Undefined behavior or not?

[Andrew Makhorin]

> No, that is not how it works.  6.2.4/2 says: [...] "The value of a
> pointer becomes indeterminate when the object it points to (or just
> past) reaches the end of its lifetime."
> 
> It does not matter whether the value of p changes in *your* system,
> or in most systems.  Using the value of p after deallocating the object
> it pointed to is undefined behaviour.
> 

Does this mean that the following fragment is invalid? (I think it
isn't.)

   void *p, *q;
   p = malloc(123);
   q = p;
   free(p);
   if (p == q) ...

Thanks.

Andrew Makhorin