linux-kernel - Re: [PATCH 5/5 ver2] debug: BUILD_BUG_ON: error on non-const expressions

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Date:	Wed, 03 Sep 2008 11:19:51 +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 ver2] debug: BUILD_BUG_ON: error on non-const
	 expressions

>>> Boaz Harrosh <bharrosh@...asas.com> 03.09.08 10:57 >>>
>Jan Beulich wrote:
>>>>> Boaz Harrosh <bharrosh@...asas.com> 02.09.08 17:57 >>>
>>> -#define BUILD_BUG_ON_ZERO(e) (sizeof(char[1 - 2 * !!(e)]) - 1)
>>> +#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
>>> +
>>> +/* Force a compilation error if condition is true */
>>> +#define BUILD_BUG_ON(e) \
>>> +	do { struct {int:-!!(e); } x __maybe_unused;} while(0)
>>  
>> As indicated before, you should at the very least use __x as the variable
>> name.
>> 
>
>The name does not matter. The scope of x is confined to the do {} while()
>and will not interfere with any local or global name.  

I'm sorry to repeat this: If x is #define-d to anything but a simple identifier,
this will break no matter that it's in a private scope. The absence of any
identifier was a benefit of the sizeof() approach here.

Jan

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