The Linux kernel is a free and open-source,

monolithic A monolith is a monument or natural feature consisting of a single massive stone or rock. Monolith or monolithic may also refer to: Architecture * Monolithic architecture, a style of construction in which a building is carved, cast or excavated ...

modular Broadly speaking, modularity is the degree to which a system's components may be separated and recombined, often with the benefit of flexibility and variety in use. The concept of modularity is used primarily to reduce complexity by breaking a s ...

, multitasking,

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 ...

operating system

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 ...

. It was originally authored in 1991 by

Linus Torvalds Linus Benedict Torvalds ( , ; born 28 December 1969) is a Finnish software engineer who is the creator and, historically, the lead developer of the Linux kernel, used by Linux distributions and other operating systems such as Android. He also ...

for his

i386 The Intel 386, originally released as 80386 and later renamed i386, is a 32-bit microprocessor introduced in 1985. The first versions had 275,000 transistors GNU operating system GNU () is an extensive collection of free software (383 packages as of January 2022), which can be used as an operating system or can be used in parts with other operating systems. The use of the completed GNU tools led to the family of operat ...

, which was written to be a free (libre) replacement for

Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, an ...

. Linux is provided under the GNU General Public License version 2 only, but it contains files under other compatible licenses. Since the late 1990s, it has been included as part of a large number of operating system distributions, many of which are commonly also called

Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which ...

. Linux is deployed on a wide variety of computing systems, such as

embedded device An embedded system is a computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is ''embedded'' as ...

mobile device A mobile device (or handheld computer) is a computer small enough to hold and operate in the hand. Mobile devices typically have a flat LCD or OLED screen, a touchscreen interface, and digital or physical buttons. They may also have a physica ...

s (including its use in the Android operating system),

personal computer A personal computer (PC) is a multi-purpose microcomputer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or tec ...

s, servers,

mainframes A mainframe computer, informally called a mainframe or big iron, is a computer used primarily by large organizations for critical applications like bulk data processing for tasks such as censuses, industry and consumer statistics, enterprise ...

, and supercomputers. It can be tailored for specific architectures and for several usage scenarios using a family of simple commands (that is, without the need of manually editing its source code before compilation); privileged users can also fine-tune kernel parameters at runtime. Most of the Linux kernel code is written using the GNU extensions of GCC to the standard C programming language and with the use of architecture-specific instructions ( ISA) in limited parts of the kernel. This produces a highly optimized executable (

vmlinux vmlinux is a statically linked executable file that contains the Linux kernel in one of the object file formats supported by Linux, which includes Executable and Linkable Format (ELF) and Common Object File Format (COFF). The vmlinux file m ...

) with respect to utilization of memory space and task execution times. Day-to-day development discussions take place on the

Linux kernel mailing list The Linux kernel mailing list (LKML) is the main electronic mailing list for Linux kernel development, where the majority of the announcements, discussions, debates, and flame wars over the kernel take place. Many other mailing lists exist to di ...

(LKML). Changes are tracked using the version control system

git Git () is a distributed version control system: tracking changes in any set of files, usually used for coordinating work among programmers collaboratively developing source code during software development. Its goals include speed, data in ...

, which was originally authored by Torvalds as a free software replacement for

BitKeeper BitKeeper is a software tool for distributed revision control of computer source code. Originally developed as proprietary software by BitMover Inc., a privately held company based in Los Gatos, California, it was released as open-source softwar ...

History

LinuxCon Europe Linus Torvalds 03 (cropped)

In April 1991,

, at the time a 21-year-old

computer science Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to Applied science, practical discipli ...

student at the

University of Helsinki The University of Helsinki ( fi, Helsingin yliopisto, sv, Helsingfors universitet, abbreviated UH) is a public research university located in Helsinki, Finland since 1829, but founded in the city of Turku (in Swedish ''Åbo'') in 1640 as the ...

Finland Finland ( fi, Suomi ; sv, Finland ), officially the Republic of Finland (; ), is a Nordic country in Northern Europe. It shares land borders with Sweden to the northwest, Norway to the north, and Russia to the east, with the Gulf of B ...

, started working on some simple ideas for an operating system inspired by UNIX, for a personal computer. He started with a task switcher in Intel 80386 assembly language and a terminal driver. On 25 August 1991, Torvalds posted the following to ''comp.os.minix'', a

newsgroup A Usenet newsgroup is a repository usually within the Usenet system, for messages posted from users in different locations using the Internet. They are discussion groups and are not devoted to publishing news. Newsgroups are technically distinct ...

Usenet Usenet () is a worldwide distributed discussion system available on computers. It was developed from the general-purpose Unix-to-Unix Copy (UUCP) dial-up network architecture. Tom Truscott and Jim Ellis conceived the idea in 1979, and it was ...

: On 17 September 1991, Torvalds prepared version 0.01 of Linux and put on the "ftp.funet.fi" – FTP server of the Finnish University and Research Network (

FUNET FUNET is the Finnish University and Research Network, a backbone network providing Internet connections for Finnish universities and polytechnics as well as other research facilities. It is governed by the state-owned CSC – IT Center for Sc ...

). It was not even executable since its code still needed Minix for compilation and play. On 5 October 1991, Torvalds announced the first "official" version of Linux, version 0.02. At this point, Linux was able to run Bash, GCC, and some other GNU utilities: After that, despite the limited functionality of the early versions, Linux rapidly gained developers and users. Many people contributed code to the project, including some developers from the MINIX community. At the time, the

GNU Project The GNU Project () is a free software, mass collaboration project announced by Richard Stallman on September 27, 1983. Its goal is to give computer users freedom and control in their use of their computers and computing devices by collaborat ...

had created many of the components required for its free UNIX replacement, the

GNU operating system GNU () is an extensive collection of free software (383 packages as of January 2022), which can be used as an operating system or can be used in parts with other operating systems. The use of the completed GNU tools led to the family of operat ...

, but its own kernel,

GNU Hurd GNU Hurd is a collection of microkernel servers written as part of GNU, for the GNU Mach microkernel. It has been under development since 1990 by the GNU Project of the Free Software Foundation, designed as a replacement for the Unix kernel, and ...

, was incomplete. For this reason, it soon adopted the Linux kernel as well. The Berkeley Software Distribution had not yet freed itself from legal encumbrances and was not competing in the space for a free OS kernel. Torvalds assigned version 0 to the kernel to indicate that it was mainly for testing and not intended for productive use. Version 0.11, released in December 1991, was the first self-hosted Linux, for it could be compiled by a computer running the same kernel. When Torvalds released version 0.12 in February 1992, he adopted the

GNU General Public License The GNU General Public License (GNU GPL or simply GPL) is a series of widely used free software licenses that guarantee end users the Four Freedoms (Free software), four freedoms to run, study, share, and modify the software. The license was th ...

version 2 (GPLv2) over his previous self-drafted license, which had not permitted commercial redistribution. In contrast to

, all

source files In computing, source code, or simply code, is any collection of code, with or without comments, written using a human-readable programming language, usually as plain text. The source code of a program is specially designed to facilitate the w ...

of Linux are freely available, including

device drivers 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 ot ...

. The initial success of Linux was driven by programmers and testers across the world. With the support of the

POSIX The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines both the system- and user-level application programming inter ...

APIs, through the libC that, whether needed, acts as an entry point to the kernel address space, Linux could run software and applications that had been developed for Unix. Linux kernel ubiquity

On 19 January 1992, the first post to the new newsgroup ''alt.os.linux'' was submitted. On 31 March 1992, the newsgroup was renamed ''comp.os.linux''. The fact that Linux is a

monolithic kernel A monolithic kernel is an operating system architecture where the entire operating system is working in kernel space. The monolithic model differs from other operating system architectures (such as the microkernel architecture) in that it alone ...

rather than a

microkernel In computer science, a microkernel (often abbreviated as μ-kernel) is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system (OS). These mechanisms include low-level address space management, ...

was the topic of a debate between

Andrew S. Tanenbaum Andrew Stuart Tanenbaum (born March 16, 1944), sometimes referred to by the handle ast, is an American-Dutch computer scientist and professor emeritus of computer science at the Vrije Universiteit Amsterdam in the Netherlands. He is the author ...

, the creator of MINIX, and Torvalds. The

Tanenbaum–Torvalds debate The Tanenbaum–Torvalds debate was a written debate between Andrew S. Tanenbaum and Linus Torvalds, regarding the Linux kernel and kernel architecture in general. Tanenbaum, the creator of Minix, began the debate in 1992 on the Usenet discus ...

started in 1992 on the

group ''comp.os.minix'' as a general discussion about kernel architectures. Linux version 0.95 was the first to be capable of running the

X Window System The X Window System (X11, or simply X) is a windowing system for bitmap displays, common on Unix-like operating systems. X provides the basic framework for a GUI environment: drawing and moving windows on the display device and interacting wit ...

. In March 1994, Linux 1.0.0 was released with 176,250 lines of code. It was the first version suitable for use in production environments. It started a versioning system for the kernel with three or four numbers separated by dots where the first represented the ''major'' release, the second was the ''minor release'', and the third was the ''revision.'' At that time odd-numbered ''minor'' releases were for development and tests, whilst even numbered ''minor'' releases were for production. The optional fourth digit indicated a set of patches to a ''revision.'' Development releases were indicated with ''-rc'' ("release candidate") suffix. The current version numbering is slightly different from the above. The even vs. odd numbering has been dropped, and a specific ''major'' version is now indicated by the first two numbers, taken as a whole. While the time-frame is open for the development of the next ''major'', the -rcN suffix is used to identify the n'th ''release candidate'' for the next version. For example, the release of the version 4.16 was preceded by seven 4.16-rcN (from -rc1 to -rc7). Once a stable release is made, its maintenance is passed off to the "stable team". Occasional updates to stable releases are identified by a three numbering scheme (e.g., 4.13.1, 4.13.2, ..., 4.13.16). After version 1.3 of the kernel, Torvalds decided that Linux had evolved enough to warrant a new ''major'' number, so he released version 2.0.0 in June 1996. The series included 41 releases. The major feature of 2.0 was support for symmetric multiprocessing (SMP) and support for more types of processors. Starting with version 2.0, Linux is configurable for selecting specific hardware targets and for enabling architecture-specific features and optimizations. The ''make *config'' family of commands of ''kbuild'' are used to enable and configure thousands of options for building ad hoc kernel executables (

) and loadable modules. Version 2.2, released on 20 January 1999, improved locking granularity and SMP management, added

m68k The Motorola 68000 series (also known as 680x0, m68000, m68k, or 68k) is a family of 32-bit complex instruction set computer (CISC) microprocessors. During the 1980s and early 1990s, they were popular in personal computers and workstations and w ...

