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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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88<p>Entities in principle are similar to simple C macros. An entity defines an
89abbreviation for a given string that you can reuse many times throughout the
90content of your document. Entities are especially useful when a given string
91may occur frequently within a document, or to confine the change needed to a
92document to a restricted area in the internal subset of the document (at the
93beginning). Example:</p>
94<pre>1 &lt;?xml version=&quot;1.0&quot;?&gt;
952 &lt;!DOCTYPE EXAMPLE SYSTEM &quot;example.dtd&quot; [
963 &lt;!ENTITY xml &quot;Extensible Markup Language&quot;&gt;
974 ]&gt;
985 &lt;EXAMPLE&gt;
996 &amp;xml;
1007 &lt;/EXAMPLE&gt;</pre>
101<p>Line 3 declares the xml entity. Line 6 uses the xml entity, by prefixing
102its name with '&amp;' and following it by ';' without any spaces added. There
103are 5 predefined entities in libxml allowing you to escape charaters with
104predefined meaning in some parts of the xml document content:
105<strong>&amp;lt;</strong> for the character '&lt;', <strong>&amp;gt;</strong>
106for the character '&gt;', <strong>&amp;apos;</strong> for the character ''',
107<strong>&amp;quot;</strong> for the character '&quot;', and
108<strong>&amp;amp;</strong> for the character '&amp;'.</p>
109<p>One of the problems related to entities is that you may want the parser to
110substitute an entity's content so that you can see the replacement text in
111your application. Or you may prefer to keep entity references as such in the
112content to be able to save the document back without losing this usually
113precious information (if the user went through the pain of explicitly
114defining entities, he may have a a rather negative attitude if you blindly
115susbtitute them as saving time). The <a href="html/libxml-parser.html#XMLSUBSTITUTEENTITIESDEFAULT">xmlSubstituteEntitiesDefault()</a>
116function allows you to check and change the behaviour, which is to not
117substitute entities by default.</p>
118<p>Here is the DOM tree built by libxml for the previous document in the
119default case:</p>
120<pre>/gnome/src/gnome-xml -&gt; ./xmllint --debug test/ent1
121DOCUMENT
122version=1.0
123 ELEMENT EXAMPLE
124 TEXT
125 content=
126 ENTITY_REF
127 INTERNAL_GENERAL_ENTITY xml
128 content=Extensible Markup Language
129 TEXT
130 content=</pre>
131<p>And here is the result when substituting entities:</p>
132<pre>/gnome/src/gnome-xml -&gt; ./tester --debug --noent test/ent1
133DOCUMENT
134version=1.0
135 ELEMENT EXAMPLE
136 TEXT
137 content= Extensible Markup Language</pre>
138<p>So, entities or no entities? Basically, it depends on your use case. I
139suggest that you keep the non-substituting default behaviour and avoid using
140entities in your XML document or data if you are not willing to handle the
141entity references elements in the DOM tree.</p>
142<p>Note that at save time libxml enforces the conversion of the predefined
143entities where necessary to prevent well-formedness problems, and will also
144transparently replace those with chars (i.e. it will not generate entity
145reference elements in the DOM tree or call the reference() SAX callback when
146finding them in the input).</p>
147<p>
148<span style="background-color: #FF0000">WARNING</span>: handling entities
149on top of the libxml SAX interface is difficult!!! If you plan to use
150non-predefined entities in your documents, then the learning cuvre to handle
151then using the SAX API may be long. If you plan to use complex documents, I
152strongly suggest you consider using the DOM interface instead and let libxml
153deal with the complexity rather than trying to do it yourself.</p>
Daniel Veillard3f4c40f2002-02-13 09:19:28 +0000[diff] [blame]154<p><a href="bugs.html">Daniel Veillard</a></p>
Daniel Veillardc9484202001-10-24 12:35:52 +0000[diff] [blame]155</td></tr></table></td></tr></table></td></tr></table></td>
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