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IO(4)			 BSD Kernel Interfaces Manual			 IO(4)

NAME
     io — I/O privilege file

SYNOPSIS
     device io

     #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;
     };

DESCRIPTION
     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) inter‐
     face 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 depen‐
     dent, 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.

LEGACY
     The /dev/io interface used to be very i386 specific and worked differ‐
     ently.  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.

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

HISTORY
     The io file appeared in FreeBSD 1.0.

BSD				 June 01, 2010				   BSD
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