14.9 Pointer Comparison

Two pointer values are equal if they point to the same location, or if they are both null. You can test for this with == and !=. Here’s a trivial example:

{
  int i;
  int *p, *q;

  p = &i;
  q = &i;
  if (p == q)
    printf ("This will be printed.\n");
  if (p != q)
    printf ("This won't be printed.\n");
}

Ordering comparisons such as > and >= operate on pointers by converting them to unsigned integers. The C standard says the two pointers must point within the same object in memory, but on GNU/Linux systems these operations simply compare the numeric values of the pointers.

The pointer values to be compared should in principle have the same type, but they are allowed to differ in limited cases. First of all, if the two pointers’ target types are nearly compatible (see Compatible Types), the comparison is allowed.

If one of the operands is void * (see Void Pointers) and the other is another pointer type, the comparison operator converts the void * pointer to the other type so as to compare them. (In standard C, this is not allowed if the other type is a function pointer type, but it works in GNU C.)

Comparison operators also allow comparing the integer 0 with a pointer value. This works by converting 0 to a null pointer of the same type as the other operand.