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22<h1>The XML C library for Gnome</h1>
23<h2>Encodings support</h2>
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Daniel Veillardbe40c8b2000-07-14 12:10:59 +000098<p>Table of Content:</p>
99<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000100<li><a href="encoding.html#What">What does internationalization support
101 mean ?</a></li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000102 <li><a href="encoding.html#internal">The internal encoding, how and
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000103 why</a></li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000104 <li><a href="encoding.html#implemente">How is it implemented ?</a></li>
105 <li><a href="encoding.html#Default">Default supported encodings</a></li>
106 <li><a href="encoding.html#extend">How to extend the existing
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000107 support</a></li>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000108</ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000109<h3><a name="What">What does internationalization support mean ?</a></h3>
Daniel Veillard238836e2003-04-07 22:57:29 +0000110<p>If you are not really familiar with Internationalization (usual shorcut is
111I18N) , Unicode, characters and glyphs, I suggest you read a <a href="http://www.tbray.org/ongoing/When/200x/2003/04/06/Unicode">presentation</a>
112by Tim Bray on Unicode and why you should care about it.</p>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000113<p>XML was designed from the start to allow the support of any character set
114by using Unicode. Any conformant XML parser has to support the UTF-8 and
115UTF-16 default encodings which can both express the full unicode ranges. UTF8
Daniel Veillard63d83142002-05-20 06:51:05 +0000116is a variable length encoding whose greatest points are to reuse the same
117encoding for ASCII and to save space for Western encodings, but it is a bit
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000118more complex to handle in practice. UTF-16 use 2 bytes per characters (and
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000119sometimes combines two pairs), it makes implementation easier, but looks a
120bit overkill for Western languages encoding. Moreover the XML specification
121allows document to be encoded in other encodings at the condition that they
Daniel Veillard63d83142002-05-20 06:51:05 +0000122are clearly labeled as such. For example the following is a wellformed XML
Daniel Veillard0d6b1702000-08-22 23:52:16 +0000123document encoded in ISO-8859 1 and using accentuated letter that we French
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000124likes for both markup and content:</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000125<pre>&lt;?xml version=&quot;1.0&quot; encoding=&quot;ISO-8859-1&quot;?&gt;
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000126&lt;très&gt;là&lt;/très&gt;</pre>
Daniel Veillard63d83142002-05-20 06:51:05 +0000127<p>Having internationalization support in libxml means the following:</p>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000128<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000129<li>the document is properly parsed</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000130 <li>informations about it's encoding are saved</li>
131 <li>it can be modified</li>
132 <li>it can be saved in its original encoding</li>
133 <li>it can also be saved in another encoding supported by libxml (for
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000134 example straight UTF8 or even an ASCII form)</li>
135</ul>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000136<p>Another very important point is that the whole libxml API, with the
137exception of a few routines to read with a specific encoding or save to a
138specific encoding, is completely agnostic about the original encoding of the
139document.</p>
Daniel Veillard63d83142002-05-20 06:51:05 +0000140<p>It should be noted too that the HTML parser embedded in libxml now obey
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000141the same rules too, the following document will be (as of 2.2.2) handled in
142an internationalized fashion by libxml too:</p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000143<pre>&lt;!DOCTYPE HTML PUBLIC &quot;-//W3C//DTD HTML 4.0 Transitional//EN&quot;
144 &quot;http://www.w3.org/TR/REC-html40/loose.dtd&quot;&gt;
145&lt;html lang=&quot;fr&quot;&gt;
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000146&lt;head&gt;
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000147 &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 +0000148&lt;/head&gt;
149&lt;body&gt;
150&lt;p&gt;W3C crée des standards pour le Web.&lt;/body&gt;
151&lt;/html&gt;</pre>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000152<h3><a name="internal">The internal encoding, how and why</a></h3>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000153<p>One of the core decision was to force all documents to be converted to a
154default internal encoding, and that encoding to be UTF-8, here are the
155rationale for those choices:</p>
156<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000157<li>keeping the native encoding in the internal form would force the libxml
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000158 users (or the code associated) to be fully aware of the encoding of the
