libxml2/doc/xml.html

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<h1 align="center">The XML library for Gnome</h1>

<h2 style="text-align: center">libxml, a.k.a. gnome-xml</h2>

<p></p>
<ul>
  <li><a href="#Introducti">Introduction</a></li>
  <li><a href="#Documentat">Documentation</a></li>
  <li><a href="#Downloads">Downloads</a></li>
  <li><a href="#News">News</a></li>
  <li><a href="#XML">XML</a></li>
  <li><a href="#tree">The tree output</a></li>
  <li><a href="#interface">The SAX interface</a></li>
  <li><a href="#library">The XML library interfaces</a>
    <ul>
      <li><a href="#Invoking">Invoking the parser: the pull way</a></li>
      <li><a href="#Invoking">Invoking the parser: the push way</a></li>
      <li><a href="#Invoking2">Invoking the parser: the SAX interface</a></li>
      <li><a href="#Building">Building a tree from scratch</a></li>
      <li><a href="#Traversing">Traversing the tree</a></li>
      <li><a href="#Modifying">Modifying the tree</a></li>
      <li><a href="#Saving">Saving the tree</a></li>
      <li><a href="#Compressio">Compression</a></li>
    </ul>
  </li>
  <li><a href="#Entities">Entities or no entities</a></li>
  <li><a href="#Namespaces">Namespaces</a></li>
  <li><a href="#Validation">Validation</a></li>
  <li><a href="#Principles">DOM principles</a></li>
  <li><a href="#real">A real example</a></li>
</ul>

<h2><a name="Introducti">Introduction</a></h2>

<p>This document describes libxml, the <a
href="http://www.w3.org/XML/">XML</a> library provided in the <a
href="http://www.gnome.org/">Gnome</a> framework. XML is a standard for
building tag-based structured documents/data.</p>

<p>Here are some key points about libxml:</p>
<ul>
  <li>The internal document repesentation is as close as possible to the <a
    href="http://www.w3.org/DOM/">DOM</a> interfaces.</li>
  <li>Libxml also has a <a href="http://www.megginson.com/SAX/index.html">SAX
    like interface</a>; the interface is designed to be compatible with <a
    href="http://www.jclark.com/xml/expat.html">Expat</a>.</li>
  <li>Libxml now includes a nearly complete <a
    href="http://www.w3.org/TR/xpath">XPath</a> implementation.</li>
  <li>Libxml exports Push and Pull type parser interfaces for both XML and
    HTML.</li>
  <li>This library is released both under the <a
    href="http://www.w3.org/Consortium/Legal/copyright-software-19980720.html">W3C
    IPR</a> and the GNU LGPL. Use either at your convenience, basically this
    should make everybody happy, if not, drop me a mail.</li>
  <li>There is <a href="upgrade.html">a  first set of instruction</a>
    concerning upgrade from libxml-1.x to libxml-2.x</li>
</ul>

<h2><a name="Documentat">Documentation</a></h2>

<p>There are some on-line resources about using libxml:</p>
<ol>
  <li>The code is commented in a way which allows <a
    href="http://xmlsoft.org/libxml.html">extensive documentation</a> to be
    automatically extracted.</li>
  <li>This page provides a global overview and <a href="#real">some
    examples</a> on how to use libxml.</li>
  <li><a href="mailto:james@daa.com.au">James Henstridge</a> wrote <a
    href="http://www.daa.com.au/~james/gnome/xml-sax/xml-sax.html">some nice
    documentation</a> explaining how to use the libxml SAX interface.</li>
  <li>George Lebl wrote <a
    href="http://www-4.ibm.com/software/developer/library/gnome3/">an article
    for IBM developerWorks</a> about using libxml.</li>
  <li>It is also a good idea to check to <a href="mailto:raph@levien.com">Raph
    Levien</a> <a href="http://levien.com/gnome/">web site</a> since he is
    building the <a href="http://levien.com/gnome/gdome.html">DOM interface
    gdome</a> on top of libxml result tree and an implementation of <a
    href="http://www.w3.org/Graphics/SVG/">SVG</a> called <a
    href="http://www.levien.com/svg/">gill</a>. Check his <a
    href="http://www.levien.com/gnome/domination.html">DOMination
  paper</a>.</li>
  <li>Check <a href="http://cvs.gnome.org/lxr/source/gnome-xml/TODO">the TODO
    file</a></li>
  <li>Read the <a href="upgrade.html">1.x to 2.x upgrade path</a>. If you are
    starting a new project using libxml you should really use the 2.x
  version.</li>
  <li>And don't forget to look at the <a href="/messages/">mailing-list
    archive</a>, too.</li>
</ol>

