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 <body bgcolor="#ffffff">
 <h1 align="center">The XML library for Gnome</h1>

 <p>This document describes the <a href="http://www.w3.org/XML/">XML</a>
 library provideed in the <a href="http://www.gnome.org/">Gnome</a> framework.
 XML is a standard to build tag based structured documents/data. </p>

 <p>The internal document repesentation is as close as possible to the <a
 href="http://www.w3.org/DOM/">DOM</a> interfaces. </p>

 <p>Libxml also has a <a href="http://www.megginson.com/SAX/index.html">SAX
 interface</a>, <a href="mailto:james@daa.com.au">James Henstridge</a> made <a
 href="http://www.daa.com.au/~james/gnome/xml-sax/xml-sax.html">a nice
 documentation</a> expaining how to use it. The interface is as compatible as
 possible with <a href="http://www.jclark.com/xml/expat.html">Expat</a>
 one.</p>

 <p>The code is commented in a <a href=""></a>way which allow <a
 href="http://rpmfind.net/veillard/XML/libxml.html">extensive documentation</a>
 to be automatically extracted.</p>

 <p>There is also a mailing-list <a
 href="xml@rufus.w3.org">xml@rufus.w3.org</a> for libxml, with an <a
 href="http://rpmfind.net/veillard/XML/messages">on-line archive</a>. To
 subscribe to this majordomo based list, send a mail to <a
 href="majordomo@rufus.w3.org">majordomo@rufus.w3.org</a> with "subscribe xml"
 in the <strong>content</strong> of the message.</p>

 <p>This library is released both under the W3C Copyright and the GNU LGP,
 basically everybody should be happy, if not, drop me a mail.</p>

 <p>People are invited to use the <a
 href="http://cvs.gnome.org/lxr/source/gdome/">gdome Gnome module to</a> get a
 full DOM interface, thanks to <a href="mailto:raph@levien.com">Raph
 Levien</a>, check his <a
 href="http://www.levien.com/gnome/domination.html">DOMination paper</a>. He
 uses it for his implementation of <a
 href="http://www.w3.org/Graphics/SVG/">SVG</a> called <a
 href="http://www.levien.com/svg/">gill</a>.</p>

 <h2>xml</h2>

 <p>XML is a standard for markup based structured documents, here is <a
 name="example">an example</a>:</p>
 <pre>&lt;?xml version="1.0"?>
 &lt;EXAMPLE prop1="gnome is great" prop2="&amp;amp; linux too">
   &lt;head>
    &lt;title>Welcome to Gnome&lt;/title>
   &lt;/head>
   &lt;chapter>
    &lt;title>The Linux adventure&lt;/title>
    &lt;p>bla bla bla ...&lt;/p>
    &lt;image href="linus.gif"/>
    &lt;p>...&lt;/p>
   &lt;/chapter>
 &lt;/EXAMPLE></pre>

 <p>The first line specify that it's an XML document and gives useful
 informations about it's encoding. Then the document is a text format whose
 structure is specified by tags between brackets. <strong>Each tag opened have
 to be closed</strong> XML is pedantic about this, not that for example the
 image tag has no content (just an attribute) and is closed by ending up the
 tag with <code>/></code>.</p>

 <h2>The tree output</h2>

 <p>The parser returns a tree built during the document analysis. The value
 returned is an <strong>xmlDocPtr</strong> (i.e. a pointer to an
 <strong>xmlDoc</strong> structure). This structure contains informations like
 the file  name, the document type, and a <strong>root</strong> pointer which
 is the root of the document (or more exactly the first child under the root
 which is the document). The tree is made of <strong>xmlNode</strong>s, chained
 in double linked lists of siblings and with childs&lt;->parent relationship.
 An xmlNode can also carry properties (a chain of xmlAttr structures). An
 attribute may have a value which is a list of TEXT or ENTITY_REF nodes.</p>

 <p>Here is an example (erroneous w.r.t. the XML spec since there should be
 only one ELEMENT under the root):</p>

 <p><img src="structure.gif" alt=" structure.gif "></p>

 <p>In the source package there is a small program (not installed by default)
 called <strong>tester</strong> which parses XML files given as argument and
 prints them back as parsed, this is useful to detect errors both in XML code
 and in the XML parser itself. It has an option <strong>--debug</strong> which
 prints the actual in-memory structure of the document, here is the result with
 the <a href="#example">example</a> given before:</p>
 <pre>DOCUMENT
 version=1.0
 standalone=true
   ELEMENT EXAMPLE
     ATTRIBUTE prop1
       TEXT
       content=gnome is great
     ATTRIBUTE prop2
       ENTITY_REF
       TEXT
       content= too
     ELEMENT head
       ELEMENT title
         TEXT
         content=Welcome to Gnome
     ELEMENT chapter
       ELEMENT title
         TEXT
         content=The Linux adventure
       ELEMENT p
         TEXT
         content=bla bla bla ...
       ELEMENT image
         ATTRIBUTE href
           TEXT
           content=linus.gif
       ELEMENT p
         TEXT
         content=...</pre>