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000159 original document, for examples when adding a text node to a document,
160 the content would have to be provided in the document encoding, i.e. the
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000161 client code would have to check it before hand, make sure it's conformant
162 to the encoding, etc ... Very hard in practice, though in some specific
163 cases this may make sense.</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000164 <li>the second decision was which encoding. From the XML spec only UTF8 and
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000165 UTF16 really makes sense as being the two only encodings for which there
Daniel Veillard63d83142002-05-20 06:51:05 +0000166 is mandatory support. UCS-4 (32 bits fixed size encoding) could be
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000167 considered an intelligent choice too since it's a direct Unicode mapping
168 support. I selected UTF-8 on the basis of efficiency and compatibility
169 with surrounding software:
170 <ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000171<li>UTF-8 while a bit more complex to convert from/to (i.e. slightly
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000172 more costly to import and export CPU wise) is also far more compact
173 than UTF-16 (and UCS-4) for a majority of the documents I see it used
174 for right now (RPM RDF catalogs, advogato data, various configuration
175 file formats, etc.) and the key point for today's computer
176 architecture is efficient uses of caches. If one nearly double the
177 memory requirement to store the same amount of data, this will trash
178 caches (main memory/external caches/internal caches) and my take is
179 that this harms the system far more than the CPU requirements needed
180 for the conversion to UTF-8</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000181 <li>Most of libxml version 1 users were using it with straight ASCII
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000182 most of the time, doing the conversion with an internal encoding
183 requiring all their code to be rewritten was a serious show-stopper
184 for using UTF-16 or UCS-4.</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000185 <li>UTF-8 is being used as the de-facto internal encoding standard for
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000186 related code like the <a href="http://www.pango.org/">pango</a>
187 upcoming Gnome text widget, and a lot of Unix code (yep another place
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000188 where Unix programmer base takes a different approach from Microsoft
189 - they are using UTF-16)</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000190 </ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000191</li>
192</ul>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000193<p>What does this mean in practice for the libxml user:</p>
194<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000195<li>xmlChar, the libxml data type is a byte, those bytes must be assembled
196 as UTF-8 valid strings. The proper way to terminate an xmlChar * string
197 is simply to append 0 byte, as usual.</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000198 <li>One just need to make sure that when using chars outside the ASCII set,
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000199 the values has been properly converted to UTF-8</li>
200</ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000201<h3><a name="implemente">How is it implemented ?</a></h3>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000202<p>Let's describe how all this works within libxml, basically the I18N
203(internationalization) support get triggered only during I/O operation, i.e.
204when reading a document or saving one. Let's look first at the reading
205sequence:</p>
206<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000207<li>when a document is processed, we usually don't know the encoding, a
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000208 simple heuristic allows to detect UTF-18 and UCS-4 from whose where the
209 ASCII range (0-0x7F) maps with ASCII</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000210 <li>the xml declaration if available is parsed, including the encoding
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000211 declaration. At that point, if the autodetected encoding is different
212 from the one declared a call to xmlSwitchEncoding() is issued.</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000213 <li>If there is no encoding declaration, then the input has to be in either
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000214 UTF-8 or UTF-16, if it is not then at some point when processing the
215 input, the converter/checker of UTF-8 form will raise an encoding error.
216 You may end-up with a garbled document, or no document at all ! Example:
217 <pre>~/XML -&gt; ./xmllint err.xml
218err.xml:1: error: Input is not proper UTF-8, indicate encoding !
219&lt;très&gt;là&lt;/très&gt;
220 ^
221err.xml:1: error: Bytes: 0xE8 0x73 0x3E 0x6C
222&lt;très&gt;là&lt;/très&gt;
223 ^</pre>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000224 </li>
225 <li>xmlSwitchEncoding() does an encoding name lookup, canonicalize it, and
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000226 then search the default registered encoding converters for that encoding.
