标签：linux
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。
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_NONE 0U #define _IOC_WRITE 1U #define _IOC_READ 2U
/* provoke compile error for invalid uses of size argument */ #define _IOC_SIZEBITS 14 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)_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)
#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 #define _IOC_SIZEBITS 14 #define _IOC_DIRBITS 2 #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. */ #define _IOC_NONE 0U #define _IOC_WRITE 1U #define _IOC_READ 2U #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 */
标签：linux
原文地址：http://blog.csdn.net/u013686019/article/details/45870347
