# Linux kernel

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Free Unix-like operating system kernel

Linux Kernel Tux, mascot of Linux, created by Larry Ewing[1] Linux kernel 6.1.0 kernel booting, under Debian Original author Linus Torvalds Developers Community contributors Linus Torvalds Release 0.02 (5 October 1991; 34 years ago (1991-10-05)) Stable release Regular: 7.1.1 / 19 June 2026[2] LTS: 6.18.21 / 2 April 2026[3] Preview release 7.1-rc7[4] / 7 June 2026 Written in C,[a] assembly, Rust[b] and others[9] Available in English License GPL-2.0-only with Linux-syscall-note[10][11][12][c] Website kernel.org Repository git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git

The **Linux kernel** is a [free and open-source](/source/Free_and_open-source_software)[16]: 4 [Unix-like](/source/Unix-like) [kernel](/source/Kernel_(operating_system)) that is used in many [computer systems](/source/Computer_system) worldwide. The kernel was created by [Linus Torvalds](/source/Linus_Torvalds) in 1991 and was soon adopted as the kernel for the [GNU](/source/GNU) [operating system](/source/Operating_system) (OS), which was created to be a [free](/source/Free_software) replacement for [Unix](/source/Unix). Since the late 1990s, it has been included in many [operating system distributions](/source/Linux_distribution), many of which are called [Linux](/source/Linux). One such Linux kernel operating system is [Android](/source/Android_(operating_system)), which is used in many mobile and embedded devices.

Most of the kernel code is written in [C](/source/C_(programming_language)) as supported by the [GNU Compiler Collection](/source/GNU_Compiler_Collection) (GCC), which has extensions beyond standard C.[16]: 18[17] The code also contains [assembly](/source/Assembly_language) code for architecture-specific logic such as optimizing memory use and task execution.[16]: 379–380 The kernel has a [modular](/source/Modular_programming) design such that modules can be integrated as [software components](/source/Software_component) – including dynamically loaded. The kernel is [monolithic](/source/Monolithic_kernel) in an architectural sense since the entire OS kernel runs in [kernel space](/source/Kernel_space).

Linux is provided under the [GNU General Public License](/source/GNU_General_Public_License) version 2, although it contains files under other [compatible licenses](/source/GPL_compatibility).[15]

## History

See also: [History of Linux](/source/History_of_Linux) and [Linux kernel version history](/source/Linux_kernel_version_history)

[Linus Torvalds](/source/Linus_Torvalds) at the LinuxCon Europe 2014

In 1991, Linus Torvalds was a [computer science](/source/Computer_science) student enrolled at the [University of Helsinki](/source/University_of_Helsinki). During his time there, he began to develop an operating system as a side-project inspired by UNIX, for a personal computer.[18] He started with a [task switcher](/source/Context_switch) in [Intel 80386 assembly language](/source/X86_assembly_language) and a [terminal driver](/source/Pseudo_terminal).[18] On 25 August 1991, Torvalds posted the following to *comp.os.minix*, a [newsgroup](/source/Usenet_newsgroup) on [Usenet](/source/Usenet):[19]

I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu) for 386(486) [AT](/source/IBM_Personal_Computer%2FAT) clones. This has been brewing since April, and is starting to get ready. I'd like any feedback on things people like/dislike in minix, as my OS resembles it somewhat (same physical layout of the file-system (due to practical reasons) among other things). I've currently ported [bash](/source/Bash_(Unix_shell))(1.08) and [gcc](/source/GNU_Compiler_Collection)(1.40), and things seem to work. This implies that I'll get something practical within a few months [...] Yes - it's free of any minix code, and it has a multi-threaded fs. It is NOT protable [*[sic](/source/Sic)*] (uses 386 task switching etc), and it probably never will support anything other than AT-harddisks, as that's all I have :-(.

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](/source/FUNET)). It was not even executable since its code still needed Minix to compile and test it.[20]

On 5 October 1991, Torvalds announced the first "official" version of Linux, version 0.02.[21][20]

[As] I mentioned a month ago, I'm working on a free version of a Minix-lookalike for AT-386 computers. It has finally reached the stage where it's even usable (though may not be depending on what you want), and I am willing to put out the sources for wider distribution. It is just version 0.02...but I've successfully run bash, gcc, gnu-make, gnu-sed, compress, etc. under it.

Linux grew rapidly as many developers, including the [MINIX](/source/MINIX) community, contributed to the project.[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*] At the time, the [GNU Project](/source/GNU_Project) had completed many components for its free UNIX replacement, [GNU](/source/GNU), but its kernel, the [GNU Hurd](/source/GNU_Hurd), was incomplete. The project adopted the Linux kernel for its OS.[22]

Torvalds labeled the kernel with major version 0 to indicate that it was not yet intended for general use.[23] Version 0.11, released in December 1991, was the first version to be [self-hosted](/source/Self-hosting_(compilers)); compiled on a computer running the Linux kernel.

