1 <?xml version="1.0" encoding="ISO-8859-1"?>
2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
3 <html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" /><style type="text/css">
4 TD {font-family: Verdana,Arial,Helvetica}
5 BODY {font-family: Verdana,Arial,Helvetica; margin-top: 2em; margin-left: 0em; margin-right: 0em}
6 H1 {font-family: Verdana,Arial,Helvetica}
7 H2 {font-family: Verdana,Arial,Helvetica}
8 H3 {font-family: Verdana,Arial,Helvetica}
9 A:link, A:visited, A:active { text-decoration: underline }
10 </style><title>Writing extensions</title></head><body bgcolor="#8b7765" text="#000000" link="#000000" vlink="#000000"><table border="0" width="100%" cellpadding="5" cellspacing="0" align="center"><tr><td width="120"><a href="http://swpat.ffii.org/"><img src="epatents.png" alt="Action against software patents" /></a></td><td width="100"><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></td><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"><h1>The XSLT C library for Gnome</h1><h2>Writing extensions</h2></td></tr></table></td></tr></table></td></tr></table><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><td valign="top" width="200" bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="1" width="100%" bgcolor="#000000"><tr><td><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Main Menu</b></center></td></tr><tr><td bgcolor="#fffacd"><form action="search.php" enctype="application/x-www-form-urlencoded" method="get"><input name="query" type="text" size="20" value="" /><input name="submit" type="submit" value="Search ..." /></form><ul><li><a href="index.html">Home</a></li><li><a href="intro.html">Introduction</a></li><li><a href="docs.html">Documentation</a></li><li><a href="bugs.html">Reporting bugs and getting help</a></li><li><a href="help.html">How to help</a></li><li><a href="downloads.html">Downloads</a></li><li><a href="FAQ.html">FAQ</a></li><li><a href="news.html">News</a></li><li><a href="xsltproc2.html">The xsltproc tool</a></li><li><a href="docbook.html">DocBook</a></li><li><a href="API.html">The programming API</a></li><li><a href="python.html">Python and bindings</a></li><li><a href="internals.html">Library internals</a></li><li><a href="extensions.html">Writing extensions</a></li><li><a href="contribs.html">Contributions</a></li><li><a href="EXSLT/index.html" style="font-weight:bold">libexslt</a></li><li><a href="xslt.html">flat page</a>, <a href="site.xsl">stylesheet</a></li><li><a href="html/index.html" style="font-weight:bold">API Menu</a></li></ul></td></tr></table><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Related links</b></center></td></tr><tr><td bgcolor="#fffacd"><ul><li><a href="tutorial/libxslttutorial.html">Tutorial</a></li><li><a href="xsltproc.html">Man page for xsltproc</a></li><li><a href="http://mail.gnome.org/archives/xslt/">Mail archive</a></li><li><a href="http://xmlsoft.org/">XML libxml</a></li><li><a href="http://phd.cs.unibo.it/gdome2/">DOM gdome2</a></li><li><a href="ftp://xmlsoft.org/">FTP</a></li><li><a href="http://www.zlatkovic.com/projects/libxml/">Windows binaries</a></li><li><a href="http://garypennington.net/libxml2/">Solaris binaries</a></li><li><a href="http://www.zveno.com/open_source/libxml2xslt.html">MacOsX binaries</a></li><li><a href="http://sourceforge.net/projects/libxml2-pas/">Pascal bindings</a></li><li><a href="http://bugzilla.gnome.org/buglist.cgi?product=libxslt">Bug Tracker</a></li><li><a href="http://xsldbg.sourceforge.net/">Xsldbg Debugger</a></li><li><a href="http://www.mod-xslt.com/mod-xslt/">Apache module</a></li></ul></td></tr></table><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>API Indexes</b></center></td></tr><tr><td bgcolor="#fffacd"><ul><li><a href="APIchunk0.html">Alphabetic</a></li><li><a href="APIconstructors.html">Constructors</a></li><li><a href="APIfunctions.html">Functions/Types</a></li><li><a href="APIfiles.html">Modules</a></li><li><a href="APIsymbols.html">Symbols</a></li></ul></td></tr></table></td></tr></table></td><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"><h3>Table of content</h3><ul><li><a href="extensions.html#Introducti">Introduction</a></li>
11 <li><a href="extensions.html#Basics">Basics</a></li>
12 <li><a href="extensions.html#Keep">Extension modules</a></li>
13 <li><a href="extensions.html#Registerin">Registering a module</a></li>
14 <li><a href="extensions.html#module">Loading a module</a></li>
15 <li><a href="extensions.html#Registerin1">Registering an extension
17 <li><a href="extensions.html#Implementi">Implementing an extension
19 <li><a href="extensions.html#Examples">Examples for extension
21 <li><a href="extensions.html#Registerin2">Registering an extension
23 <li><a href="extensions.html#Implementi1">Implementing an extension
25 <li><a href="extensions.html#Example">Example for extension
27 <li><a href="extensions.html#shutdown">The shutdown of a module</a></li>
28 <li><a href="extensions.html#Future">Future work</a></li>
29 </ul><h3><a name="Introducti1" id="Introducti1">Introduction</a></h3><p>This document describes the work needed to write extensions to the
