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21<h1>The XML C library for Gnome</h1>
22<h2>Entities or no entities</h2>
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85<p>Entities in principle are similar to simple C macros. An entity defines an
86abbreviation for a given string that you can reuse many times throughout the
87content of your document. Entities are especially useful when a given string
88may occur frequently within a document, or to confine the change needed to a
89document to a restricted area in the internal subset of the document (at the
90beginning). Example:</p>
91<pre>1 &lt;?xml version=&quot;1.0&quot;?&gt;
922 &lt;!DOCTYPE EXAMPLE SYSTEM &quot;example.dtd&quot; [
933 &lt;!ENTITY xml &quot;Extensible Markup Language&quot;&gt;
944 ]&gt;
955 &lt;EXAMPLE&gt;
966 &amp;xml;
977 &lt;/EXAMPLE&gt;</pre>
98<p>Line 3 declares the xml entity. Line 6 uses the xml entity, by prefixing
99its name with '&amp;' and following it by ';' without any spaces added. There
100are 5 predefined entities in libxml allowing you to escape charaters with
101predefined meaning in some parts of the xml document content:
102<strong>&amp;lt;</strong> for the character '&lt;', <strong>&amp;gt;</strong>
103for the character '&gt;', <strong>&amp;apos;</strong> for the character ''',
104<strong>&amp;quot;</strong> for the character '&quot;', and
105<strong>&amp;amp;</strong> for the character '&amp;'.</p>
106<p>One of the problems related to entities is that you may want the parser to
107substitute an entity's content so that you can see the replacement text in
108your application. Or you may prefer to keep entity references as such in the
109content to be able to save the document back without losing this usually
110precious information (if the user went through the pain of explicitly
111defining entities, he may have a a rather negative attitude if you blindly
112susbtitute them as saving time). The <a href="html/libxml-parser.html#XMLSUBSTITUTEENTITIESDEFAULT">xmlSubstituteEntitiesDefault()</a>
113function allows you to check and change the behaviour, which is to not
114substitute entities by default.</p>
115<p>Here is the DOM tree built by libxml for the previous document in the
116default case:</p>
117<pre>/gnome/src/gnome-xml -&gt; ./xmllint --debug test/ent1
118DOCUMENT
119version=1.0
120 ELEMENT EXAMPLE
121 TEXT
122 content=
123 ENTITY_REF
124 INTERNAL_GENERAL_ENTITY xml
125 content=Extensible Markup Language
126 TEXT
127 content=</pre>
128<p>And here is the result when substituting entities:</p>
129<pre>/gnome/src/gnome-xml -&gt; ./tester --debug --noent test/ent1
130DOCUMENT
131version=1.0
132 ELEMENT EXAMPLE
133 TEXT
134 content= Extensible Markup Language</pre>
135<p>So, entities or no entities? Basically, it depends on your use case. I
136suggest that you keep the non-substituting default behaviour and avoid using
137entities in your XML document or data if you are not willing to handle the
138entity references elements in the DOM tree.</p>
139<p>Note that at save time libxml enforces the conversion of the predefined
140entities where necessary to prevent well-formedness problems, and will also
141transparently replace those with chars (i.e. it will not generate entity
142reference elements in the DOM tree or call the reference() SAX callback when
143finding them in the input).</p>
144<p>
145<span style="background-color: #FF0000">WARNING</span>: handling entities
146on top of the libxml SAX interface is difficult!!! If you plan to use
147non-predefined entities in your documents, then the learning cuvre to handle
148then using the SAX API may be long. If you plan to use complex documents, I
149strongly suggest you consider using the DOM interface instead and let libxml
150deal with the complexity rather than trying to do it yourself.</p>
151<p><a href="mailto:daniel@veillard.com">Daniel Veillard</a></p>
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