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753 lines
29 KiB
HTML
753 lines
29 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
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"http://www.w3.org/TR/REC-html40/loose.dtd">
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<html>
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<head>
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<title>The XML library for Gnome</title>
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<meta name="GENERATOR" content="amaya V2.1">
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</head>
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<body bgcolor="#ffffff">
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<h1 align="center">The XML library for Gnome</h1>
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<h2 style="text-align: center">libxml, a.k.a. gnome-xml</h2>
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<p></p>
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<p>This document describes the <a href="http://www.w3.org/XML/">XML</a>
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library provideed in the <a href="http://www.gnome.org/">Gnome</a> framework.
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XML is a standard to build tag based structured documents/data.</p>
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<p>The internal document repesentation is as close as possible to the <a
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href="http://www.w3.org/DOM/">DOM</a> interfaces.</p>
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<p>Libxml also has a <a href="http://www.megginson.com/SAX/index.html">SAX
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interface</a>, <a href="mailto:james@daa.com.au">James Henstridge</a> made <a
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href="http://www.daa.com.au/~james/gnome/xml-sax/xml-sax.html">a nice
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documentation</a> expaining how to use it. The interface is as compatible as
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possible with <a href="http://www.jclark.com/xml/expat.html">Expat</a>
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one.</p>
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<p>There is also a mailing-list <a
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href="mailto:xml@rufus.w3.org">xml@rufus.w3.org</a> for libxml, with an <a
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href="http://rpmfind.net/veillard/XML/messages">on-line archive</a>. To
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subscribe to this majordomo based list, send a mail to <a
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href="mailto:majordomo@rufus.w3.org">majordomo@rufus.w3.org</a> with
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"subscribe xml" in the <strong>content</strong> of the message.</p>
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<p>This library is released both under the W3C Copyright and the GNU LGP,
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basically everybody should be happy, if not, drop me a mail.</p>
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<p>People are invited to use the <a
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href="http://cvs.gnome.org/lxr/source/gdome/">gdome Gnome module to</a> get a
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full DOM interface, thanks to <a href="mailto:raph@levien.com">Raph
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Levien</a>, check his <a
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href="http://www.levien.com/gnome/domination.html">DOMination paper</a>. He
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uses it for his implementation of <a
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href="http://www.w3.org/Graphics/SVG/">SVG</a> called <a
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href="http://www.levien.com/svg/">gill</a>.</p>
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<h2>Extensive documentation</h2>
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<p>The code is commented in a <a href=""></a>way which allow <a
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href="http://rpmfind.net/veillard/XML/libxml.html">extensive documentation</a>
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to be automatically extracted.</p>
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<p>At some point I will change the back-end to produce XML documentation in
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addition to SGML Docbook and HTML.</p>
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<h2>XML</h2>
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<p><a href="http://www.w3.org/TR/REC-xml">XML is a standard</a> for markup
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based structured documents, here is <a name="example">an example</a>:</p>
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<pre><?xml version="1.0"?>
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<EXAMPLE prop1="gnome is great" prop2="&amp; linux too">
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<head>
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<title>Welcome to Gnome</title>
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</head>
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<chapter>
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<title>The Linux adventure</title>
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<p>bla bla bla ...</p>
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<image href="linus.gif"/>
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<p>...</p>
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</chapter>
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</EXAMPLE></pre>
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<p>The first line specify that it's an XML document and gives useful
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informations about it's encoding. Then the document is a text format whose
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structure is specified by tags between brackets. <strong>Each tag opened have
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to be closed</strong> XML is pedantic about this, not that for example the
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image tag has no content (just an attribute) and is closed by ending up the
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tag with <code>/></code>.</p>
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<p>XML can be applied sucessfully to a wide range or usage from long term
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structured document maintenance where it follows the steps of SGML to simple
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data encoding mechanism like configuration file format (glade), spreadsheets
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(gnumeric), or even shorter lived document like in WebDAV where it is used to
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encode remote call between a client and a server.</p>
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<h2>The tree output</h2>
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<p>The parser returns a tree built during the document analysis. The value
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returned is an <strong>xmlDocPtr</strong> (i.e. a pointer to an
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<strong>xmlDoc</strong> structure). This structure contains informations like
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the file name, the document type, and a <strong>root</strong> pointer which
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is the root of the document (or more exactly the first child under the root
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which is the document). The tree is made of <strong>xmlNode</strong>s, chained
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in double linked lists of siblings and with childs<->parent relationship.
