Include Guard still inserting Global Variables

Question

I have 3 *.c files (file1.c, file2.c and file3.c) and 1 *.h file (file3.h) in a project (Visual Studio).

/*******************************
file3.h
********************************/
#ifndef FILE3_H
#define FILE3_H
int gintVariable = 400;
#endif

/*******************************
file1.c
********************************/
#include "file3.h"
#include <stdio.h>
#include <conio.h>

int modifyGlobalVariable(void);
void printGlobalVariable(void);

int main(void)
{
    modifyGlobalVariable();
    printGlobalVariable();
    printf("Global variable: %d\n", gintVariable++);
    getch();
    return 0;
}

/*******************************
file2.c
********************************/
#include "file3.h"                      

int modifyGlobalVariable(void) 
{ 
    return gintVariable++; 
}

/*******************************
file3.c
********************************/
#include "file3.h"
#include <stdio.h>

void printGlobalVariable(void)
{
    printf("Global: %d\n", gintVariable++);
}

When I build the solution in VS, it is giving error as "_gintVariable already defined in file1.obj".

I did check in the pre-processor output, the gintVariable is included in all the *.c files even though I have included include guards.

What mistake I am doing?

Answer 1

You should use 'extern' while declaring a global variable in header file. Define it in any one of *.c file. This should fix the issue.

For more on header files, read How do I use extern to share variables between source files?

Answer 2

Including guards prevents multiple inclusion (or, more precisely, multiple compilation of the .h file content) in a single translation unit.

It is useful against this problem:

/* glob.h */
#ifndef H_GLOB
#define H_GLOB

struct s { int i; };

#endif

/* f.h */
#ifndef H_F
#define H_F

#include "glob.h"

struct s f(void);

#endif

/* g.h */
#ifndef H_G
#define H_G

#include "glob.h"

struct s g(void);

#endif

/* c.c */
#include "f.h" /* includes "glob.h" */
#include "g.h" /* includes "glob.h" */

void c(void) {
    struct s s1 = f();
    struct s s2 = g();
}

The inclusions is like a diamond:

    glob.h
   /     \
f.h      g.h
   \     /
     c.c