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.Dd November 08, 1998
.Dt KLD 4
.\".Os FreeBSD 3.0
.Sh NAME
.Nm kld
.Nd dynamic kernel linker facility
.Sh DESCRIPTION
The LKM (Loadable Kernel Modules) facility has been deprecated in FreeBSD
3.0 and above in favor of the 
.Nm
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.
.Pp
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.
.Pp
The
.Tn FreeBSD
system makes extensive use of loadable kernel modules, and provides loadable
versions of most filesystems, the
.Tn NFS
client and server, all the screen-savers, and the
.Tn iBCS2
and
.Tn Linux
emulators.
.Nm
modules are placed by default in the
.Pa /modules
directory.
.Pp
The
.Nm
interface is used through the
.Xr kldload 8 ,
.Xr kldunload 8
and
.Xr kldstat 8
programs.
.Pp
The 
.Xr kldload 8
program can load either
.Xr a.out 5
or ELF formatted loadable modules.
The 
.Xr kldunload 8
program unloads any given loaded module, if no other module is dependent
upon the given module.
The
.Xr kldstat 8
program is used to check the status of the modules currently loaded into the
system.
.Sh "MODULE TYPES"
.Bl -ohang
.It Em "Device Driver modules"
New block and character device
drivers may be loaded into the system with
.Nm KLD .
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 using
.Xr MAKEDEV 8
or 
.Xr mknod 8 
(if the device is not supported by the
.Xr MAKEDEV 8
script) or, by writing a
shell script to run 
.Xr kldload 8
which should run the appropriate program to create the devices when the
driver has been successfully loaded.
.El
.Sh FILES
.Bl -tag -width /usr/include/sys/module.h -compact
.It Pa /modules
directory containing module binaries shipped with the system
.It Pa /usr/include/sys/module.h
file containing definitions required to compile a 
.Nm
module 
.It Pa /usr/share/examples/kld
example source code implementing a sample kld module
.Sh SEE ALSO
.Xr kldload 8 ,
.Xr kldstat 8 ,
.Xr kldunload 8
.Sh BUGS
If a module B, is dependent on another module A, but is not compiled with
module A as a dependency, then
.Xr kldload 8
fails to load module B, even if module A is already present in the system.
.Pp
If multiple modules are dependent on module A, and are compiled with module
A as a dependency, then
.Xr kldload 8
loads an instance of module A when any of the modules are loaded.
.Pp
If a custom entry point is used for a module, and the module is compiled as
an 
.Sq ELF
binary, then
.Xr kldload 8
fails to execute the entry point.
.Pp
.Xr kldload 8
returns the cryptic message
.Sq ENOEXEC Po Exec format error Pc
for any error encountered while loading a module.
.Pp
When system internal interfaces change, old modules often cannot
detect this, and such modules when loaded will often cause crashes or
mysterious failures.
.Sh AUTHORS
The
.Nm
facility was originally implemented by
.An Doug Rabson Aq dfr@FreeBSD.org .
.Sh HISTORY
The
.Nm
facility appeared in FreeBSD 3.0 and was designed as a replacement for the
.Xr lkm 4
facility, which was similar in functionality to the loadable kernel modules
facility provided by
.Tn SunOS
4.1.3.