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An xmlNode can also carry properties (a chain of xmlAttr structures). An
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attribute may have a value which is a list of TEXT or ENTITY_REF nodes.</p>
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<p>Here is an example (erroneous w.r.t. the XML spec since there should be
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only one ELEMENT under the root):</p>
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<p><img src="structure.gif" alt=" structure.gif "></p>
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<p>In the source package there is a small program (not installed by default)
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called <strong>tester</strong> which parses XML files given as argument and
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prints them back as parsed, this is useful to detect errors both in XML code
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and in the XML parser itself. It has an option <strong>--debug</strong> which
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prints the actual in-memory structure of the document, here is the result with
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the <a href="#example">example</a> given before:</p>
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<pre>DOCUMENT
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version=1.0
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standalone=true
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ELEMENT EXAMPLE
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ATTRIBUTE prop1
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TEXT
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content=gnome is great
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ATTRIBUTE prop2
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ENTITY_REF
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TEXT
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content= too
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ELEMENT head
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ELEMENT title
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TEXT
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content=Welcome to Gnome
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ELEMENT chapter
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ELEMENT title
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TEXT
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content=The Linux adventure
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ELEMENT p
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TEXT
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content=bla bla bla ...
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ELEMENT image
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ATTRIBUTE href
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TEXT
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content=linus.gif
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ELEMENT p
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TEXT
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content=...</pre>
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<p>This should be useful to learn the internal representation model.</p>
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<h2>The SAX interface</h2>
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<p>Sometimes the DOM tree output is just to large to fit reasonably into
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memory. In that case and if you don't expect to save back the XML document
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loaded using libxml, it's better to use the SAX interface of libxml. SAX is a
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<strong>callback based interface</strong> to the parser. Before parsing, the
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application layer register a customized set of callbacks which will be called
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by the library as it progresses through the XML input.</p>
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<p>To get a more detailed step-by-step guidance on using the SAX interface of
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libxml, <a href="mailto:james@daa.com.au">James Henstridge</a> made <a
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href="http://www.daa.com.au/~james/gnome/xml-sax/xml-sax.html">a nice
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documentation.</a></p>
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<p>You can debug the SAX behaviour by using the <strong>testSAX</strong>
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program located in the gnome-xml module (it's usually not shipped in the
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binary packages of libxml, but you can also find it in the tar source
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distribution). Here is the sequence of callback that would be generated when
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parsing the example given before as reported by testSAX:</p>
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<pre>SAX.setDocumentLocator()
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SAX.startDocument()
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SAX.getEntity(amp)
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SAX.startElement(EXAMPLE, prop1='gnome is great', prop2='&amp; linux too')
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SAX.characters( , 3)
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SAX.startElement(head)
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SAX.characters( , 4)
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SAX.startElement(title)
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SAX.characters(Welcome to Gnome, 16)
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SAX.endElement(title)
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SAX.characters( , 3)
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SAX.endElement(head)
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SAX.characters( , 3)
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SAX.startElement(chapter)
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SAX.characters( , 4)
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SAX.startElement(title)
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SAX.characters(The Linux adventure, 19)
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SAX.endElement(title)
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SAX.characters( , 4)
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SAX.startElement(p)
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SAX.characters(bla bla bla ..., 15)
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SAX.endElement(p)
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SAX.characters( , 4)
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SAX.startElement(image, href='linus.gif')
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SAX.endElement(image)
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SAX.characters( , 4)
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SAX.startElement(p)
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SAX.characters(..., 3)
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SAX.endElement(p)
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SAX.characters( , 3)
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SAX.endElement(chapter)
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SAX.characters( , 1)
