DTD

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DTD

Document Type Definition (DTD)

A '''Document Type Definition''' is a construction in an XML document which defines the terms for the document. A DTD may be embedded directly in the XML document itself, or it may be a standalone document to which the XML document makes reference.

A DTD defines the members of an XML document. There are four members which may be defined in the DTD: elements, entities, attributes, and ?notations.

Elements

Elements are the data-containing constructs which actually format the XML document. An example of an element may be the <title> element or the <body> element. Note that elements correspond to the tags used in the creation of the XML document. An element definition would appear as follows:

<!ELEMENT body (#PCDATA,section,paragraph)*>

Entities

Entities are like constants which may be reused throughout the XML document, or within the DTD itself. An example of an entity definition would be as follows:

<!ENTITY % inline "bold,italic,underline,strikethru,sub,super">

This could then be reused in other definitions:

<!ELEMENT body (#PCDATA,&inline;)*>

Attributes

Attributes are like ?properties which can be added to tags in order to provide more information about the structure of the information. At attribute definition might be as follows:

<!ATTLIST link
  id IDREF #REQUIRED
  href CDATA #REQUIRED>

?Notations

?Notations contain information about an attribute or entity which required accessing an external application to process data. An example of a ?notation declaration would be as follows:

<!NOTATION tiff SYSTEM "/usr/local/bin/display.exe">

Used together, these four members completely define a valid XML document.

last edited (December 29, 2002) by towercpu, Number of views: 3133, Current Rev: 5 (Diff)

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