MEM(7D)MEM(7D)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 physi‐
cal memory of the computer.
The file /dev/kmem is a special file that provides access to the vir‐
tual 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 inter‐
preted as kernel virtual memory addresses. A reference to a non-exis‐
tent 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 environ‐
ment. 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 combi‐
nation of llseek(2) and read(2) or write(2).
ERRORS
EFAULT
Occurs when trying to write(2) a read-only location (allk‐
mem), 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 asso‐
ciated with an I/O device.
/dev/allkmem
Provides access to the virtual address space of the
operating system kernel, including memory that is asso‐
ciated with an I/O device.
SEE ALSOllseek(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 hard‐
ware device is extremely dangerous and may result in a system panic if
kernel data structures are damaged or if device state is changed.
Feb 18, 2002 MEM(7D)
