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   10 </style><title>Encodings support</title></head><body bgcolor="#8b7765" text="#000000" link="#a06060" vlink="#000000"><table border="0" width="100%" cellpadding="5" cellspacing="0" align="center"><tr><td width="120"><a href="http://swpat.ffii.org/"><img src="epatents.png" alt="Action against software patents" /></a></td><td width="180"><a href="http://www.gnome.org/"><img src="gnome2.png" alt="Gnome2 Logo" /></a><a href="http://www.w3.org/Status"><img src="w3c.png" alt="W3C Logo" /></a><a href="http://www.redhat.com/"><img src="redhat.gif" alt="Red Hat Logo" /></a><div align="left"><a href="http://xmlsoft.org/"><img src="Libxml2-Logo-180x168.gif" alt="Made with Libxml2 Logo" /></a></div></td><td><table border="0" width="90%" cellpadding="2" cellspacing="0" align="center" bgcolor="#000000"><tr><td><table width="100%" border="0" cellspacing="1" cellpadding="3" bgcolor="#fffacd"><tr><td align="center"><h1>The XML C parser and toolkit of Gnome</h1><h2>Encodings support</h2></td></tr></table></td></tr></table></td></tr></table><table border="0" cellpadding="4" cellspacing="0" width="100%" align="center"><tr><td bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="2" width="100%"><tr><td valign="top" width="200" bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="1" width="100%" bgcolor="#000000"><tr><td><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Main Menu</b></center></td></tr><tr><td bgcolor="#fffacd"><form action="search.php" enctype="application/x-www-form-urlencoded" method="get"><input name="query" type="text" size="20" value="" /><input name="submit" type="submit" value="Search ..." /></form><ul><li><a href="index.html">Home</a></li><li><a href="html/index.html">Reference Manual</a></li><li><a href="intro.html">Introduction</a></li><li><a href="FAQ.html">FAQ</a></li><li><a href="docs.html" style="font-weight:bold">Developer Menu</a></li><li><a href="bugs.html">Reporting bugs and getting help</a></li><li><a href="help.html">How to help</a></li><li><a href="downloads.html">Downloads</a></li><li><a href="news.html">Releases</a></li><li><a href="XMLinfo.html">XML</a></li><li><a href="XSLT.html">XSLT</a></li><li><a href="xmldtd.html">Validation &amp; DTDs</a></li><li><a href="encoding.html">Encodings support</a></li><li><a href="catalog.html">Catalog support</a></li><li><a href="namespaces.html">Namespaces</a></li><li><a href="contribs.html">Contributions</a></li><li><a href="examples/index.html" style="font-weight:bold">Code Examples</a></li><li><a href="html/index.html" style="font-weight:bold">API Menu</a></li><li><a href="guidelines.html">XML Guidelines</a></li><li><a href="ChangeLog.html">Recent Changes</a></li></ul></td></tr></table><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Related links</b></center></td></tr><tr><td bgcolor="#fffacd"><ul><li><a href="http://mail.gnome.org/archives/xml/">Mail archive</a></li><li><a href="http://xmlsoft.org/XSLT/">XSLT libxslt</a></li><li><a href="http://phd.cs.unibo.it/gdome2/">DOM gdome2</a></li><li><a href="http://www.aleksey.com/xmlsec/">XML-DSig xmlsec</a></li><li><a href="ftp://xmlsoft.org/">FTP</a></li><li><a href="http://www.zlatkovic.com/projects/libxml/">Windows binaries</a></li><li><a href="http://opencsw.org/packages/libxml2">Solaris binaries</a></li><li><a href="http://www.explain.com.au/oss/libxml2xslt.html">MacOsX binaries</a></li><li><a href="http://lxml.de/">lxml Python bindings</a></li><li><a href="http://cpan.uwinnipeg.ca/dist/XML-LibXML">Perl bindings</a></li><li><a href="http://libxmlplusplus.sourceforge.net/">C++ bindings</a></li><li><a href="http://www.zend.com/php5/articles/php5-xmlphp.php#Heading4">PHP bindings</a></li><li><a href="http://sourceforge.net/projects/libxml2-pas/">Pascal bindings</a></li><li><a href="http://libxml.rubyforge.org/">Ruby bindings</a></li><li><a href="http://tclxml.sourceforge.net/">Tcl bindings</a></li><li><a href="http://bugzilla.gnome.org/buglist.cgi?product=libxml2">Bug Tracker</a></li></ul></td></tr></table></td></tr></table></td><td valign="top" bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="1" width="100%"><tr><td><table border="0" cellspacing="0" cellpadding="1" width="100%" bgcolor="#000000"><tr><td><table border="0" cellpadding="3" cellspacing="1" width="100%"><tr><td bgcolor="#fffacd"><p>If you are not really familiar with Internationalization (usual shortcut
   11 is I18N) , Unicode, characters and glyphs, I suggest you read a <a href="http://www.tbray.org/ongoing/When/200x/2003/04/06/Unicode">presentation</a>
   12 by Tim Bray on Unicode and why you should care about it.</p><p>If you don't understand why <b>it does not make sense to have a string
   13 without knowing what encoding it uses</b>, then as Joel Spolsky said <a href="http://www.joelonsoftware.com/articles/Unicode.html">please do not
   14 write another line of code until you finish reading that article.</a>. It is
