[src/trunk]: src/share/misc explain NULL casts

[christos <christos%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/f63e68f007ce
branches:  trunk
changeset: 769140:f63e68f007ce
user:      christos <christos%NetBSD.org@localhost>
date:      Thu Sep 01 09:33:01 2011 +0000

description:
explain NULL casts

diffstat:

 share/misc/style |  14 ++++++++++++--
 1 files changed, 12 insertions(+), 2 deletions(-)

diffs (35 lines):

diff -r b8e0c942e6fd -r f63e68f007ce share/misc/style
--- a/share/misc/style  Thu Sep 01 09:04:08 2011 +0000
+++ b/share/misc/style  Thu Sep 01 09:33:01 2011 +0000
@@ -1,4 +1,4 @@
-/* $NetBSD: style,v 1.48 2011/08/27 17:48:11 joerg Exp $ */
+/* $NetBSD: style,v 1.49 2011/09/01 09:33:01 christos Exp $ */
 
 /*
  * The revision control tag appears first, with a blank line after it.
@@ -30,7 +30,7 @@
 #include <sys/cdefs.h>
 __COPYRIGHT("@(#) Copyright (c) 2008\
  The NetBSD Foundation, inc. All rights reserved.");
-__RCSID("$NetBSD: style,v 1.48 2011/08/27 17:48:11 joerg Exp $");
+__RCSID("$NetBSD: style,v 1.49 2011/09/01 09:33:01 christos Exp $");
 
 /*
  * VERY important single-line comments look like this.
@@ -354,6 +354,16 @@
         * not:
         *      !(p = f())
         *
+        * The notable exception here is varyadic functions. Since our
+        * code is designed to compile and work on different environments
+        * where we don't have control over the NULL definition (on NetBSD
+        * it is defined as ((void *)0), but on other systems it can be
+        * defined as (0) and both definitions are valid under ANSI C), it
+        * it advised to cast NULL to a pointer on varyadic functions,
+        * because on machines where sizeof(pointer) != sizeof(int) and in
+        * the absence of a prototype in scope, passing an un-casted NULL,
+        * will result in passing an int on the stack instead of a pointer.
+        *
         * Don't use `!' for tests unless it's a boolean.
         * E.g. use "if (*p == '\0')", not "if (!*p)".
         *