From owner-freebsd-net  Wed Sep  5 13:28: 3 2001
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Date: Wed, 5 Sep 2001 13:59:58 -0700 (PDT)
From: Julian Elischer <julian@elischer.org>
To: "Ramasubramanian Ramamoorthy (EXT-NRC/Boston)" <EXT-Ramamoorthy.Ramasubramanian@nokia.com>
Cc: freebsd-net@freebsd.org
Subject: RE: Reg. mbuf structure
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On Wed, 5 Sep 2001, Ramasubramanian Ramamoorthy (EXT-NRC/Boston) wrote:

> Hi,
> I had mailed regarding extracting the same data from the skbuff
> structure in Linux and the mbuf structure in BSD. I am calculating the
> checksum in two machines, one running BSD and the other running Red Hat.
> I need to calculate the checksum at both the machines, for which I
> should be extracting the same data from these structures and send that
> to the algorithm, which calculates the checksum.
> 
> Can you tell me how we can confirm that we are extracting the same data
> from both these structures. It would be of great help, if anybody could
> help me in this regard soon.
> 

Do you know how much data you are supposed to be extracting?
Do you have an IP header or something on the front you want to 
ignore?
I assume this is a kernel module.

Here is a data extraction function from kern/uipc_mbuf.c
You can modify this function to checksum intead of copying..

or since this is always in the kernel, you could just call it.

/*
 * Copy data from an mbuf chain starting "off" bytes from the beginning,
 * continuing for "len" bytes, into the indicated buffer.
 */
void
m_copydata(m, off, len, cp)
        register struct mbuf *m;
        register int off;
        register int len;
        caddr_t cp;
{
        register unsigned count;

        while (off > 0) {
                if (off < m->m_len)
                        break;
                off -= m->m_len;
                m = m->m_next;
        }
        while (len > 0) {
                count = min(m->m_len - off, len);
                bcopy(mtod(m, caddr_t) + off, cp, count);
                len -= count;
                cp += count;
                off = 0;
                m = m->m_next;
        }
}






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