30 standard 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><p>Before starting reading this document it is highly recommended to get
31 familiar with <a href="internals.html">the libxslt internals</a>.</p><p>Note: this documentation is by definition incomplete and I am not good at
32 spelling, grammar, so patches and suggestions are <a href="mailto:veillard@redhat.com">really welcome</a>.</p><h3><a name="Basics" id="Basics">Basics</a></h3><p>The <a href="http://www.w3.org/TR/xslt">XSLT specification</a> provides
33 two <a href="http://www.w3.org/TR/xslt">ways to extend an XSLT engine</a>:</p><ul><li>providing <a href="http://www.w3.org/TR/xslt">new extension
34 functions</a> which can be called from XPath expressions</li>
35 <li>providing <a href="http://www.w3.org/TR/xslt">new extension
36 elements</a> which can be inserted in stylesheets</li>
37 </ul><p>In both cases the extensions need to be associated to a new namespace,
38 i.e. an URI used as the name for the extension's namespace (there is no need
39 to have a resource there for this to work).</p><p>libxslt provides a few extensions itself, either in libxslt namespace
40 "http://xmlsoft.org/XSLT/" or in other namespace for well known extensions
41 provided by other XSLT processors like Saxon, Xalan or XT.</p><h3><a name="Keep" id="Keep">Extension modules</a></h3><p>Since extensions are bound to a namespace name, usually sets of extensions
42 coming from a given source are using the same namespace name defining in
43 practice a group of extensions providing elements, functions or both. From
44 libxslt point of view those are considered as an "extension module", and most
45 of the APIs work at a module point of view.</p><p>Registration of new functions or elements are bound to the activation of
46 the module, this is currently done by declaring the namespace as an extension
47 by using the attribute <code>extension-element-prefixes</code> on the
48 <code><a href="http://www.w3.org/TR/xslt">xsl:stylesheet</a></code>
49 element.</p><p>And extension module is defined by 3 objects:</p><ul><li>the namespace name associated</li>
50 <li>an initialization function</li>
51 <li>a shutdown function</li>
52 </ul><h3><a name="Registerin" id="Registerin">Registering a module</a></h3><p>Currently a libxslt module has to be compiled within the application using
53 libxslt, there is no code to load dynamically shared libraries associated to
54 namespace (this may be added but is likely to become a portability
55 nightmare).</p><p>So the current way to register a module is to link the code implementing
56 it with the application and to call a registration function:</p><pre>int xsltRegisterExtModule(const xmlChar *URI,
57 xsltExtInitFunction initFunc,
58 xsltExtShutdownFunction shutdownFunc);</pre><p>The associated header is read by:</p><pre>#include<libxslt/extensions.h></pre><p>which also defines the type for the initialization and shutdown
59 functions</p><h3><a name="module" id="module">Loading a module</a></h3><p>Once the module URI has been registered and if the XSLT processor detects
60 that a given stylesheet needs the functionalities of an extended module, this
61 one is initialized.</p><p>The xsltExtInitFunction type defines the interface for an initialization
63 * xsltExtInitFunction:
64 * @ctxt: an XSLT transformation context
65 * @URI: the namespace URI for the extension
67 * A function called at initialization time of an XSLT
70 * Returns a pointer to the module specific data for this
73 typedef void *(*xsltExtInitFunction)(xsltTransformContextPtr ctxt,
74 const xmlChar *URI);</pre><p>There are 3 things to notice:</p><ul><li>the function gets passed the namespace name URI as an argument, this
75 allow a single function to provide the initialization for multiple
77 <li>it also gets passed a transformation context, the initialization is
78 done at run time before any processing occurs on the stylesheet but it
79 will be invoked separately each time for each transformation</li>
80 <li>it returns a pointer, this can be used to store module specific
81 informations which can be retrieved later when a function or an element
82 from the extension are used, an obvious example is a connection to a
83 database which should be kept and reused along the transformation. NULL
84 is a perfectly valid return, there is no way to indicate a failure at
86 </ul><p>What this function is expected to do is:</p><ul><li>prepare the context for this module (like opening the database
88 <li>register the extensions specific to this module</li>
89 </ul><h3><a name="Registerin1" id="Registerin1">Registering an extension function</a></h3><p>There is a single call to do this registration:</p><pre>int xsltRegisterExtFunction(xsltTransformContextPtr ctxt,
92 xmlXPathEvalFunc function);</pre><p>The registration is bound to a single transformation instance referred by
93 ctxt, name is the UTF8 encoded name for the NCName of the function, and URI
94 is the namespace name for the extension (no checking is done, a module could
