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14<title>Entities or no entities</title>
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22<h1>The XML C library for Gnome</h1>
23<h2>Entities or no entities</h2>
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89<p>Entities in principle are similar to simple C macros. An entity defines an
90abbreviation for a given string that you can reuse many times throughout the
91content of your document. Entities are especially useful when a given string
92may occur frequently within a document, or to confine the change needed to a
93document to a restricted area in the internal subset of the document (at the
94beginning). Example:</p>
95<pre>1 &lt;?xml version=&quot;1.0&quot;?&gt;
962 &lt;!DOCTYPE EXAMPLE SYSTEM &quot;example.dtd&quot; [
973 &lt;!ENTITY xml &quot;Extensible Markup Language&quot;&gt;
984 ]&gt;
995 &lt;EXAMPLE&gt;
1006 &amp;xml;
1017 &lt;/EXAMPLE&gt;</pre>
102<p>Line 3 declares the xml entity. Line 6 uses the xml entity, by prefixing
103its name with '&amp;' and following it by ';' without any spaces added. There
Daniel Veillard63d83142002-05-20 06:51:05 +0000[diff] [blame^]104are 5 predefined entities in libxml allowing you to escape characters with
Daniel Veillardc9484202001-10-24 12:35:52 +0000[diff] [blame]105predefined meaning in some parts of the xml document content:
106<strong>&amp;lt;</strong> for the character '&lt;', <strong>&amp;gt;</strong>
107for the character '&gt;', <strong>&amp;apos;</strong> for the character ''',
108<strong>&amp;quot;</strong> for the character '&quot;', and
109<strong>&amp;amp;</strong> for the character '&amp;'.</p>
110<p>One of the problems related to entities is that you may want the parser to
111substitute an entity's content so that you can see the replacement text in
112your application. Or you may prefer to keep entity references as such in the
113content to be able to save the document back without losing this usually
114precious information (if the user went through the pain of explicitly
115defining entities, he may have a a rather negative attitude if you blindly
Daniel Veillard63d83142002-05-20 06:51:05 +0000[diff] [blame^]116substitute them as saving time). The <a href="html/libxml-parser.html#XMLSUBSTITUTEENTITIESDEFAULT">xmlSubstituteEntitiesDefault()</a>
Daniel Veillardc9484202001-10-24 12:35:52 +0000[diff] [blame]117function allows you to check and change the behaviour, which is to not
118substitute entities by default.</p>
119<p>Here is the DOM tree built by libxml for the previous document in the
120default case:</p>
121<pre>/gnome/src/gnome-xml -&gt; ./xmllint --debug test/ent1
122DOCUMENT
123version=1.0
124 ELEMENT EXAMPLE
125 TEXT
126 content=
127 ENTITY_REF
128 INTERNAL_GENERAL_ENTITY xml
129 content=Extensible Markup Language
130 TEXT
131 content=</pre>
132<p>And here is the result when substituting entities:</p>
133<pre>/gnome/src/gnome-xml -&gt; ./tester --debug --noent test/ent1
134DOCUMENT
135version=1.0
136 ELEMENT EXAMPLE
137 TEXT
138 content= Extensible Markup Language</pre>
139<p>So, entities or no entities? Basically, it depends on your use case. I
140suggest that you keep the non-substituting default behaviour and avoid using
141entities in your XML document or data if you are not willing to handle the
142entity references elements in the DOM tree.</p>
143<p>Note that at save time libxml enforces the conversion of the predefined
144entities where necessary to prevent well-formedness problems, and will also
145transparently replace those with chars (i.e. it will not generate entity
146reference elements in the DOM tree or call the reference() SAX callback when
147finding them in the input).</p>
148<p>
149<span style="background-color: #FF0000">WARNING</span>: handling entities
150on top of the libxml SAX interface is difficult!!! If you plan to use
Daniel Veillard63d83142002-05-20 06:51:05 +0000[diff] [blame^]151non-predefined entities in your documents, then the learning curve to handle
Daniel Veillardc9484202001-10-24 12:35:52 +0000[diff] [blame]152then using the SAX API may be long. If you plan to use complex documents, I
153strongly suggest you consider using the DOM interface instead and let libxml
154deal with the complexity rather than trying to do it yourself.</p>
Daniel Veillard3f4c40f2002-02-13 09:19:28 +0000[diff] [blame]155<p><a href="bugs.html">Daniel Veillard</a></p>
Daniel Veillardc9484202001-10-24 12:35:52 +0000[diff] [blame]156</td></tr></table></td></tr></table></td></tr></table></td>
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