netdev - Re: [PATCH net-next v2] geneve: Use empty braces for addr6 initializer

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Date:   Fri, 16 Nov 2018 15:04:32 +0100
From:   Stefano Brivio <sbrivio@...hat.com>
To:     Nathan Chancellor <natechancellor@...il.com>,
        Joe Perches <joe@...ches.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Sabrina Dubroca <sd@...asysnail.net>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v2] geneve: Use empty braces for addr6
 initializer

On Mon, 12 Nov 2018 23:11:47 -0700
Nathan Chancellor <natechancellor@...il.com> wrote:

> Clang warns:
> 
> drivers/net/geneve.c:428:29: error: suggest braces around initialization
> of subobject [-Werror,-Wmissing-braces]
>                 struct in6_addr addr6 = { 0 };
>                                           ^
>                                           {}
> 
> Most initializations of structs in the kernel seem to use this format.

Actually, even with this, we get a warning with gcc 4.4 and 4.8. I tried a
few compilers:

$ gcc-4.4 --version | head -n1
rhel-6.9-gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-18)
$ gcc-4.8 --version | head -n1
rhel-7.5-gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-28)
$ gcc-7.3 --version | head -n1
gcc-7.3-gcc (GCC) 7.3.0
$ gcc-8.2 --version | head -n1
gcc (Debian 8.2.0-9) 8.2.0
$ clang --version | head -n1
clang version 6.0.1-9.2 (tags/RELEASE_601/final)

$ cat init.c
#include <linux/in6.h>
 
int main()
{
        struct in6_addr addr6 = INIT;
 
        return addr6.in6_u.u6_addr8[0];
}

$ gcc-4.4 -DINIT="{ }" -S -Wall -Wextra -o init init.c 2>&1 | grep warning | head -n1
init.c:5: warning: missing initializer
$ gcc-4.4 -DINIT="{ 0 }" -S -Wall -Wextra -o init init.c 2>&1 | grep warning | head -n1
init.c:5: warning: missing braces around initializer
$ gcc-4.4 -DINIT="{ { { 0 } } }" -S -Wall -Wextra -o init init.c 2>&1 | grep warning | head -n1

$ gcc-4.8 -DINIT="{ }" -S -Wall -Wextra -o init init.c 2>&1 | grep warning | head -n1
init.c:5:16: warning: missing initializer for field 'in6_u' of 'struct in6_addr' [-Wmissing-field-initializers]
$ gcc-4.8 -DINIT="{ 0 }" -S -Wall -Wextra -o init init.c 2>&1 | grep warning | head -n1
init.c:5:16: warning: missing braces around initializer [-Wmissing-braces]
$ gcc-4.8 -DINIT="{ { { 0 } } }" -S -Wall -Wextra -o init init.c 2>&1 | grep warning | head -n1

$ gcc-7.3 -DINIT="{ }" -S -Wall -Wextra -o init init.c 2>&1 | grep warning | head -n1
$ gcc-7.3 -DINIT="{ 0 }" -S -Wall -Wextra -o init init.c 2>&1 | grep warning | head -n1
$ gcc-7.3 -DINIT="{ { { 0 } } }" -S -Wall -Wextra -o init init.c 2>&1 | grep warning | head -n1

$ gcc-8.2 -DINIT="{ }" -S -Wall -Wextra -o init init.c 2>&1 | grep warning | head -n1
$ gcc-8.2 -DINIT="{ 0 }" -S -Wall -Wextra -o init init.c 2>&1 | grep warning | head -n1
$ gcc-8.2 -DINIT="{ { { 0 } } }" -S -Wall -Wextra -o init init.c 2>&1 | grep warning | head -n1

$ clang -DINIT="{ }" -S -Wall -Wextra -o init init.c 2>&1 | grep warning | head -n1
$ clang -DINIT="{ 0 }" -S -Wall -Wextra -o init init.c 2>&1 | grep warning | head -n1
init.c:5:33: warning: suggest braces around initialization of subobject [-Wmissing-braces]
$ clang -DINIT="{ { { 0 } } }" -S -Wall -Wextra -o init init.c 2>&1 | grep warning | head -n1

So { { { 0 } } } seems to be the safest option. We could go with static
but it looks even uglier to me.

Joe, suggestions?

-- 
Stefano