PowerPC PowerPC (with the backronym Performance Optimization With Enhanced RISC – Performance Computing, sometimes abbreviated as PPC) is a reduced instruction set computer (RISC) instruction set architecture (ISA) created by the 1991 Apple Inc., App ...

, Sparc64,

Alpha Alpha (uppercase , lowercase ; grc, ἄλφα, ''álpha'', or ell, άλφα, álfa) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter aleph , whic ...

, and other 64-bit platforms support. Furthermore, it added new

file systems 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 stored and retrieved. Without a file system, data placed in a storage medium would be one larg ...

including

Microsoft Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washing ...

NTFS New Technology File System (NTFS) is a proprietary journaling file system developed by Microsoft. Starting with Windows NT 3.1, it is the default file system of the Windows NT family. It superseded File Allocation Table (FAT) as the preferred fil ...

read-only capability. In 1999, IBM published its patches to the Linux 2.2.13 code for the support of the

S/390 The IBM System/390 is a discontinued mainframe product family implementing the ESA/390, the fifth generation of the System/360 instruction set architecture. The first computers to use the ESA/390 were the Enterprise System/9000 (ES/90 ...

architecture. Version 2.4.0, released on 4 January 2001, contained support for ISA

Plug and Play In computing, a plug and play (PnP) device or computer bus is one with a specification that facilitates the recognition of a hardware component in a system without the need for physical device configuration or user intervention in resolving resou ...

USB Universal Serial Bus (USB) is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication and power supply (interfacing) between computers, peripherals and other computers. A broad ...

, and PC Cards. Linux 2.4 added support for the

Pentium 4 Pentium 4 is a series of single-core CPUs for desktops, laptops and entry-level servers manufactured by Intel. The processors were shipped from November 20, 2000 until August 8, 2008. The production of Netburst processors was active from 200 ...

and

Itanium Itanium ( ) is a discontinued family of 64-bit Intel microprocessors that implement the Intel Itanium architecture (formerly called IA-64). Launched in June 2001, Intel marketed the processors for enterprise servers and high-performance comput ...

(the latter introduced the

ia64 IA-64 (Intel Itanium architecture) is the instruction set architecture (ISA) of the Itanium family of 64-bit Intel microprocessors. The basic ISA specification originated at Hewlett-Packard (HP), and was subsequently implemented by Intel in coll ...

ISA that was jointly developed by Intel and Hewlett-Packard to supersede the older

PA-RISC PA-RISC is an instruction set architecture (ISA) developed by Hewlett-Packard. As the name implies, it is a reduced instruction set computer (RISC) architecture, where the PA stands for Precision Architecture. The design is also referred to as ...

), and for the newer 64-bit MIPS processor. Development for 2.4.''x'' changed a bit in that more features were made available throughout the duration of the series, including support for

Bluetooth Bluetooth is a short-range wireless technology standard that is used for exchanging data between fixed and mobile devices over short distances and building personal area networks (PANs). In the most widely used mode, transmission power is limi ...

Logical Volume Manager In computer storage, logical volume management or LVM provides a method of allocating space on mass-storage devices that is more flexible than conventional partitioning schemes to store volumes. In particular, a volume manager can concatenate ...

(LVM) version 1,

RAID Raid, RAID or Raids may refer to: Attack * Raid (military), a sudden attack behind the enemy's lines without the intention of holding ground * Corporate raid, a type of hostile takeover in business * Panty raid, a prankish raid by male college ...

support,

InterMezzo In music, an intermezzo (, , plural form: intermezzi), in the most general sense, is a composition which fits between other musical or dramatic entities, such as acts of a play or movements of a larger musical work. In music history, the term ha ...

and

ext3 ext3, or third extended filesystem, is a journaled file system that is commonly used by the Linux kernel. It used to be the default file system for many popular Linux distributions. Stephen Tweedie first revealed that he was working on ext ...

file systems. Version 2.6.0 was released on 17 December 2003. The development for 2.6.''x'' changed further towards including new features throughout the duration of the series. Among the changes that have been made in the 2.6 series are: integration of

µClinux μClinux is a variation of the Linux kernel, previously maintained as a fork, that targets microcontrollers without a memory management unit (MMU). It was integrated into the mainline kernel as of 2.5.46; the project continues to develop patch ...

into the mainline kernel sources, PAE support, support for several new lines of

CPUs A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, and ...

, integration of Advanced Linux Sound Architecture (ALSA) into the mainline kernel sources, support for up to 2³² users (up from 2¹⁶), support for up to 2²⁹ process IDs (64-bit only, 32-bit arches still limited to 2¹⁵), substantially increased the number of device types and the number of devices of each type, improved

64-bit In computer architecture, 64-bit integers, memory addresses, or other data units are those that are 64 bits wide. Also, 64-bit CPUs and ALUs are those that are based on processor registers, address buses, or data buses of that size. A compu ...

support, support for file systems which support file sizes of up to 16

terabyte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable uni ...

s, in-kernel preemption, support for the

Native POSIX Thread Library The Native POSIX Thread Library (NPTL) is an implementation of the POSIX Threads specification for the Linux operating system. History Before the 2.6 version of the Linux kernel, processes were the schedulable entities, and there were no special f ...

(NPTL),

User-mode Linux User-mode Linux (UML) is an architectural port of the Linux kernel to its own system call interface, which enables multiple virtual Linux kernel-based operating systems (known as guests) to run as an application within a normal Linux system (know ...

integration into the mainline kernel sources,

SELinux Security-Enhanced Linux (SELinux) is a Linux kernel security module that provides a mechanism for supporting access control security policies, including mandatory access controls (MAC). SELinux is a set of kernel modifications and user-space ...

integration into the mainline kernel sources,

InfiniBand InfiniBand (IB) is a computer networking communications standard used in high-performance computing that features very high throughput and very low latency. It is used for data interconnect both among and within computers. InfiniBand is also use ...

support, and considerably more. Also notable are the addition of a wide selection of file systems starting with the 2.6.''x'' releases: now, the kernel supports a large number of file systems, some that have been designed for Linux, like

ext4 ext4 (fourth extended filesystem) is a journaling file system for Linux, developed as the successor to ext3. ext4 was initially a series of backward-compatible extensions to ext3, many of them originally developed by Cluster File Systems for ...

FUSE Fuse or FUSE may refer to: Devices * Fuse (electrical), a device used in electrical systems to protect against excessive current ** Fuse (automotive), a class of fuses for vehicles * Fuse (hydraulic), a device used in hydraulic systems to protect ...

Btrfs Btrfs (pronounced as "better F S", "butter F S", "b-tree F S", or simply by spelling it out) is a computer storage format that combines a file system based on the copy-on-write (COW) principle with a logical volume manager (not to be confused ...

, and others that are native of other operating systems like JFS,

XFS XFS is a high-performance 64-bit journaling file system created by Silicon Graphics, Inc (SGI) in 1993. It was the default file system in SGI's IRIX operating system starting with its version 5.3. XFS was ported to the Linux kernel in 2001; as ...

, Minix,

Xenix Xenix is a discontinued version of the Unix operating system for various microcomputer platforms, licensed by Microsoft from AT&T Corporation in the late 1970s. The Santa Cruz Operation (SCO) later acquired exclusive rights to the software, and ...

, Irix,

Solaris Solaris may refer to: Arts and entertainment Literature, television and film * ''Solaris'' (novel), a 1961 science fiction novel by Stanisław Lem ** ''Solaris'' (1968 film), directed by Boris Nirenburg ** ''Solaris'' (1972 film), directed by ...

System V Unix System V (pronounced: "System Five") is one of the first commercial versions of the Unix operating system. It was originally developed by AT&T and first released in 1983. Four major versions of System V were released, numbered 1, 2, 3, an ...

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 serv ...

and

MS-DOS MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few ope ...

. In 2005 the ''stable team'' was formed as a response to the lack of a kernel tree where people could work on bug fixes, and it would keep updating ''stable'' versions. In February 2008 the ''linux-next'' tree was created to serve as a place where patches aimed to be merged during the next development cycle gathered. Several subsystem maintainers also adopted the suffix ''-next'' for trees containing code which they mean to submit for inclusion in the next release cycle. , the in-development version of Linux is held in an unstable branch named ''linux-next''. Linux used to be maintained without the help of an automated

source code management In software engineering, version control (also known as revision control, source control, or source code management) is a class of systems responsible for managing changes to computer programs, documents, large web sites, or other collections o ...

system until, in 2002, development switched to

. It was freely available for Linux developers but it was not

free software Free software or libre software is computer software distributed under terms that allow users to run the software for any purpose as well as to study, change, and distribute it and any adapted versions. Free software is a matter of liberty, no ...

. In 2005, because of efforts to

reverse-engineer Reverse engineering (also known as backwards engineering or back engineering) is a process or method through which one attempts to understand through deductive reasoning how a previously made device, process, system, or piece of software accompl ...

it, the company which owned the software revoked its support of the Linux community. In response, Torvalds and others wrote

Git Git () is a distributed version control system: tracking changes in any set of files, usually used for coordinating work among programmers collaboratively developing source code during software development. Its goals include speed, data in ...

. The new system was written within weeks, and in two months the first official kernel made using it was released. Details on the history of the 2.6 kernel series can be found in the ChangeLog files on the 2.6 kernel series source code release area of

kernel.org kernel.org is the main distribution point of source code for the Linux kernel, which is the base of the Linux operating system. Website The website and related infrastructure, which are operated by the Linux Kernel Organization, host the reposi ...

