Ioctl Numbers 2 September 1999 Michael Elizabeth 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 (copy_from_user) _IOR an ioctl with read parameters (copy_to_user) _IOWR an ioctl with both write and read parameters. 'Write' and 'read' are from the user's point of view, just 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. Because of the large number of drivers, many drivers share a partial letter with other drivers. If you are writing a driver for a new device and need a letter, pick an unused block with enough room for expansion: 32 to 256 ioctl commands. You can register the block 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 OK, as long as it is unique. Some devices are irregular and don't follow any convention at all. Following this 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 this convention is used to define the ioctl numbers. (5) When following the convention, the driver code can use generic code to copy the parameters between user and kernel space. This table lists ioctls visible from user land for Linux/i386. It contains most drivers up to 2.3.14, but I know I am missing some. Code Seq# Include File Comments ======================================================== 0x00 00-1F linux/fs.h conflict! 0x00 00-1F scsi/scsi_ioctl.h conflict! 0x00 00-1F linux/fb.h conflict! 0x00 00-1F linux/wavefront.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 linux/blkpg.h 0x20 all drivers/cdrom/cm206.h 0x22 all scsi/sg.h '1' 00-1F PPS kit from Ulrich Windl '8' all SNP8023 advanced NIC card 'A' 00-1F linux/apm_bios.h 'B' C0-FF advanced bbus 'C' all linux/soundcard.h 'F' all linux/fb.h 'I' all linux/isdn.h 'J' 00-1F drivers/scsi/gdth_ioctl.h 'K' all linux/kd.h 'L' 00-1F linux/loop.h 'L' E0-FF linux/ppdd.h encrypted disk device driver 'M' all linux/soundcard.h conflict! 'M' 00-1F linux/isicom.h conflict! 'P' all linux/soundcard.h 'Q' all linux/soundcard.h 'R' 00-1F linux/random.h 'S' all linux/cdrom.h conflict! 'S' 80-81 scsi/scsi_ioctl.h conflict! 'S' 82-FF scsi/scsi.h conflict! 'T' all linux/soundcard.h conflict! 'T' all asm-i386/ioctls.h conflict! 'U' all linux/drivers/usb/usb.h 'V' all linux/vt.h 'W' 00-1F linux/watchdog.h conflict! 'W' 00-1F linux/wanrouter.h conflict! 'Y' all linux/cyclades.h 'a' all ATM on linux 'b' 00-FF bit3 vme host bridge 'c' 00-7F linux/comstats.h conflict! 'c' 00-7F linux/coda.h conflict! 'd' 00-1F linux/devfs_fs.h conflict! 'd' 00-DF linux/video_decoder.h conflict! 'd' F0-FF linux/digi1.h 'e' all linux/digi1.h conflict! 'e' 00-1F linux/video_encoder.h conflict! 'e' 00-1F net/irda/irtty.h conflict! 'f' 00-1F linux/ext2_fs.h 'i' 00-3F linux/i2o.h 'j' 00-3F linux/joystick.h 'k' all asm-sparc/kbio.h asm-sparc64/kbio.h 'l' 00-3F linux/tcfs_fs.h transparent cryptographic file system 'm' all linux/mtio.h conflict! 'm' all linux/soundcard.h conflict! 'm' all linux/synclink.h conflict! 'm' 00-1F net/irda/irmod.h conflict! 'n' all linux/ncp_fs.h 'p' 00-3F linux/mc146818rtc.h 'p' 40-7F linux/nvram.h 'p' 80-9F user-space parport 'q' 00-1F linux/videotext.h conflict! 'q' 80-FF Internet PhoneJACK, Internet LineJACK 'r' 00-1F linux/msdos_fs.h 's' all linux/cdk.h 't' 00-7F linux/if_ppp.h 't' 80-8F linux/isdn_ppp.h 'u' 00-1F linux/smb_fs.h 'v' 00-1F linux/ext2_fs.h conflict! 'v' all linux/videodev.h conflict! 'w' all CERN SCI driver 'z' 00-3F CAN bus card 'z' 40-7F CAN bus card 0x80 00-1F linux/fb.h 0x89 00-06 asm-i386/sockios.h 0x89 0B-DF linux/sockios.h 0x89 E0-EF linux/sockios.h SIOCPROTOPRIVATE range 0x89 F0-FF linux/sockios.h SIOCDEVPRIVATE range 0x8B all linux/wireless.h 0x8C 00-3F WiNRADiO driver 0x90 00 drivers/cdrom/sbpcd.h 0x93 60-7F linux/auto_fs.h 0x99 00-0F 537-Addinboard driver 0xA0 all linux/sdp/sdp.h Industrial Device Project 0xA3 00-1F Philips SAA7146 dirver in development: 0xA3 80-8F Port ACL in development: 0xA3 90-9F linux/dtlk.h 0xAB 00-1F linux/nbd.h 0xAC 00-1F linux/raw.h 0xAD 00 Netfilter device in development: 0xB0 all RATIO devices in development: