multiple definition of a const int

Question

I have a header file A.h in which i define a const int ID = 4;. I have included this header file in to C files A.c and main.c. I have used header guards #ifndef A_H #define A_H etc. But I get the error multiple definition of ID when I try to compile the code. I read somewhere that this can, in most cases, be avoided by using #pragma once but I still get the error. My question is that how can I define variables in C? Should I have to move definition of ID to C file but then I have to declare it in every file I use? Or using extern the only way in this situation?

Answer 1

In C you are only allowed to have a single definition per object. Include guards and similar don't help on this if you have multiple .o files (compilation units). Each of them has a copy, which is not allowed.

If you don't need the address of that object and you are only interested in its constant value you can replace it by

enum { ID = 4 };

This defines a named value ID of type int that you easily can put in a header file.

Answer 2

Yes, using extern is the only solution. pragma or include guards guard against multiple inclusion in the same translation unit, this is a multiple definition error.