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;  
}