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SAX.endElement(EXAMPLE)
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SAX.endDocument()</pre>
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<p>Most of the other functionnalities of libxml are based on the DOM tree
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building facility, so nearly everything up to the end of this document
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presuppose the use of the standard DOM tree build. Note that the DOM tree
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itself is built by a set of registered default callbacks, without internal
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specific interface.</p>
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<h2>The XML library interfaces</h2>
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<p>This section is directly intended to help programmers getting bootstrapped
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using the XML library from the C language. It doesn't intent to be extensive,
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I hope the automatically generated docs will provide the completeness
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required, but as a separated set of documents. The interfaces of the XML
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library are by principle low level, there is nearly zero abstration. Those
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interested in a higher level API should <a href="#DOM">look at DOM</a>.</p>
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<h3>Invoking the parser</h3>
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<p>Usually, the first thing to do is to read an XML input, the parser accepts
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to parse both memory mapped documents or direct files. The functions are
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defined in "parser.h":</p>
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<dl>
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<dt><code>xmlDocPtr xmlParseMemory(char *buffer, int size);</code></dt>
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<dd><p>parse a zero terminated string containing the document</p>
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</dd>
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</dl>
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<dl>
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<dt><code>xmlDocPtr xmlParseFile(const char *filename);</code></dt>
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<dd><p>parse an XML document contained in a file (possibly compressed)</p>
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</dd>
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</dl>
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<p>This returns a pointer to the document structure (or NULL in case of
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failure).</p>
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<p>A couple of comments can be made, first this mean that the parser is
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memory-hungry, first to load the document in memory, second to build the tree.
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Reading a document without building the tree will be possible in the future by
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pluggin the code to the SAX interface (see SAX.c).</p>
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<h3>Building a tree from scratch</h3>
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<p>The other way to get an XML tree in memory is by building it. Basically
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there is a set of functions dedicated to building new elements, those are also
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described in "tree.h", here is for example the piece of code producing the
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example used before:</p>
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<pre> xmlDocPtr doc;
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xmlNodePtr tree, subtree;
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doc = xmlNewDoc("1.0");
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doc->root = xmlNewDocNode(doc, NULL, "EXAMPLE", NULL);
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xmlSetProp(doc->root, "prop1", "gnome is great");
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xmlSetProp(doc->root, "prop2", "&linux; too");
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tree = xmlNewChild(doc->root, NULL, "head", NULL);
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subtree = xmlNewChild(tree, NULL, "title", "Welcome to Gnome");
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tree = xmlNewChild(doc->root, NULL, "chapter", NULL);
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subtree = xmlNewChild(tree, NULL, "title", "The Linux adventure");
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subtree = xmlNewChild(tree, NULL, "p", "bla bla bla ...");
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subtree = xmlNewChild(tree, NULL, "image", NULL);
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xmlSetProp(subtree, "href", "linus.gif");</pre>
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<p>Not really rocket science ...</p>
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<h3>Traversing the tree</h3>
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<p>Basically by including "tree.h" your code has access to the internal
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structure of all the element of the tree. The names should be somewhat simple
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like <strong>parent</strong>, <strong>childs</strong>, <strong>next</strong>,
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<strong>prev</strong>, <strong>properties</strong>, etc... For example still
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with the previous example:</p>
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<pre><code>doc->root->childs->childs</code></pre>
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<p>points to the title element,</p>
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<pre>doc->root->childs->next->child->child</pre>
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<p>points to the text node containing the chapter titlle "The Linux adventure"
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and</p>
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<pre>doc->root->properties->next->val</pre>
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<p>points to the entity reference containing the value of "&linux" at the
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beginning of the second attribute of the root element "EXAMPLE".</p>
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<h3>Modifying the tree</h3>
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<p>functions are provided to read and write the document content:</p>
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<dl>
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<dt><code>xmlAttrPtr xmlSetProp(xmlNodePtr node, const CHAR *name, const
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CHAR *value);</code></dt>
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<dd><p>This set (or change) an attribute carried by an ELEMENT node the
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value can be NULL</p>
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</dd>
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</dl>
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<dl>
