Overview of the event subsystem
Adrian Chadd <adrian@creative.net.au>

$Id: event.txt 33 2005-10-02 20:50:00Z knight $

One of the things that immediately struck me whilst first looking at the
code was that the ircd periodically scheduled things in io_loop() but
it did them manually. This is very wasteful and very tedious.

Therefore, an event system was added to hybrid. src/event.c contains an
event system ported from the squid web cache. It is pretty self contained,
and only a few things (debugging, time resolution) needed changing.

An event is scheduled through eventAdd() or eventAddIsh() :

eventAdd(const char *name, EVH * func, void *arg, time_t when, int weight)
eventAddIsh(const char *name, EVH * func, void *arg, time_t delta_ish,
  int weight)

after 'when' (or delta_ish) seconds has elapsed from the time the above
functions are called, the 'func' is called with the given data 'arg'. The
event is then deleted.

To delete an event, use eventDelete() :

eventDelete(EVH * func, void *arg)

An event is identified by its callback function and data pair.

Events are run through eventRun(). This is designed to be called *BEFORE*
your IO handlers, to let events scheduled immediately (ie a time of 0)
to initiate IO before the IO handlers are called.

(Believe me, its useful.)



Example:

Say you have something which must be called every 15 seconds.

* You would first define the callback in your module:

static EVH foo_periodic_event;
static int initialised = 0;

* You would then add the event in your initialization function:

void foo_init(void)
{
    if (!initialised) {
        eventAdd("foo_periodic_event", foo_periodic_event, NULL, 0, 0);
        initialised = 1;
    }
}

  This will force the event to be called the next time eventRun() is called,
  rather than waiting 15 seconds.

* You then define your event callback:

static void
foo_periodic_event(void *data)
{
    /* We'd do our work here */

    /* Then we'd finish */
    eventAdd("foo_periodic_event", foo_periodic_event, NULL, 15, 0);
}


Notes:

* I really should change the timeout value to be in milliseconds. Squid used
  a double, but Dianora had something against floating point code in the main
  loop (which is understandable). If someone wants a fun task .. :-)

* Note that the 'name' parameter to eventAdd() / eventAddIsh() is a const
  char *, and is *not copied* but *referenced*. Therefore, it is in your
  best interest to use string constants.

* /stats E for an oper shows pending events. Thanks Diane!
