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

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Date:   Mon, 16 Aug 2021 09:55:53 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        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 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That's out of date.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.

regards,
dan carpenter