When Torvalds released version 0.12 in January 1992, he adopted the [GNU General Public License](/source/GNU_General_Public_License) version 2 (GPLv2) over his previous self-drafted license, which had not permitted commercial redistribution. GPL took effect as of 1 February 1992.[24] In contrast to [Unix](/source/Unix), all [source files](/source/Source_files) of Linux are freely available, including [device drivers](/source/Device_drivers).[25]

The initial success of Linux was driven by programmers and testers across the world. Because Linux implemented Unix-like system call semantics and exposed the standard [POSIX](/source/POSIX) APIs through an implementation of the [C standard library](/source/C_standard_library), Linux could run software and applications that had been originally developed for Unix.[26]

The Linux kernel supports various hardware architectures, providing a common platform for software, including [proprietary software](/source/Proprietary_software).

On 19 January 1992, the first post to the new newsgroup *alt.os.linux* was submitted.[27] On 31 March 1992, the newsgroup was renamed *comp.os.linux*.[28]

The fact that Linux is a [monolithic kernel](/source/Monolithic_kernel) rather than a [microkernel](/source/Microkernel) was the topic of a debate between [Andrew S. Tanenbaum](/source/Andrew_S._Tanenbaum), the creator of MINIX, and Torvalds.[29] The [Tanenbaum–Torvalds debate](/source/Tanenbaum%E2%80%93Torvalds_debate) started in 1992 on the [Usenet](/source/Usenet) group *comp.os.minix* as a general discussion about kernel architectures.[30][31]

Version 0.96 released in May 1992 was the first capable of running the [X Window System](/source/X_Window_System).[32][33] In March 1994, Linux 1.0.0 was released with 176,250 lines of code.[34] As indicated by the version number, it was the first version considered suitable for a [production environment](/source/Deployment_environment).[23] In June 1996, after release 1.3, Torvalds decided that Linux had evolved enough to warrant a new major number, and so labeled the next release as version 2.0.0.[35][36] Significant features of 2.0 included [symmetric multiprocessing](/source/Symmetric_multiprocessing) (SMP), support for more processors types and support for selecting specific hardware targets and for enabling architecture-specific features and optimizations.[26] The *make *config* family of commands of *kbuild* enable and configure options for building ad hoc kernel executables ([vmlinux](/source/Vmlinux)) and loadable modules.[37][38]

Version 2.2, released on 20 January 1999,[39] improved locking granularity and SMP management, added [m68k](/source/M68k), [PowerPC](/source/PowerPC), [Sparc64](/source/SPARC), [Alpha](/source/DEC_Alpha), and other 64-bit platforms support.[40] Furthermore, it added new [file systems](/source/File_systems) including [Microsoft](/source/Microsoft)'s [NTFS](/source/NTFS) read-only capability.[40] In 1999, IBM published its patches to the Linux 2.2.13 code for the support of the [S/390](/source/IBM_System%2F390) architecture.[41]

Version 2.4.0, released on 4 January 2001,[42] contained support for [ISA](/source/Industry_Standard_Architecture) [Plug and Play](/source/Plug_and_Play), [USB](/source/Universal_Serial_Bus), and [PC Cards](/source/PC_Card). Linux 2.4 added support for the [Pentium 4](/source/Pentium_4) and [Itanium](/source/Itanium) (the latter introduced the [ia64](/source/IA-64) ISA that was jointly developed by Intel and Hewlett-Packard to supersede the older [PA-RISC](/source/PA-RISC)), and for the newer [64-bit MIPS](/source/MIPS_architecture) processor.[43] Development for 2.4.*x* changed a bit in that more features were made available throughout the series, including support for [Bluetooth](/source/Bluetooth), [Logical Volume Manager](/source/Logical_Volume_Manager_(Linux)) (LVM) version 1, [RAID](/source/RAID) support, [InterMezzo](/source/InterMezzo_(file_system)) and [ext3](/source/Ext3) file systems.

Version 2.6.0 was released on 17 December 2003.[44] The development for 2.6.*x* changed further towards including new features throughout the series. Among the changes that have been made in the 2.6 series are: integration of [μClinux](/source/%CE%9CClinux) into the mainline kernel sources, [PAE](/source/Physical_Address_Extension) support, support for several new lines of [CPUs](/source/Central_processing_unit), 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 architectures still limited to 215),[45] substantially increased the number of device types and the number of devices of each type, improved [64-bit](/source/64-bit) support, support for [file systems](/source/File_system) that support file sizes of up to 16 [terabytes](/source/Terabyte), in-kernel [preemption](/source/Preemption_(computing)), support for the [Native POSIX Thread Library](/source/Native_POSIX_Thread_Library) (NPTL), [User-mode Linux](/source/User-mode_Linux) integration into the mainline kernel sources, [SELinux](/source/Security-Enhanced_Linux) integration into the mainline kernel sources, [InfiniBand](/source/InfiniBand) support, and considerably more.

