Re: self contains no data

On 13/09/2012 15:21, Bruce Miller wrote:
> On 09/13/2012 05:49 AM, Nick Wellnhofer wrote:
>> On 13/09/2012 00:33, Bruce Miller wrote:
>>> I did find something curious.  So, XML::LibXML nodes are
>>> references to integers.  I don't know if those numbers "mean"
>>> anything --- in ancient times, they were constant, and
>>> you could test node equality by $$a == $$b.
>>> At some point (an update related to thread safety?),
>>> they're no longer constant, and there's a specific
>>> method $a->isSameNode($b).  Would be interested in the back-story.
>>
>> These numbers are raw C pointers.
>
> Ah, I'd suspected that....
> but they're actually moving around a bit in modern C?  interesting...
> [If I fetch the "same" nodes from a document at several points
> in the program, I'll get different values]

That seems strange indeed. I can't remember the internals of 
XML::LibXML, but maybe it's normal. Different values across program runs 
are expected because of address space layout randomization.

>>> At any rate, printing out the dereferenced integer while walking
>>> the tree gives me nice 8-digit numbers like 54824480...
>>> until the broken node which has 12 digits (249739024344)
>>> and then I get a segfault before any of the Node methods
>>> can tell me anything about the node!
>>
>> The 12-digit number seems to be a corrupted pointer, so the previous
>> operation on that node is probably the culprit. Can you tell what the
>> previous operation is? Or how the node is created if it's not
>> manipulated in any way?
>
> The structure is roughly:
>    <Math>
>      <XMath>
>        <XMTok>
>        ...
> In this case, the pointer is (or should be) to the <XMath>
> and was obtained either by $m->firstChild or $m->childNodes
> (where $m is to the <Math>); both fail.
>
> The children/structure under the <XMath> has been manipulated
> and rearranged, rather vigorously.

One of those manipulations probably triggers a bug in either libxml2 or 
the Perl bindings. But without further details it's impossible to know 
what's going on.

Nick

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