String: Convert String To Integer atoi

Problem: Implement atoi function in C language and give the test cases. atoi function converts a string to an integer. The function prototype is as follows:

         int my_atoi(const char str[]);

Solution: The key is to be able to find out the int value of a numeric character.
ASCII value of a numeric character - ASCII value of the character '0' = int value of numeric character.
Ex: '8' - '0' = 8

Code:

#define MIN_INT -2147483648
#define MAX_INT 2147483647

int my_atoi(const char str[])
{
    if(str == NULL) return 0;
    
    int len = strlen(str);
    
    if(len <= 0) return 0;

    int index = 0;
    
    //skip leading spaces
    while(str[index] == ' ') index++;

    bool isNeg = str[index] == '-';
    int outNum = 0;

    if(isNeg)
    {
        index++;
        // skip white space after the sign
        while(str[index] == ' ') index++;
    }

    while(index < len)
    {
        char currentChar = str[index++];
        if(currentChar >= '0' && currentChar <= '9')
        {
            int oldValue = outNum;
            int charVal = currentChar - '0';
            outNum *= 10;
            outNum += charVal;

            //overflow underflow detection
            if(outNum < oldValue)
            {
                if(isNeg)
                    outNum = MIN_INT;
                else
                    outNum = MAX_INT;
                return outNum;
            }
        }
        else
            break;
    }
    if(isNeg)
        outNum = outNum * -1;
    return outNum;
}

atoi Test Cases:

Input         : Output
""            : 0
"0"           : 0
"1"           : 1
"-1"          : -1
"10"          : 10
"-10"         : -10
"1234567890"  : 1234567890
"23 45"       : 23
" 99"         : 99
" -66"        : -66
"- 77"        : -77
"55 "         : 55
"-2147483648" : -2147483648 (MIN)
"2147483647"  : 2147483647  (MAX)
"2147483648"  : 2147483647   (overflow)
"-2147483649" : -2147483648  (underflow)
"abc*"        : 0
"23ab"        : 23
"23ab34"      : 23
"b31"         : 0