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KLD(4)			 BSD Kernel Interfaces Manual			KLD(4)

     kld -- dynamic kernel linker facility

     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	sev-
     eral 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.	kld
     modules are placed	by default in the /modules directory.

     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 sys-

     Device Driver modules
     New block and character device drivers may	be loaded into the system with
     kld.  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	using MAKEDEV(8) or mknod(8) (if the device is not supported
     by	the MAKEDEV(8) script) or, by writing a	shell script to	run kldload(8)
     which should run the appropriate program to create	the devices when the
     driver has	been successfully loaded.

     /modules			directory containing module binaries shipped
				with the system
     /usr/include/sys/module.h	file containing	definitions required to	com-
				pile a kld module
     /usr/share/examples/kld	example	source code implementing a sample kld

     kldfind(2), kldfirstmod(2), kldload(2), kldnext(2), kldstat(2),
     kldunload(2), kldload(8), kldstat(8), kldunload(8)

     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 mod-
     ule 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

     The kld facility was originally implemented by Doug Rabson

     The kld facility appeared in FreeBSD 3.0 and was designed as a replace-
     ment for the lkm(4) facility, which was similar in	functionality to the
     loadable kernel modules facility provided by SunOS	4.1.3.

BSD			       November	8, 1998				   BSD


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