[deliverable/linux.git] / Documentation / ioctl / ioctl-decoding.txt

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 #


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])
Commit	Line	Data
cf1b939e CE	1	To decode a hex IOCTL code:
cf1b939e CE	2
d9195881	3	Most architectures use this generic format, but check
cf1b939e CE	4	include/ARCH/ioctl.h for specifics, e.g. powerpc
	5	uses 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
d9195881	21	So for example 0x82187201 is a read with arg length of 0x218,
cf1b939e CE	22	character 'r' function 1. Grepping the source reveals this is:
	23
	24	#define VFAT_IOCTL_READDIR_BOTH _IOR('r', 1, struct dirent [2])