. The 20th anniversary of Linux was celebrated by Torvalds in July 2011 with the release of the 3.0.0 kernel version. As 2.6 had been the version number for 8 years, a new ''uname26'' personality that reports 3.x as 2.6.40+x had to be added to the kernel so that old programs would work. Version 3.0 was released on 22 July 2011. On 30 May 2011, Torvalds announced that the big change was "NOTHING. Absolutely nothing." and asked, "...let's make sure we really make the next release not just an all new shiny number, but a good kernel too." After the expected 6–7 weeks of the development process, it would be released near the 20th anniversary of Linux. On 11 December 2012, Torvalds decided to reduce kernel complexity by removing support for

i386 The Intel 386, originally released as 80386 and later renamed i386, is a 32-bit microprocessor introduced in 1985. The first versions had 275,000 transistors The same series unified support for the

ARM In human anatomy, the arm refers to the upper limb in common usage, although academically the term specifically means the upper arm between the glenohumeral joint (shoulder joint) and the elbow joint. The distal part of the upper limb between th ...

processor. Version 3.11, released on 2 September 2013, adds many new features such as new flag for to reduce temporary file vulnerabilities, experimental AMD Radeon dynamic power management, low-latency network polling, and

zswap zswap is a Linux kernel feature that provides a compressed write-back Cache (computing), cache for Paging, swapped pages, as a form of virtual memory compression. Instead of moving memory pages to a swap device when they are to be swapped ou ...

(compressed swap cache). The numbering change from 2.6.39 to 3.0, and from 3.19 to 4.0, involved no meaningful technical differentiation. The major version number was increased to avoid large minor numbers. Stable 3.x.y kernels were released until 3.19 in February 2015. In April 2015, Torvalds released kernel version 4.0. By February 2015, Linux had received contributions from nearly 12,000 programmers from more than 1,200 companies, including some of the world's largest software and hardware vendors. Version 4.1 of Linux, released in June 2015, contains over 19.5 million lines of code contributed by almost 14,000 programmers. A total of 1,991 developers, of whom 334 were first-time collaborators, added more than 553,000 lines of code to version 5.8, breaking the record previously held by version 4.9. According to the Stack Overflow's annual Developer Survey of 2019, more than the 53% of all respondents have developed software for

and about 27% for Android, although only about 25% develop with Linux-based operating systems. Most websites run on Linux-based operating systems, and all of the world's 500 most powerful supercomputers use some kind of OS based on Linux.

Linux distributions A Linux distribution (often abbreviated as distro) is an operating system made from a software collection that includes the Linux kernel and, often, a package management system. Linux users usually obtain their operating system by downloading one ...

bundle the kernel with

system software System software is software designed to provide a platform for other software. Examples of system software include operating systems (OS) like macOS, Linux, Android and Microsoft Windows, computational science software, game engines, search engin ...

(e.g., the

GNU C Library The GNU C Library, commonly known as glibc, is the GNU Project's implementation of the C standard library. Despite its name, it now also directly supports C++ (and, indirectly, other programming languages). It was started in the 1980s by ...

, systemd, and others

utilities A public utility company (usually just utility) is an organization that maintains the infrastructure for a public service (often also providing a service using that infrastructure). Public utilities are subject to forms of public control and ...

and daemons) and a wide selection of

application software Application may refer to: Mathematics and computing * Application software, computer software designed to help the user to perform specific tasks ** Application layer, an abstraction layer that specifies protocols and interface methods used in a ...

, but their

usage share Usage share may refer to: *Usage share of BitTorrent clients *Usage share of instant messaging clients *Usage share of operating systems *Usage share of web browsers *Usage share of web search engines See also *Market share Market share is t ...

in desktops is low in comparison to other operating systems. Android, which accounts for the majority of the installed base of all operating systems for mobile devices, is responsible for the rising usage of the Linux kernel, together with its wide use in a large variety of

embedded devices An embedded system is a computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is ''embedded'' as ...

Architecture and features

Linux is a

with a modular design (e.g., it can insert and remove loadable kernel modules at runtime), supporting most features once only available in closed source kernels of non-free operating systems. The rest of the article makes use of the UNIX and Unix-like operating systems convention on the official manual pages. The numbers that follow the name of commands, interfaces, and other features, have the purpose of specifying the section (i.e., the type of the OS' component or feature) they belong to (e.g., refers to a system call, while refers to a userspace library wrapper). The following list and the subsequent sections describe a non-comprehensive overview of Linux architectural design and of some of its noteworthy features. * Concurrent computing and (with the availability of enough CPU cores for tasks that are ready to run) even true parallel execution of many processes at once (each of them having one or more threads of execution) on SMP and

NUMA Nuclear mitotic apparatus protein 1 is a protein that in humans is encoded by the ''NUMA1'' gene. Interactions Nuclear mitotic apparatus protein 1 has been shown to interact with PIM1, Band 4.1, GPSM2 G-protein-signaling modulator 2, also call ...

architectures. * Selection and configuration of hundreds of kernel features and drivers (using one of the family of commands, before running compilation), modification of kernel parameters before booting (usually by inserting instructions into the lines of the

GRUB2 GNU GRUB (short for GNU GRand Unified Bootloader, commonly referred to as GRUB) is a boot loader package from the GNU Project. GRUB is the reference implementation of the Free Software Foundation's Multiboot Specification, which provides a us ...

menu), and fine tuning of kernel behavior at run-time (using the interface to ). * Configuration (again using the commands) and run-time modifications of the policies (via , , and the family of syscalls) of the task schedulers that allow

preemptive multitasking In computing, preemption is the act of temporarily interrupting an executing task, with the intention of resuming it at a later time. This interrupt is done by an external scheduler with no assistance or cooperation from the task. This preemp ...

(both in

user mode A modern computer operating system usually segregates virtual memory into user space and kernel space. Primarily, this separation serves to provide memory protection and hardware protection from malicious or errant software behaviour. Kernel ...

and, since the 2.6 series, in

kernel mode In computer science, hierarchical protection domains, often called protection rings, are mechanisms to protect data and functionality from faults (by improving fault tolerance) and malicious behavior (by providing computer security). Compute ...

); the Completely Fair Scheduler (CFS) is the default scheduler of Linux since 2007 and it uses a red-black tree which can search, insert and delete process information ( task struct) with O(log n)

time complexity In computer science, the time complexity is the computational complexity that describes the amount of computer time it takes to run an algorithm. Time complexity is commonly estimated by counting the number of elementary operations performed by t ...

, where ''n'' is the number of runnable tasks. * Advanced

memory management Memory management is a form of resource management applied to computer memory. The essential requirement of memory management is to provide ways to dynamically allocate portions of memory to programs at their request, and free it for reuse when ...

with

paged virtual memory In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very ...

. * Inter-process communications and synchronization mechanism. * A

virtual filesystem A virtual file system (VFS) or virtual filesystem switch is an abstract layer on top of a more concrete file system. The purpose of a VFS is to allow client applications to access different types of concrete file systems in a uniform way. A VFS ...

on top of several concrete filesystems (

, JFS,

FAT32 File Allocation Table (FAT) is a file system developed for personal computers. Originally developed in 1977 for use on floppy disks, it was adapted for use on hard disks and other devices. It is often supported for compatibility reasons by ...

, and many more). * Configurable I/O schedulers, syscall that manipulates the underlying device parameters of special files (it is a non standard system call, since arguments, returns, and semantics depends on the device driver in question), support for POSIX asynchronous I/O (however, because they scale poorly with multithreaded applications, a family of Linux specific I/O system calls () had to be created for the management of asynchronous I/O contexts suitable for concurrently processing). *

OS-level virtualization OS-level virtualization is an operating system (OS) paradigm in which the kernel allows the existence of multiple isolated user space instances, called ''containers'' (LXC, Solaris containers, Docker, Podman), ''zones'' (Solaris containers), '' ...

(with

Linux-VServer Linux-VServer is a virtual private server implementation that was created by adding operating system-level virtualization capabilities to the Linux kernel. It is developed and distributed as open-source software. Details The project was started ...

paravirtualization In computing, paravirtualization or para-virtualization is a virtualization technique that presents a software interface to the virtual machines which is similar, yet not identical, to the underlying hardware–software interface. The intent o ...

and

hardware-assisted virtualization In computing, hardware-assisted virtualization is a platform virtualization approach that enables efficient full virtualization using help from hardware capabilities, primarily from the host processors. A full virtualization is used to emulate a ...

(with KVM or Xen, and using

QEMU QEMU is a free and open-source emulator (Quick EMUlator). It emulates the machine's central processing unit, processor through dynamic binary translation and provides a set of different hardware and device models for the machine, enabling it t ...

for hardware emulation); On the Xen

hypervisor A hypervisor (also known as a virtual machine monitor, VMM, or virtualizer) is a type of computer software, firmware or hardware that creates and runs virtual machines. A computer on which a hypervisor runs one or more virtual machines is calle ...

, the Linux kernel provides support to build Linux distributions (such as openSuSE Leap and many others) that work as ''Dom0'', that are virtual machine host servers that provide the management environment for the user's virtual machines (''DomU''). *I/O Virtualization with

VFIO In virtualization, input/output virtualization (I/O virtualization) is a methodology to simplify management, lower costs and improve performance of servers in enterprise environments. I/O virtualization environments are created by abstracting the ...

and

SR-IOV In virtualization, single root input/output virtualization (SR-IOV) is a specification that allows the isolation of PCI Express resources for manageability and performance reasons. Details A single physical PCI Express bus can be shared in a virt ...

