1 #include <stdio.h>
2 #include <string.h>
3
4 #define DATA_MAX_LEN 14 /* 最大数据长度 */
5
6 struct data_sum
7 {
8 char data[DATA_MAX_LEN]; /* 数据 */
9 unsigned short checksum; /* 校验和 */
10 };
11
12 unsigned short ip_checksum(unsigned short *buf, int buf_len)
13 {
14 unsigned long checksum = 0;
15
16 while (buf_len > 1)
17 {
18 checksum += *buf++;
19 buf_len -= sizeof(unsigned short);
20 }
21
22 if (buf_len)
23 {
24 checksum += *(unsigned char *)buf;
25 }
26
27 checksum = (checksum >> 16) + (checksum & 0xffff);
28 checksum += (checksum >> 16);
29
30 return (unsigned short)(~checksum);
31 }
32
33 int main(int argc, char **argv)
34 {
35 struct data_sum msg = { "adddddaa", 0 };
36 int length = sizeof(msg);
37
38 printf("length = %d\n",length);
39
40 /* checksum = 0, 计算校验和 */
41 msg.checksum = ip_checksum((unsigned short *)&msg, length);
42 printf("Calculate check sum : 0x%x\n", msg.checksum);
43
44 /* 包含 checksum 值, 验证校验和 */
45 msg.checksum = ip_checksum((unsigned short *)&msg, length);
46 printf("Verify check sum : 0x%x\n", msg.checksum);
47
48 msg.data[0]='w';
49 /* 包含 checksum 值, 验证校验和 因为改变了其值，校验和不为0 */
50 msg.checksum = ip_checksum((unsigned short *)&msg, length);
51 printf("Verify check sum : 0x%x\n", msg.checksum);
52
53 return 0;
54 }
