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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 contructs 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><mydoc xmlns="http://mydoc.example.org/schemas/"> |
| <elem1>...</elem1> |
| <elem2>...</elem2> |
| </mydoc></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 |
| atributes 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>"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. <foo:text> may be exactly the |
| same as <bar:text> 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="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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