in_cksum

[loukopoulos klearchos]

I am writting a program with raw sockets.
the headers I 've included are:

#include <stdio.h>
#include<stdlib.h>

#include<linux/ip.h>
#include<linux/icmp.h>
#include<netinet/in.h>
#include<sys/types.h>
#include<sys/socket.h>

and I use in_cksum once

when linking I see
undefined reference to `in_cksum'

???????????????????????????????
why ?
where is in_cksum defined ??????


thank U