判断IP地址是否合法

/* return 1 if string contain only digits, else return 0 */
int valid_digit(char *ip_str)
{
    while (*ip_str) {
        if (*ip_str >= '0' && *ip_str <= '9')
            ++ip_str;
        else
            return 0;
    }
    return 1;
}  

/* return 1 if IP string is valid, else return 0 */
int is_valid_ip(char *ip_str)
{
    int i, num, dots = 0;
    char *ptr;  

    if (ip_str == NULL)
        return 0;  

    // See following link for strtok()
    // http://pubs.opengroup.org/onlinepubs/009695399/functions/strtok_r.html
    ptr = strtok(ip_str, DELIM);  

    if (ptr == NULL)
        return 0;  

    while (ptr) {  

        /* after parsing string, it must contain only digits */
        if (!valid_digit(ptr))
            return 0;  

        num = atoi(ptr);  

        /* check for valid IP */
        if (num >= 0 && num <= 255) {
            /* parse remaining string */
            ptr = strtok(NULL, DELIM);
            if (ptr != NULL)
                ++dots;
        } else
            return 0;
    }  

    /* valid IP string must contain 3 dots */
    if (dots != 3)
        return 0;
    return 1;
}