linux-kernel - Re: [PATCH 1/2] move offsetofend() from vfio.h to stddef.h

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Message-ID: <20150309154538.GA21578@gmail.com>
Date:	Mon, 9 Mar 2015 16:45:38 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Denys Vlasenko <vda.linux@...glemail.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] move offsetofend() from vfio.h to stddef.h


* Andy Lutomirski <luto@...capital.net> wrote:

> > So I might be missing something, but what generic uses does it 
> > have, beyond structures that have some rare size related 
> > weirdness, such as alignment attributes? In 99% of the cases:
> >
> >    sizeof(struct) == offsetofend(struct, last_member)
> >
> > right?
> 
> struct foo {
>     u64 a;
>     char b;
> };
> 
> sizeof(struct foo) will be 16, but offsetofend(struct foo, b) will be
> 9 on most platforms, right?

I knew I missed something obvious :-)

Let me attempt to get it right:

When the next byte after the last member of a structure is not aligned 
to the largest alignment requirement of any structure member, then the 
structure grows (is padded) and offsetofend() < sizeof().

'packed' or 'aligned' attributes will modify the largest alignment 
requirement value so they are a common but not only mechanism for this 
to be the case.

Thanks,

	Ingo
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