encoding.html revision 66f68e716ba29bf333cb0c83d401b7a06afe2197
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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>Table of Content:</p><ol><li><a href="encoding.html#What">What does internationalization support 11 mean ?</a></li> 12 <li><a href="encoding.html#internal">The internal encoding, how and 13 why</a></li> 14 <li><a href="encoding.html#implemente">How is it implemented ?</a></li> 15 <li><a href="encoding.html#Default">Default supported encodings</a></li> 16 <li><a href="encoding.html#extend">How to extend the existing 17 support</a></li> 18</ol><h3><a name="What" id="What">What does internationalization support mean ?</a></h3><p>If you are not really familiar with Internationalization (usual shorcut is 19I18N) , Unicode, characters and glyphs, I suggest you read a <a href="http://www.tbray.org/ongoing/When/200x/2003/04/06/Unicode">presentation</a> 20by Tim Bray on Unicode and why you should care about it.</p><p>XML was designed from the start to allow the support of any character set 21by using Unicode. Any conformant XML parser has to support the UTF-8 and 22UTF-16 default encodings which can both express the full unicode ranges. UTF8 23is a variable length encoding whose greatest points are to reuse the same 24encoding for ASCII and to save space for Western encodings, but it is a bit 25more complex to handle in practice. UTF-16 use 2 bytes per characters (and 26sometimes combines two pairs), it makes implementation easier, but looks a 27bit overkill for Western languages encoding. Moreover the XML specification 28allows document to be encoded in other encodings at the condition that they 29are clearly labeled as such. For example the following is a wellformed XML 30document encoded in ISO-8859 1 and using accentuated letter that we French 31likes for both markup and content:</p><pre><?xml version="1.0" encoding="ISO-8859-1"?> 32<tr�s>l�</tr�s></pre><p>Having internationalization support in libxml2 means the following:</p><ul><li>the document is properly parsed</li> 33 <li>informations about it's encoding are saved</li> 34 <li>it can be modified</li> 35 <li>it can be saved in its original encoding</li> 36 <li>it can also be saved in another encoding supported by libxml2 (for 37 example straight UTF8 or even an ASCII form)</li> 38</ul><p>Another very important point is that the whole libxml2 API, with the 39exception of a few routines to read with a specific encoding or save to a 40specific encoding, is completely agnostic about the original encoding of the 41document.</p><p>It should be noted too that the HTML parser embedded in libxml2 now obey 42the same rules too, the following document will be (as of 2.2.2) handled in 43an internationalized fashion by libxml2 too:</p><pre><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" 44 "http://www.w3.org/TR/REC-html40/loose.dtd">; 45<html lang="fr"> 46<head> 47 <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"> 48</head> 49<body> 50<p>W3C cr�e des standards pour le Web.</body> 51</html></pre><h3><a name="internal" id="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 52default internal encoding, and that encoding to be UTF-8, here are the 53rationale for those choices:</p><ul><li>keeping the native encoding in the internal form would force the libxml 54 users (or the code associated) to be fully aware of the encoding of the 55 original document, for examples when adding a text node to a document, 56 the content would have to be provided in the document encoding, i.e. the 57 client code would have to check it before hand, make sure it's conformant 58 to the encoding, etc ... Very hard in practice, though in some specific 59 cases this may make sense.</li> 60 <li>the second decision was which encoding. From the XML spec only UTF8 and 61 UTF16 really makes sense as being the two only encodings for which there 62 is mandatory support. UCS-4 (32 bits fixed size encoding) could be 63 considered an intelligent choice too since it's a direct Unicode mapping 64 support. I selected UTF-8 on the basis of efficiency and compatibility 65 with surrounding software: 66 <ul><li>UTF-8 while a bit more complex to convert from/to (i.e. slightly 67 more costly to import and export CPU wise) is also far more compact 68 than UTF-16 (and UCS-4) for a majority of the documents I see it used 69 for right now (RPM RDF catalogs, advogato data, various configuration 70 file formats, etc.) and the key point for today's computer 71 architecture is efficient uses of caches. If one nearly double the 72 memory requirement to store the same amount of data, this will trash 73 caches (main memory/external caches/internal caches) and my take is 74 that this harms the system far more than the CPU requirements needed 75 for the conversion to UTF-8</li> 76 <li>Most of libxml2 version 1 users were using it with straight ASCII 77 most of the time, doing the conversion with an internal encoding 78 requiring all their code to be rewritten was a serious show-stopper 79 for using UTF-16 or UCS-4.