227 If it's not within the default set and iconv() support has been compiled
228 it, it will ask iconv for such an encoder. If this fails then the parser
229 will report an error and stops processing:
230 <pre>~/XML -&gt; ./xmllint err2.xml
231err2.xml:1: error: Unsupported encoding UnsupportedEnc
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000232&lt;?xml version=&quot;1.0&quot; encoding=&quot;UnsupportedEnc&quot;?&gt;
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000233 ^</pre>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000234 </li>
235 <li>From that point the encoder processes progressively the input (it is
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000236 plugged as a front-end to the I/O module) for that entity. It captures
237 and convert on-the-fly the document to be parsed to UTF-8. The parser
238 itself just does UTF-8 checking of this input and process it
239 transparently. The only difference is that the encoding information has
240 been added to the parsing context (more precisely to the input
241 corresponding to this entity).</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000242 <li>The result (when using DOM) is an internal form completely in UTF-8
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000243 with just an encoding information on the document node.</li>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000244</ol>
Daniel Veillard63d83142002-05-20 06:51:05 +0000245<p>Ok then what happens when saving the document (assuming you
246collected/built an xmlDoc DOM like structure) ? It depends on the function
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000247called, xmlSaveFile() will just try to save in the original encoding, while
248xmlSaveFileTo() and xmlSaveFileEnc() can optionally save to a given
249encoding:</p>
250<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000251<li>if no encoding is given, libxml will look for an encoding value
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000252 associated to the document and if it exists will try to save to that
253 encoding,
254 <p>otherwise everything is written in the internal form, i.e. UTF-8</p>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000255 </li>
256 <li>so if an encoding was specified, either at the API level or on the
Daniel Veillard63d83142002-05-20 06:51:05 +0000257 document, libxml will again canonicalize the encoding name, lookup for a
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000258 converter in the registered set or through iconv. If not found the
259 function will return an error code</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000260 <li>the converter is placed before the I/O buffer layer, as another kind of
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000261 buffer, then libxml will simply push the UTF-8 serialization to through
262 that buffer, which will then progressively be converted and pushed onto
263 the I/O layer.</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000264 <li>It is possible that the converter code fails on some input, for example
Daniel Veillard63d83142002-05-20 06:51:05 +0000265 trying to push an UTF-8 encoded Chinese character through the UTF-8 to
Daniel Veillard0d6b1702000-08-22 23:52:16 +0000266 ISO-8859-1 converter won't work. Since the encoders are progressive they
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000267 will just report the error and the number of bytes converted, at that
268 point libxml will decode the offending character, remove it from the
269 buffer and replace it with the associated charRef encoding &amp;#123; and
Daniel Veillard63d83142002-05-20 06:51:05 +0000270 resume the conversion. This guarantees that any document will be saved
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000271 without losses (except for markup names where this is not legal, this is
Daniel Veillard63d83142002-05-20 06:51:05 +0000272 a problem in the current version, in practice avoid using non-ascii
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000273 characters for tags or attributes names @@). A special &quot;ascii&quot; encoding
Daniel Veillard0d6b1702000-08-22 23:52:16 +0000274 name is used to save documents to a pure ascii form can be used when
275 portability is really crucial</li>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000276</ol>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000277<p>Here is a few examples based on the same test document:</p>
278<pre>~/XML -&gt; ./xmllint isolat1
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000279&lt;?xml version=&quot;1.0&quot; encoding=&quot;ISO-8859-1&quot;?&gt;
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000280&lt;très&gt;là&lt;/très&gt;
281~/XML -&gt; ./xmllint --encode UTF-8 isolat1
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000282&lt;?xml version=&quot;1.0&quot; encoding=&quot;UTF-8&quot;?&gt;
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000283&lt;très&gt;là  &lt;/très&gt;
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000284~/XML -&gt; </pre>
