c - What is the best way to return an error from a function when I'm already returning a value?

Question

I wrote a function in C that converts a string to an integer and returns the integer. When I call the function I also want it to let me know if the string is not a valid number. In the past I returned -1 when this error occurred, because I didn't need to convert strings to negative numbers. But now I want it to convert strings to negative numbers, so what is the best way to report the error?

In case I wasn't clear about this: I don't want this function to report the error to the user, I want it to report the error to the code that called the function. ("Report" might be the wrong word to use...)

Here's the code:

s32 intval(const char *string) {
    bool negative = false;
    u32 current_char = 0;

    if (string[0] == '-') {
        negative = true;
        current_char = 1;
    }

    s32 num = 0;
    while (string[current_char]) {
        if (string[current_char] < '0' || string[current_char] > '9') {
            // Return an error here.. but how?
        }

        num *= 10;
        num += string[current_char] - '0';
        current_char++;
    }

    if (negative) {
        num = -num;
    }

    return num;
}

Answer 1

There are several ways. All have their pluses and minuses.

• Have the function return an error code and pass in a pointer to a location to return the result. The nice thing about this there's no overloading of the result. The bad thing is that you can't use the real result of the function directly in an expression.

  Evan Teran suggested a variation of this that has the caller pass a pointer to a success variable (which can be optionally NULL if the caller doesn't care) and returns the actual value from the function. This has the advantage of allowing the function to be used directly in expressions when the caller is OK with a default value in an error result or knows that the function cannot fail.

• Use a special 'sentinel' return value to indicate an error, such as a negative number (if normal return values cannot be negative) or INT_MAX or INT_MIN if good values cannot be that extreme. Sometimes to get more detailed error information a call to another function (such as GetLastError()) or a global variable needs to be consulted (such as errno). This doesn't work well when your return value has no invalid values, and is considered bad form in general by many people.

  An example function that uses this technique is getc(), which returns EOF if end of file is reached or an error is encountered.

• Have the function never return an error indication directly, but require the caller to query another function or global. This is similar to how VB's "On Error Goto Next" mode works - and it's pretty much universally considered a bad way to go.

• Yet another way to go is to have a 'default' value. For example, the atoi() function, which has pretty much the same functionality that your intval() function, will return 0 when it is unable to convert any characters (it's different from your function in that it consumes characters to convert until it reaches the end of string or a character that is not a digit).

  The obvious drawback here is that it can be tricky to tell if an actual value has been converted or if junk has been passed to atoi().

  I'm not a huge fan of this way to handle errors.

I'll update as other options cross my mind...