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To: cygwin AT cygwin DOT com
Subject: Thoughts on the wcwidth confusion
Organization: WiseMo A/S
Message-ID: <19c6f9b4-5f09-6929-891c-d25ebe48af82@wisemo.com>
Date: Mon, 1 Jun 2026 17:34:35 +0200
MIME-Version: 1.0
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From: Jakob Bohm via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Jakob Bohm <jb-cygwin AT wisemo DOT com>
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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.

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
-- 
Jakob Bohm, CIO, Partner, WiseMo A/S. https://www.wisemo.com
Transformervej 29, 2860 Søborg, Denmark. Direct +45 31 13 16 10
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