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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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Daniel Veillardc9484202001-10-24 12:35:52 +0000[diff] [blame]50<li><a href="entities.html">Entities or no entities</a></li>
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87<p>Entities in principle are similar to simple C macros. An entity defines an
88abbreviation for a given string that you can reuse many times throughout the
89content of your document. Entities are especially useful when a given string
90may occur frequently within a document, or to confine the change needed to a
91document to a restricted area in the internal subset of the document (at the
92beginning). Example:</p>
93<pre>1 &lt;?xml version=&quot;1.0&quot;?&gt;
942 &lt;!DOCTYPE EXAMPLE SYSTEM &quot;example.dtd&quot; [
953 &lt;!ENTITY xml &quot;Extensible Markup Language&quot;&gt;
964 ]&gt;
975 &lt;EXAMPLE&gt;
986 &amp;xml;
997 &lt;/EXAMPLE&gt;</pre>
100<p>Line 3 declares the xml entity. Line 6 uses the xml entity, by prefixing
101its name with '&amp;' and following it by ';' without any spaces added. There
102are 5 predefined entities in libxml allowing you to escape charaters with
103predefined meaning in some parts of the xml document content:
104<strong>&amp;lt;</strong> for the character '&lt;', <strong>&amp;gt;</strong>
105for the character '&gt;', <strong>&amp;apos;</strong> for the character ''',
106<strong>&amp;quot;</strong> for the character '&quot;', and
107<strong>&amp;amp;</strong> for the character '&amp;'.</p>
108<p>One of the problems related to entities is that you may want the parser to
109substitute an entity's content so that you can see the replacement text in
110your application. Or you may prefer to keep entity references as such in the
111content to be able to save the document back without losing this usually
112precious information (if the user went through the pain of explicitly
113defining entities, he may have a a rather negative attitude if you blindly
114susbtitute them as saving time). The <a href="html/libxml-parser.html#XMLSUBSTITUTEENTITIESDEFAULT">xmlSubstituteEntitiesDefault()</a>
115function allows you to check and change the behaviour, which is to not
116substitute entities by default.</p>
117<p>Here is the DOM tree built by libxml for the previous document in the
118default case:</p>
119<pre>/gnome/src/gnome-xml -&gt; ./xmllint --debug test/ent1
120DOCUMENT
121version=1.0
122 ELEMENT EXAMPLE
123 TEXT
124 content=
125 ENTITY_REF
126 INTERNAL_GENERAL_ENTITY xml
127 content=Extensible Markup Language
128 TEXT
129 content=</pre>
130<p>And here is the result when substituting entities:</p>
131<pre>/gnome/src/gnome-xml -&gt; ./tester --debug --noent test/ent1
132DOCUMENT
133version=1.0
134 ELEMENT EXAMPLE
135 TEXT
136 content= Extensible Markup Language</pre>
137<p>So, entities or no entities? Basically, it depends on your use case. I
138suggest that you keep the non-substituting default behaviour and avoid using
139entities in your XML document or data if you are not willing to handle the
140entity references elements in the DOM tree.</p>
141<p>Note that at save time libxml enforces the conversion of the predefined
142entities where necessary to prevent well-formedness problems, and will also
143transparently replace those with chars (i.e. it will not generate entity
144reference elements in the DOM tree or call the reference() SAX callback when
145finding them in the input).</p>
146<p>
147<span style="background-color: #FF0000">WARNING</span>: handling entities
148on top of the libxml SAX interface is difficult!!! If you plan to use
149non-predefined entities in your documents, then the learning cuvre to handle
150then using the SAX API may be long. If you plan to use complex documents, I
151strongly suggest you consider using the DOM interface instead and let libxml
152deal with the complexity rather than trying to do it yourself.</p>
Daniel Veillard3f4c40f2002-02-13 09:19:28 +0000[diff] [blame]153<p><a href="bugs.html">Daniel Veillard</a></p>
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