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1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/1999/REC-html401-19991224/loose.dtd">
2<html>
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13<title>Writing extensions</title>
14</head>
15<body bgcolor="#8b7765" text="#000000" link="#000000" vlink="#000000">
16<table border="0" width="100%" cellpadding="5" cellspacing="0" align="center"><tr>
17<td width="100">
18<a href="http://www.gnome.org/"><img src="gnome2.png" alt="Gnome2 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/XSLT/"><img src="Libxslt-Logo-180x168.gif" alt="Made with Libxslt Logo"></a></div>
19</td>
20<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">
21<h1>The XSLT C library for Gnome</h1>
22<h2>Writing extensions</h2>
23</td></tr></table></td></tr></table></td>
24</tr></table>
25<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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27<table width="100%" border="0" cellspacing="1" cellpadding="3">
28<tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Main Menu</b></center></td></tr>
29<tr><td bgcolor="#fffacd">
30<form action="search.php" enctype="application/x-www-form-urlencoded" method="GET">
31<input name="query" type="TEXT" size="20" value=""><input name="submit" type="submit" value="Search ...">
32</form>
33<ul>
34<li><a href="index.html">Home</a></li>
35<li><a href="intro.html">Introduction</a></li>
36<li><a href="docs.html">Documentation</a></li>
37<li><a href="bugs.html">Reporting bugs and getting help</a></li>
38<li><a href="help.html">How to help</a></li>
39<li><a href="downloads.html">Downloads</a></li>
40<li><a href="FAQ.html">FAQ</a></li>
41<li><a href="news.html">News</a></li>
42<li><a href="xsltproc2.html">The xsltproc tool</a></li>
43<li><a href="docbook.html">DocBook</a></li>
44<li><a href="API.html">The programming API</a></li>
45<li><a href="python.html">Python and bindings</a></li>
46<li><a href="internals.html">Library internals</a></li>
47<li><a href="extensions.html">Writing extensions</a></li>
48<li><a href="contribs.html">Contributions</a></li>
49<li>
50<a href="xslt.html">flat page</a>, <a href="site.xsl">stylesheet</a>
51</li>
52</ul>
53</td></tr>
54</table>
55<table width="100%" border="0" cellspacing="1" cellpadding="3">
56<tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Related links</b></center></td></tr>
57<tr><td bgcolor="#fffacd"><ul>
58<li><a href="tutorial/libxslttutorial.html">Tutorial</a></li>
59<li><a href="xsltproc.html">Man page for xsltproc</a></li>
60<li><a href="http://mail.gnome.org/archives/xslt/">Mail archive</a></li>
61<li><a href="http://xmlsoft.org/">XML libxml</a></li>
62<li><a href="http://phd.cs.unibo.it/gdome2/">DOM gdome2</a></li>
63<li><a href="ftp://xmlsoft.org/">FTP</a></li>
64<li><a href="http://www.zlatkovic.com/projects/libxml/">Windows binaries</a></li>
65<li><a href="http://garypennington.net/libxml2/">Solaris binaries</a></li>
66<li><a href="http://www.zveno.com/open_source/libxml2xslt.html">MacOsX binaries</a></li>
67<li><a href="http://sourceforge.net/projects/libxml2-pas/">Pascal bindings</a></li>
68<li><a href="http://bugzilla.gnome.org/buglist.cgi?product=libxslt">Bug Tracker</a></li>
69<li><a href="http://xsldbg.sourceforge.net/">Xsldbg Debugger</a></li>
70</ul></td></tr>
71</table>
72<table width="100%" border="0" cellspacing="1" cellpadding="3">
73<tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>API Indexes</b></center></td></tr>
74<tr><td bgcolor="#fffacd"><ul>
75<li><a href="APIchunk0.html">Alphabetic</a></li>
76<li><a href="APIconstructors.html">Constructors</a></li>
77<li><a href="APIfunctions.html">Functions/Types</a></li>
78<li><a href="APIfiles.html">Modules</a></li>
79<li><a href="APIsymbols.html">Symbols</a></li>
80</ul></td></tr>
81</table>
82</td></tr></table></td>
83<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">
84<h3>Table  of content</h3>
85<ul>
86<li><a href="extensions.html#Introducti">Introduction</a></li>
87  <li><a href="extensions.html#Basics">Basics</a></li>
88  <li><a href="extensions.html#Keep">Extension modules</a></li>
89  <li><a href="extensions.html#Registerin">Registering a module</a></li>
