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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
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
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
Kernel modules may only be loaded or unloaded if the system security
level kern.securelevel is less than one.
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).
/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
kldfind(2), kldfirstmod(2), kldload(2), kldnext(2), kldstat(2),
kldunload(2), devfs(5), devd(8), kldload(8), kldstat(8), kldunload(8),
The kld facility appeared in FreeBSD 3.0 and was designed as a
replacement for the lkm(4) facility, which was similar in functionality
to the loadable kernel modules facility provided by SunOS 4.1.3.
The kld facility was originally implemented by Doug Rabson
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