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Re: RFC: kernel coding style: prefer array to &array[0] ?

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  Re: RFC: kernel coding style: prefer array to &array[0] ? Dmitry Torokhov <dmitry.torokhov@gmail.com> - 2015-07-02 00:40 +0200
    Re: RFC: kernel coding style: prefer array to &array[0] ? Joe Perches <joe@perches.com> - 2015-07-02 01:10 +0200

#1175585 — Re: RFC: kernel coding style: prefer array to &array[0] ?

On Wed, Jul 01, 2015 at 07:53:44AM -0700, Joe Perches wrote:
> On Wed, 2015-07-01 at 14:26 +0200, Julia Lawall wrote:
> > On Wed, 1 Jul 2015, Dan Carpenter wrote:
> > > On Wed, Jul 01, 2015 at 01:54:29PM +0200, Clemens Ladisch wrote:
> > > > Joe Perches wrote:
> > > > > It seems most in-kernel uses are 'array' rather than '&array[0]'
> > > > >
> > > > > Most of the time, using array is simpler to read than &array[0].
> > > > >
> > > > > Exceptions exists when addresses for consecutive members are
> > > > > used like func(&array[0], &array[1]);
> > > >
> > > > I use '&array[0]' when I want to get a pointer to a single object that
> > > > happens to be the first one in an array.
> > >
> > > Yeah.  Of course, you're right.  Otherwise it ends up confusing static
> > > checkers if you want the first element or the whole array.
> 
> Right.
> 
> > > > > Should this preference be put into checkpatch and/or CodingStyle?
> 
> And checkpatch will have no idea what the prototype
> for any function is, so this transform is better left
> for smarter tools like coccinelle.
> 
> The proper answer here is no.
> 
> > > > How about the following low-hanging fruit?
> > > >
> > > >   foo(..., &array[0], ARRAY_SIZE(array), ...)
> > >
> > > Yes, to this also.  I doubt checkpatch.pl will find a meaningful number
> > > of these but doing that is annoying thing.
> > 
> > Atcually, I find 236 of them, in 48 files.
> 
> The uses I found:
> 
> drivers/input/touchscreen nas a few

I got curious so I ran the proposed patch over drivers/input/touchscreen
and it produced the following gems:

CHECK: Using addressof array 'data' index [0] may be simpler as 'data'
#49: FILE: drivers/input/touchscreen/dynapro.c:49:
+#define DYNAPRO_GET_TOUCHED(data) (DYNAPRO_FORMAT_TOUCH_BIT & data[0])


CHECK: Using addressof array 'mtouch->data' index [0] may be simpler as
'mtouch->data'
#97: FILE: drivers/input/touchscreen/mtouch.c:97:
+       if (MTOUCH_FORMAT_TABLET_STATUS_BIT & mtouch->data[0])

... etc.


While below can be written as just "msg" in many cases when you parse
several fields in the structure the original is actually cleaner:

CHECK: Using addressof array 'msg' index [0] may be simpler as 'msg'
#38: FILE: drivers/input/touchscreen/ipaq-micro-ts.c:38:
+                                be16_to_cpup((__be16 *) &msg[0]));

I'd be OK with changing cases like:

CHECK: Using addressof array 'buf' index [0] may be simpler as 'buf'
#232: FILE: drivers/input/touchscreen/zforce_ts.c:232:
+       return zforce_send_wait(ts, &buf[0], ARRAY_SIZE(buf));

Thanks.

-- 
Dmitry
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#1175603

On Wed, 2015-07-01 at 15:33 -0700, Dmitry Torokhov wrote:
> On Wed, Jul 01, 2015 at 07:53:44AM -0700, Joe Perches wrote:
> > On Wed, 2015-07-01 at 14:26 +0200, Julia Lawall wrote:
> > > On Wed, 1 Jul 2015, Dan Carpenter wrote:
> > > > On Wed, Jul 01, 2015 at 01:54:29PM +0200, Clemens Ladisch wrote:
> > > > > Joe Perches wrote:
> > > > > > It seems most in-kernel uses are 'array' rather than '&array[0]'
> > > > > >
> > > > > > Most of the time, using array is simpler to read than &array[0].
> > > > > >
> > > > > > Exceptions exists when addresses for consecutive members are
> > > > > > used like func(&array[0], &array[1]);
> > > > >
> > > > > I use '&array[0]' when I want to get a pointer to a single object that
> > > > > happens to be the first one in an array.
> > > >
> > > > Yeah.  Of course, you're right.  Otherwise it ends up confusing static
> > > > checkers if you want the first element or the whole array.
> > 
> > Right.
> > 
> > > > > > Should this preference be put into checkpatch and/or CodingStyle?
> > 
> > And checkpatch will have no idea what the prototype
> > for any function is, so this transform is better left
> > for smarter tools like coccinelle.
> > 
> > The proper answer here is no.
[]
> CHECK: Using addressof array 'mtouch->data' index [0] may be simpler as
> 'mtouch->data'
> #97: FILE: drivers/input/touchscreen/mtouch.c:97:
> +       if (MTOUCH_FORMAT_TABLET_STATUS_BIT & mtouch->data[0])

The joys of perl parsing.

> CHECK: Using addressof array 'msg' index [0] may be simpler as 'msg'
> #38: FILE: drivers/input/touchscreen/ipaq-micro-ts.c:38:
> +                                be16_to_cpup((__be16 *) &msg[0]));

That's using the first member in an array.

> I'd be OK with changing cases like:
> 
> CHECK: Using addressof array 'buf' index [0] may be simpler as 'buf'
> #232: FILE: drivers/input/touchscreen/zforce_ts.c:232:
> +       return zforce_send_wait(ts, &buf[0], ARRAY_SIZE(buf));

I think cases like those are sensible to change.

cheers, Joe

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