| <?xml version="1.0" encoding="ISO-8859-1"?> |
| <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> |
| <html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" /><link rel="SHORTCUT ICON" href="/favicon.ico" /><style type="text/css"> |
| TD {font-family: Verdana,Arial,Helvetica} |
| BODY {font-family: Verdana,Arial,Helvetica; margin-top: 2em; margin-left: 0em; margin-right: 0em} |
| H1 {font-family: Verdana,Arial,Helvetica} |
| H2 {font-family: Verdana,Arial,Helvetica} |
| H3 {font-family: Verdana,Arial,Helvetica} |
| A:link, A:visited, A:active { text-decoration: underline } |
| </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 & 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://www.blastwave.org/packages.php/libxml2">Solaris binaries</a></li><li><a href="http://www.explain.com.au/oss/libxml2xslt.html">MacOsX binaries</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 shortcutis |
| 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>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 |
| stringwithout knowing what encoding it uses</b>, then as Joel Spolsky said <a href="http://www.joelonsoftware.com/articles/Unicode.html">please do notwrite |
| another line of code until you finish reading that article.</a>. It isa |
| prerequisite to understand this page, and avoid a lot of problems |
| withlibxml2, XML or text processing in general.</p><p>Table of Content:</p><ol><li><a href="encoding.html#What">What does internationalization supportmean |
| ?</a></li> |
| <li><a href="encoding.html#internal">The internal encoding, how |
| andwhy</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 |
| existingsupport</a></li> |
| </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 |
| setby using Unicode. Any conformant XML parser has to support the UTF-8 |
| andUTF-16 default encodings which can both express the full unicode ranges. |
| UTF8is a variable length encoding whose greatest points are to reuse the |
| sameencoding for ASCII and to save space for Western encodings, but it is a |
| bitmore complex to handle in practice. UTF-16 use 2 bytes per character |
| (andsometimes combines two pairs), it makes implementation easier, but looks |
| abit overkill for Western languages encoding. Moreover the XML |
| specificationallows the document to be encoded in other encodings at the |
| condition thatthey are clearly labeled as such. For example the following is |
| a wellformedXML document encoded in ISO-8859-1 and using accentuated letters |
| that weFrench like for both markup and content:</p><pre><?xml version="1.0" encoding="ISO-8859-1"?> |
| <très>là</très></pre><p>Having internationalization support in libxml2 means the following:</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 libxml2 |
| (forexample straight UTF8 or even an ASCII form)</li> |
| </ul><p>Another very important point is that the whole libxml2 API, with |
| theexception of a few routines to read with a specific encoding or save to |
| aspecific encoding, is completely agnostic about the original encoding of |
| thedocument.</p><p>It should be noted too that the HTML parser embedded in libxml2 now |
| obeythe same rules too, the following document will be (as of 2.2.2) handled |
| inan internationalized fashion by libxml2 too:</p><pre><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" |
| "http://www.w3.org/TR/REC-html40/loose.dtd"> |
| <html lang="fr"> |
| <head> |
| <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"> |
| </head> |
| <body> |
| <p>W3C crée des standards pour le Web.</body> |
| </html></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 |
| adefault internal encoding, and that encoding to be UTF-8, here are |
| therationales for those choices:</p><ul><li>keeping the native encoding in the internal form would force the |
| libxmlusers (or the code associated) to be fully aware of the encoding of |
| theoriginal document, for examples when adding a text node to a |
| document,the content would have to be provided in the document encoding, |
| i.e. theclient code would have to check it before hand, make sure it's |
| conformantto the encoding, etc ... Very hard in practice, though in some |
| specificcases this may make sense.</li> |
| <li>the second decision was which encoding. From the XML spec only UTF8 |
| andUTF16 really makes sense as being the two only encodings for which |
| thereis mandatory support. UCS-4 (32 bits fixed size encoding) could |
| beconsidered an intelligent choice too since it's a direct Unicode |
| mappingsupport. I selected UTF-8 on the basis of efficiency and |
| compatibilitywith surrounding software: |
| <ul><li>UTF-8 while a bit more complex to convert from/to (i.e. |
| slightlymore costly to import and export CPU wise) is also far more |
| compactthan UTF-16 (and UCS-4) for a majority of the documents I see |
| it usedfor right now (RPM RDF catalogs, advogato data, various |
| configurationfile formats, etc.) and the key point for today's |
| computerarchitecture is efficient uses of caches. If one nearly |
| double thememory requirement to store the same amount of data, this |
| will trashcaches (main memory/external caches/internal caches) and my |
| take isthat this harms the system far more than the CPU requirements |
| neededfor the conversion to UTF-8</li> |
| <li>Most of libxml2 version 1 users were using it with straight |
| ASCIImost of the time, doing the conversion with an internal |
| encodingrequiring all their code to be rewritten was a serious |
| show-stopperfor using UTF-16 or UCS-4.</li> |
| <li>UTF-8 is being used as the de-facto internal encoding standard |
| forrelated code like the <a href="http://www.pango.org/">pango</a>upcoming Gnome text widget, and |
| a lot of Unix code (yet another placewhere 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 libxml2 user:</p><ul><li>xmlChar, the libxml2 data type is a byte, those bytes must be |
| assembledas UTF-8 valid strings. The proper way to terminate an xmlChar * |
| stringis 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" id="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 |
