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LKM(4)                 FreeBSD Kernel Interfaces Manual                 LKM(4)

     lkm - loadable kernel module facility

     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
     several 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 FreeBSD system makes extensive use of loadable kernel modules, and
     provides loadable versions of most filesystems, the NFS client and
     server, all the screen-savers, and the iBCS2 and Linux emulators.
     Modules distributed with the system are found in the /lkm directory.

     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.

     System Call modules
     System calls may be replaced by loading new ones via the lkm interface.
     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 be 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 successfully loaded.

     Execution classes
     Also known as image activators, execution classes are the mechanisms by
     which the execve(2) system call is able to recognize an executable file's
     format and load it into memory.

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

     /dev/lkm                 lkm interface device
     /lkm                     directory containing module binaries shipped
                              with the system
     /usr/include/sys/lkm.h   file containing definitions of module types
     /usr/share/examples/lkm  example source code implementing two of the
                              module types

     modload(8), modstat(8), modunload(8)

     If a module fails to initialize itself correctly, and the system is able
     to detect this failure, it will panic immediately.

     When system internal interfaces change, old modules often cannot detect
     this, and such modules when loaded will often cause crashes or mysterious

     The lkm facility was originally implemented by Terrence R. Lambert.
     Loadable filesystems were implemented by
     Garrett Wollman, and loadable execution classes were implemented by
     David Greenman,
     Soren Schmidt, and
     Garrett Wollman.

     The lkm facility was designed to be similar in functionality to the
     loadable kernel modules facility provided by SunOS 4.1.3.

FreeBSD 2.0                    January 17, 1996                    FreeBSD 2.0


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