ELisp: Get universal-argument

How to Get Universal Argument

To make your command aware of Emacs: Universal Argument (prefix arg) , there are 3 simple ways:

• In the function body, check the builtin variable current-prefix-arg. It holds the value of universal argument.

• Add capital P as prompt string to ELisp: Interactive Form in your function. It passes the value of current-prefix-arg as your function's first parameter.
• Add lowercase p as prompt string to ELisp: Interactive Form in your function. It passes converted numerical value of current-prefix-arg to your function's first argument.

(defun xf (x)
  "print value of `current-prefix-arg'"
  (interactive "P")
  (message "%s" x))

(defun xg ()
  "print value of `current-prefix-arg'"
  (interactive)
  (message "%s" current-prefix-arg))

;; try
;; M-x xg
;; C-u M-x xg
;; C-u C-u M-x xg
;; C-u 1 M-x xg
;; C-u 2 M-x xg

Possible Values of Universal Argument

Here's the possible values of current-prefix-arg and (prefix-numeric-value current-prefix-arg)

Convert current-prefix-arg to number

The function prefix-numeric-value converts current-prefix-arg to number.

(defun xh ()
 "print numerical prefix arg received"
 (message "%s" (prefix-numeric-value current-prefix-arg) )
)

Reference

• Prefix Command Arguments (ELISP Manual)