Winsock 入门 计算校验和 示例

 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 }