Re: using libmagic in Emacs?

[Juri Linkov]

>     For instance, Java archive files have the .jar extension but build on
>     the ZIP file format, so they can be visited in Emacs with the help of
>     `archive-mode'.  Enterprise Java archives with the .ear extension
>     and Web application Java archives with the .war extension all are
>     based on the ZIP file format as well as OpenDocument files with
>     extensions .odt .ods .odb .odp .odg .odf, Firefox add-ons (.xpi),
>     Keyhole Markup (.kmz), and many other file types that can be potentially
>     opened in Emacs if were identified as archive files by libmagic.
>
> How hard would it be to change the code in Emacs to recognize these
> using the existing mechanism?

Not hard at all.  For the ZIP file format it's just one line
in `magic-fallback-mode-alist'.  Unlike `image-type-auto-detected-p'
from the preloaded image.el, we can't use `archive-find-type' in
`magic-fallback-mode-alist' because arc-mode.el is not preloaded.
So we have to copy archive magic numbers manually from arc-mode.el
to `magic-fallback-mode-alist'.

Index: lisp/files.el
===================================================================
RCS file: /sources/emacs/emacs/lisp/files.el,v
retrieving revision 1.1069
diff -u -r1.1069 files.el
--- lisp/files.el       17 Aug 2009 23:40:22 -0000      1.1069
+++ lisp/files.el       25 Aug 2009 20:36:28 -0000
@@ -2399,6 +2399,7 @@
 
 (defvar magic-fallback-mode-alist
   `((image-type-auto-detected-p . image-mode)
+    ("\\(PK00\\)?[P]K\003\004" . archive-mode) ; zip
     ;; The < comes before the groups (but the first) to reduce backtracking.
     ;; TODO: UTF-16 <?xml may be preceded by a BOM 0xff 0xfe or 0xfe 0xff.
     ;; We use [ \t\r\n] instead of `\\s ' to make regex overflow less likely.

-- 
Juri Linkov
http://www.jurta.org/emacs/