Ioctl Numbers 29 May 1996 Michael Chastain If you are adding new ioctl's to the kernel, you should use the _IO macros defined in : _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 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 the 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 pre2.0.9. Code Seq# Include File Comments ======================================================== 0x00 01-02 linux/fs.h conflict! 0x00 01-04 scsi/scsi_ioctl.h conflict! 0x02 all linux/fd.h 0x03 all linux/hdreg.h 0x04 all linux/umsdos_fs.h 0x06 all linux/lp.h 0x09 all linux/md.h 0x12 all linux/fs.h 0x20 all linux/cm206.h 0x22 all linux/scc.h conflict! (version 2.01 of z8530drv) 0x22 all scsi/sg.h conflict! 'A' all linux/apm_bios.h 'B' all linux/baycom.h 'C' all linux/soundcard.h 'F' all linux/fb.h 'I' all linux/isdn.h 'K' all linux/kd.h 'L' all linux/loop.h 'M' all linux/soundcard.h 'P' all linux/soundcard.h 'Q' all linux/soundcard.h 'R' all linux/random.h 'S' 00-1F linux/cdrom.h 'S' 20-7F linux/ucdrom.h 'S' 80-81 scsi/scsi_ioctl.h 'S' 82-FF scsi/scsi.h 'T' all linux/soundcard.h conflict! 'T' all asm-i386/ioctls.h conflict! 'V' all linux/vt.h 'Y' all linux/cyclades.h 'Z' all linux/scc.h version 2.2 of z8530drv 'a' all various, see http://lrcwww.epfl.ch/linux-atm/magic.html 'c' all linux/comstats.h 'f' all linux/ext2_fs.h 'm' all linux/mtio.h conflict! 'm' all linux/soundcard.h conflict! 'n' all linux/ncp_fs.h 'p' all linux/mc146818rtc.h 'r' all linux/msdos_fs.h 's' all linux/cdk.h 't' 00-7F linux/if_ppp.h 't' 80-8F linux/isdn_ppp.h 'u' all linux/smb_fs.h 'v' all linux/ext2_fs.h 0x89 00-0F asm-i386/sockios.h 0x89 10-FF linux/sockios.h 0x90 00 linux/sbpcd.h