MEM(4D) Devices MEM(4D)

NAME

mem, kmem, allkmem - physical or virtual memory access

SYNOPSIS

/dev/mem


/dev/kmem


/dev/allkmem

DESCRIPTION

The file /dev/mem is a special file that provides access to the
physical memory of the computer.


The file /dev/kmem is a special file that provides access to the
virtual address space of the operating system kernel, excluding
memory that is associated with an I/O device.


The file /dev/allkmem is a special file that provides access to the
virtual address space of the operating system kernel, including
memory that is associated with an I/O device. You can use any of
these devices to examine and modify the system.


Byte addresses in /dev/mem are interpreted as physical memory
addresses. Byte addresses in /dev/kmem and /dev/allkmem are
interpreted as kernel virtual memory addresses. A reference to a non-
existent location returns an error. See ERRORS for more information.


The file /dev/mem accesses physical memory; the size of the file is
equal to the amount of physical memory in the computer. This size may
be larger than 4GB on a system running the 32-bit operating
environment. In this case, you can access memory beyond 4GB using a
series of read(2) and write(2) calls, a pread64() or pwrite64() call,
or a combination of llseek(2) and read(2) or write(2).

ERRORS

EFAULT
Occurs when trying to write(2) a read-only location
(allkmem), read(2) a write-only location (allkmem), or
read(2) or write(2) a non-existent or unimplemented
location (mem, kmem, allkmem).


EIO
Occurs when trying to read(2) or write(2) a memory location
that is associated with an I/O device using the /dev/kmem
special file.


ENXIO
Results from attempting to mmap(2) a non-existent physical
(mem) or virtual (kmem, allkmem) memory address.

FILES

/dev/mem
Provides access to the computer's physical memory.


/dev/kmem
Provides access to the virtual address space of the
operating system kernel, excluding memory that is
associated with an I/O device.


/dev/allkmem
Provides access to the virtual address space of the
operating system kernel, including memory that is
associated with an I/O device.

SEE ALSO

llseek(2), mmap(2), read(2), write(2)

WARNINGS

Using these devices to modify (that is, write to) the address space
of a live running operating system or to modify the state of a
hardware device is extremely dangerous and may result in a system
panic if kernel data structures are damaged or if device state is
changed.

February 18, 2002 MEM(4D)