. Virtual Function I/O (VFIO) exposes direct device access to user space in a secure memory (IOMMU) protected environment. With VFIO, a VM Guest can directly access hardware devices on the VM Host Server. This technique improves performance, if compared both to Full virtualization and Paravirtualization. However, with VFIO, devices cannot be shared with multiple VM guests. Single Root I/O Virtualization (SR-IOV) combines the performance gains of VFIO and the ability to share a device with several VM Guests (but it requires special hardware that must be capable to appear to two or more VM guests as different devices). * Security mechanisms for discretionary and

mandatory access control In computer security, mandatory access control (MAC) refers to a type of access control by which the operating system or database constrains the ability of a ''subject'' or ''initiator'' to access or generally perform some sort of operation on a ...

(SELinux, AppArmor, POSIX ACLs, and others). * Several types of layered

communication protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synchroniza ...

s (including the

Internet protocol suite The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the set of communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the sui ...

). *

Asymmetric multiprocessing An asymmetric multiprocessing (AMP or ASMP) system is a multiprocessor computer system where not all of the multiple interconnected central processing units (CPUs) are treated equally. For example, a system might allow (either at the hardware or ope ...

via the

RPMsg An rpmsg file is a restricted-permission message with an extension of rpmsg. It is used to implement Information Rights Management, IRM for Outlook messages with the aim of controlling access to content via encryption and access controls, and re ...

subsystem. Most Device drivers and kernel extensions run in kernel space ( ring 0 in many CPU

architectures Architecture is the art and technique of designing and building, as distinguished from the skills associated with construction. It is both the process and the product of sketching, conceiving, planning, designing, and constructing buildings o ...

), with full access to the hardware. Some exceptions run in

user space A modern computer operating system usually segregates virtual memory into user space and kernel space. Primarily, this separation serves to provide memory protection and hardware protection from malicious or errant software behaviour. Kernel ...

; notable examples are filesystems based on

/CUSE, and parts of UIO. Furthermore, the

and Wayland, the windowing system and display server protocols that most people use with Linux, do not run within the kernel. Differently, the actual interfacing with

GPUs A graphics processing unit (GPU) is a specialized electronic circuit designed to manipulate and alter memory to accelerate the creation of images in a frame buffer intended for output to a display device. GPUs are used in embedded systems, mobil ...

of graphics cards is an in-kernel subsystem called Direct Rendering Manager (DRM). Unlike standard monolithic kernels, device drivers are easily configured as

modules Broadly speaking, modularity is the degree to which a system's components may be separated and recombined, often with the benefit of flexibility and variety in use. The concept of modularity is used primarily to reduce complexity by breaking a s ...

, and loaded or unloaded while the system is running and can also be pre-empted under certain conditions in order to handle

hardware interrupt In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to ''interrupt'' currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted, ...

s correctly and to better support symmetric multiprocessing. By choice, Linux has no stable device driver

application binary interface In computer software, an application binary interface (ABI) is an interface between two binary program modules. Often, one of these modules is a library or operating system facility, and the other is a program that is being run by a user. An ...

. Linux typically makes use of

memory protection Memory protection is a way to control memory access rights on a computer, and is a part of most modern instruction set architectures and operating systems. The main purpose of memory protection is to prevent a process from accessing memory that h ...

and

virtual memory In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very ...

and can also handle

non-uniform memory access Non-uniform memory access (NUMA) is a computer memory design used in multiprocessing, where the memory access time depends on the memory location relative to the processor. Under NUMA, a processor can access its own local memory faster than non ...

, however the project has absorbed

μClinux μClinux is a variation of the Linux kernel, previously maintained as a fork, that targets microcontrollers without a memory management unit (MMU). It was integrated into the mainline kernel as of 2.5.46; the project continues to develop patc ...

which also makes it possible to run Linux on microcontrollers without virtual memory. The hardware is represented in the file hierarchy. User applications interact with device drivers via entries in the or directories. Processes information as well are mapped to the file system through the directory.

Interfaces

Linux is a clone of UNIX, and aims toward

and Single UNIX Specification compliance. The kernel also provides system calls and other interfaces that are Linux-specific. In order to be included in the official kernel, the code must comply with a set of licensing rules. The Linux

Application binary interface In computer software, an application binary interface (ABI) is an interface between two binary program modules. Often, one of these modules is a library or operating system facility, and the other is a program that is being run by a user. An ...

(ABI) between the kernel and the user space has four degrees of stability (stable, testing, obsolete, removed); however, the

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 are expected to never change in order to not break the

userspace A modern computer operating system usually segregates virtual memory into user space and kernel space. Primarily, this separation serves to provide memory protection and hardware protection from malicious or errant software behaviour. Kernel ...

programs that rely on them. Loadable kernel modules (LKMs), by design, cannot rely on a stable ABI. Therefore, they must always be recompiled whenever a new kernel executable is installed in a system, otherwise they will not be loaded. In-tree drivers that are configured to become an integral part of the kernel executable (

) are statically linked by the building process. There is also no guarantee of stability of source-level in-kernel API and, because of this, device drivers code, as well as the code of any other kernel subsystem, must be kept updated with kernel evolution. Any developer who makes an API change is required to fix any code that breaks as the result of their change.

Kernel-to-userspace API

The set of the Linux kernel API that regards the interfaces exposed to user applications is fundamentally composed of UNIX and Linux-specific

s. A system call is an entry point into the Linux kernel. For example, among the Linux-specific ones there is the family of the system calls. Most extensions must be enabled by defining the _GNU_SOURCE macro in a

header file Many programming languages and other computer files have a directive, often called include (sometimes copy or import), that causes the contents of the specified file to be inserted into the original file. These included files are called copybooks ...

or when the user-land code is being compiled. System calls can only be invoked by using assembly instructions which enable the transition from unprivileged user space to privileged kernel space in ring 0. For this reason, the C standard library (libC) acts as a wrapper to most Linux system calls, by exposing C functions that, only whether it is needed, can transparently enter into the kernel which will execute on behalf of the calling process. For those system calls not exposed by libC, e.g. the ''fast userspace mutex'' (

futex In computing, a futex (short for "fast userspace mutex") is a kernel system call that programmers can use to implement basic locking, or as a building block for higher-level locking abstractions such as semaphores and POSIX mutexes or condition ...

), the library provides a function called which can be used to explicitly invoke them. Pseudo filesystems (e.g., the sysfs and

procfs The proc filesystem (procfs) is a special filesystem in Unix-like operating systems that presents information about processes and other system information in a hierarchical file-like structure, providing a more convenient and standardized meth ...

filesystems) and

special file In Unix-like operating systems, a device file or special file is an interface to a device driver that appears in a file system as if it were an ordinary file. There are also special files in DOS, OS/2, and Windows. These special files allow ...

s (e.g., /dev/random, /dev/sda, /dev/tty, and many others) constitute another layer of interface to kernel data structures representing hardware or logical (software) devices.

Kernel-to-userspace ABI

Because of the differences existing between the hundreds of various implementations of the Linux OS, executable objects, even though they are compiled, assembled, and linked for running on a specific hardware architecture (that is, they use the ISA of the target hardware), often cannot run on different Linux Distributions. This issue is mainly due to distribution-specific configurations and a set of patches applied to the code of the Linux kernel, differences in system libraries, services (daemons), filesystem hierarchies, and environment variables. The main standard concerning application and binary compatibility of Linux distributions is the

Linux Standard Base The Linux Standard Base (LSB) was a joint project by several Linux distributions under the organizational structure of the Linux Foundation to standardize the software system structure, including the Filesystem Hierarchy Standard used in the ...

(LSB). However, the LSB goes beyond what concerns the Linux kernel, because it also defines the desktop specifications, the X libraries and Qt that have little to do with it. The LSB version 5 is built upon several standards and drafts (POSIX, SUS, X/Open, File System Hierarchy (FHS), and others). The parts of the LSB largely relevant to the kernel are the ''General ABI'' (gABI), especially the System V ABI and the Executable and Linking Format (ELF), and the ''Processor Specific ABI'' (psABI), for example the ''Core Specification for X86-64.'' The standard ABI for how x86_64 user programs invoke system calls is to load the syscall number into the ''rax'' register, and the other parameters into ''rdi'', ''rsi'', ''rdx'', ''r10'', ''r8'', and ''r9'', and finally to put the ''syscall'' assembly instruction in the code.

In-kernel API

There are several kernel internal APIs utilized between the different subsystems. Some are available only within the kernel subsystems, while a somewhat limited set of in-kernel symbols (i.e., variables, data structures, and functions) is exposed also to dynamically loadable modules (e.g., device drivers loaded on demand) whether they're exported with the and macros (the latter reserved to modules released under a GPL-compatible license). Linux provides in-kernel APIs that manipulate data structures (e.g., linked lists,

radix tree In computer science, a radix tree (also radix trie or compact prefix tree or compressed trie) is a data structure that represents a space-optimized trie (prefix tree) in which each node that is the only child is merged with its parent. The resul ...

s, red-black trees, queues) or perform common routines (e.g., copy data from and to user space, allocate memory, print lines to the system log, and so on) that have remained stable at least since Linux version 2.6. In-kernel APIs include libraries of low-level common services used by device drivers: * SCSI Interfaces and

libATA libATA is a library used inside the Linux kernel to support ATA host controllers and devices. libATA provides an ATA driver API, class transports for ATA and ATAPI devices, and SCSI / ATA Translation for ATA devices according to the T10 SAT spe ...

respectively, a peer-to-peer packet based communication protocol for storage devices attached to USB, SATA, SAS, Fibre Channel, FireWire, ATAPI device, and an in-kernel library to support TA host controllers and devices. * Direct Rendering Manager (DRM) and Kernel Mode Setting (KMS) for interfacing with GPUs and supporting the needs of modern 3D-accelerated video hardware, and for setting screen resolution, color depth and refresh rate * DMA buffers ( DMA-BUF) for sharing buffers for hardware direct memory access across multiple device drivers and subsystems *

Video4Linux Video4Linux (V4L for short) is a collection of device drivers and an API for supporting realtime video capture on Linux systems. It supports many USB webcams, TV tuners, and related devices, standardizing their output, so programmers can easily ...

for video capture hardware *

Advanced Linux Sound Architecture Advanced Linux Sound Architecture (ALSA) is a software framework and part of the Linux kernel that provides an application programming interface (API) for sound card device drivers. Some of the goals of the ALSA project at its inception w ...

