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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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73<p>Entities in principle are similar to simple C macros. An entity defines an
74abbreviation for a given string that you can reuse many times throughout the
75content of your document. Entities are especially useful when a given string
76may occur frequently within a document, or to confine the change needed to a
77document to a restricted area in the internal subset of the document (at the
78beginning). Example:</p>
79<pre>1 &lt;?xml version=&quot;1.0&quot;?&gt;
802 &lt;!DOCTYPE EXAMPLE SYSTEM &quot;example.dtd&quot; [
813 &lt;!ENTITY xml &quot;Extensible Markup Language&quot;&gt;
824 ]&gt;
835 &lt;EXAMPLE&gt;
846 &amp;xml;
857 &lt;/EXAMPLE&gt;</pre>
86<p>Line 3 declares the xml entity. Line 6 uses the xml entity, by prefixing
87its name with '&amp;' and following it by ';' without any spaces added. There
88are 5 predefined entities in libxml allowing you to escape charaters with
89predefined meaning in some parts of the xml document content:
90<strong>&amp;lt;</strong> for the character '&lt;', <strong>&amp;gt;</strong>
91for the character '&gt;', <strong>&amp;apos;</strong> for the character ''',
92<strong>&amp;quot;</strong> for the character '&quot;', and
93<strong>&amp;amp;</strong> for the character '&amp;'.</p>
94<p>One of the problems related to entities is that you may want the parser to
95substitute an entity's content so that you can see the replacement text in
96your application. Or you may prefer to keep entity references as such in the
97content to be able to save the document back without losing this usually
98precious information (if the user went through the pain of explicitly
99defining entities, he may have a a rather negative attitude if you blindly
100susbtitute them as saving time). The <a href="html/libxml-parser.html#XMLSUBSTITUTEENTITIESDEFAULT">xmlSubstituteEntitiesDefault()</a>
101function allows you to check and change the behaviour, which is to not
102substitute entities by default.</p>
103<p>Here is the DOM tree built by libxml for the previous document in the
104default case:</p>
105<pre>/gnome/src/gnome-xml -&gt; ./xmllint --debug test/ent1
106DOCUMENT
107version=1.0
108 ELEMENT EXAMPLE
109 TEXT
110 content=
111 ENTITY_REF
112 INTERNAL_GENERAL_ENTITY xml
113 content=Extensible Markup Language
114 TEXT
115 content=</pre>
116<p>And here is the result when substituting entities:</p>
117<pre>/gnome/src/gnome-xml -&gt; ./tester --debug --noent test/ent1
118DOCUMENT
119version=1.0
120 ELEMENT EXAMPLE
121 TEXT
122 content= Extensible Markup Language</pre>
123<p>So, entities or no entities? Basically, it depends on your use case. I
124suggest that you keep the non-substituting default behaviour and avoid using
125entities in your XML document or data if you are not willing to handle the
126entity references elements in the DOM tree.</p>
127<p>Note that at save time libxml enforces the conversion of the predefined
128entities where necessary to prevent well-formedness problems, and will also
129transparently replace those with chars (i.e. it will not generate entity
130reference elements in the DOM tree or call the reference() SAX callback when
131finding them in the input).</p>
132<p>
133<span style="background-color: #FF0000">WARNING</span>: handling entities
134on top of the libxml SAX interface is difficult!!! If you plan to use
135non-predefined entities in your documents, then the learning cuvre to handle
136then using the SAX API may be long. If you plan to use complex documents, I
137strongly suggest you consider using the DOM interface instead and let libxml
138deal with the complexity rather than trying to do it yourself.</p>
139<p><a href="mailto:daniel@veillard.com">Daniel Veillard</a></p>
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