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<article>
<articleinfo>
<title>libxslt Tutorial</title>
<copyright>
<year>2001</year>
<holder>John Fleck</holder>
</copyright>
<legalnotice id="legalnotice">
<para>Permission is granted to copy, distribute and/or modify this
document under the terms of the <citetitle>GNU Free Documentation
License</citetitle>, Version 1.1 or any later version
published by the Free Software Foundation with no Invariant
Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of
the license can be found <ulink type="http"
url="http://www.gnu.org/copyleft/fdl.html">here</ulink>.</para>
</legalnotice>
<author>
<firstname>John</firstname>
<surname>Fleck</surname>
</author>
<releaseinfo>
This is version 0.4 of the libxslt Tutorial
</releaseinfo>
</articleinfo>
<abstract>
<para>A tutorial on building a simple application using the
<application>libxslt</application> library to perform
<acronym>XSLT</acronym> transformations to convert an
<acronym>XML</acronym> file into <acronym>HTML</acronym>.</para>
</abstract>
<sect1 id="introduction">
<title>Introduction</title>
<para>The Extensible Markup Language (<acronym>XML</acronym>) is a World
Wide Web Consortium standard for the exchange of structured data in text
form. Its popularity stems from its universality. Any computer can
read a text file. With the proper tools, any computer can read any other
computer's <acronym>XML</acronym> files.
</para>
<para>One of the most important of those tools is <acronym>XSLT</acronym>:
Extensible Stylesheet Language Transformations. <acronym>XSLT</acronym>
is a declarative language that allows you to
translate your <acronym>XML</acronym> into arbitrary text output
using a stylesheet. <application>libxslt</application> provides the
functions to perform the transformation.
</para>
<para><application>libxslt</application> is a free C language library
written by Daniel Veillard for the <acronym>GNOME</acronym> project
allowing you to write programs that perform <acronym>XSLT</acronym>
transformations.
<note>
<para>
While <application>libxslt</application> was written
under the auspices of the <acronym>GNOME</acronym> project, it does not
depend on any <acronym>GNOME</acronym> libraries. None are used in the
example in this tutorial.
</para>
</note>
</para>
<para>This tutorial illustrates a simple program that reads an
<acronym>XML</acronym> file, applies a stylesheet and saves the resulting
output. This is not a program you would want to create
yourself. <application>xsltproc</application>, which is included with the
<application>libxslt</application> package, does the same thing and is
more robust and full-featured. The program written for this tutorial is a
stripped-down version of <application>xsltproc</application> designed to
illustrate the functionality of <application>libxslt</application>.
</para>
<para>The full code for <application>xsltproc</application> is in
<filename>xsltproc.c</filename> in the <application>libxslt</application>
distribution. It also is available <ulink
url="http://cvs.gnome.org/lxr/source/libxslt/libxslt/xsltproc.c">on the
web</ulink>.
</para>
<para>References:
<itemizedlist>
<listitem>
<para><ulink url="http://www.w3.org/XML/">W3C <acronym>XML</acronym> page</ulink></para>
</listitem>
<listitem>
<para><ulink url="http://www.w3.org/Style/XSL/">W3C
<acronym>XSL</acronym> page.</ulink></para>
</listitem>
<listitem>
<para><ulink url="http://xmlsoft.org/XSLT/">libxslt</ulink></para>
</listitem>
</itemizedlist>
</para>
</sect1>
<sect1 id="functions">
<title>Primary Functions</title>
<para>To transform an <acronym>XML</acronym> file, you must perform three
functions:
<orderedlist>
<listitem>
<para>parse the input file</para>
</listitem>
<listitem>
<para>parse the stylesheet</para>
</listitem>
<listitem>
<para>apply the stylesheet</para>
</listitem>
</orderedlist>
</para>
<sect2 id="preparing">
<title>Preparing to Parse</title>
<para>Before you can begin parsing input files or stylesheets, there are
several steps you need to take to set up entity handling. These steps are
not unique to <application>libxslt</application>. Any
<application>libxml2</application> program that parses
<acronym>XML</acronym> files would need to take similar steps.
</para>
<para>First, you need set up some <application>libxml</application>
housekeeping. Pass the integer value <parameter>1</parameter> to the
<function>xmlSubstituteEntitiesDefault</function> function, which tells
the <application>libxml2</application> parser to substitute entities as
it parses your file. (Passing <parameter>0</parameter> causes
<application>libxml2</application> to not perform entity substitution.)
