cygwin 3.6.4 breaks mbrtowc

[Brian Inglis]

On 2025-09-29 16:52, Mark Geisert via Cygwin wrote:
> Hello Thomas,
> Apologies for the late response to your report.
> 
> On 7/21/2025 8:25 PM, Thomas Wolff via Cygwin wrote:
>> mbrtowc is broken in 3.6.4 which breaks non-BMP display in mintty.
>> Test case below.
>> Thomas
>>
>> #include <locale.h>
>> #include <wchar.h>
>> #include <stdio.h>
>>
>> void mb(unsigned char c)
>> {
>>    wchar_t wc;
>>    int ret = mbrtowc(&wc, &c, 1, 0);
>>    printf("%02X -> %04X : %d\n", c, wc, ret);
>> }
>>
>> void main ()
>> {
>>    setlocale (LC_CTYPE, "");
>>
>>    mb(0xF0);
>>    mb(0x9F);
>>    mb(0x98);
>>    mb(0x8E);
>> }
> 
> Running your testcase gives different output between 3.6.4 and 3.7.0-dev-139 but 
> I'm unsure the latter is correct.  Can you comment please?
> 
> On 3.6.4:
> ~ ./a
> F0 -> 0000 : -2
> 9F -> 0000 : -2
> 98 -> 0000 : -2
> 8E -> D83D : 3
> 
> On 3.7.0-dev-139:
> ~ ./a
> F0 -> 0000 : -2
> 9F -> 0000 : -2
> 98 -> D83D : 1
> 8E -> DE0E : 1

A code point converter I have agrees with the latter under 3.6.4, but the 
mbrtowc() return values should not be 1, 1, but 2, 1 possibly; otherwise you 
don't know when the character is complete:

$ utf8cp $'\xf0\x9f\x98\x8e'  😎
  😎  U+01f60e  f0 9f 98 8e  d83d de0e
  😎  U+01f60e  f0 9f 98 8e  d83d de0e
expanded:
f0 == 1111 0/000 => 4 bytes - 3 bits 0 00                      -- 0
9f == 10/01 1111 => 3       - 6          01 1111               -- 1f
98 == 10/01 1000 => 2       - 6                 0110 00        --   60
                       hi 10 - 1101 10/00 0011 1101                      == d8 3d
8e == 10/00 1110 => 1       - 6                        00 1110 --    0e
                       lo 10 -             1101 11/10 0000 1110          == de 0e

-- 
Take care. Thanks, Brian Inglis              Calgary, Alberta, Canada

La perfection est atteinte                   Perfection is achieved
non pas lorsqu'il n'y a plus rien à ajouter  not when there is no more to add
mais lorsqu'il n'y a plus rien à retrancher  but when there is no more to cut
                                 -- Antoine de Saint-Exupéry