bug#20258: 24.5; format-time-string miscounting of multibyte characters

bug#20258: 24.5; format-time-string miscounting of multibyte characters

[Gunnar Horrigmo]

As the subject says, format-time-string miscounts multibyte characters.
Simple example with nb_NO.utf8 locale, where ø is two bytes:

(format-time-string "%6a" (date-to-time "Sat Apr  4 16:14:40 2015"))
"  lø."

(length (format-time-string "%6a" (date-to-time "Sat Apr  4 16:14:40 2015")))
5

Let me know if you need more info.

--Gunnar




In GNU Emacs 24.5.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.14.10)
 of 2015-04-01 on lumpy-gravy.uio.no
Repository revision: 1b70aa634c9ce117fed418894b54b1f2647bda1c
Windowing system distributor `StarNet Communications Corp.', version 11.0.14000
System Description:	Fedora release 21 (Twenty One)

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-- 
Gunnar

bug#20258: 24.5; format-time-string miscounting of multibyte characters

[Eli Zaretskii]

> From: Gunnar Horrigmo <gunnar.horrigmo@usit.uio.no>
> Date: Sat, 04 Apr 2015 16:33:50 +0200
> 
> 
> As the subject says, format-time-string miscounts multibyte characters.
> Simple example with nb_NO.utf8 locale, where ø is two bytes:
> 
> (format-time-string "%6a" (date-to-time "Sat Apr  4 16:14:40 2015"))
> "  lø."
> 
> (length (format-time-string "%6a" (date-to-time "Sat Apr  4 16:14:40 2015")))
> 5

'length' counts characters, not bytes.  If you need to count bytes,
use 'string-bytes' instead:

  (string-bytes "  lø.") => 6

bug#20258: 24.5; format-time-string miscounting of multibyte characters

[Stefan Monnier]

> 'length' counts characters, not bytes.  If you need to count bytes,
> use 'string-bytes' instead:

>   (string-bytes "  lø.") => 6

And in 99% of the cases, using length-bytes doesn't do what you think
(it doesn't count the number of bytes that it would take in your
favorite coding-system, but the number of bytes it takes within Emacs's
internal encoding).
If you want to know how many bytes it would take in your locale's
encoding, then you need:

   (length (encode-coding-string <thestring> locale-coding-system))


        Stefan

bug#20258: 24.5; format-time-string miscounting of multibyte characters

[Andreas Schwab]

Gunnar Horrigmo <gunnar.horrigmo@usit.uio.no> writes:

> As the subject says, format-time-string miscounts multibyte characters.
> Simple example with nb_NO.utf8 locale, where ø is two bytes:
>
> (format-time-string "%6a" (date-to-time "Sat Apr  4 16:14:40 2015"))
> "  lø."

This is a limitation of the underlying strftime, which operates on
bytes, not characters.  This could be fixed by using wcsftime instead.

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

bug#20258: 24.5; format-time-string miscounting of multibyte characters

[Stefan Kangas]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Gunnar Horrigmo <gunnar.horrigmo@usit.uio.no>
>> Date: Sat, 04 Apr 2015 16:33:50 +0200
>>
>>
>> As the subject says, format-time-string miscounts multibyte characters.
>> Simple example with nb_NO.utf8 locale, where ø is two bytes:
>>
>> (format-time-string "%6a" (date-to-time "Sat Apr  4 16:14:40 2015"))
>> "  lø."
>>
>> (length (format-time-string "%6a" (date-to-time "Sat Apr  4 16:14:40 2015")))
>> 5
>
> 'length' counts characters, not bytes.  If you need to count bytes,
> use 'string-bytes' instead:
>
>   (string-bytes "  lø.") => 6

I can see no bug here, only a misunderstanding about the length
function.  I'm therefore closing this bug.  If that's incorrect, please
reopen this bug report.

Best regards,
Stefan Kangas

bug#20258: 24.5; format-time-string miscounting of multibyte characters

[Lars Ingebrigtsen]

Stefan Kangas <stefan@marxist.se> writes:

>>> As the subject says, format-time-string miscounts multibyte characters.
>>> Simple example with nb_NO.utf8 locale, where ø is two bytes:
>>>
>>> (format-time-string "%6a" (date-to-time "Sat Apr  4 16:14:40 2015"))
>>> "  lø."
>>>
>>> (length (format-time-string "%6a" (date-to-time "Sat Apr  4 16:14:40 2015")))
>>> 5
>>
>> 'length' counts characters, not bytes.  If you need to count bytes,
>> use 'string-bytes' instead:
>>
>>   (string-bytes "  lø.") => 6
>
> I can see no bug here, only a misunderstanding about the length
> function.  I'm therefore closing this bug.  If that's incorrect, please
> reopen this bug report.

