linux-kernel - unix(7) and MSG

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Message-Id: <200701111541.03667.dkabs@mobotix.com>
Date:	Thu, 11 Jan 2007 15:41:03 +0100
From:	Daniel Kabs <dkabs@...otix.com>
To:	linux-kernel@...r.kernel.org
Subject: unix(7) and MSG_TRUNC semantics

Hello!

For IPC, I use unix domain datagram sockets. I receive messages by calling 
recv(). The man page recv(2) tells me about the flags argument to a recv 
call, namely:
 MSG_TRUNC
      Return  the  real  length of the packet, even when it was longer
      than the passed buffer. Only valid for packet sockets.
Thus I used recv() with flags MSG_TRUNC|MSG_PEEK in order to detect 
message truncation due to insufficient buffer size.

Strangely enough, MSG_TRUNC seems to get ignored by the kernel: If the 
message received is larger than the receive buffer I supplied, the 
function returns the size of the buffer. I reckon, the function should 
return the real message size instead.

To work around this problem, I use the ioctl FIONREAD instead.

On the other hand, in this mailing list, I found an old bug report 
describing the same problem using UDP sockets:

http://groups.google.com/group/fa.linux.kernel/browse_frm/thread/fb6acbb527507e26/ad0b2ba33b6b66fa

UDP sockets seem to have been patched by now. From linux/net/ipv4/udp.c:
 udp_recvmsg()
  ...
        err = copied;
        if (flags & MSG_TRUNC)
                err = skb->len - sizeof(struct udphdr);
   ...

Why doesn't unix_dgram_recvmsg() in linux/net/unix/af_unix.c contain code 
to this effect? Is this a feature or a bug?

Cheers
Daniel Kabs
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