linux-kernel - Re: [PATCH] Use a zero-size array in the struct devres

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Message-ID: <tnx1wk2mm82.fsf@arm.com>
Date:	Tue, 06 Mar 2007 15:21:33 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Tejun Heo <htejun@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Use a zero-size array in the struct devres

Tejun Heo <htejun@...il.com> wrote:
> Catalin Marinas wrote:
>> It sounds to me like the alignment of an incomplete type is not
>> guaranteed (as you can't even enquire about it, though I might be
>> wrong). This is probably dependent on the compiler as well - with
>> gcc-3.3 on x86, __alignof__(unsigned long long) is 8 but the
>> "offsetof(struct test, data)" below is 12 (and it doesn't make any
>> difference whether it is a 0-size array or not):
>> 
>> struct test {
>>     unsigned long a;
>>     unsigned long b;
>>     unsigned long c;
>>     unsigned long long data[];
>> };
>
> data[0] and data[1] or whatever will also give you offset of 12.  Dunno
> why, but it is.  Anyways, whatever the wording in the manual is,
> flexible arrays just have to have the required alignment to do its job
> as advertised.  :-)

On ARM (with gcc-4) it gives 16 for both offsetof and sizeof.

>> I did a quick grep through the kernel and it looks like Linux mainly
>> uses 0-size rather than flexible arrays at the end of a structure
>> (>500 vs ~14). This might be for historical reasons but it's not a big
>> issue in modifying them.
>
> I think it's mostly historical.  Flexible array is still a relatively
> new thing.  I don't mind changing devres to zero sized array, but please
> explain in the commit message and as a comment that the choice is for
> kmemleak's container_of(), and cc Greg K-H as the change should probably
> go through his tree.

OK. I'll probably wait until I post a new version of kmemleak against
2.6.21-rcX.

Thanks.

-- 
Catalin
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