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10</style><title>Validation &amp; DTDs</title></head><body bgcolor="#8b7765" text="#000000" link="#000000" vlink="#000000"><table border="0" width="100%" cellpadding="5" cellspacing="0" align="center"><tr><td width="120"><a href="http://swpat.ffii.org/"><img src="epatents.png" alt="Action against software patents" /></a></td><td width="180"><a href="http://www.gnome.org/"><img src="gnome2.png" alt="Gnome2 Logo" /></a><a href="http://www.w3.org/Status"><img src="w3c.png" alt="W3C Logo" /></a><a href="http://www.redhat.com/"><img src="redhat.gif" alt="Red Hat Logo" /></a><div align="left"><a href="http://xmlsoft.org/"><img src="Libxml2-Logo-180x168.gif" alt="Made with Libxml2 Logo" /></a></div></td><td><table border="0" width="90%" cellpadding="2" cellspacing="0" align="center" bgcolor="#000000"><tr><td><table width="100%" border="0" cellspacing="1" cellpadding="3" bgcolor="#fffacd"><tr><td align="center"><h1>The XML C parser and toolkit of Gnome</h1><h2>Validation &amp; DTDs</h2></td></tr></table></td></tr></table></td></tr></table><table border="0" cellpadding="4" cellspacing="0" width="100%" align="center"><tr><td bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="2" width="100%"><tr><td valign="top" width="200" bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="1" width="100%" bgcolor="#000000"><tr><td><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Main Menu</b></center></td></tr><tr><td bgcolor="#fffacd"><form action="search.php" enctype="application/x-www-form-urlencoded" method="get"><input name="query" type="text" size="20" value="" /><input name="submit" type="submit" value="Search ..." /></form><ul><li><a href="index.html">Home</a></li><li><a href="intro.html">Introduction</a></li><li><a href="FAQ.html">FAQ</a></li><li><a href="docs.html" style="font-weight:bold">Developer Menu</a></li><li><a href="bugs.html">Reporting bugs and getting help</a></li><li><a href="help.html">How to help</a></li><li><a href="downloads.html">Downloads</a></li><li><a href="news.html">News</a></li><li><a href="XMLinfo.html">XML</a></li><li><a href="XSLT.html">XSLT</a></li><li><a href="xmldtd.html">Validation &amp; DTDs</a></li><li><a href="encoding.html">Encodings support</a></li><li><a href="catalog.html">Catalog support</a></li><li><a href="namespaces.html">Namespaces</a></li><li><a href="contribs.html">Contributions</a></li><li><a href="examples/index.html" style="font-weight:bold">Code Examples</a></li><li><a href="html/index.html" style="font-weight:bold">API Menu</a></li><li><a href="guidelines.html">XML Guidelines</a></li></ul></td></tr></table><table width="100%" border="0" cellspacing="1" cellpadding="3"><tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Related links</b></center></td></tr><tr><td bgcolor="#fffacd"><ul><li><a href="http://mail.gnome.org/archives/xml/">Mail archive</a></li><li><a href="http://xmlsoft.org/XSLT/">XSLT libxslt</a></li><li><a href="http://phd.cs.unibo.it/gdome2/">DOM gdome2</a></li><li><a href="http://www.aleksey.com/xmlsec/">XML-DSig xmlsec</a></li><li><a href="ftp://xmlsoft.org/">FTP</a></li><li><a href="http://www.zlatkovic.com/projects/libxml/">Windows binaries</a></li><li><a href="http://garypennington.net/libxml2/">Solaris binaries</a></li><li><a href="http://www.zveno.com/open_source/libxml2xslt.html">MacOsX binaries</a></li><li><a href="http://sourceforge.net/projects/libxml2-pas/">Pascal bindings</a></li><li><a href="http://bugzilla.gnome.org/buglist.cgi?product=libxml2">Bug Tracker</a></li></ul></td></tr></table></td></tr></table></td><td valign="top" bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="1" width="100%"><tr><td><table border="0" cellspacing="0" cellpadding="1" width="100%" bgcolor="#000000"><tr><td><table border="0" cellpadding="3" cellspacing="1" width="100%"><tr><td bgcolor="#fffacd"><p>Table of Content:</p><ol><li><a href="#General5">General overview</a></li>
11  <li><a href="#definition">The definition</a></li>
12  <li><a href="#Simple">Simple rules</a>
13    <ol><li><a href="#reference">How to reference a DTD from a document</a></li>
14      <li><a href="#Declaring">Declaring elements</a></li>
15      <li><a href="#Declaring1">Declaring attributes</a></li>
16    </ol></li>
17  <li><a href="#Some">Some examples</a></li>
18  <li><a href="#validate">How to validate</a></li>
19  <li><a href="#Other">Other resources</a></li>
20</ol><h3><a name="General5" id="General5">General overview</a></h3><p>Well what is validation and what is a DTD ?</p><p>DTD is the acronym for Document Type Definition. This is a description of
21the content for a family of XML files. This is part of the XML 1.0
22specification, and allows one to describe and verify that a given document
23instance conforms to the set of rules detailing its structure and content.</p><p>Validation is the process of checking a document against a DTD (more
24generally against a set of construction rules).</p><p>The validation process and building DTDs are the two most difficult parts
25of the XML life cycle. Briefly a DTD defines all the possible elements to be
26found within your document, what is the formal shape of your document tree
27(by defining the allowed content of an element; either text, a regular
28expression for the allowed list of children, or mixed content i.e. both text
29and children). The DTD also defines the valid attributes for all elements and
30the types of those attributes.</p><h3><a name="definition1" id="definition1">The definition</a></h3><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
31Rev1</a>):</p><ul><li><a href="http://www.w3.org/TR/REC-xml#elemdecls">Declaring
32  elements</a></li>
33  <li><a href="http://www.w3.org/TR/REC-xml#attdecls">Declaring
34  attributes</a></li>
35</ul><p>(unfortunately) all this is inherited from the SGML world, the syntax is
