about pre-compile #include directive

[nanger]

main.cpp

  #include "one.h";
  #include "two.h";
  ...

there are other files:
one.h
one.cpp
two.h
two.cpp

if there is a variable defined in one.cpp

  int a;

and declared in one.h

  extern int a;

and there is a varialbe defined in two.cpp

  int b;

but the variable b is not declared in two.h

then, I think a is a global variable in main.cpp
b is a global variable in two.cpp but not visible in main.cpp
if I want a to be a global variable in two.cpp, what should I do?
thank you!

[Zhuge]

I would answer, but it really sounds like you are rephrasing a homework question. At least you didn't simply copy and paste...

I would compare the two syntaxes you have been given and see what the differences are, then look up what that difference does for you.

[nanger]

no, I'm just puzzled by such stuff
I elaborated the question in such a way in order to make it clear to everyone who want to give a hand
thank u!

[Bazzy]

If you try to compile it you should have linker errors.
All the source files are linked so it will result that 'b' was defined twice