Root/Documentation/ioctl/ioctl-decoding.txt

1To decode a hex IOCTL code:
2
3Most architectures use this generic format, but check
4include/ARCH/ioctl.h for specifics, e.g. powerpc
5uses 3 bits to encode read/write and 13 bits for size.
6
7 bits meaning
8 31-30 00 - no parameters: uses _IO macro
9    10 - read: _IOR
10    01 - write: _IOW
11    11 - read/write: _IOWR
12
13 29-16 size of arguments
14
15 15-8 ascii character supposedly
16    unique to each driver
17
18 7-0 function #
19
20
21So for example 0x82187201 is a read with arg length of 0x218,
22character 'r' function 1. Grepping the source reveals this is:
23
24#define VFAT_IOCTL_READDIR_BOTH _IOR('r', 1, struct dirent [2])
25

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