kmem(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.