cygwin 3.6.4 breaks mbrtowc

Mon Sep 29 22:52:46 GMT 2025

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
Thanks much,
..mark