diff options
| author | Mike Hommey <glandium@debian.org> | 2006-06-08 10:59:26 +0200 |
|---|---|---|
| committer | Mike Hommey <glandium@debian.org> | 2006-06-08 10:59:26 +0200 |
| commit | a7e9d3f37d5e9fba4b9acaa43e7c12b6d9a669ae (patch) | |
| tree | f70d9afbc2bc584860755cb0f933a4405ee3c338 /doc/encoding.html | |
| parent | 07a67fa4bcc1b8bf2651ab41e5fc54a05059cf7e (diff) | |
| download | libxml2-a7e9d3f37d5e9fba4b9acaa43e7c12b6d9a669ae.tar.gz | |
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libxml2/branches/upstream/current.
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| -rw-r--r-- | doc/encoding.html | 312 |
1 files changed, 156 insertions, 156 deletions
diff --git a/doc/encoding.html b/doc/encoding.html index 1f4558d..8db787e 100644 --- a/doc/encoding.html +++ b/doc/encoding.html @@ -7,44 +7,44 @@ H1 {font-family: Verdana,Arial,Helvetica} H2 {font-family: Verdana,Arial,Helvetica} H3 {font-family: Verdana,Arial,Helvetica} A:link, A:visited, A:active { text-decoration: underline } -</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 & 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://www.blastwave.org/packages.php/libxml2">Solaris binaries</a></li><li><a href="http://www.explain.com.au/oss/libxml2xslt.html">MacOsX binaries</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 -is 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> -by 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 -without knowing what encoding it uses</b>, then as Joel Spolsky said <a href="http://www.joelonsoftware.com/articles/Unicode.html">please do not -write another line of code until you finish reading that article.</a>. It is -a prerequisite to understand this page, and avoid a lot of problems with -libxml2, XML or text processing in general.</p><p>Table of Content:</p><ol><li><a href="encoding.html#What">What does internationalization support - mean ?</a></li> - <li><a href="encoding.html#internal">The internal encoding, how and - why</a></li> +</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 & 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://www.blastwave.org/packages.php/libxml2">Solaris binaries</a></li><li><a href="http://www.explain.com.au/oss/libxml2xslt.html">MacOsX binaries</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 +shortcutisI18N) , Unicode, characters and glyphs, I suggest you read a <a href="http://www.tbray.org/ongoing/When/200x/2003/04/06/Unicode">presentation</a>byTim +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 +astringwithout knowing what encoding it uses</b>, then as Joel Spolsky said +<a href="http://www.joelonsoftware.com/articles/Unicode.html">please do +notwriteanother line of code until you finish reading that article.</a>. It +isaprerequisite to understand this page, and avoid a lot of +problemswithlibxml2, XML or text processing in general.</p><p>Table of Content:</p><ol><li><a href="encoding.html#What">What does internationalization + supportmean?</a></li> + <li><a href="encoding.html#internal">The internal encoding, + howandwhy</a></li> <li><a href="encoding.html#implemente">How is it implemented ?</a></li> <li><a href="encoding.html#Default">Default supported encodings</a></li> - <li><a href="encoding.html#extend">How to extend the existing - support</a></li> -</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 -by using Unicode. Any conformant XML parser has to support the UTF-8 and -UTF-16 default encodings which can both express the full unicode ranges. UTF8 -is a variable length encoding whose greatest points are to reuse the same -encoding for ASCII and to save space for Western encodings, but it is a bit -more complex to handle in practice. UTF-16 use 2 bytes per character (and -sometimes combines two pairs), it makes implementation easier, but looks a -bit overkill for Western languages encoding. Moreover the XML specification -allows the document to be encoded in other encodings at the condition that -they are clearly labeled as such. For example the following is a wellformed -XML document encoded in ISO-8859-1 and using accentuated letters that we -French like for both markup and content:</p><pre><?xml version="1.0" encoding="ISO-8859-1"?> + <li><a href="encoding.html#extend">How to extend theexistingsupport</a></li> +</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 charactersetby +using Unicode. Any conformant XML parser has to support the UTF-8andUTF-16 +default encodings which can both express the full unicode ranges.UTF8is a +variable length encoding whose greatest points are to reuse thesameencoding +for ASCII and to save space for Western encodings, but it is abitmore complex +to handle in practice. UTF-16 use 2 bytes per character(andsometimes combines +two pairs), it makes implementation easier, but looksabit overkill for +Western languages encoding. Moreover the XMLspecificationallows the document +to be encoded in other encodings at thecondition thatthey are clearly labeled +as such. For example the following isa wellformedXML document encoded in +ISO-8859-1 and using accentuated lettersthat weFrench like for both markup +and content:</p><pre><?xml version="1.0" encoding="ISO-8859-1"?