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</style><title>Entities or no entities</title></head><body bgcolor="#8b7765" text="#000000" link="#a06060" vlink="#000000"> |    | The XML C parser and toolkit of GnomeEntities or no entities |
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| Entities in principle are similar to simple C macros. An entity defines an |
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abbreviation for a given string that you can reuse many times throughout the
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content of your document. Entities are especially useful when a given string
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may occur frequently within a document, or to confine the change needed to a
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document to a restricted area in the internal subset of the document (at the
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beginning). Example:1 <?xml version="1.0"?>
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2 <!DOCTYPE EXAMPLE SYSTEM "example.dtd" [
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3 <!ENTITY xml "Extensible Markup Language">
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4 ]>
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5 <EXAMPLE>
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6 &xml;
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7 </EXAMPLE>Line 3 declares the xml entity. Line 6 uses the xml entity, by prefixing
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its name with '&' and following it by ';' without any spaces added. There
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are 5 predefined entities in libxml2 allowing you to escape characters with
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predefined meaning in some parts of the xml document content:
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< for the character '<', >
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for the character '>', ' for the character ''',
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" for the character '"', and
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& for the character '&'.One of the problems related to entities is that you may want the parser to
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substitute an entity's content so that you can see the replacement text in
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your application. Or you may prefer to keep entity references as such in the
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content to be able to save the document back without losing this usually
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precious information (if the user went through the pain of explicitly
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defining entities, he may have a a rather negative attitude if you blindly
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substitute them as saving time). The xmlSubstituteEntitiesDefault()
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function allows you to check and change the behaviour, which is to not
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substitute entities by default.Here is the DOM tree built by libxml2 for the previous document in the
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default case:/gnome/src/gnome-xml -> ./xmllint --debug test/ent1
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DOCUMENT
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version=1.0
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ELEMENT EXAMPLE
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TEXT
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content=
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ENTITY_REF
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INTERNAL_GENERAL_ENTITY xml
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content=Extensible Markup Language
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TEXT
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content=And here is the result when substituting entities: /gnome/src/gnome-xml -> ./tester --debug --noent test/ent1
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DOCUMENT
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version=1.0
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ELEMENT EXAMPLE
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TEXT
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content= Extensible Markup LanguageSo, entities or no entities? Basically, it depends on your use case. I
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suggest that you keep the non-substituting default behaviour and avoid using
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entities in your XML document or data if you are not willing to handle the
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entity references elements in the DOM tree.Note that at save time libxml2 enforces the conversion of the predefined
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entities where necessary to prevent well-formedness problems, and will also
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transparently replace those with chars (i.e. it will not generate entity
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reference elements in the DOM tree or call the reference() SAX callback when
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finding them in the input).WARNING: handling entities
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on top of the libxml2 SAX interface is difficult!!! If you plan to use
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non-predefined entities in your documents, then the learning curve to handle
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then using the SAX API may be long. If you plan to use complex documents, I
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strongly suggest you consider using the DOM interface instead and let libxml
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deal with the complexity rather than trying to do it yourself.Daniel Veillard </body></html>
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