libxml2/doc/xmldtd.html

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<h1 align="center">Libxml DTD support</h1>

<p>Location: <a
href="http://xmlsoft.org/xmlio.html">http://xmlsoft.org/xmldtd.html</a></p>

<p>Libxml home page: <a href="http://xmlsoft.org/">http://xmlsoft.org/</a></p>

<p>Mailing-list archive:  <a
href="http://xmlsoft.org/messages/">http://xmlsoft.org/messages/</a></p>

<p>Version: $Revision$</p>

<p>Table of Content:</p>
<ol>
  <li><a href="#General">General overview</a></li>
  <li><a href="#definition">The definition</a></li>
  <li><a href="#Simple">Simple rules</a>
    <ol>
      <li><a href="#reference">How to reference a DTD from a document</a></li>
      <li><a href="#Declaring">Declaring elements</a></li>
      <li><a href="#Declaring1">Declaring attributes</a></li>
    </ol>
  </li>
  <li><a href="#Some">Some examples</a></li>
  <li><a href="#validate">How to validate</a></li>
  <li><a href="#Other">Other resources</a></li>
</ol>

<h2><a name="General">General overview</a></h2>

<p>DTD is the acronym for Document Type Definition. This is a description of
the content for a familly of XML files. This is part of the XML 1.0
specification, and alows to describe and check that a given document instance
conforms to a set of rules detailing its structure and content.</p>

<h2><a name="definition">The definition</a></h2>

<p>The <a href="http://www.w3.org/TR/REC-xml">W3C XML Recommendation</a> (<a
href="http://www.xml.com/axml/axml.html">Tim Bray's annotated version of
Rev1</a>):</p>
<ul>
  <li><a href="http://www.w3.org/TR/REC-xml#elemdecls">Declaring
  elements</a></li>
  <li><a href="http://www.w3.org/TR/REC-xml#attdecls">Declaring
  attributes</a></li>
</ul>

<p>(unfortunately) all this is inherited from the SGML world, the syntax is
ancient...</p>

<h2><a name="Simple">Simple rules</a></h2>

<p>Writing DTD can be done in multiple ways, the rules to build them if you
need something fixed or something which can evolve over time can be radically
different. Really complex DTD like Docbook ones are flexible but quite harder
to design. I will just focuse on DTDs for a formats with a fixed simple
structure. It is just a set of basic rules, and definitely not exhaustive nor
useable for complex DTD design.</p>

<h3><a name="reference">How to reference a DTD from a document</a>:</h3>

<p>Assuming the top element of the document is <code>spec</code> and the dtd
is placed in the file <code>mydtd</code> in the subdirectory <code>dtds</code>
of the directory from where the document were loaded:</p>

<p><code>&lt;!DOCTYPE spec SYSTEM "dtds/mydtd"&gt;</code></p>

<p>Notes:</p>
<ul>
  <li>the system string is actually an URI-Reference (as defined in <a
    href="http://www.ietf.org/rfc/rfc2396.txt">RFC 2396</a>) so you can use a
    full URL string indicating the location of your DTD on the Web, this is a
    really good thing to do if you want others to validate your document</li>
  <li>it is also possible to associate a <code>PUBLIC</code> identifier (a
    magic string) so that the DTd is looked up in catalogs on the client side
    without having to locate it on the web</li>
  <li>a dtd contains a set of elements and attributes declarations, but they
    don't define what the root of the document should be. This is explicitely
    told to the parser/validator as the first element of the
    <code>DOCTYPE</code> declaration.</li>
</ul>

<h3><a name="Declaring">Declaring elements</a>:</h3>

<p>The following declares an element <code>spec</code>:</p>

<p><code>&lt;!ELEMENT spec (front, body, back?)&gt;</code></p>

<p>it also expresses that the spec element contains one <code>front</code>,
one <code>body</code> and one optionnal <code>back</code> children elements in
this order. The declaration of one element of the structure and its content
are done in a single declaration. Similary the following declares
<code>div1</code> elements:</p>

