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

Russell King (Oracle)
2021-08-16T07:23:55Z

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 That's out of date.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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