Re: [xml-dev] [Summary] Why is Encoding Metadata (e.g. encoding="UTF-8")

["Pete Cordell" <petexmldev@tech-know-ware.com>]

Not quite I'm afraid :-)  You can also get little endian UCS-2 and little 
endian UCS-4, UTF-16 little endian and various permutations thereof.  e.g.

UCS-2 LE is 00111100 00000000
UCS-4 LE is 00111100 00000000 00000000 00000000

(although I think support for UCS-4 is optional.)

In our implementation we basically take the tables contained in 
http://www.w3.org/TR/REC-xml/#sec-guessing and convert them into an if-else 
based decision tree so that we can read a byte at a time and makes 
successive deductions about the encoding in use.  This is an implementation 
issue though, and grabbing the first 4 bytes is also likely to work (subject 
to there being 4 bytes available!).

Note also, that the prolog (the bit that may contain the xml-decl - 
http://www.w3.org/TR/REC-xml/#NT-prolog) may just consist of white space, 
hence the opening character may be a whitespace character also.

HTH,

Pete.
--
=============================================
Pete Cordell
Codalogic
for XML Schema to C++ data binding visit
 http://www.codalogic.com/lmx/
=============================================

----- Original Message ----- 
From: "Rudick, Tom" <tmrudick@mitre.org>
To: <xml-dev@lists.xml.org>
Sent: Thursday, September 20, 2007 6:05 PM
Subject: RE: [xml-dev] [Summary] Why is Encoding Metadata (e.g. 
encoding="UTF-8") put Inside the XML Document?


So we know that the first character in an xml document must be <.
Which has the ASCII value of 60.

So a parser will keep reading in bytes until it gets up to 60.

ASCII is 00111100
UCS-2 is 00000000 00111100

So with ASCII (or UTF-8), we encounter 60 which is in the first byte.
After that characters will be considered to be one-byte long until we
read in the correct encoding attribute.

With UCS-2, read up to 60, see that it took two bytes, and now all
characters are two-bytes long.

Is this correct?

Thanks again,
-Tom

-----Original Message-----
From: Philippe Poulard [mailto:philippe.poulard@sophia.inria.fr]
Sent: Thursday, September 20, 2007 12:00 PM
To: Rudick, Tom
Cc: xml-dev@lists.xml.org
Subject: Re: [xml-dev] [Summary] Why is Encoding Metadata (e.g.
encoding="UTF-8") put Inside the XML Document?

Rudick, Tom a écrit :
> If the HTTP headers do not indicate what the encoding of the document
> is, you must read the document (at least the first line) and figure
out
> what the encoding is.  However, how is this accomplished?  If you
don't
> know the encoding of the document to begin with, how can you read
even
> the first line?
>
> After reading this http://www.w3.org/TR/REC-xml/#sec-guessing, it
seems
> that instead of reading what <?xml encoding="utf-8"?> has to say,
> parsers simply look at the first few octets of the document and
compare
> it to several known encodings of the text <?xml.  Then, they just
> continue to read the rest of the document.

Not exactly : the first few octets will indicate if <?xml
encoding="blah-blah"?> is coded on 1, 2, or even 4 bytes (for UCS) ;
the
charset of the sequence <?xml encoding="blah-blah"?> is limited to
ASCII-7 bits, which is fortunately compatible with UTF-8, ISO-8859-1
and
some others, and easily decodable if coded on 2 or 4 bytes, because the

same sequence is mapped to ASCII-7 bits, whatever the number of bytes
(zero-extension) ; for example :
Bits Encoding    Hex Dec Char
  7   US-ASCII     41  65  A                              1000001
  8   ASCII 8bits  41  65  A                             01000001
16   UCS-2        41  65  A                    00000000 01000001
32   UCS-4        41  65  A  00000000 00000000 00000000 01000001

So, the encoding can be read (if any)

I guess some parsers have additional heuristics for reading
successfully
the sequence <?xml encoding="blah-blah"?> ; maybe some try-catch to
apply with the set of charset they know ?

-- 
Cordialement,

               ///
              (. .)
  --------ooO--(_)--Ooo--------
|      Philippe Poulard       |
  -----------------------------
  http://reflex.gforge.inria.fr/
        Have the RefleX !

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