#define PerlIO_fileno(f) sffileno(f)
#define PerlIO_clearerr(f) sfclrerr(f)
#define PerlIO_flush(f) sfsync(f)
-#define PerlIO_tell(f) sftell(f)
+#define PerlIO_tell(f) sfseek(f,0,1|SF_SHARE)
#define PerlIO_seek(f,o,w) sfseek(f,o,w)
#define PerlIO_rewind(f) (void) sfseek((f),0L,0)
#define PerlIO_tmpfile() sftmp(0)
#define PerlIO_has_cntptr(f) 1
#define PerlIO_get_ptr(f) ((f)->next)
#define PerlIO_get_cnt(f) ((f)->endr - (f)->next)
-#define PerlIO_canset_cnt(f) 1
-#define PerlIO_fast_gets(f) 1
-#define PerlIO_set_ptrcnt(f,p,c) ((f)->next = (unsigned char *)(p))
-#define PerlIO_set_cnt(f,c) 1
+#define PerlIO_canset_cnt(f) 0
+#define PerlIO_fast_gets(f) 1
+#define PerlIO_set_ptrcnt(f,p,c) STMT_START {(f)->next = (unsigned char *)(p); assert(FILE_cnt(f) == (c))} STMT_END
+#define PerlIO_set_cnt(f,c) Perl_croak(aTHX_ "Cannot set 'cnt' of FILE * on this system");
#define PerlIO_has_base(f) 1
#define PerlIO_get_base(f) ((f)->data)