(ALSA) for sound cards *

New API New API (also referred to as NAPI) is an interface to use interrupt mitigation techniques for networking devices in the Linux kernel. Such an approach is intended to reduce the overhead of packet receiving. The idea is to defer incoming message h ...

for

network interface controller A network interface controller (NIC, also known as a network interface card, network adapter, LAN adapter or physical network interface, and by similar terms) is a computer hardware component that connects a computer to a computer network. Ear ...

s *

mac80211 A wireless network interface controller (WNIC) is a network interface controller which connects to a wireless network, such as Wi-Fi or Bluetooth, rather than a wired network, such as a Token Ring or Ethernet. A WNIC, just like other NICs, wor ...

and cfg80211 - for wireless network interface controllers

In-kernel ABI

The Linux developers chose not to maintain a stable in-kernel ABI. Modules compiled for a specific version of the kernel cannot be loaded into another version without being recompiled, assuming that the in-kernel API has remained the same at the source level; otherwise, the module code must also be modified accordingly.

Processes and threads

Linux creates processes by means of the or by the newer system calls. Depending on the given parameters, the new entity can share most or none of the resources of the caller. These syscalls can create new entities ranging from new independent processes (each having a special identifier called ''TGID'' within the ''task_struct'' data structure in kernel space, although that same identifier is called ''PID'' in userspace), to new threads of execution within the calling process (by using the parameter). In this latter case the new entity owns the same ''TGID'' of the calling process and consequently has also the same ''PID'' in userspace. If the executable is dynamically linked to shared libraries, a

dynamic linker In computing, a dynamic linker is the part of an operating system that loads and links the shared libraries needed by an executable when it is executed (at " run time"), by copying the content of libraries from persistent storage to RAM, filling ...

(for ELF objects, it is typically ) is used to find and load the needed objects, prepare the program to run and then run it. The

, simply known as the NPTL, provides the standard POSIX threads interface (''pthreads'') to userspace. Whenever a new thread is created using the pthread_create(3) POSIX interface, the family of system calls must also be given the address of the function that the new thread must jump to. The Linux kernel provides the (acronym for "Fast user-space mutexes") mechanisms for fast user-space locking and synchronization; the majority of the operations are performed in userspace but it may be necessary to communicate with the kernel using the system call. A very special category of threads is the so-called ''kernel threads''. They must not be confused with the above-mentioned threads of execution of the user's processes. Kernel threads exist only in kernel space and their only purpose is to concurrently run kernel tasks. Differently, whenever an independent process is created, the syscalls return exactly to the next instruction of the same program, concurrently in ''parent'' process and in ''child's'' one (i.e., one program, two processes). Different return values (one per process) enable the program to know in which of the two processes it is currently executing. Programs need this information because the child process, a few steps after process duplication, usually invokes the system call (possibly via the family of wrapper functions in glibC) and replace the program that is currently being run by the calling process with a new program, with newly initialized stack, heap, and (initialized and uninitialized) data segments. When it is done, it results in two processes that run two different programs. Depending on the effective user id (''euid''), and on the effective group id (''egid''), a process running with user zero privileges (''root'', the system administrator, owns the identifier 0) can perform everything (e.g., kill all the other processes or recursively wipe out whole filesystems), instead non zero user processes cannot. divides the privileges traditionally associated with superuser into distinct units, which can be independently enabled and disabled by the parent process or dropped by the child itself.

Scheduling and preemption

The Linux scheduler is modular, in the sense that it enables different scheduling classes and policies. Scheduler classes are plugable scheduler algorithms that can be registered with the base scheduler code. Each class schedules different types of processes. The core code of the scheduler iterates over each class in order of priority and chooses the highest priority scheduler that has a schedulable entity of type struct sched_entity ready to run. Entities may be threads, group of threads, and even all the processes of a specific user. Linux provides both ''user preemption'' as well as full ''kernel preemption''. Preemption reduces latency, increases responsiveness, and makes Linux more suitable for desktop and

real-time Real-time or real time describes various operations in computing or other processes that must guarantee response times within a specified time (deadline), usually a relatively short time. A real-time process is generally one that happens in defined ...

applications. For normal tasks, by default, the kernel uses the

Completely Fair Scheduler The Completely Fair Scheduler (CFS) is a process scheduler that was merged into the 2.6.23 (October 2007) release of the Linux kernel and is the default scheduler of the tasks of the SCHED_NORMAL class (i.e., tasks that have no real-time execution ...

(CFS) class, introduced in the 2.6.23 version of the kernel. Internally this default-scheduler class is defined in a macro of a C header as SCHED_NORMAL. In other POSIX kernels, a similar policy known as SCHED_OTHER allocates CPU timeslices (i.e, it assigns absolute slices of the processor time depending on either predetermined or dynamically computed priority of each process). The Linux CFS does away with absolute timeslices and assigns a fair proportion of CPU time, as a function of parameters like the total number of runnable processes and the time they have already run; this function also takes into account a kind of weight that depends on their relative priorities (nice values). With user preemption, the kernel scheduler can replace the current process with the execution of a context switch to a different one that therefore acquires the computing resources for running (CPU, memory, and more). It makes it according to the CFS algorithm (in particular, it uses a variable called for sorting entities and then chooses the one that has the smaller vruntime, - i.e., the schedulable entity that has had the least share of CPU time), to the active scheduler policy and to the relative priorities. With kernel preemption, the kernel can preempt itself when an interrupt handler returns, when kernel tasks block, and whenever a subsystem explicitly calls the schedule() function. The kernel also contains two POSIX-compliant real-time scheduling classes named SCHED_FIFO (realtime

first-in-first-out FIFO and LIFO accounting are methods used in managing inventory and financial matters involving the amount of money a company has to have tied up within inventory of produced goods, raw materials, parts, components, or feedstocks. They are used t ...

) and SCHED_RR (realtime round-robin), both of which take precedence over the default class. An additional scheduling policy known as

SCHED DEADLINE 

SCHED_DEADLINE is a CPU scheduler available in the  Linux kernel since version 3.14,
Linux Weekly News, Deadline scheduling for Lin ...

, implementing the earliest deadline first scheduling, earliest deadline first algorithm (EDF), was added in kernel version 3.14, released on 30 March 2014. SCHED_DEADLINE takes precedence over all the other scheduling classes. Real-time PREEMPT_RT patches, included into the mainline Linux since version 2.6, provide a deterministic scheduler, the removal of preemption and interrupts disabling (where possible), PI Mutexes (i.e., locking primitives that avoid priority inversion), support for high precision event timers (

HPET The High Precision Event Timer (HPET) is a hardware timer available in modern x86-compatible personal computers. Compared to older types of timers available in the x86 architecture, HPET allows more efficient processing of highly timing-sensitiv ...

), preemptive Read-copy-update, (forced) IRQ threads, and other minor features.

Concurrency and synchronization

The kernel has different causes of concurrency (e.g., interrupts, bottom halves, preemption of kernel and users tasks, symmetrical multiprocessing). For protecting critical regions (sections of code that must be executed atomically), shared memory locations (like

global variable In computer programming, a global variable is a variable with global scope, meaning that it is visible (hence accessible) throughout the program, unless shadowed. The set of all global variables is known as the ''global environment'' or ''global s ...

s and other data structures with global scope), and regions of memory that are asynchronously modifiable by hardware (e.g., having the C volatile

type qualifier In the C, C++, and D programming languages, a type qualifier is a keyword that is applied to a type, resulting in a ''qualified type.'' For example, const int is a qualified type representing a constant integer, while int is the corresponding u ...

), Linux provides a large set of tools. They consist of atomic types (which can only be manipulated by a set of specific operators),

spinlock In software engineering, a spinlock is a lock that causes a thread trying to acquire it to simply wait in a loop ("spin") while repeatedly checking whether the lock is available. Since the thread remains active but is not performing a useful task, ...

s, semaphores, mutexes, and lockless algorithms (e.g., RCUs). Most lock-less algorithms are built on top of

memory barrier In computing, a memory barrier, also known as a membar, memory fence or fence instruction, is a type of barrier instruction that causes a central processing unit (CPU) or compiler to enforce an ordering constraint on memory operations issued b ...

s for the purpose of enforcing

memory ordering Memory ordering describes the order of accesses to computer memory by a CPU. The term can refer either to the memory ordering generated by the compiler during compile time, or to the memory ordering generated by a CPU during runtime. In modern mic ...

and prevent undesired side effects due to compiler optimization. PREEMPT_RT code included in mainline Linux provide ''RT-mutexes'', a special kind of Mutex which do not disable preemption and have support for priority inheritance. Almost all locks are changed into sleeping locks when using configuration for realtime operation.

Priority inheritance In real-time computing, priority inheritance is a method for eliminating unbounded priority inversion. Using this programming method, a process scheduling algorithm increases the priority of a process (A) to the maximum priority of any other proce ...

avoids priority inversion by granting a low-priority task which holds a contended lock the priority of a higher-priority waiter until that lock is released. Linux includes a kernel lock validator called ''Lockdep''.

Interrupts management

The management of the

interrupt In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to ''interrupt'' currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted, ...

s, although it could be seen as a single job, is divided in two separate parts. This split in two is due to the different time constraints and to the synchronization needs of the tasks whose the management is composed of. The first part is made up of an asynchronous

interrupt service routine In computer systems programming, an interrupt handler, also known as an interrupt service routine or ISR, is a special block of code associated with a specific interrupt condition. Interrupt handlers are initiated by hardware interrupts, softwar ...

that in Linux is known as the ''top half'', while the second part is carried out by one of three types of the so-called ''bottom halves'' (''softirq'', ''tasklets,'' and ''work queues''). Linux interrupts service routines can be nested (i.e., a new IRQ can trap into a high priority ISR that preempts any other lower priority ISRs).

