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

Question

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

Answer 1

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.