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First version of the encoding doc, Daniel.

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Daniel Veillard 2000-07-14 12:10:59 +00:00
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<p>Mailing-list archive: <a
href="http://xmlsoft.org/messages/">http://xmlsoft.org/messages/</a></p>
<p>Version: $Version$</p>
<p>Version: $Revision$</p>
<p>Table of Content:</p>
<ul>

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
"http://www.w3.org/TR/REC-html40/loose.dtd">
<html>
<head>
<title>Libxml Internationalization support</title>
<meta name="GENERATOR" content="amaya V3.2">
<meta http-equiv="Content-Type" content="text/html">
</head>
<body bgcolor="#ffffff">
<h1 align="center">Libxml Internationalization support</h1>
<p>Location: <a
href="http://xmlsoft.org/encoding.html">http://xmlsoft.org/encoding.html</a></p>
<p>Libxml home page: <a href="http://xmlsoft.org/">http://xmlsoft.org/</a></p>
<p>Mailing-list archive: <a
href="http://xmlsoft.org/messages/">http://xmlsoft.org/messages/</a></p>
<p>Version: $Revision$</p>
<p>Table of Content:</p>
<ol>
<li><a href="#What">What does internationalization support mean ?</a></li>
<li><a href="#internal">The internal encoding, how and why</a></li>
<li><a href="#implemente">How is it implemented ?</a></li>
<li><a href="#Default">Default supported encodings</a></li>
<li><a href="#extend">How to extend the existing support</a></li>
</ol>
<h2><a name="What">What does internationalization support mean ?</a></h2>
<p>XML was designed from the start to allow the support of any character set
by using Unicode. Any conformant XML parser has to support the UTF-8 and
UTF-16 default encodings which can both express the full unicode ranges. UTF8
is a variable length encoding whose greatest point are to resuse the same
emcoding for ASCII and to save space for Western encodings, but it is a bit
more complex to handle in practice. UTF-16 use 2 bytes per characters (and
sometimes combines two pairs), it makes implementation easier, but looks a bit
overkill for Western languages encoding. Moreover the XML specification allows
document to be encoded in other encodings at the condition that they are
clearly labelled as such. For example the following is a wellformed XML
document encoded in ISO-Latin 1 and using accentuated letter that we French
likes for both markup and content:</p>
<pre>&lt;?xml version="1.0" encoding="ISO-8859-1"?&gt;
&lt;très&gt;là&lt;/très&gt;</pre>
<p> Having internationalization support in libxml means the foolowing:</p>
<ul>
<li>the document is properly parsed</li>
<li>informations about it's encoding are saved</li>
<li>it can be modified</li>
<li>it can be saved in its original encoding</li>
<li>it can also be saved in another encoding supported by libxml (for
example straight UTF8 or even an ASCII form)</li>
</ul>
<p>Another very important point is that the whole libxml API, with the
exception of a few routines to read with a specific encoding or save to a
specific encoding, is completely agnostic about the original encoding of the
document.</p>
<p>It should be noted too that the HTML parser embedded in libxml now obbey
the same rules too, the following document will be (as of 2.2.2) handled in
an internationalized fashion by libxml too:</p>
<pre>&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
"http://www.w3.org/TR/REC-html40/loose.dtd"&gt;
&lt;html lang="fr"&gt;
&lt;head&gt;
&lt;META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-latin-1"&gt;
&lt;/head&gt;
&lt;body&gt;
&lt;p&gt;W3C crée des standards pour le Web.&lt;/body&gt;
&lt;/html&gt;</pre>
<h2><a name="internal">The internal encoding, how and why</a></h2>
<p>One of the core decision was to force all documents to be converted to a
default internal encoding, and that encoding to be UTF-8, here are the
rationale for those choices:</p>
<ul>
<li>keeping the native encoding in the internal form would force the libxml
users (or the code associated) to be fully aware of the encoding of the
original document, for examples when adding a text node to a document, the
content would have to be provided in the document encoding, i.e. the
client code would have to check it before hand, make sure it's conformant
to the encoding, etc ... Very hard in practice, though in some specific
cases this may make sense.</li>
<li>the second decision was which encoding. From the XML spec only UTF8 and
UTF16 really makes sense as being the two only encodings for which there
is amndatory support. UCS-4 (32 bits fixed size encoding) could be
