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

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Message-ID: <1b94e688-a070-998a-3014-96bcbaed4cae@wanadoo.fr>
Date:   Sat, 14 Aug 2021 15:59:22 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Dan Carpenter <dan.carpenter@...cle.com>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Leon Romanovsky <leon@...nel.org>
Cc:     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}

Hi all,

Le 05/08/2021 à 12:43, Dan Carpenter a écrit :
> The "= {};" style empty struct initializer is preferred over = {0}.
> It avoids the situation where the first struct member is a pointer and
> that generates a Sparse warning about assigning using zero instead of
> NULL.  Also it's just nicer to look at.
> 
> Some people complain that {} is less portable but the kernel has
> different portability requirements from userspace so this is not a
> issue that we care about.
> 
> Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
> ---
>   scripts/checkpatch.pl | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
> index 461d4221e4a4..32c8a0ca6fd0 100755
> --- a/scripts/checkpatch.pl
> +++ b/scripts/checkpatch.pl
> @@ -4029,6 +4029,12 @@ sub process {
>   			     "Using $1 is unnecessary\n" . $herecurr);
>   		}
>   
> +# prefer = {}; to = {0};
> +		if ($line =~ /= \{ *0 *\}/) {
> +			WARN("ZERO_INITIALIZER",
> +			     "= {} is preferred over = {0}\n" . $herecurr);
> +		}
> +
>   # Check for potential 'bare' types
>   		my ($stat, $cond, $line_nr_next, $remain_next, $off_next,
>   		    $realline_next);
> 

[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.

To test, I wrote a trivial C program, compiled it with gcc -S and looked 
at the assembly files.

Maybe, if it is an undefined behavior, other compilers behave 
differently than gcc.

However, the 2 persons listed bellow have a much better Linux and C 
background than me. So it is likely that my testings were too naive.

Can someone provide some rational or compiler output that confirms that 
{} and {0} are not the same?

Because if confirmed, I guess that there is some clean-up work to do all 
over the code, not only to please Sparse!

Thanks in advance.
CJ

[1]: Russell King - 
https://lore.kernel.org/netdev/YRFGxxkNyJDxoGWu@shredder/T/#efe1b6c7862b7ca9588c2734f04be5ef94e03d446

[2]: Leon Romanovsky - 
https://lore.kernel.org/netdev/YRFGxxkNyJDxoGWu@shredder/T/#efe1b6c7862b7ca9588c2734f04be5ef94e03d446