extern  int x;   /* ******** Declaration of x ********** */

void f1()        /* Function f1() that uses global var x */
{
   x = 4;        /* ******** Access error !!!! ********* */
}

int x;           /* Definition of global var x */

void f2()        /* Function f2() that uses global var x */
{
   x = 4;
}