linux-kernel - Re: AF_UNIX MSG

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Date:	Tue, 8 Jan 2008 23:46:15 +0000
From:	"Tom Spink" <tspink@...il.com>
To:	"Brent Casavant" <bcasavan@....com>
Cc:	"Rick Jones" <rick.jones2@...com>, netdev@...r.kernel.org,
	"David Miller" <davem@...emloft.net>, linux-kernel@...r.kernel.org
Subject: Re: AF_UNIX MSG_PEEK bug?

On 08/01/2008, Tom Spink <tspink@...il.com> wrote:
> On 08/01/2008, Brent Casavant <bcasavan@....com> wrote:
> > On Tue, 8 Jan 2008, Tom Spink wrote:
> >
> > > Where in the code is the message length being sent across the socket?
> >
> > In do_producer(), there are the following lines in the main loop:
> >
> >         /* Send random lengths of data */
> >         messages[i].length = (rand() % MAXLEN) + sizeof(size_t);
> >         iov[i].iov_len = messages[i].length;
> >
> > The entire "struct sockmsg" is sent across the socket, so the first
> > size_t in each message contains the length of the entire message
> > (including the size_t).  This size gets picked up at the
> > recv(...,MSG_PEEK) line in do_consumer().
> >
> > Thanks,
> > Brent
> >
> > --
> > Brent Casavant                          All music is folk music.  I ain't
> > bcasavan@....com                        never heard a horse sing a song.
> > Silicon Graphics, Inc.                    -- Louis Armstrong
> >
>
> Hi,
>
> But you're not consuming the size_t on the other end.  You're only
> peeking it, i.e. you're doing the recv to peek at the message, but
> never calling recv to remove that data from the queue... or am I
> missing something?
>
> --
> Regards,
> Tom Spink
> University of Edinburgh
>

Ach.  I *am* missing something... and what I'm missing is my
understanding of the sendmsg and recvmsg calls.

A quick Google has sorted me out.

-- 
Regards,
Tom Spink
University of Edinburgh
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