From: "David Schwartz" <davids@webmaster.com>
To: <davidel@xmailserver.org>
Cc: <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH/RFC] POLLHUP tinkering ...
Date: Sun, 8 Jan 2006 19:13:04 -0800
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> From: Davide Libenzi <davidel@xmailserver.org>
> Date: Sun, 8 Jan 2006 16:02:10 -0800 (PST)

> > But if and hangup happened with some data (data + FIN), they won't
> > receive any more events for the Linux poll subsystem (and epoll,
> > when using the event triggered interface), so they are forced to
> > issue an extra read() after the loop to detect the EOF
> > condition. Besides from the extra read() overhead, the code does not
> > come exactly pretty.

> The extra last read is always necessary, it's an error synchronization
> barrier.  Did you know that?

	If there is an error, an error event must be returned. An edge-triggered
interface must report every event that occurs with an indication of that
type.

> If a partial read or write hits an error, the successful amount of
> bytes read or written before the error occurred is returned.  Then any
> subsequent read or write will report the error immediately.

	If the connection closes and the edge-triggered interface does not give a
HUP indication, then it is broken.

	A HUP is not a read event and signalling a read is not sufficient.

	DS


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