<h3>Reporting bugs and getting help</h3>

<p>Well, bugs or missing features are always possible, and I will make a point
of fixing them in a timely fashion. The best way to report a bug is to <a
href="http://bugs.gnome.org/db/pa/lgnome-xml.html">use the Gnome bug tracking
database</a>. I look at reports there regularly and it's good to have a
reminder when a bug is still open. Check the <a
href="http://bugs.gnome.org/Reporting.html">instructions on reporting bugs</a>
and be sure to specify that the bug is for the package gnome-xml.</p>

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

<p>Alternatively, you can just send the bug to the <a
href="mailto:xml@rufus.w3.org">xml@rufus.w3.org</a> list.</p>

<p>Of course, bugs reports with a suggested patch for fixing them will
probably be processed faster.</p>

<p>If you're looking for help, a quick look at <a
href="http://xmlsoft.org/messages/#407">the list archive</a> may actually
provide the answer, I usually send source samples when answering libxml usage
questions. The <a href="http://xmlsoft.org/book1.html">auto-generated
documentantion</a> is not as polished as I would like (i need to learn more
about Docbook), but it's a good starting point.</p>

<h2><a name="Downloads">Downloads</a></h2>

<p>The latest versions of libxml can be found on <a
href="ftp://rpmfind.net/pub/libxml/">rpmfind.net</a> or on the <a
href="ftp://ftp.gnome.org/pub/GNOME/MIRRORS.html">Gnome FTP server</a> either
as a <a href="ftp://ftp.gnome.org/pub/GNOME/stable/sources/libxml/">source
archive</a> or <a href="ftp://ftp.gnome.org/pub/GNOME/contrib/rpms/">RPMs
packages</a>. (NOTE that you need both the <a
href="http://rpmfind.net/linux/RPM/libxml.html">libxml</a> and <a
href="http://rpmfind.net/linux/RPM/libxml-devel.html">libxml-devel</a>
packages installed to compile applications using libxml.)</p>

<p><a name="Snapshot">Snapshot:</a></p>
<ul>
  <li>Code from the W3C cvs base libxml <a
    href="ftp://rpmfind.net/pub/libxml/cvs-snapshot.tar.gz">cvs-snapshot.tar.gz</a></li>
  <li>Docs, content of the web site, the list archive included <a
    href="ftp://rpmfind.net/pub/libxml/libxml-docs.tar.gz">libxml-docs.tar.gz</a></li>
</ul>

<p><a name="Contribs">Contribs:</a></p>

<p>I do accept external contributions, especially if compiling on another
platform, get in touch with me to upload the package. I will keep them in the
<a href="ftp://rpmfind.net/pub/libxml/contribs/">contrib directory</a></p>

<p>Libxml is also available from 2 CVs bases:</p>
<ul>
  <li><p>The <a href="http://dev.w3.org/cvsweb/XML/">W3C CVS base</a>,
    available read-only using the CVS pserver authentification (I tend to use
    this base for my own development, so it's updated more regularly, but the
    content may not be as stable):</p>
    <pre>CVSROOT=:pserver:anonymous@dev.w3.org:/sources/public
    password: anonymous
    module: XML</pre>
  </li>
  <li><p>The <a
    href="http://cvs.gnome.org/bonsai/rview.cgi?cvsroot=/cvs/gnome&dir=gnome-xml">Gnome
    CVS base</a>. Check the <a
    href="http://developer.gnome.org/tools/cvs.html">Gnome CVS Tools</a> page;
    the CVS module is <b>gnome-xml</b>.</p>
  </li>
</ul>

<h2><a name="News">News</a></h2>

<h3>CVS only : check the <a
href="http://cvs.gnome.org/lxr/source/gnome-xml/ChangeLog">Changelog</a> file
for really accurate description</h3>
<ul>
  <li>working on HTML and XML links recognition layers, get in touch with me
    if you want to test those.</li>
  <li>So far the feedback on the libxml2 beta is positive</li>
</ul>

