#include <stdio.h>

   #include <unistd.h>

   #include <linux/if_ether.h>

   #include <linux/ip.h>

   #include <linux/udp.h>

   #include <linux/types.h>

   unsigned short check_sum(unsigned char *data, int len);

  int main()

  {

          unsigned char data[] = {

          0x45, 0x00,

          0x00, 0x20, 0x00, 0x00, 0x40, 0x00, 0x40, 0x11,

          0x00, 0x00, 0xc0, 0xa8, 0x1f, 0x72, 0xc0, 0xa8,

          0x1f, 0x7a

          };

          unsigned short ret = ;

          ret = check_sum(data, );

          printf("check sum is %x\n", htons(ret));

  }

  unsigned short check_sum(unsigned char *data, int len)

  {

          unsigned short ret = ;

          int i = ;

          int sum = ;

          unsigned short *p = (short *)data;

          for(i=; i<len/; i++)

                  sum = sum + ntohs(p[i]);

          if(sum > 0xffff){

                  sum = (sum & 0x0ffff) + ((sum & 0xf0000) >> );

          }

          ret = (short)sum;

          return ~ret;

  }