diff --git a/Size.xs b/Size.xs
index ff585e0..60417ac 100644
--- a/Size.xs
+++ b/Size.xs
@@ -21,14 +21,14 @@ MODULE = Term::Size		PACKAGE = Term::Size
 PROTOTYPES: DISABLE
 
 void
-chars( f = stdin )
-	FILE *f;
+chars( f = PerlIO_stdin() )
+	PerlIO *f;
 
 	PREINIT:
 	struct winsize w = { 0, 0, 0, 0 };
 
 	PPCODE:
-	if (ioctl(fileno(f), TIOCGWINSZ, &w) == -1)
+	if (ioctl(PerlIO_fileno(f), TIOCGWINSZ, &w) == -1)
 		XSRETURN_NO;
 
 	XPUSHs(sv_2mortal(newSViv(w.ws_col)));
@@ -36,14 +36,14 @@ chars( f = stdin )
 		XPUSHs(sv_2mortal(newSViv(w.ws_row)));
 
 void
-pixels( f = stdin )
-	FILE *f;
+pixels( f = PerlIO_stdin() )
+	PerlIO *f;
 
 	PREINIT:
 	struct winsize w = { 0, 0, 0, 0 };
 
 	PPCODE:
-	if (ioctl(fileno(f), TIOCGWINSZ, &w) == -1)
+	if (ioctl(PerlIO_fileno(f), TIOCGWINSZ, &w) == -1)
 		XSRETURN_NO;
 
 	XPUSHs(sv_2mortal(newSViv(w.ws_xpixel)));