[xml] how to validate xml with external schema

[xjuri volny cz]

Hi everybody,
does anybody know the way, how to validate XML document with XML schema
given as a parametr using libxml2? I know how to validate XML document
which includes schema adress, viz. 
http://groups.google.cz/group/comp.text.xml/browse_thread/thread/54d101bb8764068f/56e5ca1fdd408556?lnk=st&q=xmlSchemaValidateDoc&rnum=11&hl=cs#56e5ca1fdd408556
But I need some functionality like:

int validate (xml_document, xsd_schema);

I have to validate it in my application, so I can not use xmllint. I
would expact using libxml2 function xmlSchemaNewValidCtxt(schema), but
as I read this function should be called with NULL parametr.

Thanks for help,
Martin