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© 2006-2013 Andrew Cooke (site) / post authors (content).

Using libxml (libxml2) with Namespaces

From: andrew cooke <andrew@...>

Date: Mon, 18 Jul 2011 11:31:46 -0400

This has just cost me more time than it should have - I was generating a
validating XML document in C, using libxml2 (via the tree API), but when I
wrote the document to a file, the xmlns namespace declaration was missing.
All the elements has a namespace prefix, but that prefix was not defined.

So an example file looked like:

  <?xml version="1.0" encoding="UTF-8"?>
  <stn:StaMessage>
    <stn:Source>...</stn:Source>
    <stn:ModuleURI>...</stn:ModuleURI>
    <stn:SentDate>2011-07-18T15:05:31</stn:SentDate>
    <stn:Station net_code="13050" sta_code="KIV1"/>
    <stn:Station net_code="z" sta_code="XYZ"/>
  </stn:StaMessage>

This is the same issue mentioned here
http://mail.gnome.org/archives/xml/2005-April/msg00116.html but the reply is
less than helpful.

Anyway... after much experimenting, it turns out that the following sequence
works:

  1 - Create the document
  2 - Create the root node
  3 - Create the namespace, passing the root node as first argument
  4 - Set the namespace on the root node using xmlSetNs
  5 - Add the root node to the document

Not all of that is important (but I am describing it exactly as I know that
order works).  The root of the problem is that constructors for both nodes and
namespaces take namespaces and nodes as arguments (respectively) which gives a
chicken-and-egg situation.  xmlSetNS cuts the Gordian knot and creates the
primordial chicken.

Also, with the above, you should not free the namespace (before, when the two
were insufficiently connected, I was freeing the namespace; now, that causes a
double-free segfault).

The final document then looks like:

  <?xml version="1.0" encoding="UTF-8"?>
  <stn:StaMessage xmlns:stn="http://www.data.scec.org/xml/station/">
    <stn:Source>...</stn:Source>
    <stn:ModuleURI>...</stn:ModuleURI>
    <stn:SentDate>2011-07-18T15:20:02</stn:SentDate>
    <stn:Station net_code="13050" sta_code="KIV1"/>
    <stn:Station net_code="z" sta_code="XYZ"/>
  </stn:StaMessage>

(I still can't work out how to set a default namespace).

Andrew

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