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<dt><code>const CHAR *xmlGetProp(xmlNodePtr node, const CHAR
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*name);</code></dt>
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<dd><p>This function returns a pointer to the property content, note that
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no extra copy is made</p>
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</dd>
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</dl>
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<p>Two functions must be used to read an write the text associated to
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elements:</p>
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<dl>
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<dt><code>xmlNodePtr xmlStringGetNodeList(xmlDocPtr doc, const CHAR
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*value);</code></dt>
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<dd><p>This function takes an "external" string and convert it to one text
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node or possibly to a list of entity and text nodes. All non-predefined
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entity references like &Gnome; will be stored internally as an
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entity node, hence the result of the function may not be a single
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node.</p>
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</dd>
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</dl>
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<dl>
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<dt><code>CHAR *xmlNodeListGetString(xmlDocPtr doc, xmlNodePtr list, int
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inLine);</code></dt>
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<dd><p>this is the dual function, which generate a new string containing
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the content of the text and entity nodes. Note the extra argument
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inLine, if set to 1 instead of returning the &Gnome; XML encoding in
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the string it will substitute it with it's value say "GNU Network Object
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Model Environment". Set it if you want to use the string for non XML
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usage like User Interface.</p>
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</dd>
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</dl>
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<h3>Saving a tree</h3>
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<p>Basically 3 options are possible:</p>
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<dl>
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<dt><code>void xmlDocDumpMemory(xmlDocPtr cur, CHAR**mem, int
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*size);</code></dt>
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<dd><p>returns a buffer where the document has been saved</p>
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</dd>
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</dl>
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<dl>
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<dt><code>extern void xmlDocDump(FILE *f, xmlDocPtr doc);</code></dt>
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<dd><p>dumps a buffer to an open file descriptor</p>
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</dd>
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</dl>
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<dl>
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<dt><code>int xmlSaveFile(const char *filename, xmlDocPtr cur);</code></dt>
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<dd><p>save the document ot a file. In that case the compression interface
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is triggered if turned on</p>
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</dd>
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</dl>
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<h3>Compression</h3>
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<p>The library handle transparently compression when doing file based
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accesses, the level of compression on saves can be tuned either globally or
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individually for one file:</p>
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<dl>
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<dt><code>int xmlGetDocCompressMode (xmlDocPtr doc);</code></dt>
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<dd><p>Get the document compression ratio (0-9)</p>
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</dd>
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</dl>
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<dl>
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<dt><code>void xmlSetDocCompressMode (xmlDocPtr doc, int mode);</code></dt>
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<dd><p>Set the document compression ratio</p>
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</dd>
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</dl>
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<dl>
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<dt><code>int xmlGetCompressMode(void);</code></dt>
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<dd><p>Get the default compression ratio</p>
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</dd>
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</dl>
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<dl>
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<dt><code>void xmlSetCompressMode(int mode);</code></dt>
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<dd><p>set the default compression ratio</p>
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</dd>
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</dl>
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<h2>Entities or no entities</h2>
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<p>Entities principle is similar to simple C macros. They define an
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abbreviation for a given string that you can reuse many time through the
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content of your document. They are especially useful when frequent occurrences
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of a given string may occur within a document or to confine the change needed
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to a document to a restricted area in the internal subset of the document (at
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the beginning). Example:</p>
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<pre>1 <?xml version="1.0"?>
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2 <!DOCTYPE EXAMPLE SYSTEM "example.dtd" [
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3 <!ENTITY xml "Extensible Markup Language">
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4 ]>
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5 <EXAMPLE>
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6 &xml;
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7 </EXAMPLE>
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</pre>
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<p>Line 3 declares the xml entity. Line 6 uses the xml entity, by prefixing
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it's name with '&' and following it by ';' without any spaces added.
