Paul Eggert <eggert@cs.ucla.edu> schrieb am So., 22. Nov. 2015 um 00:25 Uhr:

> Philipp Stephani wrote:
> > I honestly can't think of a situation where version checking would work
> but
> > size checking wouldn't.
>
> For example, a structure contains an off_t value, and time_t grows from 32
> to
> 64-bits.  The designer knew this might be a problem because of Y2038
> issues, and
> so created padding for the time_t to grow into.
>
> Another example: suppose we change ptrdiff_t back to int.
>

Such issues are why we generally prefer fixed-size integers.
Currently the structures contain only pointers and pointer-sized integers.
If we are careful to keep it that way, sizes can only ever increase.


>
> Another example: suppose we want the same module to work in both narrow
> and wide
> int Emacs, so we allocate storage for wide integers even though half of
> them are
> not used in narrow platforms.
>

The module interface never exposes Lisp_Objects directly, so it's immune to
such considerations.


>
> What you're saying, if I understand it, is that we promise that we'll
> never make
> any changes like that.  This sounds overly constraining, at least for now.
>

Not really, because there are no cases where we'd need to e.g. expose an
off_t field. We only need three types of fields:
- A field for version checking (size should be reasonably fixed)
- A pointer-sized field pointing to private data (size is fixed within a
single address space)
- Function pointer fields for interacting with Emacs (size is fixed within
the address space)
If we ever need to expose an off_t value, it will be in the form of another
function pointer.
Even if, for whatever reason, we must expose an off_t value directly, we
can insert appropriate padding to make sure the size increases.


> While the interface is experimental we'll retain the freedom to make
> arbitrary
> changes to it. When it settles down we can think about promises about
> never ever
> making changes in the future. In the meantime the size field is
> experimental and
> maybe we'll think of uses for it other than sizes.
>

The module interface is supposed to be stable at least for the lifetime of
Emacs 25. At least from our side it's not meant to be experimental.