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22<h1>The XML C library for Gnome</h1>
23<h2>Namespaces</h2>
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90<p>The libxml library implements <a href="http://www.w3.org/TR/REC-xml-names/">XML namespaces</a> support by
Daniel Veillard63d83142002-05-20 06:51:05 +0000[diff] [blame]91recognizing namespace constructs in the input, and does namespace lookup
Daniel Veillardc9484202001-10-24 12:35:52 +0000[diff] [blame]92automatically when building the DOM tree. A namespace declaration is
93associated with an in-memory structure and all elements or attributes within
94that namespace point to it. Hence testing the namespace is a simple and fast
95equality operation at the user level.</p>
96<p>I suggest that people using libxml use a namespace, and declare it in the
97root element of their document as the default namespace. Then they don't need
98to use the prefix in the content but we will have a basis for future semantic
99refinement and merging of data from different sources. This doesn't increase
100the size of the XML output significantly, but significantly increases its
101value in the long-term. Example:</p>
102<pre>&lt;mydoc xmlns=&quot;http://mydoc.example.org/schemas/&quot;&gt;
103 &lt;elem1&gt;...&lt;/elem1&gt;
104 &lt;elem2&gt;...&lt;/elem2&gt;
105&lt;/mydoc&gt;</pre>
106<p>The namespace value has to be an absolute URL, but the URL doesn't have to
107point to any existing resource on the Web. It will bind all the element and
Daniel Veillardc0801af2002-05-28 16:28:42 +0000[diff] [blame]108attributes with that URL. I suggest to use an URL within a domain you
109control, and that the URL should contain some kind of version information if
110possible. For example, <code>&quot;http://www.gnome.org/gnumeric/1.0/&quot;</code> is a
111good namespace scheme.</p>
Daniel Veillardc9484202001-10-24 12:35:52 +0000[diff] [blame]112<p>Then when you load a file, make sure that a namespace carrying the
113version-independent prefix is installed on the root element of your document,
114and if the version information don't match something you know, warn the user
115and be liberal in what you accept as the input. Also do *not* try to base
116namespace checking on the prefix value. &lt;foo:text&gt; may be exactly the
117same as &lt;bar:text&gt; in another document. What really matters is the URI
118associated with the element or the attribute, not the prefix string (which is
119just a shortcut for the full URI). In libxml, element and attributes have an
120<code>ns</code> field pointing to an xmlNs structure detailing the namespace
121prefix and its URI.</p>
122<p>@@Interfaces@@</p>
123<p>@@Examples@@</p>
124<p>Usually people object to using namespaces together with validity checking.
125I will try to make sure that using namespaces won't break validity checking,
126so even if you plan to use or currently are using validation I strongly
127suggest adding namespaces to your document. A default namespace scheme
128<code>xmlns=&quot;http://....&quot;</code> should not break validity even on less
129flexible parsers. Using namespaces to mix and differentiate content coming
130from multiple DTDs will certainly break current validation schemes. I will
131try to provide ways to do this, but this may not be portable or
132standardized.</p>
Daniel Veillard3f4c40f2002-02-13 09:19:28 +0000[diff] [blame]133<p><a href="bugs.html">Daniel Veillard</a></p>
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