Starting with 2.6.x releases, the kernel supported a large number of file systems. Some were designed for Linux, like [ext3](/source/Ext3), [ext4](/source/Ext4), [FUSE](/source/Filesystem_in_Userspace), and [Btrfs](/source/Btrfs).[46] Others were native to other operating systems like [JFS](/source/JFS_(file_system)), [XFS](/source/XFS), Minix, [Xenix](/source/Xenix), [Irix](/source/IRIX), [Solaris](/source/Solaris_(operating_system)), [System V](/source/System_V), [Windows](/source/Microsoft_Windows), and [MS-DOS](/source/MS-DOS).[47]

Though development had not used a [version control system](/source/Version_control) thus far, in 2002, Linux developers adopted [BitKeeper](/source/BitKeeper), which was made freely available to them even though it was not [free software](/source/Free_software). In 2005, because of efforts to [reverse-engineer](/source/Reverse-engineer) it, the company that owned the software revoked its support of the Linux community. In response, Torvalds and others wrote [Git](/source/Git_(software)). The new system was written within weeks, and in two months the first official kernel made using it was released.[48]

In 2005 the *stable team* was formed as a response to the lack of a kernel tree where people could work on [bug fixes](/source/Software_bug), and it would keep updating *stable* versions.[49] 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.[50][51] Several subsystem maintainers also adopted the suffix *-next* for trees containing code that they mean to submit for inclusion in the next release cycle. As of January 2014[\[update\]](https://en.wikipedia.org/w/index.php?title=Linux_kernel&action=edit), the in-development version of Linux is held in an unstable branch named *linux-next*.[52]

The 20th anniversary of Linux was celebrated by Torvalds in July 2011 with the release of version 3.0.0.[35] 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.[53]

Version 3.0 was released on 22 July 2011.[54] 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."[55] 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](/source/I386) processors—specifically by not having to [emulate](/source/Emulator)[56] the [atomic](/source/Atomic_(computer_science)) [CMPXCHG](/source/X86_instruction_listings#Added_with_80486) instruction introduced with the [i486](/source/I486) to allow reliable [mutexes](/source/Mutex)—making the 3.7 kernel series the last one still supporting the original processor.[57][58] The same series unified support for the [ARM](/source/ARM_architecture) processor.[59]

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 simply to avoid large minor numbers.[54][60] Stable 3.x.y kernels were released until 3.19 in February 2015. Version 3.11, released on 2 September 2013,[61] added many new features such as new O_TMPFILE flag for [open(2)](https://man7.org/linux/man-pages/man2/open.2.html) to reduce temporary file vulnerabilities, experimental AMD [Radeon](/source/Radeon) dynamic power management, low-latency network polling, and [zswap](/source/Zswap) (compressed swap cache).[62]

In April 2015, Torvalds released kernel version 4.0.[35] 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.[63] Version 4.1 of Linux, released in June 2015, contains over 19.5 million lines of code contributed by almost 14,000 programmers.[64]

Linus Torvalds announced that kernel version 4.22 would instead be numbered 5.0 in March 2019, stating that "'5.0' doesn't mean anything more than that the 4.x numbers started getting big enough that I ran out of fingers and toes."[65] It featured many major additions such as support for the AMD Radeon [FreeSync](/source/FreeSync) and [NVIDIA](/source/Nvidia) Xavier display, fixes for [F2FS](/source/F2FS), [EXT4](/source/Ext4) and [XFS](/source/XFS), restored support for swap files on the [Btrfs](/source/Btrfs) [file system](/source/File_system) and continued work on the [Intel](/source/Intel) [Icelake](/source/Ice_Lake_(microprocessor)) Gen11 graphics and on the [NXP](/source/NXP_Semiconductors) [i.MX8](/source/I.MX) SoCs.[66][67] This release was noticeably larger than the rest, Torvalds mentioning that "The overall changes for all of the 5.0 release are much bigger."[65]

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.[68]

## Popularity

Main article: [Usage share of operating systems](/source/Usage_share_of_operating_systems)

According to the Stack Overflow's annual Developer Survey of 2019, more than 53% of all respondents have developed software for [Linux](/source/Linux) and about 27% for [Android](/source/Android_(operating_system)),[69] although only about 25% develop with Linux-based operating systems.[70]

Most websites run on [Linux-based operating systems](/source/Linux),[71][72] and all of the [world's 500 most powerful supercomputers](/source/TOP500) run on Linux.[73]

[Linux distributions](/source/Linux_distribution) bundle the kernel with [system software](/source/System_software) (e.g., the [GNU C Library](/source/GNU_C_Library), [systemd](/source/Systemd), and other [Unix](/source/Unix) [utilities](/source/Utility_software) and [daemons](/source/Daemon_(computing))) and a wide selection of [application software](/source/Application_software), but their [usage share](/source/Usage_share_of_desktop_operating_systems) in desktops is low in comparison to other operating systems.

