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258 lines
14 KiB
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<title>Validation & DTDs</title>
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<table border="0" width="100%" cellpadding="5" cellspacing="0" align="center"><tr>
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<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>
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<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">
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<h1>The XML C library for Gnome</h1>
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<h2>Validation & DTDs</h2>
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</td></tr></table></td></tr></table></td>
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<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>
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<li><a href="index.html">Home</a></li>
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<li><a href="interface.html">The SAX interface</a></li>
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<li><a href="xmldtd.html">Validation & DTDs</a></li>
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<li><a href="xmlmem.html">Memory Management</a></li>
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<li><a href="encoding.html">Encodings support</a></li>
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<li><a href="namespaces.html">Namespaces</a></li>
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<li><a href="upgrade.html">Upgrading 1.x code</a></li>
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<li><a href="threads.html">Thread safety</a></li>
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<li><a href="DOM.html">DOM Principles</a></li>
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<li><a href="example.html">A real example</a></li>
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<table width="100%" border="0" cellspacing="1" cellpadding="3">
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<tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>Related links</b></center></td></tr>
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<tr><td bgcolor="#fffacd"><ul>
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<li><a href="http://mail.gnome.org/archives/xml/">Mail archive</a></li>
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<li><a href="http://xmlsoft.org/XSLT/">XSLT libxslt</a></li>
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<li><a href="http://phd.cs.unibo.it/gdome2/">DOM gdome2</a></li>
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<li><a href="http://www.aleksey.com/xmlsec/">XML-DSig xmlsec</a></li>
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<li><a href="ftp://xmlsoft.org/">FTP</a></li>
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<li><a href="http://www.zlatkovic.com/projects/libxml/">Windows binaries</a></li>
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<li><a href="http://garypennington.net/libxml2/">Solaris binaries</a></li>
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<li><a href="http://www.zveno.com/open_source/libxml2xslt.html">MacOsX binaries</a></li>
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<li><a href="http://sourceforge.net/projects/libxml2-pas/">Pascal bindings</a></li>
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<li><a href="http://bugzilla.gnome.org/buglist.cgi?product=libxml&product=libxml2">Bug Tracker</a></li>
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</ul></td></tr>
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<table width="100%" border="0" cellspacing="1" cellpadding="3">
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<tr><td colspan="1" bgcolor="#eecfa1" align="center"><center><b>API Indexes</b></center></td></tr>
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<li><a href="APIchunk0.html">Alphabetic</a></li>
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<li><a href="APIconstructors.html">Constructors</a></li>
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<li><a href="APIfunctions.html">Functions/Types</a></li>
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<li><a href="APIfiles.html">Modules</a></li>
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<li><a href="APIsymbols.html">Symbols</a></li>
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</ul></td></tr>
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</table>
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</td></tr></table></td>
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<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">
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<p>Table of Content:</p>
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<ol>
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<li><a href="#General5">General overview</a></li>
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<li><a href="#definition">The definition</a></li>
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<li>
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<a href="#Simple">Simple rules</a>
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<ol>
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<li><a href="#reference">How to reference a DTD from a document</a></li>
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<li><a href="#Declaring">Declaring elements</a></li>
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<li><a href="#Declaring1">Declaring attributes</a></li>
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</ol>
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</li>
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<li><a href="#Some">Some examples</a></li>
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<li><a href="#validate">How to validate</a></li>
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<li><a href="#Other">Other resources</a></li>
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</ol>
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<h3><a name="General5">General overview</a></h3>
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<p>Well what is validation and what is a DTD ?</p>
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<p>DTD is the acronym for Document Type Definition. This is a description of
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the content for a family of XML files. This is part of the XML 1.0
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specification, and allows one to describe and verify that a given document
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instance conforms to the set of rules detailing its structure and content.</p>
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<p>Validation is the process of checking a document against a DTD (more
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generally against a set of construction rules).</p>
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<p>The validation process and building DTDs are the two most difficult parts
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of the XML life cycle. Briefly a DTD defines all the possible elements to be
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found within your document, what is the formal shape of your document tree
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(by defining the allowed content of an element; either text, a regular
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expression for the allowed list of children, or mixed content i.e. both text
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and children). The DTD also defines the valid attributes for all elements and
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the types of those attributes.</p>
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<h3><a name="definition1">The definition</a></h3>
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<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
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Rev1</a>):</p>
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<ul>
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<li><a href="http://www.w3.org/TR/REC-xml#elemdecls">Declaring
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elements</a></li>
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<li><a href="http://www.w3.org/TR/REC-xml#attdecls">Declaring
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attributes</a></li>
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</ul>
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<p>(unfortunately) all this is inherited from the SGML world, the syntax is
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ancient...</p>
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<h3><a name="Simple1">Simple rules</a></h3>
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<p>Writing DTDs can be done in many ways. The rules to build them if you need
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something permanent or something which can evolve over time can be radically
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different. Really complex DTDs like DocBook ones are flexible but quite
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harder to design. I will just focus on DTDs for a formats with a fixed simple
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structure. It is just a set of basic rules, and definitely not exhaustive nor
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usable for complex DTD design.</p>
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<h4>
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<a name="reference1">How to reference a DTD from a document</a>:</h4>
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<p>Assuming the top element of the document is <code>spec</code> and the dtd
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is placed in the file <code>mydtd</code> in the subdirectory
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<code>dtds</code> of the directory from where the document were loaded:</p>
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<p><code><!DOCTYPE spec SYSTEM "dtds/mydtd"></code></p>
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<p>Notes:</p>
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<ul>
