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NAME

     io - I/O privilege file

SYNOPSIS

     device io

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).  Any process that holds a file descriptor on /dev/io open
     will get its 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.

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

FILES

     /dev/io

SEE ALSO

     i386_get_ioperm(2), i386_set_ioperm(2), mem(4)

HISTORY

     The io file appeared in FreeBSD 1.0.