判断一个ip地址合法性（基础c，不用库函数）

#include <stdio.h>
int judge(char *strIp);
int main()
{
    char a[];
    while()
    {
        scanf("%s", a);
        if( == judge(a))
        {
            printf("this is a correct ip\n");
        }
    }
}

int judge(char *strIp)
{
　　 if(NULL == strIp)
　　　{
　　　　　　return -1;
　　　}
    //IP must start with 0-9
    if(*strIp > '' || *strIp < '')
    {
        return -;
    }
    int num = ;
    int dotNum = ;
    do
    {
        if(*strIp >= '' && *strIp <= '')
        {
            num =  * num + *strIp - '';
        }
        else if(('.' == *strIp) || ('\0' == *strIp))
        {
            //filter the string including '..' or ending with '.'
            if('.' == *(strIp - ))
            {
                return -;
            }
            //filter the section number large than 255 or little than 0
            if(num <  || num > )
            {
                return -;
            }
            num = ;
            //record the number of right dots
            if('.' == *strIp)
            {
                dotNum++;
            }
        }
        else
        {
            //Can't have chars excluding '0' - '9' and '.'
            return -;
        }
    }while(*(strIp++))

    if( == dotNum)
    {
        // ip is correct
        return ;
    }
    else
    {
        //the string is too short for ip
        return -;
    }

}