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gerrit-public.fairphone.software / platform / external / libxml2 / 293416828e1467f877d9dd928f174dcf81b103bc / . / doc / encoding.html
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Daniel Veillard1177ca42003-04-26 22:29:54 +0000[diff] [blame]1<?xml version="1.0" encoding="ISO-8859-1"?>
2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
3<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">
Daniel Veillard373a4752002-02-21 14:46:29 +0000[diff] [blame]4TD {font-family: Verdana,Arial,Helvetica}
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Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]9A:link, A:visited, A:active { text-decoration: underline }
Daniel Veillard45e21e22009-08-20 19:40:03 +0200[diff] [blame]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://codespeak.net/lxml/">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
Daniel Veillardf781dba2006-06-09 13:34:49 +0000[diff] [blame]11is 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>
12by 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
13without knowing what encoding it uses</b>, then as Joel Spolsky said <a href="http://www.joelonsoftware.com/articles/Unicode.html">please do not
14write another line of code until you finish reading that article.</a>. It is
15a prerequisite to understand this page, and avoid a lot of problems with
16libxml2, XML or text processing in general.</p><p>Table of Content:</p><ol><li><a href="encoding.html#What">What does internationalization support
17 mean ?</a></li>
18 <li><a href="encoding.html#internal">The internal encoding, how and
19 why</a></li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]20 <li><a href="encoding.html#implemente">How is it implemented ?</a></li>
21 <li><a href="encoding.html#Default">Default supported encodings</a></li>
Daniel Veillardf781dba2006-06-09 13:34:49 +0000[diff] [blame]22 <li><a href="encoding.html#extend">How to extend the existing
23 support</a></li>
24</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
25by using Unicode. Any conformant XML parser has to support the UTF-8 and
26UTF-16 default encodings which can both express the full unicode ranges. UTF8
27is a variable length encoding whose greatest points are to reuse the same
28encoding for ASCII and to save space for Western encodings, but it is a bit
29more complex to handle in practice. UTF-16 use 2 bytes per character (and
30sometimes combines two pairs), it makes implementation easier, but looks a
31bit overkill for Western languages encoding. Moreover the XML specification
32allows the document to be encoded in other encodings at the condition that
33they are clearly labeled as such. For example the following is a wellformed
34XML document encoded in ISO-8859-1 and using accentuated letters that we
35French like for both markup and content:</p><pre>&lt;?xml version="1.0" encoding="ISO-8859-1"?&gt;
Daniel Veillard29341682009-09-10 18:23:39 +0200[diff] [blame^]36&lt;très&gt;là&lt;/très&gt;</pre><p>Having internationalization support in libxml2 means the following:</p><ul><li>the document is properly parsed</li>
William M. Brack43a87292007-02-15 20:41:02 +0000[diff] [blame]37 <li>information about it's encoding is saved</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]38 <li>it can be modified</li>
39 <li>it can be saved in its original encoding</li>
Daniel Veillardf781dba2006-06-09 13:34:49 +0000[diff] [blame]40 <li>it can also be saved in another encoding supported by libxml2 (for
41 example straight UTF8 or even an ASCII form)</li>
42</ul><p>Another very important point is that the whole libxml2 API, with the
43exception of a few routines to read with a specific encoding or save to a
44specific encoding, is completely agnostic about the original encoding of the
45document.</p><p>It should be noted too that the HTML parser embedded in libxml2 now obey
46the same rules too, the following document will be (as of 2.2.2) handled in
47an internationalized fashion by libxml2 too:</p><pre>&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
Daniel Veillard024f1992003-12-10 16:43:49 +0000[diff] [blame]48 "http://www.w3.org/TR/REC-html40/loose.dtd"&gt;
49&lt;html lang="fr"&gt;
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]50&lt;head&gt;
Daniel Veillard024f1992003-12-10 16:43:49 +0000[diff] [blame]51 &lt;META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"&gt;
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]52&lt;/head&gt;
53&lt;body&gt;
Daniel Veillard29341682009-09-10 18:23:39 +0200[diff] [blame^]54&lt;p&gt;W3C crée des standards pour le Web.&lt;/body&gt;
Daniel Veillardf781dba2006-06-09 13:34:49 +0000[diff] [blame]55&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
56default internal encoding, and that encoding to be UTF-8, here are the
57rationales for those choices:</p><ul><li>keeping the native encoding in the internal form would force the libxml
58 users (or the code associated) to be fully aware of the encoding of the
59 original document, for examples when adding a text node to a document,
60 the content would have to be provided in the document encoding, i.e. the
61 client code would have to check it before hand, make sure it's conformant
62 to the encoding, etc ... Very hard in practice, though in some specific
63 cases this may make sense.</li>
64 <li>the second decision was which encoding. From the XML spec only UTF8 and