> <très>là</très></pre><p>Having internationalization support in libxml2 means the following:</p><ul><li>the document is properly parsed</li> <li>informations about it's encoding are saved</li> <li>it can be modified</li> <li>it can be saved in its original encoding</li> - <li>it can also be saved in another encoding supported by libxml2 (for - example straight UTF8 or even an ASCII form)</li> -</ul><p>Another very important point is that the whole libxml2 API, with the -exception of a few routines to read with a specific encoding or save to a -specific encoding, is completely agnostic about the original encoding of the -document.</p><p>It should be noted too that the HTML parser embedded in libxml2 now obey -the same rules too, the following document will be (as of 2.2.2) handled in -an internationalized fashion by libxml2 too:</p><pre><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" + <li>it can also be saved in another encoding supported by + libxml2(forexample straight UTF8 or even an ASCII form)</li> +</ul><p>Another very important point is that the whole libxml2 API, +withtheexception of a few routines to read with a specific encoding or save +toaspecific encoding, is completely agnostic about the original encoding +ofthedocument.</p><p>It should be noted too that the HTML parser embedded in libxml2 nowobeythe +same rules too, the following document will be (as of 2.2.2) handledinan +internationalized fashion by libxml2 too:</p><pre><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/REC-html40/loose.dtd"> <html lang="fr"> <head> @@ -52,59 +52,60 @@ an internationalized fashion by libxml2 too:</p><pre><!DOCTYPE HTML PUBLIC "- </head> <body> <p>W3C crée des standards pour le Web.</body> -</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 -default internal encoding, and that encoding to be UTF-8, here are the -rationales for those choices:</p><ul><li>keeping the native encoding in the internal form would force the libxml - users (or the code associated) to be fully aware of the encoding of the - original document, for examples when adding a text node to a document, - the content would have to be provided in the document encoding, i.e. the - client code would have to check it before hand, make sure it's conformant - to the encoding, etc ... Very hard in practice, though in some specific - cases this may make sense.</li> - <li>the second decision was which encoding. From the XML spec only UTF8 and - UTF16 really makes sense as being the two only encodings for which there - is mandatory support. UCS-4 (32 bits fixed size encoding) could be - considered an intelligent choice too since it's a direct Unicode mapping - support. I selected UTF-8 on the basis of efficiency and compatibility - with surrounding software: - <ul><li>UTF-8 while a bit more complex to convert from/to (i.e. slightly - more costly to import and export CPU wise) is also far more compact - than UTF-16 (and UCS-4) for a majority of the documents I see it used - for right now (RPM RDF catalogs, advogato data, various configuration - file formats, etc.) and the key point for today's computer - architecture is efficient uses of caches. If one nearly double the - memory requirement to store the same amount of data, this will trash - caches (main memory/external caches/internal caches) and my take is - that this harms the system far more than the CPU requirements needed - for the conversion to UTF-8</li> - <li>Most of libxml2 version 1 users were using it with straight ASCII - most of the time, doing the conversion with an internal encoding - requiring all their code to be rewritten was a serious show-stopper - for using UTF-16 or UCS-4.</li> - <li>UTF-8 is being used as the de-facto internal encoding standard for - related code like the <a href="http://www.pango.org/">pango</a> - upcoming Gnome text widget, and a lot of Unix code (yet another place - where Unix programmer base takes a different approach from Microsoft - - they are using UTF-16)</li> +</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 +toadefault internal encoding, and that encoding to be UTF-8, here +aretherationales for those choices:</p><ul><li>keeping the native encoding in the internal form would force + thelibxmlusers (or the code associated) to be fully aware of the encoding + oftheoriginal document, for examples when adding a text node to + adocument,the content would have to be provided in the document + encoding,i.e. theclient code would have to check it before hand, make + sure it'sconformantto the encoding, etc ... Very hard in practice, though + in somespecificcases this may make sense.