Undefined reference to global variable during linking

Question

I am trying to compile a program which is divided into 3 modules, corresponding to 3 source files: a.c, b.c, and z.c. z.c contains the main() function, which calls functions in a.c and b.c. Furthermore, a function in a.c calls a function in b.c, and viceversa. Finally, there is a global variable count which is used by the three modules and is defined in a separate header file, global.h.

The code of the source files is the following:

a.c

#include "global.h"
#include "b.h"
#include "a.h"

int functAb() {
    functB();
    functA();
    return 0;
}

int functA() {
    count++;
    printf("A:%d\n", count);
    return 0;
}

b.c

#include "global.h"
#include "a.h"
#include "b.h"

int functBa() {
    functA();
    functB();
    return 0;
}

int functB() {
    count++;
    printf("B:%d\n", count);
    return 0;
}

z.c

#include "a.h"
#include "b.h"
#include "global.h"

int main() {
    count = 0;
    functAb();
    functBa();
    return 0;
}

The header files:

a.h

#ifndef A_H
#define A_H

#include <stdio.h>

int functA();
int functAb();

#endif

b.h

#ifndef B_H
#define B_H

#include <stdio.h>

int functB();
int functBa();

#endif

global.h

#ifndef GLOBAL_H
#define GLOBAL_H

extern int count;

#endif

And, finally, the makefile that reproduces my error:

CC = gcc
CFLAGS = -O3 -march=native -Wall -Wno-unused-result

z:  a.o b.o z.o global.h
    $(CC) -o z a.o b.o z.o $(CFLAGS)
a.o:    a.c b.h global.h
    $(CC) -c a.c $(CFLAGS)
b.o:    b.c a.h global.h
    $(CC) -c b.c $(CFLAGS)
z.o:    z.c a.h global.h
    $(CC) -c z.c $(CFLAGS)

With this, I can compile the objects a.o, b.o, and z.o fine, but, when linking with make z, I get undefined reference to 'count' in all of them:

z.o: In function `main':
z.c:(.text.startup+0x8): undefined reference to `count'
a.o: In function `functAb':
a.c:(.text+0xd): undefined reference to `count'
a.c:(.text+0x22): undefined reference to `count'
a.o: In function `functA':
a.c:(.text+0x46): undefined reference to `count'
a.c:(.text+0x5b): undefined reference to `count'
b.o:b.c:(.text+0xd): more undefined references to `count' follow
collect2: ld returned 1 exit status

I managed to reproduce the error in my actual code in this minimal example, so I guess there is a problem in the dependencies between modules, but I can't spot it. Can anyone point me in the right direction?

Solution 1

Change your z.c to

#include "a.h"
#include "b.h"
#include "global.h"

int count; /* Definition here */
int main() {
    count = 0;
    functAb();
    functBa();
    return 0;
}

From global.h, all your files inherit the declaration of variable count but the definition is missing from all files.

You must add the definition to one of the file as int count = some_value;

Solution 2

You have declared count, not defined it.

extern is a part of declaration, not a definition.

To be explicit, extern is a storage-class specifier and used at declaration.

You need to define int count somewhere in your source files.

Solution 3

You have to add int count; to your z.c file. This because of declaring a variable in header file as extern tells to the compiler that the variable will be declared in another file, but the variable is not declared yet and will be resolved b linker.

Then you need to declare the variable somewhere.

Comments

[Taozi]

Can "int count" be inside the main function?

[Mohit Jain]

@Taozi No, it will make the count a local variable. We need a global definition.

[Matt Montag]

yep, it's diabolical. Sorry to all the beginners :)

[Daniel N.]

In my case, when I compile inside a Linux distro I have no issues at all, but when I try to compile the same program with cygwin, I get this error. I found that indeed my variables are declared inside a function and that maybe is the root cause of my headaches, but why does it Linux compile without complain?