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21<h1>The XML C library for Gnome</h1>
22<h2>Namespaces</h2>
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85<p>The libxml library implements <a href="http://www.w3.org/TR/REC-xml-names/">XML namespaces</a> support by
86recognizing namespace contructs in the input, and does namespace lookup
87automatically when building the DOM tree. A namespace declaration is
88associated with an in-memory structure and all elements or attributes within
89that namespace point to it. Hence testing the namespace is a simple and fast
90equality operation at the user level.</p>
91<p>I suggest that people using libxml use a namespace, and declare it in the
92root element of their document as the default namespace. Then they don't need
93to use the prefix in the content but we will have a basis for future semantic
94refinement and merging of data from different sources. This doesn't increase
95the size of the XML output significantly, but significantly increases its
96value in the long-term. Example:</p>
97<pre>&lt;mydoc xmlns=&quot;http://mydoc.example.org/schemas/&quot;&gt;
98 &lt;elem1&gt;...&lt;/elem1&gt;
99 &lt;elem2&gt;...&lt;/elem2&gt;
100&lt;/mydoc&gt;</pre>
101<p>The namespace value has to be an absolute URL, but the URL doesn't have to
102point to any existing resource on the Web. It will bind all the element and
103atributes with that URL. I suggest to use an URL within a domain you control,
104and that the URL should contain some kind of version information if possible.
105For example, <code>&quot;http://www.gnome.org/gnumeric/1.0/&quot;</code> is a good
106namespace scheme.</p>
107<p>Then when you load a file, make sure that a namespace carrying the
108version-independent prefix is installed on the root element of your document,
109and if the version information don't match something you know, warn the user
110and be liberal in what you accept as the input. Also do *not* try to base
111namespace checking on the prefix value. &lt;foo:text&gt; may be exactly the
112same as &lt;bar:text&gt; in another document. What really matters is the URI
113associated with the element or the attribute, not the prefix string (which is
114just a shortcut for the full URI). In libxml, element and attributes have an
115<code>ns</code> field pointing to an xmlNs structure detailing the namespace
116prefix and its URI.</p>
117<p>@@Interfaces@@</p>
118<p>@@Examples@@</p>
119<p>Usually people object to using namespaces together with validity checking.
120I will try to make sure that using namespaces won't break validity checking,
121so even if you plan to use or currently are using validation I strongly
122suggest adding namespaces to your document. A default namespace scheme
123<code>xmlns=&quot;http://....&quot;</code> should not break validity even on less
124flexible parsers. Using namespaces to mix and differentiate content coming
125from multiple DTDs will certainly break current validation schemes. I will
126try to provide ways to do this, but this may not be portable or
127standardized.</p>
Daniel Veillard3f4c40f2002-02-13 09:19:28 +0000[diff] [blame]128<p><a href="bugs.html">Daniel Veillard</a></p>
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