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

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.

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 doesn't seem to work for me. How does this solution work?

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; 

Header guards are only good for a single compilation unit, i.e., source file. If you happen to include a header file multiple times, perhaps because all headers included from main.c in turn include stdio.h then guards will help.

If you have the definition of a function f in x.h which is included by main.c and util.c, then it is like copying and pasting the definition of f into main.c when creating main.o and doing the same for util.c to create util.o. Then the linker will complain and this happens despite your header guards. Having multiple #include "x.h" statements in main.c is possible of course because of these guards.

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.

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

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?

Maybe X_H is already defined somewhere else? I just ran into this issue, where Xlib defines X_H in /usr/include/X11/X.h.

To check, you can call gcc -dM -E (if you are using gcc), e.g. in the buildsystem I’m using that works with CC=gcc CFLAGS="-dM -E" make. If the output file contains #define X_H even though you remove it from your file (use Y_H for example), then it is already defined outside your source code.