Subject: Re: [PATCH v2] media: cedrus: don't initialize pointers with zero

From: Mauro Carvalho Chehab <[email-masked]>
Date: Fri, 7 Dec 2018 11:41:50 -0200

Em Fri, 07 Dec 2018 14:21:44 +0100
Paul Kocialkowski <[email-masked]> escreveu:

> Hi,
> 
> On Fri, 2018-12-07 at 08:03 -0500, 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
> > 
> > As the right initialization would be, instead:
> > 
> > 	struct foo f = { NULL };  
> 
> Thanks for sharing these details, it's definitely interesting and good
> to know :)

Yeah, that's something that was bothering for quite a while, as I've
seen patches using either one of the ways. It took me a while to
do some research, and having it documented at the patch helps, as
we should now handle it the same way for similar stuff :-)

> 
> > Another way to initialize it with gcc is to use:
> > 
> > 	struct foo f = {};
> > 
> > That seems to be a gcc extension, but clang also does the right thing,
> > and that's a clean way for doing it.
> > 
> > Anyway, I decided to check upstream what's the most commonly pattern.
> > The "= {}" 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
> > 
> > Meaning that developers have split options on that.
> > 
> > So, let's opt to the simpler form.  
> 
> Acked-by: Paul Kocialkowski <[email-masked]>

Applied, thanks!

> 
> > Signed-off-by: Mauro Carvalho Chehab <[email-masked]>
> > ---
> >  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..b7c918fa5fd1 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 = {};
> >  
> >  		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..f10c25f5460e 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 = {};
> >  	struct media_request *src_req;
> >  	unsigned long flags;
> >    

Thanks,
Mauro

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