<h3>2.0.0: Apr 3 2000</h3>
<ul>
  <li>First public release of libxml2. If you are using libxml, it's a good
    idea to check the 1.x to 2.x upgrade instructions</li>
  <li>The include are now located under $prefix/include/libxml (instead of
    $prefix/include/gnome-xml), they also are referenced by
    <pre>#include &lt;libxml/xxx.h></pre>
    <p>instead of </p>
    <pre>#include "xxx.h"</pre>
  </li>
  <li>a new URI module for parsing URIs and following strictly RFC 2396</li>
  <li>the memory allocation routines used by libxml can now be overloaded
    dynamically by using xmlMemSetup()</li>
  <li>The previously CVS only tool tester has been renamed
    <strong>xmllint</strong> and is now installed as part of the libxml2
    package</li>
  <li></li>
</ul>

<p> </p>

<h3>2.0.0beta: Mar 14 2000</h3>
<ul>
  <li>This is a first Beta release of libxml version 2</li>
  <li>It's available only from<a href="ftp://rpmfind.net/pub/libxml/">
    rpmfind.net FTP</a>, it's packaged as libxml2-2.0.0beta and available as
    tar and RPMs</li>
  <li>This version is now the head in the Gnome CVS base, the old one is
    available under the tag LIB_XML_1_X</li>
  <li>This includes a very large set of changes. Froma  programmatic point of
    view applications should not have to be modified too much, check the <a
    href="upgrade.html">upgrade page</a></li>
  <li>Some interfaces may changes (especially a bit about encoding).</li>
  <li>the updates includes:
    <ul>
      <li>fix I18N support. ISO-Latin-x/UTF-8/UTF-16 (nearly) seems correctly
        handled now</li>
      <li>Better handling of entities, especially well formedness checking and
        proper PEref extensions in external subsets</li>
      <li>DTD conditional sections</li>
      <li>Validation now correcly handle entities content</li>
      <li><a href="http://rpmfind.net/tools/gdome/messages/0039.html">change
        structures to accomodate DOM</a></li>
    </ul>
  </li>
  <li>Serious progress were made toward compliance, <a
    href="conf/result.html">here are the result of the test</a> against the
    OASIS testsuite (except the japanese tests since I don't support that
    encoding yet). This URL is rebuilt every couple of hours using the CVS
    head version.</li>
</ul>

<h3>1.8.7: Mar 6 2000</h3>
<ul>
  <li>This is a bug fix release:</li>
  <li>It is possible to disable the ignorable blanks heuristic used by
    libxml-1.x, a new function  xmlKeepBlanksDefault(0) will allow this. Note
    that for adherence to XML spec, this behaviour will be disabled by default
    in 2.x . The same function will allow to keep compatibility for old
  code.</li>
  <li>Blanks in &lt;a>  &lt;/a> constructs are not ignored anymore, avoiding
    heuristic is really the Right Way :-\</li>
  <li>The unchecked use of snprintf which was breaking libxml-1.8.6
    compilation on some platforms has been fixed</li>
  <li>nanoftp.c nanohttp.c: Fixed '#' and '?' stripping when processing
  URIs</li>
</ul>

<h3>1.8.6: Jan 31 2000</h3>
<ul>
  <li>added a nanoFTP transport module, debugged until the new version of <a
    href="http://rpmfind.net/linux/rpm2html/rpmfind.html">rpmfind</a> can use
    it without troubles</li>
</ul>

<h3>1.8.5: Jan 21 2000</h3>
<ul>
  <li>adding APIs to parse a well balanced chunk of XML (production <a
    href="http://www.w3.org/TR/REC-xml#NT-content">[43] content</a> of the XML
    spec)</li>
  <li>fixed a hideous bug in xmlGetProp pointed by Rune.Djurhuus@fast.no</li>
  <li>Jody Goldberg &lt;jgoldberg@home.com> provided another patch trying to
    solve the zlib checks problems</li>
  <li>The current state in gnome CVS base is expected to ship as 1.8.5 with
    gnumeric soon</li>
</ul>

<h3>1.8.4: Jan 13 2000</h3>
<ul>
  <li>bug fixes, reintroduced xmlNewGlobalNs(), fixed xmlNewNs()</li>
  <li>all exit() call should have been removed from libxml</li>
  <li>fixed a problem with INCLUDE_WINSOCK on WIN32 platform</li>
  <li>added newDocFragment()</li>
</ul>

