Re: [xml] libxml2 xmlNode management

[Michael Stahl <Michael X Stahl Oracle COM>]

On 27/04/2011 16:55, Alex Bligh wrote:
> So, questions:
>
> 1. Can an xmlNode that has children in document tree be "pruned" from
>    one document and added to another together with its children? Is this
>    a simple matter of doing an xmlAddChild? (ie are operations like
>    xmlAddChild safe with xmlNodes that already form part of the same
>    or a different document).

there is a function xmlSetTreeDoc which should do this (i've never used it):
http://xmlsoft.org/html/libxml-tree.html#xmlSetTreeDoc

i guess you also have to call xmlUnlinkNode on the original first, then
xmlSetTreeDoc, and then xmlAddChild with some node in the target document
to insert the original node.

i don't believe xmlAddChild with nodes from 2 different documents will
work; you'll probably sometimes then get nodes that are linked into both
documents.
hmmm, just had a look at xmlAddChild, actually it does this:
    if (cur->doc != parent->doc) {
        xmlSetTreeDoc(cur, parent->doc);
    }
so you don't actually have to call it yourself.

> 2. xmlNodes can be created without adding them to an xmlDocument.
>    Can such an xmlNode have children? If so how do I prune a subtree

AFAIK nodes without a document may have children.
but i guess then the children better also have no document.

>    of xmlNodes off an existing tree without adding them to a new
>    one (i.e. how do I reverse xmlAddChild)? I think this is
>    xmlUnlinkNode though the documentation refers to this as
>    unlinking it from its "current context". Does that mean it is
>    unlinked from just its parent? Or its children too?

xmlUnlinkNode will unlink the node from both its parent and its siblings,
but not from its children or the document.

so xmlUnlinkNode is basically (almost) the reverse of xmlAddChild.

> 3. If I want to prune a node and its children and delete them
>    how can I do that without risking a memory leak? Does xmlUnlinkNode
>    followed by xmlFreeNode (which is described as recursive) do that. If the
>    node is unlinked, how can it have children?

yes, xmlUnlinkNode + xmlFreeNode should do it.
as already mentioned, xmlUnlinkNode leaves children alone.

note also that xmlFreeDoc will not free nodes that have been unlinked; the
only element nodes freed by xmlFreeDoc are those reachable from the root
element.

> 4. Is it safe to xmlFreeNode a node which is part of a tree?

you first have to call xmlUnlinkNode.
then xmlFreeNode should be safe.

> I get the feeling some calls (e.g. xmlAddChild) will deal intelligently
> with the node's existing parents or children, but some (perhaps e.g.
> xmlFreeNode) will not. Is there a list somewhere, or perhaps better
> an idiots guide to how these tree manipulation operations work?

i've also asked myself this question, when i debugged OOo's libxml-based
DOM implementation some months ago, which needs to do all sorts of tree
manipulation (and was often doing it wrong); mainly i just read the
implementation when the documentation wasn't clear enough :)

regards,
 michael

-- 
"If you think good architecture is expensive, try bad architecture."
 -- Brian Foote and Joseph Yoder, "Big Ball Of Mud"