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<?xml version="1.0"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" [
<!ENTITY KEYWORD SYSTEM "includekeyword.c">
<!ENTITY STORY SYSTEM "includestory.xml">
<!ENTITY ADDKEYWORD SYSTEM "includeaddkeyword.c">
<!ENTITY ADDATTRIBUTE SYSTEM "includeaddattribute.c">
]>
<article>
<articleinfo>
<title>Libxml Tutorial</title>
<author>
<firstname>John</firstname>
<surname>Fleck</surname>
</author>
<copyright>
<year>2002</year>
<holder>John Fleck</holder>
</copyright>
<revhistory>
<revision>
<revnumber>1</revnumber>
<date>June 4,2002</date>
</revision>
</revhistory>
</articleinfo>
<abstract>
<para>Libxml is a freely licensed C language library for handling
<acronym>XML</acronym>, portable across a large number of platforms. This
tutorial provides examples of its basic functions.</para>
</abstract>
<sect1 id="introduction">
<title>Introduction</title>
<para>Libxml is a C language library implementing functions for reading,
creating and manipulating <acronym>XML</acronym> data. This tutorial
provides example code and explanations of its basic functionality.</para>
<para>Libxml and more details about its use are available on <ulink
url="http://www.xmlsoft.org/">the project home page</ulink>. Included there is complete <ulink url="http://xmlsoft.org/html/libxml-lib.html">
<acronym>API</acronym> documentation</ulink>. This tutorial is not meant
to substitute for that complete documentation, but to illustrate the
functions needed to use the library to perform basic operations.
<!--
Links to
other resources can be found in <xref linkend="furtherresources" />.
-->
</para>
<para>The tutorial is based on a simple <acronym>XML</acronym> application I
use for articles I write. The format includes metadata and the body
of the article.</para>
<para>The example code in this tutorial demonstrates how to:
<itemizedlist>
<listitem>
<para>Parse the document.</para>
</listitem>
<listitem>
<para>Extract the text within a specified element.</para>
</listitem>
<listitem>
<para>Add an element and its content.</para>
</listitem>
<listitem>
<para>Extract the value of an attribute.</para>
</listitem>
<listitem>
<para>Add an attribute.</para>
</listitem>
</itemizedlist>
</para>
<para>Full code for the examples is included in the appendices.</para>
</sect1>
<sect1 id="xmltutorialdatatypes">
<title>Data Types</title>
<para><application>Libxml</application> declares a number of datatypes we
will encounter repeatedly, hiding the messy stuff so you do not have to deal
with it unless you have some specific need.</para>
<para>
<variablelist>
<varlistentry>
<term><ulink
url="http://xmlsoft.org/html/libxml-tree.html#XMLCHAR">xmlChar</ulink></term>
<listitem>
<para>A basic replacement for char, a byte in a UTF-8 encoded
string.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<ulink url="http://xmlsoft.org/html/libxml-tree.html#XMLDOC">xmlDoc</ulink></term>
<listitem>
<para>A structure containing the tree created by a parsed doc. <ulink
url="http://xmlsoft.org/html/libxml-tree.html#XMLDOCPTR">xmlDocPtr</ulink>
is a pointer to the structure.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><ulink
url="http://xmlsoft.org/html/libxml-tree.html#XMLNODEPTR">xmlNodePtr</ulink>
and <ulink url="http://xmlsoft.org/html/libxml-tree.html#XMLNODE">xmlNode</ulink></term>
<listitem>
<para>A structure containing a single node. <ulink
url="http://xmlsoft.org/html/libxml-tree.html#XMLNODEPTR">xmlNodePtr</ulink>
is a pointer to the structure, and is used in traversing the document tree.</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</sect1>
<sect1 id="xmltutorialparsing">
<title>Parsing the file</title>
<para>Parsing the file requires only the name of the file and a single
function call, plus error checking. Full code: <xref
linkend="keywordappendix" /></para>
<para>
<programlisting>
<co id="declaredoc" /> xmlDocPtr doc;
<co id="declarenode" /> xmlNodePtr cur;
<co id="parsefile" /> doc = xmlParseFile(docname);
<co id="checkparseerror" /> if (doc == NULL ) {
fprintf(stderr,"Document not parsed successfully. \n");
