[tom_]

If storing the metadata in the heap, 0 bytes often doesn't even end up a special case. You need to have a case for allocations of some arbitrary number of bytes, and 0 is an arbitrary number of bytes.

Another option is to treat them as being of size 1.

(In theory you could do endless allocations of size 0, and eventually you'd run out of space, even though you've allocated 0 bytes in total. But you end up in exactly that situation, whatever the allocation size, if you don't take bookkeeping overhead into account!)