sd_event_add_time

systemd
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 *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

`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.

### 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.

`-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).

## 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