> most implementations of malloc would write some kind of book-keeping header before the returned object (observe that on amd64 glibc, malloc(3) for large alloc

> most implementations of malloc would write some kind of book-keeping header before the returned object (observe that on amd64 glibc, malloc(3) for large allocations returns pointers that are offset +0x10 from page alignment, that is where the two word allocated block header lives). Hmm. You're right about glibc, but macOS for instance reliably returns page-aligned pointers for large allocations. I wonder how Windows behaves.