Something separate that occurred to me - many systems have empty sections of address space, those addresses can't back `malloc(1)` allocations but they could back `malloc(0)` allocations with a unique address. I doubt any C runtime out there will actually do that, but in theory it could be done.

> allocating a few bytes that weren't asked for.

FWIW the alignment guarantees of `malloc()` mean it often will have to allocate more than you ask for (before C23 anyway). You can't 'legally' use this space, but `malloc()` also can't repurpose it for other allocations because it's not suitably aligned.

That said I still agree it's a hack compared to just using `malloc(1)` for this purpose, it's well-defined and functionally equivalent if you're looking for a unique address. The fact that you don't know what `malloc(0)` is going to do makes it pretty useless anyway.

The only requirement which seems reasonable to me, is that the address be unique. Since the allocation size is zero, it should never be accessed for read or write, but the address itself may need to be used for comparisons.

If you’re pointing to a zero sized data it shouldn’t matter what it’s pointing to. Even outside valid address space. Because you shouldn’t be reading or writing more than 0 bytes anyway.

> or allocating a few bytes that weren't asked for.

You are always allocating bytes you weren't asked for: the allocation metadata and some extra bytes to satisfy the alignment requirement. If you absolutely don't want to allocate memory, you probably shouldn't have called malloc() in the first place :)

You can copy from a zero sized pointer with memcpy, but not NULL.