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"> 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>Namespaces</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 software patents" 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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>The libxml2 library implements <a href="http://www.w3.org/TR/REC-xml-names/">XML namespaces</a> support by 11recognizing namespace constructs in the input, and does namespace lookup 12automatically when building the DOM tree. A namespace declaration is 13associated with an in-memory structure and all elements or attributes within 14that namespace point to it. Hence testing the namespace is a simple and fast 15equality operation at the user level.</p><p>I suggest that people using libxml2 use a namespace, and declare it in the 16root element of their document as the default namespace. Then they don't need 17to use the prefix in the content but we will have a basis for future semantic 18refinement and merging of data from different sources. This doesn't increase 19the size of the XML output significantly, but significantly increases its 20value in the long-term. Example:</p><pre><mydoc xmlns="http://mydoc.example.org/schemas/"> 21 <elem1>...</elem1> 22 <elem2>...</elem2> 23</mydoc></pre><p>The namespace value has to be an absolute URL, but the URL doesn't have to 24point to any existing resource on the Web. It will bind all the element and 25attributes with that URL. I suggest to use an URL within a domain you 26control, and that the URL should contain some kind of version information if 27possible. For example, <code>"http://www.gnome.org/gnumeric/1.0/"</code> is a 28good namespace scheme.</p><p>Then when you load a file, make sure that a namespace carrying the 29version-independent prefix is installed on the root element of your document, 30and if the version information don't match something you know, warn the user 31and be liberal in what you accept as the input. Also do *not* try to base 32namespace checking on the prefix value. <foo:text> may be exactly the 33same as <bar:text> in another document. What really matters is the URI 34associated with the element or the attribute, not the prefix string (which is 35just a shortcut for the full URI). In libxml, element and attributes have an 36<code>ns</code> field pointing to an xmlNs structure detailing the namespace 37prefix and its URI.</p><p>@@Interfaces@@</p><pre>xmlNodePtr node; 38if(!strncmp(node->name,"mytag",5) 39 && node->ns 40 && !strcmp(node->ns->href,"http://www.mysite.com/myns/1.0")) { 41 ... 42}</pre><p>Usually people object to using namespaces together with validity checking. 43I will try to make sure that using namespaces won't break validity checking, 44so even if you plan to use or currently are using validation I strongly 45suggest adding namespaces to your document. A default namespace scheme 46<code>xmlns="http://...."</code> should not break validity even on less 47flexible parsers. Using namespaces to mix and differentiate content coming 48from multiple DTDs will certainly break current validation schemes. To check 49such documents one needs to use schema-validation, which is supported in 50libxml2 as well. See <a href="http://www.relaxng.org/">relagx-ng</a> and <a href="http://www.w3c.org/XML/Schema">w3c-schema</a>.</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> 51
