LXR linux-old/MAGIC

   1This file is a registry of magic numbers which are in use.  When you
   2add a magic number to a structure, you should also add it to this
   3file, since it is best if the magic numbers used by various structures
   4are unique.
   5
   6It is a *very* good idea to protect kernel data structures with magic
   7numbers.  This allows you to check at run time whether (a) a structure
   8has been clobbered, or (b) you've passed the wrong structure to a
   9routine.  This last is especially useful --- particularly when you are
  10passing pointers to structures via a void * pointer.  The tty code,
  11for example, does this frequently to pass driver-specific and line
  12discipline-specific structures back and forth.
  13
  14The way to use magic numbers is to declare then at the beginning of
  15the structure, like so:
  16
  17struct tty_ldisc {
  18        int     magic;
  19        ...
  20};
  21
  22Please follow this discipline when you are adding future enhancements
  23to the kernel!  It has saved me countless hours of debugging,
  24especially in the screw cases where an array has been overrun and
  25structures following the array have been overwritten.  Using this
  26discipline, these cases get detected quickly and safely.
  27
  28You should also register the magic number used by ioctls in the table
  29below.  This allows ioctls on inappropriate devices to fail, rather than
  30doing something completely unexpected.  The magic number is the first
  31argument to the _IO family of macros (see include/linux/ioctl.h) or
  32in general bits 8..15 of the ioctl number.  Where ioctls are done on
  33devices with a major device number, it is recommended that you use the
  34major device number as the ioctl magic value (e.g. hd, lp).
  35
  36                                        Theodore Ts'o
  37                                        31-Mar-94
  38
  39Magic Name          Number  Structure            File
  40===========================================================================
  41FASYNC_MAGIC        0x4601  struct fasync_struct include/linux/fs.h
  42PTY_MAGIC           0x5001  struct pty_struct    drivers/char/pty.c
  43PPP_MAGIC           0x5002  struct ppp_struct    include/linux/ppp.h
  44TTY_MAGIC           0x5401  struct tty_struct    include/linux/tty.h
  45TTY_DRIVER_MAGIC    0x5402  struct tty_driver    include/linux/tty_driver.h
  46TTY_LDISC_MAGIC     0x5403  struct tty_ldisc     include/linux/tty_ldisc.h
  47SERIAL_MAGIC        0x5301  struct async_struct  include/linux/serial.h
  48SLIP_MAGIC          0x5302  struct slip          drivers/net/slip.h
  49
  50Ioctl   Include File    Comments
  51========================================================
  520x00    fd.h
  530x03    hdreg.h
  540x06    lp.h
  550x12    fs.h
  56'C'     soundcard.h
  57'K'     kd.h
  58'M'     soundcard.h
  59'P'     soundcard.h
  60'Q'     soundcard.h
  61'S'     cdrom.h
  62'S'     fs.h            subcodes from 0x80, no conflict
  63'T'     soundcard.h     conflicts with termios.h
  64'T'     termios.h       conflicts with soundcard.h
  65'T'     ppp.h           subcodes from 0x90, no conflict
  66'V'     vt.h
  67'f'     ext2_fs.h
  68'm'     mtio.h          conflicts with soundcard.h
  69'm'     soundcard.h     conflicts with mtio.h
  70's'     pcsp.h
  71'v'     ext2_fs.h
  720x89    sockios.h
  73
  74