== for pointer comparison

Question

I quote from \"The C Programming Language\" by Kernighan & Ritchie:

Any pointer can be meaningfully compared for equality or inequality with zero.

Answer 1

You cannot use pointer comparison for comparing pointers that point into different arrays.

So:

int arr[5] = {1, 2, 3, 4, 5};

int * p = &arr[0];

int anotherarr[] = {1, 2};

int * pf = &anotherarr[0];

You cannot do if (p == pf) since p and pf do not point into the same array. This will lead to undefined behaviour.

You can rely on pointer comparison if they point within the same array.

Not sure about the arithmetic case myself.