Why is it illegal to call offsetof() on pointer member?

From there I know that it is illegal in C to call offsetof on a pointer, but why is it undefined behaviour? Standard implementation

#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

seems suitable even for pointer MEMBER, I can't get why it doesn't work.

Example on the architecture where int is 4 bytes:

#include <stddef.h>
#include <stdio.h>

struct foo {
    int a;
    int *ptr;
};

int main() {
    printf("Offset of ptr: %zu\n", offsetof(struct foo, ptr));
    return 0;
}

That code will return 8, whereas if ptr was just int, it would return 4. But in both cases that member is on the same offset of 4.

Edit: I did wrong assumption and the mentioned link is invalid. The problem lied in structure padding, as was explained

The comment you link to is for C++ and not C. It does not apply to C.

Further, it does not say offsetof cannot be used on (not “called”; it is not a function) a pointer member (that is, a member of the structure that is a pointer). It says offsetof cannot be used on a member pointer.

A member pointer is a C++ thing that does not exist in C. It is a special kind of pointer that refers to a member within a class context. (It is not a stand-alone pointer that can be used by itself.)

So the reason you cannot use offsetof on a member pointer in C is that no such thing exists in C.