Emacs executes `c-mode-hook` twice

Question

Here's my emacs config

(require 'cc-mode)

(defun ffy-c-mode-hook ()
  "This is settings for the C/C++ mode"
  (message "ffy-c-mode-hook executed")
  (electric-pair-mode +1)
  (electric-indent-local-mode +1)
  (if electric-indent-mode
      (let ((binding (key-binding (kbd "RET"))))
        (local-set-key (kbd "RET") (key-binding (kbd "C-j")))
        (local-set-key (kbd "C-j") binding)))
  (c-toggle-hungry-state +1)
  (c-set-style "gnu")
  (setq c-basic-offset 4))

(add-hook 'c-mode-hook #'ffy-c-mode-hook)

Apparently c-mode-hook is executed twice, because when I open a C file I see:

user-error: Beginning of history; no preceding item
ffy-c-mode-hook executed [2 times]

Is it a feature or a bug ? No other hooks are executed multiple times AFAIK. Plus it seems that now I can't toggle features in the hook.

I looked at the variable c-mode-hook and it its value is (ffy-c-mode-hook)

The versions of Emacs and CC-mode are

GNU Emacs 24.5.1 (i686-pc-mingw32) of 2015-04-11 on LEG570
Using CC Mode version 5.32.5

Stacktrace on ffy-c-mode-hook

ffy-c-mode-hook()
run-hooks(change-major-mode-after-body-hook prog-mode-hook c-mode-common-hook c-mode-hook c-mode-hook)
apply(run-hooks (change-major-mode-after-body-hook prog-mode-hook c-mode-common-hook c-mode-hook c-mode-hook))
run-mode-hooks(c-mode-hook)
c-mode()
set-auto-mode-0(c-mode nil)
set-auto-mode()

Does this still happen if you invoke Emacs using `emacs -Q`, then evaluate the snippet above, then load a C file? — ChrisGPT was on strike, Aug 31 '15 at 20:28
@Chris Yes. it happens. I added a stacktrace to the question. — EvgeniySharapov, Aug 31 '15 at 21:02
I think this is a bug in emacs. There's a discussion here: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=16759 — andygavin, Sep 01 '15 at 19:21
Related question: http://stackoverflow.com/questions/11911266/c-mode-hook-and-c-mode-hook-appears-to-run-twice — andygavin, Sep 09 '15 at 10:53

andygavin · Accepted Answer · 2015-09-02T09:36:53.597

2

Following my comment about the bug report http://debbugs.gnu.org/cgi/bugreport.cgi?bug=16759 I don't think that language hooks are guaranteed to be executed once for a number of language modes. I suspect the issue is due to using define-derived-mode which is a lisp macro for defining a mode that already includes calls to hooks, this means the hooks called in the mode will be an additional execution.

You might want to try the initialization hook. The manual says:

Variable: c-initialization-hook

Hook run only once per Emacs session, when CC Mode is initialized. This is a good place to change key bindings (or add new ones) in any of the CC Mode key maps. See Sample Init File.

The sample it gives is here: https://www.gnu.org/software/emacs/manual/html_node/ccmode/Sample-Init-File.html#Sample-Init-File

edited Sep 02 '15 at 09:36

answered Sep 01 '15 at 19:19

andygavin

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It's very unusual for a mode defined with `define-derived-mode` to also explicitly run its mode hooks. In the (vast) majority of cases, this is left up to the macro-generated code (and modes not defined with the macro can invariably be depended upon to mimic the functionality where hooks are concerned). You might find the write-up here useful: http://stackoverflow.com/a/19295380/324105 – phils Sep 08 '15 at 14:25
Do you mean if it did it would be a bug? – andygavin Sep 09 '15 at 10:49
I'd certainly argue for it to be considered a bug -- but it looks like it's already been accepted as such in the discussion you've linked to, with ideas having been put forward for how to replace the unexpected call with something cleaner (and presumably the same can be done to any other instances, should any exist). – phils Sep 09 '15 at 11:53

Emacs executes `c-mode-hook` twice

1 Answers1