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

The os-style global errno variable is also popular. Use errno.h.

If errno is non-zero, something went wrong.

Here's a man page reference for errno.