netdev - Re: [RFC] Zero-length write() does not generate a datagram on connected socket

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Date:	Tue, 25 Sep 2007 22:36:09 -0700
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	netdev@...r.kernel.org
Subject: Re: [RFC] Zero-length write() does not generate a datagram on
 connected socket

On Wed, 26 Sep 2007 11:18:39 +0800
Herbert Xu <herbert@...dor.apana.org.au> wrote:

> Stephen Hemminger <shemminger@...ux-foundation.org> wrote:
> > The bug http://bugzilla.kernel.org/show_bug.cgi?id=5731
> > describes an issue where write() can't be used to generate a zero-length
> > datagram (but send, and sendto do work).
> > 
> > I think the following is needed:
> > 
> > --- a/net/socket.c      2007-08-20 09:54:28.000000000 -0700
> > +++ b/net/socket.c      2007-09-24 15:31:25.000000000 -0700
> > @@ -777,8 +777,11 @@ static ssize_t sock_aio_write(struct kio
> >        if (pos != 0)
> >                return -ESPIPE;
> > 
> > -       if (iocb->ki_left == 0) /* Match SYS5 behaviour */
> > -               return 0;
> > +       if (unlikely(iocb->ki_left == 0)) {
> > +               struct socket *sock = iocb->ki_filp->private_data;
> > +               if (sock->type == SOCK_STREAM)
> > +                       return 0;
> > +       }
> 
> I'm not sure whether all STREAM protocols treat zero-length
> sends as no-ops.  What about SCTP?
> 
> Put it another way, do we really need to keep the short-circuit
> for SOCK_STREAM?
> 
> Cheers,

Stream is defined as sequence of bytes. So short circuit makes sense
If the application wants message boundaries it needs to use SOCK_SEQPACKET.
I was paranoid about possible breakage in TCP or SCTP. But since
send(s, buf, 0, 0) already filters through, I guess it doesn't matter.

-- 
Stephen Hemminger <shemminger@...ux-foundation.org>
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