 <p>This should be useful to learn the internal representation model.</p>

 <h2>The XML library interfaces</h2>

 <p>This section is directly intended to help programmers getting bootstrapped
 using the XML library from the C language. It doesn't intent to be extensive,
 I hope the automatically generated docs will provide the completeness
 required, but as a separated set of documents. The interfaces of the XML
 library are by principle low level, there is nearly zero abstration. Those
 interested in a higher level API should <a href="#DOM">look at DOM</a>
 (unfortunately not completed).</p>

 <h3>Invoking the parser</h3>

 <p>Usually, the first thing to do is to read an XML input, the parser accepts
 to parse both memory mapped documents or direct files. The functions are
 defined in "parser.h":</p>
 <dl>
   <dt><code>xmlDocPtr xmlParseMemory(char *buffer, int size);</code></dt>
     <dd><p>parse a zero terminated string containing the document</p>
     </dd>
 </dl>
 <dl>
   <dt><code>xmlDocPtr xmlParseFile(const char *filename);</code></dt>
     <dd><p>parse an XML document contained in a file (possibly compressed)</p>
     </dd>
 </dl>

 <p>This returns a pointer to the document structure (or NULL in case of
 failure).</p>

 <p>A couple of comments can be made, first this mean that the parser is
 memory-hungry, first to load the document in memory, second to build the tree.
 Reading a document without building the tree will be possible in the future by
 pluggin the code to the SAX interface (see SAX.c).</p>

 <h3>Building a tree from scratch</h3>

 <p>The other way to get an XML tree in memory is by building it. Basically
 there is a set of functions dedicated to building new elements, those are also
 described in "tree.h", here is for example the piece of code producing the
 example used before:</p>
 <pre>    xmlDocPtr doc;
     xmlNodePtr tree, subtree;

     doc = xmlNewDoc("1.0");
     doc->root = xmlNewDocNode(doc, NULL, "EXAMPLE", NULL);
     xmlSetProp(doc->root, "prop1", "gnome is great");
     xmlSetProp(doc->root, "prop2", "&amp;linux; too");
     tree = xmlNewChild(doc->root, NULL, "head", NULL);
     subtree = xmlNewChild(tree, NULL, "title", "Welcome to Gnome");
     tree = xmlNewChild(doc->root, NULL, "chapter", NULL);
     subtree = xmlNewChild(tree, NULL, "title", "The Linux adventure");
     subtree = xmlNewChild(tree, NULL, "p", "bla bla bla ...");
     subtree = xmlNewChild(tree, NULL, "image", NULL);
     xmlSetProp(subtree, "href", "linus.gif");</pre>

 <p>Not really rocket science ...</p>

 <h3>Traversing the tree</h3>

 <p>Basically by including "tree.h" your code has access to the internal
 structure of all the element of the tree. The names should be somewhat simple
 like <strong>parent</strong>, <strong>childs</strong>, <strong>next</strong>,
 <strong>prev</strong>, <strong>properties</strong>, etc... For example still
 with the previous example:</p>
 <pre><code>doc->root->childs->childs</code></pre>

 <p>points to the title element,</p>
 <pre>doc->root->childs->next->child->child</pre>

 <p>points to the text node containing the chapter titlle "The Linux adventure"
 and</p>
 <pre>doc->root->properties->next->val</pre>

 <p>points to the entity reference containing the value of "&amp;linux" at the
 beginning of the second attribute of the root element "EXAMPLE".</p>

 <h3>Modifying the tree</h3>

 <p>functions are provided to read and write the document content:</p>
 <dl>
   <dt><code>xmlAttrPtr xmlSetProp(xmlNodePtr node, const CHAR *name, const
   CHAR *value);</code></dt>
     <dd><p>This set (or change) an attribute carried by an ELEMENT node the
       value can be NULL</p>
     </dd>
 </dl>
 <dl>
   <dt><code>const CHAR *xmlGetProp(xmlNodePtr node, const CHAR
   *name);</code></dt>
     <dd><p>This function returns a pointer to the property content, note that
       no extra copy is made</p>
     </dd>
 </dl>

 <p>Two functions must be used to read an write the text associated to
 elements:</p>
 <dl>
   <dt><code>xmlNodePtr xmlStringGetNodeList(xmlDocPtr doc, const CHAR
   *value);</code></dt>
     <dd><p>This function takes an "external" string and convert it to one text
       node or possibly to a list of entity and text nodes. All non-predefined
       entity references like &amp;Gnome; will be stored internally as an
       entity node, hence the result of the function may not be a single
       node.</p>
     </dd>
 </dl>
 <dl>
   <dt><code>CHAR *xmlNodeListGetString(xmlDocPtr doc, xmlNodePtr list, int
   inLine);</code></dt>
     <dd><p>this is the dual function, which generate a new string containing
       the content of the text and entity nodes. Note the extra argument
       inLine, if set to 1 instead of returning the &amp;Gnome; XML encoding in
       the string it will substitute it with it's value say "GNU Network Object
       Model Environment". Set it if you want to use the string for non XML
       usage like User Interface.</p>
     </dd>
 </dl>

