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   10 </style><title>Validation &amp; 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 &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="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 &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><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>
   11   <li><a href="#General5">General overview</a></li>
   12   <li><a href="#definition">The definition</a></li>
   13   <li><a href="#Simple">Simple rules</a>
   14     <ol>
   15       <li><a href="#reference">How to reference a DTD from a document</a></li>
   16       <li><a href="#Declaring">Declaring elements</a></li>
   17       <li><a href="#Declaring1">Declaring attributes</a></li>
   18     </ol>
   19   </li>
   20   <li><a href="#Some">Some examples</a></li>
   21   <li><a href="#validate">How to validate</a></li>
   22   <li><a href="#Other">Other resources</a></li>
   23 </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
   24 the content for a family of XML files. This is part of the XML 1.0
   25 specification, and allows one to describe and verify that a given document
   26 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
   27 generally against a set of construction rules).</p><p>The validation process and building DTDs are the two most difficult parts
   28 of the XML life cycle. Briefly a DTD defines all the possible elements to be
   29 found within your document, what is the formal shape of your document tree
   30 (by defining the allowed content of an element; either text, a regular
   31 expression for the allowed list of children, or mixed content i.e. both text
   32 and children). The DTD also defines the valid attributes for all elements and
   33 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
   34 Rev1</a>):</p><ul>
   35   <li><a href="http://www.w3.org/TR/REC-xml#elemdecls">Declaring
   36   elements</a></li>
   37   <li><a href="http://www.w3.org/TR/REC-xml#attdecls">Declaring
   38   attributes</a></li>
   39 </ul><p>(unfortunately) all this is inherited from the SGML world, the syntax is
   40 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
   41 something permanent or something which can evolve over time can be radically
   42 different. Really complex DTDs like DocBook ones are flexible but quite
   43 harder to design. I will just focus on DTDs for a formats with a fixed simple
   44 structure. It is just a set of basic rules, and definitely not exhaustive nor
   45 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
   46 is placed in the file <code>mydtd</code> in the subdirectory
   47 <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>
   48   <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
   49     full URL string indicating the location of your DTD on the Web. This is a
   50     really good thing to do if you want others to validate your document.</li>
   51   <li>It is also possible to associate a <code>PUBLIC</code> identifier (a
   52     magic string) so that the DTD is looked up in catalogs on the client side
   53     without having to locate it on the web.</li>
   54   <li>A DTD contains a set of element and attribute declarations, but they
   55     don't define what the root of the document should be. This is explicitly
   56     told to the parser/validator as the first element of the
   57     <code>DOCTYPE</code> declaration.</li>
   58 </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>,
   59 one <code>body</code> and one optional <code>back</code> children elements in
   60 this order. The declaration of one element of the structure and its content
   61 are done in a single declaration. Similarly the following declares
   62 <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
   63 <code>p</code>, <code>list</code>s and <code>note</code>s and then an
   64 optional <code>div2</code>. And last but not least an element can contain
   65 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
   66 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>,
   67 <code>b</code>, <code>i </code>or <code>em</code> elements in no particular
   68 order.</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>
   69 attribute containing text (<code>CDATA</code>) and which is optional
   70 (<code>#IMPLIED</code>). The attribute value can also be defined within a
   71 set:</p><p><code>&lt;!ATTLIST list type (bullets|ordered|glossary)
   72 "ordered"&gt;</code></p><p>means <code>list</code> element have a <code>type</code> attribute with 3
   73 allowed values "bullets", "ordered" or "glossary" and which default to
   74 "ordered" if the attribute is not explicitly specified.</p><p>The content type of an attribute can be text (<code>CDATA</code>),
   75 anchor/reference/references
   76 (<code>ID</code>/<code>IDREF</code>/<code>IDREFS</code>), entity(ies)
   77 (<code>ENTITY</code>/<code>ENTITIES</code>) or name(s)
   78 (<code>NMTOKEN</code>/<code>NMTOKENS</code>). The following defines that a
   79 <code>chapter</code> element can have an optional <code>id</code> attribute
   80 of type <code>ID</code>, usable for reference from attribute of type
   81 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
   82 </code>meaning that the attribute has to be given, <code>#IMPLIED</code>
   83 meaning that it is optional, or the default value (possibly prefixed by
   84 <code>#FIXED</code> if it is the only allowed).</p><p>Notes:</p><ul>
   85   <li>Usually the attributes pertaining to a given element are declared in a
   86     single expression, but it is just a convention adopted by a lot of DTD
   87     writers:
   88     <pre>&lt;!ATTLIST termdef
   89           id      ID      #REQUIRED
   90           name    CDATA   #IMPLIED&gt;</pre>
   91     <p>The previous construct defines both <code>id</code> and
   92     <code>name</code> attributes for the element <code>termdef</code>.</p>
   93   </li>
   94 </ul><h3><a name="Some1" id="Some1">Some examples</a></h3><p>The directory <code>test/valid/dtds/</code> in the libxml2 distribution
   95 contains some complex DTD examples. The example in the file
   96 <code>test/valid/dia.xml</code> shows an XML file where the simple DTD is
   97 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
   98 <code>--valid</code> option turns-on validation of the files given as input.
   99 For example the following validates a copy of the first revision of the XML
  100 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)
  101 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
  102 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
  103 will just list one for now, others pointers welcome:</p><ul>
  104   <li><a href="http://www.xml101.com:8081/dtd/">XML-101 DTD</a></li>
  105 </ul><p>I suggest looking at the examples found under test/valid/dtd and any of
  106 the large number of books available on XML. The dia example in test/valid
  107 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>