Re: [xml] xmlAddChild question

[Daniel Veillard <veillard redhat com>]

On Tue, Apr 10, 2012 at 05:50:07PM -0500, Bruce Sinclair wrote:
> The description of xmlAddChild says:
>
>  
>
> "Add a new node to @parent, at the end of the child (or property) list
> merging adjacent TEXT nodes (in which case @cur is freed)."
>
>  
>
> and the description of xmlAddChildList says:
>
>  
>
> "Add a list of node at the end of the child list of the parent merging
> adjacent TEXT nodes (@cur may be freed)"
>
>  
>
> Assuming that @cur is freed in the case of "merging adjacent text nodes"
> (as the sentence seems to read and bolstered by the "may" in the second
> description above), how is one to know whether adjacent TEXT nodes were
> merged in order to know whether @cur is freed?  
>
>  
>
> We had a case of a memory leak because we generally assumed @cur was
> freed, but when we added an empty value text node, it was not.  We don't
> generally have "adjacent TEXT nodes", so it seems that "merging adjacent
> TEXT nodes" is not a qualification of whether @cur is freed, which would
> mean no information is necessary on the status of node merges.  Our
> experience is that the @cur is freed if the function succeeds, except in
> the case of adding an empty text node (even though the add child
> function succeeds in this case).  Can you clarify when @cur is freed or
> not freed?

  In general cur is *not* freed, it is linked to the parent at the
end of the children list. Only when the last item of that list is a text
node and cur is a text node, then libxml2 will merge the two and as a
result cur will be freed.
  In general cur will be freed when the parent is freed (usually when
the containing document is freed).
  The case that you must account for is when the function fails
(in which case it will return NULL) and in that case cur is not linked
to the parent and won't be freed when the document is freed, and there
you have a leak. You must watch xmlAddChild() return value !
  The warning about mergind of text nodes is that people were trying
to use @cur after the call, which worked most of the time, until
text merging happen in which case they were referencing freed memory
and crash !

Daniel

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