   15 a prerequisite to understand this page, and avoid a lot of problems with
   16 libxml2, XML or text processing in general.</p><p>Table of Content:</p><ol>
   17   <li><a href="encoding.html#What">What does internationalization support
   18     mean ?</a></li>
   19   <li><a href="encoding.html#internal">The internal encoding, how and
   20   why</a></li>
   21   <li><a href="encoding.html#implemente">How is it implemented ?</a></li>
   22   <li><a href="encoding.html#Default">Default supported encodings</a></li>
   23   <li><a href="encoding.html#extend">How to extend the existing
   24   support</a></li>
   25 </ol><h3><a name="What" id="What">What does internationalization support mean ?</a></h3><p>XML was designed from the start to allow the support of any character set
   26 by using Unicode. Any conformant XML parser has to support the UTF-8 and
   27 UTF-16 default encodings which can both express the full unicode ranges. UTF8
   28 is a variable length encoding whose greatest points are to reuse the same
   29 encoding for ASCII and to save space for Western encodings, but it is a bit
   30 more complex to handle in practice. UTF-16 use 2 bytes per character (and
   31 sometimes combines two pairs), it makes implementation easier, but looks a
   32 bit overkill for Western languages encoding. Moreover the XML specification
   33 allows the document to be encoded in other encodings at the condition that
   34 they are clearly labeled as such. For example the following is a wellformed
   35 XML document encoded in ISO-8859-1 and using accentuated letters that we
   36 French like for both markup and content:</p><pre>&lt;?xml version="1.0" encoding="ISO-8859-1"?&gt;
   37 &lt;très&gt;&lt;/très&gt;</pre><p>Having internationalization support in libxml2 means the following:</p><ul>
   38   <li>the document is properly parsed</li>
   39   <li>information about it's encoding is saved</li>
   40   <li>it can be modified</li>
   41   <li>it can be saved in its original encoding</li>
   42   <li>it can also be saved in another encoding supported by libxml2 (for
   43     example straight UTF8 or even an ASCII form)</li>
   44 </ul><p>Another very important point is that the whole libxml2 API, with the
   45 exception of a few routines to read with a specific encoding or save to a
   46 specific encoding, is completely agnostic about the original encoding of the
   47 document.</p><p>It should be noted too that the HTML parser embedded in libxml2 now obey
   48 the same rules too, the following document will be (as of 2.2.2) handled  in
   49 an internationalized fashion by libxml2 too:</p><pre>&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
   50                       "http://www.w3.org/TR/REC-html40/loose.dtd"&gt;
   51 &lt;html lang="fr"&gt;
   52 &lt;head&gt;
   53   &lt;META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"&gt;
   54 &lt;/head&gt;
   55 &lt;body&gt;
   56 &lt;p&gt;W3C crée des standards pour le Web.&lt;/body&gt;
   57 &lt;/html&gt;</pre><h3><a name="internal" id="internal">The internal encoding, how and why</a></h3><p>One of the core decisions was to force all documents to be converted to a
   58 default internal encoding, and that encoding to be UTF-8, here are the
   59 rationales for those choices:</p><ul>
   60   <li>keeping the native encoding in the internal form would force the libxml
   61     users (or the code associated) to be fully aware of the encoding of the
   62     original document, for examples when adding a text node to a document,
   63     the content would have to be provided in the document encoding, i.e. the
   64     client code would have to check it before hand, make sure it's conformant
   65     to the encoding, etc ... Very hard in practice, though in some specific
   66     cases this may make sense.</li>
   67   <li>the second decision was which encoding. From the XML spec only UTF8 and
   68     UTF16 really makes sense as being the two only encodings for which there
   69     is mandatory support. UCS-4 (32 bits fixed size encoding) could be
   70     considered an intelligent choice too since it's a direct Unicode mapping
   71     support. I selected UTF-8 on the basis of efficiency and compatibility
   72     with surrounding software:
   73     <ul>
   74       <li>UTF-8 while a bit more complex to convert from/to (i.e. slightly
   75         more costly to import and export CPU wise) is also far more compact
   76         than UTF-16 (and UCS-4) for a majority of the documents I see it used
   77         for right now (RPM RDF catalogs, advogato data, various configuration
   78         file formats, etc.) and the key point for today's computer
   79         architecture is efficient uses of caches. If one nearly double the
   80         memory requirement to store the same amount of data, this will trash
   81         caches (main memory/external caches/internal caches) and my take is
