How do define global variable visible across several .c files

Question

assume there is a header file： a.h and source file : a1.c a2.c ... an.c; n>=1， which means it may only have one source file, or have several source files.

my question

Answer 1

As you said, variable must be defined in the c file and declaration should be located in header file.
You have to define a global variable in any c file and declare 'extern' in header file.

Example)

1) define global variable in a1.c
    int g;

2) declare global variable in a.h
    extern int g;

3) include header file in other c files
    #include "a.h"
    // to do something