65 UTF16 really makes sense as being the two only encodings for which there
66 is mandatory support. UCS-4 (32 bits fixed size encoding) could be
67 considered an intelligent choice too since it's a direct Unicode mapping
68 support. I selected UTF-8 on the basis of efficiency and compatibility
69 with surrounding software:
70 <ul><li>UTF-8 while a bit more complex to convert from/to (i.e. slightly
71 more costly to import and export CPU wise) is also far more compact
72 than UTF-16 (and UCS-4) for a majority of the documents I see it used
73 for right now (RPM RDF catalogs, advogato data, various configuration
74 file formats, etc.) and the key point for today's computer
75 architecture is efficient uses of caches. If one nearly double the
76 memory requirement to store the same amount of data, this will trash
77 caches (main memory/external caches/internal caches) and my take is
78 that this harms the system far more than the CPU requirements needed
79 for the conversion to UTF-8</li>
80 <li>Most of libxml2 version 1 users were using it with straight ASCII
81 most of the time, doing the conversion with an internal encoding
82 requiring all their code to be rewritten was a serious show-stopper
83 for using UTF-16 or UCS-4.</li>
84 <li>UTF-8 is being used as the de-facto internal encoding standard for
85 related code like the <a href="http://www.pango.org/">pango</a>
86 upcoming Gnome text widget, and a lot of Unix code (yet another place
87 where Unix programmer base takes a different approach from Microsoft
88 - they are using UTF-16)</li>
Daniel Veillard1177ca42003-04-26 22:29:54 +0000[diff] [blame]89 </ul></li>
Daniel Veillardf781dba2006-06-09 13:34:49 +0000[diff] [blame]90</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
91 as UTF-8 valid strings. The proper way to terminate an xmlChar * string
92 is simply to append 0 byte, as usual.</li>
93 <li>One just need to make sure that when using chars outside the ASCII set,
94 the values has been properly converted to UTF-8</li>
95</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
96(internationalization) support get triggered only during I/O operation, i.e.
97when reading a document or saving one. Let's look first at the reading
98sequence:</p><ol><li>when a document is processed, we usually don't know the encoding, a
99 simple heuristic allows to detect UTF-16 and UCS-4 from encodings where
100 the ASCII range (0-0x7F) maps with ASCII</li>
101 <li>the xml declaration if available is parsed, including the encoding
102 declaration. At that point, if the autodetected encoding is different
103 from the one declared a call to xmlSwitchEncoding() is issued.</li>
104 <li>If there is no encoding declaration, then the input has to be in either
105 UTF-8 or UTF-16, if it is not then at some point when processing the
106 input, the converter/checker of UTF-8 form will raise an encoding error.
107 You may end-up with a garbled document, or no document at all ! Example:
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]108 <pre>~/XML -&gt; ./xmllint err.xml
109err.xml:1: error: Input is not proper UTF-8, indicate encoding !
Daniel Veillard29341682009-09-10 18:23:39 +0200[diff] [blame^]110&lt;très&gt;là&lt;/très&gt;
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]111 ^
112err.xml:1: error: Bytes: 0xE8 0x73 0x3E 0x6C
Daniel Veillard29341682009-09-10 18:23:39 +0200[diff] [blame^]113&lt;très&gt;là&lt;/très&gt;
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]114 ^</pre>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]115 </li>
Daniel Veillardf781dba2006-06-09 13:34:49 +0000[diff] [blame]116 <li>xmlSwitchEncoding() does an encoding name lookup, canonicalize it, and
117 then search the default registered encoding converters for that encoding.
118 If it's not within the default set and iconv() support has been compiled
119 it, it will ask iconv for such an encoder. If this fails then the parser
120 will report an error and stops processing:
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]121 <pre>~/XML -&gt; ./xmllint err2.xml
122err2.xml:1: error: Unsupported encoding UnsupportedEnc
Daniel Veillard024f1992003-12-10 16:43:49 +0000[diff] [blame]123&lt;?xml version="1.0" encoding="UnsupportedEnc"?&gt;
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]124 ^</pre>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]125 </li>
Daniel Veillardf781dba2006-06-09 13:34:49 +0000[diff] [blame]126 <li>From that point the encoder processes progressively the input (it is
127 plugged as a front-end to the I/O module) for that entity. It captures
128 and converts on-the-fly the document to be parsed to UTF-8. The parser
129 itself just does UTF-8 checking of this input and process it
130 transparently. The only difference is that the encoding information has
131 been added to the parsing context (more precisely to the input
132 corresponding to this entity).</li>
133 <li>The result (when using DOM) is an internal form completely in UTF-8
134 with just an encoding information on the document node.</li>
135</ol><p>Ok then what happens when saving the document (assuming you
136collected/built an xmlDoc DOM like structure) ? It depends on the function
137called, xmlSaveFile() will just try to save in the original encoding, while
138xmlSaveFileTo() and xmlSaveFileEnc() can optionally save to a given
139encoding:</p><ol><li>if no encoding is given, libxml2 will look for an encoding value
140 associated to the document and if it exists will try to save to that
