Messages in this thread | | | Date | Mon, 7 Jul 2003 21:03:15 +0100 | From | Jamie Lokier <> | Subject | Re: epoll vs stdin/stdout |
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Eric Varsanyi wrote: > Epoll's API/impl is great as it is IMO, not suggesting need for change, I was > hoping there was a good standard trick someone worked up to get around > this specifc end case of stdin/stdout usually being dups but sometimes > not. Porting my event system over to use epoll was easy/straightforward > except for this one minor hitch.
Easy: if it's a read event, it's stdin; if it's a write event, it's stdout :)
You've raised an interesting problem. It is easy to fix this in the specific case of stdin/stdout, however what happens when your process is passed a pair of fds from some other process (or more than one process, using AF_UNIX), and told to read one and write the other? What happens when you have 10 fds from different sources, some for reading and some for writing (quite typical in a complex server)?
With the epoll API, your process has to know whether any paids or fds correspond to the same file *, in order to decide whether to register one interested in READ+WRITE or two interests separately.
Unfortunately I cannot think of a way for a process to know, in general, whether two fds that it is passed correspond to the same file *. Well, apart from trying epoll on it and seeing what happens :/
Perhaps this indicates the epoll API _is_ flawed. Epoll maintains this state mapping:
file * -> (event mask, event states)
when it ought to maintain this:
(file *, event type) -> event state
In other words, perhaps epoll should be keeping registered interest in read events and registered interest in write events completely separate.
I suspect changing the API to do that wouldn't even break any of the existing apps.
Davide, what do you think?
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