36ancient...</p><h3><a name="Simple1" id="Simple1">Simple rules</a></h3><p>Writing DTDs can be done in many ways. The rules to build them if you need
37something permanent or something which can evolve over time can be radically
38different. Really complex DTDs like DocBook ones are flexible but quite
39harder to design. I will just focus on DTDs for a formats with a fixed simple
40structure. It is just a set of basic rules, and definitely not exhaustive nor
41usable for complex DTD design.</p><h4><a name="reference1" id="reference1">How to reference a DTD from a document</a>:</h4><p>Assuming the top element of the document is <code>spec</code> and the dtd
42is placed in the file <code>mydtd</code> in the subdirectory
43<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
44    full URL string indicating the location of your DTD on the Web. This is a
45    really good thing to do if you want others to validate your document.</li>
46  <li>It is also possible to associate a <code>PUBLIC</code> identifier (a
47    magic string) so that the DTD is looked up in catalogs on the client side
48    without having to locate it on the web.</li>
49  <li>A DTD contains a set of element and attribute declarations, but they
50    don't define what the root of the document should be. This is explicitly
51    told to the parser/validator as the first element of the
52    <code>DOCTYPE</code> declaration.</li>
53</ul><h4><a name="Declaring2" id="Declaring2">Declaring elements</a>:</h4><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>,
54one <code>body</code> and one optional <code>back</code> children elements in
55this order. The declaration of one element of the structure and its content
56are done in a single declaration. Similarly the following declares
57<code>div1</code> elements:</p><p><code>&lt;!ELEMENT div1 (head, (p | list | note)*, div2?)&gt;</code></p><p>which means div1 contains one <code>head</code> then a series of optional
58<code>p</code>, <code>list</code>s and <code>note</code>s and then an
59optional <code>div2</code>. And last but not least an element can contain
60text:</p><p><code>&lt;!ELEMENT b (#PCDATA)&gt;</code></p><p><code>b</code> contains text or being of mixed content (text and elements
61in 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>,
62<code>b</code>, <code>i </code>or <code>em</code> elements in no particular
63order.</p><h4><a name="Declaring1" id="Declaring1">Declaring attributes</a>:</h4><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>
64attribute containing text (<code>CDATA</code>) and which is optional
65(<code>#IMPLIED</code>). The attribute value can also be defined within a
66set:</p><p><code>&lt;!ATTLIST list type (bullets|ordered|glossary)
67"ordered"&gt;</code></p><p>means <code>list</code> element have a <code>type</code> attribute with 3
68allowed values "bullets", "ordered" or "glossary" and which default to
69"ordered" if the attribute is not explicitly specified.</p><p>The content type of an attribute can be text (<code>CDATA</code>),
70anchor/reference/references
71(<code>ID</code>/<code>IDREF</code>/<code>IDREFS</code>), entity(ies)
72(<code>ENTITY</code>/<code>ENTITIES</code>) or name(s)
73(<code>NMTOKEN</code>/<code>NMTOKENS</code>). The following defines that a
74<code>chapter</code> element can have an optional <code>id</code> attribute
75of type <code>ID</code>, usable for reference from attribute of type
76IDREF:</p><p><code>&lt;!ATTLIST chapter id ID #IMPLIED&gt;</code></p><p>The last value of an attribute definition can be <code>#REQUIRED
77</code>meaning that the attribute has to be given, <code>#IMPLIED</code>
78meaning that it is optional, or the default value (possibly prefixed by
79<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
80    single expression, but it is just a convention adopted by a lot of DTD
81    writers:
82    <pre>&lt;!ATTLIST termdef
83          id      ID      #REQUIRED
84          name    CDATA   #IMPLIED&gt;</pre>
85    <p>The previous construct defines both <code>id</code> and
86    <code>name</code> attributes for the element <code>termdef</code>.</p>
87  </li>
88</ul><h3><a name="Some1" id="Some1">Some examples</a></h3><p>The directory <code>test/valid/dtds/</code> in the libxml2 distribution
89contains some complex DTD examples. The example in the file
90<code>test/valid/dia.xml</code> shows an XML file where the simple DTD is
91directly included within the document.</p><h3><a name="validate1" id="validate1">How to validate</a></h3><p>The simplest way is to use the xmllint program included with libxml. The
92<code>--valid</code> option turns-on validation of the files given as input.
93For example the following validates a copy of the first revision of the XML
941.0 specification:</p><p><code>xmllint --valid --noout test/valid/REC-xml-19980210.xml</code></p><p>the -- noout is used to disable output of the resulting tree.</p><p>The <code>--dtdvalid dtd</code> allows validation of the document(s)
95against a given DTD.</p><p>Libxml2 exports an API to handle DTDs and validation, check the <a href="http://xmlsoft.org/html/libxml-valid.html">associated
96description</a>.</p><h3><a name="Other1" id="Other1">Other resources</a></h3><p>DTDs are as old as SGML. So there may be a number of examples on-line, I
97will just list one for now, others pointers welcome:</p><ul><li><a href="http://www.xml101.com:8081/dtd/">XML-101 DTD</a></li>
98</ul><p>I suggest looking at the examples found under test/valid/dtd and any of
99the large number of books available on XML. The dia example in test/valid
100should be both simple and complete enough to allow you to build your own.</p><p></p><p><a href="bugs.html">Daniel Veillard</a></p></td></tr></table></td></tr></table></td></tr></table></td></tr></table></td></tr></table></body></html>
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