after the invocation, even if the SD_EVENT_ON mode was requested before. 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().

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 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 used for the exit code passed 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(). 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.

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

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

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.


The selected clock is not supported by the event loop implementation.


The passed event source is not a timer event source.


These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.

See Also

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_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), clock_gettime(2), timerfd_create(2), prctl(2)