 <h3>Saving a tree</h3>

 <p>Basically 3 options are possible:</p>
 <dl>
   <dt><code>void xmlDocDumpMemory(xmlDocPtr cur, CHAR**mem, int
   *size);</code></dt>
     <dd><p>returns a buffer where the document has been saved</p>
     </dd>
 </dl>
 <dl>
   <dt><code>extern void xmlDocDump(FILE *f, xmlDocPtr doc);</code></dt>
     <dd><p>dumps a buffer to an open file descriptor</p>
     </dd>
 </dl>
 <dl>
   <dt><code>int xmlSaveFile(const char *filename, xmlDocPtr cur);</code></dt>
     <dd><p>save the document ot a file. In that case the compression interface
       is triggered if turned on</p>
     </dd>
 </dl>

 <h3>Compression</h3>

 <p>The library handle transparently compression when doing file based
 accesses, the level of compression on saves can be tuned either globally or
 individually for one file:</p>
 <dl>
   <dt><code>int  xmlGetDocCompressMode (xmlDocPtr doc);</code></dt>
     <dd><p>Get the document compression ratio (0-9)</p>
     </dd>
 </dl>
 <dl>
   <dt><code>void xmlSetDocCompressMode (xmlDocPtr doc, int mode);</code></dt>
     <dd><p>Set the document compression ratio</p>
     </dd>
 </dl>
 <dl>
   <dt><code>int  xmlGetCompressMode(void);</code></dt>
     <dd><p>Get the default compression ratio</p>
     </dd>
 </dl>
 <dl>
   <dt><code>void xmlSetCompressMode(int mode);</code></dt>
     <dd><p>set the default compression ratio</p>
     </dd>
 </dl>

 <h2><a name="DOM">DOM Principles</a></h2>

 <p><a href="http://www.w3.org/DOM/">DOM</a> stands for the <em>Document Object
 Model</em> this is an API for accessing XML or HTML structured documents.
 Native support for DOM in Gnome is on the way (module gnome-dom), and it will
 be based on gnome-xml. This will be a far cleaner interface to manipulate XML
 files within Gnome since it won't expose the internal structure. DOM defiles a
 set of IDL (or Java) interfaces allowing to traverse and manipulate a
 document. The DOM library will allow accessing and modifying "live" documents
 presents on other programs like this:</p>

 <p><img src="DOM.gif" alt=" DOM.gif "></p>

 <p>This should help greatly doing things like modifying a gnumeric spreadsheet
 embedded in a GWP document for example.</p>

 <h3><a name="Example">A real example</a></h3>

 <p>Here is a real size example, where the actual content of the application
 data is not kept in the DOM tree but uses internal structures. It is based on
 a proposal to keep a database of jobs related to Gnome, with an XML based
 storage structure. Here is an <a href="gjobs.xml">XML encoded jobs
 base</a>:</p>
 <pre>&lt;?xml version="1.0"?>
 &lt;gjob:Helping xmlns:gjob="http://www.gnome.org/some-location">
   &lt;gjob:Jobs>

     &lt;gjob:Job>
       &lt;gjob:Project ID="3"/>
       &lt;gjob:Application>GBackup&lt;/gjob:Application>
       &lt;gjob:Category>Development&lt;/gjob:Category>

       &lt;gjob:Update>
         &lt;gjob:Status>Open&lt;/gjob:Status>
         &lt;gjob:Modified>Mon, 07 Jun 1999 20:27:45 -0400 MET DST&lt;/gjob:Modified>
         &lt;gjob:Salary>USD 0.00&lt;/gjob:Salary>
       &lt;/gjob:Update>

       &lt;gjob:Developers>
         &lt;gjob:Developer>
         &lt;/gjob:Developer>
       &lt;/gjob:Developers>

       &lt;gjob:Contact>
         &lt;gjob:Person>Nathan Clemons&lt;/gjob:Person>
         &lt;gjob:Email>nathan@windsofstorm.net&lt;/gjob:Email>
         &lt;gjob:Company>
         &lt;/gjob:Company>
         &lt;gjob:Organisation>
         &lt;/gjob:Organisation>
         &lt;gjob:Webpage>
         &lt;/gjob:Webpage>
         &lt;gjob:Snailmail>
         &lt;/gjob:Snailmail>
         &lt;gjob:Phone>
         &lt;/gjob:Phone>
       &lt;/gjob:Contact>

       &lt;gjob:Requirements>
       The program should be released as free software, under the GPL.
       &lt;/gjob:Requirements>

       &lt;gjob:Skills>
       &lt;/gjob:Skills>

       &lt;gjob:Details>
       A GNOME based system that will allow a superuser to configure
       compressed and uncompressed files and/or file systems to be backed
       up with a supported media in the system.  This should be able to
       perform via find commands generating a list of files that are passed
       to tar, dd, cpio, cp, gzip, etc., to be directed to the tape machine
       or via operations performed on the filesystem itself. Email
       notification and GUI status display very important.
       &lt;/gjob:Details>

