How do I change read/write mode for a file using Emacs?

If a file is set to read only mode, how do I change it to write mode and vice versa from within Emacs?

> M-x read-only-mode

in very old versions of Emacs, the command was:

> M-x toggle-read-only

On my Windows box, that amounts to Alt-x to bring up the meta prompt and typing "read-only-mode" to call the correct elisp function.

If you are using the default keyboard bindings,

> C-x C-q

(which you read aloud as "Control-X Control-Q") will have the same effect. Remember, however, given that emacs is essentially infinitely re-configurable, your mileage may vary.

Following up from the commentary: you should note that the writeable status of the buffer does not change the writeable permission of the file. If you try to write out to a read only file, you'll see a confirmation message. However, if you own the file, you can write out your changes without changing the permissions on the file.

This is very convenient if you'd like to make a quick change to a file without having to go through the multiple steps of add write permission, write out changes, remove write permission. I tend to forget that last step, leaving potentially critical files open for accidental changes later on.

Be sure you're not confusing 'file' with 'buffer'. You can set buffers to read-only and back again with C-x C-q (toggle-read-only). If you have permission to read, but not write, a file, the buffer you get when you visit the file (C-x C-f or find-file) will be put in read-only mode automatically. If you want to change the permissions on a file in the file system, perhaps start with dired on the directory that contains the file. Documentation for dired can be found in info; C-h i (emacs)dired RET.

What I found is M-x set-file-modes filename mode

It worked at my Windows Vista box. For example: M-x set-file-modes <RET> ReadOnlyFile.txt <RET> 0666

CTRL + X + CTRL + Q     

As mentioned up there by somebody else: M-x toggle-read-only would work.

However, this is now deprecated and M-x read-only-mode is the current way to do it, that it is set to C-x C-q keybinding.

If only the buffer (and not the file) is read-only, you can use toggle-read-only, which is usually bound to C-x C-q.

If the file itself is read-only, however, you may find the following function useful:

(defun set-buffer-file-writable ()
  "Make the file shown in the current buffer writable.
Make the buffer writable as well."
  (interactive)
  (unix-output "chmod" "+w" (buffer-file-name))
  (toggle-read-only nil)
  (message (trim-right '(?\n) (unix-output "ls" "-l" (buffer-file-name)))))

The function depends on unix-output and trim-right:

(defun unix-output (command &rest args)
  "Run a unix command and, if it returns 0, return the output as a string.
Otherwise, signal an error.  The error message is the first line of the output."
  (let ((output-buffer (generate-new-buffer "*stdout*")))
    (unwind-protect
	 (let ((return-value (apply 'call-process command nil
				    output-buffer nil args)))
	   (set-buffer output-buffer)
	   (save-excursion 
	     (unless (= return-value 0)
	       (goto-char (point-min))
	       (end-of-line)
	       (if (= (point-min) (point))
		   (error "Command failed: %s%s" command
			  (with-output-to-string
			      (dolist (arg args)
				(princ " ")
				(princ arg))))
		   (error "%s" (buffer-substring-no-properties (point-min) 
							       (point)))))
	     (buffer-substring-no-properties (point-min) (point-max))))
      (kill-buffer output-buffer))))

(defun trim-right (bag string &optional start end)
  (setq bag (if (eq bag t) '(?\  ?\n ?\t ?\v ?\r ?\f) bag)
	start (or start 0)
	end (or end (length string)))
  (while (and (> end 0)
	      (member (aref string (1- end)) bag))
    (decf end))
  (substring string start end))

Place the functions in your ~/.emacs.el, evaluate them (or restart emacs). You can then make the file in the current buffer writable with M-x set-buffer-file-writable.

If you are looking at a directory of files (dired), then you can use Shift + M on a filename and enter the modespec, the same attributes used in the chmod command.
M modespec <RET>

See the other useful commands on files in a directory listing at http://www.gnu.org/s/libtool/manual/emacs/Operating-on-Files.html

I tried out Vebjorn Ljosa's solution, and it turned out that at least in my Emacs (22.3.1) there isn't such function as 'trim-right', which is used for removing an useless newline at the end of chmod output.

Removing the call to 'trim-right' helped, but made the status row "bounce" because of the extra newline.

C-x C-q is useless. Because your also need the permission to save a file.

I use Spacemacs. It gives me a convenient function to solve this question. The code is following.

(defun spacemacs/sudo-edit (&optional arg)
  (interactive "p")
  (if (or arg (not buffer-file-name))
      (find-file (concat "/sudo:root@localhost:" (ido-read-file-name "File: ")))
    (find-alternate-file (concat "/sudo:root@localhost:" buffer-file-name))))

I call spacemacs/sudo-edit to open a file in emacs and input my password, I can change the file without read-only mode.

You can write a new function like spacemacs/sudo-edit.