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 X.c and Y.c.

Note that I am not including this in X.h or Y.h.

The files X.c and Y.c get compiled into object files which are archived into a static library called libXY.a.

When I include X.h and Y.h in Z.h, and when I link to libXY.a, I cannot compile Z.c without errors:

/* Z.h */

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

I get the following compilation errors when trying to compile Z.c:

/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 #includes 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)