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

From: Ingo Molnar <mingo@kernel.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Denys Vlasenko <vda.linux@googlemail.com>,
	Denys Vlasenko <dvlasenk@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
	Oleg Nesterov <oleg@redhat.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Alexei Starovoitov <ast@plumgrid.com>,
	Will Drewry <wad@chromium.org>, Kees Cook <keescook@chromium.org>,
	X86 ML <x86@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/2] move offsetofend() from vfio.h to stddef.h
Date: Mon, 9 Mar 2015 16:45:38 +0100	[thread overview]
Message-ID: <20150309154538.GA21578@gmail.com> (raw)
In-Reply-To: <CALCETrXRjpary2PRzxbyjcwJcHUu5Pr4ELiK8+=_feNxooMoPg@mail.gmail.com>

* Andy Lutomirski <luto@amacapital.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