But the issue here is that "%6a" should give you a string that's six
characters long, I think?  Admittedly the doc string is vague here:

---
A field width N is an unsigned decimal integer with a leading digit nonzero.
%NX is like %X, but takes up at least N positions.
---

But the natural interpretation of "positions" isn't bytes, I think, and
if is, then the doc string should say so.

(let ((system-time-locale "nb_NO.UTF-8"))
  (format-time-string "%6a" (date-to-time "Sat Apr  4 16:14:40 2015")))
=> "  lø."

(if you have that locale in /etc/locale.gen.)

But I seem to remember from previous discussions that this quirk is in
the C strftime function?  And Emacs just call it?  I haven't checked.
But this means that you can't use format-time-string to line stuff up,
but have to use `format':

(let ((system-time-locale "nb_NO.UTF-8"))
  (format "%6s" (format-time-string "%a" (date-to-time "Sat Apr  4 16:14:40 2015"))))
=> "   lø."

So I think what WIDTH means should be said explicitly in the doc string.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

bug#20258: 24.5; format-time-string miscounting of multibyte characters

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Mon, 30 Sep 2019 05:09:08 +0200
> Cc: stefan@marxist.se, gunnar.horrigmo@usit.uio.no
> 
> A field width N is an unsigned decimal integer with a leading digit nonzero.
> %NX is like %X, but takes up at least N positions.
> ---
> 
> But the natural interpretation of "positions" isn't bytes, I think, and
> if is, then the doc string should say so.
> 
> (let ((system-time-locale "nb_NO.UTF-8"))
>   (format-time-string "%6a" (date-to-time "Sat Apr  4 16:14:40 2015")))
> => "  lø."
> 
> (if you have that locale in /etc/locale.gen.)
> 
> But I seem to remember from previous discussions that this quirk is in
> the C strftime function?  And Emacs just call it?

Yes, that's true.

> So I think what WIDTH means should be said explicitly in the doc string.

It can only warn that WIDTH _might_ be measured in bytes, since the
underlying implementation of strftime just might DTRT.  Or not.

I think this should be raised as a bug to glibc developers, as their
documentation says "characters", according to my reading.

bug#20258: 24.5; format-time-string miscounting of multibyte characters

[Andreas Schwab]

On Sep 30 2019, Eli Zaretskii <eliz@gnu.org> wrote:

> I think this should be raised as a bug to glibc developers, as their
> documentation says "characters", according to my reading.

The POSIX descriptions says bytes.

Andreas.

-- 
Andreas Schwab, SUSE Labs, schwab@suse.de
GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA B9D7
"And now for something completely different."

bug#20258: 24.5; format-time-string miscounting of multibyte characters

[Eli Zaretskii]

> From: Andreas Schwab <schwab@suse.de>
> Cc: Lars Ingebrigtsen <larsi@gnus.org>,  20258@debbugs.gnu.org,  stefan@marxist.se,  gunnar.horrigmo@usit.uio.no
> Date: Mon, 30 Sep 2019 10:41:23 +0200
> 
> On Sep 30 2019, Eli Zaretskii <eliz@gnu.org> wrote:
> 
> > I think this should be raised as a bug to glibc developers, as their
> > documentation says "characters", according to my reading.
> 
> The POSIX descriptions says bytes.

Right.  So it might be a glibc documentation bug (or maybe the glibc
manual I have here is outdated).

And there is the issue with non-glibc implementations.

Thanks.

bug#20258: 24.5; format-time-string miscounting of multibyte characters

[Lars Ingebrigtsen]

Eli Zaretskii <eliz@gnu.org> writes:

>> The POSIX descriptions says bytes.
>
> Right.  So it might be a glibc documentation bug (or maybe the glibc
> manual I have here is outdated).
>
> And there is the issue with non-glibc implementations.

What about something appropriately vague like the following patch to
draw attention to the issue:

diff --git a/src/timefns.c b/src/timefns.c
index 330d5623f0..20f7ccb7d7 100644
--- a/src/timefns.c
+++ b/src/timefns.c
@@ -1437,8 +1437,11 @@ DEFUN ("format-time-string", Fformat_time_string, Sformat_time_string, 1, 3, 0,
 `^' Use upper case characters if possible.
 `#' Use opposite case characters if possible.
 
