LKM(4)                    OpenBSD Programmer's Manual                   LKM(4)

NAME
     LKM - Loadable Kernel Modules interface

SYNOPSIS
     option LKM

DESCRIPTION
     Loadable kernel modules allow the system administrator to dynamically add
     and remove functionality from a running system.  This ability also helps
     software developers to develop new parts of the kernel without constantly
     rebooting to test their changes.

     Various types of modules can be loaded into the system.  There are sever-
     al defined module types, listed below, which can be added to the system
     in a predefined way.  In addition, there is a generic type, for which the
     module itself handles loading and unloading.

     The LKM interface is used by performing ioctl(2) calls on the /dev/lkm
     device.  Normally all operations involving Loadable Kernel Modules are
     handled by the modload(8), modunload(8), and modstat(8) programs.  Users
     should never have to interact with /dev/lkm directly.  The loading and
     unloading of modules is also dependent on the system securelevel(7).

MODULE TYPES
     System Call modules
             System calls may be replaced by loading new ones via the LKM in-
             terface.  All system calls may be replaced, but special care
             should be taken with the ioctl(2) system call, as it is used to
             load and unload modules.

             When a system call module is unloaded, the system call which was
             replaced by the loadable module is returned to its rightful place
             in the system call table.

     Virtual File System modules
             Virtual file systems may be added via the LKM interface.

     Device Driver modules
             New block and character device drivers may be loaded into the
             system with LKM.  The major problem with loading a device driver
             is that the driver's device nodes must exist for the devices to
             be accessed.  They are usually created by instructing modload(8)
             to run an appropriate program when the driver has been success-
             fully loaded.

     Execution Interpreters
             Execution interpreters allow the loading and execution of bina-
             ries which are normally not usable by the operating system.

     Miscellaneous modules
             Miscellaneous modules are modules for which there are not cur-
             rently well-defined or well-used interfaces for extension.  The
             user is expected to write their own loader to manipulate whatever
             kernel data structures necessary to enable and disable the new
             module when it is loaded and unloaded.

FILES
     /dev/lkm                   LKM interface device.
     /usr/include/sys/lkm.h     File containing definitions of module types.
     /usr/share/lkm             Example source code implementing several of
                                the modules types.

SEE ALSO
     securelevel(7), modload(8), modstat(8), modunload(8)

HISTORY
     The LKM facility was designed to be similar in functionality to the load-
     able kernel modules facility provided by SunOS 4.1.3.

AUTHORS
     Terrence R. Lambert <terry@cs.weber.edu>

BUGS
     Loading a bogus module is likely to kill your machine.

     Loadable streams modules should and will be implemented when a streams
     implementation is written.

OpenBSD 4.7                     August 2, 2008                               2
