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/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
