atio (考虑溢出问题)

#include "stdio.h"
#include "math.h"
long int atoi(char * str)
{
    unsigned long int intNum = 0;
    unsigned long int maxNum = (long int)pow(2,(int)(8 * sizeof(long int) -1 )) -1 ;
    int sign = 1;
    if(NULL == str)
    {
        return 0;
    }
    while (' ' == *str || '\t' == *str)
    {
        str++;
    }
    if( '-' == *str )
    {
        sign = -1;
        str++;
    } 
    else if( '+' == *str)
    {
        str++;
    }
    while(*str >= '0' && *str <= '9' )
    {
        intNum = intNum *10 + (*str - '0');
        if(-1 == sign && intNum > maxNum + 1)
        {
            return sign*(maxNum+1);
        }
        else if(1 == sign && intNum > maxNum )
        {
            return sign*maxNum;
        }
        else 
        {
            str++;
        }
        
    }
    return sign * intNum;
}
int main()
{
    char str[] = "        -01234567989999999x99999999999999";
    printf("%d",atoi(str));
    return 0;
}