KLD(4) BSD Kernel Interfaces Manual KLD(4)NAMEkld — 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 sev‐
eral 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 DragonFly system makes extensive use of loadable kernel modules, and
provides loadable versions of most filesystems, the NFS client and
server, all the screen-savers, and the Linux emulator. kld modules are
placed by default in the /boot/modules directory.
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 sys‐
tem.
FILES
/boot/modules directory containing module binaries shipped with the
system
<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 ALSOkldfind(2), kldfirstmod(2), kldload(2), kldnext(2), kldstat(2),
kldunload(2), kldload(8), kldstat(8), kldunload(8)HISTORY
The kld facility appeared in FreeBSD 3.0 and was designed as a replace‐
ment for the lkm(4) 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 mod‐
ule 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 ‘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.
BSD August 6, 2009 BSD