90  <li><a href="extensions.html#module">Loading a module</a></li>
91  <li><a href="extensions.html#Registerin1">Registering an extension
92    function</a></li>
93  <li><a href="extensions.html#Implementi">Implementing an extension
94    function</a></li>
95  <li><a href="extensions.html#Examples">Examples for extension
96  functions</a></li>
97  <li><a href="extensions.html#Registerin2">Registering an extension
98    element</a></li>
99  <li><a href="extensions.html#Implementi1">Implementing an extension
100    element</a></li>
101  <li><a href="extensions.html#Example">Example for extension
102  elements</a></li>
103  <li><a href="extensions.html#shutdown">The shutdown of a module</a></li>
104  <li><a href="extensions.html#Future">Future work</a></li>
105</ul>
106<h3><a name="Introducti1">Introduction</a></h3>
107<p>This document describes the work needed to write extensions to the
108standard XSLT library for use with <a href="http://xmlsoft.org/XSLT/">libxslt</a>, the <a href="http://www.w3.org/TR/xslt">XSLT</a> C library developed for the <a href="http://www.gnome.org/">Gnome</a> project.</p>
109<p>Before starting reading this document it is highly recommended to get
110familiar with <a href="internals.html">the libxslt internals</a>.</p>
111<p>Note: this documentation is by definition incomplete and I am not good at
112spelling, grammar, so patches and suggestions are <a href="mailto:veillard@redhat.com">really welcome</a>.</p>
113<h3><a name="Basics">Basics</a></h3>
114<p>The <a href="http://www.w3.org/TR/xslt">XSLT specification</a> provides
115two <a href="http://www.w3.org/TR/xslt">ways to extend an XSLT engine</a>:</p>
116<ul>
117<li>providing <a href="http://www.w3.org/TR/xslt">new extension
118    functions</a> which can be called from XPath expressions</li>
119  <li>providing <a href="http://www.w3.org/TR/xslt">new extension
120    elements</a> which can be inserted in stylesheets</li>
121</ul>
122<p>In both cases the extensions need to be associated to a new namespace,
123i.e. an URI used as the name for the extension's namespace (there is no need
124to have a resource there for this to work).</p>
125<p>libxslt provides a few extensions itself, either in libxslt namespace
126&quot;http://xmlsoft.org/XSLT/&quot; or in other namespace for well known extensions
127provided by other XSLT processors like Saxon, Xalan or XT.</p>
128<h3><a name="Keep">Extension modules</a></h3>
129<p>Since extensions are bound to a namespace name, usually sets of extensions
130coming from a given source are using the same namespace name defining in
131practice a group of extensions providing elements, functions or both. From
132libxslt point of view those are considered as an &quot;extension module&quot;, and most
133of the APIs work at a module point of view.</p>
134<p>Registration of new functions or elements are bound to the activation of
135the module, this is currently done by declaring the namespace as an extension
136by using the attribute  <code>extension-element-prefixes</code> on the
137<code><a href="http://www.w3.org/TR/xslt">xsl:stylesheet</a></code>
138element.</p>
139<p>And extension module is defined by 3 objects:</p>
140<ul>
141<li>the namespace name associated</li>
142  <li>an initialization function</li>
143  <li>a shutdown function</li>
144</ul>
145<h3><a name="Registerin">Registering a module</a></h3>
146<p>Currently a libxslt module has to be compiled within the application using
147libxslt, there is no code to load dynamically shared libraries associated to
148namespace (this may be added but is likely to become a portability
149nightmare).</p>
150<p>So the current way to register a module is to link the code implementing
151it with the application and to call a registration function:</p>
152<pre>int xsltRegisterExtModule(const xmlChar *URI,
153                          xsltExtInitFunction initFunc,
154                          xsltExtShutdownFunction shutdownFunc);</pre>
155<p>The associated header is read by:</p>
156<pre>#include&lt;libxslt/extensions.h&gt;</pre>
157<p>which also defines the type for the initialization and shutdown
158functions</p>
