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

Take a look at how the standard library deals with this problem:

long  strtol(const  char  * restrict str,  char **restrict endptr, int base);

Here, after the call the endptr points at the first character that could not be parsed. If endptr == str, then no characters were converted, and this is a problem.