1 <?xml version="1.0" encoding="UTF-8"?> 2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3 <html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><link rel="SHORTCUT ICON" href="/favicon.ico" /><style type="text/css"> 4 TD {font-family: Verdana,Arial,Helvetica} 5 BODY {font-family: Verdana,Arial,Helvetica; margin-top: 2em; margin-left: 0em; margin-right: 0em} 6 H1 {font-family: Verdana,Arial,Helvetica} 7 H2 {font-family: Verdana,Arial,Helvetica} 8 H3 {font-family: Verdana,Arial,Helvetica} 9 A:link, A:visited, A:active { text-decoration: underline } 10 </style><title>Entities or no entities</title></head><body bgcolor="#8b7765" text="#000000" link="#a06060" vlink="#000000"><table border="0" width="100%" cellpadding="5" cellspacing="0" align="center"><tr><td width="120"><a href="http://swpat.ffii.org/"><img src="epatents.png" alt="Action against 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Python bindings</a></li><li><a href="http://cpan.uwinnipeg.ca/dist/XML-LibXML">Perl bindings</a></li><li><a href="http://libxmlplusplus.sourceforge.net/">C++ bindings</a></li><li><a href="http://www.zend.com/php5/articles/php5-xmlphp.php#Heading4">PHP bindings</a></li><li><a href="http://sourceforge.net/projects/libxml2-pas/">Pascal bindings</a></li><li><a href="http://libxml.rubyforge.org/">Ruby bindings</a></li><li><a href="http://tclxml.sourceforge.net/">Tcl bindings</a></li><li><a href="http://bugzilla.gnome.org/buglist.cgi?product=libxml2">Bug Tracker</a></li></ul></td></tr></table></td></tr></table></td><td valign="top" bgcolor="#8b7765"><table border="0" cellspacing="0" cellpadding="1" width="100%"><tr><td><table border="0" cellspacing="0" cellpadding="1" width="100%" bgcolor="#000000"><tr><td><table border="0" cellpadding="3" cellspacing="1" width="100%"><tr><td bgcolor="#fffacd"><p>Entities in principle are similar to simple C macros. An entity defines an 11 abbreviation for a given string that you can reuse many times throughout the 12 content of your document. Entities are especially useful when a given string 13 may occur frequently within a document, or to confine the change needed to a 14 document to a restricted area in the internal subset of the document (at the 15 beginning). Example:</p><pre>1 <?xml version="1.0"?> 16 2 <!DOCTYPE EXAMPLE SYSTEM "example.dtd" [ 17 3 <!ENTITY xml "Extensible Markup Language"> 18 4 ]> 19 5 <EXAMPLE> 20 6 &xml; 21 7 </EXAMPLE></pre><p>Line 3 declares the xml entity. Line 6 uses the xml entity, by prefixing 22 its name with '&' and following it by ';' without any spaces added. There 23 are 5 predefined entities in libxml2 allowing you to escape characters with 24 predefined meaning in some parts of the xml document content: 25 <strong>&lt;</strong> for the character '<', <strong>&gt;</strong> 26 for the character '>', <strong>&apos;</strong> for the character ''', 27 <strong>&quot;</strong> for the character '"', and 28 <strong>&amp;</strong> for the character '&'.</p><p>One of the problems related to entities is that you may want the parser to 29 substitute an entity's content so that you can see the replacement text in 30 your application. Or you may prefer to keep entity references as such in the 31 content to be able to save the document back without losing this usually 32 precious information (if the user went through the pain of explicitly 33 defining entities, he may have a a rather negative attitude if you blindly 34 substitute them as saving time). The <a href="html/libxml-parser.html#xmlSubstituteEntitiesDefault">xmlSubstituteEntitiesDefault()</a> 35 function allows you to check and change the behaviour, which is to not 36 substitute entities by default.</p><p>Here is the DOM tree built by libxml2 for the previous document in the 37 default case:</p><pre>/gnome/src/gnome-xml -> ./xmllint --debug test/ent1 38 DOCUMENT 39 version=1.0 40 ELEMENT EXAMPLE 41 TEXT 42 content= 43 ENTITY_REF 44 INTERNAL_GENERAL_ENTITY xml 45 content=Extensible Markup Language 46 TEXT 47 content=</pre><p>And here is the result when substituting entities:</p><pre>/gnome/src/gnome-xml -> ./tester --debug --noent test/ent1 48 DOCUMENT 49 version=1.0 50 ELEMENT EXAMPLE 51 TEXT 52 content= Extensible Markup Language</pre><p>So, entities or no entities? Basically, it depends on your use case. I 53 suggest that you keep the non-substituting default behaviour and avoid using 54 entities in your XML document or data if you are not willing to handle the 55 entity references elements in the DOM tree.</p><p>Note that at save time libxml2 enforces the conversion of the predefined 56 entities where necessary to prevent well-formedness problems, and will also 57 transparently replace those with chars (i.e. it will not generate entity 58 reference elements in the DOM tree or call the reference() SAX callback when 59 finding them in the input).</p><p><span style="background-color: #FF0000">WARNING</span>: handling entities 60 on top of the libxml2 SAX interface is difficult!!! If you plan to use 61 non-predefined entities in your documents, then the learning curve to handle 62 then using the SAX API may be long. If you plan to use complex documents, I 63 strongly suggest you consider using the DOM interface instead and let libxml 64 deal with the complexity rather than trying to do it yourself.</p><p><a href="bugs.html">Daniel Veillard</a></p></td></tr></table></td></tr></table></td></tr></table></td></tr></table></td></tr></table></body></html> 65
