C the extern keyword

I just had the error src/main.c:46: multiple definition of '_led' again. I figured the extern keyword is coming for my rescue.

What Failed?

In *.h files I wrote const Led led; to "announce" that the according C file provides this variable. But if multiple places #include ... this header file this clashes.

Of course one can surround the entire header file with #ifndef MAKE_UP_A_NAME, but to me this feels like a symptom for hiding something we can do better.

The Solution

So I discovered extern. And by doing so I also learned why const Led led; is just not a good idea.

int var;  ->  declaration and definition
extern int var;  -> declaration

In short extern int var; just announces that var will be defined at some point in time. In comparison int var; does define var already, which is not what we want, because if that happens twice the compiler complains. Why? Because all code, from all files can be imagined as getting thrown into one bit file and then there should be no duplicates definitions of anything.