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Re: Dynamic loading progress

From: Philipp Stephani
Subject: Re: Dynamic loading progress
Date: Fri, 20 Nov 2015 22:36:56 +0000

Paul Eggert <address@hidden> 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.
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.

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