</li> + <li>the second decision was which encoding. From the XML spec only + UTF8andUTF16 really makes sense as being the two only encodings for + whichthereis mandatory support. UCS-4 (32 bits fixed size encoding) + couldbeconsidered an intelligent choice too since it's a direct + Unicodemappingsupport. I selected UTF-8 on the basis of efficiency + andcompatibilitywith surrounding software: + <ul><li>UTF-8 while a bit more complex to convert from/to (i.e.slightlymore + costly to import and export CPU wise) is also far morecompactthan + UTF-16 (and UCS-4) for a majority of the documents I seeit usedfor + right now (RPM RDF catalogs, advogato data, variousconfigurationfile + formats, etc.) and the key point for today'scomputerarchitecture is + efficient uses of caches. If one nearlydouble thememory requirement + to store the same amount of data, thiswill trashcaches (main + memory/external caches/internal caches) and mytake isthat this harms + the system far more than the CPU requirementsneededfor the conversion + to UTF-8</li> + <li>Most of libxml2 version 1 users were using it with + straightASCIImost of the time, doing the conversion with an + internalencodingrequiring all their code to be rewritten was a + seriousshow-stopperfor using UTF-16 or UCS-4.</li> + <li>UTF-8 is being used as the de-facto internal encoding + standardforrelated code like the <a href="http://www.pango.org/">pango</a>upcoming Gnome text widget, + anda lot of Unix code (yet another placewhere Unix programmer base + takesa different approach from Microsoft- they are using UTF-16)</li> </ul></li> -</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 - as UTF-8 valid strings. The proper way to terminate an xmlChar * string - is simply to append 0 byte, as usual.</li> - <li>One just need to make sure that when using chars outside the ASCII set, - the values has been properly converted to UTF-8</li> -</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 -(internationalization) support get triggered only during I/O operation, i.e. -when reading a document or saving one. Let's look first at the reading -sequence:</p><ol><li>when a document is processed, we usually don't know the encoding, a - simple heuristic allows to detect UTF-16 and UCS-4 from encodings where - the ASCII range (0-0x7F) maps with ASCII</li> - <li>the xml declaration if available is parsed, including the encoding - declaration. At that point, if the autodetected encoding is different - from the one declared a call to xmlSwitchEncoding() is issued.</li> - <li>If there is no encoding declaration, then the input has to be in either - UTF-8 or UTF-16, if it is not then at some point when processing the - input, the converter/checker of UTF-8 form will raise an encoding error. - You may end-up with a garbled document, or no document at all ! Example: +</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 + beassembledas UTF-8 valid strings. The proper way to terminate an xmlChar + *stringis simply to append 0 byte, as usual.</li> + <li>One just need to make sure that when using chars outside the + ASCIIset,the values has been properly converted to UTF-8</li> +</ul><h3><a name="implemente" id="implemente">How is it implemented ?</a></h3><p>Let's describe how all this works within libxml, basically +theI18N(internationalization) support get triggered only during I/O +operation,i.e.when reading a document or saving one. Let's look first at +thereadingsequence:</p><ol><li>when a document is processed, we usually don't know the + encoding,asimple heuristic allows to detect UTF-16 and UCS-4 from + encodingswherethe ASCII range (0-0x7F) maps with ASCII</li> + <li>the xml declaration if available is parsed, including + theencodingdeclaration. At that point, if the autodetected encoding + isdifferentfrom the one declared a call to xmlSwitchEncoding() + isissued.</li> + <li>If there is no encoding declaration, then the input has to be + ineitherUTF-8 or UTF-16, if it is not then at some point when + processingtheinput, the converter/checker of UTF-8 form will raise an + encodingerror.You may end-up with a garbled document, or no document at + all !Example: <pre>~/XML -> ./xmllint err.xml err.xml:1: error: Input is not proper UTF-8, indicate encoding ! <très>là</très> @@ -113,94 +114,93 @@ err.xml:1: error: Bytes: 0xE8 0x73 0x3E 0x6C <très>là</très> ^</pre> </li> - <li>xmlSwitchEncoding() does an encoding name lookup, canonicalize it, and - then search the default registered encoding converters for that encoding. - If it's not within the default set and iconv() support has been compiled - it, it will ask iconv for such an encoder. If this fails then the parser - will report an error and stops processing: + <li>xmlSwitchEncoding() does an encoding name lookup, canonicalize + it,andthen search the default registered encoding converters for + thatencoding.If it's not within the default set and iconv() support has + beencompiledit, it will ask iconv for such an encoder. If this fails then + theparserwill report an error and stops processing: <pre>~/XML -> ./xmllint err2.xml err2.xml:1: error: Unsupported encoding UnsupportedEnc <?xml version="1.0" encoding="UnsupportedEnc"?