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 = 0;

    while (buf_len > 1)
    {
        checksum += *buf++;
        buf_len -= sizeof(unsigned short);
    }

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

    checksum = (checksum >> 16) + (checksum & 0xffff);
    checksum += (checksum >> 16);

    return (unsigned short)(~checksum);
}

int main(int argc, char **argv)
{
    struct data_sum msg = { "adddddaa", 0 };
    int length = sizeof(msg);

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

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

    msg.data[0]='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
", msg.checksum);

    return 0;
}