linux-kernel - Re: [00/02] add BUILD_BUG_DECL assertion (for 3.4??)

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Date:	Sun, 08 Apr 2012 15:52:15 -0700
From:	Joe Perches <joe@...ches.com>
To:	Jim Cromie <jim.cromie@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [00/02] add BUILD_BUG_DECL assertion (for 3.4??)

On Sun, 2012-04-08 at 16:38 -0600, Jim Cromie wrote:

> this patch (0001) adds new bug.h macro, BUILD_BUG_DECL(name, cond),
> which unlike other *BUG* assertions is usable at file scope.  Its
> primary purpose is to enforce identical sizes of 2 separate arrays,
> which but for considerations of packing/padding/section, would be
> together in a struct.
> 
> const char const *names[] = { "bart", "lisa", "homer", "marge" };
> int a[] = {1,2,3,4};
> int b[] = {1,2,3,5};
> long d[] = {1,2};
> 
> BUILD_BUG_DECL(foo, ARRAY_SIZE(a) != ARRAY_SIZE(b));
> BUILD_BUG_DECL(buz, sizeof(a) != sizeof(b));	// good
> BUILD_BUG_DECL(a, sizeof(a) != sizeof(d));	// ok on x32, error x64
> BUILD_BUG_DECL(b, ARRAY_SIZE(a) != ARRAY_SIZE(names));	// good
> 
> macro expands as:
> static __attribute__ ((__section__(".init.data"))) struct {
>        int BUILD_BUG_DECL_buz[1 - 2*!!(sizeof(a) != sizeof(b))];
> } BUILD_BUG_DECL_buz[0] __attribute__((unused));
> 

If possible, it might be better to wrap the
declarations themselves in a macro that ensures
the sizes are the same.

Something like:

declare_same_size_arrays(
	typeof array1[] = {...},
	typeof array2[] = {...}
);


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