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Date: Fri, 12 Jun 2026 12:15:20 +0200
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Subject: Re: wcwidth broken with gcc 16
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From: Thomas Wolff via Cygwin <cygwin@cygwin.com>
Reply-To: Thomas Wolff <towo@towo.net>
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Hi Doug,

Am 02.06.2026 um 20:43 schrieb Doug Henderson via Cygwin:
>   On Tue, Jun 2, 2026 at 1:46 AM Thomas Wolff via Cygwin <cygwin@cygwin.com>
> wrote:
>
> Thomas Wolff, et. al.,
>
> I have been carefully following this discussion of technical issues, API
> conformance, and related issues,.
>
> There has been no significant discussion of the user experience that
> depends on the resolution of this problem.
>
> Will non-BMP Unicode code points display correctly in terminal windows
> (that use an appropriate font), e.g. mintty?
I noticed the problem when I compiled mintty on a system where I had 
installed the test release of gcc 16 and various display widths were 
wrong, actually also within the BMP. So a fix was needed and it turned 
out that the issue was raised by, as I still think, wrong sign-extension 
of a 16-bit value to a 32-bit parameter by gcc 16 where previous gcc 
versions had only applied unsigned bit extension. Well, for subtle 
reasons, this was not accepted as a bug in gcc, so the cygwin API had to 
be adjusted.
About non-BMP characters, requesting their width with the wcwidth 
function never used to work (my first plan was to make it work but this 
was rejected as not standard-conformant and in fact would not have been 
portable so actually useless in the first place). If a program wants to 
determine the locale width of a non-BMP character on a UTF-16-based 
system, it needs to split it into its Unicode surrogates and use the 
wcswidth function. This was also done by mintty already, so there was no 
non-BMP-specific problem.
This will be handled under-the-hood by mintty, applications can use 
UTF-8 4-byte sequences towards the terminal of course.

> Non-BMP Unicode code points include emojis, mathematical script glyphs, and many others.
>
> For me, I care if a small Python script like:
>
> $ type main.py
> print("U+01D49E ‹𝒞›  GC=Lu    MATHEMATICAL SCRIPT CAPITAL C")
>
> outputs:
> U+01D49E ‹𝒞›  GC=Lu    MATHEMATICAL SCRIPT CAPITAL C
Your test case does not cover the problem. Plain output will always 
appear properly as long as you don't reach the end of the line.
The question is whether for example python has an idea of current cursor 
position and whether it is consistent with output.
Maybe you can try to evaluate that too.

> on Windows Terminal, as expected
>
> and when running cygwin in a mintty window:
>
> $ uname -a
> CYGWIN_NT-10.0-26200 mercury 3.6.9-1.x86_64 2026-04-21 15:46 UTC x86_64
> Cygwin
> $ mintty --version
> mintty 3.8.2 (Cygwin-x86_64)
> $ date
> Jun  2, 2026 11:36:33
> $ python3 -V
> Python 3.12.12
> $ cat main.py
> print("U+01D49E ‹𝒞›  GC=Lu    MATHEMATICAL SCRIPT CAPITAL C")
> $ python3 main.py
> U+01D49E ‹𝒞›  GC=Lu    MATHEMATICAL SCRIPT CAPITAL C
>
> correctly displays the Mathematical Script Capital C glyph as seen in the
> attached screen capture.
>
> If the resolution of this problem changes "our" user experience, that will
> be another problem.
>
> As an after thought, I may be seeing success due to the byte stream
> containing UTF-8 4-byte sequences all the way through Windows 11 APIs,
> without any conversions to UTF-16 or UTF-32.
>
> I am not so up-to-date with C as to rattle off a demo in seconds, as I can
> in Python. I would like to see such a minimal C demo program that I can try
> with cmd in WT, with MINGW64 in mintty, and with cygwin in mintty. The demo
> should send UTF-8 4-byte, UTF-16 2-short, and UTF-32 1-long characters, if
> possible. TIA.
>
> Just my take,
> Doug
>
> -- Doug Henderson, Calgary, Alberta, Canada - from gmail.com
>


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