141 encoding,
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]142 <p>otherwise everything is written in the internal form, i.e. UTF-8</p>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]143 </li>
Daniel Veillardf781dba2006-06-09 13:34:49 +0000[diff] [blame]144 <li>so if an encoding was specified, either at the API level or on the
145 document, libxml2 will again canonicalize the encoding name, lookup for a
146 converter in the registered set or through iconv. If not found the
147 function will return an error code</li>
148 <li>the converter is placed before the I/O buffer layer, as another kind of
149 buffer, then libxml2 will simply push the UTF-8 serialization to through
150 that buffer, which will then progressively be converted and pushed onto
151 the I/O layer.</li>
152 <li>It is possible that the converter code fails on some input, for example
153 trying to push an UTF-8 encoded Chinese character through the UTF-8 to
154 ISO-8859-1 converter won't work. Since the encoders are progressive they
155 will just report the error and the number of bytes converted, at that
156 point libxml2 will decode the offending character, remove it from the
157 buffer and replace it with the associated charRef encoding &amp;#123; and
158 resume the conversion. This guarantees that any document will be saved
159 without losses (except for markup names where this is not legal, this is
160 a problem in the current version, in practice avoid using non-ascii
161 characters for tag or attribute names). A special "ascii" encoding name
162 is used to save documents to a pure ascii form can be used when
163 portability is really crucial</li>
Daniel Veillardabfca612004-01-07 23:38:02 +0000[diff] [blame]164</ol><p>Here are a few examples based on the same test document:</p><pre>~/XML -&gt; ./xmllint isolat1
Daniel Veillard024f1992003-12-10 16:43:49 +0000[diff] [blame]165&lt;?xml version="1.0" encoding="ISO-8859-1"?&gt;
Daniel Veillard29341682009-09-10 18:23:39 +0200[diff] [blame^]166&lt;très&gt;là&lt;/très&gt;
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]167~/XML -&gt; ./xmllint --encode UTF-8 isolat1
Daniel Veillard024f1992003-12-10 16:43:49 +0000[diff] [blame]168&lt;?xml version="1.0" encoding="UTF-8"?&gt;
Daniel Veillard29341682009-09-10 18:23:39 +0200[diff] [blame^]169&lt;très&gt;là  &lt;/très&gt;
Daniel Veillardf781dba2006-06-09 13:34:49 +0000[diff] [blame]170~/XML -&gt; </pre><p>The same processing is applied (and reuse most of the code) for HTML I18N
171processing. Looking up and modifying the content encoding is a bit more
172difficult since it is located in a &lt;meta&gt; tag under the &lt;head&gt;,
173so a couple of functions htmlGetMetaEncoding() and htmlSetMetaEncoding() have
174been provided. The parser also attempts to switch encoding on the fly when
175detecting such a tag on input. Except for that the processing is the same
176(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
177(located in encoding.c):</p><ol><li>UTF-8 is supported by default (null handlers)</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]178 <li>UTF-16, both little and big endian</li>
179 <li>ISO-Latin-1 (ISO-8859-1) covering most western languages</li>
180 <li>ASCII, useful mostly for saving</li>
Daniel Veillardf781dba2006-06-09 13:34:49 +0000[diff] [blame]181 <li>HTML, a specific handler for the conversion of UTF-8 to ASCII with HTML
182 predefined entities like &amp;copy; for the Copyright sign.</li>
183</ol><p>More over when compiled on an Unix platform with iconv support the full
184set of encodings supported by iconv can be instantly be used by libxml. On a
185linux machine with glibc-2.1 the list of supported encodings and aliases fill
1863 full pages, and include UCS-4, the full set of ISO-Latin encodings, and the
187various Japanese ones.</p><p>To convert from the UTF-8 values returned from the API to another encoding
188then 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
189POSIX <a href="http://www.opengroup.org/onlinepubs/009695399/functions/iconv.html">iconv()</a>
190API directly.</p><h4>Encoding aliases</h4><p>From 2.2.3, libxml2 has support to register encoding names aliases. The
191goal is to be able to parse document whose encoding is supported but where
192the name differs (for example from the default set of names accepted by
193iconv). The following functions allow to register and handle new aliases for
194existing encodings. Once registered libxml2 will automatically lookup the
195aliases when handling a document:</p><ul><li>int xmlAddEncodingAlias(const char *name, const char *alias);</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000[diff] [blame]196 <li>int xmlDelEncodingAlias(const char *alias);</li>
197 <li>const char * xmlGetEncodingAlias(const char *alias);</li>
198 <li>void xmlCleanupEncodingAliases(void);</li>
Daniel Veillardf781dba2006-06-09 13:34:49 +0000[diff] [blame]199</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
200(assuming it is buggy) should not be hard, just write input and output
201conversion routines to/from UTF-8, and register them using
202xmlNewCharEncodingHandler(name, xxxToUTF8, UTF8Toxxx), and they will be
203called automatically if the parser(s) encounter such an encoding name
204(register it uppercase, this will help). The description of the encoders,
205their arguments and expected return values are described in the encoding.h
206header.</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>