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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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86<p>Entities in principle are similar to simple C macros. An entity defines an
87abbreviation for a given string that you can reuse many times throughout the
88content of your document. Entities are especially useful when a given string
89may occur frequently within a document, or to confine the change needed to a
90document to a restricted area in the internal subset of the document (at the
91beginning). Example:</p>
92<pre>1 &lt;?xml version=&quot;1.0&quot;?&gt;
932 &lt;!DOCTYPE EXAMPLE SYSTEM &quot;example.dtd&quot; [
943 &lt;!ENTITY xml &quot;Extensible Markup Language&quot;&gt;
954 ]&gt;
965 &lt;EXAMPLE&gt;
976 &amp;xml;
987 &lt;/EXAMPLE&gt;</pre>
99<p>Line 3 declares the xml entity. Line 6 uses the xml entity, by prefixing
100its name with '&amp;' and following it by ';' without any spaces added. There
101are 5 predefined entities in libxml allowing you to escape charaters with
102predefined meaning in some parts of the xml document content:
103<strong>&amp;lt;</strong> for the character '&lt;', <strong>&amp;gt;</strong>
104for the character '&gt;', <strong>&amp;apos;</strong> for the character ''',
105<strong>&amp;quot;</strong> for the character '&quot;', and
106<strong>&amp;amp;</strong> for the character '&amp;'.</p>
107<p>One of the problems related to entities is that you may want the parser to
108substitute an entity's content so that you can see the replacement text in
109your application. Or you may prefer to keep entity references as such in the
110content to be able to save the document back without losing this usually
111precious information (if the user went through the pain of explicitly
112defining entities, he may have a a rather negative attitude if you blindly
113susbtitute them as saving time). The <a href="html/libxml-parser.html#XMLSUBSTITUTEENTITIESDEFAULT">xmlSubstituteEntitiesDefault()</a>
114function allows you to check and change the behaviour, which is to not
115substitute entities by default.</p>
116<p>Here is the DOM tree built by libxml for the previous document in the
117default case:</p>
118<pre>/gnome/src/gnome-xml -&gt; ./xmllint --debug test/ent1
119DOCUMENT
120version=1.0
121 ELEMENT EXAMPLE
122 TEXT
123 content=
124 ENTITY_REF
125 INTERNAL_GENERAL_ENTITY xml
126 content=Extensible Markup Language
127 TEXT
128 content=</pre>
129<p>And here is the result when substituting entities:</p>
130<pre>/gnome/src/gnome-xml -&gt; ./tester --debug --noent test/ent1
131DOCUMENT
132version=1.0
133 ELEMENT EXAMPLE
134 TEXT
135 content= Extensible Markup Language</pre>
136<p>So, entities or no entities? Basically, it depends on your use case. I
137suggest that you keep the non-substituting default behaviour and avoid using
138entities in your XML document or data if you are not willing to handle the
139entity references elements in the DOM tree.</p>
140<p>Note that at save time libxml enforces the conversion of the predefined
141entities where necessary to prevent well-formedness problems, and will also
142transparently replace those with chars (i.e. it will not generate entity
143reference elements in the DOM tree or call the reference() SAX callback when
144finding them in the input).</p>
145<p>
146<span style="background-color: #FF0000">WARNING</span>: handling entities
147on top of the libxml SAX interface is difficult!!! If you plan to use
148non-predefined entities in your documents, then the learning cuvre to handle
149then using the SAX API may be long. If you plan to use complex documents, I
150strongly suggest you consider using the DOM interface instead and let libxml
151deal with the complexity rather than trying to do it yourself.</p>
Daniel Veillard3f4c40f2002-02-13 09:19:28 +0000[diff] [blame]152<p><a href="bugs.html">Daniel Veillard</a></p>
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