What is XML ?

XML is a W3C Recommendation for building tag-based structured documents/data.

Tags like in HTML/SGML : <myTag>

Structured: XML requires proper nesting and closing

<myTag> bla bla <unclosed> bla bla </myTag>
<myTag> bla bla <nesting> bla bla </myTag> </nesting>

Why is XML useful for free software ?

Well defined, strong I18N support, open specification
vendor independant, and getting wide industry acceptance
ease reuse of data between projects
allow to share code

A few examples:

config file format (GConf, gphoto, ...)
data format (Gnumeric, abiword, advogato, packages, ...)
client-server protocols (WebDav, rpmfind, ...)

What is libxml

Libxml is an XML toolkit:

developped within the Gnome (gnome-xml) project, and also as one of W3C software
available under LGPL and the W3C IPR
present in Gnome and W3C CVS bases
Web page is at http://xmlsoft.org/
Packaged as libxml on most Linux distributions
Extremely portable (Windows, Unices, Embedded targets, PSion, ...)
Version 1.0 shipped beginning of 1999. Version 2.0 released beginning of April
conformance tested against the OASis test suite

Libxml main interfaces

The parser: does Well-Formeness and DTD validation, handle namespaces, can operate in pull or push modes
SAX: callback based interface (opening, closing, characters ...)
DOM tree: build a full in-memory tree followingly DOM interfaces
HTML: an HTML parser, generating either SAX callbacks or a DOM tree
Tree: routines to create/modify/save a DOM tree. Being able to save back parsed documents influenced a lot of the design of libxml
URI module
I/0 interface: modular with basic FTP and HTTP built-in
I18N handlers: defaults UTF8 UTF16 and ISO-Latin, uses iconv if found
XPath expression language to query XML documents
Debug module (including a small shell) and memory usage checking

Libxml architecture overview

libxml architecture drawing

A basic example

<?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>

The DOM tree

Internationalization

XML requires support for the following encodings:

UTF8: includes the ASCII range and use a variable lenght encoding for the rest of the full Unicode set
UTF16: uses 16bits chars (and sometime surrogates)

Libxml v1 was very weak in that area. Libxml2 has native support for both encodings and ISO-Latin-x

If iconv library is found at compile time, it is used to add support to a large (and expandable) set of encodings including the most common Japanese encodings.

Libxml can save to a specific encoding, if a character cannot be encoded it is converted to a char ref on the fly like ሴ

On the workbench

Started but not completed:

XPath: a sample and currently incomplete implementation

Todo:

XPointer: built on top of XPath allow to address any portion of an XML document
XLink detection: detect XML hypertext links and implement a simple link database
Schemas: new version obsoleting DTDs inherited from SGML

Add-ons worked on:

Gdome: the DOM interface
Gtkhtml2: new version of the Gnome HTML widget

Existing use

Libxml is already in use in a number of environments:

rpm2html/rpmfind: generation/maintenance/client for 300Megs of RDF encoded linux software package metadata
configuration files for programs, metadata (gconf, glade, nautilus)
data format for gnome programs (gill, gnumeric, gphoto, ...)
embedded system (TV desktop set)
handling of satellite images

However most users just don't give feedback, but patches are coming (and usually accepted !)

Future

Focuse on implementing future XML specifications

Would like a basic rendering widget (gtkhtml2)

Would like volunteers for an XSLT implementation