   82         that this harms the system far more than the CPU requirements needed
   83         for the conversion to UTF-8</li>
   84       <li>Most of libxml2 version 1 users were using it with straight ASCII
   85         most of the time, doing the conversion with an internal encoding
   86         requiring all their code to be rewritten was a serious show-stopper
   87         for using UTF-16 or UCS-4.</li>
   88       <li>UTF-8 is being used as the de-facto internal encoding standard for
   89         related code like the <a href="http://www.pango.org/">pango</a>
   90         upcoming Gnome text widget, and a lot of Unix code (yet another place
   91         where Unix programmer base takes a different approach from Microsoft
   92         - they are using UTF-16)</li>
   93     </ul>
   94   </li>
   95 </ul><p>What does this mean in practice for the libxml2 user:</p><ul>
   96   <li>xmlChar, the libxml2 data type is a byte, those bytes must be assembled
   97     as UTF-8 valid strings. The proper way to terminate an xmlChar * string
   98     is simply to append 0 byte, as usual.</li>
   99   <li>One just need to make sure that when using chars outside the ASCII set,
  100     the values has been properly converted to UTF-8</li>
  101 </ul><h3><a name="implemente" id="implemente">How is it implemented ?</a></h3><p>Let's describe how all this works within libxml, basically the I18N
  102 (internationalization) support get triggered only during I/O operation, i.e.
  103 when reading a document or saving one. Let's look first at the reading
  104 sequence:</p><ol>
  105   <li>when a document is processed, we usually don't know the encoding, a
  106     simple heuristic allows to detect UTF-16 and UCS-4 from encodings where
  107     the ASCII range (0-0x7F) maps with ASCII</li>
  108   <li>the xml declaration if available is parsed, including the encoding
  109     declaration. At that point, if the autodetected encoding is different
  110     from the one declared a call to xmlSwitchEncoding() is issued.</li>
  111   <li>If there is no encoding declaration, then the input has to be in either
  112     UTF-8 or UTF-16, if it is not then at some point when processing the
  113     input, the converter/checker of UTF-8 form will raise an encoding error.
  114     You may end-up with a garbled document, or no document at all ! Example:
  115     <pre>~/XML -&gt; ./xmllint err.xml 
  116 err.xml:1: error: Input is not proper UTF-8, indicate encoding !
  117 &lt;très&gt;là &lt;/très&gt;
  118    ^
  119 err.xml:1: error: Bytes: 0xE8 0x73 0x3E 0x6C
  120 &lt;très&gt;là &lt;/très&gt;
  121    ^</pre>
  122   </li>
  123   <li>xmlSwitchEncoding() does an encoding name lookup, canonicalize it, and
  124     then search the default registered encoding converters for that encoding.
  125     If it's not within the default set and iconv() support has been compiled
  126     it, it will ask iconv for such an encoder. If this fails then the parser
  127     will report an error and stops processing:
  128     <pre>~/XML -&gt; ./xmllint err2.xml 
  129 err2.xml:1: error: Unsupported encoding UnsupportedEnc
  130 &lt;?xml version="1.0" encoding="UnsupportedEnc"?&gt;
  131                                              ^</pre>
  132   </li>
  133   <li>From that point the encoder processes progressively the input (it is
  134     plugged as a front-end to the I/O module) for that entity. It captures
  135     and converts on-the-fly the document to be parsed to UTF-8. The parser
  136     itself just does UTF-8 checking of this input and process it
  137     transparently. The only difference is that the encoding information has
  138     been added to the parsing context (more precisely to the input
  139     corresponding to this entity).</li>
  140   <li>The result (when using DOM) is an internal form completely in UTF-8
  141     with just an encoding information on the document node.</li>
  142 </ol><p>Ok then what happens when saving the document (assuming you
  143 collected/built an xmlDoc DOM like structure) ? It depends on the function
  144 called, xmlSaveFile() will just try to save in the original encoding, while
  145 xmlSaveFileTo() and xmlSaveFileEnc() can optionally save to a given
  146 encoding:</p><ol>
  147   <li>if no encoding is given, libxml2 will look for an encoding value
  148     associated to the document and if it exists will try to save to that
  149     encoding,
  150     <p>otherwise everything is written in the internal form, i.e. UTF-8</p>
  151   </li>
  152   <li>so if an encoding was specified, either at the API level or on the
  153     document, libxml2 will again canonicalize the encoding name, lookup for a
  154     converter in the registered set or through iconv. If not found the
  155     function will return an error code</li>
  156   <li>the converter is placed before the I/O buffer layer, as another kind of
