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<h3>Table of content</h3>
<ul>
<li><a href="extensions.html#Introducti">Introduction</a></li>
<li><a href="extensions.html#Basics">Basics</a></li>
<li><a href="extensions.html#Keep">Extension modules</a></li>
<li><a href="extensions.html#Registerin">Registering a module</a></li>
<li><a href="extensions.html#module">Loading a module</a></li>
<li><a href="extensions.html#Registerin1">Registering an extension
function</a></li>
<li><a href="extensions.html#Implementi">Implementing an extension
function</a></li>
<li><a href="extensions.html#Examples">Examples for extension
functions</a></li>
<li><a href="extensions.html#Registerin2">Registering an extension
element</a></li>
<li><a href="extensions.html#Implementi1">Implementing an extension
element</a></li>
<li><a href="extensions.html#Example">Example for extension
elements</a></li>
<li><a href="extensions.html#shutdown">The shutdown of a module</a></li>
<li><a href="extensions.html#Future">Future work</a></li>
</ul>
<h3><a name="Introducti1">Introduction</a></h3>
<p>This document describes the work needed to write extensions to the
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 developped for the <a href="http://www.gnome.org/">Gnome</a> project.</p>
<p>Before starting reading this document it is highly recommended to get
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
spelling, grammar, so patches and suggestions are <a href="mailto:veillard@redhat.com">really welcome</a>.</p>
<h3><a name="Basics">Basics</a></h3>
<p>The <a href="http://www.w3.org/TR/xslt">XSLT specification</a> provides
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
functions</a> which can be called from XPath expressions</li>
<li>providing <a href="http://www.w3.org/TR/xslt">new extension
elements</a> which can be inserted in stylesheets</li>
</ul>
<p>In both cases the extensions need to be associated to a new namespace,
i.e. an URI used as the name for the extension's namespace (there is no need
to have a resource there for this to work).</p>
<p>libxslt provides a few extensions itself, either in libxslt namespace
"http://xmlsoft.org/XSLT/" or in other namespace for well known extensions
provided by other XSLT processors like Saxon, Xalan or XT.</p>
<h3><a name="Keep">Extension modules</a></h3>
<p>Since extensions are bound to a namespace name, usually sets of extensions
coming from a given source are using the same namespace name defining in
practice a group of extensions providing elements, functions or both. From
libxslt point of view those are considered as an "extension module", and most
of the APIs work at a module point of view.</p>
<p>Registration of new functions or elements are bound to the activation of
the module, this is currently done by declaring the namespace as an extension
by using the attribute <code>extension-element-prefixes</code> on the
<code><a href="http://www.w3.org/TR/xslt">xsl:stylesheet</a></code>
element.</p>
<p>And extension module is defined by 3 objects:</p>
<ul>
<li>the namespace name associated</li>
<li>an initialization function</li>
<li>a shutdown function</li>
</ul>
<h3><a name="Registerin">Registering a module</a></h3>
<p>Currently a libxslt module has to be compiled within the application using
libxslt, there is no code to load dynamically shared libraries associated to
namespace (this may be added but is likely to become a portability
nightmare).</p>
<p>So the current way to register a module is to link the code implementing
it with the application and to call a registration function:</p>
<pre>int xsltRegisterExtModule(const xmlChar *URI,
xsltExtInitFunction initFunc,
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
functions</p>
<h3><a name="module">Loading a module</a></h3>
<p>Once the module URI has been registered and if the XSLT processor detects
that a given stylesheet needs the functionalities of an extended module, this
one is initialized.</p>
<p>The xsltExtInitFunction type defines the interface for an initialization
function:</p>
<pre>/**
* xsltExtInitFunction:
* @ctxt: an XSLT transformation context
* @URI: the namespace URI for the extension
*
* A function called at initialization time of an XSLT
* extension module
*
* Returns a pointer to the module specific data for this
* transformation
*/
typedef void *(*xsltExtInitFunction)(xsltTransformContextPtr ctxt,
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
allow a single function to provide the initialization for multiple
logical modules</li>
<li>it also gets passed a transformation context, the initialization is
done at run time before any processing occurs on the stylesheet but it
will be invoked separately each time for each transformation</li>
<li>it returns a pointer, this can be used to store module specific
informations which can be retrieved later when a function or an element
from the extension are used, an obvious example is a connection to a
database which should be kept and reused along the transformation. NULL
is a perfectly valid return, there is no way to indicate a failure at
this level</li>
</ul>
<p>What this function is expected to do is:</p>
<ul>
<li>prepare the context for this module (like opening the database
connection)</li>
<li>register the extensions specific to this module</li>
</ul>
