Provided by: freebsd-manpages_7.2-1_all
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). 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.
i386_get_ioperm(2), i386_set_ioperm(2), mem(4)
The io file appeared in FreeBSD 1.0.