[Android](/source/Android_(operating_system)), which runs on a modified Linux kernel, accounts for the majority of mobile device operating systems,[74][75][76] and is increasingly being used in [embedded devices](/source/Linux_on_embedded_systems), making it a significant driver of Linux adoption.[26]

## Value

Redevelopment costs of Linux kernel

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](/source/COCOMO) person-month estimation model.[77] 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).[78]

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).[79] 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.[80]

As of 7 March 2011[\[update\]](https://en.wikipedia.org/w/index.php?title=Linux_kernel&action=edit), using then-current [LOC](/source/Source_lines_of_code) (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 as of 26 September 2018[\[update\]](https://en.wikipedia.org/w/index.php?title=Linux_kernel&action=edit), 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 that it would cost approximately $14,725,449,000 (£11,191,341,000) to rewrite the existing code.[81]

## Distribution

Most who use Linux do so via a [Linux distribution](/source/Linux_distribution). Some distributions ship the vanilla or stable kernel. Several vendors (such as [Red Hat](/source/Red_Hat) and [Debian](/source/Debian)) maintain a customized source tree. These are usually updated at a slower pace than 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 that had not been released in the vanilla version the distribution vendor started basing its branch from.

## Developers

### Community

Million lines of codeKernel Version010203040501.0.02.2.02.5.04.06.17.1Million lines of codeGrowth of the Linux kernel by number of lines of code

Graph of the sizes of Linux Kernel versions in millions of lines of code.[82] View [source data](https://commons.wikimedia.org/wiki/Data:Growth_of_the_Linux_kernel_by_number_of_lines_of_code.tab).

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.[83]

"Instead of a roadmap, there are technical guidelines. Instead of a central resource allocation, there are persons and companies who all have a stake in the further development of the Linux kernel, quite independently from one another: People like Linus Torvalds and I don’t plan the kernel evolution. We don’t sit there and think up the roadmap for the next two years, then assign resources to the various new features. That's because we don’t have any resources. The resources are all owned by the various corporations who use and contribute to Linux, as well as by the various independent contributors out there. It's those people who own the resources who decide..."

— [Andrew Morton](/source/Andrew_Morton_(computer_programmer)), 2005

[Intel](/source/Intel)

*None*

[Red Hat](/source/Red_Hat)

[Linaro](/source/Linaro)

*Unknown*

[IBM](/source/IBM)

*Consultants*

[Samsung](/source/Samsung)

SUSE

Google

*nearly 500 other companies*

Corporate affiliation of contributions to the Linux kernel, 4.8–4.13[83]

### Conflict

Notable conflicts among Linux kernel developers:

- In July 2007, [Con Kolivas](/source/Con_Kolivas) announced that he would cease developing for the Linux kernel.[84][85]

- In July 2009, [Alan Cox](/source/Alan_Cox_(computer_programmer)) quit his role as the [TTY](/source/Tty_(Unix)) layer maintainer after disagreement with Torvalds.[86]

- 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.[87] This made some Linux users upset.[88]

- In June 2012, Torvalds made it very clear that he [did not agree](/source/The_finger) with [NVIDIA](/source/NVIDIA) releasing its drivers as closed.[89]

- In April 2014, Torvalds banned [Kay Sievers](/source/Kay_Sievers) from submitting patches to the Linux kernel for failing to deal with [bugs](/source/Software_bug) that caused [systemd](/source/Systemd) to negatively interact with the kernel.[90]

- In October 2014, [Lennart Poettering](/source/Lennart_Poettering) accused Torvalds of tolerating the rough discussion style on Linux kernel related mailing lists and of being a bad role model.[91]

- In March 2015, Christoph Hellwig filed a lawsuit against [VMware](/source/VMware) for infringement of the copyright on the Linux kernel.[92] Linus Torvalds made it clear that he did not agree with this and similar initiatives by calling lawyers a festering disease.[93]

- In April 2021, a team from the [University of Minnesota](/source/University_of_Minnesota) was found to be submitting "bad faith" patches to the kernel as part of its 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.[94][95]