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There are 5 predefined entities in libxml allowing to escape charaters with
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predefined meaning in some parts of the xml document content:
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<strong>&lt;</strong> for the letter '<', <strong>&gt;</strong> for
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the letter '>', <strong>&apos;</strong> for the letter ''',
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<strong>&quot;</strong> for the letter '"', and
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<strong>&amp;</strong> for the letter '&'.</p>
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<p>One of the problems related to entities is that you may want the parser to
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substitute entities content to see the replacement text in your application,
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or you may prefer keeping entities references as such in the content to be
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able to save the document back without loosing this usually precious
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information (if the user went through the pain of explicitley defining
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entities, he may have a a rather negative attitude if you blindly susbtitute
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them as saving time). The function <a
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href="gnome-xml-parser.html#XMLSUBSTITUTEENTITIESDEFAULT">xmlSubstituteEntitiesDefault()</a>
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allows to check and change the behaviour, which is to not substitute entities
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by default.</p>
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<p>Here is the DOM tree built by libxml for the previous document in the
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default case:</p>
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<pre>/gnome/src/gnome-xml -> ./tester --debug test/ent1
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DOCUMENT
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version=1.0
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ELEMENT EXAMPLE
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TEXT
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content=
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ENTITY_REF
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INTERNAL_GENERAL_ENTITY xml
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content=Extensible Markup Language
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TEXT
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content=</pre>
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<p>And here is the result when substituting entities:</p>
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<pre>/gnome/src/gnome-xml -> ./tester --debug --noent test/ent1
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DOCUMENT
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version=1.0
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ELEMENT EXAMPLE
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TEXT
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content= Extensible Markup Language</pre>
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<p>So entities or no entities ? Basically it depends on your use case, I
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suggest to keep the non-substituting default behaviour and avoid using
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entities in your XML document or data if you are not willing to handle the
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entity references elements in the DOM tree.</p>
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<p>Note that at save time libxml enforce the conversion of the predefined
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entities where necessary to prevent well-formedness problems, and will also
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transparently replace those with chars (i.e. will not generate entity
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reference elements in the DOM tree nor call the reference() SAX callback when
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finding them in the input).</p>
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<h2>Namespaces</h2>
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<p>The libxml library implement namespace @@ support by recognizing namespace
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contructs in the input, and does namespace lookup automatically when building
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the DOM tree. A namespace declaration is associated with an in-memory
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structure and all elements or attributes within that namespace point to it.
|
|
Hence testing the namespace is a simple and fast equality operation at the
|
|
user level. </p>
|
|
|
|
<p>I suggest it that people using libxml use a namespace, and declare it on
|
|
the root element of their document as the default namespace. Then they dont
|
|
need to happend the prefix in the content but we will have a basis for future
|
|
semantic refinement and merging of data from different sources. This doesn't
|
|
augment significantly the size of the XML output, but significantly increase
|
|
it's value in the long-term.</p>
|
|
|
|
<p>Concerning the namespace value, this has to be an URL, but this doesn't
|
|
have to point to any existing resource on the Web. I suggest using an URL
|
|
within a domain you control, which makes sense and if possible holding some
|
|
kind of versionning informations. For example
|
|
<code>"http://www.gnome.org/gnumeric/1.0"</code> is a good namespace scheme.
|
|
Then when you load a file, make sure that a namespace carrying the
|
|
version-independant prefix is installed on the root element of your document,
|
|
and if the version information don't match something you know, warn the user
|
|
and be liberal in what you accept as the input. Also do *not* try to base
|
|
namespace checking on the prefix value <foo:text> may be exactly the same
|
|
as <bar:text> in another document, what really matter is the URI
|
|
associated with the element or the attribute, not the prefix string which is
|
|
just a shortcut for the full URI.</p>
|
|
|
|
<p>@@Interfaces@@</p>
|
|
|
|
<p>@@Examples@@</p>
|
|
|
|
<p>Usually people object using namespace in the case of validation, I object
|
|
this and will make sure that using namespaces won't break validity checking,
|
|
so even is you plan or are using validation I strongly suggest to add
|
|
namespaces to your document. A default namespace scheme
|
|
<code>xmlns="http://...."</code> should not break validity even on less
|
|
flexible parsers. Now using namespace to mix and differenciate content coming
|
|
from mutliple Dtd will certainly break current validation schemes, I will try
|
|
to provide ways to do this, but this may not be portable or standardized.</p>
|
|
|
|
<h2>Validation, or are you afraid of DTDs ?</h2>
|
|
|
|
<p>Well what is validation and what is a DTD ?</p>
|
|
|
|
<p>Validation is the process of checking a document against a set of
|
|
construction rules, a <strong>DTD</strong> (Document Type Definition) is such
|
|
a set of rules.</p>
|
|
|
|
<p>The validation process and building DTDs are the two most difficult parts
|
|
of XML life cycle. Briefly a DTD defines all the possibles element to be
|
|
found within your document, what is the formal shape of your document tree (by
|
|
defining the allowed content of an element, either text, a regular expression
|
|
for the allowed list of children, or mixed content i.e. both text and childs).
