RFM: How to make a buffer-local var in C layer

[phillip.lord@russet.org.uk (Phillip Lord)]

Sorry for the total newbie noise, but I am trying to add a buffer-local
variable to the undo machinary. In emacs-lisp I would do...

(defvar-local undo-buffer-undoably-changed nil
  "Non-nil means that that the buffer has had a recent undo-able change.

Recent means since the value of this variable was last set explicitly to nil,
usually as part of the undo machinary.")

My C is very poor (I'm learning it as I go). But I *think* I need to do
this in the C layer, as I am going to use and set this directly from
within undo.c. Or am I wrong about this? Is it easier to just use the
lisp above and access it's value in C (not sure how do do that either).

If I need to do this in C, I have worked out how to add new variables to
Emacs in C, but it's the buffer-local bit that is causing problems. Do,
I need to do follow all the steps in buffer.c, which look something like

- call it "buffer-undoably-changed"
- add to the structure in buffer.h
- add a bset-* method
- do DEFVAR_PER_BUFFER in syms_of_buffer_c

The background to this is in this thread here:

https://lists.gnu.org/archive/html/emacs-devel/2015-09/msg00013.html

Sorry for dumb questions; this bit of Emacs is totally new to me.

Phil