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13<title>Entities or no entities</title>
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21<h1>The XML C library for Gnome</h1>
22<h2>Entities or no entities</h2>
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75<p>Entities in principle are similar to simple C macros. An entity defines an
76abbreviation for a given string that you can reuse many times throughout the
77content of your document. Entities are especially useful when a given string
78may occur frequently within a document, or to confine the change needed to a
79document to a restricted area in the internal subset of the document (at the
80beginning). Example:</p>
81<pre>1 &lt;?xml version=&quot;1.0&quot;?&gt;
822 &lt;!DOCTYPE EXAMPLE SYSTEM &quot;example.dtd&quot; [
833 &lt;!ENTITY xml &quot;Extensible Markup Language&quot;&gt;
844 ]&gt;
855 &lt;EXAMPLE&gt;
866 &amp;xml;
877 &lt;/EXAMPLE&gt;</pre>
88<p>Line 3 declares the xml entity. Line 6 uses the xml entity, by prefixing
89its name with '&amp;' and following it by ';' without any spaces added. There
90are 5 predefined entities in libxml allowing you to escape charaters with
91predefined meaning in some parts of the xml document content:
92<strong>&amp;lt;</strong> for the character '&lt;', <strong>&amp;gt;</strong>
93for the character '&gt;', <strong>&amp;apos;</strong> for the character ''',
94<strong>&amp;quot;</strong> for the character '&quot;', and
95<strong>&amp;amp;</strong> for the character '&amp;'.</p>
96<p>One of the problems related to entities is that you may want the parser to
97substitute an entity's content so that you can see the replacement text in
98your application. Or you may prefer to keep entity references as such in the
99content to be able to save the document back without losing this usually
100precious information (if the user went through the pain of explicitly
101defining entities, he may have a a rather negative attitude if you blindly
102susbtitute them as saving time). The <a href="html/libxml-parser.html#XMLSUBSTITUTEENTITIESDEFAULT">xmlSubstituteEntitiesDefault()</a>
103function allows you to check and change the behaviour, which is to not
104substitute entities by default.</p>
105<p>Here is the DOM tree built by libxml for the previous document in the
106default case:</p>
107<pre>/gnome/src/gnome-xml -&gt; ./xmllint --debug test/ent1
108DOCUMENT
109version=1.0
110 ELEMENT EXAMPLE
111 TEXT
112 content=
113 ENTITY_REF
114 INTERNAL_GENERAL_ENTITY xml
115 content=Extensible Markup Language
116 TEXT
117 content=</pre>
118<p>And here is the result when substituting entities:</p>
119<pre>/gnome/src/gnome-xml -&gt; ./tester --debug --noent test/ent1
120DOCUMENT
121version=1.0
122 ELEMENT EXAMPLE
123 TEXT
124 content= Extensible Markup Language</pre>
125<p>So, entities or no entities? Basically, it depends on your use case. I
126suggest that you keep the non-substituting default behaviour and avoid using
127entities in your XML document or data if you are not willing to handle the
128entity references elements in the DOM tree.</p>
129<p>Note that at save time libxml enforces the conversion of the predefined
130entities where necessary to prevent well-formedness problems, and will also
131transparently replace those with chars (i.e. it will not generate entity
132reference elements in the DOM tree or call the reference() SAX callback when
133finding them in the input).</p>
134<p>
135<span style="background-color: #FF0000">WARNING</span>: handling entities
136on top of the libxml SAX interface is difficult!!! If you plan to use
137non-predefined entities in your documents, then the learning cuvre to handle
138then using the SAX API may be long. If you plan to use complex documents, I
139strongly suggest you consider using the DOM interface instead and let libxml
140deal with the complexity rather than trying to do it yourself.</p>
141<p><a href="mailto:daniel@veillard.com">Daniel Veillard</a></p>
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