Description¶

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.

Return Value¶

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

Notes¶

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.

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_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)