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KLD(4)                 FreeBSD Kernel Interfaces Manual                 KLD(4)

     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 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 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 sys-

     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 com-
                                pile 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 replace-
     ment 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 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 `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

FreeBSD 6.2                    November 8, 1998                    FreeBSD 6.2


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