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Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/REC-html40/loose.dtd">
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13<title>Encodings support</title>
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21<h1>The XML C library for Gnome</h1>
22<h2>Encodings support</h2>
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Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]74<p>Table of Content:</p>
75<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]76<li><a href="encoding.html#What">What does internationalization support
77 mean ?</a></li>
78<li><a href="encoding.html#internal">The internal encoding, how and
79 why</a></li>
80<li><a href="encoding.html#implemente">How is it implemented ?</a></li>
81<li><a href="encoding.html#Default">Default supported encodings</a></li>
82<li><a href="encoding.html#extend">How to extend the existing
83 support</a></li>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]84</ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]85<h3><a name="What">What does internationalization support mean ?</a></h3>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]86<p>XML was designed from the start to allow the support of any character set
87by using Unicode. Any conformant XML parser has to support the UTF-8 and
88UTF-16 default encodings which can both express the full unicode ranges. UTF8
89is a variable length encoding whose greatest point are to resuse the same
90emcoding for ASCII and to save space for Western encodings, but it is a bit
91more complex to handle in practice. UTF-16 use 2 bytes per characters (and
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]92sometimes combines two pairs), it makes implementation easier, but looks a
93bit overkill for Western languages encoding. Moreover the XML specification
94allows document to be encoded in other encodings at the condition that they
95are clearly labelled as such. For example the following is a wellformed XML
Daniel Veillard0d6b1702000-08-22 23:52:16 +0000[diff] [blame]96document encoded in ISO-8859 1 and using accentuated letter that we French
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]97likes for both markup and content:</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]98<pre>&lt;?xml version=&quot;1.0&quot; encoding=&quot;ISO-8859-1&quot;?&gt;
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]99&lt;très&gt;là&lt;/très&gt;</pre>
Daniel Veillard0d6b1702000-08-22 23:52:16 +0000[diff] [blame]100<p>Having internationalization support in libxml means the foolowing:</p>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]101<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]102<li>the document is properly parsed</li>
103<li>informations about it's encoding are saved</li>
104<li>it can be modified</li>
105<li>it can be saved in its original encoding</li>
106<li>it can also be saved in another encoding supported by libxml (for
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]107 example straight UTF8 or even an ASCII form)</li>
108</ul>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]109<p>Another very important point is that the whole libxml API, with the
110exception of a few routines to read with a specific encoding or save to a
111specific encoding, is completely agnostic about the original encoding of the
112document.</p>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]113<p>It should be noted too that the HTML parser embedded in libxml now obbey
114the same rules too, the following document will be (as of 2.2.2) handled in
115an internationalized fashion by libxml too:</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]116<pre>&lt;!DOCTYPE HTML PUBLIC &quot;-//W3C//DTD HTML 4.0 Transitional//EN&quot;
117 &quot;http://www.w3.org/TR/REC-html40/loose.dtd&quot;&gt;
118&lt;html lang=&quot;fr&quot;&gt;
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]119&lt;head&gt;
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]120 &lt;META HTTP-EQUIV=&quot;Content-Type&quot; CONTENT=&quot;text/html; charset=ISO-8859-1&quot;&gt;
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]121&lt;/head&gt;
122&lt;body&gt;
123&lt;p&gt;W3C crée des standards pour le Web.&lt;/body&gt;
124&lt;/html&gt;</pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]125<h3><a name="internal">The internal encoding, how and why</a></h3>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]126<p>One of the core decision was to force all documents to be converted to a
127default internal encoding, and that encoding to be UTF-8, here are the
128rationale for those choices:</p>
129<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]130<li>keeping the native encoding in the internal form would force the libxml
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]131 users (or the code associated) to be fully aware of the encoding of the
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]132 original document, for examples when adding a text node to a document,
133 the content would have to be provided in the document encoding, i.e. the
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]134 client code would have to check it before hand, make sure it's conformant
135 to the encoding, etc ... Very hard in practice, though in some specific
136 cases this may make sense.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]137<li>the second decision was which encoding. From the XML spec only UTF8 and
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]138 UTF16 really makes sense as being the two only encodings for which there
139 is amndatory support. UCS-4 (32 bits fixed size encoding) could be
140 considered an intelligent choice too since it's a direct Unicode mapping
141 support. I selected UTF-8 on the basis of efficiency and compatibility
142 with surrounding software:
143 <ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]144<li>UTF-8 while a bit more complex to convert from/to (i.e. slightly
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]145 more costly to import and export CPU wise) is also far more compact
146 than UTF-16 (and UCS-4) for a majority of the documents I see it used
147 for right now (RPM RDF catalogs, advogato data, various configuration
148 file formats, etc.) and the key point for today's computer
149 architecture is efficient uses of caches. If one nearly double the
150 memory requirement to store the same amount of data, this will trash
151 caches (main memory/external caches/internal caches) and my take is
152 that this harms the system far more than the CPU requirements needed
153 for the conversion to UTF-8</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]154<li>Most of libxml version 1 users were using it with straight ASCII
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]155 most of the time, doing the conversion with an internal encoding
156 requiring all their code to be rewritten was a serious show-stopper
157 for using UTF-16 or UCS-4.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]158<li>UTF-8 is being used as the de-facto internal encoding standard for
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]159 related code like the <a href="http://www.pango.org/">pango</a>
160 upcoming Gnome text widget, and a lot of Unix code (yep another place
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]161 where Unix programmer base takes a different approach from Microsoft
162 - they are using UTF-16)</li>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]163</ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]164</li>
165</ul>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]166<p>What does this mean in practice for the libxml user:</p>
167<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]168<li>xmlChar, the libxml data type is a byte, those bytes must be assembled
169 as UTF-8 valid strings. The proper way to terminate an xmlChar * string
170 is simply to append 0 byte, as usual.</li>
171<li>One just need to make sure that when using chars outside the ASCII set,
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]172 the values has been properly converted to UTF-8</li>
173</ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]174<h3><a name="implemente">How is it implemented ?</a></h3>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]175<p>Let's describe how all this works within libxml, basically the I18N
176(internationalization) support get triggered only during I/O operation, i.e.
