Re: 16-bit wchar_t on Windows and Cygwin

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

On Feb  2 12:29, Bruno Haible wrote:
> Hello Eric,
> 
> > ... POSIX requires that 1 wchar_t corresponds to 1 character
> > ...
> > > What consequences does this have?
> > > 
> > >   1) All code that uses the functions from <wctype.h> (wide character
> > >      classification and mapping) or wcwidth() malfunctions on strings that
> > >      contains Unicode characters outside the BMP, i.e. outside the range
> > >      U+0000..U+FFFF.
> > 
> > Not necessarily.  Such code falls outside of POSIX, but it may still be
> > a well-behaved extension if given sane behavior for how to deal with
> > surrogates.
> 
> No. Code that uses <wctype.h> and wcwidth() is written precisely according
> to POSIX. The problem is that this code cannot work correctly when wchar_t[]
> is in UTF-16 encoding. There simply is no way to define these functions
> in a reasonable way for surrogates.
> 
> For example:
>   U+1031E = 0xD800 0xDF1E   is a letter (iswalpha should be true)
>   U+10320 = 0xD800 0xDF20   is not a letter (iswalpha should be false)
>   U+1D31E = 0xD834 0xDF1E   is not a letter (iswalpha should be false)
>   U+1D320 = 0xD834 0xDF20   is not a letter (iswalpha should be false)
>   U+1D71E = 0xD835 0xDF1E   is a letter (iswalpha should be true)
>   U+1D720 = 0xD835 0xDF20   is a letter (iswalpha should be true)
> There is no way that a system can provide this information through a
> function 'iswalpha' that takes a single wchar_t argument.

iswalpha takes wint_t, not wchar_t.  Since sizeof (wint_t) is 4 byte,
the function can return the correct value, provided that the application
converts the UTF-16 surrogate to UTF-32 before calling iswalpha.

> We agree that it is a bug. And it is caused by
>   - the fact that Cygwin's wchar_t[] encoding is UTF-16, and
>   - there is no way to define the <wctype.h> POSIX functions sanely in this
>     setting, and

See above.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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