1<?xml version="1.0" encoding="UTF-8"?> 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=UTF-8" /><link rel="SHORTCUT ICON" href="/favicon.ico" /><style type="text/css"> 4TD {font-family: Verdana,Arial,Helvetica} 5BODY {font-family: Verdana,Arial,Helvetica; margin-top: 2em; margin-left: 0em; margin-right: 0em} 6H1 {font-family: Verdana,Arial,Helvetica} 7H2 {font-family: Verdana,Arial,Helvetica} 8H3 {font-family: Verdana,Arial,Helvetica} 9A:link, A:visited, A:active { text-decoration: underline } 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" 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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 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> 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 26by using Unicode. Any conformant XML parser has to support the UTF-8 and 27UTF-16 default encodings which can both express the full unicode ranges. UTF8 28is a variable length encoding whose greatest points are to reuse the same 29encoding for ASCII and to save space for Western encodings, but it is a bit 30more complex to handle in practice. UTF-16 use 2 bytes per character (and 31sometimes combines two pairs), it makes implementation easier, but looks a 32bit overkill for Western languages encoding. Moreover the XML specification 33allows the document to be encoded in other encodings at the condition that 34they are clearly labeled as such. For example the following is a wellformed 35XML document encoded in ISO-8859-1 and using accentuated letters that we 36French like for both markup and content:</p><pre><?xml version="1.0" encoding="ISO-8859-1"?> 37<très>là </très></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 45exception of a few routines to read with a specific encoding or save to a 46specific encoding, is completely agnostic about the original encoding of the 47document.</p><p>It should be noted too that the HTML parser embedded in libxml2 now obey 48the same rules too, the following document will be (as of 2.2.2) handled in 49an internationalized fashion by libxml2 too:</p><pre><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" 50 "http://www.w3.org/TR/REC-html40/loose.dtd"> 51<html lang="fr"> 52<head> 53 <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"> 54</head> 55<body> 56<p>W3C crée des standards pour le Web.</body> 57</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 58default internal encoding, and that encoding to be UTF-8, here are the 59rationales 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. 103when reading a document or saving one. Let's look first at the reading 104sequence:</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 -> ./xmllint err.xml 116err.xml:1: error: Input is not proper UTF-8, indicate encoding ! 117<très>là </très> 118 ^ 119err.xml:1: error: Bytes: 0xE8 0x73 0x3E 0x6C 120<très>là </très> 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 -> ./xmllint err2.xml 129err2.xml:1: error: Unsupported encoding UnsupportedEnc 130<?xml version="1.0" encoding="UnsupportedEnc"?> 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 143collected/built an xmlDoc DOM like structure) ? It depends on the function 144called, xmlSaveFile() will just try to save in the original encoding, while 145xmlSaveFileTo() and xmlSaveFileEnc() can optionally save to a given 146encoding:</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 &#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 173terminal using ISO-8859-1 as the text encoding:</p><pre>~/XML -> ./xmllint isolat1 174<?xml version="1.0" encoding="ISO-8859-1"?> 175<très>là </très> 176~/XML -> ./xmllint --encode UTF-8 isolat1 177<?xml version="1.0" encoding="UTF-8"?> 178<très>là </très> 179~/XML -> </pre><p>The same processing is applied (and reuse most of the code) for HTML I18N 180processing. Looking up and modifying the content encoding is a bit more 181difficult since it is located in a <meta> tag under the <head>, 182so a couple of functions htmlGetMetaEncoding() and htmlSetMetaEncoding() have 183been provided. The parser also attempts to switch encoding on the fly when 184detecting 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 &copy; for the Copyright sign.</li> 193</ol><p>More over when compiled on an Unix platform with iconv support the full 194set of encodings supported by iconv can be instantly be used by libxml. On a 195linux machine with glibc-2.1 the list of supported encodings and aliases fill 1963 full pages, and include UCS-4, the full set of ISO-Latin encodings, and the 197various Japanese ones.</p><p>To convert from the UTF-8 values returned from the API to another encoding 198then 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 199POSIX <a href="http://www.opengroup.org/onlinepubs/009695399/functions/iconv.html">iconv()</a> 200API directly.</p><h4>Encoding aliases</h4><p>From 2.2.3, libxml2 has support to register encoding names aliases. The 201goal is to be able to parse document whose encoding is supported but where 202the name differs (for example from the default set of names accepted by 203iconv). The following functions allow to register and handle new aliases for 204existing encodings. Once registered libxml2 will automatically lookup the 205aliases 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 212conversion routines to/from UTF-8, and register them using 213xmlNewCharEncodingHandler(name, xxxToUTF8, UTF8Toxxx), and they will be 214called automatically if the parser(s) encounter such an encoding name 215(register it uppercase, this will help). The description of the encoders, 216their arguments and expected return values are described in the encoding.h 217header.</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> 218
