Do not use this API unless you compile libxml together with you program.
The FILE* structure is incompatible btw. different crt versions and so you cannot guarantee that you program links to the same one as the compiled libxml2.
(Someone correct me if I'm wrong.)
You can use the xmlSaveFormatFile.. API just as well. It will even convert UTF8-Paths you pass it to UTF-16 windows paths.
best regards / mit freundlichen Grüßen,
(ps: these email disclaimers are generally not very well recieved here :-)