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To decode a hex IOCTL code:

Most architectures use this generic format, but check 

include/ARCH/ioctl.h for specifics, e.g. powerpc 

uses 3 bits to encode read/write and 13 bits for size.

bits meaning 

31-30 00 - no parameters: uses _IO macro 

10 - read: _IOR 

01 - write: _IOW 

11 - read/write: _IOWR

29-16 size of arguments

15-8 ascii character supposedly 

unique to each driver

7-0 function #

下面这个解释要看清楚 

So for example 0x82187201 is a read with arg length of　0x218, 

character ‘r’ function 1. Grepping the source reveals this is:

＃define VFAT_IOCTL_READDIR_BOTH _IOR(‘r’, 1, struct dirent [2])