Winsock 入门 计算校验和 示例

 #include <stdio.h>
 #include <string.h>

 #define DATA_MAX_LEN    14   /* 最大数据长度 */

 struct data_sum
 {
     char data[DATA_MAX_LEN]; /* 数据 */
     unsigned short checksum; /* 校验和 */
 };

 unsigned short ip_checksum(unsigned short *buf, int buf_len)
 {
     unsigned long checksum = ;

     while (buf_len > )
     {
         checksum += *buf++;
         buf_len -= sizeof(unsigned short);
     }

     if (buf_len)
     {
         checksum += *(unsigned char *)buf;
     }

     checksum = (checksum >> ) + (checksum & 0xffff);
     checksum += (checksum >> );

     return (unsigned short)(~checksum);
 }

 int main(int argc, char **argv)
 {
     struct data_sum msg = { "adddddaa",  };
     int length = sizeof(msg);

     printf("length = %d\n",length);

     /* checksum = 0, 计算校验和 */
     msg.checksum = ip_checksum((unsigned short *)&msg, length);
     printf("Calculate check sum : 0x%x\n", msg.checksum);

     /* 包含 checksum 值, 验证校验和 */
     msg.checksum = ip_checksum((unsigned short *)&msg, length);
     printf("Verify check sum : 0x%x\n", msg.checksum);

     msg.data[]='w';
         /* 包含 checksum 值, 验证校验和 因为改变了其值，校验和不为0 */
     msg.checksum = ip_checksum((unsigned short *)&msg, length);
     printf("Verify check sum : 0x%x\n", msg.checksum);

     return ;
 }