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307 lines
15 KiB
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<title>The parser interfaces</title>
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<table border="0" width="100%" cellpadding="5" cellspacing="0" align="center"><tr>
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<td width="180">
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<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>
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</td>
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<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">
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<h1>The XML C library for Gnome</h1>
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<h2>The parser interfaces</h2>
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</td></tr></table></td></tr></table></td>
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</tr></table>
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<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>
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<td valign="top" width="200" bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="1" width="100%" bgcolor="#000000"><tr><td>
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<table width="100%" border="0" cellspacing="1" cellpadding="3">
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<tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Main Menu</b></center></td></tr>
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<tr><td bgcolor="#fffacd"><ul>
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<li><a href="index.html">Home</a></li>
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<li><a href="architecture.html">libxml architecture</a></li>
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<li><a href="library.html">The parser interfaces</a></li>
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<table width="100%" border="0" cellspacing="1" cellpadding="3">
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<tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>API Indexes</b></center></td></tr>
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<form action="search.php" enctype="application/x-www-form-urlencoded" method="GET">
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<li><a href="APIchunk0.html">Alphabetic</a></li>
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<table width="100%" border="0" cellspacing="1" cellpadding="3">
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<tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Related links</b></center></td></tr>
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<tr><td bgcolor="#fffacd"><ul>
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<li><a href="http://mail.gnome.org/archives/xml/">Mail archive</a></li>
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<li><a href="http://xmlsoft.org/XSLT/">XSLT libxslt</a></li>
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<li><a href="http://phd.cs.unibo.it/gdome2/">DOM gdome2</a></li>
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<li><a href="http://www.aleksey.com/xmlsec/">XML-DSig xmlsec</a></li>
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<li><a href="ftp://xmlsoft.org/">FTP</a></li>
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<li><a href="http://www.fh-frankfurt.de/~igor/projects/libxml/">Windows binaries</a></li>
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<li><a href="http://garypennington.net/libxml2/">Solaris binaries</a></li>
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<li><a href="http://www.zveno.com/open_source/libxml2xslt.html">MacOsX binaries</a></li>
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<li><a href="http://sourceforge.net/projects/libxml2-pas/">Pascal bindings</a></li>
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<li><a href="http://bugzilla.gnome.org/buglist.cgi?product=libxml&product=libxml2">Bug Tracker</a></li>
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</ul></td></tr>
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</table>
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</td></tr></table></td>
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<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">
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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 is not intended to be
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extensive. I hope the automatically generated documents will provide the
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completeness required, but as a separate set of documents. The interfaces of
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the XML library are by principle low level, there is nearly zero abstraction.
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Those interested in a higher level API should <a href="#DOM">look at
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DOM</a>.</p>
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<p>The <a href="html/libxml-parser.html">parser interfaces for XML</a> are
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separated from the <a href="html/libxml-htmlparser.html">HTML parser
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interfaces</a>. Let's have a look at how the XML parser can be called:</p>
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<h3><a name="Invoking">Invoking the parser : the pull method</a></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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documents either from in-memory strings or from 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>
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<p>Parse a null-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>
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<p>Parse an XML document contained in a (possibly compressed)
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file.</p>
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</dd>
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</dl>
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<p>The parser returns a pointer to the document structure (or NULL in case of
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failure).</p>
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<h3 id="Invoking1">Invoking the parser: the push method</h3>
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<p>In order for the application to keep the control when the document is
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being fetched (which is common for GUI based programs) libxml provides a push
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interface, too, as of version 1.8.3. Here are the interface functions:</p>
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<pre>xmlParserCtxtPtr xmlCreatePushParserCtxt(xmlSAXHandlerPtr sax,
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void *user_data,
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const char *chunk,
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int size,
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const char *filename);
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int xmlParseChunk (xmlParserCtxtPtr ctxt,
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const char *chunk,
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int size,
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int terminate);</pre>
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<p>and here is a simple example showing how to use the interface:</p>
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<pre> FILE *f;
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f = fopen(filename, "r");
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if (f != NULL) {
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int res, size = 1024;
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char chars[1024];
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xmlParserCtxtPtr ctxt;
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res = fread(chars, 1, 4, f);
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if (res > 0) {
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ctxt = xmlCreatePushParserCtxt(NULL, NULL,
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chars, res, filename);
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while ((res = fread(chars, 1, size, f)) > 0) {
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xmlParseChunk(ctxt, chars, res, 0);
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}
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xmlParseChunk(ctxt, chars, 0, 1);
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doc = ctxt->myDoc;
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xmlFreeParserCtxt(ctxt);
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}
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}</pre>
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<p>The HTML parser embedded into libxml also has a push interface; the
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functions are just prefixed by "html" rather than "xml".</p>
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<h3 id="Invoking2">Invoking the parser: the SAX interface</h3>
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<p>The tree-building interface makes the parser memory-hungry, first loading
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the document in memory and then building the tree itself. Reading a document
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without building the tree is possible using the SAX interfaces (see SAX.h and
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<a href="http://www.daa.com.au/~james/gnome/xml-sax/xml-sax.html">James
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Henstridge's documentation</a>). Note also that the push interface can be
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limited to SAX: just use the two first arguments of
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<code>xmlCreatePushParserCtxt()</code>.</p>
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<h3><a name="Building">Building a tree from scratch</a></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. (These are
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also described in <libxml/tree.h>.) For example, here is a piece of
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code that produces the XML document used in the previous examples:</p>
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<pre> #include <libxml/tree.h>
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xmlDocPtr doc;
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xmlNodePtr tree, subtree;
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doc = xmlNewDoc("1.0");
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doc->children = xmlNewDocNode(doc, NULL, "EXAMPLE", NULL);
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xmlSetProp(doc->children, "prop1", "gnome is great");
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xmlSetProp(doc->children, "prop2", "& linux too");
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tree = xmlNewChild(doc->children, NULL, "head", NULL);
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subtree = xmlNewChild(tree, NULL, "title", "Welcome to Gnome");
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tree = xmlNewChild(doc->children, 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><a name="Traversing">Traversing the tree</a></h3>
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<p>Basically by <a href="html/libxml-tree.html">including "tree.h"</a> your
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code has access to the internal structure of all the elements of the tree.
