linux-kernel - Re: [PATCH] checkpatch: prefer = {} initializations to = {0}

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Message-ID: <20210816072344.GG22278@shell.armlinux.org.uk>
Date:   Mon, 16 Aug 2021 08:23:45 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Leon Romanovsky <leon@...nel.org>,
        Joe Perches <joe@...ches.com>,
        Dwaipayan Ray <dwaipayanray1@...il.com>,
        Andy Whitcroft <apw@...onical.com>,
        Lukas Bulwahn <lukas.bulwahn@...il.com>,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        Julia Lawall <julia.lawall@...ia.fr>
Subject: Re: [PATCH] checkpatch: prefer = {} initializations to = {0}

On Mon, Aug 16, 2021 at 09:55:53AM +0300, Dan Carpenter wrote:
> On Sat, Aug 14, 2021 at 02:52:31PM +0000, Al Viro wrote:
> > On Sat, Aug 14, 2021 at 03:59:22PM +0200, Christophe JAILLET wrote:
> > 
> > > > +# prefer = {}; to = {0};
> > > > +		if ($line =~ /= \{ *0 *\}/) {
> > > > +			WARN("ZERO_INITIALIZER",
> > > > +			     "= {} is preferred over = {0}\n" . $herecurr);
> > 
> > Sigh...  "is preferred over" by whom?  Use the active voice, would you?
> > 
> > > [1] and [2] state that {} and {0} don't have the same effect. So if correct,
> > > this is not only a matter of style.
> > > 
> > > When testing with gcc 10.3.0, I arrived at the conclusion that both {} and
> > > {0} HAVE the same behavior (i.e the whole structure and included structures
> > > are completely zeroed) and I don't have a C standard to check what the rules
> > > are.
> > > gcc online doc didn't help me either.
> > 
> > http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf, but empty
> > initializer-list is gccism anyway.
> > 
> > Section 6.7.8 is the one to look through there.
> 
> That's out of date.  It changed in C11.  Both = { 0 } and = { } will
> clear out struct holes. The = { } GCC extension has always initialized
> struct holes.
> 
> http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf
> 
> For partial initializations then all the padding is zeroed.
> Unfortunately if you fully initialize the struct then padding is not
> initialized.

If we're going to discuss which C standard applies to the kernel,
then...

As Kbuild passes -std=gnu89, the kernel expects C89 behaviour with
GNU extensions from the compiler, both C99 and C11 are not that
relevant, although the GNU extensions include some bits from these
standards.

-- 
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