159<h3><a name="module">Loading a module</a></h3>
160<p>Once the module URI has been registered and if the XSLT processor detects
161that a given stylesheet needs the functionalities of an extended module, this
162one is initialized.</p>
163<p>The xsltExtInitFunction type defines the interface for an initialization
164function:</p>
165<pre>/**
166 * xsltExtInitFunction:
167 * @ctxt:  an XSLT transformation context
168 * @URI:  the namespace URI for the extension
169 *
170 * A function called at initialization time of an XSLT
171 * extension module
172 *
173 * Returns a pointer to the module specific data for this
174 * transformation
175 */
176typedef void *(*xsltExtInitFunction)(xsltTransformContextPtr ctxt,
177                                     const xmlChar *URI);</pre>
178<p>There are 3 things to notice:</p>
179<ul>
180<li>the function gets passed the namespace name URI as an argument, this
181    allow a single function to provide the initialization for multiple
182    logical modules</li>
183  <li>it also gets passed a transformation context, the initialization is
184    done at run time before any processing occurs on the stylesheet but it
185    will be invoked separately each time for each transformation</li>
186  <li>it returns a pointer, this can be used to store module specific
187    informations which can be retrieved later when a function or an element
188    from the extension are used, an obvious example is a connection to a
189    database which should be kept and reused along the transformation. NULL
190    is a perfectly valid return, there is no way to indicate a failure at
191    this level</li>
192</ul>
193<p>What this function is expected to do is:</p>
194<ul>
195<li>prepare the context for this module (like opening the database
196    connection)</li>
197  <li>register the extensions specific to this module</li>
198</ul>
199<h3><a name="Registerin1">Registering an extension function</a></h3>
200<p>There is a single call to do this registration:</p>
201<pre>int xsltRegisterExtFunction(xsltTransformContextPtr ctxt,
202                            const xmlChar *name,
203                            const xmlChar *URI,
204                            xmlXPathEvalFunc function);</pre>
205<p>The registration is bound to a single transformation instance referred by
206ctxt, name is the UTF8 encoded name for the NCName of the function, and URI
207is the namespace name for the extension (no checking is done, a module could
208register functions or elements from a different namespace, but it is not
209recommended).</p>
210<h3><a name="Implementi">Implementing an extension function</a></h3>
211<p>The implementation of the function must have the signature of a libxml
212XPath function:</p>
213<pre>/**
214 * xmlXPathEvalFunc:
215 * @ctxt: an XPath parser context
216 * @nargs: the number of arguments passed to the function
217 *
218 * an XPath evaluation function, the parameters are on the
219 * XPath context stack
220 */
221
222typedef void (*xmlXPathEvalFunc)(xmlXPathParserContextPtr ctxt,
223                                 int nargs);</pre>
224<p>The context passed to an XPath function is not an XSLT context but an <a href="internals.html#XPath1">XPath context</a>. However it is possible to
225find one from the other:</p>
226<ul>
227<li>The function xsltXPathGetTransformContext provide this lookup facility:
228    <pre>xsltTransformContextPtr
229         xsltXPathGetTransformContext
230                          (xmlXPathParserContextPtr ctxt);</pre>
231  </li>
232  <li>The <code>xmlXPathContextPtr</code> associated to an
233    <code>xsltTransformContext</code> is stored in the <code>xpathCtxt</code>
234    field.</li>
235</ul>
236<p>The first thing an extension function may want to do is to check the
237arguments passed on the stack, the <code>nargs</code> will precise how many
238of them were provided on the XPath expression. The macros valuePop will
239extract them from the XPath stack:</p>
240<pre>#include &lt;libxml/xpath.h&gt;
241#include &lt;libxml/xpathInternals.h&gt;
242
243xmlXPathObjectPtr obj = valuePop(ctxt); </pre>
244<p>Note that <code>ctxt</code> is the XPath context not the XSLT one. It is
