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Message-ID: <1449187382.17296.16.camel@perches.com>
Subject: Re: [PATCH v3 04/14] lib/vsprintf.c: expand field_width to 24 bits
From: Joe Perches <joe@perches.com>
To: Andrew Morton <akpm@linux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Kees Cook <keescook@chromium.org>,
        Martin Kletzander <mkletzan@redhat.com>,
        Andy Shevchenko <andy.shevchenko@gmail.com>,
        Maurizio Lombardi <mlombard@redhat.com>, Tejun Heo <tj@kernel.org>,
        linux-kernel@vger.kernel.org
Date: Thu, 03 Dec 2015 16:03:02 -0800
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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))]