How do I hook fundamental-mode and all modes derived from it for modifying syntax for underscores

Question

Here is the definition of fundamental-mode:

(defun fundamental-mode ()
  "Major mode not specialized for anything in particular.
Other major modes are defined by comparison with this one."
  (interactive)
  (kill-all-local-variables)
  (run-mode-hooks))

There does not seem to be a fundamental-mode-hook defined.

I would like to call some code for all "fundamental-mode" buffers and all derived modes from it, without having to itemize all such modes and explicitly call add-hook on all of their mode functions.

Is there a way or am I stuck paying the itemization cost? If yes, then how do I do this inside all modes:

(modify-syntax-entry ?_ "w")

because after decades of Emacs usage, I've finally grown tired of my cursor getting hung up on every single underscore for every time I use M-f (ALT+f), across all new modes that are invented and that I install into my setup.

Answer 1

1st: Dealing with your actual problem of setting the syntax for ?_ in modes derived from fundamental-mode.

There are no commands setting the syntax table in fundamental-mode. Therefore, it uses the (standard-syntax-table). If the syntax for ?_ is not defined in a mode it uses the entry in its inherited syntax table. By default, (standard-syntax-table) is inherited. Since there are no circular references for syntax tables (standard-syntax-table) is eventually the fallback for all modes.

If we assume that modes setting the syntax for ?_ do it for a good reason it should be sufficient to use the following line in your init files:

(modify-syntax-entry ?_ " " (standard-syntax-table))

(This works at least in Emacs 27.1.90.)

2nd: The user is not supposed to customize fundamental-mode. So there is no hook for that mode.
Furthermore, fundamental-mode does not really play a role in the inheritance hierarchy. For all modes but fundamental-mode itself (derived-mode-p 'fundamental-mode) evaluates to nil.

Nevertheless, there is after-change-major-mode-hook that is run at the very end of major mode functions. That allows you to do general settings for all modes.