177when reading a document or saving one. Let's look first at the reading
178sequence:</p>
179<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]180<li>when a document is processed, we usually don't know the encoding, a
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]181 simple heuristic allows to detect UTF-18 and UCS-4 from whose where the
182 ASCII range (0-0x7F) maps with ASCII</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]183<li>the xml declaration if available is parsed, including the encoding
184 declaration. At that point, if the autodetected encoding is different
185 from the one declared a call to xmlSwitchEncoding() is issued.</li>
186<li>If there is no encoding declaration, then the input has to be in either
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]187 UTF-8 or UTF-16, if it is not then at some point when processing the
188 input, the converter/checker of UTF-8 form will raise an encoding error.
189 You may end-up with a garbled document, or no document at all ! Example:
190 <pre>~/XML -&gt; ./xmllint err.xml
191err.xml:1: error: Input is not proper UTF-8, indicate encoding !
192&lt;très&gt;là&lt;/très&gt;
193 ^
194err.xml:1: error: Bytes: 0xE8 0x73 0x3E 0x6C
195&lt;très&gt;là&lt;/très&gt;
196 ^</pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]197</li>
198<li>xmlSwitchEncoding() does an encoding name lookup, canonalize it, and
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]199 then search the default registered encoding converters for that encoding.
200 If it's not within the default set and iconv() support has been compiled
201 it, it will ask iconv for such an encoder. If this fails then the parser
202 will report an error and stops processing:
203 <pre>~/XML -&gt; ./xmllint err2.xml
204err2.xml:1: error: Unsupported encoding UnsupportedEnc
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]205&lt;?xml version=&quot;1.0&quot; encoding=&quot;UnsupportedEnc&quot;?&gt;
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]206 ^</pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]207</li>
208<li>From that point the encoder process progressingly the input (it is
209 plugged as a front-end to the I/O module) for that entity. It captures
210 and convert on-the-fly the document to be parsed to UTF-8. The parser
211 itself just does UTF-8 checking of this input and process it
212 transparently. The only difference is that the encoding information has
213 been added to the parsing context (more precisely to the input
214 corresponding to this entity).</li>
215<li>The result (when using DOM) is an internal form completely in UTF-8
216 with just an encoding information on the document node.</li>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]217</ol>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]218<p>Ok then what's happen when saving the document (assuming you
219colllected/built an xmlDoc DOM like structure) ? It depends on the function
220called, xmlSaveFile() will just try to save in the original encoding, while
221xmlSaveFileTo() and xmlSaveFileEnc() can optionally save to a given
222encoding:</p>
223<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]224<li>if no encoding is given, libxml will look for an encoding value
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]225 associated to the document and if it exists will try to save to that
226 encoding,
227 <p>otherwise everything is written in the internal form, i.e. UTF-8</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]228</li>
229<li>so if an encoding was specified, either at the API level or on the
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]230 document, libxml will again canonalize the encoding name, lookup for a
231 converter in the registered set or through iconv. If not found the
232 function will return an error code</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]233<li>the converter is placed before the I/O buffer layer, as another kind of
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]234 buffer, then libxml will simply push the UTF-8 serialization to through
235 that buffer, which will then progressively be converted and pushed onto
236 the I/O layer.</li>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]237<li>It is possible that the converter code fails on some input, for example
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]238 trying to push an UTF-8 encoded chinese character through the UTF-8 to
Daniel Veillard0d6b1702000-08-22 23:52:16 +0000[diff] [blame]239 ISO-8859-1 converter won't work. Since the encoders are progressive they
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]240 will just report the error and the number of bytes converted, at that
241 point libxml will decode the offending character, remove it from the
242 buffer and replace it with the associated charRef encoding &amp;#123; and
243 resume the convertion. This guarante that any document will be saved
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]244 without losses (except for markup names where this is not legal, this is
245 a problem in the current version, in pactice avoid using non-ascci
246 characters for tags or attributes names @@). A special &quot;ascii&quot; encoding
Daniel Veillard0d6b1702000-08-22 23:52:16 +0000[diff] [blame]247 name is used to save documents to a pure ascii form can be used when
248 portability is really crucial</li>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]249</ol>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]250<p>Here is a few examples based on the same test document:</p>
