Thread: Few questions about header file...

I think you will find this helpful on what object files are and how they are made:
https://www.cs.umd.edu/class/spring2.../makefile.html

The #ifndef wrapping code is called a header guard, it keeps things from being declared more than once. It's basically a "one shot" shield, equivalent to something like :

Code:

if identifier doesn't exist {
    Create identifier
    ... More Code ...
}

The first time the compiler reads the file, the identifier doesn't exist, and so it runs the code. During the course of running the code it defines the identifier (the line directly after, #define FOO_H, the identifier could be any unique valid identifier). The next time it reads the file the identifier is already defined, so it doesn't cause the declarations you have made in the code to be declared again.