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" <tobias@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx >http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
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