     &lt;/gjob:Job>

   &lt;/gjob:Jobs>
 &lt;/gjob:Helping>
 </pre>

 <p>While loading the XML file into an internal DOM tree is a matter of calling
 only a couple of functions, browsing the tree to gather the informations and
 generate the internals structures is harder, and more error prone.</p>

 <p>The suggested principle is to be tolerant with respect to the input
 structure. For example the ordering of the attributes is not significant, Cthe
 XML specification is clear about it. It's also usually a good idea to not be
 dependant of the orders of the childs of a given node, unless it really makes
 things harder. Here is some code to parse the informations for a person:</p>
 <pre>/*
  * A person record
  */
 typedef struct person {
     char *name;
     char *email;
     char *company;
     char *organisation;
     char *smail;
     char *webPage;
     char *phone;
 } person, *personPtr;

 /*
  * And the code needed to parse it
  */
 personPtr parsePerson(xmlDocPtr doc, xmlNsPtr ns, xmlNodePtr cur) {
     personPtr ret = NULL;

 DEBUG("parsePerson\n");
     /*
      * allocate the struct
      */
     ret = (personPtr) malloc(sizeof(person));
     if (ret == NULL) {
         fprintf(stderr,"out of memory\n");
         return(NULL);
     }
     memset(ret, 0, sizeof(person));

     /* We don't care what the top level element name is */
     cur = cur->childs;
     while (cur != NULL) {
         if ((!strcmp(cur->name, "Person")) &amp;&amp; (cur->ns == ns))
             ret->name = xmlNodeListGetString(doc, cur->childs, 1);
         if ((!strcmp(cur->name, "Email")) &amp;&amp; (cur->ns == ns))
             ret->email = xmlNodeListGetString(doc, cur->childs, 1);
         cur = cur->next;
     }

     return(ret);
 }</pre>

 <p>Here is a couple of things to notice:</p>
 <ul>
   <li>Usually a recursive parsing style is the more convenient one, XML data
     being by nature subject to repetitive constructs and usualy exibit highly
     stuctured patterns.</li>
   <li>The two arguments of type <em>xmlDocPtr</em> and <em>xmlNsPtr</em>, i.e.
     the pointer to the global XML document and the namespace reserved to the
     application. Document wide information are needed for example to decode
     entities and it's a good coding practice to define a namespace for your
     application set of data and test that the element and attributes you're
     analyzing actually pertains to your application space. This is done by a
     simple equality test (cur->ns == ns).</li>
   <li>To retrieve text and attributes value, it is suggested to use the
     function <em>xmlNodeListGetString</em> to gather all the text and entity
     reference nodes generated by the DOM output and produce an single text
     string.</li>
 </ul>

 <p>Here is another piece of code used to parse another level of the
 structure:</p>
 <pre>/*
  * a Description for a Job
  */
 typedef struct job {
     char *projectID;
     char *application;
     char *category;
     personPtr contact;
     int nbDevelopers;
     personPtr developers[100]; /* using dynamic alloc is left as an exercise */
 } job, *jobPtr;

 /*
  * And the code needed to parse it
  */
 jobPtr parseJob(xmlDocPtr doc, xmlNsPtr ns, xmlNodePtr cur) {
     jobPtr ret = NULL;

 DEBUG("parseJob\n");
     /*
      * allocate the struct
      */
     ret = (jobPtr) malloc(sizeof(job));
     if (ret == NULL) {
         fprintf(stderr,"out of memory\n");
         return(NULL);
     }
     memset(ret, 0, sizeof(job));

     /* We don't care what the top level element name is */
     cur = cur->childs;
     while (cur != NULL) {

         if ((!strcmp(cur->name, "Project")) &amp;&amp; (cur->ns == ns)) {
             ret->projectID = xmlGetProp(cur, "ID");
             if (ret->projectID == NULL) {
                 fprintf(stderr, "Project has no ID\n");
             }
         }
         if ((!strcmp(cur->name, "Application")) &amp;&amp; (cur->ns == ns))
             ret->application = xmlNodeListGetString(doc, cur->childs, 1);
         if ((!strcmp(cur->name, "Category")) &amp;&amp; (cur->ns == ns))
             ret->category = xmlNodeListGetString(doc, cur->childs, 1);
         if ((!strcmp(cur->name, "Contact")) &amp;&amp; (cur->ns == ns))
             ret->contact = parsePerson(doc, ns, cur);
         cur = cur->next;
     }

     return(ret);
 }</pre>

 <p>One can notice that once used to it, writing this kind of code is quite
 simple, but boring. Ultimately, it could be possble to write stubbers taking
 either C data structure definitions, a set of XML examples or an XML DTD and
 produce the code needed to import and export the content between C data and
 XML storage. This is left as an exercise to the reader :-)</p>

