Messages in this thread | | | Date | Mon, 7 Jul 2003 15:11:23 -0700 (PDT) | From | Davide Libenzi <> | Subject | Re: epoll vs stdin/stdout |
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On Mon, 7 Jul 2003, Jamie Lokier wrote:
> 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.
It has to keep (file*, fd) as hashing key. That will work out just fine.
> I suspect changing the API to do that wouldn't even break any of the > existing apps. > > Davide, what do you think?
Not even thinking changing the API since it'll break existing apps. The above trick will do it. Going to test it ...
- Davide
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