- In February 2025, [Hector Martin](/source/Hector_Martin_(hacker)) resigned as maintainer of the [Asahi Linux](/source/Asahi_Linux) project (that added support for [Apple silicon](/source/Apple_silicon) systems to Linux) and related kernel subsystems over disagreements about the use of the [Rust](/source/Rust_(programming_language)) programming language in the kernel.[96]

- In June 2025, the [Bcachefs](/source/Bcachefs) filesystem was creating tension with Torvalds[97] and in September 2025 it was removed from the Linux kernel starting with version 6.18.[98]

Prominent Linux kernel developers have been aware of the importance of avoiding conflicts between developers.[99] For a long time there was no code of conduct for kernel developers due to opposition by Torvalds.[100] A Linux Kernel *Code of Conflict* was introduced on 8 March 2015.[101] It was replaced on 16 September 2018 by a new *Code of Conduct* based on the [Contributor Covenant](/source/Contributor_Covenant). This coincided with a public apology by Torvalds and a brief break from kernel development.[102][103] 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'.[104]

Developers who feel treated unfairly can report this to the [Linux Foundation](/source/Linux_Foundation) Technical Advisory Board.[105] In July 2013, the maintainer of the USB 3.0 driver [Sage Sharp](/source/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".[106] 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.[107]

Laurent Pinchart asked developers for feedback on their experiences 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](/source/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.[108]

## Development

Linux is evolution, not [intelligent design](/source/Intelligent_design)!

— Linus Torvalds, 2005[109][110][111]

### Codebase

The kernel source code, a.k.a. source tree, is managed in the [Git](/source/Git) [version control system](/source/Version_control_system) – also created by Torvalds.[112]

As of 2021[\[update\]](https://en.wikipedia.org/w/index.php?title=Linux_kernel&action=edit), 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," including architecture-specific code, kernel code, and memory management code, while 60% is drivers.

### Contributions

Contributions are submitted as patches, in the form of text messages on the [Linux kernel mailing list](/source/Linux_kernel_mailing_list) (LKML) (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.[113] Linux is distributed also in [GNU zip](/source/GNU_zip) (gzip) and [bzip2](/source/Bzip2) formats.

A developer who wants to change the Linux kernel writes and tests a code 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 [source code](/source/Source_code). 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 [Git](/source/Git) 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.[114] 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*.[115]

As with many large open-source software projects, developers are required to adhere to the [Contributor Covenant](/source/Contributor_Covenant), a [code of conduct](/source/Code_of_conduct) intended to address harassment of minority contributors.[116][117] Additionally, to prevent offense the use of [inclusive terminology](/source/Inclusive_language) within the source code is mandated.[118]

### Programming language

See also: [Rust for Linux](/source/Rust_for_Linux)

Linux is written in a special [C programming language](/source/C_(programming_language)) supported by [GCC](/source/GNU_Compiler_Collection), a compiler that extends the C standard in many ways, for example using [inline sections of code](/source/Inline_assembler) written in the [assembly language](/source/Assembly_language) (in GCC's "AT&T-style" syntax) of the target architecture.

In September 2021, the GCC version requirement for compiling and building the Linux kernel increased from GCC 4.9 to 5.1, allowing the potential for the kernel to be moved from using C code based on the [C89](/source/ANSI_C#C89) standard to using code written with the [C11](/source/C11_(C_standard_revision)) standard,[119] with the migration to the standard taking place in March 2022, with the release of Linux 5.18.[120]

Initial support for the [Rust](/source/Rust_(programming_language)) programming language was added in Linux 6.1,[121] which was released in December 2022,[122] with later kernel versions, such as Linux 6.2 and Linux 6.3, further improving the support.[123][124]

### Coding style

Since 2002, code must adhere to the 21 rules of the *Linux Kernel Coding Style.*[125][126]

### Versioning

As for most software, the kernel is versioned as a series of dot-separated numbers.

For early versions, the version consisted of three or four dot-separated numbers called *major release*, *minor release* and *revision.*[16]: 9 At that time, odd-numbered minor releases were for development and testing, while even numbered minor releases for production. The optional fourth digit indicated a patch level.[23] Development releases were indicated with a release candidate suffix (*-rc*).