245then possible to examine the content of the value. Check <a href="internals.html#Descriptio">the description of XPath objects</a> if
246necessary. The following is a common sequcnce checking whether the argument
247passed is a string and converting it using the built-in XPath
248<code>string()</code> function if this is not the case:</p>
249<pre>if (obj-&gt;type != XPATH_STRING) {
250    valuePush(ctxt, obj);
251    xmlXPathStringFunction(ctxt, 1);
252    obj = valuePop(ctxt);
253}</pre>
254<p>Most common XPath functions are available directly at the C level and are
255exported either in <code>&lt;libxml/xpath.h&gt;</code> or in
256<code>&lt;libxml/xpathInternals.h&gt;</code>.</p>
257<p>The extension function may also need to retrieve the data associated to
258this module instance (the database connection in the previous example) this
259can be done using the xsltGetExtData:</p>
260<pre>void * xsltGetExtData(xsltTransformContextPtr ctxt,
261                      const xmlChar *URI);</pre>
262<p>again the URI to be provided is the one used which was used when
263registering the module.</p>
264<p>Once the function finishes, don't forget to:</p>
265<ul>
266<li>push the return value on the stack using <code>valuePush(ctxt,
267    obj)</code>
268</li>
269  <li>deallocate the parameters passed to the function using
270    <code>xmlXPathFreeObject(obj)</code>
271</li>
272</ul>
273<h3><a name="Examples">Examples for extension functions</a></h3>
274<p>The module libxslt/functions.c containsthe sources of the XSLT built-in
275functions, including document(), key(), generate-id(), etc. as well as a full
276example module at the end. Here is the test function implementation for the
277libxslt:test function:</p>
278<pre>/**
279 * xsltExtFunctionTest:
280 * @ctxt:  the XPath Parser context
281 * @nargs:  the number of arguments
282 *
283 * function libxslt:test() for testing the extensions support.
284 */
285static void
286xsltExtFunctionTest(xmlXPathParserContextPtr ctxt, int nargs)
287{
288    xsltTransformContextPtr tctxt;
289    void *data;
290
291    tctxt = xsltXPathGetTransformContext(ctxt);
292    if (tctxt == NULL) {
293        xsltGenericError(xsltGenericErrorContext,
294            &quot;xsltExtFunctionTest: failed to get the transformation context\n&quot;);
295        return;
296    }
297    data = xsltGetExtData(tctxt, (const xmlChar *) XSLT_DEFAULT_URL);
298    if (data == NULL) {
299        xsltGenericError(xsltGenericErrorContext,
300            &quot;xsltExtFunctionTest: failed to get module data\n&quot;);
301        return;
302    }
303#ifdef WITH_XSLT_DEBUG_FUNCTION
304    xsltGenericDebug(xsltGenericDebugContext,
305                     &quot;libxslt:test() called with %d args\n&quot;, nargs);
306#endif
307}</pre>
308<h3><a name="Registerin2">Registering an extension element</a></h3>
309<p>There is a single call to do this registration:</p>
310<pre>int xsltRegisterExtElement(xsltTransformContextPtr ctxt,
311                           const xmlChar *name,
312                           const xmlChar *URI,
313                           xsltTransformFunction function);</pre>
314<p>It is similar to the mechanism used to register an extension function,
315except that the signature of an extension element implementation is
316different.</p>
317<p>The registration is bound to a single transformation instance referred by
318ctxt, name is the UTF8 encoded name for the NCName of the element, and URI is
319the namespace name for the extension (no checking is done, a module could
320register elements for a different namespace, but it is not recommended).</p>
321<h3><a name="Implementi1">Implementing an extension element</a></h3>
322<p>The implementation of the element must have the signature of an XSLT
323transformation function:</p>
324<pre>/**
325 * xsltTransformFunction:
326 * @ctxt: the XSLT transformation context
327 * @node: the input node
328 * @inst: the stylesheet node
329 * @comp: the compiled information from the stylesheet
330 *
331 * signature of the function associated to elements part of the
332 * stylesheet language like xsl:if or xsl:apply-templates.
