linux-kernel - Re: unexpected extra pollout events from epoll

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Date:	Sun, 26 Oct 2008 18:18:00 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Paul P <ppak_98@...oo.com>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: unexpected extra pollout events from epoll

On Sun, 26 Oct 2008, Paul P wrote:

> > You do that by writing data until it's finished, or you
> > get EAGAIN. If you
> > get EAGAIN, you listen for EPOLLOUT.
> > Reading is same, but you'd wait for EPOLLIN.
> 
> I've got a few questions about this approach.  The most logical 
> way to do this seems to be:
> 
> 1) Leave the epoll_wait with the EPOLLIN|EPOLLOUT event flags and
>  use epoll_ctl to switch the interest mask for each fd between EPOLLIN 
> and EPOLLOUT on a per fd basis.

Which version of epoll do you have? The epoll_wait() function does not 
accept an event mask (like you write above, EPOLLIN|EPOLLOUT). It never 
had.
But yes, you'd switch interest with epoll_ctl().



> 2) When I'm ready to write, I do a write and if it does not fully 
> write and I get the EAGAIN flag, I switch the fd with epoll_ctl(fd,MOD,EPOLLOUT). 

As optimization, if the EPOLLOUT bit is already set, you don't need to 
keep calling epoll_ctl(fd,MOD,EPOLLOUT).



> However, I get strange behavior when I tried adding fd's with only the 
> EPOLLIN interest mask. If I use epoll_wait with both the EPOLLIN and 
> EPOLLOUT interest mask, but add fd's with only the EPOLLIN interest mask,
> I still seem to get EPOLLOUT events on the fd.

Again, how the heck do you "use epoll_wait with both the EPOLLIN and 
EPOLLOUT"?!? There is not such a thing.




> So, I'm a little confused.

>From the wording above, that doesn't seem like a wrong guess.



- Davide


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