From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: bytes received from recvmsg doesn't match FIONREAD
Date: Thu, 11 Oct 2007 16:35:06 -0700 (PDT)
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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.