Hexadecimal string to Decimal integer conversion

17, Sep, 2010.
Question:
Given a string that contains hexadecimal number, write a function htoi() that returns the equivalent integer.

// Here is one function

int getValue(char const* hexString, int const radix)

{

    int         loopIndex;      // Loop iterator

    int         signFlag = 1;   // Keeps track of sign

    signed int  retValue = 0;   // Final value

    int         currentDigit;   // Current digit under process

    int         len = strlen(hexString);    // Lenght of string

    // Check for sign

    if(hexString[0] == '-')

    {

        // Advance the base

        hexString++;

        // Remove '-' from count

        len--;

        // Sing is -ve

        signFlag = -1;

    }

    // Iterate from left to right

    // Accumulate the weight of each digit upon occurance of next digit

    for(loopIndex = 0; loopIndex < len; loopIndex++)

    {

        currentDigit = toupper(hexString[loopIndex]) - ASCII_ZERO;

        // Is the digit between A through F?

        if(currentDigit > 9)

        {

            // Adjust numerical value

            currentDigit -= (ASCII_A - '9');

        }

        // Based on Horner's principle

        retValue = retValue * radix + currentDigit;

    }

    // Flip the sign, if required

    retValue *= signFlag;

    // This is the final value

    return retValue;

}
Examples:


getValue("1011", 2);    // Binary string to integer
getValue("12345", 10);  // Decimal string to integer
getValue("ABCDE", 16);  // Hexadecimal string to integer


Error handling is omitted.

1. Overflow and underflow (if string contains more than 10 characters on 32 bit machines, i.e. length = [32*log(2) + 1] digits in general, which is the characteristic).

2. Invalid String (containing unexpected characters).

3. String not terminated with NUL character.

4. String is NULL.

5. What if a kernel space buffer is passed as hex string?

6. Is it reentrant?

7. Who will be the consumers of the function, and possible ways of misusing it?

8. What changes are required if wide character buffer (Unicode) is passed (portability issue).

9. Any optimizations without compromising on the functionality.