Re: (e)poll and (rd)hup questions

From: Michael Noisternig
Date: Tue Aug 26 2008 - 15:38:24 EST


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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