post-command-hook and prefix argument

Question

I have a non-interactive function F to do post processing after each command. F is buffer locally hooked to post-command-hook.

Whenever F is hooked the prefix argument is eaten and the next command does not get it.

How can I preserve the prefix argument when hooking a function to post-command-hook ?

EDIT: There is no issue with the prefix argument and the function F being hooked as the selected answer shows.

My issue was that the function (let's call it A) that did not worked properly could be either called directly (which had a working behavior) or called from a push-button (in this case the prefix argument was not passed to A).

Answer 1

test-fn is run first, at which time the variable my-universal-arg is set (if there was a universal argument called before pressing the f5 key). Then, the post-command-hook gets triggered and it picks up the variable my-universal-arg (if that variable was set by calling a universal argument before pressing the f5key), and then the message plays displaying the argument and the variable my-universal-arg is then set back to nil.

NOTE:  If the variable my-universal-arg is made to be buffer-local, then the appropriate functions will need to either be occurring in the target buffer or something like with-current-buffer . . . would need to be used to pick up the variable bound to the corresponding buffer.

(defvar my-universal-arg nil)

(defun test-fn (arg)
(interactive "P")
  (if arg
    (setq my-universal-arg arg)
    (message "No `arg` was specified.")))

(define-key global-map [f5] 'test-fn)

(defun post-command-hook-fn ()
  (when my-universal-arg
    (message "This is `my-universal-arg`:  %s" my-universal-arg)
    (setq my-universal-arg nil)))

(add-hook 'post-command-hook 'post-command-hook-fn)