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NAME

     kld — dynamic kernel linker facility

DESCRIPTION

     The LKM (Loadable Kernel Modules) facility has been deprecated in FreeBSD 3.0 and above in
     favor of the kld interface.  This interface, like its predecessor, allows 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 file systems, the NFS client and server, all the screen-savers, and the
     iBCS2 and Linux emulators.  kld modules are placed by default in the /boot/kernel directory
     along with their matching kernel.

     The kld interface is used through the kldload(8), kldunload(8) and kldstat(8) programs.

     The kldload(8) program can load either a.out(5) or ELF formatted loadable modules.  The
     kldunload(8) program unloads any given loaded module, if no other module is dependent upon
     the given module.  The kldstat(8) program is used to check the status of the modules
     currently loaded into the system.

     Kernel modules may only be loaded or unloaded if the system security level kern.securelevel
     is less than one.

MODULE TYPES

     Device Driver modules
     New block and character device drivers may be loaded into the system with kld.  Device nodes
     for the loaded drivers are automatically created when a module is loaded and destroyed when
     it is unloaded by devfs(5).  You can specify userland programs that will run when new
     devices become available as a result of loading modules, or existing devices go away when
     modules are unloaded, by configuring devd(8).

FILES

     /boot/kernel               directory containing module binaries built for the kernel also
                                residing in the directory.
     /usr/include/sys/module.h  file containing definitions required to compile a kld module
     /usr/share/examples/kld    example source code implementing a sample kld module

SEE ALSO

     kldfind(2), kldfirstmod(2), kldload(2), kldnext(2), kldstat(2), kldunload(2), devfs(5),
     devd(8), kldload(8), kldstat(8), kldunload(8), sysctl(8)

HISTORY

     The kld facility appeared in FreeBSD 3.0 and was designed as a replacement for the lkm
     facility, which was similar in functionality to the loadable kernel modules facility
     provided by SunOS 4.1.3.

AUTHORS

     The kld facility was originally implemented by Doug Rabson <dfr@FreeBSD.org>.

BUGS

     If a module B, is dependent on another module A, but is not compiled with module A as a
     dependency, then kldload(8) fails to load module B, even if module A is already present in
     the system.

     If multiple modules are dependent on module A, and are compiled with module A as a
     dependency, then kldload(8) loads an instance of module A when any of the modules are
     loaded.

     If a custom entry point is used for a module, and the module is compiled as an ‘ELF’ binary,
     then kldload(8) fails to execute the entry point.

     kldload(8) returns the cryptic message ‘ENOEXEC (Exec format error)’ for any error
     encountered while loading a module.

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