Emacs comment-region in C mode

Question

In GNU Emacs, is there a good way to change the comment-region command in C mode from ``` /* This is a comment which extends */ /* over more than one line in C. */ ``` to ``` /* This is a comment which extends over more than one line in C. */ ``` ? I have tried ``` (setq comment-multi-line t) ``` but this does not help. There is a section on multi-line comments in the Emacs manual, but it does not mention anything.

Accepted Answer

Since Emacs 21, there's been a module named `'newcomment`, which has different comment styles (see the variable `'comment-styles`. This setting gets close to what you want:

```
(setq comment-style 'multi-line)
```

(Note: you should probably make that setting in `'c-mode-hook`).

However, none of the settings make the comments look like what you want.

The easiest way I saw to get what you want is to add this hack:

```
(defadvice comment-region-internal (before comment-region-internal-hack-ccs activate)
  "override 4th argument to be just spaces"
  (when (eq major-mode 'c-mode)  ; some condition here
    (let ((arg (ad-get-arg 4)))
      (when arg
        (ad-set-arg 4 (make-string (length arg) ?\ ))))))
```

The current settings for `comment-style` always prefix the comment lines with " * " (if not the whole " /* ").

If you don't have Emacs 21, I suppose you could simply download `newcomment.el` from the repository. I don't know if it works as-is in earlier versions of Emacs, but it might be worth a shot, though upgrading Emacs would be a better solution.

My hack breaks the `'uncomment-region`. A proper fix would be to change `'comment-padright`. That would take a little more research so as not to break other things. The above hack only changes behavior in `'c-mode` (adjust the condition to your liking).

Emacs comment-region in C mode

Solution

Problem