KLD(4)

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 DragonFly system makes extensive use of loadable kernel modules, and provides loadable versions of most filesystems, the NFS client and server, and all the screen-savers. kld modules are placed by default in the /boot/kernel 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 system.

FILES
/boot/kernel
    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 ALSO
kldfind(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 replacement 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 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 ‘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.