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SubjectRe: [rfc] epoll interface change and glibc bits ...
On Tue, 19 Nov 2002, Edgar Toernig wrote:

> Davide Libenzi wrote:
> >
> >
> >
> >
> >
> >
> > it is going to change though with the latest talks about the interface.

Yes, man pages needs some work after the interface will be fixed. I hope
to fix it this week.

> Remove the waitqueue stuff from epoll.2. It has meaning only to
> linux kernel developers and noone else.

Sigh, I liked it :)

> What about adding an fd twice to the epoll-set? Do you get an
> error, will it override the previous settings for that fd, will
> it be ignored, or is it registered twice and you get two results
> for that fd?

You get EEXIST
Well, there's the remote possibility, trying very badly from two threads,
to add the same fd twice. It is an harmless condition though.

> Can two epoll-sets wait for the same fd?

Yes. Not suggested though.

> Are events reported to both epoll-fds?


> Is the epoll-fd itself poll/epoll/selectable?


> Can I build cluster of epoll-sets?

Uh ?!

> What happens if the epollfd is put into its own fd set?

You might find your machine a little bit frozen :)
Either 1) I remove the read lock from poll() or 2) I check the condition
at insetion time to avoid it. I very much prefer 2)

> Can I send the epoll-fd over a unix-socket to another
> process?

I'd say yes. SCM_RIGHTS should simply do an in-kernel file* to remote task
descriptor mapping.

> Then, please add more details of how events are generated. You
> say, that an inactive-to-active transition causes an event. What
> is the starting point of the collection? (I guess, all transitions

The starting point are the bits found at insertion time.

> between two epoll_wait calls.) There could be a couple of transi-
> tions on an fd between two epoll_wait calls. Are these events com-
> bined into a single reported event or is each single edge reported?

Yes, they'll be combined.

> Does an operation on an fd effect the already collected but not yet
> reported events?

You can do two operations on an existing fd. Remove is meaninless for this
case. Modify will re-read available bits.

> About epoll_wait: it looks like a "read with timeout" call. Is that
> really necessary or wouldn't a normal read(2) work as well? Similar
> for epoll_ctl: couldn't a write(2) to the epoll-fd do the same?

IMHO distinc functions are more clear than magic read/write operations.

- Davide

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