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<h1>The XML C library for Gnome</h1>
<h2>Entities or no entities</h2>
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<p>Entities in principle are similar to simple C macros. An entity defines an
abbreviation for a given string that you can reuse many times throughout the
content of your document. Entities are especially useful when a given string
may occur frequently within a document, or to confine the change needed to a
document to a restricted area in the internal subset of the document (at the
beginning). Example:</p>
<pre>1 &lt;?xml version=&quot;1.0&quot;?&gt;
2 &lt;!DOCTYPE EXAMPLE SYSTEM &quot;example.dtd&quot; [
3 &lt;!ENTITY xml &quot;Extensible Markup Language&quot;&gt;
4 ]&gt;
5 &lt;EXAMPLE&gt;
6    &amp;xml;
7 &lt;/EXAMPLE&gt;</pre>
<p>Line 3 declares the xml entity. Line 6 uses the xml entity, by prefixing
its name with '&amp;' and following it by ';' without any spaces added. There
are 5 predefined entities in libxml allowing you to escape charaters with
predefined meaning in some parts of the xml document content:
<strong>&amp;lt;</strong> for the character '&lt;', <strong>&amp;gt;</strong>
for the character '&gt;',  <strong>&amp;apos;</strong> for the character ''',
<strong>&amp;quot;</strong> for the character '&quot;', and
<strong>&amp;amp;</strong> for the character '&amp;'.</p>
<p>One of the problems related to entities is that you may want the parser to
substitute an entity's content so that you can see the replacement text in
your application. Or you may prefer to keep entity references as such in the
content to be able to save the document back without losing this usually
precious information (if the user went through the pain of explicitly
defining entities, he may have a a rather negative attitude if you blindly
susbtitute them as saving time). The <a href="html/libxml-parser.html#XMLSUBSTITUTEENTITIESDEFAULT">xmlSubstituteEntitiesDefault()</a>
function allows you to check and change the behaviour, which is to not
substitute entities by default.</p>
<p>Here is the DOM tree built by libxml for the previous document in the
default case:</p>
<pre>/gnome/src/gnome-xml -&gt; /xmllint --debug test/ent1
DOCUMENT
version=1.0
   ELEMENT EXAMPLE
     TEXT
     content=
     ENTITY_REF
       INTERNAL_GENERAL_ENTITY xml
       content=Extensible Markup Language
     TEXT
     content=</pre>
<p>And here is the result when substituting entities:</p>
<pre>/gnome/src/gnome-xml -&gt; /tester --debug --noent test/ent1
DOCUMENT
version=1.0
   ELEMENT EXAMPLE
     TEXT
     content=     Extensible Markup Language</pre>
<p>So, entities or no entities? Basically, it depends on your use case. I
suggest that you keep the non-substituting default behaviour and avoid using
entities in your XML document or data if you are not willing to handle the
entity references elements in the DOM tree.</p>
<p>Note that at save time libxml enforces the conversion of the predefined
entities where necessary to prevent well-formedness problems, and will also
transparently replace those with chars (i.e. it will not generate entity
reference elements in the DOM tree or call the reference() SAX callback when
finding them in the input).</p>
<p>
<span style="background-color: #FF0000">WARNING</span>: handling entities
on top of the libxml SAX interface is difficult!!! If you plan to use
non-predefined entities in your documents, then the learning cuvre to handle
then using the SAX API may be long. If you plan to use complex documents, I
strongly suggest you consider using the DOM interface instead and let libxml
deal with the complexity rather than trying to do it yourself.</p>
<p><a href="mailto:daniel@veillard.com">Daniel Veillard</a></p>
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