The current versioning conventions are different. The odd/even number implying dev/prod has been dropped, and a major version is indicated by the first two numbers together. 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.[127] For example, the release of the version 4.16 was preceded by seven 4.16-rcN (from -rc1 to -rc7). Once a stable version is released, its maintenance is passed to the *stable team*. Updates to a stable release are identified by a three-number scheme (e.g., 4.16.1, 4.16.2, ...).[127]

### Toolchain

The kernel is usually built with the [GNU toolchain](/source/GNU_toolchain). The GNU C compiler, GNU cc, part of the [GNU Compiler Collection](/source/GNU_Compiler_Collection) (GCC), is the default [compiler](/source/Compiler) for mainline Linux. Sequencing is handled by GNU [make](/source/Make_(Unix)). The [GNU Assembler](/source/GNU_Assembler) (often called GAS or GNU as) outputs the [object files](/source/Object_file) from the GCC generated [assembly](/source/Assembly_language) code. Finally, the [GNU Linker](/source/GNU_linker) (GNU ld) produces a statically linked executable kernel file called [vmlinux](/source/Vmlinux). Both as and ld are part of [GNU Binary Utilities](/source/GNU_Binary_Utilities) (binutils).

GNU cc was for a long time the only compiler capable of correctly building Linux. In 2004, [Intel](/source/Intel) claimed to have modified the kernel so that [its C compiler](/source/Intel_C%2B%2B_Compiler) was also capable of compiling it.[128] There was another such reported success in 2009, with a modified 2.6.22 version.[129][130] Support for the Intel compiler has been dropped in 2023.[131]

Since 2010, effort has been underway to build Linux with [Clang](/source/Clang), an alternative compiler for the C language;[132] as of 12 April 2014, the official kernel could almost be compiled by Clang.[133][134] The project dedicated to this effort is named *LLVMLinux* after the [LLVM](/source/LLVM) compiler infrastructure upon which Clang is built.[135] 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.[136]

In 2017, developers completed upstreaming patches to support building the Linux kernel with [Clang](/source/Clang) in the 4.15 release, having [backported](/source/Backporting) support for [X86-64](/source/X86-64) and [AArch64](/source/AArch64) to the 4.4, 4.9, and 4.14 branches of the stable kernel tree. Google's [Pixel 2](/source/Pixel_2) shipped with the first [Clang](/source/Clang) built [Linux](/source/Linux) kernel,[137] though patches for [Pixel (1st generation)](/source/Pixel_(1st_generation)) did exist.[138] 2018 saw [ChromeOS](/source/ChromeOS) move to building kernels with [Clang](/source/Clang) by default,[139] while [Android](/source/Android_(operating_system)) made [Clang](/source/Clang)[140] and [LLVM](/source/LLVM)'s linker LLD[141] required for kernel builds in 2019. [Google](/source/Google) moved its production kernel used throughout its datacenters to being built with [Clang](/source/Clang) in 2020.[142] The *[ClangBuiltLinux](https://clangbuiltlinux.github.io/)* group coordinates fixes to both [Linux](/source/Linux) and [LLVM](/source/LLVM) to ensure compatibility, both composed of members from *LLVMLinux* and having upstreamed patches from *LLVMLinux*.

### Debugging

Main articles: [KGDB](/source/KGDB), [Kernel panic](/source/Kernel_panic), and [Linux kernel oops](/source/Linux_kernel_oops)

Linux kernel panic output

As with any software, problems with the Linux kernel can be difficult to [troubleshoot](/source/Troubleshoot). Common challenges relate to userspace vs. kernel space access, misuse of synchronization primitives, and incorrect hardware management.[16]: 364

An [oops](/source/Linux_kernel_oops) is a non-fatal error in the kernel. After such an error, operations continue with suspect reliability.[143]

A panic (generated by [panic()](/source/Kernel_panic)) is a fatal error. After such an error, the kernel prints a message and halts the computer.[16]: 371

The kernel provides for *debugging by printing* via [printk](/source/Printk)(), which stores messages in a circular buffer (overwriting older entries with newer). The syslog(2) system call provides for reading and clearing the message buffer and for setting the maximum *log level* of the messages to be sent to the console.[144] Kernel messages are also exported to userland through the */dev/kmsg* interface.[145]

The *[ftrace](/source/Ftrace)* mechanism allow for debugging by tracing. It is used for monitoring and debugging Linux at runtime and it can analyze user space latencies due to kernel misbehavior.[146][147][148][149] Furthermore, *ftrace* allows users to trace Linux at boot-time.[150]

*kprobes* and *kretprobes* can break into kernel execution (like debuggers in userspace) and collect information non-disruptively.[151] *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.[152]

With [KGDB](/source/KGDB) Linux can be debugged in much the same way as userspace programs. KGDB requires an additional machine that runs [GDB](/source/GNU_Debugger#Remote_debugging) and that is connected to the target to be debugged using a [serial cable](/source/Serial_cable) or [Ethernet](/source/Ethernet).[153]

### Change process

The Linux kernel project integrates new code on a rolling basis. Standard operating procedure is that software checked into the project must work and [compile](/source/Compiling) without error.

Each kernel subsystem is assigned a maintainer who is responsible for reviewing patches against the kernel code standards and keeping a queue of patches that can be submitted to Torvalds within a merge window that is usually several weeks.