considered an intelligent choice too since it's a direct Unicode mapping
support. I selected UTF-8 on the basis of efficiency and compatibility
with surrounding software:
<ul>
<li>UTF-8 while a bit more complex to convert from/to (i.e. slightly
more costly to import and export CPU wise) is also far more compact
than UTF-16 (and UCS-4) for a majority of the documents I see it used
for right now (RPM RDF catalogs, advogato data, various configuration
file formats, etc.) and the key point for today's computer
architecture is efficient uses of caches. If one nearly double the
memory requirement to store the same amount of data, this will trash
caches (main memory/external caches/internal caches) and my take is
that this harms the system far more than the CPU requirements needed
for the conversion to UTF-8</li>
<li>Most of libxml version 1 users were using it with straight ASCII
most of the time, doing the conversion with an internal encoding
requiring all their code to be rewritten was a serious show-stopper
for using UTF-16 or UCS-4.</li>
<li>UTF-8 is being used as the de-facto internal encoding standard for
related code like the <a href="http://www.pango.org/">pango</a>
upcoming Gnome text widget, and a lot of Unix code (yep another place
where Unix programmer base takes a different approach from Microsoft -
they are using UTF-16)</li>
</ul>
</li>
</ul>
<p>What does this mean in practice for the libxml user:</p>
<ul>
<li>xmlChar, the libxml data type is a byte, those bytes must be assembled
as UTF-8 valid strings. The proper way to terminate an xmlChar * string is
simply to append 0 byte, as usual.</li>
<li> One just need to make sure that when using chars outside the ASCII set,
the values has been properly converted to UTF-8</li>
</ul>
<h2><a name="implemente">How is it implemented ?</a></h2>
<p>Let's describe how all this works within libxml, basically the I18N
(internationalization) support get triggered only during I/O operation, i.e.
when reading a document or saving one. Let's look first at the reading
sequence:</p>
<ol>
<li>when a document is processed, we usually don't know the encoding, a
simple heuristic allows to detect UTF-18 and UCS-4 from whose where the
ASCII range (0-0x7F) maps with ASCII</li>
<li>the xml declaration if available is parsed, including the encoding
declaration. At that point, if the autodetected encoding is different from
the one declared a call to xmlSwitchEncoding() is issued.</li>
<li>If there is no encoding declaration, then the input has to be in either
UTF-8 or UTF-16, if it is not then at some point when processing the
input, the converter/checker of UTF-8 form will raise an encoding error.
You may end-up with a garbled document, or no document at all ! Example:
<pre>~/XML -&gt; ./xmllint err.xml
err.xml:1: error: Input is not proper UTF-8, indicate encoding !
&lt;très&gt;là&lt;/très&gt;
^
err.xml:1: error: Bytes: 0xE8 0x73 0x3E 0x6C
&lt;très&gt;là&lt;/très&gt;
^</pre>
</li>
<li>xmlSwitchEncoding() does an encoding name lookup, canonalize it, and
then search the default registered encoding converters for that encoding.
If it's not within the default set and iconv() support has been compiled
it, it will ask iconv for such an encoder. If this fails then the parser
will report an error and stops processing:
<pre>~/XML -&gt; ./xmllint err2.xml
err2.xml:1: error: Unsupported encoding UnsupportedEnc
&lt;?xml version="1.0" encoding="UnsupportedEnc"?&gt;
^</pre>
</li>
<li> From that point the encoder process progressingly the input (it is
plugged as a front-end to the I/O module) for that entity. It captures and
convert on-the-fly the document to be parsed to UTF-8. The parser itself
just does UTF-8 checking of this input and process it transparently. The
only difference is that the encoding information has been added to the
parsing context (more precisely to the input corresponding to this
entity).</li>
<li>The result (when using DOM) is an internal form completely in UTF-8 with
just an encoding information on the document node.</li>
</ol>
<p>Ok then what's happen when saving the document (assuming you
colllected/built an xmlDoc DOM like structure) ? It depends on the function
called, xmlSaveFile() will just try to save in the original encoding, while
xmlSaveFileTo() and xmlSaveFileEnc() can optionally save to a given
encoding:</p>
<ol>
<li> if no encoding is given, libxml will look for an encoding value
associated to the document and if it exists will try to save to that
encoding,