Daniel Veillard0d6b1702000-08-22 23:52:16 +0000285<p>The same processing is applied (and reuse most of the code) for HTML I18N
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000286processing. Looking up and modifying the content encoding is a bit more
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000287difficult since it is located in a &lt;meta&gt; tag under the &lt;head&gt;,
288so a couple of functions htmlGetMetaEncoding() and htmlSetMetaEncoding() have
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000289been provided. The parser also attempts to switch encoding on the fly when
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000290detecting such a tag on input. Except for that the processing is the same
291(and again reuses the same code).</p>
292<h3><a name="Default">Default supported encodings</a></h3>
293<p>libxml has a set of default converters for the following encodings
294(located in encoding.c):</p>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000295<ol>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000296<li>UTF-8 is supported by default (null handlers)</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000297 <li>UTF-16, both little and big endian</li>
298 <li>ISO-Latin-1 (ISO-8859-1) covering most western languages</li>
299 <li>ASCII, useful mostly for saving</li>
300 <li>HTML, a specific handler for the conversion of UTF-8 to ASCII with HTML
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000301 predefined entities like &amp;copy; for the Copyright sign.</li>
302</ol>
Daniel Veillardc0801af2002-05-28 16:28:42 +0000303<p>More over when compiled on an Unix platform with iconv support the full
304set of encodings supported by iconv can be instantly be used by libxml. On a
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000305linux machine with glibc-2.1 the list of supported encodings and aliases fill
3063 full pages, and include UCS-4, the full set of ISO-Latin encodings, and the
307various Japanese ones.</p>
308<h4>Encoding aliases</h4>
309<p>From 2.2.3, libxml has support to register encoding names aliases. The
310goal is to be able to parse document whose encoding is supported but where
311the name differs (for example from the default set of names accepted by
312iconv). The following functions allow to register and handle new aliases for
313existing encodings. Once registered libxml will automatically lookup the
314aliases when handling a document:</p>
Daniel Veillard088f4282000-08-25 23:46:50 +0000315<ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000316<li>int xmlAddEncodingAlias(const char *name, const char *alias);</li>
Daniel Veillard0b28e882002-07-24 23:47:05 +0000317 <li>int xmlDelEncodingAlias(const char *alias);</li>
318 <li>const char * xmlGetEncodingAlias(const char *alias);</li>
319 <li>void xmlCleanupEncodingAliases(void);</li>
Daniel Veillard088f4282000-08-25 23:46:50 +0000320</ul>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000321<h3><a name="extend">How to extend the existing support</a></h3>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000322<p>Well adding support for new encoding, or overriding one of the encoders
323(assuming it is buggy) should not be hard, just write an input and output
324conversion routines to/from UTF-8, and register them using
325xmlNewCharEncodingHandler(name, xxxToUTF8, UTF8Toxxx), and they will be
326called automatically if the parser(s) encounter such an encoding name
327(register it uppercase, this will help). The description of the encoders,
328their arguments and expected return values are described in the encoding.h
329header.</p>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000330<p>A quick note on the topic of subverting the parser to use a different
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000331internal encoding than UTF-8, in some case people will absolutely want to
332keep the internal encoding different, I think it's still possible (but the
333encoding must be compliant with ASCII on the same subrange) though I didn't
334tried it. The key is to override the default conversion routines (by
335registering null encoders/decoders for your charsets), and bypass the UTF-8
336checking of the parser by setting the parser context charset
337(ctxt-&gt;charset) to something different than XML_CHAR_ENCODING_UTF8, but
Daniel Veillard63d83142002-05-20 06:51:05 +0000338there is no guarantee that this will work. You may also have some troubles
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000339saving back.</p>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000340<p>Basically proper I18N support is important, this requires at least
341libxml-2.0.0, but a lot of features and corrections are really available only
342starting 2.2.</p>
Daniel Veillard3f4c40f2002-02-13 09:19:28 +0000343<p><a href="bugs.html">Daniel Veillard</a></p>
Daniel Veillardb8cfbd12001-10-25 10:53:28 +0000344</td></tr></table></td></tr></table></td></tr></table></td>
345</tr></table></td></tr></table>
Daniel Veillardbe40c8b2000-07-14 12:10:59 +0000346</body>
347</html>