Like which one? Pretty much every single one I know, you pass a buffer and a pointer to a buffer size; the first time you call with buffer NULL and the required buffer size is put in the buffer size; then you allocate the buffer and call again with the proper buffer allocated.
QueryDosDevice? You're right that mostly length is passed back, but esp in libs you get this problem you pass memory you own, some underlying function fails with "buffer too small", but buffer length was not in/out in the interface. Places where this can happen are not always obvious from reading the msdn.
