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| author | William Joye <wjoye@cfa.harvard.edu> | 2016-10-19 18:28:08 (GMT) |
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| committer | William Joye <wjoye@cfa.harvard.edu> | 2016-10-19 18:28:08 (GMT) |
| commit | 7f402e480a04b974617c04142993d3919457d342 (patch) | |
| tree | a39cf901d49b9767df3fe076ebf8a06c92277995 /libxml2/doc/encoding.html | |
| parent | 818fd731f62993dc8aa5d07b82e1d1b2c04aae7d (diff) | |
| parent | 6c75f0dc676d73c4cbcdca5a9cf05c6797a2ac0b (diff) | |
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Merge commit '6c75f0dc676d73c4cbcdca5a9cf05c6797a2ac0b' as 'libxml2'
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| -rw-r--r-- | libxml2/doc/encoding.html | 217 |
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diff --git a/libxml2/doc/encoding.html b/libxml2/doc/encoding.html new file mode 100644 index 0000000..7c7953f --- /dev/null +++ b/libxml2/doc/encoding.html @@ -0,0 +1,217 @@ +<?xml version="1.0" encoding="UTF-8"?> +<!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=UTF-8" /><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://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 +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> +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 +without knowing what encoding it uses</b>, then as Joel Spolsky said <a href="http://www.joelonsoftware.com/articles/Unicode.html">please do not +write another line of code until you finish reading that article.</a>. It is +a prerequisite to understand this page, and avoid a lot of problems with +libxml2, XML or text processing in general.</p><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" id="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 points are to reuse the same +encoding 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 character (and +sometimes combines two pairs), it makes implementation easier, but looks a +bit overkill for Western languages encoding. Moreover the XML specification +allows the document to be encoded in other encodings at the condition that +they are clearly labeled as such. For example the following is a wellformed +XML document encoded in ISO-8859-1 and using accentuated letters that we +French 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>information about it's encoding is 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 (for + example straight UTF8 or even an ASCII form)</li> +</ul><p>Another very important point is that the whole libxml2 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 libxml2 now obey +the same rules too, the following document will be (as of 2.2.2) handled in +an 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 a +default internal encoding, and that encoding to be UTF-8, here are the +rationales 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 mandatory 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 libxml2 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 (yet 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 libxml2 user:</p><ul> + <li>xmlChar, the libxml2 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" 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 reading +sequence:</p><ol> + <li>when a document is processed, we usually don't know the encoding, a + simple heuristic allows to detect UTF-16 and UCS-4 from encodings 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 -> ./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, 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 -> ./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 is + plugged as a front-end to the I/O module) for that entity. It captures + and converts 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 happens when saving the document (assuming you +collected/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, libxml2 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, libxml2 will again canonicalize 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 libxml2 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 libxml2 will decode the offending character, remove it from the + buffer and replace it with the associated charRef encoding &#123; and + resume the conversion. This guarantees 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 practice avoid using non-ascii + characters for tag or attribute names). 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 are a few examples based on the same test document and assumin a +terminal using ISO-8859-1 as the text encoding:</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 I18N +processing. Looking up and modifying the content encoding is a bit more +difficult since it is located in a <meta> tag under the <head>, +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" 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 HTML + predefined entities like &copy; for the Copyright sign.</li> +</ol><p>More over when compiled on an Unix platform 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><p>To convert from the UTF-8 values returned from the API to another encoding +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 +POSIX <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. 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 libxml2 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" 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 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 href="bugs.html">Daniel Veillard</a></p></td></tr></table></td></tr></table></td></tr></table></td></tr></table></td></tr></table></body></html> |