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.Dd January 13, 2014
.Dt KLD 4
.Os
.Sh NAME
.Nm kld
.Nd dynamic kernel linker facility
.Sh DESCRIPTION
The LKM (Loadable Kernel Modules) facility has been deprecated in
.Fx 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
.Fx
system makes extensive use of loadable kernel modules, and provides loadable
versions of most file systems, the
.Tn NFS
client and server, all the screen-savers, and the
.Tn Linux
emulator.
.Nm
modules are placed by default in the
.Pa /boot/kernel
directory along with their matching kernel.
.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.
.Pp
Kernel modules may only be loaded or unloaded if the system security level
.Va kern.securelevel
is less than one.
.Sh "MODULE TYPES"
.Bl -ohang
.It Em "Device Driver modules"
New block and character device
drivers may be loaded into the system with
.Nm .
Device nodes for the loaded drivers are automatically created when a
module is loaded and destroyed when it is unloaded by
.Xr devfs 4 .
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
.Xr devd 8 .
.El
.Sh FILES
.Bl -tag -width /usr/include/sys/module.h -compact
.It Pa /boot/kernel
directory containing module binaries built for the kernel also
residing in the directory.
.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
.El
.Sh SEE ALSO
.Xr kldfind 2 ,
.Xr kldfirstmod 2 ,
.Xr kldload 2 ,
.Xr kldnext 2 ,
.Xr kldstat 2 ,
.Xr kldunload 2 ,
.Xr devfs 4 ,
.Xr devd 8 ,
.Xr kldload 8 ,
.Xr kldstat 8 ,
.Xr kldunload 8 ,
.Xr sysctl 8
.Sh HISTORY
The
.Nm
facility appeared in
.Fx 3.0
and was designed as a replacement for the
.Nm lkm
facility, which was similar in functionality to the loadable kernel modules
facility provided by
.Tn SunOS
4.1.3.
.Sh AUTHORS
The
.Nm
facility was originally implemented by
.An Doug Rabson Aq Mt dfr@FreeBSD.org .
.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
points the user to read
.Xr dmesg 8
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.