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12<title>The XML library interfaces</title>
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20<h1>The XML C library for Gnome</h1>
21<h2>The XML library interfaces</h2>
22</td></tr></table></td></tr></table></td>
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Daniel Veillard594cf0b2001-10-25 08:09:12 +0000[diff] [blame^]25<td valign="top" width="200" bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="1" width="100%" bgcolor="#000000"><tr><td>
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Daniel Veillardc9484202001-10-24 12:35:52 +0000[diff] [blame]27<tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Main Menu</b></center></td></tr>
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39<li><a href="architecture.html">An overview of libxml architecture</a></li>
40<li><a href="tree.html">The tree output</a></li>
41<li><a href="interface.html">The SAX interface</a></li>
42<li><a href="library.html">The XML library interfaces</a></li>
43<li><a href="entities.html">Entities or no entities</a></li>
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59<table width="100%" border="0" cellspacing="1" cellpadding="3">
60<tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Related links</b></center></td></tr>
61<tr><td bgcolor="#fffacd"><ul style="margin-left: -2pt">
62<li><a href="http://mail.gnome.org/archives/xml/">Mail archive</a></li>
63<li><a href="http://xmlsoft.org/XSLT/">XSLT libxslt</a></li>
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66<li><a href="http://www.fh-frankfurt.de/~igor/projects/libxml/">Windows binaries</a></li>
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Daniel Veillardc9484202001-10-24 12:35:52 +0000[diff] [blame]71<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">
72<p>This section is directly intended to help programmers getting bootstrapped
73using the XML library from the C language. It is not intended to be
74extensive. I hope the automatically generated documents will provide the
75completeness required, but as a separate set of documents. The interfaces of
76the XML library are by principle low level, there is nearly zero abstraction.
77Those interested in a higher level API should <a href="#DOM">look at
78DOM</a>.</p>
79<p>The <a href="html/libxml-parser.html">parser interfaces for XML</a> are
80separated from the <a href="html/libxml-htmlparser.html">HTML parser
81interfaces</a>. Let's have a look at how the XML parser can be called:</p>
82<h3><a name="Invoking">Invoking the parser : the pull method</a></h3>
83<p>Usually, the first thing to do is to read an XML input. The parser accepts
84documents either from in-memory strings or from files. The functions are
85defined in &quot;parser.h&quot;:</p>
86<dl>
87<dt><code>xmlDocPtr xmlParseMemory(char *buffer, int size);</code></dt>
88<dd><p>Parse a null-terminated string containing the document.</p></dd>
89</dl>
90<dl>
91<dt><code>xmlDocPtr xmlParseFile(const char *filename);</code></dt>
92<dd><p>Parse an XML document contained in a (possibly compressed)
93 file.</p></dd>
94</dl>
95<p>The parser returns a pointer to the document structure (or NULL in case of
96failure).</p>
97<h3 id="Invoking1">Invoking the parser: the push method</h3>
98<p>In order for the application to keep the control when the document is
99being fetched (which is common for GUI based programs) libxml provides a push
100interface, too, as of version 1.8.3. Here are the interface functions:</p>
101<pre>xmlParserCtxtPtr xmlCreatePushParserCtxt(xmlSAXHandlerPtr sax,
102 void *user_data,
103 const char *chunk,
104 int size,
105 const char *filename);
106int xmlParseChunk (xmlParserCtxtPtr ctxt,
107 const char *chunk,
108 int size,
109 int terminate);</pre>
110<p>and here is a simple example showing how to use the interface:</p>
111<pre> FILE *f;
112
113 f = fopen(filename, &quot;r&quot;);
114 if (f != NULL) {
115 int res, size = 1024;
116 char chars[1024];
117 xmlParserCtxtPtr ctxt;
118
119 res = fread(chars, 1, 4, f);
120 if (res &gt; 0) {
121 ctxt = xmlCreatePushParserCtxt(NULL, NULL,
122 chars, res, filename);
123 while ((res = fread(chars, 1, size, f)) &gt; 0) {
124 xmlParseChunk(ctxt, chars, res, 0);
125 }
126 xmlParseChunk(ctxt, chars, 0, 1);
127 doc = ctxt-&gt;myDoc;
128 xmlFreeParserCtxt(ctxt);
129 }
130 }</pre>
131<p>The HTML parser embedded into libxml also has a push interface; the
132functions are just prefixed by &quot;html&quot; rather than &quot;xml&quot;.</p>
133<h3 id="Invoking2">Invoking the parser: the SAX interface</h3>
134<p>The tree-building interface makes the parser memory-hungry, first loading
135the document in memory and then building the tree itself. Reading a document
136without building the tree is possible using the SAX interfaces (see SAX.h and
137<a href="http://www.daa.com.au/~james/gnome/xml-sax/xml-sax.html">James
138Henstridge's documentation</a>). Note also that the push interface can be
139limited to SAX: just use the two first arguments of
140<code>xmlCreatePushParserCtxt()</code>.</p>
141<h3><a name="Building">Building a tree from scratch</a></h3>
142<p>The other way to get an XML tree in memory is by building it. Basically
143there is a set of functions dedicated to building new elements. (These are
144also described in &lt;libxml/tree.h&gt;.) For example, here is a piece of
145code that produces the XML document used in the previous examples:</p>
146<pre> #include &lt;libxml/tree.h&gt;
147 xmlDocPtr doc;
148 xmlNodePtr tree, subtree;
149
150 doc = xmlNewDoc(&quot;1.0&quot;);
151 doc-&gt;children = xmlNewDocNode(doc, NULL, &quot;EXAMPLE&quot;, NULL);
152 xmlSetProp(doc-&gt;children, &quot;prop1&quot;, &quot;gnome is great&quot;);
153 xmlSetProp(doc-&gt;children, &quot;prop2&quot;, &quot;&amp; linux too&quot;);
154 tree = xmlNewChild(doc-&gt;children, NULL, &quot;head&quot;, NULL);
155 subtree = xmlNewChild(tree, NULL, &quot;title&quot;, &quot;Welcome to Gnome&quot;);
156 tree = xmlNewChild(doc-&gt;children, NULL, &quot;chapter&quot;, NULL);
157 subtree = xmlNewChild(tree, NULL, &quot;title&quot;, &quot;The Linux adventure&quot;);
158 subtree = xmlNewChild(tree, NULL, &quot;p&quot;, &quot;bla bla bla ...&quot;);
159 subtree = xmlNewChild(tree, NULL, &quot;image&quot;, NULL);
160 xmlSetProp(subtree, &quot;href&quot;, &quot;linus.gif&quot;);</pre>
161<p>Not really rocket science ...</p>
162<h3><a name="Traversing">Traversing the tree</a></h3>
163<p>Basically by <a href="html/libxml-tree.html">including &quot;tree.h&quot;</a> your
164code has access to the internal structure of all the elements of the tree.
