Think you !
Oh , I was wrong . How careless I was.
I have fixed it :
#define _IOC_NRMASK (((1 << _IOC_NRBITS)-1) << _IOC_NRSHIFT) // if the
_IOC_NRBITS is 8 ,then the result is 0XFF
#define _IOC_DIR(nr) ((nr & _IOC_DIRMASK) >> _IOC_DIRSHIFT) // when
decode , keep the same.
Now I think this will spend more time than the kernel code when executed. It
performs an extra operation ">>", but I think it better to understand.
_IOC_DIRMASK 0xC0000000
_IOC_TYPEMASK 0x0000FF00
_IOC_NRMASK 0x000000FF
_IOC_SIZEMASK 0x3FFF0000
At 2013-03-30 16:41:55,"Tobias Boege" <tob...@gambas-buch.de> wrote:
>On Sat, 30 Mar 2013, RS wrote:
>> it defines in the kernel: #define _IOC_NRMASK ((1 << _IOC_NRBITS)-1)
>> //define ... #define _IOC_NRSHIFT 0 ... #define _IOC_DIR(nr)
>> (((nr) >> _IOC_DIRSHIFT) & _IOC_DIRMASK) //when decode
>>
>> why not define it like this:
>> #define _IOC_NRSHIFT 0
>> ...
>> #define _IOC_NRMASK ((_IOC_NRSHIFT >> _IOC_NRBITS) - _IOC_NRSHIFT)
>> //define
>> ...
>> #define _IOC_DIR(nr) ((nr & _IOC_DIRMASK) >> _IOC_DIRSHIFT) //
>> when decode
>>
>>
>> I think it is better for the word "mask" .
>>
>
>It can't be better this way because it's wrong. Let's compute a bit:
>
>You are referring include/uapi/asm-generic/ioctl.h AFAICS:
>
> #define _IOC_NRBITS 8
> #define _IOC_NRSHIFT 0
> #define _IOC_NRMASK ((1 << _IOC_NRBITS)-1)
>
>This gives _IOC_NRMASK = ((1 << 8) - 1) = 0xff. But supposing your
>
> _IOC_NRMASK ((_IOC_NRSHIFT >> _IOC_NRBITS) - _IOC_NRSHIFT)
>
>it yields _IOC_NRMASK = ((0 >> 8) - 0) = 0x00.
>
>You see? With your mask you'll never get any bits out of the ioctl number
>because it's just wrong.
>
>The same applies to _IOC_DIR(nr). You can't just exchange bitwise ands and
>shifts - you'd have to change the constants for this... and there is really
>no point in it.
>
>Regards,
>Tobi
>
>
>_______________________________________________
>Kernelnewbies mailing list
>Kernelnewbies@kernelnewbies.org
>http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
_______________________________________________
Kernelnewbies mailing list
Kernelnewbies@kernelnewbies.org
http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
