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SubjectRe: [PATCH 5/6] zsmalloc: remove unnecessary type casting
On 04/25/2012 01:56 PM, Greg Kroah-Hartman wrote:

> On Wed, Apr 25, 2012 at 09:35:25AM -0400, Nitin Gupta wrote:
>> On 04/25/2012 02:23 AM, Minchan Kim wrote:
>>
>>> Let's remove unnecessary type casting of (void *).
>>>
>>> Signed-off-by: Minchan Kim <minchan@kernel.org>
>>> ---
>>> drivers/staging/zsmalloc/zsmalloc-main.c | 3 +--
>>> 1 file changed, 1 insertion(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c b/drivers/staging/zsmalloc/zsmalloc-main.c
>>> index b7d31cc..ff089f8 100644
>>> --- a/drivers/staging/zsmalloc/zsmalloc-main.c
>>> +++ b/drivers/staging/zsmalloc/zsmalloc-main.c
>>> @@ -644,8 +644,7 @@ void zs_free(struct zs_pool *pool, void *obj)
>>> spin_lock(&class->lock);
>>>
>>> /* Insert this object in containing zspage's freelist */
>>> - link = (struct link_free *)((unsigned char *)kmap_atomic(f_page)
>>> - + f_offset);
>>> + link = (struct link_free *)(kmap_atomic(f_page) + f_offset);
>>> link->next = first_page->freelist;
>>> kunmap_atomic(link);
>>> first_page->freelist = obj;
>>
>>
>>
>> Incrementing a void pointer looks weired and should not be allowed by C
>> compilers though gcc and clang seem to allow this without any warnings.
>> (fortunately C++ forbids incrementing void pointers)
>
> Huh? A void pointer can safely be incremented by C I thought, do you
> have a pointer to where in the reference it says it is "unspecified"?
>


Arithmetic on void pointers and function pointers is listed as a GNU C
extension, so I don't think these operations are part of C standard.
From info gcc (section 6.23):

"""
6.23 Arithmetic on `void'- and Function-Pointers
In GNU C, addition and subtraction operations are supported on pointers
to `void' and on pointers to functions. This is done by treating the
size of a `void' or of a function as 1.


A consequence of this is that `sizeof' is also allowed on `void' and on
function types, and returns 1.
The option `-Wpointer-arith' requests a warning if these extensions are
used.
"""


>> So, we should keep this cast to unsigned char pointer to avoid relying
>> on a non-standard, compiler specific behavior.
>
> I do agree about this, more people are starting to build the kernel with
> other compilers than gcc, so it would be nice to ensure that we get
> stuff like this right.
>


As an example, MSVC does not support arithmetic on void pointers:
http://stackoverflow.com/questions/1864352/pointer-arithmetic-when-void-has-unknown-size

Thanks,
Nitin


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