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<h1>The XML C library for Gnome</h1>
<h2>Encodings support</h2>
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<p>Table of Content:</p>
<ol>
<li><a href="encoding.html#What">What does internationalization support
    mean ?</a></li>
<li><a href="encoding.html#internal">The internal encoding, how and
  why</a></li>
<li><a href="encoding.html#implemente">How is it implemented ?</a></li>
<li><a href="encoding.html#Default">Default supported encodings</a></li>
<li><a href="encoding.html#extend">How to extend the existing
  support</a></li>
</ol>
<h3><a name="What">What does internationalization support mean ?</a></h3>
<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-8859 1 and using accentuated letter that we French
likes for both markup and content:</p>
<pre>&lt;?xml version=&quot;1.0&quot; encoding=&quot;ISO-8859-1&quot;?&gt;
&lt;tr�s&gt;l�&lt;/tr�s&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 &quot;-//W3C//DTD HTML 4.0 Transitional//EN&quot;
                      &quot;http://www.w3.org/TR/REC-html40/loose.dtd">;
&lt;html lang=&quot;fr&quot;&gt;
&lt;head&gt;
  &lt;META HTTP-EQUIV=&quot;Content-Type&quot; CONTENT=&quot;text/html; charset=ISO-8859-1&quot;&gt;
&lt;/head&gt;
&lt;body&gt;
&lt;p&gt;W3C cr�e des standards pour le Web.&lt;/body&gt;
&lt;/html&gt;</pre>
<h3><a name="internal">The internal encoding, how and why</a></h3>
<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>
<h3><a name="implemente">How is it implemented ?</a></h3>
<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;tr�s&gt;l�&lt;/tr�s&gt;
   ^
err.xml:1: error: Bytes: 0xE8 0x73 0x3E 0x6C
&lt;tr�s&gt;l�&lt;/tr�s&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=&quot;1.0&quot; encoding=&quot;UnsupportedEnc&quot;?&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-8859-1 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 (except for markup names where this is not legal, this is
    a problem in the current version, in pactice avoid using non-ascci
    characters for tags or attributes names  @@). A special &quot;ascii&quot; 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=&quot;1.0&quot; encoding=&quot;ISO-8859-1&quot;?&gt;
&lt;tr�s&gt;l�&lt;/tr�s&gt;
~/XML -&gt; /xmllint --encode UTF-8 isolat1
&lt;?xml version=&quot;1.0&quot; encoding=&quot;UTF-8&quot;?&gt;
&lt;très&gt;l� �&lt;/très&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>
<h3><a name="Default">Default supported encodings</a></h3>
<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>
<h4>Encoding aliases</h4>
<p>From 2.2.3, libxml has support to register encoding names aliases. The
goal is to be able to parse document whose encoding is supported but where
the name differs (for example from the default set of names accepted by
iconv). The following functions allow to register and handle new aliases for
existing encodings. Once registered libxml will automatically lookup the
aliases when handling a document:</p>
<ul>
<li>int xmlAddEncodingAlias(const char *name, const char *alias);</li>
<li>int xmlDelEncodingAlias(const char *alias);</li>
<li>const char * xmlGetEncodingAlias(const char *alias);</li>
<li>void xmlCleanupEncodingAliases(void);</li>
</ul>
<h3><a name="extend">How to extend the existing support</a></h3>
<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="bugs.html">Daniel Veillard</a></p>
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