> ^</pre> </li> - <li>From that point the encoder processes progressively the input (it is - plugged as a front-end to the I/O module) for that entity. It captures - and converts on-the-fly the document to be parsed to UTF-8. The parser - itself just does UTF-8 checking of this input and process it - transparently. The only difference is that the encoding information has - been added to the parsing context (more precisely to the input - corresponding to this entity).</li> - <li>The result (when using DOM) is an internal form completely in UTF-8 - with just an encoding information on the document node.</li> -</ol><p>Ok then what happens when saving the document (assuming you -collected/built an xmlDoc DOM like structure) ? It depends on the function -called, xmlSaveFile() will just try to save in the original encoding, while -xmlSaveFileTo() and xmlSaveFileEnc() can optionally save to a given -encoding:</p><ol><li>if no encoding is given, libxml2 will look for an encoding value - associated to the document and if it exists will try to save to that - encoding, + <li>From that point the encoder processes progressively the input + (itisplugged as a front-end to the I/O module) for that entity. + Itcapturesand converts on-the-fly the document to be parsed to UTF-8. + Theparseritself just does UTF-8 checking of this input and + processittransparently. The only difference is that the encoding + informationhasbeen added to the parsing context (more precisely to + theinputcorresponding to this entity).</li> + <li>The result (when using DOM) is an internal form completely in + UTF-8withjust an encoding information on the document node.</li> +</ol><p>Ok then what happens when saving the document (assuming +youcollected/builtan xmlDoc DOM like structure) ? It depends on the +functioncalled,xmlSaveFile() will just try to save in the original +encoding,whilexmlSaveFileTo() and xmlSaveFileEnc() can optionally save to +agivenencoding:</p><ol><li>if no encoding is given, libxml2 will look for an + encodingvalueassociated to the document and if it exists will try to save + tothatencoding, <p>otherwise everything is written in the internal form, i.e. UTF-8</p> </li> - <li>so if an encoding was specified, either at the API level or on the - document, libxml2 will again canonicalize the encoding name, lookup for a - converter in the registered set or through iconv. If not found the - function will return an error code</li> - <li>the converter is placed before the I/O buffer layer, as another kind of - buffer, then libxml2 will simply push the UTF-8 serialization to through - that buffer, which will then progressively be converted and pushed onto - the I/O layer.</li> - <li>It is possible that the converter code fails on some input, for example - trying to push an UTF-8 encoded Chinese character through the UTF-8 to - ISO-8859-1 converter won't work. Since the encoders are progressive they - will just report the error and the number of bytes converted, at that - point libxml2 will decode the offending character, remove it from the - buffer and replace it with the associated charRef encoding &#123; and - resume the conversion. This guarantees that any document will be saved - without losses (except for markup names where this is not legal, this is - a problem in the current version, in practice avoid using non-ascii - characters for tag or attribute names). A special "ascii" encoding name - is used to save documents to a pure ascii form can be used when - portability is really crucial</li> + <li>so if an encoding was specified, either at the API level or + onthedocument, libxml2 will again canonicalize the encoding name, + lookupfor aconverter in the registered set or through iconv. If not + foundthefunction will return an error code</li> + <li>the converter is placed before the I/O buffer layer, as another + kindofbuffer, then libxml2 will simply push the UTF-8 serialization + tothroughthat buffer, which will then progressively be converted and + pushedontothe I/O layer.</li> + <li>It is possible that the converter code fails on some input, + forexampletrying to push an UTF-8 encoded Chinese character through + theUTF-8 toISO-8859-1 converter won't work. Since the encoders + areprogressive theywill just report the error and the number of + bytesconverted, at thatpoint libxml2 will decode the offending + character,remove it from thebuffer and replace it with the associated + charRefencoding &#123; andresume the conversion. This guarantees that + anydocument will be savedwithout losses (except for markup names where + thisis not legal, this isa problem in the current version, in practice + avoidusing non-asciicharacters for tag or attribute names). A special + "ascii"encoding nameis used to save documents to a pure ascii form can be + usedwhenportability is really crucial</li> </ol><p>Here are a few examples based on the same test document:</p><pre>~/XML -> ./xmllint isolat1 <?xml version="1.0" encoding="ISO-8859-1"?> <très>là</très> ~/XML -> ./xmllint --encode UTF-8 isolat1 <?xml version="1.0" encoding="UTF-8"?