linux-kernel - Re: Problem with unix sockets: SOCK_DGRAM ignores MSG

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Date:	Wed, 7 Feb 2007 10:31:57 +0100
From:	Daniel Kabs <dkabs@...otix.com>
To:	David Miller <davem@...emloft.net>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Problem with unix sockets: SOCK_DGRAM ignores MSG_TRUNC

On Monday 05 February 2007 01:52, David Miller wrote:
> What UDP is doing is different, it's returning the full packet length
> when the packet is larger then the given buffer size, but it does this
> irregardless of whether you set MSG_TRUNC in the recvmsg() passed-in
> flags.  UDP itself sets the MSG_TRUNC flag when it detects this
> situation.

I hope that I am researching the appropriate kernel source tree, but if I 
am not mistaken, this behaviour was introduced with change 1.66 according 
to bitkeeper:
  http://linux.bkbits.net:8080/linux-2.6/net/ipv4/udp.c?PAGE=history

I think the bitkeeper comment on that change (1.66) is misleading:
  "[UDP]: Return true length if user specifies MSG_TRUNC."

As you said, this is wrong. The true length is returned, regardless 
whether the user specified MSG_TRUNC.

Exploring this topic I learned a lot about browsing and reading kernel 
source code, e.g. that commit statements have to be taken with a grain of 
salt :-)

Cheers
Daniel
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