前缀长度转成子网掩码

#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <string.h>
#include <stdlib.h>

int mask_fun(int mask_num)
{
        int i,j, a, b;
        unsigned char ipv6[16] = {0};
        unsigned char ipv6_b[16] = {0};
        unsigned char tmp_str[128] = {0};

        a = mask_num / 8;
        b = mask_num % 8;

        for(i = 0; i < a; i++) {
                ipv6[i] = 0xff;
        }
        for (j = 0; j < b; j++) {
                ipv6[i] |= 0x80 >> j;
        }

        for (i = 0; i < 16; i++){
                printf("ipv6[%d]:%x \n",15-i, ipv6[15-i]);
        }
        printf("\n===========================\n");
        inet_ntop(AF_INET6,ipv6, tmp_str, sizeof(tmp_str));
        printf("\nsubnet mask : %s \n", tmp_str);


        return 1;

}


int main(int argc, char * argv[])
{
        int mask_num;

        if(argc != 2) {
                printf("%s <mask_bit_num>\n", argv[0]);
                return -1;
        }

        mask_num = atoi(argv[1]);

        mask_fun( mask_num);
        return 1;
}