Sharing a variable across multiple files

Hi all,

I have specific question about usage of extern. I want to share a
single variable say int a; across multiple files.

I would define it in file1.c as

int a=0;
int main()
{
    .........
   a= 20;
  ..........
}

In another file2.c I would use it as,

int main()
{
  extern int a;
  ................
  printf("value of a=%d\n", a);

  ..................
}

Is this usage right. If yes, how do I run these as two separate
processes so that value changed in file1 is always reflected in file2
whenever it is used in fil2.c. How do I link these two processes put
it in another way.

Regards,
Aruna