ioctl.h 分析

我自己畫了個解析圖。。。不要嫌棄醜啊。。

。

哈哈

type

The magic number. Just choose one number (after consultingioctl-number.txt ) and use it throughout the driver. This field is eight bits wide (_IOC_TYPEBITS).

number.

The ordinal (sequential) number. It’s eight bits ( _IOC_NRBITS) wide.

direction

The direction of data transfer,if the particular command involves a data transfer. The possible values are _IOC_NONE (no data transfer), _IOC_READ, _IOC_WRITE, and _IOC_READ|_IOC_WRITE(data is transferred both ways). Data transfer is seen from the application’s point of view; _IOC_READ means reading from the device, so the driver must write to user space. Note that the field is a bitmask,so _IOC_ READ and _IOC_WRITE can be extracted using a logical AND operation.

size

The size of user data involved.The width of this field is architecture dependent, but is usually 13 or 14 bits.You can find its value for your specific architecture in the macro _IOC_SIZEBITS. It’s not mandatory that you use the size field—the kernel does not check it—but it is a goodidea.Proper use of this field can help detect user-space programming errors and enable you to implement backward compatibility if you ever need to change the size of the relevant data item. If you need larger data structures, however,you can just ignore the size field. We’ll see how this field is used soon.

#ifndef _UAPI_ASM_GENERIC_IOCTL_H
#define _UAPI_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)           //0xff
#define _IOC_TYPEMASK	((1 << _IOC_TYPEBITS)-1)     //0xff
#define _IOC_SIZEMASK	((1 << _IOC_SIZEBITS)-1)     //0x3fff
#define _IOC_DIRMASK	((1 << _IOC_DIRBITS)-1)      //0x3

/*這部分是NR TYPE SIZE DIR 段在32bit數據中儲存位置相對於起始0位置的偏移量*/
#define _IOC_NRSHIFT	0
#define _IOC_TYPESHIFT	(_IOC_NRSHIFT+_IOC_NRBITS)   //0x8
#define _IOC_SIZESHIFT	(_IOC_TYPESHIFT+_IOC_TYPEBITS)//0x10
#define _IOC_DIRSHIFT	(_IOC_SIZESHIFT+_IOC_SIZEBITS)//0x1E

/*
 * Direction bits, which any architecture can choose to override
 * before including this file.
 */
/*io 讀寫權限宏*/
#ifndef _IOC_NONE
# define _IOC_NONE	0U
#endif

#ifndef _IOC_WRITE
# define _IOC_WRITE	1U
#endif

#ifndef _IOC_READ
# define _IOC_READ	2U
#endif

/*_IOC 適用於將dir, type nr size 這四個信息合成到一個32bit的數據中*/
#define _IOC(dir,type,nr,size) 	(((dir)  << _IOC_DIRSHIFT) | 	 ((type) << _IOC_TYPESHIFT) | 	 ((nr)   << _IOC_NRSHIFT) | 	 ((size) << _IOC_SIZESHIFT))

#ifndef __KERNEL__
#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)))
	/*一下三個BAD結尾的宏定義我也沒看明確為什麽最後一個參數是sizeof(size) 就得跟一個BAD*/
#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.. */
	/*從32bit的數據中解碼出DIR TYPE NR SIZE，非常easy沒啥講的*/
#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 /* _UAPI_ASM_GENERIC_IOCTL_H */

ioctl.h 分析