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

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Message-ID: <CAJfuBxxBY3Mq3EnuginFw_JkK3AE15V-u6C7uvi2L03tFfHOXQ@mail.gmail.com>
Date:	Mon, 9 Apr 2012 13:52:16 -0600
From:	Jim Cromie <jim.cromie@...il.com>
To:	Joe Perches <joe@...ches.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [00/02] add BUILD_BUG_DECL assertion (for 3.4??)

On Sun, Apr 8, 2012 at 8:37 PM, Joe Perches <joe@...ches.com> wrote:
> On Sun, 2012-04-08 at 17:59 -0600, Jim Cromie wrote:
>> On Sun, Apr 8, 2012 at 4:52 PM, Joe Perches <joe@...ches.com> wrote:
>> > 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.


>> >> BUILD_BUG_DECL(b, ARRAY_SIZE(a) != ARRAY_SIZE(names));        // good

>> >
>> > 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[] = {...}
>> > );
>> >
>> >
>>
>> Unless Im mis-reading you, this has a couple disadvantages:
>>
>> - bigger patches where its added.
>> granted these are mostly whitespace, but theyre less trivial to inspect.
>
> Not a problem in my view, it moves the
> related declarations closer together.
>
>> - not useful for array definitions which are not contiguous
>> granted thats a minority case, and the definitions could often be moved,
>> but not always.
>
> Discontiguous array definitions must be ugly.

Ugly ?
too pejorative IMO

each array defn is a single statement.
there may be functions between the 2 defns being compared.

Maybe not ideal,

>
>> Do you see advantages other than stylistic ones ?
>
> Not really.
>
> Contiguous declarations.
> No need for other markings.
>
> Seems useful enough.
>

OK.  Id expect your construct to be built upon mine,
we'd still need to start with something.

Also, mine is usable for things yours isnt.
I dont have a good example, but a simple/silly one is:

BUILD_BUG_DECL( wifi_channels_must_be_14,
                  ARRAY_SIZE(channels_table) == 14 );
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