Error when building c code with -D

Question

I get the following errors when I'm trying to build my c code files with -D. But, if I build it without -D, it works. I don't know why. Thanks. (My machine is ubuntu12.10, 32bit)

gcc c1.c c2.c -D DEBUG

/tmp/ccX04EIf.o:(.data+0x0): multiple definition of `g'
/tmp/cc0j9MoU.o:(.data+0x0): first defined here
collect2: ld returned 1 exit status

Here is my source code:

global.h

#ifdef DEBUG 
    int g = 23;
    static int init = 1;
#else
    int g;
    static int init = 0;
#endif

c1.c

#include "global.h"
int f() {
    return g + 1;
}

c2.c

#include <stdio.h>
#include "global.h"

int main() {
    if (!init) {
        g = 37;
    }
    int t = f();
    printf("calling f yields %d\n", t);

    return 0;
}

Answer 1

You define the variable g in the header file, that means it will be defined in all source files that includes the header file.

Instead declare it, like

extern int g;

and then define it in a single source file.

Answer 2

The difference between the two code paths (with and without DEBUG) is the initialization of the g variable. Without it, it is only a "tentative" definition, and so the symbol is only generated in the file that really uses it, namely c2.o.

With initialization it is a real definition of the symbol, and so it is also generated in c1.o.

As a rule of thumb, header files should never contain data definitions, only declarations. Data "declarations-only" should be made with the keyword extern. You'd then need to define any such symbol in exactly one of your .c files.