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authorChuck Ebbert <76306.1226@compuserve.com>2006-12-10 02:18:57 -0800
committerLinus Torvalds <torvalds@woody.osdl.org>2006-12-10 09:55:40 -0800
commitcf1b939e41c28a31f2052e5c0a6619b5711ad7d0 (patch)
treed8ab99d8381b6b3b441f4e2cee545779e7cee24a /Documentation/ioctl
parent0dd4e5b8e197f9d10e171e9f4aa9ed72e4a6d00c (diff)
[PATCH] Document how to decode an IOCTL number
Document how to decode a binary IOCTL number. Signed-off-by: Chuck Ebbert <76306.1226@compuserve.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Diffstat (limited to 'Documentation/ioctl')
-rw-r--r--Documentation/ioctl/ioctl-decoding.txt24
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diff --git a/Documentation/ioctl/ioctl-decoding.txt b/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])