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Date:	Mon, 1 Jun 2026 04:11:52 +0200
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Subject:	Re: wcwidth broken with gcc 16
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From:	Thomas Wolff via Cygwin <cygwin AT cygwin DOT com>
Reply-To:	Thomas Wolff <towo AT towo DOT net>
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Hi Achim,

Am 31.05.2026 um 20:25 schrieb ASSI via Cygwin:
> Thomas Wolff via Cygwin writes:
>> cygwin has changed it already by defining wchar_t as 16-bit. My fix
>> only reverts this.
> Nope.  That's not a Cygwin thing and wchar_t can be anything, even less
> than 16bit, as far as the standards are concerned.  Yes, that's messy,
> but it is what it is.
>
>>> The fix for your bug is easy enough
>> It's just not a fix because it narrows the parameter to 16 bits and
>> thus breaks current wcwidth functionality.
> It doesn't break anything that wasn't already broken before.  There can
> be no correct Cygwin program that can use an argument to wcwidth that
> has more than 16 bits (and your own example doesn't use any of these
> anyway).  The public interface always was wchar_t (nee unsigned short on
> Cygwin) and the extension to the win_t in the implementation function in
> newlib libc just wasn't working as you (still?) seem to think it does
> when you mismatch the types.  The type extension of function arguments
> (via register sharing) on x86_64 is undefined, the resulting upper 16
> bits in this case (wchar_t --> wint_t) can be anything: zero extension,
> sign extension or even random bits.  This is just not something that can
> work reliably.  I don't know what aarch64 does on that front.
>
> In your example, you use an int as argument, which is used as a function
> argument declared as short unsigned (via wchar_t, so it gets truncated
> to 16 bit, at least logically) and then used an int again inside the
> actual implementation of that function.  This is wrong as the prototype
> and the implementation don't match, but the ABI only guarantees that the
> lower 16 bit of the original argument transfer in that case.  Even if
> zero extension was used you'd lose any upper bits in the original
> argument anyway (which you say works for your example and I'd argue that
> there can't be any upper bits anyway on Cygwin, see aboive), which is
> exactly what will _definitely_ happen if the libc implementation stub is
> reverted to the correct signature.
You explain what I had already conceded before, that I had overlooked 
the prototype and the intended functionality did not work:
   printf("%d\n", wcwidth(0x13000));  // should be 1
   printf("%d\n", wcwidth(0x1F500));  // should be 2

So why not establish that functionality with an easy tweak that does not 
interfere with anything?

> Regards,
> Achim.


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