int foo(int n, int (*a)[n]) { return (\*a)[5]; }
  int main() {
    int a[3];
    return foo(ARRAY_SIZE(a), &a);
  }

That syntax is why array overflows remain the #1 problem with C bugs in shipped code. It isn't any better than:

  int foo(size_t n, int* a) { assert(5 < n); return a[5]; }
  int main() {
    int a[3];
    return foo(ARRAY_SIZE(a), a);
  }

as the array dimension has to be handled separately from the pointer.

Contrast with how simple it is in D:

    int foo(int[] a) { return a[5]; }
    int main() {
        int[3] a;
        return foo(a);
    }

and the proof is shown by array overflow bugs in the wild are stopped cold. It can be that simple and effective in C.

\* what operator is this? I have never seen it. Where can I read about it?

My guess is that it was intended to escape the * since unescaped * in regular text on HN results in italics. Since the text in question is in a code block, though, that escaping is not needed.