Ubuntu Manpage: sd_event_add_time, sd_event_add_time_relative, sd_event_source_get

Provided by: libsystemd-dev_255.4-1ubuntu8.4_amd64

NAME

       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

SYNOPSIS

       #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, 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.

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.

HISTORY