333 */
334typedef void (*xsltTransformFunction)
335                          (xsltTransformContextPtr ctxt,
336                           xmlNodePtr node,
337                           xmlNodePtr inst,
338                           xsltStylePreCompPtr comp);</pre>
339<p>The first argument is the XSLT transformation context. The second and
340third arguments are xmlNodePtr i.e. internal memory <a href="internals.html#libxml">representation of  XML nodes</a>. They are
341respectively <code>node</code> from the the input document being transformed
342by the stylesheet and <code>inst</code> the extension element in the
343stylesheet. The last argument is <code>comp</code> a pointer to a precompiled
344representation of <code>inst</code> but usually for extension function this
345value is <code>NULL</code> by default (it could be added and associated to
346the instruction in <code>inst-&gt;_private</code>).</p>
347<p>The same functions are available from a function implementing an extension
348element as in an extension function, including
349<code>xsltGetExtData()</code>.</p>
350<p>The goal of extension element being usually to enrich the generated
351output, it is expected that they will grow the currently generated output
352tree, this can be done by grabbing ctxt-&gt;insert which is the current
353libxml node being generated (Note this can also be the intermediate value
354tree being built for example to initialize a variable, the processing should
355be similar). The functions for libxml tree manipulation from <a href="http://xmlsoft.org/html/libxml-tree.html">&lt;libxml/tree.h&gt;</a> can
356be employed to extend or modify the tree, but it is required to preserve the
357insertion node and its ancestors since there is existing pointers to those
358elements still in use in the XSLT template execution stack.</p>
359<h3><a name="Example">Example for extension elements</a></h3>
360<p>The module libxslt/transform.c containsthe sources of the XSLT built-in
361elements, including xsl:element, xsl:attribute, xsl:if, etc. There is a small
362but full example in functions.c providing the implementation for the
363libxslt:test element, it will output a comment in the result tree:</p>
364<pre>/**
365 * xsltExtElementTest:
366 * @ctxt:  an XSLT processing context
367 * @node:  The current node
368 * @inst:  the instruction in the stylesheet
369 * @comp:  precomputed informations
370 *
371 * Process a libxslt:test node
372 */
373static void
374xsltExtElementTest(xsltTransformContextPtr ctxt, xmlNodePtr node,
375                   xmlNodePtr inst,
376                   xsltStylePreCompPtr comp)
377{
378    xmlNodePtr comment;
379
380    if (ctxt == NULL) {
381        xsltGenericError(xsltGenericErrorContext,
382                         &quot;xsltExtElementTest: no transformation context\n&quot;);
383        return;
384    }
385    if (node == NULL) {
386        xsltGenericError(xsltGenericErrorContext,
387                         &quot;xsltExtElementTest: no current node\n&quot;);
388        return;
389    }
390    if (inst == NULL) {
391        xsltGenericError(xsltGenericErrorContext,
392                         &quot;xsltExtElementTest: no instruction\n&quot;);
393        return;
394    }
395    if (ctxt-&gt;insert == NULL) {
396        xsltGenericError(xsltGenericErrorContext,
397                         &quot;xsltExtElementTest: no insertion point\n&quot;);
398        return;
399    }
400    comment =
401        xmlNewComment((const xmlChar *)
402                      &quot;libxslt:test element test worked&quot;);
403    xmlAddChild(ctxt-&gt;insert, comment);
404}</pre>
405<h3><a name="shutdown">The shutdown of a module</a></h3>
406<p>When the XSLT processor ends a transformation, the shutdown function (if
407it exists) of all the modules initialized are called.The
408xsltExtShutdownFunction type defines the interface for a shutdown
409function:</p>
410<pre>/**
411 * xsltExtShutdownFunction:
412 * @ctxt:  an XSLT transformation context
413 * @URI:  the namespace URI for the extension
414 * @data:  the data associated to this module
415 *
416 * A function called at shutdown time of an XSLT extension module
417 */
418typedef void (*xsltExtShutdownFunction) (xsltTransformContextPtr ctxt,
419                                         const xmlChar *URI,
420                                         void *data);</pre>
421<p>this is really similar to a module initialization function except a third
422argument is passed, it's the value that was returned by the initialization
423function. This allow to deallocate resources from the module for example
424close the connection to the database to keep the same example.</p>
425<h3><a name="Future">Future work</a></h3>
426<p>Well some of the pieces missing:</p>
427<ul>
428<li>a way to load shared libraries to instanciate new modules</li>
429  <li>a better detection of extension function usage and their registration
430    without having to use the extension prefix which ought to be reserved to
431    element extensions.</li>
432  <li>more examples</li>
433  <li>implementations of the <a href="http://www.exslt.org/">EXSLT</a> common
434    extension libraries, Thomas Broyer nearly finished implementing them.</li>
435</ul>
436<p></p>
437<p><a href="bugs.html">Daniel Veillard</a></p>
438</td></tr></table></td></tr></table></td></tr></table></td>
439</tr></table></td></tr></table>
440</body>
441</html>
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