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ioctl problem

Donn Cave donn at u.washington.edu
Thu Feb 15 13:04:56 EST 2001 
Quoth "Eduardo Ferro" <eferro at inicia.es>:

| Have anybody some example of the use of ioctl method when this ioctl
| return some data in a estructure.
| I use icotl to control the cdrom, and the ioctls without parameters works
| well, but i don`t know how to use the ioctls that return me data

  >>> import fcntl
  >>> print fcntl.ioctl.__doc__
  ioctl(fd, opt, [arg])

  Perform the requested operation on file descriptor fd.  The operation
  is defined by op and is operating system dependent.  Typically these
  codes can be retrieved from the library module IOCTL.  The argument arg
  is optional, and defaults to 0; it may be an int or a string. If arg is
  given as a string, the return value of ioctl is a string of that length,
  containing the resulting value put in the arg buffer by the operating system.
  The length of the arg string is not allowed to exceed 1024 bytes. If the arg
  given is an integer or if none is specified, the result value is an integer
  corresponding to the return value of the ioctl call in the C code.

That means you must lay your data out as a string of bytes.  Pass any
string of that length to ioctl(), then use the "struct" module's byte
unpacking functions to convert the return string to the data type you
want to work with.

  >>> import fcntl
  >>> n = fcntl.ioctl(0, 1074295912, 'XXXXXXXX')
  >>> n
  '\030\000P\000\360\000 \003'
  >>> import struct
  >>> struct.unpack('hhhh', n)
  (24, 80, 240, 800)

That's the TIOCGWINSZ ioctl (on NetBSD 1.5.)

	Donn Cave, donn at u.washington.edu
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