NAME¶
kld —
dynamic kernel linker
facility
DESCRIPTION¶
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.
MODULE TYPES¶
- 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).
FILES¶
- /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
SEE ALSO¶
kldfind(2),
kldfirstmod(2),
kldload(2),
kldnext(2),
kldstat(2),
kldunload(2),
devfs(5),
devd(8),
kldload(8),
kldstat(8),
kldunload(8),
sysctl(8)
HISTORY¶
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.
AUTHORS¶
The
kld facility was originally implemented by
Doug Rabson ⟨dfr@FreeBSD.org⟩.
BUGS¶
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
failures.