165The names should be somewhat simple like <strong>parent</strong>,
166<strong>children</strong>, <strong>next</strong>, <strong>prev</strong>,
167<strong>properties</strong>, etc... For example, still with the previous
168example:</p>
169<pre><code>doc-&gt;children-&gt;children-&gt;children</code></pre>
170<p>points to the title element,</p>
171<pre>doc-&gt;children-&gt;children-&gt;next-&gt;children-&gt;children</pre>
172<p>points to the text node containing the chapter title &quot;The Linux
173adventure&quot;.</p>
174<p>
175<strong>NOTE</strong>: XML allows <em>PI</em>s and <em>comments</em> to be
176present before the document root, so <code>doc-&gt;children</code> may point
177to an element which is not the document Root Element; a function
178<code>xmlDocGetRootElement()</code> was added for this purpose.</p>
179<h3><a name="Modifying">Modifying the tree</a></h3>
180<p>Functions are provided for reading and writing the document content. Here
181is an excerpt from the <a href="html/libxml-tree.html">tree API</a>:</p>
182<dl>
183<dt><code>xmlAttrPtr xmlSetProp(xmlNodePtr node, const xmlChar *name, const
184 xmlChar *value);</code></dt>
185<dd><p>This sets (or changes) an attribute carried by an ELEMENT node.
186 The value can be NULL.</p></dd>
187</dl>
188<dl>
189<dt><code>const xmlChar *xmlGetProp(xmlNodePtr node, const xmlChar
190 *name);</code></dt>
191<dd><p>This function returns a pointer to new copy of the property
192 content. Note that the user must deallocate the result.</p></dd>
193</dl>
194<p>Two functions are provided for reading and writing the text associated
195with elements:</p>
196<dl>
197<dt><code>xmlNodePtr xmlStringGetNodeList(xmlDocPtr doc, const xmlChar
198 *value);</code></dt>
199<dd><p>This function takes an &quot;external&quot; string and converts it to one
200 text node or possibly to a list of entity and text nodes. All
201 non-predefined entity references like &amp;Gnome; will be stored
202 internally as entity nodes, hence the result of the function may not be
203 a single node.</p></dd>
204</dl>
205<dl>
206<dt><code>xmlChar *xmlNodeListGetString(xmlDocPtr doc, xmlNodePtr list, int
207 inLine);</code></dt>
208<dd><p>This function is the inverse of
209 <code>xmlStringGetNodeList()</code>. It generates a new string
210 containing the content of the text and entity nodes. Note the extra
211 argument inLine. If this argument is set to 1, the function will expand
212 entity references. For example, instead of returning the &amp;Gnome;
213 XML encoding in the string, it will substitute it with its value (say,
214 &quot;GNU Network Object Model Environment&quot;).</p></dd>
215</dl>
216<h3><a name="Saving">Saving a tree</a></h3>
217<p>Basically 3 options are possible:</p>
218<dl>
219<dt><code>void xmlDocDumpMemory(xmlDocPtr cur, xmlChar**mem, int
220 *size);</code></dt>
221<dd><p>Returns a buffer into which the document has been saved.</p></dd>
222</dl>
223<dl>
224<dt><code>extern void xmlDocDump(FILE *f, xmlDocPtr doc);</code></dt>
225<dd><p>Dumps a document to an open file descriptor.</p></dd>
226</dl>
227<dl>
228<dt><code>int xmlSaveFile(const char *filename, xmlDocPtr cur);</code></dt>
229<dd><p>Saves the document to a file. In this case, the compression
230 interface is triggered if it has been turned on.</p></dd>
231</dl>
232<h3><a name="Compressio">Compression</a></h3>
233<p>The library transparently handles compression when doing file-based
234accesses. The level of compression on saves can be turned on either globally
235or individually for one file:</p>
236<dl>
237<dt><code>int xmlGetDocCompressMode (xmlDocPtr doc);</code></dt>
238<dd><p>Gets the document compression ratio (0-9).</p></dd>
239</dl>
240<dl>
241<dt><code>void xmlSetDocCompressMode (xmlDocPtr doc, int mode);</code></dt>
242<dd><p>Sets the document compression ratio.</p></dd>
243</dl>
244<dl>
245<dt><code>int xmlGetCompressMode(void);</code></dt>
246<dd><p>Gets the default compression ratio.</p></dd>
247</dl>
248<dl>
249<dt><code>void xmlSetCompressMode(int mode);</code></dt>
250<dd><p>Sets the default compression ratio.</p></dd>
251</dl>
252<p><a href="mailto:daniel@veillard.com">Daniel Veillard</a></p>
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