File: ioctl-decoding.rst

package info (click to toggle)
linux 6.1.8-1
  • links: PTS, VCS
  • area: main
  • in suites: bookworm
  • size: 1,488,076 kB
  • sloc: ansic: 23,401,844; asm: 266,744; sh: 108,976; makefile: 49,705; python: 36,927; perl: 36,810; cpp: 6,044; yacc: 4,904; lex: 2,722; awk: 1,440; ruby: 25; sed: 5
file content (31 lines) | stat: -rw-r--r-- 830 bytes parent folder | download | duplicates (19)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
==============================
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])