 <p>Feel free to use <a href="gjobread.c">the code for the full C parsing
 example</a> as a template,</p>

 <p> <a href="mailto:Daniel.Veillard@w3.org">Daniel Veillard</a></p>
 </body>
 </html>
	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
	"http://www.w3.org/TR/REC-html40/loose.dtd">
	<html>
	<head>
	<title>The XML library for Gnome</title>
	<meta name="GENERATOR" content="amaya V2.1">
	</head>

	<body bgcolor="#ffffff">
	<h1 align="center">The XML library for Gnome</h1>

	<p>This document describes the <a href="http://www.w3.org/XML/">XML</a>
	library provideed in the <a href="http://www.gnome.org/">Gnome</a> framework.
	XML is a standard to build tag based structured documents/data. </p>

	<p>The internal document repesentation is as close as possible to the <a
	href="http://www.w3.org/DOM/">DOM</a> interfaces. </p>

	<p>Libxml also has a <a href="http://www.megginson.com/SAX/index.html">SAX
	interface</a>, <a href="mailto:james@daa.com.au">James Henstridge</a> made <a
	href="http://www.daa.com.au/~james/gnome/xml-sax/xml-sax.html">a nice
	documentation</a> expaining how to use it. The interface is as compatible as
	possible with <a href="http://www.jclark.com/xml/expat.html">Expat</a>
	one.</p>

	<p>The code is commented in a <a href=""></a>way which allow <a
	href="http://rpmfind.net/veillard/XML/libxml.html">extensive documentation</a>
	to be automatically extracted.</p>

	<p>There is also a mailing-list <a
	href="xml@rufus.w3.org">xml@rufus.w3.org</a> for libxml, with an <a
	href="http://rpmfind.net/veillard/XML/messages">on-line archive</a>. To
	subscribe to this majordomo based list, send a mail to <a
	href="majordomo@rufus.w3.org">majordomo@rufus.w3.org</a> with "subscribe xml"
	in the <strong>content</strong> of the message.</p>

	<p>This library is released both under the W3C Copyright and the GNU LGP,
	basically everybody should be happy, if not, drop me a mail.</p>

	<p>People are invited to use the <a
	href="http://cvs.gnome.org/lxr/source/gdome/">gdome Gnome module to</a> get a
	full DOM interface, thanks to <a href="mailto:raph@levien.com">Raph
	Levien</a>, check his <a
	href="http://www.levien.com/gnome/domination.html">DOMination paper</a>. He
	uses it for his implementation of <a
	href="http://www.w3.org/Graphics/SVG/">SVG</a> called <a
	href="http://www.levien.com/svg/">gill</a>.</p>

	<h2>xml</h2>

	<p>XML is a standard for markup based structured documents, here is <a
	name="example">an example</a>:</p>
	<pre><?xml version="1.0"?>
	<EXAMPLE prop1="gnome is great" prop2="&amp; linux too">
	<head>
	<title>Welcome to Gnome</title>
	</head>
	<chapter>
	<title>The Linux adventure</title>
	<p>bla bla bla ...</p>
	<image href="linus.gif"/>
	<p>...</p>
	</chapter>
	</EXAMPLE></pre>

	<p>The first line specify that it's an XML document and gives useful
	informations about it's encoding. Then the document is a text format whose
	structure is specified by tags between brackets. <strong>Each tag opened have
	to be closed</strong> XML is pedantic about this, not that for example the
	image tag has no content (just an attribute) and is closed by ending up the
	tag with <code>/></code>.</p>

	<h2>The tree output</h2>

	<p>The parser returns a tree built during the document analysis. The value
	returned is an <strong>xmlDocPtr</strong> (i.e. a pointer to an
	<strong>xmlDoc</strong> structure). This structure contains informations like
	the file name, the document type, and a <strong>root</strong> pointer which
	is the root of the document (or more exactly the first child under the root
	which is the document). The tree is made of <strong>xmlNode</strong>s, chained
	in double linked lists of siblings and with childs<->parent relationship.
	An xmlNode can also carry properties (a chain of xmlAttr structures). An
	attribute may have a value which is a list of TEXT or ENTITY_REF nodes.</p>

	<p>Here is an example (erroneous w.r.t. the XML spec since there should be
	only one ELEMENT under the root):</p>

	<p><img src="structure.gif" alt=" structure.gif "></p>

	<p>In the source package there is a small program (not installed by default)
	called <strong>tester</strong> which parses XML files given as argument and
	prints them back as parsed, this is useful to detect errors both in XML code
	and in the XML parser itself. It has an option <strong>--debug</strong> which
	prints the actual in-memory structure of the document, here is the result with
	the <a href="#example">example</a> given before:</p>
	<pre>DOCUMENT
	version=1.0
	standalone=true
	ELEMENT EXAMPLE
	ATTRIBUTE prop1
	TEXT
	content=gnome is great
	ATTRIBUTE prop2
	ENTITY_REF
	TEXT
	content= too
	ELEMENT head
	ELEMENT title
	TEXT
	content=Welcome to Gnome
	ELEMENT chapter
	ELEMENT title
	TEXT
	content=The Linux adventure
	ELEMENT p
	TEXT
	content=bla bla bla ...
	ELEMENT image
	ATTRIBUTE href
	TEXT
	content=linus.gif
	ELEMENT p
	TEXT
	content=...</pre>

