Why can't I declare and assign global variable in a header file, even when using #ifndef HEADER_H

Question

I saw some similar questions, but in none of them, the #ifndef HEADER_H was mentioned.

I have a header file and 2 C files: constants.h main.c mylib.c

In constants.h:

#ifndef CONSTANTS_H
#define CONSTANTS_H

const int NUM_OF_ITEMS = 22;

#endif

In mylib.c:

#include "constants.h"
... code ...

In main.c:

#include "constants.h"

int main() {
    ... code ...
}

When I compile using the command: gcc main.c mylib.c -o main, I get the following error:

/tmp/ccl55fv3.o:(.rodata+0x0): multiple definition of `NUM_OF_ITEMS'
/tmp/ccyZhu6F.o:(.rodata+0x0): first defined here
collect2: ld returned 1 exit status

• I mentioned #ifndef, so why it happens??
• Is there anything to do except for splitting it to constants.h for declaration and constants.c for assignment?

Answer 1

Each translation unit (read .c source file) into which you include the header file results in a new definition of the global variable. That is why the linker objects to there being multiple definitions. Remember that #include performs text insertion of the included file. From the perspective of the compiler, each of your translation units contains distinct definitions of NUM_OF_ITEMS.

You need to define it in exactly one translation unit. You might consider adding a constants.c which contained the definition of NUM_OF_ITEMS, and leaving just a declaration in the header file.

Alternatively you could use a macro:

#define NUM_OF_ITEMS 22

Or as Jens suggests in the comment below, an enumeration constant.

Or as you yourself suggest, with static linkage so that the object is internal to each translation unit.