Linking issue with “multiple definition of” compilation error

Question

I have the following "constants" header:

/* constants.h */

#ifdef __cplusplus 
extern "C" {
#endif

#pragma once

#ifndef CONSTANTS_H
#define CONSTANTS_H

const char * kFoo = "foo";
const char * kBar = "bar";

#endif

#ifdef __cplusplus
}
#endif

I am #include -ing this header in files Xc and Yc .

Note that I am not including this in Xh or Yh .

The files Xc and Yc get compiled into object files which are archived into a static library called libXY.a .

When I include Xh and Yh in Zh , and when I link to libXY.a , I cannot compile Zc without errors:

/* Z.h */

#include "X.h"
#include "Y.h"

I get the following compilation errors when trying to compile Zc :

/path/to/libXY.a(X.o):(.data+0x0): multiple definition of `kFoo`
/path/to/libXY.a(Y.o):(.data+0x0): first defined here
/path/to/libXY.a(X.o):(.data+0x8): multiple definition of `kBar`
/path/to/libXY.a(Y.o):(.data+0x8): first defined here

I have tried setting kFoo and kBar to extern , but that does not help.

How would I resolve multiple definitions, when I am only including the constants once (via the header guard #ifndef CONSTANTS_H )?

Answer 1

How would I resolve multiple definitions, when I am only including the constants once (via the header guard #ifndef CONSTANTS_H)?

With this in constants.h :

const char * kFoo = "foo";

a definition for kFoo will be emitted in every translation that #include s constants.h . Thus, multiple definitions, which then result in link errors.

As asaelr noted (+1), you would solve it like this:

constants.h

extern const char* const kFoo;

constants.c

const char* const kFoo = "foo";

(note that i also made the pointer const, which is usually what you want to do in this case)

Answer 2

You should not define variables in header file. define them in one of the source files, and declare them ( extern ) in the header file.

(You wrote "I have tried setting kFoo and kBar to extern, but that does not help." I guess that you didn't define them in a source file)