Re: Problem with unix sockets: SOCK_DGRAM ignores MSG_TRUNC

[David Miller <davem@davemloft.net>]

From: Daniel Kabs <dkabs@mobotix.com>
Date: Mon, 29 Jan 2007 12:59:49 +0100

> 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.

What part of "Only valid for packet sockets" from the manual page
escapes you?  :-))

It's a feature which only was meant to be valid for AF_PACKET sockets.

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 checked the history and our AF_UNIX sockets have always behaved like
this.