Re: [PATCH] media: cedrus: don't initialize pointers with zero

[Ian Arkver <ian.arkver.dev@gmail.com>]

On 07/12/2018 11:37, Hans Verkuil wrote:
> On 12/07/2018 12:31 PM, Mauro Carvalho Chehab wrote:
>> Em Fri, 7 Dec 2018 12:14:50 +0100
>> Hans Verkuil <hverkuil@xs4all.nl> escreveu:
>>
>>> On 12/07/2018 11:56 AM, Mauro Carvalho Chehab wrote:
>>>> A common mistake is to assume that initializing a var with:
>>>> 	struct foo f = { 0 };
>>>>
>>>> Would initialize a zeroed struct. Actually, what this does is
>>>> to initialize the first element of the struct to zero.
>>>>
>>>> According to C99 Standard 6.7.8.21:
>>>>
>>>>      "If there are fewer initializers in a brace-enclosed
>>>>       list than there are elements or members of an aggregate,
>>>>       or fewer characters in a string literal used to initialize
>>>>       an array of known size than there are elements in the array,
>>>>       the remainder of the aggregate shall be initialized implicitly
>>>>       the same as objects that have static storage duration."
>>>>
>>>> So, in practice, it could zero the entire struct, but, if the
>>>> first element is not an integer, it will produce warnings:
>>>>
>>>> 	drivers/staging/media/sunxi/cedrus/cedrus.c:drivers/staging/media/sunxi/cedrus/cedrus.c:78:49:  warning: Using plain integer as NULL pointer
>>>> 	drivers/staging/media/sunxi/cedrus/cedrus_dec.c:drivers/staging/media/sunxi/cedrus/cedrus_dec.c:29:35:  warning: Using plain integer as NULL pointer
>>>>
>>>> A proper way to initialize it with gcc is to use:
>>>>
>>>> 	struct foo f = { };
>>>>
>>>> But that seems to be a gcc extension. So, I decided to check upstream
>>>
>>> No, this is not a gcc extension. It's part of the latest C standard.
>>
>> Sure? Where the C standard spec states that? I've been seeking for
>> such info for a while, as '= {}' is also my personal preference.
> 
> I believe it was added in C11, but I've loaned the book that stated
> that to someone else, so I can't check.

Sadly I don't see mention of empty initializer lists in section 6.7.9 of
ISO/IEC 9899:2011, though I've only got a draft version.

I had a play with Compiler Explorer[1] and it seems like clang is OK
with the {} form though just out of interest msvc isn't. I didn't try
exploring other command line options.

[1] https://gcc.godbolt.org/z/XIDC7W

Regards,
Ian
> 
> In any case, it's used everywhere:
> 
> git grep '= *{ *}' drivers/
> 
> So it is really pointless to *not* use it.
> 
> Regards,
> 
> 	Hans
> 
>> I tried to build the Kernel with clang, just to be sure that this
>> won't cause issues with the clang support, but, unfortunately, the
>> clang compiler (not even the upstream version) is able to build
>> the upstream Kernel yet, as it lacks asm-goto support (there is an
>> OOT patch for llvm solving it - but it sounds too much effort for
>> my time to have to build llvm from their sources just for a simple
>> check like this).
>>
>>> It's used all over in the kernel, so please use {} instead of { NULL }.
>>>
>>> Personally I think {} is a fantastic invention and I wish C++ had it as
>>> well.
>>
>> Fully agreed on that.
>>
>>>
>>> Regards,
>>>
>>> 	Hans
>>>
>>>> what's the most commonly pattern. The gcc pattern has about 2000 entries:
>>>>
>>>> 	$ git grep -E "=\s*\{\s*\}"|wc -l
>>>> 	1951
>>>>
>>>> The standard-C compliant pattern has about 2500 entries:
>>>>
>>>> 	$ git grep -E "=\s*\{\s*NULL\s*\}"|wc -l
>>>> 	137
>>>> 	$ git grep -E "=\s*\{\s*0\s*\}"|wc -l
>>>> 	2323
>>>>
>>>> So, let's initialize those structs with:
>>>> 	 = { NULL }
>>>>
>>>> Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
>>>> ---
>>>>   drivers/staging/media/sunxi/cedrus/cedrus.c     | 2 +-
>>>>   drivers/staging/media/sunxi/cedrus/cedrus_dec.c | 2 +-
>>>>   2 files changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/staging/media/sunxi/cedrus/cedrus.c b/drivers/staging/media/sunxi/cedrus/cedrus.c
>>>> index b538eb0321d8..44c45c687503 100644
>>>> --- a/drivers/staging/media/sunxi/cedrus/cedrus.c
>>>> +++ b/drivers/staging/media/sunxi/cedrus/cedrus.c
>>>> @@ -75,7 +75,7 @@ static int cedrus_init_ctrls(struct cedrus_dev *dev, struct cedrus_ctx *ctx)
>>>>   	memset(ctx->ctrls, 0, ctrl_size);
>>>>   
>>>>   	for (i = 0; i < CEDRUS_CONTROLS_COUNT; i++) {
>>>> -		struct v4l2_ctrl_config cfg = { 0 };
>>>> +		struct v4l2_ctrl_config cfg = { NULL };
>>>>   
>>>>   		cfg.elem_size = cedrus_controls[i].elem_size;
>>>>   		cfg.id = cedrus_controls[i].id;
>>>> diff --git a/drivers/staging/media/sunxi/cedrus/cedrus_dec.c b/drivers/staging/media/sunxi/cedrus/cedrus_dec.c
>>>> index e40180a33951..4099a42dba2d 100644
>>>> --- a/drivers/staging/media/sunxi/cedrus/cedrus_dec.c
>>>> +++ b/drivers/staging/media/sunxi/cedrus/cedrus_dec.c
>>>> @@ -26,7 +26,7 @@ void cedrus_device_run(void *priv)
>>>>   {
>>>>   	struct cedrus_ctx *ctx = priv;
>>>>   	struct cedrus_dev *dev = ctx->dev;
>>>> -	struct cedrus_run run = { 0 };
>>>> +	struct cedrus_run run = { NULL };
>>>>   	struct media_request *src_req;
>>>>   	unsigned long flags;
>>>>   
>>>>    
>>>
>>
>>
>>
>> Thanks,
>> Mauro
>>
>