return;
}
<co id="getrootelement" /> cur = xmlDocGetRootElement(doc);
<co id="checkemptyerror" /> if (cur == NULL) {
fprintf(stderr,"empty document\n");
xmlFreeDoc(doc);
return;
}
<co id="checkroottype" /> if (xmlStrcmp(cur->name, (const xmlChar *) "story")) {
fprintf(stderr,"document of the wrong type, root node != story");
xmlFreeDoc(doc);
return;
}
</programlisting>
<calloutlist>
<callout arearefs="declaredoc">
<para>Declare the pointer that will point to your parsed document.</para>
</callout>
<callout arearefs="declarenode">
<para>Declare a node pointer (you'll need this in order to
interact with individual nodes).</para>
</callout>
<callout arearefs="checkparseerror">
<para>Check to see that the document was successfully parsed.</para>
</callout>
<callout arearefs="getrootelement">
<para>Retrieve the document's root element.</para>
</callout>
<callout arearefs="checkemptyerror">
<para>Check to make sure the document actually contains something.</para>
</callout>
<callout arearefs="checkroottype">
<para>In our case, we need to make sure the document is the right
type. &quot;story&quot; is the root type of my documents.</para>
</callout>
</calloutlist>
</para>
</sect1>
<sect1 id="xmltutorialgettext">
<title>Retrieving Element Content</title>
<para>Retrieving the content of an element involves traversing the document
tree until you find what you are looking for. In this case, we are looking
for an element called &quot;keyword&quot; contained within element called &quot;story&quot;. The
process to find the node we are interested in involves tediously walking the
tree. We assume you already have an xmlDocPtr called <varname>doc</varname>
and an xmlNodPtr called <varname>cur</varname>.</para>
<para>
<programlisting>
<co id="getchildnode" /> cur = cur->xmlChildrenNode;
<co id="huntstoryinfo" /> while (cur != NULL) {
if ((!xmlStrcmp(cur->name, (const xmlChar *)"storyinfo"))){
parseStory (doc, cur);
}
cur = cur->next;
}
</programlisting>
<calloutlist>
<callout arearefs="getchildnode">
<para>Get the first child node of <varname>cur</varname>. At this
point, <varname>cur</varname> points at the document root, which is
the element &quot;story&quot;.</para>
</callout>
<callout arearefs="huntstoryinfo">
<para>This loop iterates through the elements that are children of
&quot;story&quot;, looking for one called &quot;storyinfo&quot;. That
is the element that will contain the &quot;keywords&quot; we are
looking for. It uses the <application>libxml</application> string
comparison
function, <function><ulink
url="http://xmlsoft.org/html/libxml-parser.html#XMLSTRCMP">xmlStrcmp</ulink></function>. If there is a match, it calls the function <function>parseStory</function>.</para>
</callout>
</calloutlist>
</para>
<para>
<programlisting>
void
parseStory (xmlDocPtr doc, xmlNodePtr cur) {
<co id="anothergetchild" /> cur = cur->xmlChildrenNode;
<co id="findkeyword" /> while (cur != NULL) {
if ((!xmlStrcmp(cur->name, (const xmlChar *)"keyword"))) {
<co id="foundkeyword" /> printf("keyword: %s\n", xmlNodeListGetString(doc, cur->xmlChildrenNode, 1));
}
cur = cur->next;
}
return;
}
</programlisting>
<calloutlist>
<callout arearefs="anothergetchild">
<para>Again we get the first child node.</para>
</callout>
<callout arearefs="findkeyword">
<para>Like the loop above, we then iterate through the nodes, looking
for one that matches the element we're interested in, in this case
&quot;keyword&quot;.</para>
</callout>
<callout arearefs="foundkeyword">
<para>When we find the &quot;keyword&quot; element, we need to print
its contents. Remember that in <acronym>XML</acronym>, the text
contained within an element is a child node of that element, so we
turn to <varname>cur-&gt;xmlChildrenNode</varname>. To retrieve it, we
use the function <function><ulink
url="http://xmlsoft.org/html/libxml-tree.html#XMLNODELISTGETSTRING">xmlNodeListGetString</ulink></function>, which also takes the <varname>doc</varname> pointer as an argument. In this case, we just print it out.</para>
</callout>
</calloutlist>
</para>
</sect1>
<sect1 id="xmltutorialwritingcontent">
