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simons |
Ioctl Numbers
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6 Aug 1996
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Michael Chastain
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If you are adding new ioctl's to the kernel, you should use the _IO
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macros defined in :
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_IO an ioctl with no parameters
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_IOW an ioctl with write parameters (from user's point of view)
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_IOR an ioctl with read parameters (from user's point of view)
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_IOWR an ioctl with both write and read parameters.
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'Write' and 'read' are from the user's point of view. This is like the
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system calls 'write' and 'read'. For example, a SET_FOO ioctl would be
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_IOW, although the kernel would actually read data from user space; a
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GET_FOO ioctl would be _IOR, although the kernel would actually write
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data to user space.
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The first argument to _IO, _IOW, _IOR, or _IOWR is an identifying letter
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or number from the table below. If you are writing a driver for a new
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device and need a letter, pick an unused letter. You can register the
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letter by patching this file and submitting the patch to Linus Torvalds.
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Or you can e-mail me at and I'll register one
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for you.
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The second argument to _IO, _IOW, _IOR, or _IOWR is a sequence number
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to distinguish ioctls from each other. The third argument is the size
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of the structure going into the kernel or coming out of the kernel.
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Some devices use their major number as the identifier; this is not
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recommended. Some devices are even more irregular and don't follow
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the convention at all.
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Following the convention is good because:
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(1) Keeping the ioctl's globally unique helps error checking:
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if a program calls an ioctl on the wrong device, it will get an
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error rather than some unexpected behaviour.
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(2) The 'strace' build procedure automatically finds ioctl numbers
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defined with _IO, _IOW, _IOR, or _IOWR.
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(3) 'strace' can decode numbers back into useful names when the
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numbers are unique.
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(4) People looking for ioctls can grep for them more easily when
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the convention is used to define the ioctl numbers.
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(5) When following the convention, the driver code can use generic
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code to call verify_area to validate parameters.
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This table lists ioctls visible from user land for Linux/i386. It is
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current to Linux 2.0.11.
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Code Seq# Include File Comments
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========================================================
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0x00 01-02 linux/fs.h conflict!
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0x00 01-04 scsi/scsi_ioctl.h conflict!
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0x02 all linux/fd.h
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0x03 all linux/hdreg.h
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0x04 all linux/umsdos_fs.h
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0x06 all linux/lp.h
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0x09 all linux/md.h
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0x12 all linux/fs.h
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0x20 all linux/cm206.h
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0x22 all linux/scc.h conflict! (version 2.01 of z8530drv)
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0x22 all scsi/sg.h conflict!
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'A' all linux/apm_bios.h
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'B' all linux/baycom.h
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'C' all linux/soundcard.h
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'F' all linux/fb.h
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'I' all linux/isdn.h
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'K' all linux/kd.h
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'L' all linux/loop.h
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'M' all linux/soundcard.h
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'P' all linux/soundcard.h
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'Q' all linux/soundcard.h
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'R' all linux/random.h
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'S' 00-1F linux/cdrom.h
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'S' 20-7F linux/ucdrom.h
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'S' 80-81 scsi/scsi_ioctl.h
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'S' 82-FF scsi/scsi.h
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'T' all linux/soundcard.h conflict!
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'T' all asm-i386/ioctls.h conflict!
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'V' all linux/vt.h
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'W' 00-1F linux/pcwd.h
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'Y' all linux/cyclades.h
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'Z' all linux/scc.h version 2.2 of z8530drv
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'a' all various, see http://lrcwww.epfl.ch/linux-atm/magic.html
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'c' all linux/comstats.h
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'f' all linux/ext2_fs.h
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'm' all linux/mtio.h conflict!
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'm' all linux/soundcard.h conflict!
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'n' all linux/ncp_fs.h
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'p' all linux/mc146818rtc.h
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'r' all linux/msdos_fs.h
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's' all linux/cdk.h
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't' 00-7F linux/if_ppp.h
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't' 80-8F linux/isdn_ppp.h
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'u' all linux/smb_fs.h
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'v' all linux/ext2_fs.h
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'w' all CERN SCI driver (in development)
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0x89 00-0F asm-i386/sockios.h
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0x89 10-FF linux/sockios.h
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0x90 00 linux/sbpcd.h
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