Is it possible to autosave temporary buffers that are not visiting a file?

Question

Suppose I'm entering a bunch of text in a buffer that's not visiting a file (it could be a new org2blog post, or just some scratch buffer). Is it possible to autosave it somewhere in case disaster strikes and Emacs dies?

Answer 1

auto-save-mode actually works with non-file buffers. It just doesn't get enabled by default -- that usually happens in (after-find-file).

So: M-x auto-save-mode RET

By default the auto-save file will be written to the default-directory of the buffer (or /var/tmp or ~/, depending on write permissions; see C-hv buffer-auto-save-file-name RET) which may be a little awkward to figure out after a crash, so setting that to something standard is probably a good idea.

The following would ensure that these auto-save files are written to your home directory (or M-x customize-variable RET my-non-file-buffer-auto-save-dir RET), if auto-save-mode is invoked interactively. That will hopefully avoid this conflicting with any other uses of auto-save-mode with non-file buffers (the code mentions Mail mode, for instance).

(defcustom my-non-file-buffer-auto-save-dir (expand-file-name "~/")
  "Directory in which to store auto-save files for non-file buffers,
when `auto-save-mode' is invoked manually.")

(defadvice auto-save-mode (around use-my-non-file-buffer-auto-save-dir)
  "Use a standard location for auto-save files for non-file buffers"
  (if (and (not buffer-file-name)
           (called-interactively-p 'any))
      (let ((default-directory my-non-file-buffer-auto-save-dir))
        ad-do-it)
    ad-do-it))
(ad-activate 'auto-save-mode)

Answer 2

phils' answer cleared things up for me, but I ended up using a somewhat different approach. I am posting it as a separate answer for documentation's sake. Here is my autosave stanza:

;; Put autosave files (ie #foo#) in one place
(defvar autosave-dir (concat "~/.emacs.d/autosave.1"))
(defvar autosave-dir-nonfile (concat "~/.emacs.d/autosave.nonfile"))
(make-directory autosave-dir t)
(make-directory autosave-dir-nonfile t)
(defun auto-save-file-name-p (filename) (string-match "^#.*#$" (file-name-nondirectory filename)))
(defun make-auto-save-file-name () 
  (if buffer-file-name (concat autosave-dir "/" "#" (file-name-nondirectory buffer-file-name) "#")
    (expand-file-name (concat autosave-dir-nonfile "/" "#%" 
                              (replace-regexp-in-string "[*]\\|/" "" (buffer-name)) "#"))))

Creating a separate directory for the non-visited file buffers is optional in this context; they could as well go in the centralized location (in this case, autosave-dir). Note also that I have to do some basic file name cleanup in case the temporary buffer name is something like "*foo/bar*" (with stars and/or slashes).

Finally, one can automatically turn on autosave in certain modes' temp buffers using something like

(add-hook 'org2blog/wp-mode-hook '(lambda () (auto-save-mode t)))