95 register functions or elements from a different namespace, but it is not
96 recommended).</p><h3><a name="Implementi" id="Implementi">Implementing an extension function</a></h3><p>The implementation of the function must have the signature of a libxml
97 XPath function:</p><pre>/**
99 * @ctxt: an XPath parser context
100 * @nargs: the number of arguments passed to the function
102 * an XPath evaluation function, the parameters are on the
103 * XPath context stack
106 typedef void (*xmlXPathEvalFunc)(xmlXPathParserContextPtr ctxt,
107 int nargs);</pre><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
108 find one from the other:</p><ul><li>The function xsltXPathGetTransformContext provide this lookup facility:
109 <pre>xsltTransformContextPtr
110 xsltXPathGetTransformContext
111 (xmlXPathParserContextPtr ctxt);</pre>
113 <li>The <code>xmlXPathContextPtr</code> associated to an
114 <code>xsltTransformContext</code> is stored in the <code>xpathCtxt</code>
116 </ul><p>The first thing an extension function may want to do is to check the
117 arguments passed on the stack, the <code>nargs</code> will precise how many
118 of them were provided on the XPath expression. The macros valuePop will
119 extract them from the XPath stack:</p><pre>#include <libxml/xpath.h>
120 #include <libxml/xpathInternals.h>
122 xmlXPathObjectPtr obj = valuePop(ctxt); </pre><p>Note that <code>ctxt</code> is the XPath context not the XSLT one. It is
123 then possible to examine the content of the value. Check <a href="internals.html#Descriptio">the description of XPath objects</a> if
124 necessary. The following is a common sequcnce checking whether the argument
125 passed is a string and converting it using the built-in XPath
126 <code>string()</code> function if this is not the case:</p><pre>if (obj->type != XPATH_STRING) {
127 valuePush(ctxt, obj);
128 xmlXPathStringFunction(ctxt, 1);
129 obj = valuePop(ctxt);
130 }</pre><p>Most common XPath functions are available directly at the C level and are
131 exported either in <code><libxml/xpath.h></code> or in
132 <code><libxml/xpathInternals.h></code>.</p><p>The extension function may also need to retrieve the data associated to
133 this module instance (the database connection in the previous example) this
134 can be done using the xsltGetExtData:</p><pre>void * xsltGetExtData(xsltTransformContextPtr ctxt,
135 const xmlChar *URI);</pre><p>again the URI to be provided is the one used which was used when
136 registering the module.</p><p>Once the function finishes, don't forget to:</p><ul><li>push the return value on the stack using <code>valuePush(ctxt,
138 <li>deallocate the parameters passed to the function using
139 <code>xmlXPathFreeObject(obj)</code></li>
140 </ul><h3><a name="Examples" id="Examples">Examples for extension functions</a></h3><p>The module libxslt/functions.c containsthe sources of the XSLT built-in
141 functions, including document(), key(), generate-id(), etc. as well as a full
142 example module at the end. Here is the test function implementation for the
143 libxslt:test function:</p><pre>/**
144 * xsltExtFunctionTest:
145 * @ctxt: the XPath Parser context
146 * @nargs: the number of arguments
148 * function libxslt:test() for testing the extensions support.
151 xsltExtFunctionTest(xmlXPathParserContextPtr ctxt, int nargs)
153 xsltTransformContextPtr tctxt;
156 tctxt = xsltXPathGetTransformContext(ctxt);
158 xsltGenericError(xsltGenericErrorContext,
159 "xsltExtFunctionTest: failed to get the transformation context\n");
162 data = xsltGetExtData(tctxt, (const xmlChar *) XSLT_DEFAULT_URL);
164 xsltGenericError(xsltGenericErrorContext,
165 "xsltExtFunctionTest: failed to get module data\n");
168 #ifdef WITH_XSLT_DEBUG_FUNCTION
169 xsltGenericDebug(xsltGenericDebugContext,
170 "libxslt:test() called with %d args\n", nargs);
172 }</pre><h3><a name="Registerin2" id="Registerin2">Registering an extension element</a></h3><p>There is a single call to do this registration:</p><pre>int xsltRegisterExtElement(xsltTransformContextPtr ctxt,
175 xsltTransformFunction function);</pre><p>It is similar to the mechanism used to register an extension function,
176 except that the signature of an extension element implementation is
177 different.</p><p>The registration is bound to a single transformation instance referred by
178 ctxt, name is the UTF8 encoded name for the NCName of the element, and URI is
179 the namespace name for the extension (no checking is done, a module could
180 register elements for a different namespace, but it is not recommended).</p><h3><a name="Implementi1" id="Implementi1">Implementing an extension element</a></h3><p>The implementation of the element must have the signature of an XSLT
181 transformation function:</p><pre>/**
182 * xsltTransformFunction:
183 * @ctxt: the XSLT transformation context
184 * @node: the input node
185 * @inst: the stylesheet node
186 * @comp: the compiled information from the stylesheet
188 * signature of the function associated to elements part of the
189 * stylesheet language like xsl:if or xsl:apply-templates.
