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