Re: Extending XML::LibXML through XS

On Sunday 30 November 2008 09:28:39 Emmanuel Rodriguez wrote:
> On Sat, Nov 29, 2008 at 11:30 PM, Petr Pajas <[email protected]> wrote:
> > Hi Emmanuel,
>
> Hi Petr,
>
> Thanks for the pointers!
>
> > PmmSvNode(node) is a macro defined in XML::LibXML's libxml-perl-mm.h that
> > expands to a function call PmmSvNodeExt(node,1) whose prototype is
> >
> > xmlNodePtr PmmSvNodeExt(SV* node, int copy);
>
> I tried this but then I get the following error at runtime:
> symbol lookup error: Xacobeo/XS/XS.so: undefined symbol: PmmSvNodeExt
>
> I managed to get around this error by linking Xacobeo/XS/XS.so with
> /usr/lib/perl5/auto/XML/LibXML/LibXML.so. I'm not too crazy about
> linking to LibXML.so as I will need to detect where the .so is.
>
> > Try adding just this prototype and either defining the macro or rewriting
> > the typemap to use the direct function call. Hopefully the dynamical
> > linker will be able to locate the symbol in XML/LibXML.so loaded with
> > XML::LibXML. If not, you'll have to copy-paste the function from
> > perl-libxml-mm.c.
>
> I tried to copy  perl-libxml-mm.c and perl-libxml-sax.c (it's needed
> by perl-libxml-mm.c) but I still have problem at runtime:
> undefined symbol: PROXY_NODE_REGISTRY_MUTEX
>
> I've checked and this mutex is declared in LibXML.c. I tried to link
> again with LibXML/LibXML.so but the error persists. The only way I
> found out to remove the error is to declare  PROXY_NODE_REGISTRY_MUTEX
> in xs file. I'm not sure that this is a great idea as the mutex seems
> to be used for thread synchronization and this will create two
> different mutexes.

Emmanuel,
definitelly don't do that. If you must, XML::LibXML has a function that 
returns a pointer to the mutex (see how this is done in XML::LibXSLT). But 
this copying full perl-libxml-mm.c is much overhead and I can't even 
guarantee neither backward nor forward binary compatibility for all the code.

> I think that I have no choice but to use the first alternative: to
> declare the prototypes of PmmSvNode and PmmSvNodeExt and to link with
> XML/LibXML/LibXML.so

Either that or, since you just need this function, copy/paste only this 
function (and a few macros used by it) from perl-libxml-mm.c should work as 
well; PmmSvNodeExt does not do much, you may even simplify the code to 
something like this (and maybe use it directly in your typemap):

struct _ProxyNode {
    xmlNodePtr node;
    xmlNodePtr owner;
    int count;
    int encoding;
};
typedef struct _ProxyNode ProxyNode;
typedef ProxyNode* ProxyNodePtr;

PmmSvNodeSimple( SV* perlnode )
{
  ProxyNodePtr proxy = NULL;
  if ( perlnode != NULL && perlnode != &PL_sv_undef &&
       sv_derived_from(perlnode, "XML::LibXML::Node")  ) {
    proxy = INT2PTR(ProxyNodePtr,SvIV(SvRV(perlnode)));
    if ( proxy != NULL ) return proxy->node;
  }
  return NULL;
}

Best,
-- Petr
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