I have C comments like this:

 *                   Includes                                                 *

I want to be able to center within the *'s like this:

 *                                  Includes                                  *

I've been unable to do this. If I try M-x narrow-to-region and use M-x center-line, it deletes all the spaces on the right hand side instead of preserving them. Using M-x picture-mode does not improve matters. I tried moving to the fill column using M-x move-to-column hoping it would fill in the spaces, but it stops at the current length in both picture and non-picture mode.

  • There's rebox which looks quite powerful. 8 years old, so handle with care. There's rebox2 in melpa.
    – Juancho
    Commented Jun 30, 2020 at 16:38
  • @Juancho does Rebox have any documentation? It looks like you have to set a numeric style for comments, and it's some number, but I can't find the number assignments. I played around it with but couldn't get it to work. Commented Jul 7, 2020 at 13:23

1 Answer 1


I never found what I was looking for, so I ended up hacking the center-line elisp code like so:

(defun c-center-comment (&optional nlines)
  "Center the boxed comment point is on, within the width specified by `fill-column'.
This means adjusting the space around the comment enclosed by asterisks (*) so that it equals
the distance between the end of the text and `fill-column'.
The argument NLINES says how many lines to center."
  (interactive "P")
  (if nlines (setq nlines (prefix-numeric-value nlines)))
  (while (not (eq nlines 0))
      (let ((lm (current-left-margin))
        (delete-char 1)
        (forward-char -1)
        (delete-char 1)
    (setq line-length (current-column))
    (if (> (- fill-column lm line-length) 0)
        (progn (indent-line-to
                 (+ lm (/ (- (- fill-column 1) lm line-length) 2)))
              (if (< (% (- (- fill-column 1) lm line-length) 2) 1)
                    (delete-char 1))
              (insert-char ?\*)
              (insert-char ?\ (+ lm (/ (- (- fill-column 1) lm line-length) 2)))
              (insert-char ?\*)))))
    (cond ((null nlines)
       (setq nlines 0))
      ((> nlines 0)
       (setq nlines (1- nlines))
       (forward-line 1))
      ((< nlines 0)
       (setq nlines (1+ nlines))
       (forward-line -1)))))

Note that this is very primitive. It assumes the line already starts and ends with an asterisk (change the insert-char commands to modify the character). If you want to center a new line, surround it with any sacrificial character, but I only wrote this for fixing existing comments, not for composing new ones.

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