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Date: Tue, 24 Nov 2009 10:06:46 +0100
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: cyg1.7 - DOS character remapping: change request.
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On Nov 24 09:50, Corinna Vinschen wrote:
> On Nov 23 20:29, Linda Walsh wrote:
> > Eric Blake wrote:
> > >But then, how would you distinguish between the valid UTF-16 replacement
> > >used to represent an invalid character, and a valid UTF-16 character
> > >representing itself?  I'm sorry, but the value of a 1-to-1 round trip
> > >mapping outweighs the convenience of displaying a glyph that looks the
> > >same but causes ambiguous round trip conversions.
> > ----
> > 
> > 	You've already broken 1-to-1 round trip compatibility by NOT
> > using an **INVALID** UTF-16 character.  You are using "the 0xf000-0xf0ff
> 
> There is no invalid UTF-16.  There could be invalid UTF-32, but that's
> not used by Windows.
> 
> > range.  This range is part of the UNICODE block 95, "Private Use Area".
> > These are *valid* unicode characters -- they are just NOT reserved for
> > a particular application.  This means they will be displayed randomly
> > and CAN be used by other applications
> 
> Right, and we use them to map characters from the base plane.  There's
> no area in the entire Unicode plane which would not conflict one way or
> the other.  We're using the same mapping as Interix does, so we're at
> least compatible with one other product.  The only alternative is
> not to map ascii chars at all and revert this change.

Oh and, btw., the conversion between base plane and private use area is
only done for system objects like filenames.  It's not done for every
multibyte to widechar conversion within the application itself.  So,
*if* you have collisions, they will only occur for filenames, which are
rather unlikely (not impossible, I know) to use these private use
characters.


Corinna

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

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