#include <stdio.h>
#include <errno.h>
// ntoh... & hton...
#include <arpa/inet.h>
#include <netinet/ip.h>

//
// the original header uses
//   unsigned int ihl:4;
//   unsigned int version:4;
//
struct my_iphdr
  {
#if __BYTE_ORDER == __LITTLE_ENDIAN
    u_int8_t ihl:4;
    u_int8_t version:4;
#elif __BYTE_ORDER == __BIG_ENDIAN
    u_int8_t version:4;
    u_int8_t ihl:4;
#else
# error	"Please fix <bits/endian.h>"
#endif
    u_int8_t tos;
    u_int16_t tot_len;
    u_int16_t id;
    u_int16_t frag_off;
    u_int8_t ttl;
    u_int8_t protocol;
    u_int16_t check;
    u_int32_t saddr;
    u_int32_t daddr;
    /*The options start here. */
  }__attribute__((__packed__));
  
void printbuf(u_int8_t *p,int len) {
    for (int i=0;i<len;i++) {
	printf("%02x ",p[i]);
	if ((i & 7) == 7)
	    printf("\n");
    }
    printf("\n");
}

int main(int argc,char **argv) {
    struct my_iphdr test;
    struct iphdr test2;
    
    printf("while my_iphdr is %d bytes long\n",
	    sizeof(struct my_iphdr));
    
    u_int8_t *p1 = (u_int8_t *)&test;
    test.version  = 4;
    test.ihl      = 5;
    test.tos      = 1;
    test.tot_len  = htons(2);
    test.id       = htons(3);
    test.frag_off = htons(4);
    test.ttl      = 5;
    test.protocol = 6;
    test.check    = htons(7);
    test.saddr    = htonl(8);
    test.daddr    = htonl(9);
    printf("my_iphdr: the first byte: 0x%02x\n",*(u_int8_t *)&test);
    u_int8_t *p2 = (u_int8_t *)&test.tos;
    printf("          tos      is as offset %d\n",p2-p1);
    p2 = (u_int8_t *)&test.tot_len;
    printf("          tot_len  is as offset %d\n",p2-p1);
    p2 = (u_int8_t *)&test.id;
    printf("          id       is as offset %d\n",p2-p1);
    p2 = (u_int8_t *)&test.frag_off;
    printf("          frag_off is as offset %d\n",p2-p1);
    p2 = (u_int8_t *)&test.ttl;
    printf("          ttl      is as offset %d\n",p2-p1);
    p2 = (u_int8_t *)&test.protocol;
    printf("          protocol is as offset %d\n",p2-p1);
    p2 = (u_int8_t *)&test.check;
    printf("          check    is as offset %d\n",p2-p1);
    p2 = (u_int8_t *)&test.saddr;
    printf("          saddr    is as offset %d\n",p2-p1);
    p2 = (u_int8_t *)&test.daddr;
    printf("          daddr    is as offset %d\n",p2-p1);
    printf("\n");
    printbuf((u_int8_t *)&test,sizeof(test));
    printf("\n");
    
    printf("the iphdr is %d bytes long\n",
	    sizeof(struct iphdr));
    test2.version  = 4;
    test2.ihl      = 5;
    test2.tos      = 1;
    test2.tot_len  = htons(2);
    test2.id       = htons(3);
    test2.frag_off = htons(4);
    test2.ttl      = 5;
    test2.protocol = 6;
    test2.check    = htons(7);
    test2.saddr    = htonl(8);
    test2.daddr    = htonl(9);
    p1 = (u_int8_t *)&test2;
    printf("iphdr:    the first byte: 0x%02x\n",*(u_int8_t *)&test2);
    p2 = (u_int8_t *)&test2.tos;
    printf("          tos      is as offset %d\n",p2-p1);
    p2 = (u_int8_t *)&test2.tot_len;
    printf("          tot_len  is as offset %d\n",p2-p1);
    p2 = (u_int8_t *)&test2.id;
    printf("          id       is as offset %d\n",p2-p1);
    p2 = (u_int8_t *)&test2.frag_off;
    printf("          frag_off is as offset %d\n",p2-p1);
    p2 = (u_int8_t *)&test2.ttl;
    printf("          ttl      is as offset %d\n",p2-p1);
    p2 = (u_int8_t *)&test2.protocol;
    printf("          protocol is as offset %d\n",p2-p1);
    p2 = (u_int8_t *)&test2.check;
    printf("          check    is as offset %d\n",p2-p1);
    p2 = (u_int8_t *)&test2.saddr;
    printf("          saddr    is as offset %d\n",p2-p1);
    p2 = (u_int8_t *)&test2.daddr;
    printf("          daddr    is as offset %d\n",p2-p1);
    printf("\n");
    printbuf((u_int8_t *)&test2,sizeof(test2));
    printf("\n");
}