[PATCH] fix wcwidth to work with gcc 16 sign extension (Re: wchar.h: tweak wcwidth prototype parameter wchar_t -> wint_t)

[Jeff Johnston]

Patch applied.  Thanks.

-- Jeff J.

On Tue, Jun 2, 2026 at 8:43 PM Thomas Wolff <towo@towo.net> wrote:

> As discussed in the cygwin thread, I withdraw my previous patch and
> provide the attached one to fix wcwidth for gcc 16.
> Thomas
>
> Am 01.06.2026 um 18:02 schrieb Thomas Wolff:
> >
> > Am 31.05.2026 um 13:57 schrieb Takashi Yano:
> >> Hi Thomas,
> >>
> >> On Sun, 31 May 2026 10:06:12 +0200
> >> Thomas Wolff wrote:
> >>> Hi Brian,
> >>>
> >>> Am 31.05.2026 um 05:50 schrieb Brian Inglis via Cygwin:
> >>>> On 2026-05-28 22:58, Thomas Wolff wrote:
> >>>>> to make it compliant with newlib and the manual page;
> >>>>> fixes cases of wrong width calculation:
> >>>>> https://cygwin.com/pipermail/cygwin/2026-April/259597.html
> >>>>> as mentioned in
> >>>>> https://cygwin.com/pipermail/cygwin/2026-May/259734.html
> >>>>> as described in
> >>>>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125451#c14
> >>>>> attachment:
> >>>> 0001-wchar.h-tweak-wcwidth-prototype-parameter-wchar_t-wi.patch
> >>>>
> >>>> The existing wcwidth declaration in newlib/libc/include/wchar.h agrees
> >>>> with
> >>>> POSIX 8 SUS V5.
> >>>>
> >>>> It is the man doc, definition, and implementation in
> >>>> newlib/libc/string/wcwidth.c which need changed to match the
> >>>> specification and return codes in:
> >>>>
> >>>>
> https://pubs.opengroup.org/onlinepubs/9799919799/functions/wcwidth.html
> >>>>
> >>> Your argument overlooks one significant deviation: in POSIX, wchar_t
> >>> has
> >>> 32 bits, in cygwin only 16.
> >>> So to make wcwidth work for *all* Unicode character code points, the 32
> >>> bit version must be used.
> >>> I tested positively that this fixes the broken test case with gcc 16 I
> >>> had reported to the cygwin list.
> >> However, newlib is not used only by Cygwin, so I think newlib itself
> >> should
> >> follow POSIX. Shouldn't we have our own wcwidth() implementation for
> >> Cygwin?
> >>
> >> On second thought, since a 16‑bit wchar_t needs to be converted to a
> >> 32‑bit
> >> Unicode code point especially for surrogate pair, we cannot use
> >> wcwidth in
> >> the same way as Linux does. I wonder what the correct approach would be.
> > I don't there is a "correct" approach as POSIX probably did not
> > consider this problem.
> > But I just responded to a cute idea on the cygwin mailing list, which
> > was unfeasible but I modified it with a proposal to return width 1 for
> > a high surrogate, remember it, and then return 1 or 0 for the low
> > surrogate, respectively.
>
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