ATTLIST

ATTLIST is a declaration used in SGML and XML document type definitions (DTDs) to specify the attributes for a particular element. It defines the attribute names, their data types, and how they default or require values, providing a mechanism for validating attribute usage in a document.

Syntax and components. The typical syntax is: <!ATTLIST element-name attribute-name AttType DefaultDecl>. AttType denotes the attribute's data type and can be CDATA, ID, IDREF, IDREFS, ENTITY, ENTITIES, NMTOKEN, NMTOKENS, NOTATION, or an enumerated list such as (value1|value2|...). DefaultDecl indicates whether the attribute is required, implied, or has a default value. It may be #REQUIRED, #IMPLIED, a quoted literal default, or FIXED followed by a value to enforce a constant default.

Types and examples. Common attribute types include CDATA for free text, ID for a unique identifier, and IDREF/IDREFS for cross-references. Enumerated types are written as a parenthesized list of allowed values, for example align (left|center|right). A NOTATION type references a NOTATION declared elsewhere in the DTD, restricting the value to a named notation. Example declarations: <!ATTLIST image src CDATA #REQUIRED>, <!ATTLIST table align (left|center|right) "left">, and, with NOTATION, <!ATTLIST graphic format NOTATION (svg|png) #IMPLIED>.

Role and scope. ATTLIST declarations constrain attribute usage for their element, supporting data validation and document consistency within a DTD. They operate alongside other DTD declarations and are part of XML’s DTD-based validation approach. In modern XML practice, many documents use XML Schema or other schema languages instead of DTDs, but ATTLIST remains a core concept in environments that rely on DTDs.