<title>Writing element content</title>
<para>Writing element content uses many of the same steps we used above
&mdash; parsing the document and walking the tree. We parse the document,
then traverse the tree to find the place we want to insert our element. For
this example, we want to again find the &quot;storyinfo&quot; element and
this time insert a keyword. Then we'll write the file to disk. Full code:
<xref linkend="addkeywordappendix" /></para>
<para>
The main difference in this example is in
<function>parseStory</function>:
<programlisting>
void
parseStory (xmlDocPtr doc, xmlNodePtr cur, char *keyword) {
<co id="addkeyword" /> xmlNewTextChild (cur, NULL, "keyword", keyword);
return;
}
</programlisting>
<calloutlist>
<callout arearefs="addkeyword">
<para>The <function><ulink
url="http://xmlsoft.org/html/libxml-tree.html#XMLNEWTEXTCHILD">xmlNewTextChild</ulink></function>
function adds a new child element at the
current node pointer's location in the
tree, specificied by <varname>cur</varname>.</para>
</callout>
</calloutlist>
</para>
<para>
Once the node has been added, we would like to write the document to
file. Is you want the element to have a namespace, you can add it here as
well. In our case, the namespace is NULL.
<programlisting>
xmlSaveFormatFile (docname, doc, 1);
</programlisting>
The first parameter is the name of the file to be written. You'll notice
it is the same as the file we just read. In this case, we just write over
the old file. The second parameter is a pointer to the xmlDoc
structure. Setting the third parameter equal to one ensures indenting on output.
</para>
</sect1>
<!--
<sect1 id="xmltutorialattribute">
<title>Retrieving Attributes</title>
<para>Retrieving</para>
</sect1>
-->
<sect1 id="xmltutorialwritingattribute">
<title>Writing Attribute</title>
<para>Writing an attribute is similar to writing text to a new element. In
this case, we'll add a reference <acronym>URI</acronym> to our
document. Full code:<xref linkend="addattributeappendix" />.</para>
<para>
A <sgmltag>reference</sgmltag> is a child of the <sgmltag>story</sgmltag>
element, so finding the place to put our new element and attribute is
simple. As soon as we do the error-checking test in our
<function>parseDoc</function>, we are in the right spot to add our
element. But before we do that, we need to make a declaration using a
datatype we have not seen yet:
<programlisting>
xmlAttrPtr newattr;
</programlisting>
We also need an extra xmlNodePtr:
<programlisting>
xmlNodePtr newnode;
</programlisting>
</para>
<para>
The rest of <function>parseDoc</function> is the same as before until we
check to see if our root element is <sgmltag>story</sgmltag>. If it is,
then we know we are at the right spot to add our element:
<programlisting>
<co id="addreferencenode" /> newnode = xmlNewTextChild (cur, NULL, "reference", NULL);
<co id="addattributenode" /> newattr = xmlNewProp (newnode, "uri", uri);
</programlisting>
<calloutlist>
<callout arearefs="addreferencenode">
<para>First we add a new node at the location of the current node
pointer, <varname>cur.</varname> using the <ulink
url="http://xmlsoft.org/html/libxml-tree.html#XMLNEWTEXTCHILD">xmlNewTextChild</ulink> function.</para>
</callout>
</calloutlist>
</para>
<para>Once the node is added, the file is written to disk just as in the
previous example in which we added an element with text content.</para>
</sect1>
<!--
<appendix id="furtherresources">
<title>Further Resources</title>
<para></para>
</appendix>
-->
<appendix id="sampledoc">
<title>Sample Document</title>
<programlisting>&STORY;</programlisting>
</appendix>
<appendix id="keywordappendix">
<title>Code for Keyword Example</title>
<para>
<programlisting>&KEYWORD;</programlisting>
</para>
</appendix>
<appendix id="addkeywordappendix">
<title>Code for Add Keyword Example</title>
<para>
<programlisting>&ADDKEYWORD;</programlisting>
</para>
</appendix>
<appendix id="addattributeappendix">
<title>Code for Add Attribute Example</title>
<para>
<programlisting>&ADDATTRIBUTE;</programlisting>
</para>
</appendix>
</article>