Multiple definitions due to multiple includes of same header file

Question

Hello,

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.

Questions is :

1.) 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??

2.) #pragma once does not work on this particular compiler, so what is the solution. Someone had posted this answer to a similar question. It doesnt seem to work for me. How does this solution work?

wishes,

-AD

Answer 1

Using a multiple inclusion guard prevents compiler errors, but you're getting a linker error. Do you have data definitions in the header file that don't use extern?

Answer 2

Using include guards prevents one compilation unit from including the header twice. E.g. if header B.h includes A.h and B.cpp includes A.h and B.h, everything from A.h would be declared twice in the compilation B.cpp if you weren't using include guards.

Your include guards prevent this from happening, all's fine till now.

But you get multiple definitions at link time, i.e. two compilation units define the same thing, this probably means you got a real definition in your header, use extern for all variables, make sure functions are either inline or are defined in the cpp file.

Answer 3

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;

Answer 4

If the functions aren't large, you can use "inline" before them and the linker won't complain.