<h3>1.8.3: Jan 5 2000</h3>
<ul>
  <li>a Push interface for the XML and HTML parsers</li>
  <li>a shell-like interface to the document tree (try tester --shell :-)</li>
  <li>lots of bug fixes and improvement added over XMas hollidays</li>
  <li>fixed the DTD parsing code to work with the xhtml DTD</li>
  <li>added xmlRemoveProp(), xmlRemoveID() and xmlRemoveRef()</li>
  <li>Fixed bugs in xmlNewNs()</li>
  <li>External entity loading code has been revamped, now it uses
    xmlLoadExternalEntity(), some fix on entities processing were added</li>
  <li>cleaned up WIN32 includes of socket stuff</li>
</ul>

<h3>1.8.2: Dec 21 1999</h3>
<ul>
  <li>I got another problem with includes and C++, I hope this issue is fixed
    for good this time</li>
  <li>Added a few tree modification functions: xmlReplaceNode,
    xmlAddPrevSibling, xmlAddNextSibling, xmlNodeSetName and
    xmlDocSetRootElement</li>
  <li>Tried to improve the HTML output with help from <a
    href="mailto:clahey@umich.edu">Chris Lahey</a></li>
</ul>

<h3>1.8.1: Dec 18 1999</h3>
<ul>
  <li>various patches to avoid troubles when using libxml with C++ compilers
    the "namespace" keyword and C escaping in include files</li>
  <li>a problem in one of the core macros IS_CHAR was corrected</li>
  <li>fixed a bug introduced in 1.8.0 breaking default namespace processing,
    and more specifically the Dia application</li>
  <li>fixed a posteriori validation (validation after parsing, or by using a
    Dtd not specified in the original document)</li>
  <li>fixed a bug in</li>
</ul>

<h3>1.8.0: Dec 12 1999</h3>
<ul>
  <li>cleanup, especially memory wise</li>
  <li>the parser should be more reliable, especially the HTML one, it should
    not crash, whatever the input !</li>
  <li>Integrated various patches, especially a speedup improvement for large
    dataset from <a href="mailto:cnygard@bellatlantic.net">Carl Nygard</a>,
    configure with --with-buffers to enable them.</li>
  <li>attribute normalization, oops should have been added long ago !</li>
  <li>attributes defaulted from Dtds should be available, xmlSetProp() now
    does entities escapting by default.</li>
</ul>

<h3>1.7.4: Oct 25 1999</h3>
<ul>
  <li>Lots of HTML improvement</li>
  <li>Fixed some errors when saving both XML and HTML</li>
  <li>More examples, the regression tests should now look clean</li>
  <li>Fixed a bug with contiguous charref</li>
</ul>

<h3>1.7.3: Sep 29 1999</h3>
<ul>
  <li>portability problems fixed</li>
  <li>snprintf was used unconditionnally, leading to link problems on system
    were it's not available, fixed</li>
</ul>

<h3>1.7.1: Sep 24 1999</h3>
<ul>
  <li>The basic type for strings manipulated by libxml has been renamed in
    1.7.1 from <strong>CHAR</strong> to <strong>xmlChar</strong>. The reason
    is that CHAR was conflicting with a predefined type on Windows. However on
    non WIN32 environment, compatibility is provided by the way of  a
    <strong>#define </strong>.</li>
  <li>Changed another error : the use of a structure field called errno, and
    leading to troubles on platforms where it's a macro</li>
</ul>

<h3>1.7.0: sep 23 1999</h3>
<ul>
  <li>Added the ability to fetch remote DTD or parsed entities, see the <a
    href="gnome-xml-nanohttp.html">nanohttp</a> module.</li>
  <li>Added an errno to report errors by another mean than a simple printf
    like callback</li>
  <li>Finished ID/IDREF support and checking when validation</li>
  <li>Serious memory leaks fixed (there is now a <a
    href="gnome-xml-xmlmemory.html">memory wrapper</a> module)</li>
  <li>Improvement of <a href="http://www.w3.org/TR/xpath">XPath</a>
    implementation</li>
  <li>Added an HTML parser front-end</li>
</ul>

