entities.html revision ccf996fca0d0a22d56b8fdd63551dd14ebeb360f
1<?xml version="1.0" encoding="ISO-8859-1"?> 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=ISO-8859-1" /><link rel="SHORTCUT ICON" href="/favicon.ico" /><style type="text/css"> 4TD {font-family: Verdana,Arial,Helvetica} 5BODY {font-family: Verdana,Arial,Helvetica; margin-top: 2em; margin-left: 0em; margin-right: 0em} 6H1 {font-family: Verdana,Arial,Helvetica} 7H2 {font-family: Verdana,Arial,Helvetica} 8H3 {font-family: Verdana,Arial,Helvetica} 9A:link, A:visited, A:active { text-decoration: underline } 10</style><title>Entities or no entities</title></head><body bgcolor="#8b7765" text="#000000" link="#000000" vlink="#000000"><table border="0" width="100%" cellpadding="5" cellspacing="0" align="center"><tr><td width="180"><a href="http://www.gnome.org/"><img src="gnome2.png" alt="Gnome2 Logo" /></a><a 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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 11abbreviation for a given string that you can reuse many times throughout the 12content of your document. Entities are especially useful when a given string 13may occur frequently within a document, or to confine the change needed to a 14document to a restricted area in the internal subset of the document (at the 15beginning). Example:</p><pre>1 <?xml version="1.0"?> 162 <!DOCTYPE EXAMPLE SYSTEM "example.dtd" [ 173 <!ENTITY xml "Extensible Markup Language"> 184 ]> 195 <EXAMPLE> 206 &xml; 217 </EXAMPLE></pre><p>Line 3 declares the xml entity. Line 6 uses the xml entity, by prefixing 22its name with '&' and following it by ';' without any spaces added. There 23are 5 predefined entities in libxml2 allowing you to escape characters with 24predefined meaning in some parts of the xml document content: 25<strong>&lt;</strong> for the character '<', <strong>&gt;</strong> 26for 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 29substitute an entity's content so that you can see the replacement text in 30your application. Or you may prefer to keep entity references as such in the 31content to be able to save the document back without losing this usually 32precious information (if the user went through the pain of explicitly 33defining entities, he may have a a rather negative attitude if you blindly 34substitute them as saving time). The <a href="html/libxml-parser.html#xmlSubstituteEntitiesDefault">xmlSubstituteEntitiesDefault()</a> 35function allows you to check and change the behaviour, which is to not 36substitute entities by default.</p><p>Here is the DOM tree built by libxml2 for the previous document in the 37default case:</p><pre>/gnome/src/gnome-xml -> /xmllint --debug test/ent1 38DOCUMENT 39version=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 48DOCUMENT 49version=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 53suggest that you keep the non-substituting default behaviour and avoid using 54entities in your XML document or data if you are not willing to handle the 55entity references elements in the DOM tree.</p><p>Note that at save time libxml2 enforces the conversion of the predefined 56entities where necessary to prevent well-formedness problems, and will also 57transparently replace those with chars (i.e. it will not generate entity 58reference elements in the DOM tree or call the reference() SAX callback when 59finding them in the input).</p><p><span style="background-color: #FF0000">WARNING</span>: handling entities 60on top of the libxml2 SAX interface is difficult!!! If you plan to use 61non-predefined entities in your documents, then the learning curve to handle 62then using the SAX API may be long. If you plan to use complex documents, I 63strongly suggest you consider using the DOM interface instead and let libxml 64deal 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
