Feedback on the proposal to change U+FFFD generation when decoding ill-formed UTF-8
Richard Wordingham via Unicode
unicode at unicode.org
Wed May 17 17:04:18 CDT 2017
On Wed, 17 May 2017 13:37:51 -0700
Doug Ewell via Unicode <unicode at unicode.org> wrote:
> Richard Wordingham wrote:
> >> It is not at all clear what the intent of the encoder was - or even
> >> if it's not just a problem with the data stream. E0 80 80 is not
> >> permitted, it's garbage. An encoder can't "intend" it.
> > It was once a legal way of encoding NUL, just like C0 E0, which is
> > still in use, and seems to be the best way of storing NUL as
> > character content in a *C string*.
> I wish I had a penny for every time I'd seen this urban legend.
> At http://doc.cat-v.org/bell_labs/utf-8_history you can read the
> original definition of UTF-8, from Ken Thompson on 1992-09-08, so long
> ago that it was still called FSS-UTF:
> "When there are multiple ways to encode a value, for example
> UCS 0, only the shortest encoding is legal."
> Unicode once permitted implementations to *decode* non-shortest forms,
> but never allowed an implementation to *create* them
> "For example, UTF-8 allows nonshortest code value sequences to be
> interpreted: a UTF-8 conformant may map the code value sequence C0 80
> (11000000₂ 10000000₂) to the Unicode value U+0000, even though a
> UTF-8 conformant process shall never generate that code value sequence
> -- it shall generate the sequence 00 (00000000₂) instead."
> This was the passage that was deleted as part of Corrigendum #1.
So it was still a legal way for a non-UTF-8-compliant process! Note
for example that a compliant implementation of full upper-casing
shall convert the canonically equivalent strings <U+1FB3 GREEK SMALL
LETTER ALPHA WITH YPOGEGRAMMENI, U+0313 COMBINING COMMA ABOVE> and
<U+1F00 GREEK SMALL LETTER ALPHA WITH PSILI, U+0345 COMBINING GREEK
YPOGEGRAMMENI> to the canonically inequivalent strings <U+0391 GREEK
CAPITAL LETTER ALPHA, U+0399 GREEK CAPITAL LETTER IOTA, U+0313> and
<U+1F08 GREEK CAPITAL LETTER ALPHA WITH PSILI, 0399 GREEK CAPITAL
LETTER IOTA>. A compliant Unicode process may not assume that this is
the right thing to do. (Or are some compliant Unicode processes
required to incorrectly believe that they are doing something they
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