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Date: Wed, 31 Jan 2007 15:04:57 -0500 (EST)
From: Garrett Wollman <wollman@khavrinen.csail.mit.edu>
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To: freebsd@j-davis.com
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Cc: freebsd-net@freebsd.org, freebsd-stable@freebsd.org
Subject: Re: send() returns error even though data is sent, TCP connection
	still alive
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In article <1170269163.22436.71.camel@dogma.v10.wvs>,
Jeff Davis  <freebsd@j-davis.com> wrote:

>You should see something like "write failed: host is down" and the
>session will terminate. Of course, when ssh exits, the TCP connection
>closes. The only way to see that it's still open and active is by
>writing (or using) an application that ignores EHOSTDOWN errors from
>write().

I agree that it's a bug.  The only time write() on a stream socket
should return the asynchronous error[1] is when the connection has
been (or is in the process of being) torn down as a result of a
subsequent timeout.  POSIX says "may fail" for these errors write()
and send() on sockets

-GAWollman

[1] There are two kinds of error returns in the socket model:
synchronous errors, like synchronous signals, are attributed to the
result of a specific system call, detected prior to syscall return,
and usually represent programming or user error (e.g., attempting to
connect() on an fd that is not a socket).  Asynchronous errors are
detected asynchronously, and merely posted to the socket without being
delivered; they may be delivered on the next socket operation.  See
XSH 2.10.10, "Pending Error".