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
