Assign a function with universal argument to a key (org-timer-start)

Question

I would like to assing the function org-timer-start with a univeral argument (Ctrl-u) to its own key.

I read here, that one should look into the source code of a function to find out how to pass a universal argument.

Where is this part in the following function, that tells me about the universal argument call?

    (defun org-timer-start (&optional offset)
      "Set the starting time for the relative timer to now.
    When called with prefix argument OFFSET, prompt the user for an offset time,
    with the default taken from a timer stamp at point, if any.
    If OFFSET is a string or an integer, it is directly taken to be the offset
    without user interaction.
    When called with a double prefix arg, all timer strings in the active
    region will be shifted by a specific amount.  You will be prompted for
    the amount, with the default to make the first timer string in
    the region 0:00:00."
      (interactive "P")
      (cond
       ((equal offset '(16))
        (call-interactively 'org-timer-change-times-in-region))
       (org-timer-countdown-timer
        (user-error "Countdown timer is running.  Cancel first"))
       (t
        (let (delta def s)
          (if (not offset)
          (setq org-timer-start-time (current-time))
        (cond
         ((integerp offset) (setq delta offset))
         ((stringp offset) (setq delta (org-timer-hms-to-secs offset)))
         (t
          (setq def (if (org-in-regexp org-timer-re)
                (match-string 0)
                  "0:00:00")
            s (read-string
               (format "Restart timer with offset [%s]: " def)))
          (unless (string-match "\\S-" s) (setq s def))
          (setq delta (org-timer-hms-to-secs (org-timer-fix-incomplete s)))))
        (setq org-timer-start-time (org-time-since delta)))
          (setq org-timer-pause-time nil)
          (org-timer-set-mode-line 'on)
          (message "Timer start time set to %s, current value is %s"
               (format-time-string "%T" org-timer-start-time)
               (org-timer-secs-to-hms (or delta 0)))
          (run-hooks 'org-timer-start-hook)))))

Do I need to make a custom fuction to call org-timer-start with a Ctrl-u prefix?

Answer 1

The part you need to look at is the interactive declaration which in this case is (interactive "P"). This means that C-u is passing the argument (4) to org-timer-start. So you need to assign the lambda

(lambda ()
  (interactive)
  (org-timer-start '(4)))

to a key in whatever keymap you want that will be active when you want to use it. The keymap you should probably choose is the one where org-timer-start is bound. You also need to make this binding after the keymap has been defined as a variable.

It seems that org-timer-start is not bound to any key by default and that the function is loaded when org-mode is loaded so it would be sensible to bind this lambda in org-mode-map as part of your customisation of org-mode.