<h3><a name="Registerin1">Registering an extension function</a></h3>
<p>There is a single call to do this registration:</p>
<pre>int xsltRegisterExtFunction(xsltTransformContextPtr ctxt,
const xmlChar *name,
const xmlChar *URI,
xmlXPathEvalFunc function);</pre>
<p>The registration is bound to a single transformation instance referred by
ctxt, name is the UTF8 encoded name for the NCName of the function, and URI
is the namespace name for the extension (no checking is done, a module could
register functions or elements from a different namespace, but it is not
recommended).</p>
<h3><a name="Implementi">Implementing an extension function</a></h3>
<p>The implementation of the function must have the signature of a libxml
XPath function:</p>
<pre>/**
* xmlXPathEvalFunc:
* @ctxt: an XPath parser context
* @nargs: the number of arguments passed to the function
*
* an XPath evaluation function, the parameters are on the
* XPath context stack
*/
typedef void (*xmlXPathEvalFunc)(xmlXPathParserContextPtr ctxt,
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
find one from the other:</p>
<ul>
<li>The function xsltXPathGetTransformContext provide this lookup facility:
<pre>xsltTransformContextPtr
xsltXPathGetTransformContext
(xmlXPathParserContextPtr ctxt);</pre>
</li>
<li>The <code>xmlXPathContextPtr</code> associated to an
<code>xsltTransformContext</code> is stored in the <code>xpathCtxt</code>
field.</li>
</ul>
<p>The first thing an extension function may want to do is to check the
arguments passed on the stack, the <code>nargs</code> will precise how many
of them were provided on the XPath expression. The macros valuePop will
extract them from the XPath stack:</p>
<pre>#include <libxml/xpath.h>
#include <libxml/xpathInternals.h>
xmlXPathObjectPtr obj = valuePop(ctxt); </pre>
<p>Note that <code>ctxt</code> is the XPath context not the XSLT one. It is
then possible to examine the content of the value. Check <a href="internals.html#Descriptio">the description of XPath objects</a> if
necessary. The following is a common sequcnce checking whether the argument
passed is a string and converting it using the built-in XPath
<code>string()</code> function if this is not the case:</p>
<pre>if (obj->type != XPATH_STRING) {
valuePush(ctxt, obj);
xmlXPathStringFunction(ctxt, 1);
obj = valuePop(ctxt);
}</pre>
<p>Most common XPath functions are available directly at the C level and are
exported either in <code><libxml/xpath.h></code> or in
<code><libxml/xpathInternals.h></code>.</p>
<p>The extension function may also need to retrieve the data associated to
this module instance (the database connection in the previous example) this
can be done using the xsltGetExtData:</p>
<pre>void * xsltGetExtData(xsltTransformContextPtr ctxt,
const xmlChar *URI);</pre>
<p>again the URI to be provided is the one used which was used when
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,
obj)</code>
</li>
<li>deallocate the parameters passed to the function using
<code>xmlXPathFreeObject(obj)</code>
</li>
</ul>
<h3><a name="Examples">Examples for extension functions</a></h3>
<p>The module libxslt/functions.c containsthe sources of the XSLT built-in
functions, including document(), key(), generate-id(), etc. as well as a full
example module at the end. Here is the test function implementation for the
libxslt:test function:</p>
<pre>/**
* xsltExtFunctionTest:
* @ctxt: the XPath Parser context
* @nargs: the number of arguments
*
* function libxslt:test() for testing the extensions support.
*/
static void
xsltExtFunctionTest(xmlXPathParserContextPtr ctxt, int nargs)
{
xsltTransformContextPtr tctxt;
void *data;
tctxt = xsltXPathGetTransformContext(ctxt);
if (tctxt == NULL) {
xsltGenericError(xsltGenericErrorContext,
"xsltExtFunctionTest: failed to get the transformation context\n");
return;
}
data = xsltGetExtData(tctxt, (const xmlChar *) XSLT_DEFAULT_URL);
if (data == NULL) {
xsltGenericError(xsltGenericErrorContext,
"xsltExtFunctionTest: failed to get module data\n");
return;
}
#ifdef WITH_XSLT_DEBUG_FUNCTION
xsltGenericDebug(xsltGenericDebugContext,
"libxslt:test() called with %d args\n", nargs);
#endif
}</pre>
<h3><a name="Registerin2">Registering an extension function</a></h3>
<p>There is a single call to do this registration:</p>
<pre>int xsltRegisterExtElement(xsltTransformContextPtr ctxt,
const xmlChar *name,
const xmlChar *URI,
xsltTransformFunction function);</pre>
<p>It is similar to the mechanism used to register an extension function,
except that the signature of an extension element implementation is
different.</p>
<p>The registration is bound to a single transformation instance referred by
ctxt, name is the UTF8 encoded name for the NCName of the element, and URI is
the namespace name for the extension (no checking is done, a module could
register elements for a different namespace, but it is not recommended).</p>
<h3><a name="Implementi1">Implementing an extension element</a></h3>
<p>The implementation of the element must have the signature of an XSLT
transformation function:</p>
<pre>/**
* xsltTransformFunction:
* @ctxt: the XSLT transformation context
* @node: the input node
* @inst: the stylesheet node
* @comp: the compiled information from the stylesheet
*
* signature of the function associated to elements part of the
* stylesheet language like xsl:if or xsl:apply-templates.
