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I am trying to modify a buffer written by desktop-save to a file. The file is named .emacs.desktop and I am trying to modify it from a Perl script. It turns out that it is saved in emacs-mule encoding.

From M-x list-coding-systems :

U -- utf-8 (alias: mule-utf-8)
  UTF-8 (no signature (BOM))
M -- emacs-mule
  Emacs 21 internal format used in buffer and string.

I would like to read the file in Perl as UTF-8, how can this be done? (I would prefer a solution that does not depend on a running Emacs server)

(An example: emacs-mule saves the letter æ as two bytes 0x81 0xe6, ISO Latin 1 saves it as a single byte 0xe6, whereas UTF-8 saves it as two bytes 0xc3 0xa6.. )

  • Can M-x revert-buffer-with-coding-system help? – Name Jul 24 '15 at 11:43
  • @Name Yes it seems like I have to consider options for doing the conversion from within Emacs.. I could not find any Perl modules that can convert between MULE and UTF-8.. so currently, it seems only Emacs knows how to do the conversion. The drawback with this approach is that the Perl script needs to rely on running a new Emacs instance in batch mode.. – Håkon Hægland Jul 24 '15 at 11:55
  • I am not sure if the problem discussed in this link stackoverflow.com/q/627661 is related to your question but it is worth looking at. – Name Jul 24 '15 at 13:46
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    You might be able to dig the details of the emacs-mule coding out of the file src/coding.c in the emacs source code. There are extensive comments in that file. – Harald Hanche-Olsen Jul 24 '15 at 15:32
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Here is the solution I ended up with:

  • Assume an Emacs server with name my_server is up and running.

  • Convert emacs-mule to utf-8 by calling emacsclient from the Perl script:

    emacsclient -s my_server -e '(my-convert-mule-utf8 \".emacs.desktop\")' >/dev/null 2>&1`
    

where my-convert-mule-utf8 is

(defun my-convert-mule-utf8 (fn)
  (let*  ((temp-fn (concat fn ".utf8"))
         (coding-system-for-write 'utf-8))
    (with-temp-file temp-fn
      (insert-file-contents fn)
      (when (re-search-forward "coding: \\(emacs-mule\\);" (line-end-position) t)
        (replace-match "utf-8" nil t nil 1)))))
  • To convert utf-8 to emacs-mule:

    emacsclient -s my_server -e '(my-convert-utf8-mule \".emacs.desktop\")' >/dev/null 2>&1
    

where my-convert-utf8-mule is:

(defun my-convert-utf8-mule (fn)
  (let* ((temp-fn (concat fn ".utf8"))
         (coding-system-for-write 'emacs-mule))
    (with-temp-file fn
      (insert-file-contents temp-fn)
      (when (re-search-forward "coding: \\(utf-8\\);" (line-end-position) t)
        (replace-match "emacs-mule" nil t nil 1)))))

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