	<p>This should be useful to learn the internal representation model.</p>

	<h2>The XML library interfaces</h2>

	<p>This section is directly intended to help programmers getting bootstrapped
	using the XML library from the C language. It doesn't intent to be extensive,
	I hope the automatically generated docs will provide the completeness
	required, but as a separated set of documents. The interfaces of the XML
	library are by principle low level, there is nearly zero abstration. Those
	interested in a higher level API should <a href="#DOM">look at DOM</a>
	(unfortunately not completed).</p>

	<h3>Invoking the parser</h3>

	<p>Usually, the first thing to do is to read an XML input, the parser accepts
	to parse both memory mapped documents or direct files. The functions are
	defined in "parser.h":</p>
	<dl>
	<dt><code>xmlDocPtr xmlParseMemory(char *buffer, int size);</code></dt>
	<dd><p>parse a zero terminated string containing the document</p>
	</dd>
	</dl>
	<dl>
	<dt><code>xmlDocPtr xmlParseFile(const char *filename);</code></dt>
	<dd><p>parse an XML document contained in a file (possibly compressed)</p>
	</dd>
	</dl>

	<p>This returns a pointer to the document structure (or NULL in case of
	failure).</p>

	<p>A couple of comments can be made, first this mean that the parser is
	memory-hungry, first to load the document in memory, second to build the tree.
	Reading a document without building the tree will be possible in the future by
	pluggin the code to the SAX interface (see SAX.c).</p>

	<h3>Building a tree from scratch</h3>

	<p>The other way to get an XML tree in memory is by building it. Basically
	there is a set of functions dedicated to building new elements, those are also
	described in "tree.h", here is for example the piece of code producing the
	example used before:</p>
	<pre> xmlDocPtr doc;
	xmlNodePtr tree, subtree;

	doc = xmlNewDoc("1.0");
	doc->root = xmlNewDocNode(doc, NULL, "EXAMPLE", NULL);
	xmlSetProp(doc->root, "prop1", "gnome is great");
	xmlSetProp(doc->root, "prop2", "&linux; too");
	tree = xmlNewChild(doc->root, NULL, "head", NULL);
	subtree = xmlNewChild(tree, NULL, "title", "Welcome to Gnome");
	tree = xmlNewChild(doc->root, NULL, "chapter", NULL);
	subtree = xmlNewChild(tree, NULL, "title", "The Linux adventure");
	subtree = xmlNewChild(tree, NULL, "p", "bla bla bla ...");
	subtree = xmlNewChild(tree, NULL, "image", NULL);
	xmlSetProp(subtree, "href", "linus.gif");</pre>

	<p>Not really rocket science ...</p>

	<h3>Traversing the tree</h3>

	<p>Basically by including "tree.h" your code has access to the internal
	structure of all the element of the tree. The names should be somewhat simple
	like <strong>parent</strong>, <strong>childs</strong>, <strong>next</strong>,
	<strong>prev</strong>, <strong>properties</strong>, etc... For example still
	with the previous example:</p>
	<pre><code>doc->root->childs->childs</code></pre>

	<p>points to the title element,</p>
	<pre>doc->root->childs->next->child->child</pre>

	<p>points to the text node containing the chapter titlle "The Linux adventure"
	and</p>
	<pre>doc->root->properties->next->val</pre>

	<p>points to the entity reference containing the value of "&linux" at the
	beginning of the second attribute of the root element "EXAMPLE".</p>

	<h3>Modifying the tree</h3>

	<p>functions are provided to read and write the document content:</p>
	<dl>
	<dt><code>xmlAttrPtr xmlSetProp(xmlNodePtr node, const CHAR *name, const
	CHAR *value);</code></dt>
	<dd><p>This set (or change) an attribute carried by an ELEMENT node the
	value can be NULL</p>
	</dd>
	</dl>
	<dl>
	<dt><code>const CHAR *xmlGetProp(xmlNodePtr node, const CHAR
	*name);</code></dt>
	<dd><p>This function returns a pointer to the property content, note that
	no extra copy is made</p>
	</dd>
	</dl>

	<p>Two functions must be used to read an write the text associated to
	elements:</p>
	<dl>
	<dt><code>xmlNodePtr xmlStringGetNodeList(xmlDocPtr doc, const CHAR
	*value);</code></dt>
	<dd><p>This function takes an "external" string and convert it to one text
	node or possibly to a list of entity and text nodes. All non-predefined
	entity references like &Gnome; will be stored internally as an
	entity node, hence the result of the function may not be a single
	node.</p>
	</dd>
	</dl>
	<dl>
	<dt><code>CHAR *xmlNodeListGetString(xmlDocPtr doc, xmlNodePtr list, int
	inLine);</code></dt>
	<dd><p>this is the dual function, which generate a new string containing
	the content of the text and entity nodes. Note the extra argument
	inLine, if set to 1 instead of returning the &Gnome; XML encoding in
	the string it will substitute it with it's value say "GNU Network Object
	Model Environment". Set it if you want to use the string for non XML
	usage like User Interface.</p>
	</dd>
	</dl>