*/
typedef void (*xsltTransformFunction)
(xsltTransformContextPtr ctxt,
xmlNodePtr node,
xmlNodePtr inst,
xsltStylePreCompPtr comp);</pre>
<p>The first argument is the XSLT transformation context. The second and
third arguments are xmlNodePtr i.e. internal memory <a href="internals.html#libxml">representation of XML nodes</a>. They are
respectively <code>node</code> from the the input document being transformed
by the stylesheet and <code>inst</code> the extension element in the
stylesheet. The last argument is <code>comp</code> a pointer to a precompiled
representation of <code>inst</code> but usually for extension function this
value is <code>NULL</code> by default (it could be added and associated to
the instruction in <code>inst->_private</code>).</p>
<p>The same functions are available from a function implementing an extension
element as in an extension function, including
<code>xsltGetExtData()</code>.</p>
<p>The goal of extension element being usually to enrich the generated
output, it is expected that they will grow the currently generated output
tree, this can be done by grabbing ctxt->insert which is the current
libxml node being generated (Note this can also be the intermediate value
tree being built for example to initialize a variable, the processing should
be similar). The functions for libxml tree manipulation from <a href="http://xmlsoft.org/html/libxml-tree.html"><libxml/tree.h></a> can
be employed to extend or modify the tree, but it is required to preserve the
insertion node and its ancestors since there is existing pointers to those
elements still in use in the XSLT template execution stack.</p>
<h3><a name="Example">Example for extension elements</a></h3>
<p>The module libxslt/transform.c containsthe sources of the XSLT built-in
elements, including xsl:element, xsl:attribute, xsl:if, etc. There is a small
but full example in functions.c providing the implementation for the
libxslt:test element, it will output a comment in the result tree:</p>
<pre>/**
* xsltExtElementTest:
* @ctxt: an XSLT processing context
* @node: The current node
* @inst: the instruction in the stylesheet
* @comp: precomputed informations
*
* Process a libxslt:test node
*/
static void
xsltExtElementTest(xsltTransformContextPtr ctxt, xmlNodePtr node,
xmlNodePtr inst,
xsltStylePreCompPtr comp)
{
xmlNodePtr comment;
if (ctxt == NULL) {
xsltGenericError(xsltGenericErrorContext,
"xsltExtElementTest: no transformation context\n");
return;
}
if (node == NULL) {
xsltGenericError(xsltGenericErrorContext,
"xsltExtElementTest: no current node\n");
return;
}
if (inst == NULL) {
xsltGenericError(xsltGenericErrorContext,
"xsltExtElementTest: no instruction\n");
return;
}
if (ctxt->insert == NULL) {
xsltGenericError(xsltGenericErrorContext,
"xsltExtElementTest: no insertion point\n");
return;
}
comment =
xmlNewComment((const xmlChar *)
"libxslt:test element test worked");
xmlAddChild(ctxt->insert, comment);
}</pre>
<h3><a name="shutdown">The shutdown of a module</a></h3>
<p>When the XSLT processor ends a transformation, the shutdown function (if
it exists) of all the modules initialized are called.The
xsltExtShutdownFunction type defines the interface for a shutdown
function:</p>
<pre>/**
* xsltExtShutdownFunction:
* @ctxt: an XSLT transformation context
* @URI: the namespace URI for the extension
* @data: the data associated to this module
*
* A function called at shutdown time of an XSLT extension module
*/
typedef void (*xsltExtShutdownFunction) (xsltTransformContextPtr ctxt,
const xmlChar *URI,
void *data);</pre>
<p>this is really similar to a module initialization function except a third
argument is passed, it's the value that was returned by the initialization
function. This allow to deallocate resources from the module for example
close the connection to the database to keep the same example.</p>
<h3><a name="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>
<li>a better detection of extension function usage and their registration
without having to use the extension prefix which ought to be reserved to
element extensions.</li>
<li>more examples</li>
<li>implementations of the <a href="http://www.exslt.org/">EXSLT</a> common
extension libraries, Thomas Broyer nearly finished implementing them.</li>
</ul>
<p>
<p><a href="mailto:daniel@veillard.com">Daniel Veillard</a></p>
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