049500715e
This is strictly user-space material at this point, so put it with the other user-space API documentation. Signed-off-by: Jonathan Corbet <corbet@lwn.net>
32 lines
830 B
ReStructuredText
32 lines
830 B
ReStructuredText
==============================
|
|
Decoding an IOCTL Magic Number
|
|
==============================
|
|
|
|
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])
|