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Date: Mon, 16 Jul 2001 21:54:05 +0200
From: Corinna Vinschen <cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: data in socketpair() channel lost if writer closes or exits without shutting down
Message-ID: <20010716215405.F25442@cygbert.vinschen.de>
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In-Reply-To: <20010716180010.2309.qmail@lizard.curl.com>; from jik@curl.com on Mon, Jul 16, 2001 at 02:00:10PM -0400

On Mon, Jul 16, 2001 at 02:00:10PM -0400, Jonathan Kamens wrote:
> The reason why things are different if the process exits is because
> Winsock runs in user space.  Windows can't leave something around
> finishing up with data transmission after the process exits.  On Unix,
> on the other hand, sockets are handled in the kernel, and the kernel
> will happily maintain that state and finish transmitting the data even
> after the process which sent it is gone.

AFAICS, that's the most important observation. I never thought about
that.

> Perhaps as a compromise we could set SO_LINGER on all new sockets with
> a short timeout, say a few seconds, to give data time to flush.

No, that's incorrect. Excerpt from W. Richard Stevens, "UNIX Network
Programming Vol. 1, Network APIs: Sockets and XTI", 2nd Edition, p. 187:

	If l_onoff is nonzero and l_linger is 0, TCP aborts the
	connection when it is closed [...]. That is, TCP discards
	any data still remaining [...]. This avoids TCP's TIME_WAIT
	state, but in doing so leaves open the possibility of
	another incarnation of this connection being created within
	2MSL seconds and having old duplicate segments from the just
	terminated connection being incorrectly delivered to the new
	incarnation [...].

> Or, slightly more complex but slightly more correct, keep track of
> whether or not a socket was at some point shared between multiple
> processes (either as a result of a fork or as a result of sending a
> file descriptor between processes (does Cygwin support that?)).  When
> a socket is first shared, we can set SO_LINGER as I described above.
> Then, when a socket is closed, check if it was ever shared and call
> shutdown if it wasn't.

Since Cygwin has currently no "device layer" (though planned for the
future) we have no sharing of data between duplicated and inherited
file descriptors, unfortunately.

> >   I just have that page "Graceful Shutdown, Linger Options, and
> >  Socket Closure" inside of MSDN, that's all.
> 
> Amusingly enough, the link to that page on the "closesocket" page is
> invalid.

http://msdn.microsoft.com/library/en-us/winsock/hh/winsock/ovrvw3_3she.asp

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Developer                                mailto:cygwin AT cygwin DOT com
Red Hat, Inc.

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