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Message-ID: <b5b175eb-1225-43cf-bda1-eef0cc9cff78@towo.net>
Date: Mon, 1 Jun 2026 17:59:14 +0200
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Subject: Re: Thoughts on the wcwidth confusion
To: cygwin AT cygwin DOT com
References: <19c6f9b4-5f09-6929-891c-d25ebe48af82 AT wisemo DOT com>
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Precedence: list
From: Thomas Wolff via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Thomas Wolff <towo AT towo DOT net>
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Errors-To: cygwin-bounces~archive-cygwin=delorie DOT com AT cygwin DOT com
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Am 01.06.2026 um 17:34 schrieb Jakob Bohm via Cygwin:
> Dear list,
>
> Having read through the recent debate around the wcwidth() POSIX API,
> wchar_t definitions, gcc-16 and cygwin, I have an idea not
> mentioned in the list so far:
>
> Using C17 types char32_t and char16_t, the situation can be
> summarized as follows:
>
> - Many, but not all POSIX systems define wchar_t as char32_t and thus
> wint_t as uint_least32_t
>
> - Win32 and thus Cygwin defines wchar_t as char16_t and thus wint_t as
> uint_least16_t
>
> - All systems considered treat wchar_t as unicode, with Win32 supporting
>  UTF-16 since the NT 5.00 (Windows 2000).
>
> - For char16_t/UTF-16, wcwidth() should use the high surrogate to
>  determine the range of unicode symbols and return a width common to
>  that range, then return 0 for the low surrogates, thereby allowing
>  computation of string width without having to first assemble surrogates
>  into full char32_t values.  Deciding if char32_t implementations should
>  still lump groups of 4 Unicode rows for UTF-16 compatibility is up to
>  each implementation.
It's a neat idea to split the width calculation over the surrogates. 
Unfortunately it does not work this way because widthness does not 
change in full 1024-byte blocks. For example, U+1F4FC is Wide, U+1F4FD 
and U+1F4FE are narrow/Neutral (N), and U+1F4FF is W again.
As a variant of your idea, wcwidth could return width 1 for every high 
surrogate, remember it, and if the subsequent invocation is a low 
surrogate, determine the combined width and return either 1 or 0.
Not quite standard behaviour, I suspect, so maybe not a good idea for 
the purists, but maybe worth some discussion.

>
> A practical solution would be for Cygwin/newlib to provide new functions
> c16width(), c32width(), c16swidth() and c32swidth(), each being the
> explicit size equivalants of their wc and wcs similarly named functions.
>
> Then wcwidth() can be a trivial inline alias of the explicit size
> equivalent for the compile target by having the newlib header checking a
> compiler or standard define indicating the chosen size of wchar_t.
>
> // possible wchar.h snippet
> //
> // C17+ required
> // For C2Y+ this should go in uchar.h
> //
> int c16width(char16_t c);
> int c32width(char32_t c);
> int c16swidth(const char16_t *s, size_t n);
> int c32swidth(const char32_t *s, size_t n);
>
> // ...
>
> // This belongs in wchar.h for C1x- compat
> //
> #if SOMETHING_MEANING_16bit_WCHAR_T
> inline int wcwidth(wchar_t c) {
>   return c16width(c);
> }
> inline int wcswidth(const wchar_t *s, size_t n)
> {
>   return c16swidth(s, n);
> }
> #else
> inline int wcwidth(wchar_t c) {
>   return c32width(c);
> }
> inline int wcswidth(const wchar_t *s, size_t n)
> {
>   return c32swidth(s, n);
> }
> #endif
>
>
> Enjoy
>
> Jakob


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