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Unibyte strings in Lisp data structures
From: |
Eli Zaretskii |
Subject: |
Unibyte strings in Lisp data structures |
Date: |
Tue, 13 Jul 2010 17:28:50 +0300 |
Take a look at jka-compr-compression-info-list: each compression
method has a magic signature there, which is the 9th element of the
vector describing that compression method.
Now evaluate this:
(multibyte-string-p (aref (car jka-compr-compression-info-list) 9))
=> nil
These magic signatures are unibyte strings. But why are they unibyte?
What code decides that they should be unibyte, when Emacs reads
jka-cmpr-hook.el? Can we rely on the fact that these strings will
always be unibyte?
I bumped into this while debugging a problem in rmailmm.el: saving
attachments whose file names end in .gz produces a file that is
gzip-compressed twice. I finally traced this to this fragment in
jka-compr.el:
;; If the contents to be written out
;; are properly compressed already,
;; don't try to compress them over again.
(not (and magic
(equal (if (stringp start)
(substring start 0 (min (length start)
(length magic)))
(let* ((from (or start (point-min)))
(to (min (or end (point-max))
(+ from (length magic)))))
(buffer-substring from to)))
magic))))
This test failed, because `magic' is a unibyte string, while
buffer-substring was returning a multibyte string.
The fix seems to be easy: modify rmail-mime-save to make the temporary
buffer it uses be a unibyte buffer. But then I started to wonder how
come `magic' is a unibyte string, and can I rely on that?
There is, of course, the alternative to convert both strings to
unibyte and compare that. Still, I think it would be good to know how
come these strings are unibyte to begin with.
- Unibyte strings in Lisp data structures,
Eli Zaretskii <=