  157     buffer, then libxml2 will simply push the UTF-8 serialization to through
  158     that buffer, which will then progressively be converted and pushed onto
  159     the I/O layer.</li>
  160   <li>It is possible that the converter code fails on some input, for example
  161     trying to push an UTF-8 encoded Chinese character through the UTF-8 to
  162     ISO-8859-1 converter won't work. Since the encoders are progressive they
  163     will just report the error and the number of bytes converted, at that
  164     point libxml2 will decode the offending character, remove it from the
  165     buffer and replace it with the associated charRef encoding &amp;#123; and
  166     resume the conversion. This guarantees that any document will be saved
  167     without losses (except for markup names where this is not legal, this is
  168     a problem in the current version, in practice avoid using non-ascii
  169     characters for tag or attribute names). A special "ascii" encoding name
  170     is used to save documents to a pure ascii form can be used when
  171     portability is really crucial</li>
  172 </ol><p>Here are a few examples based on the same test document and assumin a
  173 terminal using ISO-8859-1 as the text encoding:</p><pre>~/XML -&gt; ./xmllint isolat1 
  174 &lt;?xml version="1.0" encoding="ISO-8859-1"?&gt;
  175 &lt;très&gt;là&lt;/très&gt;
  176 ~/XML -&gt; ./xmllint --encode UTF-8 isolat1 
  177 &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  178 &lt;très&gt;là  &lt;/très&gt;
  179 ~/XML -&gt; </pre><p>The same processing is applied (and reuse most of the code) for HTML I18N
  180 processing. Looking up and modifying the content encoding is a bit more
  181 difficult since it is located in a &lt;meta&gt; tag under the &lt;head&gt;,
  182 so a couple of functions htmlGetMetaEncoding() and htmlSetMetaEncoding() have
  183 been provided. The parser also attempts to switch encoding on the fly when
  184 detecting such a tag on input. Except for that the processing is the same
  185 (and again reuses the same code).</p><h3><a name="Default" id="Default">Default supported encodings</a></h3><p>libxml2 has a set of default converters for the following encodings
  186 (located in encoding.c):</p><ol>
  187   <li>UTF-8 is supported by default (null handlers)</li>
  188   <li>UTF-16, both little and big endian</li>
  189   <li>ISO-Latin-1 (ISO-8859-1) covering most western languages</li>
  190   <li>ASCII, useful mostly for saving</li>
  191   <li>HTML, a specific handler for the conversion of UTF-8 to ASCII with HTML
  192     predefined entities like &amp;copy; for the Copyright sign.</li>
  193 </ol><p>More over when compiled on an Unix platform with iconv support the full
  194 set of encodings supported by iconv can be instantly be used by libxml. On a
  195 linux machine with glibc-2.1 the list of supported encodings and aliases fill
  196 3 full pages, and include UCS-4, the full set of ISO-Latin encodings, and the
  197 various Japanese ones.</p><p>To convert from the UTF-8 values returned from the API to another encoding
  198 then it is possible to use the function provided from <a href="html/libxml-encoding.html">the encoding module</a> like <a href="html/libxml-encoding.html#UTF8Toisolat1">UTF8Toisolat1</a>, or use the
  199 POSIX <a href="http://www.opengroup.org/onlinepubs/009695399/functions/iconv.html">iconv()</a>
  200 API directly.</p><h4>Encoding aliases</h4><p>From 2.2.3, libxml2 has support to register encoding names aliases. The
  201 goal is to be able to parse document whose encoding is supported but where
  202 the name differs (for example from the default set of names accepted by
  203 iconv). The following functions allow to register and handle new aliases for
  204 existing encodings. Once registered libxml2 will automatically lookup the
  205 aliases when handling a document:</p><ul>
  206   <li>int xmlAddEncodingAlias(const char *name, const char *alias);</li>
  207   <li>int xmlDelEncodingAlias(const char *alias);</li>
  208   <li>const char * xmlGetEncodingAlias(const char *alias);</li>
  209   <li>void xmlCleanupEncodingAliases(void);</li>
  210 </ul><h3><a name="extend" id="extend">How to extend the existing support</a></h3><p>Well adding support for new encoding, or overriding one of the encoders
  211 (assuming it is buggy) should not be hard, just write input and output
  212 conversion routines to/from UTF-8, and register them using
  213 xmlNewCharEncodingHandler(name, xxxToUTF8, UTF8Toxxx),  and they will be
  214 called automatically if the parser(s) encounter such an encoding name
  215 (register it uppercase, this will help). The description of the encoders,
  216 their arguments and expected return values are described in the encoding.h
  217 header.</p><p><a href="bugs.html">Daniel Veillard</a></p></td></tr></table></td></tr></table></td></tr></table></td></tr></table></td></tr></table></body></html>