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| </style><title>Validation & DTDs</title></head><body bgcolor="#8b7765" text="#000000" link="#a06060" 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 & 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="html/index.html">Reference Manual</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">Releases</a></li><li><a href="XMLinfo.html">XML</a></li><li><a href="XSLT.html">XSLT</a></li><li><a href="xmldtd.html">Validation & 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><li><a href="ChangeLog.html">Recent Changes</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://opencsw.org/packages/libxml2">Solaris binaries</a></li><li><a href="http://www.explain.com.au/oss/libxml2xslt.html">MacOsX binaries</a></li><li><a href="http://lxml.de/">lxml Python bindings</a></li><li><a href="http://cpan.uwinnipeg.ca/dist/XML-LibXML">Perl bindings</a></li><li><a href="http://libxmlplusplus.sourceforge.net/">C++ bindings</a></li><li><a href="http://www.zend.com/php5/articles/php5-xmlphp.php#Heading4">PHP bindings</a></li><li><a href="http://sourceforge.net/projects/libxml2-pas/">Pascal bindings</a></li><li><a href="http://libxml.rubyforge.org/">Ruby bindings</a></li><li><a href="http://tclxml.sourceforge.net/">Tcl 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> |
| <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><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 |
| the content for a family of XML files. This is part of the XML 1.0 |
| specification, and allows one to describe and verify that a given document |
| instance 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 |
| generally against a set of construction rules).</p><p>The validation process and building DTDs are the two most difficult parts |
| of the XML life cycle. Briefly a DTD defines all the possible elements 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 children). The DTD also defines the valid attributes for all elements and |
| the 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 |
| 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><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 |
| something permanent or something which can evolve over time can be radically |
| different. Really complex DTDs like DocBook ones are flexible but quite |
| harder to design. I will just focus on DTDs for a formats with a fixed simple |
| structure. It is just a set of basic rules, and definitely not exhaustive nor |
| usable 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 |
| 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><!DOCTYPE spec SYSTEM "dtds/mydtd"></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 element and attribute declarations, but they |
| don't define what the root of the document should be. This is explicitly |
| told to the parser/validator as the first element of the |
| <code>DOCTYPE</code> declaration.</li> |
| </ul><h4><a name="Declaring2" id="Declaring2">Declaring elements</a>:</h4><p>The following declares an element <code>spec</code>:</p><p><code><!ELEMENT spec (front, body, back?)></code></p><p>It also expresses that the spec element contains one <code>front</code>, |
| one <code>body</code> and one optional <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. Similarly the following declares |
| <code>div1</code> elements:</p><p><code><!ELEMENT div1 (head, (p | list | note)*, div2?)></code></p><p>which 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><!ELEMENT b (#PCDATA)></code></p><p><code>b</code> contains text or being of mixed content (text and elements |
| in no particular order):</p><p><code><!ELEMENT p (#PCDATA|a|ul|b|i|em)*></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><h4><a name="Declaring1" id="Declaring1">Declaring attributes</a>:</h4><p>Again the attributes declaration includes their content definition:</p><p><code><!ATTLIST termdef name CDATA #IMPLIED></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 optional |
| (<code>#IMPLIED</code>). The attribute value can also be defined within a |
| set:</p><p><code><!ATTLIST list type (bullets|ordered|glossary) |
| "ordered"></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 explicitly 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><!ATTLIST chapter id ID #IMPLIED></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><!ATTLIST termdef |
| id ID #REQUIRED |
| name CDATA #IMPLIED></pre> |
| <p>The previous construct defines both <code>id</code> and |
| <code>name</code> attributes for the element <code>termdef</code>.</p> |
| </li> |
| </ul><h3><a name="Some1" id="Some1">Some examples</a></h3><p>The directory <code>test/valid/dtds/</code> in the libxml2 distribution |
| contains some complex DTD examples. The example in the file |
| <code>test/valid/dia.xml</code> shows an XML file where the simple DTD is |
| directly 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 |
| <code>--valid</code> option turns-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 disable output of the resulting tree.</p><p>The <code>--dtdvalid dtd</code> allows validation of the document(s) |
| against 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 |
| description</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 |
| 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>I suggest looking at the examples found under test/valid/dtd and any of |
| the large number of books available on XML. The dia example in test/valid |
| should 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> |