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22<h1>The XML C library for Gnome</h1>
23<h2>Namespaces</h2>
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98<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]99recognizing namespace constructs in the input, and does namespace lookup
Daniel Veillardc9484202001-10-24 12:35:52 +0000[diff] [blame]100automatically when building the DOM tree. A namespace declaration is
101associated with an in-memory structure and all elements or attributes within
102that namespace point to it. Hence testing the namespace is a simple and fast
103equality operation at the user level.</p>
104<p>I suggest that people using libxml use a namespace, and declare it in the
105root element of their document as the default namespace. Then they don't need
106to use the prefix in the content but we will have a basis for future semantic
107refinement and merging of data from different sources. This doesn't increase
108the size of the XML output significantly, but significantly increases its
109value in the long-term. Example:</p>
110<pre>&lt;mydoc xmlns=&quot;http://mydoc.example.org/schemas/&quot;&gt;
111 &lt;elem1&gt;...&lt;/elem1&gt;
112 &lt;elem2&gt;...&lt;/elem2&gt;
113&lt;/mydoc&gt;</pre>
114<p>The namespace value has to be an absolute URL, but the URL doesn't have to
115point to any existing resource on the Web. It will bind all the element and
Daniel Veillardc0801af2002-05-28 16:28:42 +0000[diff] [blame]116attributes with that URL. I suggest to use an URL within a domain you
117control, and that the URL should contain some kind of version information if
118possible. For example, <code>&quot;http://www.gnome.org/gnumeric/1.0/&quot;</code> is a
119good namespace scheme.</p>
Daniel Veillardc9484202001-10-24 12:35:52 +0000[diff] [blame]120<p>Then when you load a file, make sure that a namespace carrying the
121version-independent prefix is installed on the root element of your document,
122and if the version information don't match something you know, warn the user
123and be liberal in what you accept as the input. Also do *not* try to base
124namespace checking on the prefix value. &lt;foo:text&gt; may be exactly the
125same as &lt;bar:text&gt; in another document. What really matters is the URI
126associated with the element or the attribute, not the prefix string (which is
127just a shortcut for the full URI). In libxml, element and attributes have an
128<code>ns</code> field pointing to an xmlNs structure detailing the namespace
129prefix and its URI.</p>
130<p>@@Interfaces@@</p>
131<p>@@Examples@@</p>
132<p>Usually people object to using namespaces together with validity checking.
133I will try to make sure that using namespaces won't break validity checking,
134so even if you plan to use or currently are using validation I strongly
135suggest adding namespaces to your document. A default namespace scheme
136<code>xmlns=&quot;http://....&quot;</code> should not break validity even on less
137flexible parsers. Using namespaces to mix and differentiate content coming
138from multiple DTDs will certainly break current validation schemes. I will
139try to provide ways to do this, but this may not be portable or
140standardized.</p>
Daniel Veillard3f4c40f2002-02-13 09:19:28 +0000[diff] [blame]141<p><a href="bugs.html">Daniel Veillard</a></p>
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