Use of global variables between two files in C

March 26, 2022

I need to use a global variable first declared in a .c file, and use later on two different .c files. Problem, I got an error message when I compile.

Here’s the declaration, there :

bool UP = false;

And here’s how I use it :

extern bool UP;

And I got this error message. If I put these variables inside the second file, it compile perfectly,but due to the fact I need these variables in another file, I need to share it.

gcc -c game.c -lGL -lGLU -lglut
gcc -o program main.o loadMap.o draw.o game.o object.o menu.o -lGL -lGLU -lglut
/usr/bin/ld: game.o: in function `game':
game.c:(.text+0x34): undefined reference to `Keyboard'
/usr/bin/ld: game.c:(.text+0x95): undefined reference to `UP'
collect2: error: ld returned 1 exit status
make: *** [makefile:10: program] Error 1

EDIT : Here’s more source code.
This is .h file where I declare my variables

#ifndef KEYBOARD_H_
#define KEYBOARD_H_

#include <stdlib.h>
#include <stdio.h>
#include <stdbool.h>


bool UP = false;
bool LEFT = false;
bool RIGHT = false;
bool DOWN = false;
bool UP_MENU = false;
bool DOWN_MENU = false;

void Keyboard(unsigned char key, int x, int y);

#endif

And I want to use it here :

void game(int mX, int mY, Player p)
{
    drawMap(mX, mY);            //afficher la carte
    drawPlay(p); //Affiche le player
    
    
    glutKeyboardFunc(Keyboard);     //fonction de glut gérant le clavier
    
    if (LEFT == true)
    {
        
        moveLeft(p);        //va se déplacer vers la gauche si on appuie sur q
        LEFT = false;
        
    }
    if (RIGHT == true)
    {
        
        moveRight(p);       //va se déplacer vers la droite si on apppuie sur d
        RIGHT = false;
    }
    if (UP == true)
    {
        moveUp(p);
        UP = false;
    }
    
    if (DOWN == true)
    {
        
        moveDown(p);
        DOWN = false;
    }


    glutPostRedisplay();//upload la position du joueur
}