How to use a function from a header file without including the header in C?

Question

Suppose I have a header file abc.h which defines a function foo(). How can I use the function foo() in the header file abc.h without using #include<abc.h> in my program?

I could come up with the following solutions

• Copy the function from the header file into the program manually. But this would fail for most of the cases where the function definition has been pre-compiled into object files. Because in that case copy pasting function prototype won't do any good because unresolved references to the function couldn't be resolved.
• Don't do anything in case of standard libraries as IDE can take care of that. But this is obviously undefined behaviour and would depend on IDE. Also for non-standard libraries it would surely fail.

So how can I use a function in a header file(not necessarily from the standard library) without including the header file?

This was an interview question so it is essentially a trick question. Not real use.

Answer 1

I believe the key word here is "defined" in the header file, not just declared. The header would have to be compiled to an object. To use it, you'd "declare" the function in your source and link with the object file.

That said, I hate interview questions like this since you're never really sure if that's what they are looking for because no one in their right mind would do this in practice.

Answer 2

But this would fail for most of the cases where the header has been pre-compiled into object files

This sentence makes no sense. A header can be used to compile an object file, but they never get compiled into object files; they are always used externally.