void foo (int a, ...)
{
  dSP ;
  ENTER ;
  SAVETMPS ;
  PUSHMARK(SP) ;

  va_list args;
  va_start(args, a);

   // how do I pass the variable arguments to Perl?

   XPUSHs(sv_2mortal(newSViv(a)));
   va_end(args);

   PUTBACK;

   perl_call_pv("My::Func", G_DISCARD);

   FREETMPS;
   LEAVE;
}