IP地址和子网掩码获得子网范围

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

static void ip_mask(char *net, int *netmask) {
    char *p;

    /* 找出第一个'/'所在字符串中的位置，截断，
     前面的字符串作为点分十进制的IP地址，
     后面的子网掩码转换为整形 */
    if ((p = strchr(net, '/')) != NULL)
        *p = '\0';
    *netmask = atoi(p+1);
}

static u_int32_t dton(u_int32_t mask) {
    u_int32_t i, c;
    int bits = sizeof(u_int32_t) * 8;

    i = ~0;
    bits -= mask;
    /* 让32位全是1的无符号数与左移bits位(右移位用0填充)得出子网掩码 */
    i <<= bits;

    return htonl(i);
}

int main(int argc, char **argv) {
    int err = 0;
    char ip[32];
    char filename[128] = {0};
    char line[128] = {0};
    u_int32_t mask, netmask;
    struct in_addr ipadd, subnet, broadcast;
    FILE *in, *out;
    int len = 0;

    if (argc >= 2) {
        strncpy(filename, argv[1], sizeof(filename)-1);
    } else {
        fprintf(stderr, "Usage: %s \n", argv[0]);
        exit(1);
    }

    if((in = fopen(filename, "r")) == NULL)
        printf("%s open failure!\n", filename), exit(-1);

    if((out = fopen("ip_out.txt", "w")) == NULL)
        printf("%s open failure!\n", filename), exit(-1);


    while((fgets(ip, 32, in)) != NULL) {

        ip_mask(ip, &mask);

        if (!(err = inet_aton(ip, &ipadd))) {
            fprintf(stderr, "IP type error!\n");
            exit(2);
        }

        if (mask > 32 || mask < 1) {
            fprintf(stderr, "Mask is out of range!\n");
            exit(3);
        }

        netmask = dton(mask);

        /* 网络地址=主机地址和掩码的与运算 */
        subnet.s_addr = ipadd.s_addr & netmask;
        /* 广播地址=网络地址和掩码的同或运算(C语言中没有同或运算, 用异或取反)*/
        broadcast.s_addr = ~(subnet.s_addr ^ netmask);

        /* 因为inet_ntoa函数不可重入，需要两次构建这个字符串
            len为第一次构建的偏移位，第二次构建字符串时跳过这个偏移位
            相应写入长度也减少这个偏移位长度 */
        len = snprintf (line, sizeof(line), "%s - ", inet_ntoa(subnet));
        snprintf (line + len, sizeof(line) - len, "%s\n", inet_ntoa(broadcast));
        fputs(line, out);

    }
    fclose(in);
    fclose(out);
    return 0;
}