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<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
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full URL string indicating the location of your DTD on the Web. This is a
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really good thing to do if you want others to validate your document.</li>
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<li>It is also possible to associate a <code>PUBLIC</code> identifier (a
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magic string) so that the DTD is looked up in catalogs on the client side
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without having to locate it on the web.</li>
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<li>A DTD contains a set of element and attribute declarations, but they
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don't define what the root of the document should be. This is explicitly
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told to the parser/validator as the first element of the
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<code>DOCTYPE</code> declaration.</li>
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</ul>
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<h4>
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<a name="Declaring2">Declaring elements</a>:</h4>
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<p>The following declares an element <code>spec</code>:</p>
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<p><code><!ELEMENT spec (front, body, back?)></code></p>
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<p>It also expresses that the spec element contains one <code>front</code>,
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one <code>body</code> and one optional <code>back</code> children elements in
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this order. The declaration of one element of the structure and its content
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are done in a single declaration. Similarly the following declares
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<code>div1</code> elements:</p>
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<p><code><!ELEMENT div1 (head, (p | list | note)*, div2?)></code></p>
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<p>which means div1 contains one <code>head</code> then a series of optional
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<code>p</code>, <code>list</code>s and <code>note</code>s and then an
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optional <code>div2</code>. And last but not least an element can contain
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text:</p>
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<p><code><!ELEMENT b (#PCDATA)></code></p>
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<p>
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<code>b</code> contains text or being of mixed content (text and elements
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in no particular order):</p>
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<p><code><!ELEMENT p (#PCDATA|a|ul|b|i|em)*></code></p>
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<p>
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<code>p </code>can contain text or <code>a</code>, <code>ul</code>,
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<code>b</code>, <code>i </code>or <code>em</code> elements in no particular
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order.</p>
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<h4>
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<a name="Declaring1">Declaring attributes</a>:</h4>
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<p>Again the attributes declaration includes their content definition:</p>
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<p><code><!ATTLIST termdef name CDATA #IMPLIED></code></p>
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<p>means that the element <code>termdef</code> can have a <code>name</code>
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attribute containing text (<code>CDATA</code>) and which is optional
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(<code>#IMPLIED</code>). The attribute value can also be defined within a
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set:</p>
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<p><code><!ATTLIST list type (bullets|ordered|glossary)
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"ordered"></code></p>
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<p>means <code>list</code> element have a <code>type</code> attribute with 3
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allowed values "bullets", "ordered" or "glossary" and which default to
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"ordered" if the attribute is not explicitly specified.</p>
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<p>The content type of an attribute can be text (<code>CDATA</code>),
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anchor/reference/references
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(<code>ID</code>/<code>IDREF</code>/<code>IDREFS</code>), entity(ies)
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(<code>ENTITY</code>/<code>ENTITIES</code>) or name(s)
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(<code>NMTOKEN</code>/<code>NMTOKENS</code>). The following defines that a
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<code>chapter</code> element can have an optional <code>id</code> attribute
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of type <code>ID</code>, usable for reference from attribute of type
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IDREF:</p>
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<p><code><!ATTLIST chapter id ID #IMPLIED></code></p>
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<p>The last value of an attribute definition can be <code>#REQUIRED
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</code>meaning that the attribute has to be given, <code>#IMPLIED</code>
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meaning that it is optional, or the default value (possibly prefixed by
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<code>#FIXED</code> if it is the only allowed).</p>
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<p>Notes:</p>
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<ul>
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<li>Usually the attributes pertaining to a given element are declared in a
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single expression, but it is just a convention adopted by a lot of DTD
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writers:
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<pre><!ATTLIST termdef
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id ID #REQUIRED
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name CDATA #IMPLIED></pre>
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<p>The previous construct defines both <code>id</code> and
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<code>name</code> attributes for the element <code>termdef</code>.</p>
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</li>
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</ul>
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<h3><a name="Some1">Some examples</a></h3>
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<p>The directory <code>test/valid/dtds/</code> in the libxml distribution
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contains some complex DTD examples. The example in the file
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<code>test/valid/dia.xml</code> shows an XML file where the simple DTD is
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directly included within the document.</p>
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<h3><a name="validate1">How to validate</a></h3>
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<p>The simplest way is to use the xmllint program included with libxml. The
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<code>--valid</code> option turns-on validation of the files given as input.
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For example the following validates a copy of the first revision of the XML
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1.0 specification:</p>
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<p><code>xmllint --valid --noout test/valid/REC-xml-19980210.xml</code></p>
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<p>the -- noout is used to disable output of the resulting tree.</p>
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<p>The <code>--dtdvalid dtd</code> allows validation of the document(s)
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against a given DTD.</p>
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<p>Libxml exports an API to handle DTDs and validation, check the <a href="http://xmlsoft.org/html/libxml-valid.html">associated
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description</a>.</p>
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<h3><a name="Other1">Other resources</a></h3>
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<p>DTDs are as old as SGML. So there may be a number of examples on-line, I
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will just list one for now, others pointers welcome:</p>
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<ul>
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<li><a href="http://www.xml101.com:8081/dtd/">XML-101 DTD</a></li>
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</ul>
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<p>I suggest looking at the examples found under test/valid/dtd and any of
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the large number of books available on XML. The dia example in test/valid
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should be both simple and complete enough to allow you to build your own.</p>
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<p></p>
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<p><a href="bugs.html">Daniel Veillard</a></p>
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