Use of void pointer arithmetic?

Use of void pointer arithmetic?

[Kalle Valo]

(Changing subject, adding Linus and linux-kernel)

Dan Carpenter <dan.carpenter@oracle.com> writes:

> On Thu, Feb 24, 2022 at 09:34:31AM +0200, Kalle Valo wrote:
>> Francesco Magliocca <franciman12@gmail.com> writes:
>> 
>> > Hi, I picked (void*) to be conformant with the other examples in htt_rx.c
>> > For example at line 1431:
>> >>    rxd = HTT_RX_BUF_TO_RX_DESC(hw,
>> >>                    (void *)msdu->data - hw->rx_desc_ops->rx_desc_size);
>> >
>> > But for me it is ok. Maybe we should fix all the occurrences of this kind.
>> 
>> Yeah, it would be good to fix the void pointer arithmetic in a separate
>> patch. I have planning to enable -Wpointer-arith in my ath10k-check and
>> ath11k-check scripts, so patches are very welcome.
>
> Void * casts simplify a lot of code.  Less noise.  More readable.
> They're more accurate in a sense because it's not a u8 at all.  The
> kernel can't compile with other compilers besides GCC and Clang so why
> care about that the C standard hasn't caught up?
>
> What does -Wpointer-arith buy us?

A good question. I have always just thought we should avoid void pointer
arithmetic due to the C standard, but now that you mention it void
pointers can indeed simplify the code. So I'm not so sure anymore.

Any opinions? Is there a kernel wide recommendation for this?

-- 
https://patchwork.kernel.org/project/linux-wireless/list/

https://wireless.wiki.kernel.org/en/developers/documentation/submittingpatches

Re: Use of void pointer arithmetic?

[Johannes Berg]

On Thu, 2022-02-24 at 11:59 +0200, Kalle Valo wrote:
> 
> A good question. I have always just thought we should avoid void pointer
> arithmetic due to the C standard, but now that you mention it void
> pointers can indeed simplify the code. So I'm not so sure anymore.
> 
> Any opinions? Is there a kernel wide recommendation for this?

The kernel only enables it with W=3, which I guess nobody uses anyway
... Originally it came from commit 4a5838ad9d2d ("kbuild: Add extra gcc
checks") with a pointer to

http://marc.info/?l=kernel-janitors&m=129802065918147&w=2

(which is offline right now due to an expired certificate ...)

but setting back my clock it seems to point to

https://lore.kernel.org/all/20110218091716.GA4384@bicker/

but the thread kind of revolves around -Wconversion.


FreeBSD does enable -Wpointer-arith which is why we've been trying to
keep iwlwifi clean as a courtesy to them, but for really Linux-only code
I dunno if there's much point. Although of course that applies also to
FreeBSD ...

johannes

Re: Use of void pointer arithmetic?

[Linus Torvalds]

On Thu, Feb 24, 2022 at 1:59 AM Kalle Valo <kvalo@kernel.org> wrote:
>
> > What does -Wpointer-arith buy us?
>
> A good question. I have always just thought we should avoid void pointer
> arithmetic due to the C standard, but now that you mention it void
> pointers can indeed simplify the code. So I'm not so sure anymore.
>
> Any opinions? Is there a kernel wide recommendation for this?

We consciously use arithmetic on 'void *' in some places, although
it's not exactly _hugely_ common.

It's much more common to turn a pointer into an 'unsigned long' and
then doing basic operations on that.

The advantage of 'void *' is that it does avoid the need to cast the
pointer back.

But at the same time it will never replace the 'cast to an actual
integer type', because the 'void *' arithmetic only does simple
addition and subtraction, and many pointer operations need more (ie
alignment needs to do the bitops).

So I think it's mostly a personal preference. I *do* think that doing
arithmetic on 'void *' is generally superior to doing it the
old-fashioned C way on 'char *' - unless, of course, 'char *' is
simply the native type in question.

So if 'char *' casts (and casting back) is the alternative, then by
all means use 'void *' as a kind of generic and type-independent "byte
pointer". That is how it is meant to be used in the gcc extension.

And no, nobody should ever use -Wpointer-arith on the kernel in
general. Our use of it is not _hugely_ common, but it's does exist,
and it's not really frowned upon.

                    Linus