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The names should be somewhat simple like <strong>parent</strong>,
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<strong>children</strong>, <strong>next</strong>, <strong>prev</strong>,
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<strong>properties</strong>, etc... For example, still with the previous
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example:</p>
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<pre><code>doc->children->children->children</code></pre>
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<p>points to the title element,</p>
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<pre>doc->children->children->next->children->children</pre>
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<p>points to the text node containing the chapter title "The Linux
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adventure".</p>
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<p>
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<strong>NOTE</strong>: XML allows <em>PI</em>s and <em>comments</em> to be
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present before the document root, so <code>doc->children</code> may point
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to an element which is not the document Root Element; a function
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<code>xmlDocGetRootElement()</code> was added for this purpose.</p>
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<h3><a name="Modifying">Modifying the tree</a></h3>
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<p>Functions are provided for reading and writing the document content. Here
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is an excerpt from the <a href="html/libxml-tree.html">tree API</a>:</p>
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<dl>
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<dt><code>xmlAttrPtr xmlSetProp(xmlNodePtr node, const xmlChar *name, const
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xmlChar *value);</code></dt>
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<dd>
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<p>This sets (or changes) an attribute carried by an ELEMENT node.
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The 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 xmlChar *xmlGetProp(xmlNodePtr node, const xmlChar
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*name);</code></dt>
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<dd>
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<p>This function returns a pointer to new copy of the property
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content. Note that the user must deallocate the result.</p>
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</dd>
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</dl>
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<p>Two functions are provided for reading and writing the text associated
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with elements:</p>
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<dl>
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<dt><code>xmlNodePtr xmlStringGetNodeList(xmlDocPtr doc, const xmlChar
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*value);</code></dt>
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<dd>
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<p>This function takes an "external" string and converts it to one
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text node or possibly to a list of entity and text nodes. All
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non-predefined entity references like &Gnome; will be stored
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internally as entity nodes, hence the result of the function may not be
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a single node.</p>
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</dd>
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</dl>
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<dl>
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<dt><code>xmlChar *xmlNodeListGetString(xmlDocPtr doc, xmlNodePtr list, int
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inLine);</code></dt>
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<dd>
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<p>This function is the inverse of
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<code>xmlStringGetNodeList()</code>. It generates a new string
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containing the content of the text and entity nodes. Note the extra
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argument inLine. If this argument is set to 1, the function will expand
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entity references. For example, instead of returning the &Gnome;
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XML encoding in the string, it will substitute it with its value (say,
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"GNU Network Object Model Environment").</p>
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</dd>
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</dl>
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<h3><a name="Saving">Saving a tree</a></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, xmlChar**mem, int
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*size);</code></dt>
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<dd>
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<p>Returns a buffer into which 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>
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<p>Dumps a document 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>
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<p>Saves the document to a file. In this case, the compression
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interface is triggered if it has been turned on.</p>
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</dd>
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</dl>
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<h3><a name="Compressio">Compression</a></h3>
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<p>The library transparently handles compression when doing file-based
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accesses. The level of compression on saves can be turned on either globally
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or 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>
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<p>Gets 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>
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<p>Sets 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>
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<p>Gets 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>
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<p>Sets the default compression ratio.</p>
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</dd>
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</dl>
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<p><a href="bugs.html">Daniel Veillard</a></p>
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</td></tr></table></td></tr></table></td></tr></table></td>
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</tr></table></td></tr></table>
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</body>
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</html>
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