b1.c
====

#include <stdio.h>

/* 
   gcc -ansi -c b*.c    => b1.o, b2.o, b3.o
   gcc -ansi -o b  b*.o => b
   The order of object files for the link command has no significance
*/
extern int a, b;

int a = 5;  /* global variable a */
int b = 2; /* global variable b - can be accesed from any file */

void f1(void);
void f2(void);

main()
{
  f1();
  f2();
  printf("\nFrom main:  a = %d  b = %d\n", a, b);
}

Output:
=======

%b

From f1:  a = 5  b = 2

From f2:  a = 5  b = 2

From main:  a = 5  b = 2






b2.c
====

#include <stdio.h>

/* int a = 9;
   This second global declaration of the same variable a
   causes the link error: multiple declaration of a
*/

/* extern int a = 9;
   It is invalid to (default) initialize a variable that
   has been declared elsewhere
*/

extern int a;

void f1(void)
{
  extern b;
  printf("\nFrom f1:  a = %d  b = %d\n", a, b);
}

b3.c
====

#include <stdio.h>

void f2(void)
{
  extern a, b;
  printf("\nFrom f2:  a = %d  b = %d\n", a, b);
}