On Feb 26, 2007, at 2:57 PM, Christian Ress wrote:
Hello,
I stumbled across libevent and it seems to fit my needs very well.
However, I wonder if this is side-effect free:
second_cb(..) { .. }
first_cb(..) {
..
struct event newev;
evtimer_set(&newev, second_cb, NULL);
event_base_set(ev_base, &newev);
event_add(&newev, &tv);
event_base_dispatch(ev_base);
}
main() {
..
ev_base = event_init();
evtimer_set(ev, first_cb, NULL);
event_add(ev, &tv);
event_base_dispatch(ev_base);
}
The point of this is creating new events 'on demand', but since I'm
calling event_dispatch when being called by event_loop, I wonder if
this might lead to recursion issues or similar bad behaviour.
I hope someone can clarify this :)
First, keep in mind something that tripped up me when I first started
using libevent - the "struct event" you pass to event_add is not
duplicated, but rather directly linked into the event_base's queues.
So if it's a timeout, it needs to live until it fires (or forever
with EV_PERSIST) or you manually call event_del(). There are a lot of
reasons that might not happen in the next invocation of
event_base_dispatch(), so this code seems dangerous.
And second...while I don't see anything off-hand in libevent that
would break based on this recursion, I don't think it was intended to
be used that way, and you always risk stack overflows if you don't
have a way to bound your recursion.
You can create new events 'on demand' with event_once. Which isn't
magic - it just internally allocates a new struct event on the heap
that adds a callback which - after calling your callback - frees the
struct event. You could accomplish the same thing yourself:
void
first_cb(int fd, short events, void *arg)
{
struct event *newev;
int res;
...
newev = malloc(sizeof(struct event));
...newev == NULL error handling goes here...
evtimer_set(newev, ...);
res = event_add(&newev, &tv);
...event_add error handling goes here...
}
void
second_cb(int fd, short events, void *arg)
{
struct event *myev = arg;
...
free(myev);
}
--
Scott Lamb <http://www.slamb.org/>
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