diff --git a/lib/libbsddialog/Makefile b/lib/libbsddialog/Makefile
index 95f8fad115a4..d5c03cb079ce 100644
--- a/lib/libbsddialog/Makefile
+++ b/lib/libbsddialog/Makefile
@@ -1,28 +1,29 @@
 BSDDIALOG=	${SRCTOP}/contrib/bsddialog
 
 .PATH:	${BSDDIALOG}/lib
 
 LIB=	bsddialog
 PRIVATELIB=	yes
 SHLIB_MAJOR=	0
 SRCS=	barbox.c \
 	formbox.c \
 	infobox.c \
 	lib_util.c \
 	lib_util.h \
 	libbsddialog.c \
 	menubox.c \
 	messagebox.c \
 	textbox.c \
 	theme.c \
 	timebox.c
 INCS=	bsddialog.h \
 	bsddialog_theme.h \
 	bsddialog_progressview.h
 MAN=	bsddialog.3
 
 LIBADD=	ncursesw tinfow formw
+CFLAGS+=	-D_XOPEN_SOURCE_EXTENDED
 
 WARNS=	2
 
 .include <bsd.lib.mk>