<p><code>&lt;!ELEMENT div1 (head, (p | list | note)*, div2*)&gt;</code></p>

<p>means div1 contains one <code>head</code> then a series of optional
<code>p</code>, <code>list</code>s and <code>note</code>s and then an optional
<code>div2</code>. And last but not least an element can contain text:</p>

<p><code>&lt;!ELEMENT b (#PCDATA)&gt;</code></p>

<p><code>b</code> contains text or being of mixed content (text and elements
in no particular order):</p>

<p><code>&lt;!ELEMENT p (#PCDATA|a|ul|b|i|em)*&gt;</code></p>

<p><code>p </code>can contain text or <code>a</code>, <code>ul</code>,
<code>b</code>, <code>i </code>or <code>em</code> elements in no particular
order.</p>

<h3><a name="Declaring1">Declaring attributes</a>:</h3>

<p>again the attributes declaration includes their content definition:</p>

<p><code>&lt;!ATTLIST termdef name CDATA #IMPLIED&gt;</code></p>

<p>means that the element <code>termdef</code> can have a <code>name</code>
attribute containing text (<code>CDATA</code>) and which is optionnal
(<code>#IMPLIED</code>). The attribute value can also be defined within a
set:</p>

<p><code>&lt;!ATTLIST list type (bullets|ordered|glossary)
"ordered"&gt;</code></p>

<p>means <code>list</code> element have a <code>type</code> attribute with 3
allowed values "bullets", "ordered" or "glossary" and which default to
"ordered" if the attribute is not explicitely specified.</p>

<p>The content type of an attribute can be text (<code>CDATA</code>),
anchor/reference/references
(<code>ID</code>/<code>IDREF</code>/<code>IDREFS</code>), entity(ies)
(<code>ENTITY</code>/<code>ENTITIES</code>) or name(s)
(<code>NMTOKEN</code>/<code>NMTOKENS</code>). The following defines that a
<code>chapter</code> element can have an optional <code>id</code> attribute of
type <code>ID</code>, usable for reference from attribute of type IDREF:</p>

<p><code>&lt;!ATTLIST chapter id ID #IMPLIED&gt;</code></p>

<p>The last value of an attribute definition can be <code>#REQUIRED
</code>meaning that the attribute has to be given, <code>#IMPLIED</code>
meaning that it is optional, or the default value (possibly prefixed by
<code>#FIXED</code> if it is the only allowed).</p>

<p>Notes:</p>
<ul>
  <li>usually the attributes pertaining to a given element are declared in a
    single expression, but it is just a convention adopted by a lot of DTD
    writers:
    <pre>&lt;!ATTLIST termdef
          id      ID      #REQUIRED
          name    CDATA   #IMPLIED&gt;</pre>
    <p>The previous construct defines both <code>id</code> and
    <code>name</code> attributes for the element <code>termdef</code></p>
  </li>
</ul>

<h2><a name="Some">Some examples</a></h2>

<p>The directory <code>test/valid/dtds/</code> in the libxml distribution
contains some complex DTD examples. The  <code>test/valid/dia.xml</code>
example shows an XML file where the simple DTD is directly included within the
document.</p>

<h2><a name="validate">How to validate</a></h2>

<p>The simplest is to use the xmllint program comming with libxml. The
<code>--valid</code> option turn on validation of the files given as input,
for example the following validates a copy of the first revision of the XML
1.0 specification:</p>

<p><code>xmllint --valid --noout test/valid/REC-xml-19980210.xml</code></p>

<p>the -- noout is used to not output the resulting tree.</p>

<p>The <code>--dtdvalid dtd</code> allows to validate the document(s) against
a given DTD.</p>

<p>Libxml exports an API to handle DTDs and validation, check the <a
href="http://xmlsoft.org/html/gnome-xml-valid.html">associated
description</a>.</p>

<h2><a name="Other">Other resources</a></h2>

<p>DTDs are as old as SGML. So there may be a number of examples on-line, I
will just list one for now, others pointers welcome:</p>
<ul>
  <li><a href="http://www.xml101.com:8081/dtd/">XML-101 DTD</a></li>
</ul>

<p></p>

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

<p>$Id$</p>
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