io — I/O privilege file

       The  special  file  /dev/io  is  a  controlled security hole that allows a process to gain I/O privileges
       (which are normally reserved for kernel-internal code).  This can be useful in order  to  write  userland
       programs that handle some hardware directly.

       The  usual  operations on the device are to open it via the open(2) interface and to send I/O requests to
       the file descriptor using the ioctl(2) syscall.

       The ioctl(2) requests available for /dev/io are mostly platform dependent, but there  are  also  some  in
       common  between  all of them.  The IODEV_PIO is used by all the architectures in order to request that an
       I/O operation be performed.  It takes a 'struct iodev_pio_req' argument that must be previously setup.

       The access member specifies the type of operation requested.  It may be:

       IODEV_PIO_READ   The operation is an "in" type.  A value will be read from the specified port  (retrieved
                        from the port member) and the result will be stored in the val member.

       IODEV_PIO_WRITE  The  operation  is a "out" type.  The value will be fetched from the val member and will
                        be written out to the specified port (defined as the port member).

       Finally, the width member specifies the size of the operand to be read/written, expressed in bytes.

       In addition to any file access permissions on /dev/io, the kernel enforces that only the  super-user  may
       open this device.

       The io file appeared in FreeBSD 1.0.

Debian                                            June 1, 2010                                             IO(4)

       The  /dev/io  interface used to be very i386 specific and worked differently.  The initial implementation
       simply raised the IOPL of the current thread when open(2) was called on the device.   This  behaviour  is
       retained in the current implementation as legacy support for both i386 and amd64 architectures.

       io — I/O privilege file

close(2), i386_get_ioperm(2), i386_set_ioperm(2), ioctl(2), open(2), mem(4)

deviceio#include<sys/types.h>#include<sys/ioctl.h>#include<dev/io/iodev.h>#include<machine/iodev.h>

       struct iodev_pio_req {
               u_int access;
               u_int port;
               u_int width;
               u_int val;
       };