| readingsequence:</p><ol><li>when a document is processed, we usually don't know the encoding, |
| asimple heuristic allows to detect UTF-16 and UCS-4 from encodings |
| wherethe ASCII range (0-0x7F) maps with ASCII</li> |
| <li>the xml declaration if available is parsed, including the |
| encodingdeclaration. At that point, if the autodetected encoding is |
| differentfrom the one declared a call to xmlSwitchEncoding() is |
| issued.</li> |
| <li>If there is no encoding declaration, then the input has to be in |
| eitherUTF-8 or UTF-16, if it is not then at some point when processing |
| theinput, 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 -> ./xmllint err.xml |
| err.xml:1: error: Input is not proper UTF-8, indicate encoding ! |
| <très>là</très> |
| ^ |
| err.xml:1: error: Bytes: 0xE8 0x73 0x3E 0x6C |
| <très>là</très> |
| ^</pre> |
| </li> |
| <li>xmlSwitchEncoding() does an encoding name lookup, canonicalize it, |
| andthen search the default registered encoding converters for that |
| encoding.If it's not within the default set and iconv() support has been |
| compiledit, it will ask iconv for such an encoder. If this fails then the |
| parserwill report an error and stops processing: |
| <pre>~/XML -> ./xmllint err2.xml |
| err2.xml:1: error: Unsupported encoding UnsupportedEnc |
| <?xml version="1.0" encoding="UnsupportedEnc"?> |
| ^</pre> |
| </li> |
| <li>From that point the encoder processes progressively the input (it |
| isplugged as a front-end to the I/O module) for that entity. It |
| capturesand converts on-the-fly the document to be parsed to UTF-8. The |
| parseritself just does UTF-8 checking of this input and process |
| ittransparently. The only difference is that the encoding information |
| hasbeen added to the parsing context (more precisely to the |
| inputcorresponding to this entity).</li> |
| <li>The result (when using DOM) is an internal form completely in UTF-8with |
| just an encoding information on the document node.</li> |
| </ol><p>Ok then what happens when saving the document (assuming youcollected/built |
| an xmlDoc DOM like structure) ? It depends on the functioncalled, |
| xmlSaveFile() will just try to save in the original encoding, |
| whilexmlSaveFileTo() and xmlSaveFileEnc() can optionally save to a |
| givenencoding:</p><ol><li>if no encoding is given, libxml2 will look for an encoding |
| valueassociated to the document and if it exists will try to save to |
| thatencoding, |
| <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 |
| thedocument, libxml2 will again canonicalize the encoding name, lookup |
| for aconverter in the registered set or through iconv. If not found |
| thefunction will return an error code</li> |
| <li>the converter is placed before the I/O buffer layer, as another kind |
| ofbuffer, then libxml2 will simply push the UTF-8 serialization to |
| throughthat buffer, which will then progressively be converted and pushed |
| ontothe I/O layer.</li> |
| <li>It is possible that the converter code fails on some input, for |
| exampletrying to push an UTF-8 encoded Chinese character through the |
| UTF-8 toISO-8859-1 converter won't work. Since the encoders are |
| progressive theywill just report the error and the number of bytes |
| converted, at thatpoint libxml2 will decode the offending character, |
| remove it from thebuffer and replace it with the associated charRef |
| encoding &#123; andresume the conversion. This guarantees that any |
| document will be savedwithout losses (except for markup names where this |
| is not legal, this isa problem in the current version, in practice avoid |
| using non-asciicharacters for tag or attribute names). A special "ascii" |
| encoding nameis used to save documents to a pure ascii form can be used |
| whenportability is really crucial</li> |
| </ol><p>Here are a few examples based on the same test document:</p><pre>~/XML -> ./xmllint isolat1 |
| <?xml version="1.0" encoding="ISO-8859-1"?> |
| <très>là</très> |
| ~/XML -> ./xmllint --encode UTF-8 isolat1 |
| <?xml version="1.0" encoding="UTF-8"?> |
| <très>là </très> |
| ~/XML -> </pre><p>The same processing is applied (and reuse most of the code) for HTML |
| I18Nprocessing. Looking up and modifying the content encoding is a bit |
| moredifficult since it is located in a <meta> tag under the |
| <head>,so a couple of functions htmlGetMetaEncoding() and |
| htmlSetMetaEncoding() havebeen provided. The parser also attempts to switch |
| encoding on the fly whendetecting such a tag on input. Except for that the |
| processing is the same(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(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 |
| HTMLpredefined entities like &copy; for the Copyright sign.</li> |
| </ol><p>More over when compiled on an Unix platform with iconv support the fullset |
| of encodings supported by iconv can be instantly be used by libxml. On alinux |
| machine with glibc-2.1 the list of supported encodings and aliases fill3 full |
| pages, and include UCS-4, the full set of ISO-Latin encodings, and thevarious |
| Japanese ones.</p><p>To convert from the UTF-8 values returned from the API to another |
| encodingthen 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 |
| thePOSIX <a href="http://www.opengroup.org/onlinepubs/009695399/functions/iconv.html">iconv()</a>API |
| directly.</p><h4>Encoding aliases</h4><p>From 2.2.3, libxml2 has support to register encoding names aliases. |
| Thegoal is to be able to parse document whose encoding is supported but |
| wherethe name differs (for example from the default set of names accepted |
| byiconv). The following functions allow to register and handle new aliases |
| forexisting encodings. Once registered libxml2 will automatically lookup |
| thealiases 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" id="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 input and |
| outputconversion routines to/from UTF-8, and register them |
| usingxmlNewCharEncodingHandler(name, xxxToUTF8, UTF8Toxxx), and they will |
| becalled 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.hheader.</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> |
