diff --git a/usr.bin/cmp/extern.h b/usr.bin/cmp/extern.h
index 84ec872e543c..a35cc3d63938 100644
--- a/usr.bin/cmp/extern.h
+++ b/usr.bin/cmp/extern.h
@@ -42,4 +42,4 @@ void	c_special(int, const char *, off_t, int, const char *, off_t);
 void	diffmsg(const char *, const char *, off_t, off_t);
 void	eofmsg(const char *);
 
-extern int lflag, sflag, xflag;
+extern int lflag, sflag, xflag, zflag;