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

Leon Romanovsky
2021-08-14T14:38:33Z

On Sat, Aug 14, 2021 at 03:59:22PM +0200, Christophe JAILLET wrote:
> 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@oracle.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.

There are number of reasons why you didn't notice any difference.
1. {} is GCC extension
2. {} was adopted in latest C standards, so need to check which one GCC 10
is using by default.
3. Main difference will be in padding - {0} will set to zero fields but
won't touch padding, while {} will zero everything.

>
>
> 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