-A field width N is an unsigned decimal integer with a leading digit nonzero.
-%NX is like %X, but takes up at least N positions.
+A field width N is an unsigned decimal integer with a leading digit
+nonzero.  %NX is like %X, but takes up at least N positions.  The
+field width is (on most systems) in bytes, not characters, so it
+depends on the locale what the width (in characters) %NX will end up
+being.
 
 The modifiers are:
 


-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

bug#20258: 24.5; format-time-string miscounting of multibyte characters

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: Andreas Schwab <schwab@suse.de>,  20258@debbugs.gnu.org,
>   stefan@marxist.se,  gunnar.horrigmo@usit.uio.no
> Date: Mon, 30 Sep 2019 15:39:15 +0200
> 
> -A field width N is an unsigned decimal integer with a leading digit nonzero.
> -%NX is like %X, but takes up at least N positions.
> +A field width N is an unsigned decimal integer with a leading digit
> +nonzero.  %NX is like %X, but takes up at least N positions.  The
> +field width is (on most systems) in bytes, not characters, so it

"is measured in bytes".  Also, I'd say "on GNU/Linux and some other
systems", which is marginally more accurate.

> +depends on the locale what the width (in characters) %NX will end up
> +being.

I would mention "non-ASCII characters" here in some way, not just the
locale, to make this more explicit.

Thanks.

bug#20258: 24.5; format-time-string miscounting of multibyte characters

[Lars Ingebrigtsen]

Eli Zaretskii <eliz@gnu.org> writes:

> "is measured in bytes".  Also, I'd say "on GNU/Linux and some other
> systems", which is marginally more accurate.

OK.

>> +depends on the locale what the width (in characters) %NX will end up
>> +being.
>
> I would mention "non-ASCII characters" here in some way, not just the
> locale, to make this more explicit.

I was pondering whether any users had a locale of *.UTF-16.  Then even
the ASCII characters will be subject to the byte/character difference,
so I thought it was best to leave even that vague.

But perhaps saying something like "especially with non-ASCII characters"
wouldn't be too misleading?

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

bug#20258: 24.5; format-time-string miscounting of multibyte characters

[Gunnar Horrigmo]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> I was pondering whether any users had a locale of *.UTF-16.

Windows users might, if that's at all relevant to the discussion.

--Gunnar

bug#20258: 24.5; format-time-string miscounting of multibyte characters

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: schwab@suse.de,  20258@debbugs.gnu.org,  stefan@marxist.se,
>   gunnar.horrigmo@usit.uio.no
> Date: Mon, 30 Sep 2019 16:12:38 +0200
> 
> > I would mention "non-ASCII characters" here in some way, not just the
> > locale, to make this more explicit.
> 
> I was pondering whether any users had a locale of *.UTF-16.

Unlikely.

> But perhaps saying something like "especially with non-ASCII characters"
> wouldn't be too misleading?

Yes, that's what I had in mind.

bug#20258: 24.5; format-time-string miscounting of multibyte characters

[Eli Zaretskii]

> From: Gunnar Horrigmo <gunnar.horrigmo@usit.uio.no>
> Cc: Eli Zaretskii <eliz@gnu.org>,  <schwab@suse.de>,
>   <20258@debbugs.gnu.org>,  <stefan@marxist.se>,
>   <gunnar.horrigmo@usit.uio.no>
> Date: Mon, 30 Sep 2019 16:30:55 +0200
> 
> Lars Ingebrigtsen <larsi@gnus.org> writes:
> 
> > I was pondering whether any users had a locale of *.UTF-16.
> 
> Windows users might

I don't think so.  AFAIK, UTF-16 is not a valid codeset of any Windows
locale.  Windows uses UTF-16 internally, and exposes it in the Windows
APIs, but APIs that came from Posix (and locale is one of them) only
support single-byte and DBCS encodings as their codeset.

bug#20258: 24.5; format-time-string miscounting of multibyte characters

[Lars Ingebrigtsen]

Eli Zaretskii <eliz@gnu.org> writes:

>> But perhaps saying something like "especially with non-ASCII characters"
>> wouldn't be too misleading?
>
> Yes, that's what I had in mind.

OK; doc string updated accordingly, and I'm closing this bug report.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no