On Monday 05 February 2007 01:52, David Miller wrote: > > 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.
Thanks for pointing that out. I thought, "packet sockets" means "datagram-oriented socket". Obviously it means PF_PACKET instead. It's hard to learn programming IPC by only reading man pages :-) > 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. Please correct me gently if I am wrong: According to the kernel source code, this behaviour was introduced with patch-2.6.8: http://www.kernel.org/diff/diffview.cgi?file=/pub/linux/kernel/v2.6/patch-2.6.8.gz;z=4325 linux/net/ipv4/udp.c + err = copied; + if (flags & MSG_TRUNC) + err = skb->len - sizeof(struct udphdr); I think local sockets (PF_UNIX) implement a different semantics: According to unix_dgram_recvmsg() in linux/net/unix/af_unix.c, in case of truncation the buffer size is returned and not the full packet length. In my opinion this implementation of local sockets (PF_UNIX) is not following the wording of the man page man 2 recv: "These calls return the number of bytes received". Am I getting something wrong here? Why not improve consistency and make unix_dgram_recvmsg() return the full packet length? So it would behave as UDP does. What do you think about adding the following code to linux/net/unix/af_unix.c: err = size; + if (flags & MSG_TRUNC) + err = skb->len; Cheers Daniel - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
