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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)
     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.

LEGACY

     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.

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.