Multiple definition of token

Question

Given the following code,

button.h

#ifndef BUTTON_H_
#define BUTTON_H_

#define true 1
#define false 0

#include <avr/io.h>
#include <avr/interrupt.h>
#include <timer0.h>

typedef struct {
    unsigned char port;
    unsigned char pin;
    unsigned long timestamp;
} BUTTONS;

BUTTONS button_1;
BUTTONS button_2;
BUTTONS button_3;

enum BUTTONS_ID{BUTTONS_ID_1,BUTTONS_ID_2,BUTTONS_ID_3,BUTTONS_ID_COUNT};

BUTTONS* button[BUTTONS_ID_COUNT] = {&button_1,&button_2,&button_3};

void Button_init(void);
#endif //BUTTON_H_

and button.c

#include <button.h>

enum BUTTONS_state{BUTTON_STATE_UNPRESSED,BUTTON_STATE_DEBOUNCING,BUTTON_STATE_PRESSED};
int state = BUTTON_STATE_UNPRESSED;

void Button_init(void){

    button[BUTTONS_ID_1]->port = PINB;
    button[BUTTONS_ID_1]->pin = PINB4;
    button[BUTTONS_ID_1]->timestamp = 0;
}

I get the following error : button.cpp : multiple definition of `button_1'. I know I must be doing something wrong. I am quite new at using structure the mistake must be coming from there. Basically I wanted to create button variable which I could access from my main program if need be. Is there a way to define them in my .h and initialize them within my .c and then access them from my main file?

Thank you

Answer 1

You've defined button1 and several other objects in your header file. If you include this header in multiple translation units (read: source files), you'll end up with one definition for each translation unit you compile. Then later, when you try to link -> KABOOM.

The simple solution is "don't put code that defines objects in your header." If you need to access them in multiple source files, you can leave the declarations, but you'll need to mark them extern. Then you can make the definition in a source file elsewhere.

Answer 2

You should not declare variables in header files. This is because when a header file is #included in a c file it is literally copied to it by the preprocessor.

So if 2 c files include the same h file, which in turn declares a variable, you end up with the same variable declared twice in both files. That's probably what happened here - you probably #included button.h in another c file.

The best way to make a variable application global is to declare it in only one file, and then declare it using extern in each c file where you want to use it.

In your example, do

BUTTONS button_1;
BUTTONS button_2; 
BUTTONS button_3;

in one c file, and in all other c files where you want to use these vars, do:

extern BUTTONS button_1;
extern BUTTONS button_2; 
extern BUTTONS button_3;

There is also other ways to do it. It's possible to use some preprocessor acrobatics and declare you variables in a header file in such a way that only in one file there are declared as global variables, and in all other files they are declared with extern. But personally I don't like this, because I do think that variables declaration do not belong in header files.

Besides, it's best to try not to use application global variables, which leads to ways of doing modular programming, low coupling and all that stuff. It's a very interesting and important topic, but is to wide for this answer.. :-)