pointer to const and normal pointer can be mixed?

Question

Is the value of *b undefined when printf() is called?

void foo(int *a) {
  const int *b = a;
  int *c = a;
  *c = 2;
  printf("%d\n", *b); // what must be *b? 1, 2 or undefined?
}

int d = 1;
foo(&d);

Answer 1

It will print 2. const int *b literally means:

Pointer to an integer whose value cannot be altered through its dereferentiation.

This does not mean that the value the pointer points to may not change. In fact it's perfectly valid to change. A likely scenario to use this, are structures that keep a read only reference to some large structure. The reference may change, but the functions working with that structure may not change what's behind the pointer.

Imagine a driver or similar that hands out a read only memory mapping of whatever data the device delivered: The address of the mapping is not constant, but since this is a read only mapping the user program may not write to it. OTOH when the device updates the data the contents of the buffer will change, but not necessarily the mapping address.