191 typedef void (*xsltTransformFunction)
192 (xsltTransformContextPtr ctxt,
195 xsltStylePreCompPtr comp);</pre><p>The first argument is the XSLT transformation context. The second and
196 third arguments are xmlNodePtr i.e. internal memory <a href="internals.html#libxml">representation of XML nodes</a>. They are
197 respectively <code>node</code> from the the input document being transformed
198 by the stylesheet and <code>inst</code> the extension element in the
199 stylesheet. The last argument is <code>comp</code> a pointer to a precompiled
200 representation of <code>inst</code> but usually for extension function this
201 value is <code>NULL</code> by default (it could be added and associated to
202 the instruction in <code>inst->_private</code>).</p><p>The same functions are available from a function implementing an extension
203 element as in an extension function, including
204 <code>xsltGetExtData()</code>.</p><p>The goal of extension element being usually to enrich the generated
205 output, it is expected that they will grow the currently generated output
206 tree, this can be done by grabbing ctxt->insert which is the current
207 libxml node being generated (Note this can also be the intermediate value
208 tree being built for example to initialize a variable, the processing should
209 be similar). The functions for libxml tree manipulation from <a href="http://xmlsoft.org/html/libxml-tree.html"><libxml/tree.h></a> can
210 be employed to extend or modify the tree, but it is required to preserve the
211 insertion node and its ancestors since there is existing pointers to those
212 elements still in use in the XSLT template execution stack.</p><h3><a name="Example" id="Example">Example for extension elements</a></h3><p>The module libxslt/transform.c containsthe sources of the XSLT built-in
213 elements, including xsl:element, xsl:attribute, xsl:if, etc. There is a small
214 but full example in functions.c providing the implementation for the
215 libxslt:test element, it will output a comment in the result tree:</p><pre>/**
216 * xsltExtElementTest:
217 * @ctxt: an XSLT processing context
218 * @node: The current node
219 * @inst: the instruction in the stylesheet
220 * @comp: precomputed informations
222 * Process a libxslt:test node
225 xsltExtElementTest(xsltTransformContextPtr ctxt, xmlNodePtr node,
227 xsltStylePreCompPtr comp)
232 xsltGenericError(xsltGenericErrorContext,
233 "xsltExtElementTest: no transformation context\n");
237 xsltGenericError(xsltGenericErrorContext,
238 "xsltExtElementTest: no current node\n");
242 xsltGenericError(xsltGenericErrorContext,
243 "xsltExtElementTest: no instruction\n");
246 if (ctxt->insert == NULL) {
247 xsltGenericError(xsltGenericErrorContext,
248 "xsltExtElementTest: no insertion point\n");
252 xmlNewComment((const xmlChar *)
253 "libxslt:test element test worked");
254 xmlAddChild(ctxt->insert, comment);
255 }</pre><h3><a name="shutdown" id="shutdown">The shutdown of a module</a></h3><p>When the XSLT processor ends a transformation, the shutdown function (if
256 it exists) of all the modules initialized are called.The
257 xsltExtShutdownFunction type defines the interface for a shutdown
258 function:</p><pre>/**
259 * xsltExtShutdownFunction:
260 * @ctxt: an XSLT transformation context
261 * @URI: the namespace URI for the extension
262 * @data: the data associated to this module
264 * A function called at shutdown time of an XSLT extension module
266 typedef void (*xsltExtShutdownFunction) (xsltTransformContextPtr ctxt,
268 void *data);</pre><p>this is really similar to a module initialization function except a third
269 argument is passed, it's the value that was returned by the initialization
270 function. This allow to deallocate resources from the module for example
271 close the connection to the database to keep the same example.</p><h3><a name="Future" id="Future">Future work</a></h3><p>Well some of the pieces missing:</p><ul><li>a way to load shared libraries to instanciate new modules</li>
272 <li>a better detection of extension function usage and their registration
273 without having to use the extension prefix which ought to be reserved to
274 element extensions.</li>
275 <li>more examples</li>
276 <li>implementations of the <a href="http://www.exslt.org/">EXSLT</a> common
277 extension libraries, Thomas Broyer nearly finished implementing them.</li>
278 </ul><p></p><p><a href="bugs.html">Daniel Veillard</a></p></td></tr></table></td></tr></table></td></tr></table></td></tr></table></td></tr></table></body></html>