Problem with unix sockets: SOCK_DGRAM ignores MSG_TRUNC

[Daniel Kabs <dkabs@mobotix.com>]

Hi,

I use unix domain datagram sockets for IPC, e.g. I receive messages by 
calling recv().

"man 2 recv" 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 think, the function should 
return the real message length instead (at least according to the manual 
page).

To work around this problem, I now 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? What is the correct semantics 
of MSG_TRUNC when used for unix sockets?

Cheers
Daniel Kabs

PS: According to some lkml archives, my previous mail did not come 
through, so I reposted it.
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