</li> 80 <li>UTF-8 is being used as the de-facto internal encoding standard for 81 related code like the <a href="http://www.pango.org/">pango</a> 82 upcoming Gnome text widget, and a lot of Unix code (yep another place 83 where Unix programmer base takes a different approach from Microsoft 84 - they are using UTF-16)</li> 85 </ul></li> 86</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 87 as UTF-8 valid strings. The proper way to terminate an xmlChar * string 88 is simply to append 0 byte, as usual.</li> 89 <li>One just need to make sure that when using chars outside the ASCII set, 90 the values has been properly converted to UTF-8</li> 91</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 92(internationalization) support get triggered only during I/O operation, i.e. 93when reading a document or saving one. Let's look first at the reading 94sequence:</p><ol><li>when a document is processed, we usually don't know the encoding, a 95 simple heuristic allows to detect UTF-16 and UCS-4 from whose where the 96 ASCII range (0-0x7F) maps with ASCII</li> 97 <li>the xml declaration if available is parsed, including the encoding 98 declaration. At that point, if the autodetected encoding is different 99 from the one declared a call to xmlSwitchEncoding() is issued.</li> 100 <li>If there is no encoding declaration, then the input has to be in either 101 UTF-8 or UTF-16, if it is not then at some point when processing the 102 input, the converter/checker of UTF-8 form will raise an encoding error. 103 You may end-up with a garbled document, or no document at all ! Example: 104 <pre>~/XML -> /xmllint err.xml 105err.xml:1: error: Input is not proper UTF-8, indicate encoding ! 106<tr�s>l�</tr�s> 107 ^ 108err.xml:1: error: Bytes: 0xE8 0x73 0x3E 0x6C 109<tr�s>l�</tr�s> 110 ^</pre> 111 </li> 112 <li>xmlSwitchEncoding() does an encoding name lookup, canonicalize it, and 113 then search the default registered encoding converters for that encoding. 114 If it's not within the default set and iconv() support has been compiled 115 it, it will ask iconv for such an encoder. If this fails then the parser 116 will report an error and stops processing: 117 <pre>~/XML -> /xmllint err2.xml 118err2.xml:1: error: Unsupported encoding UnsupportedEnc 119<?xml version="1.0" encoding="UnsupportedEnc"?> 120 ^</pre> 121 </li> 122 <li>From that point the encoder processes progressively the input (it is 123 plugged as a front-end to the I/O module) for that entity. It captures 124 and convert on-the-fly the document to be parsed to UTF-8. The parser 125 itself just does UTF-8 checking of this input and process it 126 transparently. The only difference is that the encoding information has 127 been added to the parsing context (more precisely to the input 128 corresponding to this entity).</li> 129 <li>The result (when using DOM) is an internal form completely in UTF-8 130 with just an encoding information on the document node.</li> 131</ol><p>Ok then what happens when saving the document (assuming you 132collected/built an xmlDoc DOM like structure) ? It depends on the function 133called, xmlSaveFile() will just try to save in the original encoding, while 134xmlSaveFileTo() and xmlSaveFileEnc() can optionally save to a given 135encoding:</p><ol><li>if no encoding is given, libxml2 will look for an encoding value 136 associated to the document and if it exists will try to save to that 137 encoding, 138 <p>otherwise everything is written in the internal form, i.e. UTF-8</p> 139 </li> 140 <li>so if an encoding was specified, either at the API level or on the 141 document, libxml2 will again canonicalize the encoding name, lookup for a 142 converter in the registered set or through iconv. If not found the 143 function will return an error code</li> 144 <li>the converter is placed before the I/O buffer layer, as another kind of 145 buffer, then libxml2 will simply push the UTF-8 serialization to through 146 that buffer, which will then progressively be converted and pushed onto 147 the I/O layer.