linux-kernel - Re: struct fanotify_event

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Message-ID: <1282310016.21419.2161.camel@acb20005.ipt.aol.com>
Date:	Fri, 20 Aug 2010 09:13:36 -0400
From:	Eric Paris <eparis@...hat.com>
To:	Andreas Schwab <schwab@...ux-m68k.org>
Cc:	Tvrtko Ursulin <tvrtko.ursulin@...hos.com>,
	Andreas Gruenbacher <agruen@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: struct fanotify_event_metadata

On Fri, 2010-08-20 at 11:16 +0200, Andreas Schwab wrote:
> Tvrtko Ursulin <tvrtko.ursulin@...hos.com> writes:
> 
> > Shrink pid field in the fanotify_event_metadata to 32-bit to match
> > the kernel representation. Pull mask field up since it logically
> > comes before event auxiliary data and also makes for a nicer
> > alignment.
> 
> That won't buy you much wrt. alignment though, due to the packed
> attribute.

Can you help me understand the packed attribute and why it hurts in this
case?  It's not going to change the alignment or placement of anything
and I saw it as just being needed to make sure everything would continue
to work in the future.

I'm going to compile a couple of test programs (I only have x86 and
x86_64) to see if I can find what the assembly is doing different but
maybe you can point me at the information more easily?

-Eric


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