root/Documentation/ioctl-number.txt

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Ioctl Numbers
6 May 1996
Michael Chastain
<mec@duracef.shout.net>

If you are adding new ioctl's to the kernel, you should use the _IO
macros defined in <linux/ioctl.h>:

    _IO    an ioctl with no parameters
    _IOW   an ioctl with write parameters (from user's point of view)
    _IOR   an ioctl with read parameters (from user's point of view)
    _IOWR  an ioctl with both write and read parameters.

'Write' and 'read' are from the user's point of view.  This is like the
system calls 'write' and 'read'.  For example, a SET_FOO ioctl would be
_IOW, although the kernel would actually read data from user space; a
GET_FOO ioctl would be _IOR, although the kernel would actually write
data to user space.

The first argument to _IO, _IOW, _IOR, or _IOWR is an identifying letter
or number from the table below.  If you are writing a driver for a new
device and need a letter, pick an unused letter.  You can register the
letter by patching this file and submitting the patch to Linus Torvalds.
Or you can e-mail me at <mec@duracef.shout.net> and I'll register one
for you.

The second argument to _IO, _IOW, _IOR, or _IOWR is a sequence number
to distinguish ioctls from each other.  The third argument is a size
of the structure going into the kernel or coming out of the kernel.

Some devices use their major number as the identifier; this is not
recommended.  Some devices are even more irregular and don't follow
the convention at all.

Following the convention is good because:

(1) Keeping the ioctl's globally unique helps error checking:
    if a program calls an ioctl on the wrong device, it will get an
    error rather than some unexpected behaviour.

(2) The 'strace' build procedure automatically finds ioctl numbers
    defined with _IO, _IOW, _IOR, or _IOWR.

(3) 'strace' can decode numbers back into useful names when the
    numbers are unique.

(4) People looking for ioctls can grep for them more easily when
    the convention is used to define the ioctl numbers.

(5) When following the convention, the driver code can use generic
    code to call verify_area to validate parameters.

This table is current to Linux 1.3.98.

Ioctl   Include File            Comments
========================================================
0x00    linux/fs.h              only FIBMAP, FIGETBSZ
0x00    linux/random.h          codes in 0x010800NN
0x00    linux/mc146818rtc.h     conflict!
0x02    linux/fd.h
0x03    linux/hdreg.h
0x04    linux/umsdos_fs.h
0x06    linux/lp.h
0x09    linux/md.h
0x12    linux/fs.h
0x20    linux/cm206.h
0x22    linux/scc.h             conflict!
0x22    scsi/sg.h               conflict!
'A'     linux/apm_bios.h
'C'     linux/soundcard.h
'F'     linux/fb.h
'I'     linux/isdn.h
'K'     linux/kd.h
'L'     linux/loop.h
'M'     linux/soundcard.h
'P'     linux/soundcard.h
'Q'     linux/soundcard.h
'S'     linux/cdrom.h           conflict!
'S'     scsi/scsi.h             conflict!
'S'     scsi/scsi_ioctl.h       conflict!
'T'     linux/soundcard.h       conflict!
'T'     asm/ioctls.h            conflict!
'V'     linux/vt.h
'Y'     linux/cyclades.h        codes in 0x004359NN
'a'     various, see http://lrcwww.epfl.ch/linux-atm/magic.html
'c'     linux/comstats.h
'f'     linux/ext2_fs.h
'm'     linux/mtio.h            conflict!
'm'     linux/soundcard.h       conflict!
'n'     linux/ncp_fs.h
'r'     linux/msdos_fs.h
's'     linux/cdk.h
't'     linux/if_ppp.h          no conflict
't'     linux/isdn_ppp.h        no conflict
'u'     linux/smb_fs.h
'v'     linux/ext2_fs.h
0x89    asm/sockios.h           no conflict
0x89    linux/sockios.h         no conflict

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