在应用层调用ioctl()函数的时候，该函数第二个参数是cmd，如下调用：
ret = ioctl(ppp_dev_fd, PPPIOCGIDLE, ip) ;看下cmd的格式：/* get idle time */#define PPPIOCGIDLE_IOR('t', 63, struct ppp_idle) 然后就有的研究了。
通过ioctl()函数的cmd，下发需要底层响应的命令。cmd命令的大小32位，分成4个部分：
bit31~bit30：“区别读写” 区，作用是区分是读取命令还是写入命令；
bit29~bit16：“数据大小”区，表示 ioctl() 中的 arg 变量传送的内存大小。
bit15~bit8 ：“魔数”(也称为"幻数")区，这个值用以与其它设备驱动程序的 ioctl 命令进行区别。
bit7~bit0 ：“区别序号”区，是区分命令的命令顺序序号。
比如，对于以上的PPPIOCGIDLE命令，，#define _IOR(type,nr,size)_IOC(_IOC_READ,(type),(nr),(_IOC_TYPECHECK(size)))_IOC_READ：这是读取命令，定义如下：/* * Direction bits. */#define _IOC_NONE0U#define _IOC_WRITE1U#define _IOC_READ2U_IOC_TYPECHECK：/* provoke compile error for invalid uses of size argument */
/* provoke compile error for invalid uses of size argument */#define _IOC_SIZEBITS14extern 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)_IOC_SIZEBITS之所以被定义为14，因为size字段占了32位中的14位（bit29~bit16）
所以，PPPIOCGIDLE命令等价于：
#define PPPIOCGIDLE <—>(2 << 30) | ('t' << 8) | (63 << 0) | (size << 16)
参考Linux内核文件：include/asm-generic/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_NRBITS8#define _IOC_TYPEBITS8#define _IOC_SIZEBITS14#define _IOC_DIRBITS2#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_NRSHIFT0#define _IOC_TYPESHIFT(_IOC_NRSHIFT+_IOC_NRBITS)#define _IOC_SIZESHIFT(_IOC_TYPESHIFT+_IOC_TYPEBITS)#define _IOC_DIRSHIFT(_IOC_SIZESHIFT+_IOC_SIZEBITS)/* * Direction bits. */#define _IOC_NONE0U#define _IOC_WRITE1U#define _IOC_READ2U#define _IOC(dir,type,nr,size) \(((dir) << _IOC_DIRSHIFT) | \ ((type) << _IOC_TYPESHIFT) | \ ((nr) << _IOC_NRSHIFT) | \ ((size) << _IOC_SIZESHIFT))/* 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)/* 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 */
影子依旧可以相亲相爱。哪一块骨骼最温暖，总能一击即中。
