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" . 

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