allkmem(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 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 sys-
tem.
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 com-
puter. 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 combi-
nation 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 phy-
sical (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.
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