<h2><a name="XML">XML</a></h2>

<p><a href="http://www.w3.org/TR/REC-xml">XML is a standard</a> for
markup-based structured documents. Here is <a name="example">an example XML
document</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 specifies that it's an XML document and gives useful
information about its encoding. Then the document is a text format whose
structure is specified by tags between brackets. <strong>Each tag opened has
to be closed</strong>. XML is pedantic about this. However, if a tag is empty
(no content), a single tag can serve as both the opening and closing tag if it
ends with <code>/></code> rather than with <code>></code>. Note that, for
example, the image tag has no content (just an attribute) and is closed by
ending the tag with <code>/></code>.</p>

<p>XML can be applied sucessfully to a wide range of uses, from long term
structured document maintenance (where it follows the steps of SGML) to simple
data encoding mechanisms like configuration file formatting (glade),
spreadsheets (gnumeric), or even shorter lived documents such as WebDAV where
it is used to encode remote calls between a client and a server.</p>

<h2><a name="tree">The tree output</a></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 information such
as 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 with respect to 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>xmllint</strong> which parses XML files given as argument and
prints them back as parsed. This is useful for detecting 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= linux 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 for learning the internal representation model.</p>

<h2><a name="interface">The SAX interface</a></h2>

<p>Sometimes the DOM tree output is just too large to fit reasonably into
memory. In that case (and if you don't expect to save back the XML document
loaded using libxml), it's better to use the SAX interface of libxml. SAX is a
<strong>callback-based interface</strong> to the parser. Before parsing, the
application layer registers a customized set of callbacks which are called by
the library as it progresses through the XML input.</p>

<p>To get more detailed step-by-step guidance on using the SAX interface of
libxml, see the
href="http://www.daa.com.au/~james/gnome/xml-sax/xml-sax.html">nice
documentation.written by <a href="mailto:james@daa.com.au">James
Henstridge</a>.</p>

<p>You can debug the SAX behaviour by using the <strong>testSAX</strong>
program located in the gnome-xml module (it's usually not shipped in the
binary packages of libxml, but you can find it in the tar source
distribution). Here is the sequence of callbacks that would be reported by
testSAX when parsing the example XML document shown earlier:</p>
<pre>SAX.setDocumentLocator()
SAX.startDocument()
SAX.getEntity(amp)
SAX.startElement(EXAMPLE, prop1='gnome is great', prop2='&amp;amp; linux too')
SAX.characters(   , 3)
SAX.startElement(head)
SAX.characters(    , 4)
SAX.startElement(title)
SAX.characters(Welcome to Gnome, 16)
SAX.endElement(title)
SAX.characters(   , 3)
SAX.endElement(head)
SAX.characters(   , 3)
SAX.startElement(chapter)
SAX.characters(    , 4)
SAX.startElement(title)
SAX.characters(The Linux adventure, 19)
SAX.endElement(title)
SAX.characters(    , 4)
SAX.startElement(p)
SAX.characters(bla bla bla ..., 15)
SAX.endElement(p)
SAX.characters(    , 4)
SAX.startElement(image, href='linus.gif')
SAX.endElement(image)
SAX.characters(    , 4)
SAX.startElement(p)
SAX.characters(..., 3)
SAX.endElement(p)
SAX.characters(   , 3)
SAX.endElement(chapter)
SAX.characters( , 1)
SAX.endElement(EXAMPLE)
SAX.endDocument()</pre>

<p>Most of the other functionalities of libxml are based on the DOM
tree-building facility, so nearly everything up to the end of this document
presupposes the use of the standard DOM tree build. Note that the DOM tree
itself is built by a set of registered default callbacks, without internal
specific interface.</p>

<h2><a name="library">The XML library interfaces</a></h2>

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

<p>The <a href="gnome-xml-parser.html">parser interfaces for XML</a> are
separated from the <a href="gnome-xml-htmlparser.html">HTML parser
interfaces</a>.  Let's have a look at how the XML parser can be called:</p>

<h3><a name="Invoking">Invoking the parser : the pull method</a></h3>

<p>Usually, the first thing to do is to read an XML input. The parser accepts
documents either from in-memory strings or from files.  The functions are
defined in "parser.h":</p>
<dl>
  <dt><code>xmlDocPtr xmlParseMemory(char *buffer, int size);</code></dt>
    <dd><p>Parse a null-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 (possibly compressed)
      file.</p>
    </dd>
</dl>