Memory management

Memory management in Linux is a complex topic. First of all, the kernel is not pageable (i.e., it is always resident in physical memory and cannot be swapped to the disk). In the kernel there is no memory protection (no ''SIGSEGV'' signals, unlike in userspace), therefore memory violations lead to instability and system crashes. Linux implements

with 4 and 5-levels

page table A page table is the data structure used by a virtual memory system in a computer operating system to store the mapping between virtual addresses and physical addresses. Virtual addresses are used by the program executed by the accessing proces ...

s. As said, only user memory space is always pageable. It maintains information about each

page frame A page, memory page, or virtual page is a fixed-length contiguous block of virtual memory, described by a single entry in the page table. It is the smallest unit of data for memory management in a virtual memory operating system. Similarly, a ...

RAM Ram, ram, or RAM may refer to: Animals * A male sheep * Ram cichlid, a freshwater tropical fish People * Ram (given name) * Ram (surname) * Ram (director) (Ramsubramaniam), an Indian Tamil film director * RAM (musician) (born 1974), Dutch * ...

in apposite data structures (of type ) that are populated immediately after boots and that are kept until shutdown, regardless of them being or not associated with virtual pages. Furthermore, it classifies all page frames in zones, according to their architecture dependent constraints and intended use. For example, pages reserved for DMA operations are in ZONE_DMA, pages that are not permanently mapped to virtual addresses are in ZONE_HIGHMEM (in x86_32 architecture this zone is for physical addresses above 896 MB, while x86_64 does not need it because x86_64 can permanently map physical pages that reside in higher addresses), and all that remains (with the exception of other less used classifications) is in ZONE_NORMAL. Small chunks of memory can be dynamically allocated via the family of kmalloc() API and freed with the appropriate variant of kfree(). vmalloc() and kvfree() are used for large virtually contiguous chunks. alloc_pages() allocates the desired number of entire pages. The Linux Storage Stack Diagram

Kernel includes SLAB, SLUB and SLOB allocators as configurable alternatives. SLUB is the newest and it is also the default allocator. It aims for simplicity and efficiency. SLUB has been made PREEMPT_RT compatible.

Supported architectures

While not originally designed to be

portable Portable may refer to: General * Portable building, a manufactured structure that is built off site and moved in upon completion of site and utility work * Portable classroom, a temporary building installed on the grounds of a school to provide ...

, Linux is now one of the most widely ported operating system kernels, running on a diverse range of systems from the ARM architecture to IBM

z/Architecture z/Architecture, initially and briefly called ESA Modal Extensions (ESAME), is IBM's 64-bit complex instruction set computer (CISC) instruction set architecture, implemented by its mainframe computers. IBM introduced its first z/Architecture ...

mainframe computer A mainframe computer, informally called a mainframe or big iron, is a computer used primarily by large organizations for critical applications like bulk data processing for tasks such as censuses, industry and consumer statistics, enterpris ...

s. The first port was performed on the Motorola 68000 platform. The modifications to the kernel were so fundamental that Torvalds viewed the Motorola version as a

fork In cutlery or kitchenware, a fork (from la, furca 'pitchfork') is a utensil, now usually made of metal, whose long handle terminates in a head that branches into several narrow and often slightly curved tine (structural), tines with which one ...

and a "Linux-like operating system". However, that moved Torvalds to lead a major restructure of the code to facilitate porting to more computing architectures. The first Linux that, in a single source tree, had code for more than i386 alone, supported the DEC

Alpha AXP Alpha (original name Alpha AXP) is a 64-bit reduced instruction set computer (RISC) instruction set architecture (ISA) developed by Digital Equipment Corporation (DEC). Alpha was designed to replace 32-bit VAX complex instruction set computers ( ...

64-bit platform. Linux runs as the main operating system on IBM's

Summit A summit is a point on a surface that is higher in elevation than all points immediately adjacent to it. The topography, topographic terms acme, apex, peak (mountain peak), and zenith are synonymous. The term (mountain top) is generally used ...

; , all of the world's 500 fastest supercomputers run some operating system based on the Linux kernel, a big change from 1998 when the first Linux supercomputer got added to the list. Linux has also been ported to various handheld devices such as Apple's iPhone 3G and iPod.

Supported devices

In 2007, the LKDDb project has been started to build a comprehensive database of hardware and protocols known by Linux kernels. The database is built automatically by static analysis of the kernel sources. Later in 2014, the Linux Hardware project was launched to automatically collect a database of all tested hardware configurations with the help of users of various Linux distributions.

Live patching

Rebootless updates can even be applied to the kernel by using

live patching A patch is a set of changes to a computer program or its supporting data designed to update, fix, or improve it. This includes fixing security vulnerabilities and other bugs, with such patches usually being called bugfixes or bug fixes. Patche ...

technologies such as

Ksplice Ksplice is an open-source extension of the Linux kernel that allows security patches to be applied to a running kernel without the need for reboots, avoiding downtimes and improving availability (a technique broadly referred to as dynamic softw ...

kpatch kpatch is a feature of the Linux kernel that implements live patching of a running kernel, which allows kernel patches to be applied while the kernel is still running. By avoiding the need for rebooting the system with a new kernel that cont ...

and

kGraft kGraft is a feature of the Linux kernel that implements live patching of a running kernel, which allows kernel patches to be applied while the kernel is still running. By avoiding the need for rebooting the system with a new kernel that cont ...

. Minimalistic foundations for live kernel patching were merged into the Linux kernel mainline in kernel version 4.0, which was released on 12 April 2015. Those foundations, known as ''livepatch'' and based primarily on the kernel's

ftrace ftrace (Function Tracer) is a tracing framework for the Linux kernel. Although its original name, Function Tracer, came from ftrace's ability to record information related to various function calls performed while the kernel is running, ftr ...

functionality, form a common core capable of supporting hot patching by both kGraft and kpatch, by providing an

application programming interface An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how t ...

(API) for kernel modules that contain hot patches and an

(ABI) for the userspace management utilities. However, the common core included into Linux kernel 4.0 supports only the

x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was intr ...

architecture and does not provide any mechanisms for ensuring

function Function or functionality may refer to: Computing * Function key, a type of key on computer keyboards * Function model, a structured representation of processes in a system * Function object or functor or functionoid, a concept of object-oriente ...

-level consistency while the hot patches are applied. , there is ongoing work on porting kpatch and kGraft to the common live patching core provided by the Linux kernel mainline.

Security

Kernel bugs present potential security issues. For example, they may allow for

privilege escalation Privilege escalation is the act of exploiting a bug, a design flaw, or a configuration oversight in an operating system or software application to gain elevated access to resources that are normally protected from an application or user. The re ...

or create

denial-of-service attack In computing, a denial-of-service attack (DoS attack) is a cyber-attack in which the perpetrator seeks to make a machine or network resource unavailable to its intended users by temporarily or indefinitely disrupting services of a host conn ...

vectors. Over the years, numerous bugs affecting system security were found and fixed. New features are frequently implemented to improve the kernel's security. Capabilities(7) have already been introduced in the section about the processes and threads. Android makes use of them and systemd gives administrators detailed control over the capabilities of processes. Linux offers a wealth of mechanisms to reduce kernel attack surface and improve security which are collectively known as the

Linux Security Modules Linux Security Modules (LSM) is a framework allowing the Linux kernel to support without bias a variety of computer security models. LSM is licensed under the terms of the GNU General Public License and is a standard part of the Linux kernel sinc ...

(LSM). They comprise the

Security-Enhanced Linux Security-Enhanced Linux (SELinux) is a Linux kernel security module that provides a mechanism for supporting access control security policies, including mandatory access controls (MAC). SELinux is a set of kernel modifications and user-space ...

(SELinux) module, whose code has been originally developed and then released to the public by the

NSA The National Security Agency (NSA) is a national-level intelligence agency of the United States Department of Defense, under the authority of the Director of National Intelligence (DNI). The NSA is responsible for global monitoring, collecti ...

, and

AppArmor AppArmor ("Application Armor") is a Linux kernel security module that allows the system administrator to restrict programs' capabilities with per-program profiles. Profiles can allow capabilities like network access, raw socket access, and the ...

among others. SELinux is now actively developed and maintained on

GitHub GitHub, Inc. () is an Internet hosting service for software development and version control using Git. It provides the distributed version control of Git plus access control, bug tracking, software feature requests, task management, continuous ...

. SELinux and AppArmor provide support to access control security policies, including

(MAC), though they profoundly differ in complexity and scope. Another security feature is the Seccomp BPF (SECure COMPuting with Berkeley Packet Filters) which works by filtering parameters and reducing the set of system calls available to user-land applications. Critics have accused kernel developers of covering up security flaws, or at least not announcing them; in 2008, Linus Torvalds responded to this with the following: Linux distributions typically release security updates to fix vulnerabilities in the Linux kernel. Many offer

long-term support Long-term support (LTS) is a product lifecycle management policy in which a stable release of computer software is maintained for a longer period of time than the standard edition. The term is typically reserved for open-source software, where i ...

releases that receive security updates for a certain Linux kernel version for an extended period of time.

Development

Developer community

The community of Linux kernel developers comprises about 5000–6000 members. According to the "2017 State of Linux Kernel Development", a study issued by the Linux Foundation, covering the commits for the releases 4.8 to 4.13, about 1500 developers were contributing from about 200-250 companies on average. The top 30 developers contributed a little more than 16% of the code. For companies, the top contributors are Intel (13.1%) and Red Hat (7.2%), Linaro (5.6%), IBM (4.1%), the second and fifth places are held by the 'none' (8.2%) and 'unknown' (4.1%) categories. As with many large open-source software projects, developers are required to adhere to the

Contributor Covenant The Contributor Covenant is a code of conduct for contributors to Free and open-source software, free/open source software projects, created by Coraline Ada Ehmke. Its stated purpose is to reduce harassment of minority, LGBT and otherwise underrepr ...

, a

code of conduct A code of conduct is a set of rules outlining the norms, rules, and responsibilities or proper practices of an individual party or an organization. Companies' codes of conduct A company code of conduct is a set of rules which is commonly writt ...

intended to address harassment of minority contributors. Additionally, to prevent offense the use of inclusive terminology within the source code is mandated.

