[PATCH] wcwidth: adjust manpage input (Re: [PATCH] fix wcwidth to work with gcc 16 sign extension)

[Thomas Wolff]

amending my code patch... sorry I didn't include this right away

Am 04.06.2026 um 17:33 schrieb 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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From b082f9a3108b5c7628a359aa7a636960457e3366 Mon Sep 17 00:00:00 2001
From: Thomas Wolff <towo@towo.net>
Date: Sat, 6 Jun 2026 00:00:00 +0000
Subject: [PATCH] wcwidth: adjust manpage source to parameter width patch

---
 newlib/libc/string/wcwidth.c | 34 +++++++++++++++++++++++++++++++---
 1 file changed, 31 insertions(+), 3 deletions(-)

diff --git a/newlib/libc/string/wcwidth.c b/newlib/libc/string/wcwidth.c
index dfcaa6c21..93cc3e829 100644
--- a/newlib/libc/string/wcwidth.c
+++ b/newlib/libc/string/wcwidth.c
@@ -7,15 +7,17 @@ INDEX
 
 SYNOPSIS
 	#include <wchar.h>
-	int wcwidth(const wint_t <[wc]>);
+	int wcwidth(const wchar_t <[wc]>);
 
 DESCRIPTION
 	The <<wcwidth>> function shall determine the number of column
 	positions required for the wide character <[wc]>. The application
 	shall ensure that the value of <[wc]> is a character representable
-	as a wint_t (combining Unicode surrogate pairs into single 21-bit
-	Unicode code points), and is a wide-character code corresponding to a
+	as a wchar_t, and is a wide-character code corresponding to a
 	valid character in the current locale.
+	Note that for a Unicode character outside the 16-bit range, 
+	the application must split it into Unicode surrogates 
+	and use the <<wcswidth>> function instead.
 
 RETURNS
 	The <<wcwidth>> function shall either return 0 (if <[wc]> is a null
@@ -23,6 +25,30 @@ RETURNS
 	be occupied by the wide-character code <[wc]>, or return -1 (if <[wc]>
 	does not correspond to a printable wide-character code).
 
+EXAMPLE
+	An application function to determine the width of a 21-bit 
+	Unicode character may look like this:
+
+		typedef unsigned int uchar_t;
+		// determine high and low surrogates of Unicode character
+		wchar_t hisurr(uchar_t xc)
+		{
+		  return 0xD800 | (((xc - 0x10000) >> 10) & 0x3FF);
+		}
+		wchar_t losurr(uchar_t xc)
+		{
+		  return 0xDC00 | (xc & 0x3FF);
+		}
+
+		// determine width of 21-bit Unicode character
+		int ucwidth(uchar_t uc)
+		{
+		  if (uc < 0x10000)
+		    return wcwidth(uc);
+		  else
+		    return wcswidth((wchar_t[]){hisurr(uc), losurr(uc)}, 2);
+		}
+
 PORTABILITY
 <<wcwidth>> has been introduced in the Single UNIX Specification Volume 2.
 <<wcwidth>> has been marked as an extension in the Single UNIX Specification Volume 3.
@@ -165,6 +191,8 @@ bisearch(wint_t ucs, const struct interval *table, int max)
 
 int
 __wcwidth (const wint_t ucs)
+// unlike wcwidth, the parameter type of __wcwidth must be 32 bits wide
+// in order to support wcswidth
 {
 #ifdef _MB_CAPABLE
   /* sorted list of non-overlapping intervals of East Asian Ambiguous chars */
-- 
2.51.0