Using EV_CUSTOM for custom events

[Zsbán Ambrus]

On 4/27/12, Konstantin Osipov <kostja.osipov at gmail.com> wrote:
> I'm looking for an efficient mechanism to asynchronously invoke
> one callback from another, or awake one watcher from another.
>
> Currently we use ev_async for that, but it involves writing
> into a pipe, and is thus slower than I would like it to be.

There are two cases.

(A) If you possibly want to signal an event from one thread to another
(or from a signal handler etc), then you must use an ev_async watcher
and signal it with ev_async_send.  This copes with the fact that the
event loop might not be in a consistent state.  The ev_async_send
function may actually need to write to a pipe (or some similar quick
system call) because otherwise it couldn't wake up the event loop if
it's currently sleeping or close to sleeping.  If, however, the event
loop is not currently sleeping, then ev_async_send avoids the system
call most of the time, provided that you're using a recent enough
version of libev.

(B) If you want to signal only from a clear state, such as from a
watcher callback in the same thread, or when the event loop is not
running, then you don't need ev_async.  In this case, you can use the
ev_feed_event function, and it doesn't really matter what kind of
watcher (eg. ev_signal) and what event (such as EV_CUSTOM) you're
using.  The caveat is that multiple events fed to the same watcher can
be collapsed to a single event.  Alternately, you could create a new
watcher, possibly a timer with zero timeout.

Ambrus