Re: [xml] Question regarding XPath

["Ethan Metsger" <emetsger obj-sys com>]

{{ apologies; I failed to copy the list. }}

On Fri, 16 Mar 2007 08:29:00 -0500, Daniel Veillard <veillard redhat com>
wrote:

>   I doubt it, getting 0 nodes really seems an indication of not getting  
> the
> right binding for the namespaces. And since you don't indicate what you  
> did
> precisely nor provide the XML target I'm just left guessing and I hate  
> that
> nearly as much as debugging someone else code.

Sorry for the confusion.

The schema I'm working with is as follows:

<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema";>
   <xs:element name="bad-format" type="empty"/>
</xs:schema>

It's about as simple as it gets.  We use a more complicated version in our
product, of course.


We parse the document successfully and create a new XPath context with it:

    xpctxt = xmlNewXPathContext (doc);

We then set the node to be the root of the document and get the namespace
list:

    xpctxt = xmlDocGetRootElement (doc);
    xpctxt = xmlGetNsList (doc, xpctxt->node);

We iterate through the namespaces, which the debugger indicates are
properly parsed.  That is, the prefix,href pair
xs,http://www.w3.org/2001/XMLSchema is found in xpctxt->namespaces.

Following your advice, we then run the XPath query, again confirmed to be
properly transmitted as "//xs:element" through the debugger.  This is
stored in the nodeName string:

    xmlXPathObjPtr res = xmlXPathEval (nodeName, xpctxt);

This returns zero nodes.


>   I have seen people in the past trying to use NULL or "" to registed the
> prefix of XPath queries, there is no way I can try to guess all error  
> case
> possible.

I do not think this is the case here.  As far as I can tell, we're setting
up the context correctly with appropriately registered namespaces and a
properly formatted query.  This XSD and query both worked with
libxml2-2.6.9.  Our source code hasn't changed.

Now while I am working with a library that I've compiled from source here,
rather than prebuilt binaries, I have only disabled iconv.  Every other
option is default, and I cannot imagine that disabling iconv would be
causing the problem.

> > This isn't terribly complicated, and it looks pretty similar to what's
> > happening in xpath1.c:
>
>   make sure it's really similar, and check with a debugger that the
> xmlXPathRegisterNs is called on the right context, and that the values
> are correct.

I have, actually.  The code from xpath1.c is a copy of the code from
debugAPI.c, as regards registering the namespaces.  It parses the strings
exactly the same way and calls the same functions.  That's why I assume
that the difference lies in the use of xmlXPathEval versus
xmlXPathEvalExpression.


The evaluation code is not trivial to step through (at least for me, being
somewhat new to libxml2).  I'm still going through it and trying to
discern where things are getting messed up.  But it looks like it's
starting out at the right place.


Best,

Ethan



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