|
|
The DTD also defines the allowed attributes for all elements and the types of
|
|
the attributes. For more detailed informations, I suggest to read the related
|
|
parts of the XML specification, the examples found under
|
|
gnome-xml/test/valid/dtd and the large amount of books available on XML. The
|
|
dia example in gnome-xml/test/valid should be both simple and complete enough
|
|
to allow you to build your own.</p>
|
|
|
|
<p>A word of warning, building a good DTD which will fit your needs of your
|
|
application in the long-term is far from trivial, however the extra level of
|
|
quality it can insure is well worth the price for some sets of applications or
|
|
if you already have already a DTD defined for your application field.</p>
|
|
|
|
<p>The validation is not completely finished but in a (very IMHO) usable
|
|
state. Until a real validation interface is defined the way to do it is to
|
|
define and set the <strong>xmlDoValidityCheckingDefaultValue</strong> external
|
|
variable to 1, this will of course be changed at some point:</p>
|
|
|
|
<p>extern int xmlDoValidityCheckingDefaultValue;</p>
|
|
|
|
<p>...</p>
|
|
|
|
<p>xmlDoValidityCheckingDefaultValue = 1;</p>
|
|
|
|
<p></p>
|
|
|
|
<p>To handle external entities, use the function
|
|
<strong>xmlSetExternalEntityLoader</strong>(xmlExternalEntityLoader f); to
|
|
link in you HTTP/FTP/Entities database library to the standard libxml
|
|
core.</p>
|
|
|
|
<p>@@interfaces@@</p>
|
|
|
|
<h2><a name="DOM">DOM Principles</a></h2>
|
|
|
|
<p><a href="http://www.w3.org/DOM/">DOM</a> stands for the <em>Document Object
|
|
Model</em> this is an API for accessing XML or HTML structured documents.
|
|
Native support for DOM in Gnome is on the way (module gnome-dom), and it will
|
|
be based on gnome-xml. This will be a far cleaner interface to manipulate XML
|
|
files within Gnome since it won't expose the internal structure. DOM defiles a
|
|
set of IDL (or Java) interfaces allowing to traverse and manipulate a
|
|
document. The DOM library will allow accessing and modifying "live" documents
|
|
presents on other programs like this:</p>
|
|
|
|
<p><img src="DOM.gif" alt=" DOM.gif "></p>
|
|
|
|
<p>This should help greatly doing things like modifying a gnumeric spreadsheet
|
|
embedded in a GWP document for example.</p>
|
|
|
|
<p>The current DOM implementation on top of libxml is the <a
|
|
href="http://cvs.gnome.org/lxr/source/gdome/">gdome Gnome module</a>, this is
|
|
a full DOM interface, thanks to <a href="mailto:raph@levien.com">Raph
|
|
Levien</a>.</p>
|
|
|
|
<p>The gnome-dom module in the Gnome CVS base is obsolete</p>
|
|
|
|
<h2><a name="Example">A real example</a></h2>
|
|
|
|
<p>Here is a real size example, where the actual content of the application
|
|
data is not kept in the DOM tree but uses internal structures. It is based on
|
|
a proposal to keep a database of jobs related to Gnome, with an XML based
|
|
storage structure. Here is an <a href="gjobs.xml">XML encoded jobs
|
|
base</a>:</p>
|
|
<pre><?xml version="1.0"?>
|
|
<gjob:Helping xmlns:gjob="http://www.gnome.org/some-location">
|
|
<gjob:Jobs>
|
|
|
|
<gjob:Job>
|
|
<gjob:Project ID="3"/>
|
|
<gjob:Application>GBackup</gjob:Application>
|
|
<gjob:Category>Development</gjob:Category>
|
|
|
|
<gjob:Update>
|
|
<gjob:Status>Open</gjob:Status>
