linux-kernel - Re: [PATCH v3 04/14] lib/vsprintf.c: expand field

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Date:	Thu, 03 Dec 2015 16:03:02 -0800
From:	Joe Perches <joe@...ches.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Kees Cook <keescook@...omium.org>,
	Martin Kletzander <mkletzan@...hat.com>,
	Andy Shevchenko <andy.shevchenko@...il.com>,
	Maurizio Lombardi <mlombard@...hat.com>,
	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 04/14] lib/vsprintf.c: expand field_width to 24 bits

On Thu, 2015-12-03 at 15:34 -0800, Andrew Morton wrote:
> I've been fiddling with a BUILD_BUG_ON which works outside functions
> using gcc's __COUNTER__ - something like
> 
> #define BBO(expr) typedef char __bbo##__COUNTER__[1-2*(!!expr)]

nit:  you need another parenthesis around expr

> BBO(1 == 1);
> BBO(2 == 2);
> 
> but that comes out as
> 
> typedef char __bbo__COUNTER__[1-2*(!!1 == 1)];
> typedef char __bbo__COUNTER__[1-2*(!!2 == 2)];
> 
> instead of
> 
> typedef char __bbo0[1-2*(!!1 == 1)];
> typedef char __bbo1[1-2*(!!2 == 2)];
> 
> There's some trick here but I've forgotten what it is.

I believe it's something like:

#define __stringify_2(a, b)	a##b
#define __stringify2(a, b)	__stringify_2(a, b)

#define BBO(expr) typedef char __stringify2(bbo, __COUNTER__)[1 - 2*(!!(expr))]
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