Source code management

The Linux development community uses

to manage the

source code In computing, source code, or simply code, is any collection of code, with or without comments, written using a human-readable programming language, usually as plain text. The source code of a program is specially designed to facilitate the w ...

. Git users clone the latest version of Torvalds' tree with and keep it up to date using . Contributions are submitted as patches, in the form of text messages on the

LKML The Linux kernel mailing list (LKML) is the main electronic mailing list for Linux kernel development, where the majority of the announcements, discussions, debates, and flame wars over the kernel take place. Many other mailing lists exist to ...

(and often also on other mailing lists dedicated to particular subsystems). The patches must conform to a set of rules and to a formal language that, among other things, describes which lines of code are to be deleted and what others are to be added to the specified files. These patches can be automatically processed so that system administrators can apply them in order to make just some changes to the code or to incrementally upgrade to the next version. Linux is distributed also in

GNU zip gzip is a file format and a software application used for file compression and decompression. The program was created by Jean-loup Gailly and Mark Adler as a free software replacement for the compress program used in early Unix systems, and i ...

(gzip) and bzip2 formats.

Submitting code to the kernel

A developer who wants to change the Linux kernel starts with developing and testing that change. Depending on how significant the change is and how many subsystems it modifies, the change will either be submitted as a single patch or in multiple patches of

. In case of a single subsystem that is maintained by a single maintainer, these patches are sent as e-mails to the maintainer of the subsystem with the appropriate mailing list in Cc. The maintainer and the readers of the mailing list will review the patches and provide feedback. Once the review process has finished the subsystem maintainer accepts the patches in the relevant

kernel tree. If the changes to the Linux kernel are bug fixes that are considered important enough, a pull request for the patches will be sent to Torvalds within a few days. Otherwise, a pull request will be sent to Torvalds during the next merge window. The merge window usually lasts two weeks and starts immediately after the release of the previous kernel version. The Git kernel source tree names all developers who have contributed to the Linux kernel in the ''Credits'' directory and all subsystem maintainers are listed in ''Maintainers''.

Programming language and coding style

Linux is written in a special C programming language supported by GCC, a compiler that extends in many ways the C standard, for example using inline sections of code written in the assembly language (in GCC's "AT&T-style" syntax) of the target architecture. Since 2002 all the code must adhere to the 21 rules comprising the ''Linux Kernel Coding Style.''

GNU toolchain

The

GNU Compiler Collection The GNU Compiler Collection (GCC) is an optimizing compiler produced by the GNU Project supporting various programming languages, hardware architectures and operating systems. The Free Software Foundation (FSF) distributes GCC as free software ...

(GCC or GNU cc) is the default compiler for the mainline Linux sources and it is invoked by a utility called Make (Unix), make. Then, the GNU Assembler (more often called GAS or GNU as) outputs the object files from the GCC generated Assembly language, assembly code. Finally, the GNU linker, GNU Linker (GNU ld) is used to produce a statically linked executable kernel file called . Both and are part of GNU Binary Utilities (binutils). The above-mentioned tools are collectively known as the GNU toolchain.

Compiler compatibility

GCC was for a long time the only compiler capable of correctly building Linux. In 2004, Intel claimed to have modified the kernel so that Intel C++ Compiler, its C compiler was also capable of compiling it. There was another such reported success in 2009, with a modified 2.6.22 version. Since 2010, effort has been underway to build Linux with Clang, an alternative compiler for the C language; as of 12 April 2014, the official kernel could almost be compiled by Clang. The project dedicated to this effort is named ''LLVMLinux'' after the LLVM compiler infrastructure upon which Clang is built. LLVMLinux does not aim to fork either Linux or the LLVM, therefore it is a meta-project composed of patches that are eventually submitted to the upstream projects. By enabling Linux to be compiled by Clang, developers may benefit from shorter compilation times. In 2017, developers completed upstreaming patches to support building the Linux kernel with Clang in the 4.15 release, having Backporting, backported support for X86-64 and AArch64 to the 4.4, 4.9, and 4.14 branches of the stable kernel tree. Google's Pixel 2 shipped with the first Clang built

kernel, though patches for Pixel (1st generation) did exist. 2018 saw ChromeOS move to building kernels with Clang by default, while Android (operating system) made Clang and LLVM's linker LLD required for kernel builds in 2019. Google moved its production kernel used throughout its datacenters to being built with Clang in 2020. Today, the
ClangBuiltLinux
' group coordinates fixes to both

and LLVM to ensure compatibility, both composed of members from ''LLVMLinux'' and having upstreamed patches from ''LLVMLinux''.

Kernel debugging

Bugs involving the Linux Kernel can be difficult to troubleshoot. This is because of the kernel's interaction with userspace and hardware; and also because they might be caused from a wider range of reasons compared to those of user programs. A few examples of the underlying causes are semantic errors in code, misuse of synchronization primitives, and incorrect hardware management. A report of a non-fatal bug in the kernel is called an "Linux kernel oops, oops"; such deviations from correct behavior of the Linux kernel may allow continued operation with compromised reliability. A critical and fatal error is reported via the function. It prints a message and then halts the kernel. One of the most common techniques used to find out bugs in code is ''debugging by printing''. For this purpose Linux provides an in-kernel API called which stores messages in a circular buffer. The system call is used for reading and/or clearing the kernel message ring buffer and for setting the maximum ''log level'' of the messages to be sent to the console (i.e., one of the eight parameters of , which tell the severity of the condition reported); usually it is invoked via the glibC wrapper . Kernel messages are also exported to userland through the ''/dev/kmsg'' interface (e.g., Systemd, systemd-journald reads that interface and by default append the messages to ). Another fundamental technique for debugging a running kernel is tracing. The ''

'' mechanism is a Linux internal tracer; it is used for monitoring and debugging Linux at runtime and it can also analyze user space latencies due to kernel misbehavior. Furthermore, ''ftrace'' allows users to trace Linux at boot-time. ''kprobes'' and ''kretprobes'' can break (like debuggers in userspace) into Linux and non-disruptively collect information. ''kprobes'' can be inserted into code at (almost) any address, while kretprobes work at function return. ''uprobes'' have similar purposes but they also have some differences in usage and implementation. With KGDB Linux can be debugged in much the same way as userspace programs. KGDB requires an additional machine that runs GNU Debugger#Remote debugging, GDB and that is connected to the target to be debugged using a serial cable or Ethernet.

Development model

The Linux kernel project integrates new code on a rolling basis. Software checked into the project must work and compiling, compile without error. Each kernel subsystem is assigned a maintainer who is responsible for reviewing patches against the kernel code standards and keeps a queue of patches that can be submitted to Linus Torvalds within a merge window of several weeks. Patches are merged by Torvalds into the source code of the prior stable Linux kernel release, creating the ''-rc'' release candidate for the next stable kernel. Once the merge window is closed only fixes to the new code in the development release are accepted. The ''-rc'' development release of the kernel goes through regression tests and once it is judged to be stable by Torvalds and the kernel subsystem maintainers a new Linux kernel is released and the development process starts all over again. Developers who feel treated unfairly can report this to the Linux Foundation's Technical Advisory Board. In July 2013, the maintainer of the USB 3.0 driver Sage Sharp asked Torvalds to address the abusive commentary in the kernel development community. In 2014, Sharp backed out of Linux kernel development, saying that "The focus on technical excellence, in combination with overloaded maintainers, and people with different cultural and social norms, means that Linux kernel maintainers are often blunt, rude, or brutal to get their job done". At the linux.conf.au (LCA) conference in 2018, developers expressed the view that the culture of the community has gotten much better in the past few years. Daniel Vetter, the maintainer of the Intel drm/i915 graphics kernel driver, commented that the "rather violent language and discussion" in the kernel community has decreased or disappeared. Laurent Pinchart asked developers for feedback on their experience with the kernel community at the 2017 Embedded Linux Conference Europe. The issues brought up were discussed a few days later at the Maintainers Summit. Concerns over the lack of consistency in how maintainers responded to patches submitted by developers were echoed by Shuah Khan, the maintainer of the kernel self-test framework. Torvalds contended that there would never be consistency in the handling of patches because different kernel subsystems have, over time, adopted different development processes. Therefore, it was agreed upon that each kernel subsystem maintainer would document the rules for patch acceptance.

Mainline Linux

The

tree of

that contains the Linux kernel is referred to as mainline Linux. Every stable kernel release originates from the mainline tree, and is frequently published on

. Mainline Linux has only solid support for a small subset of the many devices that run Linux. Non-mainline support is provided by independent projects, such as Yocto Project, Yocto or Linaro, but in many cases the kernel from the device vendor is needed. Using a vendor kernel likely requires a board support package. Maintaining a kernel tree outside of mainline Linux has proven to be difficult. ''Mainlining'' refers to the effort of adding support for a device to the mainline kernel, while there was formerly only support in a fork or no support at all. This usually includes adding drivers or device tree files. When this is finished, the feature or security fix is considered ''mainlined''.

Linux-like kernel

The maintainer of the stable branch, Greg Kroah-Hartman, has applied the term ''Linux-like'' to Downstream (software development), downstream kernel forks by vendors that add millions of lines of code to the mainline kernel. In 2019, Google stated that they wanted to use the mainline Linux kernel in Android so the number of kernel forks would be reduced. The term Linux-like has also been applied to the Embeddable Linux Kernel Subset, which does not include the full mainline Linux kernel but a small modified subset of the code.

