.\"
.\" Copyright (c) 1996 Joerg Wunsch
.\"
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.\" $FreeBSD: src/share/man/man4/man4.i386/io.4,v 1.6.2.4 2001/09/22 19:58:01 iedowse Exp $
.\"
.Dd January 1, 1996
.Dt IO 4
.Os
.Sh NAME
.Nm io
.Nd I/O privilege file
.Sh DESCRIPTION
The special file
.Pa /dev/io
is a controlled security hole that allows a process to gain I/O
privileges
(which are normally reserved for kernel-internal code).
Any process that holds a file descriptor on
.Pa /dev/io
open will get its
.Em IOPL
bits in the flag register set, thus allowing it to perform direct
I/O operations.  This can be useful in order to write userland
programs that handle some hardware directly.
Note that even read-only access will grant the full I/O privileges.
.Pp
In addition to any file access permissions on
.Pa /dev/io ,
the kernel enforces that only the super-user may open this device.
.Sh FILES
.Bl -tag -width Pa -compact
.It Pa /dev/io
.El
.Sh SEE ALSO
.Xr mem 4
.Sh HISTORY
The
.Nm
file appeared in
.Fx 1.0 .