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
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.
- 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 module
kldfind(2),
kldfirstmod(2),
kldload(2),
kldnext(2),
kldstat(2),
kldunload(2),
devfs(5),
devd(8),
kldload(8),
kldstat(8),
kldunload(8),
sysctl(8)
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.
The
kld facility was originally implemented by
Doug Rabson
<
[email protected]>.
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) points the user to read
dmesg(8) 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.