Paul Eggert <eggert@cs.ucla.edu> schrieb am Fr., 20. Nov. 2015 um 21:48 Uhr:

> Philipp Stephani wrote:
> > This member will typically be used as follows:
> >
> > if (env->size >= sizeof *env) {
> >    // good, object is at least as big as expected
> > } else {
> >    // Emacs is too old
> > }
>
>
> I'm leery of recommending this sort of checking.  In practice incompatible
> structure layout changes so often involve something more than merely
> extending a
> structure, that a size check is not worth the trouble;


No such incompatible changes can be allowed to the emacs_env and
emacs_runtime structures. Only additions at the end of the structures are
allowed. This is independent of how the version checking is made.


> you really want a version
> check.  I might be persuaded that it's a good idea


See Daniel's initial design (
https://lists.gnu.org/archive/html/emacs-devel/2015-02/msg00960.html) for
rationale.


> if I saw some code doing it
> effectively, but currently there is no such code in the Emacs sources


Not inside Emacs, but e.g. the Windows API uses this approach extensively
for version checking, and has been forward- and backward-compatible for
decades with it.


> (the
> module test function doesn't check sizes -- why not?).
>

 Just an omission, we'll fix that. All modules need to do version checks so
they don't run into UB if they use a more recent emacs-module.h than the
Emacs in which they are loaded.