diff --git a/bin/csh/exec.c b/bin/csh/exec.c
index 1f402cccc545..a37e1a823266 100644
--- a/bin/csh/exec.c
+++ b/bin/csh/exec.c
@@ -1,4 +1,4 @@
-/* $NetBSD: exec.c,v 1.30 2016/03/16 22:25:05 christos Exp $ */
+/* $NetBSD: exec.c,v 1.31 2016/03/16 22:35:44 christos Exp $ */
 
 /*-
  * Copyright (c) 1980, 1991, 1993
@@ -34,7 +34,7 @@
 #if 0
 static char sccsid[] = "@(#)exec.c	8.3 (Berkeley) 5/23/95";
 #else
-__RCSID("$NetBSD: exec.c,v 1.30 2016/03/16 22:25:05 christos Exp $");
+__RCSID("$NetBSD: exec.c,v 1.31 2016/03/16 22:35:44 christos Exp $");
 #endif
 #endif /* not lint */
 
@@ -90,6 +90,7 @@ static Char *expath;		/* Path for exerr */
 static unsigned char xhash[HSHSIZ / 8];
 
 #define hash(a, b) (((a) * HSHMUL + (b)) & HSHMASK)
+/* these macros eval their arguments multiple times, so be careful */
 #define bit(h, b) ((h)[(b) >> 3] & 1 << ((b) & 7))	/* bit test */
 #define bis(h, b) ((h)[(b) >> 3] = \
     (unsigned char)((1 << ((b) & 7)) | (h)[(b) >> 3]))/* bit set */