| David Brazdil | 0f672f6 | 2019-12-10 10:32:29 +0000 | [diff] [blame^] | 1 | ============================== |
| 2 | Decoding an IOCTL Magic Number |
| 3 | ============================== |
| 4 | |
| 5 | To decode a hex IOCTL code: |
| 6 | |
| 7 | Most architectures use this generic format, but check |
| 8 | include/ARCH/ioctl.h for specifics, e.g. powerpc |
| 9 | uses 3 bits to encode read/write and 13 bits for size. |
| 10 | |
| 11 | ====== ================================== |
| 12 | bits meaning |
| 13 | ====== ================================== |
| 14 | 31-30 00 - no parameters: uses _IO macro |
| 15 | 10 - read: _IOR |
| 16 | 01 - write: _IOW |
| 17 | 11 - read/write: _IOWR |
| 18 | |
| 19 | 29-16 size of arguments |
| 20 | |
| 21 | 15-8 ascii character supposedly |
| 22 | unique to each driver |
| 23 | |
| 24 | 7-0 function # |
| 25 | ====== ================================== |
| 26 | |
| 27 | |
| 28 | So for example 0x82187201 is a read with arg length of 0x218, |
| 29 | character 'r' function 1. Grepping the source reveals this is:: |
| 30 | |
| 31 | #define VFAT_IOCTL_READDIR_BOTH _IOR('r', 1, struct dirent [2]) |