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22<h1>The XML C library for Gnome</h1>
23<h2>Namespaces</h2>
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88<p>The libxml library implements <a href="http://www.w3.org/TR/REC-xml-names/">XML namespaces</a> support by
89recognizing namespace contructs in the input, and does namespace lookup
90automatically when building the DOM tree. A namespace declaration is
91associated with an in-memory structure and all elements or attributes within
92that namespace point to it. Hence testing the namespace is a simple and fast
93equality operation at the user level.</p>
94<p>I suggest that people using libxml use a namespace, and declare it in the
95root element of their document as the default namespace. Then they don't need
96to use the prefix in the content but we will have a basis for future semantic
97refinement and merging of data from different sources. This doesn't increase
98the size of the XML output significantly, but significantly increases its
99value in the long-term. Example:</p>
100<pre>&lt;mydoc xmlns=&quot;http://mydoc.example.org/schemas/&quot;&gt;
101 &lt;elem1&gt;...&lt;/elem1&gt;
102 &lt;elem2&gt;...&lt;/elem2&gt;
103&lt;/mydoc&gt;</pre>
104<p>The namespace value has to be an absolute URL, but the URL doesn't have to
105point to any existing resource on the Web. It will bind all the element and
106atributes with that URL. I suggest to use an URL within a domain you control,
107and that the URL should contain some kind of version information if possible.
108For example, <code>&quot;http://www.gnome.org/gnumeric/1.0/&quot;</code> is a good
109namespace scheme.</p>
110<p>Then when you load a file, make sure that a namespace carrying the
111version-independent prefix is installed on the root element of your document,
112and if the version information don't match something you know, warn the user
113and be liberal in what you accept as the input. Also do *not* try to base
114namespace checking on the prefix value. &lt;foo:text&gt; may be exactly the
115same as &lt;bar:text&gt; in another document. What really matters is the URI
116associated with the element or the attribute, not the prefix string (which is
117just a shortcut for the full URI). In libxml, element and attributes have an
118<code>ns</code> field pointing to an xmlNs structure detailing the namespace
119prefix and its URI.</p>
120<p>@@Interfaces@@</p>
121<p>@@Examples@@</p>
122<p>Usually people object to using namespaces together with validity checking.
123I will try to make sure that using namespaces won't break validity checking,
124so even if you plan to use or currently are using validation I strongly
125suggest adding namespaces to your document. A default namespace scheme
126<code>xmlns=&quot;http://....&quot;</code> should not break validity even on less
127flexible parsers. Using namespaces to mix and differentiate content coming
128from multiple DTDs will certainly break current validation schemes. I will
129try to provide ways to do this, but this may not be portable or
130standardized.</p>
Daniel Veillard3f4c40f2002-02-13 09:19:28 +0000[diff] [blame]131<p><a href="bugs.html">Daniel Veillard</a></p>
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