scal.c

this is a gcc file, you can download it and learn its usetility, for further detail please look at t

int g1, g2;

void
write_at (addr, off, val)
     int *addr;
     int off;
     int val;
{
  g2 = 1;
  addr[off] = val;
  g2++;
}

main ()
{
  g2 = 12;
  write_at (&g1, &g2 - &g1, 12345);
  printf ("%d\n", g2);
}