If i include a header file, where does the header file ( or more correctly - the compiler ) look in to find the function definitions and stuff that are declared in the header file? Because header file is only for declaring things, the actual code must reside somewhere else? I know the standard libraries reside in the "Include" directory in the compiler's directory. But if i have a custom source file and a header file, i include the header file, but where should i put the custom source file so it can be found by the compiler? The Include dir? Anywhere else?