computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, ...

, a loadable kernel module (LKM) is an

object file An object file is a computer file containing object code, that is, machine code output of an assembler or compiler. The object code is usually relocatable, and not usually directly executable. There are various formats for object files, and the ...

that contains code to

extend Extension, extend or extended may refer to: Mathematics Logic or set theory * Axiom of extensionality * Extensible cardinal * Extension (model theory) * Extension (predicate logic), the set of tuples of values that satisfy the predicate * Ext ...

the running

kernel Kernel may refer to: Computing * Kernel (operating system), the central component of most operating systems * Kernel (image processing), a matrix used for image convolution * Compute kernel, in GPGPU programming * Kernel method, in machine learn ...

, or so-called ''base kernel'', of an

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...

. LKMs are typically used to add support for new hardware (as

device driver In computing, a device driver is a computer program that operates or controls a particular type of device that is attached to a computer or automaton. A driver provides a software interface to hardware devices, enabling operating systems and o ...

s) and/or

filesystem In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is Computer data storage, stored and retrieved. Without a file system, data placed in a storage me ...

s, or for adding

system call In computing, a system call (commonly abbreviated to syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, acc ...

s. When the functionality provided by an LKM is no longer required, it can be unloaded in order to free

memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remember ...

and other resources. Most current

Unix-like A Unix-like (sometimes referred to as UN*X or *nix) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Unix-li ...

systems and

Microsoft Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for ...

support loadable kernel modules under different names, such as kernel loadable module (kld) in

FreeBSD FreeBSD is a free and open-source Unix-like operating system descended from the Berkeley Software Distribution (BSD), which was based on Research Unix. The first version of FreeBSD was released in 1993. In 2005, FreeBSD was the most popular ...

, kernel extension (kext) in

macOS macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and la ...

(although support for third-party modules is being dropped), kernel extension module in

AIX Aix or AIX may refer to: Computing * AIX, a line of IBM computer operating systems *An Alternate Index, for a Virtual Storage Access Method Key Sequenced Data Set * Athens Internet Exchange, a European Internet exchange point Places Belgiu ...

, kernel-mode driver in

Windows NT Windows NT is a proprietary graphical operating system produced by Microsoft, the first version of which was released on July 27, 1993. It is a processor-independent, multiprocessing and multi-user operating system. The first version of Win ...

and downloadable kernel module (DKM) in

VxWorks VxWorks is a real-time operating system (or RTOS) developed as proprietary software by Wind River Systems, a wholly-owned subsidiary of Aptiv. First released in 1987, VxWorks is designed for use in embedded systems requiring real-time, determi ...

. They are also known as kernel loadable modules (or KLM), and simply as kernel modules (KMOD).

Advantages

Without loadable kernel modules, an operating system would have to include all possible anticipated functionality compiled directly into the base kernel. Much of that functionality would reside in memory without being used, wasting memory, and would require that users rebuild and reboot the base kernel every time they require new functionality.

Disadvantages

One minor criticism of preferring a modular kernel over a static kernel is the so-called '' fragmentation penalty''. The base kernel is always unpacked into real contiguous

by its setup routines; thus, the base kernel code is never fragmented. Once the system is in a state in which modules may be inserted, for example once the

s have been

mount Mount is often used as part of the name of specific mountains, e.g. Mount Everest. Mount or Mounts may also refer to: Places * Mount, Cornwall, a village in Warleggan parish, England * Mount, Perranzabuloe, a hamlet in Perranzabuloe parish, ...

ed that contain the modules, it is likely that any new kernel code insertion will cause the kernel to become fragmented, thereby introducing a minor performance penalty by using more TLB entries, causing more TLB misses.

Implementations in different operating systems

Linux

Loadable kernel modules in Linux are loaded (and unloaded) by the

modprobe 

modprobe is a Linux program originally written by Rusty Russell and used to add a loadable kernel module to the Linux kernel or to remove a loadable kernel module from the kernel. It is commonly used indirectly: udev relies upon modprobe to load d ...

command. They are located in /lib/modules or /usr/lib/modules and have had the extension .ko ("kernel object") since version 2.6 (previous versions used the .o extension). The

lsmod 
lsmod is a command on Linux systems. It shows which loadable kernel modules are currently loaded.

An example terminal print after running lsmod command: 
 Module                  Size  Used by
 af_packet              27392  2 
  8139too            ...

command lists the loaded kernel modules. In emergency cases, when the system fails to boot due to e.g. broken modules, specific modules can be enabled or disabled by modifying the kernel boot parameters list (for example, if using GRUB, by pressing 'e' in the GRUB start menu, then editing the kernel parameter line).

License issues

In the opinion of Linux maintainers, LKM are derived works of the kernel. The Linux maintainers tolerate the distribution of

proprietary {{Short pages monitor