How do I avoid the "multiple definition of ..." Error in this case

Question

Boardcomputer.h:

#ifndef BOARDCOMPUTER_H
#define BOARDCOMPUTER_H

#include <Arduino.h>
#include <TFT_eSPI.h>

TFT_eSPI disp = TFT_eSPI();

...

#endif

Boardcomputer.cpp:

#include <Boardcomputer.h>

^^use disp functions^^
...

Error:

.pio\build\EmmaChip\lib782\libBoardcomputer.a(Boardcomputer.cpp.o):(.bss.disp+0x0): multiple definition of `disp'
.pio\build\EmmaChip\src\main.cpp.o:(.bss.disp+0x0): first defined here
collect2.exe: error: ld returned 1 exit status
*** [.pio\build\EmmaChip\firmware.elf] Error 1

Here is what I tried:

• make an extra Header-File and add #pragma once at the top
• make an extra Lib and add the typical #ifndef; #define; #endif lines

Do you have some ideas to fix this problem? I don't define 'disp' multiple times. Just in one Header.

Answer 1

The problem is here:

#ifndef BOARDCOMPUTER_H
#define BOARDCOMPUTER_H

#include <Arduino.h>
#include <TFT_eSPI.h>

TFT_eSPI disp = TFT_eSPI();    
...    
#endif

You define disp in a header file. Defining variables in header files is plain wrong, it cannot work (except if you include the .h file in a single .c file, but then the .h file is pretty pointless anyway) and it leads to these "multiple definition" issues at link time.

This happens here:

• you include Boardcomputer.h in foo.c, therefore foo.o will contain a variable disp. The code compile fine.
• you include Boardcomputer.h in bar.c, therefore bar.o will contain a variable disp. The code compile fine.
• you link foo.o and bar.o in order to produce an executable, and the linker sees two variables disp, one in bar.o and one in foo.o, hence the multiple definition error

---

You want this:

Boardcomputer.h

#ifndef BOARDCOMPUTER_H
#define BOARDCOMPUTER_H

#include <Arduino.h>
#include <TFT_eSPI.h>

extern TFT_eSPI disp;     // declare it here
    
#endif

Boardcomputer.c

#include "Boardcomputer.h"
...
TFT_eSPI disp = TFT_eSPI();   // define it here
...