[xml] node "reuse" from one doc in another

[Liebhart Heinz <Heinz Liebhart kapsch net>]

I'm quite new to libxml usage, but it seems to be a handy way to deal with XML documents solving my problem. 
I thought at least :-)

My little problem to solve: I'm getting some input XML doc (each with one out of several possible root 
elements) , shall wrap this into a "cover document" by adding some control information to it and hand over 
the "wrapper XML" to the next processing step.

For a proof of concept I've been starting with this:

   doc_in = xmlReadMemory(msg_xml, strlen(msg_xml), NULL, NULL, 0);
   root_node_in = xmlDocGetRootElement(doc_in);

   doc = xmlNewDoc(BAD_CAST "1.0");
   root_node = xmlNewNode(NULL, BAD_CAST "received");
   xmlNewProp(root_node, BAD_CAST "time", BAD_CAST "2017-03-01 17:57:21");
   xmlNewProp(root_node, BAD_CAST "from", BAD_CAST "32"); 
   xmlNewProp(root_node, BAD_CAST "type", BAD_CAST "control"); 
   xmlDocSetRootElement(doc, root_node);
        
   /* now add extracted node from input to new doc */
   xmlAddChild(root_node, root_node_in); /* <<--- causes the problem */

   /* show me result */
   xmlDocDump(stdout, doc_in);
   xmlDocDump(stdout, doc);
   xmlFreeDoc(doc); /* <<--- where it crashes */

the result is perfectly fitting my expectations, till I hit the xmlDocFree statement giving me 
*** Error in `./main2': free(): invalid pointer: 0x099680de ***
Aborted (core dumped)

I've tried to check every pointer, the one indicated isn't visible to me at all...
> From value range my expectation is it's somewhere in the control structure of doc.
If I leave out the node insertion there is no error on freeing the doc(s). 
To be honest I have no clue what I'm doing wrong, but it seems I haven't understood internal behavior of 
libxml yet :-(

Any helpful ideas? Is there be a better way doing this? 
I'm trying not to produce much overhead and trying not to parse XML myself ;-)

Thanks,
Heinz



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