Commit cf1b939e authored by Chuck Ebbert's avatar Chuck Ebbert Committed by Linus Torvalds

[PATCH] Document how to decode an IOCTL number

Document how to decode a binary IOCTL number.
Signed-off-by: default avatarChuck Ebbert <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 0dd4e5b8
To decode a hex IOCTL code:
Most architecures 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 #
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])
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