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Date: Mon, 22 Mar 2010 17:01:00 +0100
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
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Subject: Re: recvfrom + MSG_PEEK = broken?
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On Mar 22 10:56, Charles Wilson wrote:
> On 3/22/2010 7:03 AM, Corinna Vinschen wrote:
> > On Mar 22 02:04, Charles Wilson wrote:
> >> I've been tracking down a problem in xinetd. It appears to be an issue
> >> with using MSG_PEEK in recvfrom() on UDP sockets.
> > 
> > Yes, that's a bug in Cygwin.  The socket event handling got messed up
> > when MSG_PEEK was used.  I fixed that in CVS so it will be in 1.7.2.
> 
> Confirmed: fixes both my testcase, and the error in xinetd. Thanks for
> the quick turnaround!  I was about ready to pull my hair out on this

Sure.  I'm glad I had a simple testcase for a socket problem.  If only
somebody would come up with an equally simple testcase for the rsync
problem...

> > The last time I had problems with MSG_PEEK was back in 2006.  Curious
> > how seldomly it's used, apparently.
> 
> Well, it didn't appear to cause a problem in xinetd wrt TCP connections,
> only UDP.

That may explain why I didn't see problems in TCP connections.
Apparently there's a difference in event handling between TCP and UDP.

For instance, with UDP, if you call recv as you did, with a one byte
buffer but more than one bytes ready to read, the event handling notices
an FD_READ event just fine, but then the call to WSARecv returns with
error(!) and WSAGetLastError() set to WSAEMSGSIZE.  For POSIX
compatibility Cygwin has to turn this into a successful call with 1 byte
read.  The next recv call then does not get another FD_READ event, even
though the previous call with MSG_PEEK didn't empty the internal recv
buffer.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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