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| <h1>The XML C library for Gnome</h1> |
| <h2>Validation & DTDs</h2> |
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| <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> |
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| <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">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 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> |
| <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 possibles element 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 allowed attributes for all elements |
| and the types of the attributes.</p> |
| <h3><a name="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">Simple rules</a></h3> |
| <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> |
| <h4> |
| <a name="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 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> |
| <h4> |
| <a name="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 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><!ELEMENT div1 (head, (p | list | note)*, div2*)></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><!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">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 optionnal |
| (<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 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><!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">Some examples</a></h3> |
| <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> |
| <h3><a name="validate1">How to validate</a></h3> |
| <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/libxml-valid.html">associated |
| description</a>.</p> |
| <h3><a name="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><a href="mailto:daniel@veillard.com">Daniel Veillard</a></p> |
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