systemd Developer Lennart Poettering lennart@poettering.net sd_event_add_time 3 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_create2 for details regarding the various types of clocks. The usec parameter takes a time value in microseconds (µs), relative to the clock's epoch, specifying when the timer shall elapse the earliest. If a time that already lies in the past is specified (including 0), the timer source is dispatched immediately in the next event loop iterations. The accuracy parameter takes an additional accuracy value in µs specifying a time the timer event may be delayed. Specify 0 for selecting the default accuracy (250ms). Specify 1µs for most accurate timers. Consider specifying 60000000µs or larger (1min) for long-running events that may be delayed substantially. Picking higher accuracy values allows the system to coalesce timer events more aggressively, thus 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 time it was triggered, however it might actually have been called at a slightly later time, subject to the specified accuracy value, the kernel timer slack (see prctl2) and additional scheduling latencies. To query the actual time the handler was called use sd_event_now3. By default, the timer will elapse once (SD_EVENT_ONESHOT), but this may be changed with sd_event_source_set_enabled3. 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_unref3, 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 to SD_EVENT_OFF with sd_event_source_set_enabled3. If the second parameter of sd_event_add_time() is passed as 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 passed as 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_exit3. 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_now3, add the desired timespan to sleep to it, and pass 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_time3 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 a timer 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, relative to the selected clock's epoch, in µs in. sd_event_source_set_time() changes the configured time value of a timer 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 a timer 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 µs in. 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 an accuracy, in µs. sd_event_source_get_time_clock() retrieves the configured clock of a timer 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. 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. -EOPNOTSUPP The selected clock is not supported by the event loop implementation. -EDOM The passed event source is not a timer event source. systemd1, sd-event3, sd_event_new3, sd_event_now3, sd_event_add_io3, sd_event_add_signal3, sd_event_add_child3, sd_event_add_defer3, sd_event_source_set_enabled3, sd_event_source_set_priority3, sd_event_source_set_userdata3, sd_event_source_set_description3, clock_gettime2, timerfd_create2, prctl2