#define PerlIOBase(f) (*(f))
#define PerlIOSelf(f,type) ((type *)PerlIOBase(f))
#define PerlIONext(f) (&(PerlIOBase(f)->next))
+#define PerlIOValid(f) ((f) && *(f))
/*--------------------------------------------------------------------------------------*/
/* Data exports - EXT rather than extern is needed for Cygwin */