NAME

       sd_event_source_set_ratelimit, sd_event_source_get_ratelimit,
       sd_event_source_is_ratelimited, sd_event_source_set_ratelimit_expire_callback - Configure
       rate limiting on event sources

SYNOPSIS

       #include <systemd/sd-event.h>

       int sd_event_source_set_ratelimit(sd_event_source *source, uint64_t interval_usec,
                                         unsigned burst);

       int sd_event_source_get_ratelimit(sd_event_source *source, uint64_t* ret_interval_usec,
                                         unsigned* ret_burst);

       int sd_event_source_is_ratelimited(sd_event_source *source);

       int sd_event_source_set_ratelimit_expire_callback(sd_event_source *source,
                                                         sd_event_handler_tcallback);

DESCRIPTION

       sd_event_source_set_ratelimit() may be used to enforce rate limiting on an event source.
       When used an event source will be temporarily turned off when it fires more often then a
       specified burst number within a specified time interval. This is useful as simple
       mechanism to avoid event source starvation if high priority event sources fire very
       frequently.

       Pass the event source to operate on as first argument, a time interval in microseconds as
       second argument and a maximum dispatch limit ("burst") as third parameter. Whenever the
       event source is dispatched more often than the specified burst within the specified
       interval it is placed in a mode similar to being disabled with
       sd_event_source_set_enabled(3) and the SD_EVENT_OFF parameter. However it is disabled only
       temporarily – once the specified interval is over regular operation resumes. It is again
       disabled temporarily once the specified rate limiting is hit the next time. If either the
       interval or the burst value are specified as zero, rate limiting is turned off. By default
       event sources do not have rate limiting enabled. Note that rate limiting and disabling via
       sd_event_source_set_enabled() are independent of each other, and an event source will only
       effect event loop wake-ups and is dispatched while it both is enabled and rate limiting is
       not in effect.

       sd_event_source_get_ratelimit() may be used to query the current rate limiting parameters
       set on the event source object source. The previously set interval and burst vales are
       returned in the second and third argument.

       sd_event_source_is_ratelimited() may be used to query whether the event source is
       currently affected by rate limiting, i.e. it has recently hit the rate limit and is
       currently temporarily disabled due to that.

       sd_event_source_set_ratelimit_expire_callback may be used to set a callback function that
       is invoked every time the event source leaves rate limited state. Note that function is
       called in the same event loop iteration in which state transition occurred.

       Rate limiting is currently implemented for I/O, timer, signal, defer and inotify event
       sources.

RETURN VALUE

       On success, sd_event_source_set_ratelimit(), sd_event_source_set_ratelimit_expire_callback
       and sd_event_source_get_ratelimit() return a non-negative integer. On failure, they return
       a negative errno-style error code.  sd_event_source_is_ratelimited returns zero if rate
       limiting is currently not in effect and greater than zero if it is in effect; it returns a
       negative errno-style error code on failure.

   Errors
       Returned errors may indicate the following problems:

       -EINVAL
           source is not a valid pointer to an sd_event_source object.

       -ECHILD
           The event loop has been created in a different process.

       -EDOM
           It was attempted to use the rate limiting feature on an event source type that does
           not support rate limiting.

       -ENOEXEC
           sd_event_source_get_ratelimit() was called on an event source that doesn't have rate
           limiting configured.

NOTES

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

SEE ALSO

       sd-event(3), sd_event_add_io(3), sd_event_add_time(3), sd_event_add_signal(3),
       sd_event_add_inotify(3), sd_event_add_defer(3), sd_event_source_set_enabled(3)