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