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21<h1>The XML C library for Gnome</h1>
22<h2>Namespaces</h2>
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86<p>The libxml library implements <a href="http://www.w3.org/TR/REC-xml-names/">XML namespaces</a> support by
87recognizing namespace contructs in the input, and does namespace lookup
88automatically when building the DOM tree. A namespace declaration is
89associated with an in-memory structure and all elements or attributes within
90that namespace point to it. Hence testing the namespace is a simple and fast
91equality operation at the user level.</p>
92<p>I suggest that people using libxml use a namespace, and declare it in the
93root element of their document as the default namespace. Then they don't need
94to use the prefix in the content but we will have a basis for future semantic
95refinement and merging of data from different sources. This doesn't increase
96the size of the XML output significantly, but significantly increases its
97value in the long-term. Example:</p>
98<pre>&lt;mydoc xmlns=&quot;http://mydoc.example.org/schemas/&quot;&gt;
99 &lt;elem1&gt;...&lt;/elem1&gt;
100 &lt;elem2&gt;...&lt;/elem2&gt;
101&lt;/mydoc&gt;</pre>
102<p>The namespace value has to be an absolute URL, but the URL doesn't have to
103point to any existing resource on the Web. It will bind all the element and
104atributes with that URL. I suggest to use an URL within a domain you control,
105and that the URL should contain some kind of version information if possible.
106For example, <code>&quot;http://www.gnome.org/gnumeric/1.0/&quot;</code> is a good
107namespace scheme.</p>
108<p>Then when you load a file, make sure that a namespace carrying the
109version-independent prefix is installed on the root element of your document,
110and if the version information don't match something you know, warn the user
111and be liberal in what you accept as the input. Also do *not* try to base
112namespace checking on the prefix value. &lt;foo:text&gt; may be exactly the
113same as &lt;bar:text&gt; in another document. What really matters is the URI
114associated with the element or the attribute, not the prefix string (which is
115just a shortcut for the full URI). In libxml, element and attributes have an
116<code>ns</code> field pointing to an xmlNs structure detailing the namespace
117prefix and its URI.</p>
118<p>@@Interfaces@@</p>
119<p>@@Examples@@</p>
120<p>Usually people object to using namespaces together with validity checking.
121I will try to make sure that using namespaces won't break validity checking,
122so even if you plan to use or currently are using validation I strongly
123suggest adding namespaces to your document. A default namespace scheme
124<code>xmlns=&quot;http://....&quot;</code> should not break validity even on less
125flexible parsers. Using namespaces to mix and differentiate content coming
126from multiple DTDs will certainly break current validation schemes. I will
127try to provide ways to do this, but this may not be portable or
128standardized.</p>
Daniel Veillard3f4c40f2002-02-13 09:19:28 +0000[diff] [blame]129<p><a href="bugs.html">Daniel Veillard</a></p>
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