	<h3>Saving a tree</h3>

	<p>Basically 3 options are possible:</p>
	<dl>
	<dt><code>void xmlDocDumpMemory(xmlDocPtr cur, CHAR**mem, int
	*size);</code></dt>
	<dd><p>returns a buffer where the document has been saved</p>
	</dd>
	</dl>
	<dl>
	<dt><code>extern void xmlDocDump(FILE *f, xmlDocPtr doc);</code></dt>
	<dd><p>dumps a buffer to an open file descriptor</p>
	</dd>
	</dl>
	<dl>
	<dt><code>int xmlSaveFile(const char *filename, xmlDocPtr cur);</code></dt>
	<dd><p>save the document ot a file. In that case the compression interface
	is triggered if turned on</p>
	</dd>
	</dl>

	<h3>Compression</h3>

	<p>The library handle transparently compression when doing file based
	accesses, the level of compression on saves can be tuned either globally or
	individually for one file:</p>
	<dl>
	<dt><code>int xmlGetDocCompressMode (xmlDocPtr doc);</code></dt>
	<dd><p>Get the document compression ratio (0-9)</p>
	</dd>
	</dl>
	<dl>
	<dt><code>void xmlSetDocCompressMode (xmlDocPtr doc, int mode);</code></dt>
	<dd><p>Set the document compression ratio</p>
	</dd>
	</dl>
	<dl>
	<dt><code>int xmlGetCompressMode(void);</code></dt>
	<dd><p>Get the default compression ratio</p>
	</dd>
	</dl>
	<dl>
	<dt><code>void xmlSetCompressMode(int mode);</code></dt>
	<dd><p>set the default compression ratio</p>
	</dd>
	</dl>

	<h2><a name="DOM">DOM Principles</a></h2>

	<p><a href="http://www.w3.org/DOM/">DOM</a> stands for the <em>Document Object
	Model</em> this is an API for accessing XML or HTML structured documents.
	Native support for DOM in Gnome is on the way (module gnome-dom), and it will
	be based on gnome-xml. This will be a far cleaner interface to manipulate XML
	files within Gnome since it won't expose the internal structure. DOM defiles a
	set of IDL (or Java) interfaces allowing to traverse and manipulate a
	document. The DOM library will allow accessing and modifying "live" documents
	presents on other programs like this:</p>

	<p><img src="DOM.gif" alt=" DOM.gif "></p>

	<p>This should help greatly doing things like modifying a gnumeric spreadsheet
	embedded in a GWP document for example.</p>

	<h3><a name="Example">A real example</a></h3>

	<p>Here is a real size example, where the actual content of the application
	data is not kept in the DOM tree but uses internal structures. It is based on
	a proposal to keep a database of jobs related to Gnome, with an XML based
	storage structure. Here is an <a href="gjobs.xml">XML encoded jobs
	base</a>:</p>
	<pre><?xml version="1.0"?>
	<gjob:Helping xmlns:gjob="http://www.gnome.org/some-location">
	<gjob:Jobs>

	<gjob:Job>
	<gjob:Project ID="3"/>
	<gjob:Application>GBackup</gjob:Application>
	<gjob:Category>Development</gjob:Category>

	<gjob:Update>
	<gjob:Status>Open</gjob:Status>
	<gjob:Modified>Mon, 07 Jun 1999 20:27:45 -0400 MET DST</gjob:Modified>
	<gjob:Salary>USD 0.00</gjob:Salary>
	</gjob:Update>

	<gjob:Developers>
	<gjob:Developer>
	</gjob:Developer>
	</gjob:Developers>

	<gjob:Contact>
	<gjob:Person>Nathan Clemons</gjob:Person>
	<gjob:Email>nathan@windsofstorm.net</gjob:Email>
	<gjob:Company>
	</gjob:Company>
	<gjob:Organisation>
	</gjob:Organisation>
	<gjob:Webpage>
	</gjob:Webpage>
	<gjob:Snailmail>
	</gjob:Snailmail>
	<gjob:Phone>
	</gjob:Phone>
	</gjob:Contact>

	<gjob:Requirements>
	The program should be released as free software, under the GPL.
	</gjob:Requirements>

	<gjob:Skills>
	</gjob:Skills>

	<gjob:Details>
	A GNOME based system that will allow a superuser to configure
	compressed and uncompressed files and/or file systems to be backed
	up with a supported media in the system. This should be able to
	perform via find commands generating a list of files that are passed
	to tar, dd, cpio, cp, gzip, etc., to be directed to the tape machine
	or via operations performed on the filesystem itself. Email
	notification and GUI status display very important.
	</gjob:Details>

