IP首部校验和--反码算术运算求和

#include
#include

typedef struct hmw_ip_header_s {
    unsigned ver : 4;
    unsigned header_len : 4;
    unsigned char differ_service;
    unsigned short int total_len;
    unsigned int flag;
    unsigned fflag : 3;
    unsigned offset : 13;
    char ttl;
    char protcol;
    short int check_sum;
    unsigned long src_ip;
    unsigned long dest_ip;
}hmw_ip_header_t;

typedef struct packet_s {
    char str[256];
    short int check_sum;
}packet_t;

/**
 * hmw_anti_code_sum - 反码算术运算求和  Anti-code operator sum
 *
 * @str:检验内容
 * @len:检验长度
 *
 * 即16位分别相加，然后把值取反 放入校验部分,len最好为2自己的倍数
 */
short int hmw_anti_code_sum(void *str, int len)
{  
    int i;
    short int *op1, ret;
    unsigned char *tmp_str;
    int real_len;

    printf("len = %d line %d\n", len, __LINE__);
   
    /* 把字符串弄成2字节的倍数的形式 */
    real_len = (len + 1) & ((~0ul) << 1);
    printf("real_len = %d\n", real_len);
    tmp_str = (unsigned char *)malloc(real_len);
    if (tmp_str == NULL) {
        printf("malloc failed!\n");
        return 0;
    }
    memset(tmp_str, 0, real_len);
    memcpy(tmp_str, str, len);
   
    ret = 0;
    for (i = 0; i < real_len; i += 2) {
        op1 = (short int *)(tmp_str + i);
        ret += *op1;
    }
   
    free(tmp_str);   
    ret = ~ret;
    printf("ret is %d\n", ret);
   
    return ret;
}

int main(void)
{
    packet_t test;
    int ret;

    memset(&test, 0, sizeof(test));
   
    printf("please input something!\n");
    scanf("%s", test.str);
    printf("str is %s\n", test.str);

    ret = hmw_anti_code_sum((unsigned char *)(&test), sizeof(test));
    test.check_sum = ret;

    if (hmw_anti_code_sum((unsigned char *)(&test), sizeof(test)) == 0) {
        printf("check ok!\n");
    } else {
        printf("check failed!\n");
    }

    return 0;
}