251<pre>~/XML -&gt; ./xmllint isolat1
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]252&lt;?xml version=&quot;1.0&quot; encoding=&quot;ISO-8859-1&quot;?&gt;
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]253&lt;très&gt;là&lt;/très&gt;
254~/XML -&gt; ./xmllint --encode UTF-8 isolat1
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]255&lt;?xml version=&quot;1.0&quot; encoding=&quot;UTF-8&quot;?&gt;
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]256&lt;très&gt;là  &lt;/très&gt;
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]257~/XML -&gt; </pre>
Daniel Veillard0d6b1702000-08-22 23:52:16 +0000[diff] [blame]258<p>The same processing is applied (and reuse most of the code) for HTML I18N
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]259processing. Looking up and modifying the content encoding is a bit more
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]260difficult since it is located in a &lt;meta&gt; tag under the &lt;head&gt;,
261so a couple of functions htmlGetMetaEncoding() and htmlSetMetaEncoding() have
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]262been provided. The parser also attempts to switch encoding on the fly when
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]263detecting such a tag on input. Except for that the processing is the same
264(and again reuses the same code).</p>
265<h3><a name="Default">Default supported encodings</a></h3>
266<p>libxml has a set of default converters for the following encodings
267(located in encoding.c):</p>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]268<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]269<li>UTF-8 is supported by default (null handlers)</li>
270<li>UTF-16, both little and big endian</li>
271<li>ISO-Latin-1 (ISO-8859-1) covering most western languages</li>
272<li>ASCII, useful mostly for saving</li>
273<li>HTML, a specific handler for the conversion of UTF-8 to ASCII with HTML
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]274 predefined entities like &amp;copy; for the Copyright sign.</li>
275</ol>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]276<p>More over when compiled on an Unix platfor with iconv support the full set
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]277of encodings supported by iconv can be instantly be used by libxml. On a
278linux machine with glibc-2.1 the list of supported encodings and aliases fill
2793 full pages, and include UCS-4, the full set of ISO-Latin encodings, and the
280various Japanese ones.</p>
281<h4>Encoding aliases</h4>
282<p>From 2.2.3, libxml has support to register encoding names aliases. The
283goal is to be able to parse document whose encoding is supported but where
284the name differs (for example from the default set of names accepted by
285iconv). The following functions allow to register and handle new aliases for
286existing encodings. Once registered libxml will automatically lookup the
287aliases when handling a document:</p>
Daniel Veillard088f4282000-08-25 23:46:50 +0000[diff] [blame]288<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]289<li>int xmlAddEncodingAlias(const char *name, const char *alias);</li>
290<li>int xmlDelEncodingAlias(const char *alias);</li>
291<li>const char * xmlGetEncodingAlias(const char *alias);</li>
292<li>void xmlCleanupEncodingAliases(void);</li>
Daniel Veillard088f4282000-08-25 23:46:50 +0000[diff] [blame]293</ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]294<h3><a name="extend">How to extend the existing support</a></h3>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]295<p>Well adding support for new encoding, or overriding one of the encoders
296(assuming it is buggy) should not be hard, just write an input and output
297conversion routines to/from UTF-8, and register them using
298xmlNewCharEncodingHandler(name, xxxToUTF8, UTF8Toxxx), and they will be
299called automatically if the parser(s) encounter such an encoding name
300(register it uppercase, this will help). The description of the encoders,
301their arguments and expected return values are described in the encoding.h
302header.</p>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]303<p>A quick note on the topic of subverting the parser to use a different
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]304internal encoding than UTF-8, in some case people will absolutely want to
305keep the internal encoding different, I think it's still possible (but the
306encoding must be compliant with ASCII on the same subrange) though I didn't
307tried it. The key is to override the default conversion routines (by
308registering null encoders/decoders for your charsets), and bypass the UTF-8
309checking of the parser by setting the parser context charset
310(ctxt-&gt;charset) to something different than XML_CHAR_ENCODING_UTF8, but
311there is no guarantee taht this will work. You may also have some troubles
312saving back.</p>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]313<p>Basically proper I18N support is important, this requires at least
314libxml-2.0.0, but a lot of features and corrections are really available only
315starting 2.2.</p>
Daniel Veillardc5d64342001-06-24 12:13:24 +0000[diff] [blame]316<p><a href="mailto:daniel@veillard.com">Daniel Veillard</a></p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000[diff] [blame]317</td></tr></table></td></tr></table></td></tr></table></td>
318</tr></table></td></tr></table>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000[diff] [blame]319</body>
320</html>