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

<h3 id="Invoking1">Invoking the parser: the push method</h3>

<p>In order for the application to keep the control when the document is been
fetched (which is common for GUI based programs) libxml provides a push
interface, too, as of version 1.8.3. Here are the interface functions:</p>
<pre>xmlParserCtxtPtr xmlCreatePushParserCtxt(xmlSAXHandlerPtr sax,
                                         void *user_data,
                                         const char *chunk,
                                         int size,
                                         const char *filename);
int              xmlParseChunk          (xmlParserCtxtPtr ctxt,
                                         const char *chunk,
                                         int size,
                                         int terminate);</pre>

<p>and here is a simple example showing how to use the interface:</p>
<pre>            FILE *f;

            f = fopen(filename, "r");
            if (f != NULL) {
                int res, size = 1024;
                char chars[1024];
                xmlParserCtxtPtr ctxt;

                res = fread(chars, 1, 4, f);
                if (res > 0) {
                    ctxt = xmlCreatePushParserCtxt(NULL, NULL,
                                chars, res, filename);
                    while ((res = fread(chars, 1, size, f)) > 0) {
                        xmlParseChunk(ctxt, chars, res, 0);
                    }
                    xmlParseChunk(ctxt, chars, 0, 1);
                    doc = ctxt->myDoc;
                    xmlFreeParserCtxt(ctxt);
                }
            }</pre>

<p>Also note that the HTML parser embedded into libxml also has a push
interface; the functions are just prefixed by "html" rather than "xml"</p>

<h3 id="Invoking2">Invoking the parser: the SAX interface</h3>

<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 is possible using the SAX
interfaces (see SAX.h and <a
href="http://www.daa.com.au/~james/gnome/xml-sax/xml-sax.html">James
Henstridge's documentation</a>). Note also that the push interface can be
limited to SAX. Just use the two first arguments of
<code>xmlCreatePushParserCtxt()</code>.</p>

<h3><a name="Building">Building a tree from scratch</a></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. (These are
also described in &lt;libxml/tree.h>.) For example, here is a piece of code
that produces the XML document used in the previous examples:</p>
<pre>    #include &lt;libxml/tree.h>
    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><a name="Traversing">Traversing the tree</a></h3>

<p>Basically by <a href="gnome-xml-tree.html">including "tree.h"</a> your code
has access to the internal structure of all the elements 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 title "The Linux
adventure".</p>

<p><strong>NOTE</strong>: XML allows <em>PI</em>s and <em>comments</em> to be
present before the document root, so <code>doc->root</code> may point to an
element which is not the document Root Element, a function
<code>xmlDocGetRootElement()</code> was added for this purpose.</p>

<h3><a name="Modifying">Modifying the tree</a></h3>

<p>Functions are provided for reading and writing the document content. Here
is an excerpt from the <a href="gnome-xml-tree.html">tree API</a>:</p>
<dl>
  <dt><code>xmlAttrPtr xmlSetProp(xmlNodePtr node, const xmlChar *name, const
  xmlChar *value);</code></dt>
    <dd><p>This sets (or changes) an attribute carried by an ELEMENT node. The
      value can be NULL.</p>
    </dd>
</dl>
<dl>
  <dt><code>const xmlChar *xmlGetProp(xmlNodePtr node, const xmlChar
  *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 are provided for reading and writing the text associated with
elements:</p>
<dl>
  <dt><code>xmlNodePtr xmlStringGetNodeList(xmlDocPtr doc, const xmlChar
  *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 entity
      nodes, hence the result of the function may not be a single node.</p>
    </dd>
</dl>
<dl>
  <dt><code>xmlChar *xmlNodeListGetString(xmlDocPtr doc, xmlNodePtr list, int
  inLine);</code></dt>
    <dd><p>This function is the inverse of
      <code>xmlStringGetNodeList()</code>. It generates a new string
      containing the content of the text and entity nodes. Note the extra
      argument inLine. If this argument is set to 1, the function will expand
      entity references.  For example, instead of returning the &amp;Gnome;
      XML encoding in the string, it will substitute it with its value (say,
      "GNU Network Object Model Environment"). Set this argument if you want
      to use the string for non-XML usage like User Interface.</p>
    </dd>
</dl>

