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Re: 16-bit wchar_t on Windows and Cygwin
- From: Corinna Vinschen <corinna-cygwin at cygwin dot com>
- To: cygwin at cygwin dot com, bug-gnulib at gnu dot org, bug-coreutils at gnu dot org
- Date: Wed, 2 Feb 2011 13:21:02 +0100
- Subject: Re: 16-bit wchar_t on Windows and Cygwin
- References: <email@example.com> <4D46EA2B.firstname.lastname@example.org> <email@example.com> <20110202121442.GC2675@calimero.vinschen.de>
- Reply-to: cygwin at cygwin dot com, bug-gnulib at gnu dot org, bug-coreutils at gnu dot org
On Feb 2 13:14, Corinna Vinschen wrote:
> 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.
And, please note the wording in SUSv4, for instance in
The wc argument is a wint_t, the value of which the application shall
ensure is a wide-character code corresponding to a valid character in
the current locale, or equal to the value of the macro WEOF. If the
argument has any other value, the behavior is undefined.
I don't see any words in that which would disallow to convert UTF-16
wchar_t surrogates to a wint_t UTF-32 value before calling one of
the wctype functions. Just like you have to be careful not to call
the ctype functions with a signed char.
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Project Co-Leader cygwin AT cygwin DOT com
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