ip_check_sum problem

[Norka Lucena]

Hi,

I am having problems computing the IP checksum with the th ip_fast_csum.
I would REALLY appreciate if any of you can help me.

This is the function I am using to compute the IP checksum.

void computeIPChecksum(struct iphdr* ip) {
  printk("*** IP checksum (before) : %d\n", ip->check);
  
  ip->check = 0;  /* it must be pre-zero'd */
  ip->check = ip_fast_csum((unsigned char *)ip, ip->ihl);

  printk("*** IP checksum (after)  : %d\n", ip->check);
}
 
Please notice that because I am not modifying the packet at all yet, I
should 
get the same result. Well, I don't. I have no clue why. 
Any thoughts?

Please, reply directly to [e-mail].
Thank you so much, in advance,

Norka