(e)poll and (rd)hup questions

[Michael Noisternig]

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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[Davide Libenzi]

Post by Michael Noisternig
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?

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


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[Michael Noisternig]

Post by Davide Libenzi
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
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--

Post by Davide Libenzi
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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