<h3><a name="Saving">Saving a tree</a></h3>

<p>Basically 3 options are possible:</p>
<dl>
  <dt><code>void xmlDocDumpMemory(xmlDocPtr cur, xmlChar**mem, int
  *size);</code></dt>
    <dd><p>Returns a buffer into which the document has been saved.</p>
    </dd>
</dl>
<dl>
  <dt><code>extern void xmlDocDump(FILE *f, xmlDocPtr doc);</code></dt>
    <dd><p>Dumps a document to an open file descriptor.</p>
    </dd>
</dl>
<dl>
  <dt><code>int xmlSaveFile(const char *filename, xmlDocPtr cur);</code></dt>
    <dd><p>Saves the document to a file. In this case, the compression
      interface is triggered if it has been turned on.</p>
    </dd>
</dl>

<h3><a name="Compressio">Compression</a></h3>

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

<h2><a name="Entities">Entities or no entities</a></h2>

<p>Entities in principle are similar to simple C macros. An entity defines an
abbreviation for a given string that you can reuse many times throughout the
content of your document. Entities are especially useful when a given string
may occur frequently within a document, or to confine the change needed to a
document to a restricted area in the internal subset of the document (at the
beginning). Example:</p>
<pre>1 &lt;?xml version="1.0"?>
2 &lt;!DOCTYPE EXAMPLE SYSTEM "example.dtd" [
3 &lt;!ENTITY xml "Extensible Markup Language">
4 ]>
5 &lt;EXAMPLE>
6    &amp;xml;
7 &lt;/EXAMPLE></pre>

<p>Line 3 declares the xml entity. Line 6 uses the xml entity, by prefixing
it's name with '&amp;' and following it by ';' without any spaces added. There
are 5 predefined entities in libxml allowing you to escape charaters with
predefined meaning in some parts of the xml document content:
<strong>&amp;lt;</strong> for the character '&lt;', <strong>&amp;gt;</strong>
for the character '>',  <strong>&amp;apos;</strong> for the character ''',
<strong>&amp;quot;</strong> for the character '"', and
<strong>&amp;amp;</strong> for the character '&amp;'.</p>

<p>One of the problems related to entities is that you may want the parser to
substitute an entity's content so that you can see the replacement text in
your application. Or you may prefer to keep entity references as such in the
content to be able to save the document back without losing this usually
precious information (if the user went through the pain of explicitly defining
entities, he may have a a rather negative attitude if you blindly susbtitute
them as saving time). The <a
href="gnome-xml-parser.html#XMLSUBSTITUTEENTITIESDEFAULT">xmlSubstituteEntitiesDefault()</a>
function allows you to check and change the behaviour, which is to not
substitute entities by default.</p>

<p>Here is the DOM tree built by libxml for the previous document in the
default case:</p>
<pre>/gnome/src/gnome-xml -> /xmllint --debug test/ent1
DOCUMENT
version=1.0
   ELEMENT EXAMPLE
     TEXT
     content=
     ENTITY_REF
       INTERNAL_GENERAL_ENTITY xml
       content=Extensible Markup Language
     TEXT
     content=</pre>

<p>And here is the result when substituting entities:</p>
<pre>/gnome/src/gnome-xml -> /tester --debug --noent test/ent1
DOCUMENT
version=1.0
   ELEMENT EXAMPLE
     TEXT
     content=     Extensible Markup Language</pre>

<p>So, entities or no entities? Basically, it depends on your use case. I
suggest that you keep the non-substituting default behaviour and avoid using
entities in your XML document or data if you are not willing to handle the
entity references elements in the DOM tree.</p>

<p>Note that at save time libxml enforce the conversion of the predefined
entities where necessary to prevent well-formedness problems, and will also
transparently replace those with chars (i.e., it will not generate entity
reference elements in the DOM tree or call the reference() SAX callback when
finding them in the input).</p>

<h2><a name="Namespaces">Namespaces</a></h2>

<p>The libxml library implements <a
href="http://www.w3.org/TR/REC-xml-names/">XML namespaces</a> support by
recognizing namespace contructs in the input, and does namespace lookup
automatically when building the DOM tree. A namespace declaration is
associated with an in-memory structure and all elements or attributes within
that namespace point to it. Hence testing the namespace is a simple and fast
equality operation at the user level.</p>