	</gjob:Job>

	</gjob:Jobs>
	</gjob:Helping>
	</pre>

	<p>While loading the XML file into an internal DOM tree is a matter of calling
	only a couple of functions, browsing the tree to gather the informations and
	generate the internals structures is harder, and more error prone.</p>

	<p>The suggested principle is to be tolerant with respect to the input
	structure. For example the ordering of the attributes is not significant, Cthe
	XML specification is clear about it. It's also usually a good idea to not be
	dependant of the orders of the childs of a given node, unless it really makes
	things harder. Here is some code to parse the informations for a person:</p>
	<pre>/*
	* A person record
	*/
	typedef struct person {
	char *name;
	char *email;
	char *company;
	char *organisation;
	char *smail;
	char *webPage;
	char *phone;
	} person, *personPtr;

	/*
	* And the code needed to parse it
	*/
	personPtr parsePerson(xmlDocPtr doc, xmlNsPtr ns, xmlNodePtr cur) {
	personPtr ret = NULL;

	DEBUG("parsePerson\n");
	/*
	* allocate the struct
	*/
	ret = (personPtr) malloc(sizeof(person));
	if (ret == NULL) {
	fprintf(stderr,"out of memory\n");
	return(NULL);
	}
	memset(ret, 0, sizeof(person));

	/* We don't care what the top level element name is */
	cur = cur->childs;
	while (cur != NULL) {
	if ((!strcmp(cur->name, "Person")) && (cur->ns == ns))
	ret->name = xmlNodeListGetString(doc, cur->childs, 1);
	if ((!strcmp(cur->name, "Email")) && (cur->ns == ns))
	ret->email = xmlNodeListGetString(doc, cur->childs, 1);
	cur = cur->next;
	}

	return(ret);
	}</pre>

	<p>Here is a couple of things to notice:</p>
	<ul>
	<li>Usually a recursive parsing style is the more convenient one, XML data
	being by nature subject to repetitive constructs and usualy exibit highly
	stuctured patterns.</li>
	<li>The two arguments of type <em>xmlDocPtr</em> and <em>xmlNsPtr</em>, i.e.
	the pointer to the global XML document and the namespace reserved to the
	application. Document wide information are needed for example to decode
	entities and it's a good coding practice to define a namespace for your
	application set of data and test that the element and attributes you're
	analyzing actually pertains to your application space. This is done by a
	simple equality test (cur->ns == ns).</li>
	<li>To retrieve text and attributes value, it is suggested to use the
	function <em>xmlNodeListGetString</em> to gather all the text and entity
	reference nodes generated by the DOM output and produce an single text
	string.</li>
	</ul>

	<p>Here is another piece of code used to parse another level of the
	structure:</p>
	<pre>/*
	* a Description for a Job
	*/
	typedef struct job {
	char *projectID;
	char *application;
	char *category;
	personPtr contact;
	int nbDevelopers;
	personPtr developers[100]; /* using dynamic alloc is left as an exercise */
	} job, *jobPtr;

	/*
	* And the code needed to parse it
	*/
	jobPtr parseJob(xmlDocPtr doc, xmlNsPtr ns, xmlNodePtr cur) {
	jobPtr ret = NULL;

	DEBUG("parseJob\n");
	/*
	* allocate the struct
	*/
	ret = (jobPtr) malloc(sizeof(job));
	if (ret == NULL) {
	fprintf(stderr,"out of memory\n");
	return(NULL);
	}
	memset(ret, 0, sizeof(job));

	/* We don't care what the top level element name is */
	cur = cur->childs;
	while (cur != NULL) {

	if ((!strcmp(cur->name, "Project")) && (cur->ns == ns)) {
	ret->projectID = xmlGetProp(cur, "ID");
	if (ret->projectID == NULL) {
	fprintf(stderr, "Project has no ID\n");
	}
	}
	if ((!strcmp(cur->name, "Application")) && (cur->ns == ns))
	ret->application = xmlNodeListGetString(doc, cur->childs, 1);
	if ((!strcmp(cur->name, "Category")) && (cur->ns == ns))
	ret->category = xmlNodeListGetString(doc, cur->childs, 1);
	if ((!strcmp(cur->name, "Contact")) && (cur->ns == ns))
	ret->contact = parsePerson(doc, ns, cur);
	cur = cur->next;
	}

	return(ret);
	}</pre>

	<p>One can notice that once used to it, writing this kind of code is quite
	simple, but boring. Ultimately, it could be possble to write stubbers taking
	either C data structure definitions, a set of XML examples or an XML DTD and
	produce the code needed to import and export the content between C data and
	XML storage. This is left as an exercise to the reader :-)</p>

	<p>Feel free to use <a href="gjobread.c">the code for the full C parsing
	example</a> as a template,</p>

	<p> <a href="mailto:Daniel.Veillard@w3.org">Daniel Veillard</a></p>
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