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21<h1>The XML C library for Gnome</h1>
22<h2>Namespaces</h2>
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73<p>The libxml library implements <a href="http://www.w3.org/TR/REC-xml-names/">XML namespaces</a> support by
74recognizing namespace contructs in the input, and does namespace lookup
75automatically when building the DOM tree. A namespace declaration is
76associated with an in-memory structure and all elements or attributes within
77that namespace point to it. Hence testing the namespace is a simple and fast
78equality operation at the user level.</p>
79<p>I suggest that people using libxml use a namespace, and declare it in the
80root element of their document as the default namespace. Then they don't need
81to use the prefix in the content but we will have a basis for future semantic
82refinement and merging of data from different sources. This doesn't increase
83the size of the XML output significantly, but significantly increases its
84value in the long-term. Example:</p>
85<pre>&lt;mydoc xmlns=&quot;http://mydoc.example.org/schemas/&quot;&gt;
86 &lt;elem1&gt;...&lt;/elem1&gt;
87 &lt;elem2&gt;...&lt;/elem2&gt;
88&lt;/mydoc&gt;</pre>
89<p>The namespace value has to be an absolute URL, but the URL doesn't have to
90point to any existing resource on the Web. It will bind all the element and
91atributes with that URL. I suggest to use an URL within a domain you control,
92and that the URL should contain some kind of version information if possible.
93For example, <code>&quot;http://www.gnome.org/gnumeric/1.0/&quot;</code> is a good
94namespace scheme.</p>
95<p>Then when you load a file, make sure that a namespace carrying the
96version-independent prefix is installed on the root element of your document,
97and if the version information don't match something you know, warn the user
98and be liberal in what you accept as the input. Also do *not* try to base
99namespace checking on the prefix value. &lt;foo:text&gt; may be exactly the
100same as &lt;bar:text&gt; in another document. What really matters is the URI
101associated with the element or the attribute, not the prefix string (which is
102just a shortcut for the full URI). In libxml, element and attributes have an
103<code>ns</code> field pointing to an xmlNs structure detailing the namespace
104prefix and its URI.</p>
105<p>@@Interfaces@@</p>
106<p>@@Examples@@</p>
107<p>Usually people object to using namespaces together with validity checking.
108I will try to make sure that using namespaces won't break validity checking,
109so even if you plan to use or currently are using validation I strongly
110suggest adding namespaces to your document. A default namespace scheme
111<code>xmlns=&quot;http://....&quot;</code> should not break validity even on less
112flexible parsers. Using namespaces to mix and differentiate content coming
113from multiple DTDs will certainly break current validation schemes. I will
114try to provide ways to do this, but this may not be portable or
115standardized.</p>
116<p><a href="mailto:daniel@veillard.com">Daniel Veillard</a></p>
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