<p>I suggest that people using libxml use a namespace, and declare it in the
root element of their document as the default namespace. Then they don't need
to use the prefix in the content but we will have a basis for future semantic
refinement and  merging of data from different sources. This doesn't augment
significantly the size of the XML output, but significantly increase its value
in the long-term. Example:</p>
<pre>&lt;mydoc xmlns="http://mydoc.example.org/schemas/">
   &lt;elem1>...&lt;/elem1>
   &lt;elem2>...&lt;/elem2>
&lt;/mydoc></pre>

<p>Concerning the namespace value, this has to be an URL, but the URL doesn't
have to point to any existing resource on the Web. It will bind all the
element and atributes with that URL. I suggest to use an URL within a domain
you control, and that the URL should contain some kind of version information
if possible. For example, <code>"http://www.gnome.org/gnumeric/1.0/"</code> is
a good namespace scheme.</p>

<p>Then when you load a file, make sure that a namespace carrying the
version-independent prefix is installed on the root element of your document,
and if the version information don't match something you know, warn the user
and be liberal in what you accept as the input. Also do *not* try to base
namespace checking on the prefix value. &lt;foo:text> may be exactly the same
as &lt;bar:text> in another document. What really matter is the URI associated
with the element or the attribute, not the prefix string (which is just a
shortcut for the full URI). In libxml element and attributes have a
<code>ns</code> field pointing to an xmlNs structure detailing the namespace
prefix and it's URI.</p>

<p>@@Interfaces@@</p>

<p>@@Examples@@</p>

<p>Usually people object using namespace in the case of validation, I object
this and will make sure that using namespaces won't break validity checking,
so even is you plan to use or currently are using validation I strongly
suggest adding namespaces to your document. A default namespace scheme
<code>xmlns="http://...."</code> should not break validity even on less
flexible parsers. Now using namespace to mix and differentiate content coming
from multiple DTDs will certainly break current validation schemes. I will try
to provide ways to do this, but this may not be portable or standardized.</p>

<h2><a name="Validation">Validation, or are you afraid of DTDs ?</a></h2>

<p>Well what is validation and what is a DTD ?</p>

<p>Validation is the process of checking a document against a set of
construction rules, a <strong>DTD</strong> (Document Type Definition) is such
a set of rules.</p>

<p>The validation process and building DTDs are the two most difficult parts
of  XML life cycle. Briefly a DTD defines all the possibles element to be
found within your document, what is the formal shape of your document tree (by
defining the allowed content of an element, either text, a regular expression
for the allowed list of children, or mixed content i.e. both text and childs).
The DTD also defines the allowed attributes for all elements and the types of
the attributes. For more detailed informations, I suggest to read the related
parts of the XML specification, the examples found under
gnome-xml/test/valid/dtd and the large amount of books available on XML. The
dia example in gnome-xml/test/valid should be both simple and complete enough
to allow you to build your own.</p>

<p>A word of warning, building a good DTD which will fit your needs of your
application in the long-term is far from trivial, however the extra level of
quality it can insure is well worth the price for some sets of applications or
if you already have already a DTD defined for your application field.</p>

<p>The validation is not completely finished but in a (very IMHO) usable
state. Until a real validation interface is defined the way to do it is to
define and set the <strong>xmlDoValidityCheckingDefaultValue</strong> external
variable to 1, this will of course be changed at some point:</p>

<p>extern int xmlDoValidityCheckingDefaultValue;</p>

<p>...</p>

<p>xmlDoValidityCheckingDefaultValue = 1;</p>

<p></p>

<p>To handle external entities, use the function
<strong>xmlSetExternalEntityLoader</strong>(xmlExternalEntityLoader f); to
link in you HTTP/FTP/Entities database library to the standard libxml
core.</p>

<p>@@interfaces@@</p>

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

<p>The current DOM implementation on top of libxml is the <a
href="http://cvs.gnome.org/lxr/source/gdome/">gdome Gnome module</a>, this is
a full DOM interface, thanks to <a href="mailto:raph@levien.com">Raph
Levien</a>.</p>

<p>The gnome-dom module in the Gnome CVS base is obsolete</p>

<h2><a name="Example"></a><a name="real">A real example</a></h2>

<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 dependent 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>#include &lt;libxml/tree.h>
/*
 * 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, it is also available with Makefile in the Gnome CVS
base under gnome-xml/example</p>

<p></p>

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

<p>$Id: xml.html,v 1.31 2000/03/24 13:41:54 veillard Exp $</p>
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