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SubjectRe: (e)poll and (rd)hup questions
David wrote:
> There's nothing synchronous in epoll WRT userspace. If you use epoll LT,
> you don't need RDHUP. RDHUP was introduced to deal with special
> connection states and epoll ET. Take a peek here for a detailed
> description about how/why RDHUP was introduced:
>
> http://lkml.org/lkml/2003/7/12/116
>
>
> - Davide

Thanks a lot for the explanation about RDHUP. Now that leaves me with
only one question which I'm still not sure about: When I get HUP can
there still be data in the kernel buffer to read out? (I guess so.)

Thanks again,
Michael

--Original message--
Michael Noisternig schrieb:
> Hello folks,
>
> I hope this is a right place to ask this question. (Sorry if it isn't,
> please hint me where to post in that case.)
>
> This has been bugging me for quite some while now. I'm using epoll in
> default level-triggered mode. I'm polling for input, output, rdhup, and
> forcibly for hup. I figured that when I get EPOLLRDHUP that doesn't mean
> there is no more data to read, presumably I must empty the kernel buffer
> by continously reading from the fd until I get zero as a result
> indicating rdhup. This means EPOLLRDHUP is delivered asynchronously and
> thus is pretty useless to me.
>
> On the other hand, EPOLLHUP seems to be delivered synchronously. If it
> isn't then this means I get constantly notified about a closed fd until
> I have read all data from the kernel buffer and remove the fd from the
> epoll set (which makes being forced to hup notifications useless to me).
>
> (And no, don't tell me to use edge-triggered mode, I have reasons to use
> level-triggered.)
>
> Question: Is my observation correct that EPOLLRDHUP is delivered
> asynchronously in contrast to EPOLLHUP? Or is EPOLLHUP delivered
> asynchronously as well?
>
> Thanks, anwers would help a lot!!!
> Michael
>


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