diff --git a/lib/libbsddialog/Makefile b/lib/libbsddialog/Makefile
index 95f8fad115a4..d5c03cb079ce 100644
--- a/lib/libbsddialog/Makefile
+++ b/lib/libbsddialog/Makefile
@@ -22,6 +22,7 @@ INCS=	bsddialog.h \
 MAN=	bsddialog.3
 
 LIBADD=	ncursesw tinfow formw
+CFLAGS+=	-D_XOPEN_SOURCE_EXTENDED
 
 WARNS=	2