linux-kernel - Re: [PATCH 5/5] debug: BUILD_BUG_ON: error on non-const expressions

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Date:	Tue, 02 Sep 2008 08:47:33 +0100
From:	"Jan Beulich" <jbeulich@...ell.com>
To:	"Boaz Harrosh" <bharrosh@...asas.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Alexey Dobriyan" <adobriyan@...il.com>,
	"Ivo van Doorn" <IvDoorn@...il.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Theodore Tso" <tytso@....edu>,
	"Rusty Russell" <rusty@...tcorp.com.au>,
	"John W. Linville" <linville@...driver.com>,
	"linux-kernel" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/5] debug: BUILD_BUG_ON: error on non-const
	 expressions

>>> Boaz Harrosh <bharrosh@...asas.com> 01.09.08 18:41 >>>
>Jan Beulich wrote:
>>>>> Boaz Harrosh <bharrosh@...asas.com> 01.09.08 17:00 >>>
>>> What is broken with my BUILD_BUG_ON_ZERO(). I tried all tests and
>>> it works fine. Do you have a test with unwanted results?
>>> (Actually it's the original one I have not touched it).
>> 
>> That's the problem - it uses the same sizeof(char[]) approach, and hence
>> has the same problems that you just try to fix for BUILD_BUG_ON().
>> 
>
>No it does not have this problem. Have you tested it? 
>I have! It works fine. (Complains on non-const expressions)

For static variables, yes. But not for automatic variables and the like:

#define BUILD_BUG_ON_ZERO(e) (sizeof(char[1 - 2 * !!(e)]) - 1)

int test(int i) {
	int x = BUILD_BUG_ON_ZERO(i);

	return x;
}

You could argue that I could place a simple BUILD_BUG_ON() later in
the code, but that easily defeats the documentation purposes the
construct also has (my general position on this is that the check should
be in or immediately before the statement that depends on the
enforced restriction).

Jan

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