</para>
<para>Second, set <varname>xmlLoadExtDtdDefaultValue</varname> equal to
<parameter>1</parameter>. This tells <application>libxml</application>
to load external entity subsets. If you do not do this and your
input file includes entities through external subsets, you will get
errors.</para>
</sect2>
<sect2 id="parsethestylesheet">
<title>Parse the Stylesheet</title>
<para>Parsing the stylesheet takes a single function call, which takes a
variable of type <type>xmlChar</type>:
<programlisting>
<varname>cur</varname> = xsltParseStylesheetFile((const xmlChar *)argv[i]);
</programlisting>
In this case, I cast the stylesheet file name, passed in as a
command line argument, to <emphasis>xmlChar</emphasis>. The return value
is of type <emphasis>xsltStylesheetPtr</emphasis>, a struct in memory
that contains the stylesheet tree and other information about the
stylesheet. It can be manipulated directly, but for this example you
will not need to.
</para>
</sect2>
<sect2 id="parseinputfile">
<title>Parse the Input File</title>
<para>Parsing the input file takes a single function call:
<programlisting>
doc = xmlParseFile(argv[i]);
</programlisting>
It returns an <emphasis>xmlDocPtr</emphasis>, a struct in memory that
contains the document tree. It can be manipulated directly, but for this
example you will not need to.
</para>
</sect2>
<sect2 id="applyingstylesheet">
<title>Applying the Stylesheet</title>
<para>Now that you have trees representing the document and the stylesheet
in memory, apply the stylesheet to the document. The
function that does this is <function>xsltApplyStylesheet</function>:
<programlisting>
res = xsltApplyStylesheet(cur, doc, params);
</programlisting>
The function takes an xsltStylesheetPtr and an
xmlDocPtr, the values returned by the previous two functions. The third
variable, <varname>params</varname> can be used to pass
<acronym>XSLT</acronym> parameters to the stylesheet. It is a
NULL-terminated array of name/value pairs of const char's.
</para>
</sect2>
<sect2 id="saveresult">
<title>Saving the result</title>
<para><application>libxslt</application> includes a family of functions to use in
saving the resulting output. For this example,
<function>xsltSaveResultToFile</function> is used, and the results are
saved to stdout:
<programlisting>
xsltSaveResultToFile(stdout, res, cur);
</programlisting>
<note>
<para><application>libxml</application> also contains output
functions, such as <function>xmlSaveFile</function>, which can be
used here. However, output-related information contained in the
stylesheet, such as a declaration of the encoding to be used, will
be lost if one of the <application>libxslt</application> save
functions is not used.</para>
</note>
</para>
</sect2>
<sect2 id="parameters">
<title>Parameters</title>
<para>
In <acronym>XSLT</acronym>, parameters may be used as a way to pass
additional information to a
stylesheet. <application>libxslt</application> accepts
<acronym>XSLT</acronym> parameters as one of the values passed to
<function>xsltApplyStylesheet</function>.
</para>
<para>
In the tutorial example and in <application>xsltproc</application>,
on which the tutorial example is based, parameters to be passed take the
form of key-value pairs. The program collects them from command line
arguments, inserting them in the array <varname>params</varname>, then
passes them to the function. The final element in the array is set to
<parameter>NULL</parameter>.
<note>
<para>
If a parameter being passed is a string rather than an
<acronym>XSLT</acronym> node, it must be escaped. For the tutorial
program, that would be done as follows:
<command>tutorial]$ ./libxslt_tutorial --param rootid "'asect1'"
stylesheet.xsl filename.xml</command>
</para>
</note>
</para>
</sect2>
<sect2 id="cleanup">
<title>Cleanup</title>
<para>After you are finished, <application>libxslt</application> and
<application>libxml</application> provide functions for deallocating
memory.
</para>
<para>
<programlisting>
xsltFreeStylesheet(cur);<co id="cleanupstylesheet" />
xmlFreeDoc(res);<co id="cleanupresults" />
xmlFreeDoc(doc);<co id="cleanupdoc" />
xsltCleanupGlobals();<co id="cleanupglobals" />
xmlCleanupParser();<co id="cleanupparser" />
</programlisting>
<calloutlist>
<callout arearefs="cleanupstylesheet">
<para>Free the memory used by your stylesheet.</para>
</callout>
<callout arearefs="cleanupresults">
<para>Free the memory used by the results document.</para>
</callout>
<callout arearefs="cleanupdoc">
<para>Free the memory used by your original document.</para>
</callout>
<callout arearefs="cleanupglobals">
<para>Free memory used by <application>libxslt</application> global
variables</para>
</callout>
<callout arearefs="cleanupparser">
<para>Free memory used by the <acronym>XML</acronym> parser</para>
</callout>
</calloutlist>
</para>
</sect2>
</sect1>
<appendix id="thecode">
<title>The Code</title>
<para><filename>libxslt_tutorial.c</filename>
<programlisting>&CODE;</programlisting>
</para>
</appendix>
</article>
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