> <très>là </très> -~/XML -> </pre><p>The same processing is applied (and reuse most of the code) for HTML I18N -processing. Looking up and modifying the content encoding is a bit more -difficult since it is located in a <meta> tag under the <head>, -so a couple of functions htmlGetMetaEncoding() and htmlSetMetaEncoding() have -been provided. The parser also attempts to switch encoding on the fly when -detecting such a tag on input. Except for that the processing is the same -(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 -(located in encoding.c):</p><ol><li>UTF-8 is supported by default (null handlers)</li> +~/XML -> </pre><p>The same processing is applied (and reuse most of the code) for +HTMLI18Nprocessing. Looking up and modifying the content encoding is a +bitmoredifficult since it is located in a <meta> tag under +the<head>,so a couple of functions htmlGetMetaEncoding() +andhtmlSetMetaEncoding() havebeen provided. The parser also attempts to +switchencoding on the fly whendetecting such a tag on input. Except for that +theprocessing is the same(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 followingencodings(located +in encoding.c):</p><ol><li>UTF-8 is supported by default (null handlers)</li> <li>UTF-16, both little and big endian</li> <li>ISO-Latin-1 (ISO-8859-1) covering most western languages</li> <li>ASCII, useful mostly for saving</li> - <li>HTML, a specific handler for the conversion of UTF-8 to ASCII with HTML - predefined entities like &copy; for the Copyright sign.</li> -</ol><p>More over when compiled on an Unix platform with iconv support the full -set of encodings supported by iconv can be instantly be used by libxml. On a -linux machine with glibc-2.1 the list of supported encodings and aliases fill -3 full pages, and include UCS-4, the full set of ISO-Latin encodings, and the -various Japanese ones.</p><p>To convert from the UTF-8 values returned from the API to another encoding -then 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 -POSIX <a href="http://www.opengroup.org/onlinepubs/009695399/functions/iconv.html">iconv()</a> -API directly.</p><h4>Encoding aliases</h4><p>From 2.2.3, libxml2 has support to register encoding names aliases. The -goal is to be able to parse document whose encoding is supported but where -the name differs (for example from the default set of names accepted by -iconv). The following functions allow to register and handle new aliases for -existing encodings. Once registered libxml2 will automatically lookup the -aliases when handling a document:</p><ul><li>int xmlAddEncodingAlias(const char *name, const char *alias);</li> + <li>HTML, a specific handler for the conversion of UTF-8 to ASCII + withHTMLpredefined entities like &copy; for the Copyright sign.</li> +</ol><p>More over when compiled on an Unix platform with iconv support the +fullsetof encodings supported by iconv can be instantly be used by libxml. On +alinuxmachine with glibc-2.1 the list of supported encodings and aliases +fill3 fullpages, and include UCS-4, the full set of ISO-Latin encodings, and +thevariousJapanese ones.</p><p>To convert from the UTF-8 values returned from the API to +anotherencodingthen 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 +usethePOSIX <a href="http://www.opengroup.org/onlinepubs/009695399/functions/iconv.html">iconv()</a>APIdirectly.</p><h4>Encoding aliases</h4><p>From 2.2.3, libxml2 has support to register encoding names aliases.Thegoal +is to be able to parse document whose encoding is supported butwherethe name +differs (for example from the default set of names acceptedbyiconv). The +following functions allow to register and handle new aliasesforexisting +encodings. Once registered libxml2 will automatically lookupthealiases when +handling a document:</p><ul><li>int xmlAddEncodingAlias(const char *name, const char *alias);</li> <li>int xmlDelEncodingAlias(const char *alias);</li> <li>const char * xmlGetEncodingAlias(const char *alias);</li> <li>void xmlCleanupEncodingAliases(void);</li> -</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 -(assuming it is buggy) should not be hard, just write input and output -conversion routines to/from UTF-8, and register them using -xmlNewCharEncodingHandler(name, xxxToUTF8, UTF8Toxxx), and they will be -called automatically if the parser(s) encounter such an encoding name -(register it uppercase, this will help). The description of the encoders, -their arguments and expected return values are described in the encoding.h -header.</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> +</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 +theencoders(assuming it is buggy) should not be hard, just write input +andoutputconversion routines to/from UTF-8, and register +themusingxmlNewCharEncodingHandler(name, xxxToUTF8, UTF8Toxxx), and they +willbecalled automatically if the parser(s) encounter such an +encodingname(register it uppercase, this will help). The description of +theencoders,their arguments and expected return values are described in +theencoding.hheader.</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> |