Patches are merged by Torvalds into the source code of the prior stable Linux kernel release, creating the release candidate (-rc) for the next stable release. 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 testing](/source/Regression_tests) and once it is considered stable by Torvalds and the subsystem maintainers, a new version is released and the development process starts over again.[154]

### Mainline Linux

The Git tree that contains the Linux kernel source code is referred to as **mainline Linux**. Every stable kernel release originates from the mainline tree,[155] and is frequently published on [kernel.org](/source/Kernel.org). 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](/source/Yocto_Project) or [Linaro](/source/Linaro), but in many cases the kernel from the device vendor is needed.[156] Using a vendor kernel likely requires a [board support package](/source/Board_support_package).

Maintaining a kernel tree outside of mainline Linux has proven to be difficult.[157]

*Mainlining* refers to the effort of adding support for a device to the mainline kernel,[158] while there was formerly only support in a fork or no support at all. This usually includes adding drivers or [device tree](/source/Device_tree) files. When this is finished, the feature or security fix is considered *mainlined*.[159]

### Linux-like kernel

The maintainer of the stable branch, [Greg Kroah-Hartman](/source/Greg_Kroah-Hartman), has applied the term *Linux-like* to [downstream](/source/Downstream_(software_development)) kernel forks by vendors that add millions of lines of code to the mainline kernel.[160] In 2019, [Google](/source/Google) stated that it wanted to use the mainline Linux kernel in [Android](/source/Android_(operating_system)) so the number of kernel forks would be reduced.[161] The term Linux-like has also been applied to the [Embeddable Linux Kernel Subset](/source/Embeddable_Linux_Kernel_Subset), which does not include the full mainline Linux kernel but a small modified subset of the code.[162]

### Linux forks

An [iPod](/source/IPod) booting [iPodLinux](/source/IPodLinux)

There are certain communities that develop kernels based on the official Linux. Some interesting bits of code from these [forks](/source/Fork_(software_development)) that include [Linux-libre](/source/Linux-libre), [Compute Node Linux](/source/Compute_Node_Linux), [INK](/source/INK_(operating_system)), [L4Linux](/source/L4Linux), [RTLinux](/source/RTLinux), and [User-Mode Linux](/source/User-mode_Linux) (UML) have been merged into the mainline.[163] Some operating systems developed for mobile phones initially used heavily modified versions of Linux, including Google [Android](/source/Android_(operating_system)), [Firefox OS](/source/Firefox_OS), HP [webOS](/source/WebOS), Nokia [Maemo](/source/Maemo) and Jolla [Sailfish OS](/source/Sailfish_OS). In 2010, the Linux community criticised Google for effectively starting its own kernel tree:[164][165]

This means that any drivers written for Android hardware platforms, can not get merged into the main kernel tree because they have dependencies on code that only lives in Google's kernel tree, causing it to fail to build in the kernel.org tree. Because of this, Google has now prevented a large chunk of hardware drivers and platform code from ever getting merged into the main kernel tree. Effectively creating a kernel branch that a number of different vendors are now relying on.[166]

— [Greg Kroah-Hartman](/source/Greg_Kroah-Hartman), 2010

Today Android uses a customized Linux[167] 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](/source/Nexus_7) can boot and run the mainline Linux.[167]

At a 2001 presentation at the [Computer History Museum](/source/Computer_History_Museum), Torvalds had this to say in response to a question about distributions of Linux using precisely the same kernel sources or not:

They're not... well they are, and they're not. There is no single kernel. Every single distribution has their own changes. That's been going on since pretty much day one. I don't know if you may remember [Yggdrasil](/source/Yggdrasil_Linux%2FGNU%2FX) was known for having quite extreme changes to the kernel and even today all of the major vendors have their own tweaks because they have some portion of the market they're interested in and quite frankly that's how it should be. Because if everybody expects one person, me, to be able to track everything that's not the point of GPL. That's not the point of having an open system. So actually the fact that a distribution decides that something is so important to them that they will add patches for even when it's not in the standard kernel, that's a really good sign for me. So that's for example how something like [ReiserFS](/source/ReiserFS) got added. And the reason why ReiserFS is the first journaling filesystem that was integrated in the standard kernel was not because I love [Hans Reiser](/source/Hans_Reiser). It was because [SUSE](/source/SUSE_Linux_Enterprise) actually started shipping with ReiserFS as their standard kernel, which told me "ok." This is actually in production use. Normal People are doing this. They must know something I don't know. So in a very real sense what a lot of distribution houses do, they are part of this "let's make our own branch" and "let's make our changes to this." And because of the GPL, I can take the best portions of them.[168]

— [Linus Torvalds](/source/Linus_Torvalds), 2001

### Long-term support

Boot messages of a Linux kernel 2.6.25.17

The latest version and older versions are maintained separately. Most of the latest kernel releases were supervised by Torvalds.[169]

The Linux kernel developer community maintains a stable kernel by applying fixes for [software bugs](/source/Software_bug) that have been discovered during the development of the subsequent stable kernel. Therefore, www.kernel.org always lists two stable kernels. The next stable Linux kernel is released about 8 to 12 weeks later.

