bytes received from recvmsg doesn't match FIONREAD

bytes received from recvmsg doesn't match FIONREAD

[Steven Dake]

I wanted to verify that the size of a multicast UDP message received
with recvmsg matches the size of the message the kernel thinks the
message is.

So I went about using the FIONREAD ioctl as follows:

res = ioctl (fd, FIONCREAD, &value);
assert (res != -1);
bytes_received = recvmsg (fd, &msg_recv, MSG_NOSIGNAL | MSG_DONTWAIT);
assert (bytes_received == value);

It appears the value and bytes_received do not match after many
thousands of runs with a UDP multicast protocol (www.openais.org).
(neither is -1)

Am I using this ioctl improperly?  Shouldn't I expect that the full
datagram is returned by recvmsg no matter the state of the blocking or
nonblocking mode of the file descriptor?

pls copy me on responses - i am not onlist.

Regards
-steve

Re: bytes received from recvmsg doesn't match FIONREAD

[David Miller]

From: Steven Dake <sdake@redhat.com>
Date: Thu, 11 Oct 2007 16:08:15 -0700

> I wanted to verify that the size of a multicast UDP message received
> with recvmsg matches the size of the message the kernel thinks the
> message is.
> 
> So I went about using the FIONREAD ioctl as follows:
> 
> res = ioctl (fd, FIONCREAD, &value);
                   ^^^^^^^^^ (typo? should be FIONREAD not FION_C_READ)
> assert (res != -1);
> bytes_received = recvmsg (fd, &msg_recv, MSG_NOSIGNAL | MSG_DONTWAIT);
> assert (bytes_received == value);

I think you want to use SIOCINQ, FIONREAD is only valid on files.

I'm surprised you didn't get an error return from ioctl() as the
VFS code seems to enforce this.

Re: bytes received from recvmsg doesn't match FIONREAD

[David Stevens]

Questions of this sort should normally be directed to linux-net mailing 
list.

>From the code you quoted, I see at least one case where it
will fail -- when the allocated buffer you pass to recvmsg is smaller than
"value" (ie. the datagram is too big for the read buffer).

If that's not the problem, I suggest you reproduce it with a small test
case and post real code and results when it happens.

                                                +-DLS