|
|
<gjob:Modified>Mon, 07 Jun 1999 20:27:45 -0400 MET DST</gjob:Modified>
|
|
<gjob:Salary>USD 0.00</gjob:Salary>
|
|
</gjob:Update>
|
|
|
|
<gjob:Developers>
|
|
<gjob:Developer>
|
|
</gjob:Developer>
|
|
</gjob:Developers>
|
|
|
|
<gjob:Contact>
|
|
<gjob:Person>Nathan Clemons</gjob:Person>
|
|
<gjob:Email>nathan@windsofstorm.net</gjob:Email>
|
|
<gjob:Company>
|
|
</gjob:Company>
|
|
<gjob:Organisation>
|
|
</gjob:Organisation>
|
|
<gjob:Webpage>
|
|
</gjob:Webpage>
|
|
<gjob:Snailmail>
|
|
</gjob:Snailmail>
|
|
<gjob:Phone>
|
|
</gjob:Phone>
|
|
</gjob:Contact>
|
|
|
|
<gjob:Requirements>
|
|
The program should be released as free software, under the GPL.
|
|
</gjob:Requirements>
|
|
|
|
<gjob:Skills>
|
|
</gjob:Skills>
|
|
|
|
<gjob:Details>
|
|
A GNOME based system that will allow a superuser to configure
|
|
compressed and uncompressed files and/or file systems to be backed
|
|
up with a supported media in the system. This should be able to
|
|
perform via find commands generating a list of files that are passed
|
|
to tar, dd, cpio, cp, gzip, etc., to be directed to the tape machine
|
|
or via operations performed on the filesystem itself. Email
|
|
notification and GUI status display very important.
|
|
</gjob:Details>
|
|
|
|
</gjob:Job>
|
|
|
|
</gjob:Jobs>
|
|
</gjob:Helping></pre>
|
|
|
|
<p>While loading the XML file into an internal DOM tree is a matter of calling
|
|
only a couple of functions, browsing the tree to gather the informations and
|
|
generate the internals structures is harder, and more error prone.</p>
|
|
|
|
<p>The suggested principle is to be tolerant with respect to the input
|
|
structure. For example the ordering of the attributes is not significant, Cthe
|
|
XML specification is clear about it. It's also usually a good idea to not be
|
|
dependant of the orders of the childs of a given node, unless it really makes
|
|
things harder. Here is some code to parse the informations for a person:</p>
|
|
<pre>/*
|
|
* A person record
|
|
*/
|
|
typedef struct person {
|
|
char *name;
|
|
char *email;
|
|
char *company;
|
|
char *organisation;
|
|
char *smail;
|
|
char *webPage;
|
|
char *phone;
|
|
} person, *personPtr;
|
|
|
|
/*
|
|
* And the code needed to parse it
|
|
*/
|
|
personPtr parsePerson(xmlDocPtr doc, xmlNsPtr ns, xmlNodePtr cur) {
|
|
personPtr ret = NULL;
|
|
|
|
DEBUG("parsePerson\n");
|
|
/*
|
|
* allocate the struct
|
|
*/
|
|
ret = (personPtr) malloc(sizeof(person));
|
|
if (ret == NULL) {
|
|
fprintf(stderr,"out of memory\n");
|
|
return(NULL);
|
|
}
|
|
memset(ret, 0, sizeof(person));
|
|
|
|
/* We don't care what the top level element name is */
|
|
cur = cur->childs;
|
|
while (cur != NULL) {
|
|
if ((!strcmp(cur->name, "Person")) && (cur->ns == ns))
|
|
ret->name = xmlNodeListGetString(doc, cur->childs, 1);
|
|
if ((!strcmp(cur->name, "Email")) && (cur->ns == ns))
|
|
ret->email = xmlNodeListGetString(doc, cur->childs, 1);
|
|
cur = cur->next;
|
|
}
|
|
|
|
return(ret);
|
|
}</pre>
|
|
|
|
<p>Here is a couple of things to notice:</p>
|
|
<ul>
|
|
<li>Usually a recursive parsing style is the more convenient one, XML data
|
|
being by nature subject to repetitive constructs and usualy exibit highly
|
|
stuctured patterns.</li>
|
|
<li>The two arguments of type <em>xmlDocPtr</em> and <em>xmlNsPtr</em>, i.e.
