Why aren't my compile guards preventing multiple definition inclusions?

Question

I have a header file x.h which is included by more than one *.c source files. This header file has some structure variables defined.

I have put multiple inclusion prevention guard at the beginning of the header file as:

#ifndef X_H
#define X_H
...
..
//header file declarations and definitons.


#endif//X_H

On building I get linker errors related to multiple definitions. I understand the problem.

Won't a multiple inclusion prevention guard at the top of header file as I have, prevent multiple inclusions of the header file x.h and thereby avoid multiple definitions of the variables that are there in x.h?
#pragma once does not work on this particular compiler, so what is the solution? Someone had posted this answer to a similar question. It doesn't seem to work for me. How does this solution work?

@Stephen: If you are already editing the question title, please also have a look at the body (and maybe tags, though not here). This avoids duplicate editing (and bumping). — Paŭlo Ebermann
@Paulo Missed it, I've been trying to watch for it though, thanks for the reminder! — Stephen

Roddy Roddy · Accepted Answer · 2008-10-30T10:15:11

If the linker is complaining, it means you have definitions rather than just declarations in your header. Here's an example of things that would be wrong.

#ifndef X_H
#define X_H

int myFunc()
{
  return 42; // Wrong! definition in header.
}

int myVar; // Wrong! definition in header.

#endif

You should split this into source and header file like this:

Header:

#ifndef X_H
#define X_H

extern int myFunc();

extern int myVar; 

#endif

C Source:

int myFunc()
{
  return 42; 
}

int myVar;

Why aren't my compile guards preventing multiple definition inclusions?

6 Answers