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 num

Answer 1

In general I prefer the way Jon Skeet proposed, ie. returning a bool (int or uint) about success and storing the result in a passed address. But your function is very similar to strtol, so I think it is a good idea to use the same (or similar) API for your function. If you give it a similar name like my_strtos32, this makes it easy to understand what the function does without any reading of the documentation.

EDIT: Since your function is explicitly 10-based, my_strtos32_base10 is a better name. As long as your function is not a bottle-neck you can then, skip your implementation. And simply wrap around strtol:

s32
my_strtos32_base10(const char *nptr, char **endptr)
{
    long ret;
    ret = strtol(nptr, endptr, 10);
    return ret;
}

If you later realize it as an bottleneck you can still optimize it for your needs.