Re: [xml] Questions on usage: xmlTextReaderCurrentDoc, XPath and xmlTextReaderRead

[Eric West <rice cruft gmail com>]

>  "William M. Brack" wrote:
>
> I'm responding to your questions on Valgrind, but leaving the
> questions on Text Reader vs. XPath to others:
Much thanks for a quick response!


> Eric West wrote:
> > [disclaimer: I am new to coding with libxml2]
>
> Good reason to rely heavily on libxml2/doc/examples for code models.
>
> > I have a test program to write and read some sample xml. It "works",
> > but I have noticed that valgrind reports some problems related 
> > to xmlTextReaderRead and xmlNewTextReaderFilename.
>
> Using Valgrind on your programs is a GOOD thing.  I only wish more
> would follow your example.
>
> > [Details below]
> >
> > My test program uses the TextReader APIs to extract XML content. At
> > parent nodes, it utilizes XPath queries to extract child and grandchild 
> > content. In pseudocode, this is
> >
> >      reader = xmlNewReaderFilename();
> >      ret = xmlReaderRead( reader);
> >      doc = xmlTextReaderCurrentDoc( reader);
> >      while ( ret == 1) {
> >            processNode( reader, doc);
> >            xmlTextReaderRead( reader);
> >       }
> >
> >       xmlFree( doc);
>
> There is room for improvement here - check what reader3.c and
> reader4.c do with the doc returned by xmlTextReaderCurrentDoc. 
> (Hint: they *don't* use xmlFree for this).

Improvement indeed! Further investigation showed that xmlFree() is 
not always the correct API for releasing memory allocated by libxml2
<sigh>The API docs are not always clear about this. Example:
for xmlXPathEval() "the xmlXPathObjectPtr resulting from the
evaluation or NULL. the caller has to free the object." Not a word
about xmlXPathFreeObject(). Replacing xmlFree() with the appropriate
object free routines started making valgrind happier.

What was not obvious in the xmlTextReader+XPath example was where
to get a pointer to the current xmlDocPtr without calling
xmlTextReaderCurrentDoc(). I did some further experimenting and
found that one may use the xmlDocPtr from the xmlNodePtr returned
by xmlTextReaderExpand(). For example:

   node = xmlTextReaderExpand( reader);
   pathContext = xmlXPathNewContext( node->doc);
   pathContext->node = node;
   obj = xmlXPathEval( BAD_CAST xmlXPathQuery, pathContext);

With this revision of my code and replacement of xmlFree() with
the appropriate variant (eg xmlXPathFreeObject, xmlXPathFreeContext)
I was able to get valgrind to give my code a clean bill of health.

Question: is the above code sequence valid??? (Just because it works,
is not much of a measure of correctness. :)  )

Thanks.

  --Eric
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