in_cksum() for ip packets with multiple mbufs

Giorgos Keramidas keramida at
Sun Oct 23 05:06:20 PDT 2005

On 2005-10-23 04:13, kamal kc <kamal_ckk at> wrote:
> /* the argument m is the (struct mbuf *) that
>  * contains the packet data
>  */
> void copy_the_memorybuffer(struct mbuf **m)
> {
>    struct mbuf *mbuf_pointer=*m;
>    struct mbuf **next_packet;
>    next_packet=&mbuf_pointer;
>    struct ip *my_ip_hdr;
>    my_ip_hdr=mtod((*next_packet),struct ip *);
>    my_ip_hdr->ip_tos=64;
>    my_ip_hdr->ip_sum=0;
>    my_ip_hdr->ip_sum=
>        in_cksum((*next_packet),(my_ip_hdr->ip_hl<<2));
>   .......
> }

Why all this pointer fun?  How do you know that it's safe to dereference
`m' when you do:

	struct mbuf *mbuf_pointer=*m;

Why are you dereferencing `m' once to obtain mbuf_pointer and then
taking the address of that to obtain next_packet, when you could
just do:

	next_packet = m;

There are also several other problems with copy_the_memorybuffer() as
it's written above:

    - It's considered bad style to mix declarations and code the way you
      have done above

    - It is probably better to return the copy of the mbuf you're
      fiddling with, instead of modifying in place a parameter of the

    - If you are not *REALLY* copying the data of the mbuf, then
      the name of copy_the_memorybuffer() is very confusing.

    - What is the magic constant 64 that is assigned to ip_tos?

What you probably want to do is something like:

    ip_set_type_of_service(struct mbuf *m)
        struct ip *iph;
        assert(m != NULL);
        iph = mtod(m, struct ip *);
        iph->ip_tos = IPTOS_PREC_IMMEDIATE;
        iph->ip_sum = 0;
        iph->ip_sum = in_cksum((uint16_t *)iph, iph->ip_hl << 2);

but that's not copying anything.

> but still it doesn't seem to work. and the problem
> is still there.

You have obviously made a lot of changes that we haven't seen yet.
Instead of posting snippets here and there, save a copy of the original
sources somewhere, then a copy of the new sources, and run diff(1) on
the two directories to extract *ALL* the changes.

	$ cd /usr/src
	$ diff -ruN src.old/ src/ > /tmp/patchfile

and put the patchfile somewhere online where we can have a look at all
the changes.

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