se/SE/stuff/P5_Automation_can-dev-res/___main/__main_extern.h

/*
The extern Storage Class
The extern storage class is used to give a reference of a global variable that is visible to ALL 
the program files. When you use 'extern', the variable cannot be initialized however, 
it points the variable name at a storage location that has been previously defined.

When you have multiple files and you define a global variable or function, which will also be used in
 other files, then extern will be used in another file to provide the reference of defined variable
  or function.  Just for understanding, extern is used to declare a global variable or 
  function in another file.

The extern modifier is most commonly used when there are two or more files sharing the same global 
variables or functions as explained below.

First File: main.c

#include <stdio.h>
 
int count ;
extern void write_extern();
 
main() {
   count = 5;
   write_extern();
}
Second File: support.c

#include <stdio.h>
 
extern int count;
 
void write_extern(void) {
   printf("count is %d\n", count);
}
Here, extern is being used to declare count in the second file, where as it has its definition in the first 
file, main.c. Now, compile these two files as follows −

$gcc main.c support.c
It will produce the executable program a.out. When this program is executed, it produces the following result −

count is 5
*/