linux-kernel - Re: [PATCH] checkpatch: Add a --strict test for structs with bool member definitions

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Date:   Wed, 11 Apr 2018 10:15:02 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Joe Perches <joe@...ches.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andy Whitcroft <apw@...onical.com>, yuankuiz@...eaurora.org,
        Linux PM <linux-pm@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        paulmck@...ux.vnet.ibm.com, Ingo Molnar <mingo@...nel.org>,
        Len Brown <len.brown@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] checkpatch: Add a --strict test for structs with bool
 member definitions

On Tue, Apr 10, 2018 at 03:00:11PM -0700, Andrew Morton wrote:
> On Tue, 10 Apr 2018 14:53:51 -0700 Joe Perches <joe@...ches.com> wrote:
> 
> > On Tue, 2018-04-10 at 14:39 -0700, Andrew Morton wrote:
> > > On Tue, 10 Apr 2018 11:19:54 -0700 Joe Perches <joe@...ches.com> wrote:
> > > 
> > > > A struct with a bool member can have different sizes on various
> > > > architectures because neither bool size nor alignment is standardized.
> > > 
> > > What's wrong with bools in structs?
> > 
> > See above.
> 
> Yeah, but so what?  `long' has different sizes on different
> architectures too.

Right, so we have ILP32/LP64 for all our 32/64 bit archs respectively.
So only 2 possible variations to consider, and if you know your bitness
you know your layout.

(+- some really unfortunate alignment exceptions, the worst of which
Arnd recently removed, hooray!)

But neither says anything about sizeof(_Bool), and the standard leaves
it undefined and only mandates it is large enough to store either 0 or
1 (and I suspect this vagueness is because there are architectures that
either have no byte addressibility or it's more expensive than word
addressibility).

Typically GCC chooses a single byte to represent _Bool, but there are no
guarantees. This means that when you care about structure layout (as we
all really should) things go wobbly when you use _Bool.

If GCC were to guarantee a 1 byte _Bool for all Linux ABIs we could
reconsider.