_IOC_NR, _IOC_TYPE, _IOC_SIZE, _IOC_DIR
_IOC_NR(), _IOC_TYPE(), _IOC_SIZE(), _IOC_DIR() 这几个宏用来取得 cmd 命令中的域，其作用如下：
_IOC_NR : 读取基数域值 (bit0~ bit7)
_IOC_TYPE : 读取魔数域值 (bit8 ~ bit15)
_IOC_SIZE : 读取数据大小域值 (bit16 ~ bit29)
_IOC_DIR : 获取读写属性域值 (bit30 ~ bit31)
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_IOC_NR() 的定义为：
引用
#define _IOC_NR(nr) (((nr) >> _IOC_NRSHIFT) & _IOC_NRMASK)
_IOC_NRSHIFT 的值为 0 ；
_IOC_NRMASK 的定义为：
引用
#define _IOC_NRMASK ((1 << _IOC_NRBITS)-1)
_IOC_NRBITS 的值被定义为 8 ，所以 _IOC_NRMASK 的值为 0xFF 。
其余的宏分析类似。
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ioctl.h
#ifndef _ASM_GENERIC_IOCTL_H
#define _ASM_GENERIC_IOCTL_H
/* ioctl command encoding: 32 bits total, command in lower 16 bits,
* size of the parameter structure in the lower 14 bits of the
* upper 16 bits.
* Encoding the size of the parameter structure in the ioctl request
* is useful for catching programs compiled with old versions
* and to avoid overwriting user space outside the user buffer area.
* The highest 2 bits are reserved for indicating the ``access mode''.
* NOTE: This limits the max parameter size to 16kB -1 !
*/
/*
* The following is for compatibility across the various Linux
* platforms. The generic ioctl numbering scheme doesn't really enforce
* a type field. De facto, however, the top 8 bits of the lower 16
* bits are indeed used as a type field, so we might just as well make
* this explicit here. Please be sure to use the decoding macros
* below from now on.
*/
#define _IOC_NRBITS 8
#define _IOC_TYPEBITS 8
/*
* Let any architecture override either of the following before
* including this file.
*/
#ifndef _IOC_SIZEBITS
# define _IOC_SIZEBITS 14
#endif
#ifndef _IOC_DIRBITS
# define _IOC_DIRBITS 2
#endif
#define _IOC_NRMASK ((1 << _IOC_NRBITS)-1)
#define _IOC_TYPEMASK ((1 << _IOC_TYPEBITS)-1)
#define _IOC_SIZEMASK ((1 << _IOC_SIZEBITS)-1)
#define _IOC_DIRMASK ((1 << _IOC_DIRBITS)-1)
#define _IOC_NRSHIFT 0
#define _IOC_TYPESHIFT (_IOC_NRSHIFT+_IOC_NRBITS)
#define _IOC_SIZESHIFT (_IOC_TYPESHIFT+_IOC_TYPEBITS)
#define _IOC_DIRSHIFT (_IOC_SIZESHIFT+_IOC_SIZEBITS)
/*
* Direction bits, which any architecture can choose to override
* before including this file.
*/
#ifndef _IOC_NONE
# define _IOC_NONE 0U
#endif
#ifndef _IOC_WRITE
# define _IOC_WRITE 1U
#endif
#ifndef _IOC_READ
# define _IOC_READ 2U
#endif
#define _IOC(dir,type,nr,size) \
(((dir) << _IOC_DIRSHIFT) | \
((type) << _IOC_TYPESHIFT) | \
((nr) << _IOC_NRSHIFT) | \
((size) << _IOC_SIZESHIFT))
#ifdef __KERNEL__
/* provoke compile error for invalid uses of size argument */
extern unsigned int __invalid_size_argument_for_IOC;
#define _IOC_TYPECHECK(t) \
((sizeof(t) == sizeof(t[1]) && \
sizeof(t) < (1 << _IOC_SIZEBITS)) ? \
sizeof(t) : __invalid_size_argument_for_IOC)
#else
#define _IOC_TYPECHECK(t) (sizeof(t))
#endif
/* used to create numbers */
#define _IO(type,nr) _IOC(_IOC_NONE,(type),(nr),0)
#define _IOR(type,nr,size) _IOC(_IOC_READ,(type),(nr),(_IOC_TYPECHECK(size)))
#define _IOW(type,nr,size) _IOC(_IOC_WRITE,(type),(nr),(_IOC_TYPECHECK(size)))
#define _IOWR(type,nr,size) _IOC(_IOC_READ|_IOC_WRITE,(type),(nr),(_IOC_TYPECHECK(size)))
#define _IOR_BAD(type,nr,size) _IOC(_IOC_READ,(type),(nr),sizeof(size))
#define _IOW_BAD(type,nr,size) _IOC(_IOC_WRITE,(type),(nr),sizeof(size))
#define _IOWR_BAD(type,nr,size) _IOC(_IOC_READ|_IOC_WRITE,(type),(nr),sizeof(size))
/* used to decode ioctl numbers.. */
#define _IOC_DIR(nr) (((nr) >> _IOC_DIRSHIFT) & _IOC_DIRMASK)
#define _IOC_TYPE(nr) (((nr) >> _IOC_TYPESHIFT) & _IOC_TYPEMASK)
#define _IOC_NR(nr) (((nr) >> _IOC_NRSHIFT) & _IOC_NRMASK)
#define _IOC_SIZE(nr) (((nr) >> _IOC_SIZESHIFT) & _IOC_SIZEMASK)
/* ...and for the drivers/sound files... */
#define IOC_IN (_IOC_WRITE << _IOC_DIRSHIFT)
#define IOC_OUT (_IOC_READ << _IOC_DIRSHIFT)
#define IOC_INOUT ((_IOC_WRITE|_IOC_READ) << _IOC_DIRSHIFT)
#define IOCSIZE_MASK (_IOC_SIZEMASK << _IOC_SIZESHIFT)
#define IOCSIZE_SHIFT (_IOC_SIZESHIFT)
#endif /* _ASM_GENERIC_IOCTL_H */