</li> 148 <li>It is possible that the converter code fails on some input, for example 149 trying to push an UTF-8 encoded Chinese character through the UTF-8 to 150 ISO-8859-1 converter won't work. Since the encoders are progressive they 151 will just report the error and the number of bytes converted, at that 152 point libxml2 will decode the offending character, remove it from the 153 buffer and replace it with the associated charRef encoding &#123; and 154 resume the conversion. This guarantees that any document will be saved 155 without losses (except for markup names where this is not legal, this is 156 a problem in the current version, in practice avoid using non-ascii 157 characters for tags or attributes names @@). A special "ascii" encoding 158 name is used to save documents to a pure ascii form can be used when 159 portability is really crucial</li> 160</ol><p>Here is a few examples based on the same test document:</p><pre>~/XML -> /xmllint isolat1 161<?xml version="1.0" encoding="ISO-8859-1"?> 162<tr�s>l�</tr�s> 163~/XML -> /xmllint --encode UTF-8 isolat1 164<?xml version="1.0" encoding="UTF-8"?> 165<très>l� �</très> 166~/XML -> </pre><p>The same processing is applied (and reuse most of the code) for HTML I18N 167processing. Looking up and modifying the content encoding is a bit more 168difficult since it is located in a <meta> tag under the <head>, 169so a couple of functions htmlGetMetaEncoding() and htmlSetMetaEncoding() have 170been provided. The parser also attempts to switch encoding on the fly when 171detecting such a tag on input. Except for that the processing is the same 172(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 173(located in encoding.c):</p><ol><li>UTF-8 is supported by default (null handlers)</li> 174 <li>UTF-16, both little and big endian</li> 175 <li>ISO-Latin-1 (ISO-8859-1) covering most western languages</li> 176 <li>ASCII, useful mostly for saving</li> 177 <li>HTML, a specific handler for the conversion of UTF-8 to ASCII with HTML 178 predefined entities like &copy; for the Copyright sign.</li> 179</ol><p>More over when compiled on an Unix platform with iconv support the full 180set of encodings supported by iconv can be instantly be used by libxml. On a 181linux machine with glibc-2.1 the list of supported encodings and aliases fill 1823 full pages, and include UCS-4, the full set of ISO-Latin encodings, and the 183various Japanese ones.</p><h4>Encoding aliases</h4><p>From 2.2.3, libxml2 has support to register encoding names aliases. The 184goal is to be able to parse document whose encoding is supported but where 185the name differs (for example from the default set of names accepted by 186iconv). The following functions allow to register and handle new aliases for 187existing encodings. Once registered libxml2 will automatically lookup the 188aliases when handling a document:</p><ul><li>int xmlAddEncodingAlias(const char *name, const char *alias);</li> 189 <li>int xmlDelEncodingAlias(const char *alias);</li> 190 <li>const char * xmlGetEncodingAlias(const char *alias);</li> 191 <li>void xmlCleanupEncodingAliases(void);</li> 192</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 193(assuming it is buggy) should not be hard, just write an input and output 194conversion routines to/from UTF-8, and register them using 195xmlNewCharEncodingHandler(name, xxxToUTF8, UTF8Toxxx), and they will be 196called automatically if the parser(s) encounter such an encoding name 197(register it uppercase, this will help). The description of the encoders, 198their arguments and expected return values are described in the encoding.h 199header.</p><p>A quick note on the topic of subverting the parser to use a different 200internal encoding than UTF-8, in some case people will absolutely want to 201keep the internal encoding different, I think it's still possible (but the 202encoding must be compliant with ASCII on the same subrange) though I didn't 203tried it. The key is to override the default conversion routines (by 204registering null encoders/decoders for your charsets), and bypass the UTF-8 205checking of the parser by setting the parser context charset 206(ctxt->charset) to something different than XML_CHAR_ENCODING_UTF8, but 207there is no guarantee that this will work. You may also have some troubles 208saving back.</p><p>Basically proper I18N support is important, this requires at least 209libxml-2.0.0, but a lot of features and corrections are really available only 210starting 2.2.</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> 211
