netdev - Re: [PATCH 2/9] ocrdma: Driver for Emulex OneConnect RDMA adapter

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Date:	Fri, 23 Mar 2012 10:54:14 -0600
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	Parav.Pandit@...lex.Com
Cc:	David.Laight@...LAB.COM, roland@...estorage.com,
	linux-rdma@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 2/9] ocrdma: Driver for Emulex OneConnect RDMA adapter

On Fri, Mar 23, 2012 at 07:03:37AM -0700, Parav.Pandit@...lex.Com wrote:

> > David is saying you will get a 12 byte struct and fieldb will be unaligned. Since
> > 12 is aligned to 4 no padding is added.
> 
> So I decided to experiment above example before implementing in
> driver. However I find structure of 16 bytes (instead of 12) with
> padding after fielda in below example.  Am I missing some compiler
> option or syntax error in attribute? Sorry to ask this silly
> question.  I tried __attribute__((__aligned__(4))); too based on
> usage in other kernel code.

I got the syntax wrong for that specific case (it is a little
unintuitive.. IMHO, capping the alignment of a container should cap
the alignment of all members, otherwise it is nonsense!):

typedef uint64_t u64_unaligned_8 __attribute__((__aligned__(4)));

struct foo {
        uint32_t fielda;
        u64_unaligned_8 fieldb;
};

struct foo2 {
        uint32_t fielda;
        uint64_t fieldb;
};

int main(int argc,const char *argv[])
{
	printf("sizeof(foo) = %zu, fieldb = %zu\n",sizeof(struct foo),
	       offsetof(struct foo,fieldb));
	printf("sizeof(foo2) = %zu, fieldb = %zu\n",sizeof(struct foo2),
	       offsetof(struct foo2,fieldb));
	return 0;
}

sizeof(foo) = 12, fieldb = 4
sizeof(foo2) = 16, fieldb = 8

gcc version 4.6.1 (Ubuntu/Linaro 4.6.1-9ubuntu3) 

Jason
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