From owner-freebsd-hackers  Fri Sep 14  1:26:28 2001
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Date: Fri, 14 Sep 2001 12:23:44 +0400
From: Yar Tikhiy <yar@freebsd.org>
To: Mike Smith <msmith@freebsd.org>
Cc: hackers@freebsd.org, msmith@mass.dis.org
Subject: Re: Driver structures & alignment
Message-ID: <20010914122344.A38530@snark.rinet.ru>
References: <yar@freebsd.org> <200109132309.f8DN9vX03967@mass.dis.org>
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In-Reply-To: <200109132309.f8DN9vX03967@mass.dis.org>; from msmith@freebsd.org on Thu, Sep 13, 2001 at 04:09:57PM -0700
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On Thu, Sep 13, 2001 at 04:09:57PM -0700, Mike Smith wrote:
> > 
> > Is there a single blessed way to define packed structures
> > for use in drivers?  I suspect that using "#pragma pack(1)"
> > will lead to alignment errors in non-Intel architectures.
> 
> Any form of packing is going to cause problems for items that are
> located in illegal fashions. 
> 
> Having said that, I recommend using __attribute__ ((packed))
> to explicitly request that a structure be packed.

Should the __attribute__ be supplied to each structure's member?
Or can it be the whole structure's attribute?
 
> > And is it OK to rely on specially crafted structures, like
> > "struct fs" in /sys/ufs/ffs/fs.h where multi-byte elements
> > are pre-aligned by the structure's design, being contiguous?
> 
> "rely" in what fashion?
 
E.g., will the following structure:
struct foo {
	int16_t	a;
	int16_t b;
	int32_t c; /* 4-byte boundary */
	int64_t d; /* 8-byte boundary */
	int8_t  e[4];
	int32_t f; /* 4-byte boundary */
};
contain alignment holes in any architecture/compiler?

-- 
Yar

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