Linux forks

There are certain communities that develop kernels based on the official Linux. Some interesting bits of code from these Fork (software development), forks that include Linux-libre, Compute Node Linux, INK (operating system), INK, L4Linux, RTLinux, and user-mode Linux, User-Mode Linux (UML) have been merged into the mainline. Some operating systems developed for mobile phones initially used heavily modified versions of Linux, including Google Android, Firefox OS, HP webOS, Nokia Maemo and Jolla Sailfish OS. In 2010, the Linux community criticised Google for effectively starting its own kernel tree: Today Android uses a customized Linux where major changes are implemented in device drivers, but some changes to the core kernel code is required. Android developers also submit patches to the official Linux that finally can boot the Android operating system. For example, a Nexus 7 can boot and run the mainline Linux. At a 2001 presentation at the Computer History Museum,

had this to say in response to a question about distributions of Linux using precisely the same kernel sources or not:

Development community conflicts

There have been several notable conflicts among Linux kernel developers. Examples of such conflicts are: * In July 2007, Con Kolivas announced that he would cease developing for the Linux kernel. * In July 2009, Alan Cox (computer programmer), Alan Cox quit his role as the Tty (Unix), TTY layer maintainer after disagreement with

. * In December 2010, there was a discussion between Linux SCSI maintainer James Bottomley and SCST maintainer Vladislav Bolkhovitin about which SCSI target stack should be included in the Linux kernel. This made some Linux users upset. * In June 2012, Torvalds made it very clear that he did not agree with NVIDIA releasing its drivers as closed. * In April 2014, Torvalds banned Kay Sievers from submitting patches to the Linux kernel for failing to deal with Software bug, bugs that caused systemd to negatively interact with the kernel. * In October 2014, Lennart Poettering accused Torvalds of tolerating the rough discussion style on Linux kernel related mailing lists and of being a bad role model. * In March 2015, Christoph Hellwig filed a lawsuit against VMware for infringement of the copyright on the Linux kernel. Linus Torvalds made it clear that he did not agree with this and similar initiatives by calling lawyers a festering disease. * In April 2021, a team from the University of Minnesota was found to be submitting "bad faith" patches to the kernel as part of their research. This resulted in the immediate reversion of all patches ever submitted by a member of the university. In addition, a warning was issued by a senior maintainer that any future patch from the university would be rejected on sight. Prominent Linux kernel developers have been aware of the importance of avoiding conflicts between developers. For a long time there was no code of conduct for kernel developers due to opposition by

. However, a Linux Kernel ''Code of Conflict'' was introduced on 8 March 2015. It was replaced on 16 September 2018 by a new ''Code of Conduct'' based on the

. This coincided with a public apology by Torvalds and a brief break from kernel development. On 30 November 2018, complying with the ''Code of Conduct'', Jarkko Sakkinen of Intel sent out patches replacing instances of "fuck" appearing in source code comments with suitable versions focused on the word 'hug'.

Codebase

, the 5.11 release of the Linux kernel had around 30.34 million lines of code. Roughly 14% of the code is part of the "core" (arch, kernel and mm directories), while 60% is drivers.

Estimated cost to redevelop

The cost to redevelop version 2.6.0 of the Linux kernel in a traditional proprietary development setting has been estimated to be US$612 million (€467M, £394M) in 2004 prices using the COCOMO person-month estimation model. In 2006, a study funded by the European Union put the redevelopment cost of kernel version 2.6.8 higher, at €882M ($1.14bn, £744M). This topic was revisited in October 2008 by Amanda McPherson, Brian Proffitt, and Ron Hale-Evans. Using David A. Wheeler's methodology, they estimated redevelopment of the 2.6.25 kernel now costs $1.3bn (part of a total $10.8bn to redevelop Fedora 9). Again, Garcia-Garcia and Alonso de Magdaleno from University of Oviedo (Spain) estimate that the value annually added to kernel was about €100M between 2005 and 2007 and €225M in 2008, it would cost also more than €1bn (about $1.4bn as of February 2010) to develop in the European Union. , using then-current Source lines of code, LOC (lines of code) of a 2.6.x Linux kernel and wage numbers with David A. Wheeler's calculations it would cost approximately $3bn (about €2.2bn) to redevelop the Linux kernel as it keeps getting bigger. An updated calculation , using then-current 20,088,609 LOC (lines of code) for the 4.14.14 Linux kernel and the current US national average programmer salary of $75,506 show it would cost approximately $14,725,449,000 dollars (£11,191,341,000) to rewrite the existing code.

Maintenance and long-term support

The latest kernel version and older kernel versions are maintained separately. Most latest kernel releases were supervised by Linus Torvalds. The Linux kernel developer community maintains a stable kernel by applying fixes for software bugs that have been discovered during the development of the subsequent stable kernel. Therefore, www.kernel.org will always list two stable kernels. The next stable Linux kernel is now released only 8 to 12 weeks later. Therefore, the Linux kernel maintainers have designated some stable kernel releases as ''longterm'', these

Linux kernels are updated with bug fixes for two or more years. , there are six longterm Linux kernels: 5.15.23, 5.10.100, 5.4.179, 4.19.229, 4.14.266, and 4.9.301. The full list of releases is at Linux kernel version history.

Relation with Linux distributions

Most Linux users run a kernel supplied by their Linux distribution. Some distributions ship the "vanilla" or "stable" kernels. However, several Linux distribution vendors (such as Red Hat and Debian) maintain another set of Linux kernel branches which are integrated into their products. These are usually updated at a slower pace compared to the "vanilla" branch, and they usually include all fixes from the relevant "stable" branch, but at the same time they can also add support for drivers or features which had not been released in the "vanilla" version the distribution vendor started basing their branch from.

Legal aspects

Licensing terms

Initially, Torvalds released Linux under a license which forbade any commercial use. This was changed in version 0.12 by a switch to the

version 2 (GPLv2). This license allows distribution and sale of possibly modified and unmodified versions of Linux but requires that all those copies be released under the same license and be accompanied by - or that, on request, free access is given to - the complete corresponding source code. Torvalds has described licensing Linux under the GPLv2 as the "best thing I ever did". The Linux kernel is licensed explicitly under

version 2 only (GPL-2.0-only) with an explicit syscall exception (Linux-syscall-note), without offering the licensee the option to choose any later version, which is a common GPL extension. Contributed code must be available under GPL compatibility, GPL-compatible license. There was considerable debate about how easily the license could be changed to use later GPL versions (including version 3), and whether this change is even desirable. Torvalds himself specifically indicated upon the release of version 2.4.0 that his own code is released only under version 2. However, the terms of the GPL state that if no version is specified, then any version may be used, and Alan Cox (computer programmer), Alan Cox pointed out that very few other Linux contributors had specified a particular version of the GPL. In September 2006, a survey of 29 key kernel programmers indicated that 28 preferred GPLv2 to the then-current GPLv3 draft. Torvalds commented, "I think a number of outsiders... believed that I personally was just the odd man out because I've been so publicly not a huge fan of the GPLv3." This group of high-profile kernel developers, including Torvalds, Greg Kroah-Hartman and Andrew Morton (computer programmer), Andrew Morton, commented on mass media about their objections to the GPLv3. They referred to clauses regarding Digital rights management, DRM/tivoization, patents, "additional restrictions" and warned a Balkanization, Balkanisation of the "Open Source Universe" by the GPLv3. Linus Torvalds, who decided not to adopt the GPLv3 for the Linux kernel, reiterated his criticism even years later.

Loadable kernel modules

It is debated whether some loadable kernel modules (LKMs) are to be considered derivative works under copyright law, and thereby whether or not they fall under the terms of the GPL. In accordance with the license rules, LKMs using only a public subset of the kernel interfaces are non-derived works, thus Linux gives system administrators the mechanisms to load out-of-tree binary objects into the kernel address space. There are some out-of-tree loadable modules that make legitimate use of the ''dma_buf'' kernel feature. GPL compliant code can certainly use it. However, a different possible use case would be Nvidia Optimus that pairs a fast GPU with an Intel integrated GPU, where the Nvidia GPU writes into the Intel framebuffer when it is active. But, Nvidia cannot use this infrastructure because it necessitates bypassing a rule that can only be used by LKMs that are also GPL. Alan Cox (computer programmer), Alan Cox replied on Linux kernel mailing list, LKML, rejecting a request from one of their engineers to remove this technical enforcement from the API. Torvalds clearly stated on the LKML that "[I] claim that binary-only kernel modules ARE derivative "by default"'". On the other hand, Torvalds has also said that "[one] gray area in particular is something like a driver that was originally written for another operating system (i.e., clearly not a derived work of Linux in origin). THAT is a gray area, and _that_ is the area where I personally believe that some modules may be considered to not be derived works simply because they weren't designed for Linux and don't depend on any special Linux behaviour". proprietary software, Proprietary graphics drivers, in particular, are heavily discussed. Whenever proprietary modules are loaded into Linux, the kernel marks itself as being "tainted", and therefore bug reports from tainted kernels will often be ignored by developers.

Firmware binary blobs

The official kernel, that is the Linus git branch at the kernel.org repository, contains binary blobs released under the terms of the GNU GPLv2 license. Linux can also search filesystems to locate binary blobs, proprietary firmware, drivers, or other executable modules, then it can load and link them into kernel space. When it is needed (e.g., for accessing boot devices or for speed) firmware can be built-in to the kernel, this means building the firmware into

; however this is not always a viable option for technical or legal issues (e.g., it is not permitted to do this with firmware that is non-GPL compatible, although this is quite common nonetheless).

Trademark

Linux is a registered trademark of

in the United States, the European Union, and some other countries. A legal battle over the trademark began in 1996, when William Della Croce, a lawyer who was never involved in the development of Linux, started requesting licensing fees for the use of the word ''Linux''. After it was proven that the word was in common use long before Della Croce's claimed first use, the trademark was awarded to Torvalds.

Notes

References

External links

* ** [//kernel.org/doc/ Linux kernel documentation index] ** [//kernel.org/doc/man-pages/ Linux kernel man pages] *
Kernel bugzilla
an
regressions
for each recent kernel version
Kernel Newbies
a source of various kernel-related information
Kernel coverage at LWN.net
an authoritative source of kernel-related information
Bootlin's Elixir Cross Referencer
a Linux kernel source code cross-reference * {{DEFAULTSORT:Linux Kernel Linux kernel, Finnish inventions Free and open-source software Free software programmed in C Free system software Linus Torvalds Monolithic kernels Operating systems Software using the GPL license Unix variants