Is there any possible use of using extern variable in a file that is not included in any other file?

Question

I've encountered many examples on the web, that have extern int x in main.c, where the main function lies.

The only use of extern I'm aware of, is to make a declaration in another file, and use it in another file, after defining it.

Like : a.h : extern int x;

a.c : int x = 5;

main.c : #include "a.h" // and start using x

The 1st case seems redundant to me.

So, Is there any possible use of using an extern variable in a file that is not included in any other file?

Answer 1

extern tells the compiler that x exists in a different module and should be linked from elsewhere. Putting it in main.c directly just avoids pulling in a header (which would be included in-line anyways)

Just like in a header, x still needs to exist in another .c module where it isn't defined extern.