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<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>Parsing XML</title><link rel="stylesheet" type="text/css" href="../manual.css"><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"><link rel="home" href="index.html" title="neon HTTP/WebDAV client library"><link rel="up" href="api.html" title="Chapter 2. The neon C language interface"><link rel="prev" href="api.html" title="Chapter 2. The neon C language interface"><link rel="next" href="ref.html" title="neon API reference"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Parsing XML</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="api.html">Prev</a> </td><th width="60%" align="center">Chapter 2. The neon C language interface</th><td width="20%" align="right"> <a accesskey="n" href="ref.html">Next</a></td></tr></table><hr></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="xml"></a>Parsing XML</h2></div></div></div><p>The neon XML interface is exposed by the <code class="filename">ne_xml.h</code> header file. This interface gives a wrapper around the standard <a class="ulink" href="http://www.saxproject.org/" target="_top">SAX</a> API used by XML parsers, with an additional abstraction, <em class="firstterm">stacked SAX handlers</em>, and also giving consistent <a class="ulink" href="http://www.w3.org/TR/REC-xml-names" target="_top">XML Namespace</a> support.</p><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="xml-sax"></a>Introduction to SAX</h3></div></div></div><p>A SAX-based parser works by emitting a sequence of <em class="firstterm">events</em> to reflect the tokens being parsed from the XML document. For example, parsing the following document fragment: </p><pre class="programlisting"> <hello>world</hello> </pre><p> results in the following events: </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><span class="emphasis"><em>start-element</em></span> "hello"</li><li class="listitem"><span class="emphasis"><em>character-data</em></span> "world"</li><li class="listitem"><span class="emphasis"><em>end-element</em></span> "hello"</li></ol></div><p> This example demonstrates the three event types used used in the subset of SAX exposed by the neon XML interface: <span class="emphasis"><em>start-element</em></span>, <span class="emphasis"><em>character-data</em></span> and <span class="emphasis"><em>end-element</em></span>. In a C API, an <span class="quote">“<span class="quote">event</span>”</span> is implemented as a function callback; three callback types are used in neon, one for each type of event.</p></div><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="xml-stacked"></a>Stacked SAX handlers</h3></div></div></div><p>WebDAV property values are represented as fragments of XML, transmitted as parts of larger XML documents over HTTP (notably in the body of the response to a <code class="literal">PROPFIND</code> request). When neon parses such documents, the SAX events generated for these property value fragments may need to be handled by the application, since neon has no knowledge of the structure of properties used by the application.</p><p>To solve this problem<a href="#ftn.foot.xml.sax" class="footnote" name="foot.xml.sax"><sup class="footnote">[1]</sup></a> the neon XML interface introduces the concept of a <em class="firstterm">SAX handler</em>. A SAX handler comprises a <span class="emphasis"><em>start-element</em></span>, <span class="emphasis"><em>character-data</em></span> and <span class="emphasis"><em>end-element</em></span> callback; the <span class="emphasis"><em>start-element</em></span> callback being defined such that each handler may <span class="emphasis"><em>accept</em></span> or <span class="emphasis"><em>decline</em></span> the <span class="emphasis"><em>start-element</em></span> event. Handlers are composed into a <em class="firstterm">handler stack</em> before parsing a document. When a new <span class="emphasis"><em>start-element</em></span> event is generated by the XML parser, neon invokes each <span class="emphasis"><em>start-element</em></span> callback in the handler stack in turn until one accepts the event. The handler which accepts the event will then be subsequently be passed <span class="emphasis"><em>character-data</em></span> events if the element contains character data, followed by an <span class="emphasis"><em>end-element</em></span> event when the element is closed. If no handler in the stack accepts a <span class="emphasis"><em>start-element</em></span> event, the branch of the tree is ignored.