<p>otherwise everything is written in the internal form, i.e. UTF-8</p>
</li>
<li>so if an encoding was specified, either at the API level or on the
document, libxml will again canonalize the encoding name, lookup for a
converter in the registered set or through iconv. If not found the
function will return an error code</li>
<li>the converter is placed before the I/O buffer layer, as another kind of
buffer, then libxml will simply push the UTF-8 serialization to through
that buffer, which will then progressively be converted and pushed onto
the I/O layer.</li>
<li>It is possible that the converter code fails on some input, for example
trying to push an UTF-8 encoded chinese character through the UTF-8 to
ISO-Latin-1 converter won't work. Since the encoders are progressive they
will just report the error and the number of bytes converted, at that
point libxml will decode the offending character, remove it from the
buffer and replace it with the associated charRef encoding &amp;#123; and
resume the convertion. This guarante that any document will be saved
without losses. A special "ascii" encoding name is used to save documents
to a pure ascii form can be used when portability is really crucial</li>
</ol>
<p>Here is a few examples based on the same test document:</p>
<pre>~/XML -&gt; ./xmllint isolat1
&lt;?xml version="1.0" encoding="ISO-8859-1"?&gt;
&lt;très&gt;là&lt;/très&gt;
~/XML -&gt; ./xmllint --encode UTF-8 isolat1
&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;très&gt;là  &lt;/très&gt;
~/XML -&gt; ./xmllint --encode ascii isolat1
&lt;?xml version="1.0" encoding="ascii"?&gt;
&lt;tr&amp;#xE8;s&gt;l&amp;#xE0;&lt;/tr&amp;#xE8;s&gt;
~/XML -&gt; </pre>
<p> The same processing is applied (and reuse most of the code) for HTML I18N
processing. Looking up and modifying the content encoding is a bit more
difficult since it is located in a &lt;meta&gt; tag under the &lt;head&gt;, so
a couple of functions htmlGetMetaEncoding() and htmlSetMetaEncoding() have
been provided. The parser also attempts to switch encoding on the fly when
detecting such a tag on input. Except for that the processing is the same (and
again reuses the same code). </p>
<h2><a name="Default">Default supported encodings</a></h2>
<p>libxml has a set of default converters for the following encodings (located
in encoding.c):</p>
<ol>
<li>UTF-8 is supported by default (null handlers)</li>
<li>UTF-16, both little and big endian</li>
<li>ISO-Latin-1 (ISO-8859-1) covering most western languages</li>
<li>ASCII, useful mostly for saving</li>
<li>HTML, a specific handler for the conversion of UTF-8 to ASCII with HTML
predefined entities like &amp;copy; for the Copyright sign.</li>
</ol>
<p>More over when compiled on an Unix platfor with iconv support the full set
of encodings supported by iconv can be instantly be used by libxml. On a linux
machine with glibc-2.1 the list of supported encodings and aliases fill 3 full
pages, and include UCS-4, the full set of ISO-Latin encodings, and the various
Japanese ones.</p>
<h2><a name="extend">How to extend the existing support</a></h2>
<p>Well adding support for new encoding, or overriding one of the encoders
(assuming it is buggy) should not be hard, just write an input and output
conversion routines to/from UTF-8, and register them using
xmlNewCharEncodingHandler(name, xxxToUTF8, UTF8Toxxx), and they will be
called automatically if the parser(s) encounter such an encoding name
(register it uppercase, this will help). The description of the encoders,
their arguments and expected return values are described in the encoding.h
header.</p>
<p>A quick note on the topic of subverting the parser to use a different
internal encoding than UTF-8, in some case people will absolutely want to keep
the internal encoding different, I think it's still possible (but the encoding
must be compliant with ASCII on the same subrange) though I didn't tried it.
The key is to override the default conversion routines (by registering null
encoders/decoders for your charsets), and bypass the UTF-8 checking of the
parser by setting the parser context charset (ctxt-&gt;charset) to something
different than XML_CHAR_ENCODING_UTF8, but there is no guarantee taht this
will work. You may also have some troubles saving back.</p>
<p>Basically proper I18N support is important, this requires at least
libxml-2.0.0, but a lot of features and corrections are really available only
starting 2.2.</p>
<p><a href="mailto:Daniel.Veillard@w3.org">Daniel Veillard</a></p>
<p>$Id$</p>
</body>
</html>