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On Tue, 24 Oct 2000, Dan Kegel wrote:
> Linus Torvalds wrote:
> > Basically, the main loop would boil down to
> > for (;;) {
> > static struct event ev_list[MAXEV];
> > get_event(ev_list, MAXEV, &tmout);
> > .. timeout handling here ..
> > }
> >
> > because get_even() would end up doing all the user-mode calls too (so
> > "get_event()" is no longer a system call: it's a system call + a for-loop
> > to call all the ID handler functions that were associated with the events
> > that triggered).
>
> Occurs to me that this stuff will be used from other languages
> than C, which might prefer to do their own event dispatching,
> and would want the system to treat event_fn as another opaque quantity.

Yes and no.

The KERNEL would treat it as just another opaque quantity: after all, it
would never ever touch the thing other than get it as part of
"bind_event()" and return it as part of "get_events()". So as far as the
kernel is concerned, both "event_fn" and "opaque" are just random
information.

However, it is very important to have every common user agree about the
meaning of the ID's in user space: the whole point of encapsulating
"event_fn" as a function pointer is that you can have different parts of
the same program use the "bind_event()" interface, and they won't step on
each others toes.

So if you program in Fortran, for example, and you expect to link against
the C library, and the C library uses events for something, then you'd
better make sure that the fortran interfaces end up using the event_fn
thing in the same wasy as the C one..

> So I don't object to a function that dispatches events, e.g.
> int get_event(&tmout);
> as long as there's also
> int get_events(ev_list, MAXEV, &tmout)
> that treats event_fn as an opaque pointer and *does not* dispatch the events.

That's a user-mode issue, and obviously, if you don't care about
compatibility with anything else you can do this. The kernel won't know,
or care. But I think it is unlikely that you'd ever use the raw events in
any other way - most environments under UNIX tend to have a C library
component somewhere, because the low-level stuff is written in C and uses
the common code that way even when the programmer isn't aware of it.

Linus

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