</p><p>To illustrate, given a handler A, which accepts the <code class="literal">cat</code> and <code class="literal">age</code> elements, and a handler B, which accepts the <code class="literal">name</code> element, the following document: </p><div class="example"><a name="xml-example"></a><p class="title"><b>Example 2.1. An example XML document</b></p><div class="example-contents"><pre class="programlisting"> <cat> <age>3</age> <name>Bob</name> </cat> </pre></div></div><p><br class="example-break"> would be parsed as follows: </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem">A <span class="emphasis"><em>start-element</em></span> "cat" → <span class="emphasis"><em>accept</em></span></li><li class="listitem">A <span class="emphasis"><em>start-element</em></span> "age" → <span class="emphasis"><em>accept</em></span></li><li class="listitem">A <span class="emphasis"><em>character-data</em></span> "3"</li><li class="listitem">A <span class="emphasis"><em>end-element</em></span> "age"</li><li class="listitem">A <span class="emphasis"><em>start-element</em></span> "name" → <span class="emphasis"><em>decline</em></span></li><li class="listitem">B <span class="emphasis"><em>start-element</em></span> "name" → <span class="emphasis"><em>accept</em></span></li><li class="listitem">B <span class="emphasis"><em>character-data</em></span> "Bob"</li><li class="listitem">B <span class="emphasis"><em>end-element</em></span> "name"</li><li class="listitem">A <span class="emphasis"><em>end-element</em></span> "cat"</li></ol></div><p>The search for a handler which will accept a <span class="emphasis"><em>start-element</em></span> event begins at the handler of the parent element and continues toward the top of the stack. For the root element, it begins at the base of the stack. In the above example, handler A is at the base, and handler B at the top; if the <code class="literal">name</code> element had any children, only B's <span class="emphasis"><em>start-element</em></span> would be invoked to accept them.</p></div><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="xml-state"></a>Maintaining state</h3></div></div></div><p>To facilitate communication between independent handlers, a <em class="firstterm">state integer</em> is associated with each element being parsed. This integer is returned by <span class="emphasis"><em>start-element</em></span> callback and is passed to the subsequent <span class="emphasis"><em>character-data</em></span> and <span class="emphasis"><em>end-element</em></span> callbacks associated with the element. The state integer of the parent element is also passed to each <span class="emphasis"><em>start-element</em></span> callback, the value zero used for the root element (which by definition has no parent).</p><p>To further extend <a class="xref" href="xml.html#xml-example" title="Example 2.1. An example XML document">Example 2.1, “An example XML document”</a>: if handler A defines that the state of the root element <code class="sgmltag-element">cat</code> will be <code class="literal">42</code>, the event trace would be as follows: </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem">A <span class="emphasis"><em>start-element</em></span> (parent = 0, "cat") → <span class="emphasis"><em>accept</em></span>, state = 42 </li><li class="listitem">A <span class="emphasis"><em>start-element</em></span> (parent = 42, "age") → <span class="emphasis"><em>accept</em></span>, state = 50 </li><li class="listitem">A <span class="emphasis"><em>character-data</em></span> (state = 50, "3")</li><li class="listitem">A <span class="emphasis"><em>end-element</em></span> (state = 50, "age")</li><li class="listitem">A <span class="emphasis"><em>start-element</em></span> (parent = 42, "name") → <span class="emphasis"><em>decline</em></span></li><li class="listitem">B <span class="emphasis"><em>start-element</em></span> (parent = 42, "name") → <span class="emphasis"><em>accept</em></span>, state = 99</li><li class="listitem">B <span class="emphasis"><em>character-data</em></span> (state = 99, "Bob")</li><li class="listitem">B <span class="emphasis"><em>end-element</em></span> (state = 99, "name")</li><li class="listitem">A <span class="emphasis"><em>end-element</em></span> (state = 42, "cat")</li></ol></div><p>To avoid collisions between state integers used by different handlers, the interface definition of any handler includes the range of integers it will use.</p></div><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="xml-ns"></a>XML namespaces</h3></div></div></div><p>To support XML namespaces, every element name is represented as a <span class="emphasis"><em>(namespace, name)</em></span> pair. The <span class="emphasis"><em>start-element</em></span> and <span class="emphasis"><em>end-element</em></span> callbacks are passed namespace and name strings accordingly. If an element in the XML document has no declared namespace, the namespace given will be the empty string, <code class="literal">""</code>.</p></div><div class="footnotes"><br><hr style="width:100; text-align:left;margin-left: 0"><div id="ftn.foot.xml.sax" class="footnote"><p><a href="#foot.xml.sax" class="para"><sup class="para">[1] </sup></a>This <span class="quote">“<span class="quote">problem</span>”</span> only needs solving because the SAX interface is so inflexible when implemented as C function callbacks; a better approach would be to use an XML parser interface which is not based on callbacks.</p></div></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="api.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="api.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ref.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 2. 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