From: "David Schwartz" <davids@webmaster.com>
To: <martijn@entmoot.nl>, <linux-kernel@vger.kernel.org>
Subject: RE: UDP recvmsg blocks after select(), 2.6 bug?
Date: Thu, 7 Oct 2004 17:19:20 -0700
Message-ID: <MDEHLPKNGKAHNMBLJOLKKEHIONAA.davids@webmaster.com>
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> > POSIX does not require the kernel to predict the future. The
> > only guarantee
> > against having a socket operation block is found in
> > non-blocking sockets.

> It is one thing to implement select()/recvmsg() in a non POSIX compliant
> way; it is another thing to make false claims about that standard. POSIX
> _does_ guarantee that a call to recvmsg() does not block after a call
> to select().

	I do not believe this.

> > Suppose, for example, that instead of using 'read' you used
> > 'recvmsg', and
> > we add an option to 'recvmsg' to allow you to read datagrams with bad
> > checksums. What should 'select' do if a datagram is received with a bad
> > checksum? It has no idea what flavor of 'recvmsg' you're going
> > to call, so
> > it can't know if your operation is going to block or not.

> This is all described in detail in the standard.

	Where, specifically, does the standard guarantee that a subsequent call to
'recvmsg' will not block?

> > No, you are incorrect. Consider, again, a 'recvmsg' flag to allow you to
> > receive messages even if they have bad checksums versus one that blocks
> > until a message with a valid checksum is received. The 'select' function
> > just isn't smart enough.
> >
> > Consider a 'select' for write on a TCP socket. How does
> > 'select' know how
> > many bytes you're going to write? Again, a 'select' hit just indicates
> > something relevant has happened, it *cannot* guarantee that a future
> > operation won't block both because 'select' has no idea what
> > operation is
> > going to take place in the future and because things can change
> > between now
> > and then.

> You really should read the standard on this..

	I have. We obviously disagree on what it says. Since you're the one
claiming a guarantee that I claim does not exist, perhaps you could cite
where you think this guarantee appears.

	DS


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