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Commit cf1b939e authored by Chuck Ebbert's avatar Chuck Ebbert Committed by Linus Torvalds
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[PATCH] Document how to decode an IOCTL number 



Document how to decode a binary IOCTL number.

Signed-off-by: default avatarChuck Ebbert <76306.1226@compuserve.com>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 0dd4e5b8

Documentation/ioctl/ioctl-decoding.txt

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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])