Re: bytes received from recvmsg doesn't match FIONREAD

[David Miller <davem@davemloft.net>]

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.