|
|
the pointer to the global XML document and the namespace reserved to the
|
|
application. Document wide information are needed for example to decode
|
|
entities and it's a good coding practice to define a namespace for your
|
|
application set of data and test that the element and attributes you're
|
|
analyzing actually pertains to your application space. This is done by a
|
|
simple equality test (cur->ns == ns).</li>
|
|
<li>To retrieve text and attributes value, it is suggested to use the
|
|
function <em>xmlNodeListGetString</em> to gather all the text and entity
|
|
reference nodes generated by the DOM output and produce an single text
|
|
string.</li>
|
|
</ul>
|
|
|
|
<p>Here is another piece of code used to parse another level of the
|
|
structure:</p>
|
|
<pre>/*
|
|
* a Description for a Job
|
|
*/
|
|
typedef struct job {
|
|
char *projectID;
|
|
char *application;
|
|
char *category;
|
|
personPtr contact;
|
|
int nbDevelopers;
|
|
personPtr developers[100]; /* using dynamic alloc is left as an exercise */
|
|
} job, *jobPtr;
|
|
|
|
/*
|
|
* And the code needed to parse it
|
|
*/
|
|
jobPtr parseJob(xmlDocPtr doc, xmlNsPtr ns, xmlNodePtr cur) {
|
|
jobPtr ret = NULL;
|
|
|
|
DEBUG("parseJob\n");
|
|
/*
|
|
* allocate the struct
|
|
*/
|
|
ret = (jobPtr) malloc(sizeof(job));
|
|
if (ret == NULL) {
|
|
fprintf(stderr,"out of memory\n");
|
|
return(NULL);
|
|
}
|
|
memset(ret, 0, sizeof(job));
|
|
|
|
/* We don't care what the top level element name is */
|
|
cur = cur->childs;
|
|
while (cur != NULL) {
|
|
|
|
if ((!strcmp(cur->name, "Project")) && (cur->ns == ns)) {
|
|
ret->projectID = xmlGetProp(cur, "ID");
|
|
if (ret->projectID == NULL) {
|
|
fprintf(stderr, "Project has no ID\n");
|
|
}
|
|
}
|
|
if ((!strcmp(cur->name, "Application")) && (cur->ns == ns))
|
|
ret->application = xmlNodeListGetString(doc, cur->childs, 1);
|
|
if ((!strcmp(cur->name, "Category")) && (cur->ns == ns))
|
|
ret->category = xmlNodeListGetString(doc, cur->childs, 1);
|
|
if ((!strcmp(cur->name, "Contact")) && (cur->ns == ns))
|
|
ret->contact = parsePerson(doc, ns, cur);
|
|
cur = cur->next;
|
|
}
|
|
|
|
return(ret);
|
|
}</pre>
|
|
|
|
<p>One can notice that once used to it, writing this kind of code is quite
|
|
simple, but boring. Ultimately, it could be possble to write stubbers taking
|
|
either C data structure definitions, a set of XML examples or an XML DTD and
|
|
produce the code needed to import and export the content between C data and
|
|
XML storage. This is left as an exercise to the reader :-)</p>
|
|
|
|
<p>Feel free to use <a href="gjobread.c">the code for the full C parsing
|
|
example</a> as a template, it is also available with Makefile in the Gnome CVS
|
|
base under gnome-xml/example</p>
|
|
|
|
<p></p>
|
|
|
|
<p><a href="mailto:Daniel.Veillard@w3.org">Daniel Veillard</a></p>
|
|
|
|
<p>$Id$</p>
|
|
</body>
|
|
</html>
|