multiple definition error

i want to define a LiquidCrystal variable out of main unit.... but when compiling the whole program, "multiple definition" error occurs... :0

Hi Wally,

You really didn't provide enough detail for us to understand what you did or what you're trying to do...

Most likely you are creating a variable or type that already exists, or you are including a header file that has a definition that already exists in another one.


i want to create a library which contains routines that use LCD functions.
my library’s begining:

#include <LiquidCrystal.h>
LiquidCrystal sysMon(12, 11, 5, 4, 3, 2);

And main unit:

#include “Arduino.h”
#include <LiquidCrystal.h>
#include “lcd_lib.cpp”

void setup()




if i compile this project in eclipse IDE, this message occurs:

…/lcd_lib.cpp:13: multiple definition of `sysMon’
./main_unit.o:(.bss.sysMon+0x0): first defined here

Ahhh. I see what you're doing... try it again without the following line:

include "lcd_lib.cpp"

You don't include CPP files into CPP files, or you will get duplicate code. The #include pre-processor operation essentially copies and pastes the included file into your CPP file before it compiles it. The compiler saw your sysMon definition twice.


i used #include direktive with header file (lcd_lib.h) and problem solved…

:slight_smile: :slight_smile:

Great! Good luck with your project. LCDs can be a lot of fun.