Some releases are [designated](/source/Linux_kernel_version_history) for [long-term support](/source/Long-term_support) as *longterm* with bug fix releases for two or more years.[170]

### Size

Some projects have attempted to reduce the size of the Linux kernel. One of them is [TinyLinux](/source/TinyLinux). In 2014, Josh Triplett started the -tiny source tree for a reduced size version.[171][172][173][174]

## Architecture and features

See also: [vmlinux](/source/Vmlinux)

Map of the Linux kernel

[Sankey diagram](/source/Sankey_diagram) of Linux Kernel Source Lines of Code

Even though seemingly contradictory, the Linux kernel is both monolithic and modular. The kernel is classified as a [monolithic kernel](/source/Monolithic_kernel) architecturally since the entire OS runs in kernel space. The design is modular since it can be assembled from [modules](/source/Loadable_kernel_module) that in some cases are loaded and unloaded at runtime.[16]: 338[175] It supports 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 of the [manual pages](/source/Man_page). The number that follows the name of a command, interface, or other feature specifies the section (i.e. the type of the OS' component or feature) it belongs to. For example execve(2) refers to a system call, and exec(3) refers to a userspace library wrapper.

The following is an overview of architectural design and of noteworthy features.

- [Concurrent computing](/source/Concurrent_computing) and (with the availability of enough CPU cores for tasks that are ready to run) even [true parallel execution](/source/Parallel_computing) of many [processes](/source/Process_(computing)) at once (each of them having one or more [threads of execution](/source/Thread_(computing))) on [SMP](/source/Symmetric_multiprocessing) and [NUMA](/source/Non-uniform_memory_access) architectures.

- Selection and configuration of hundreds of kernel features and drivers (using one of the make *config family of commands before building),[176][38][37] modification of kernel parameters before [boot](/source/Linux_boot) (usually by inserting instructions into the lines of a [bootloader](/source/Bootloader) menu or configuration file), and fine tuning of kernel behavior at run-time (using the sysctl(8) interface to /proc/sys/).[177][178][179]

- Configuration (again using the make *config commands) and run-time modifications of the policies[180] (via nice(2), setpriority(2), and the family of sched_*(2) syscalls) of the [task schedulers](/source/Scheduling_(computing)) that allow [preemptive multitasking](/source/Preemptive_multitasking) (both in [user mode](/source/User_mode) and, since the 2.6 series, in [kernel mode](/source/Kernel_mode)[181][182]); the [earliest eligible virtual deadline first scheduling](/source/Earliest_eligible_virtual_deadline_first_scheduling) (EEVDF) scheduler,[183] is the default scheduler of Linux since 2023 and it uses a [red-black tree](/source/Red%E2%80%93black_tree) that can search, insert and delete process information ([task struct](/source/Task_struct)) with [O(log n)](/source/Big_O_notation) [time complexity](/source/Time_complexity), where *n* is the number of runnable tasks.[184][185]

- Advanced [memory management](/source/Memory_management) with [paged virtual memory](/source/Virtual_memory) and a multi-generational least recently used (MGLRU) [page replacement algorithm](/source/Page_replacement_algorithm).

- [Inter-process communications](/source/Inter-process_communication) and [synchronization](/source/Synchronization_(computer_science)) mechanism.

- A [virtual filesystem](/source/Virtual_file_system) on top of several concrete filesystems ([ext4](/source/Ext4), [Btrfs](/source/Btrfs), [XFS](/source/XFS), [JFS](/source/JFS_(file_system)), [FAT32](/source/File_Allocation_Table), and many more).

- Configurable I/O schedulers, ioctl(2)[186] 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[187] (because they scale poorly with multithreaded applications, however, a family of Linux specific I/O system calls (io_*(2)[188]) had to be created for the management of asynchronous I/O contexts suitable for concurrent processing).

- [OS-level virtualization](/source/OS-level_virtualization) (with [Linux-VServer](/source/Linux-VServer)), [paravirtualization](/source/Paravirtualization) and [hardware-assisted virtualization](/source/Hardware-assisted_virtualization) (with [KVM](/source/Kernel-based_Virtual_Machine) or [Xen](/source/Xen), and using [QEMU](/source/QEMU) for hardware emulation);[189][190][191][192][193][194] On the Xen [hypervisor](/source/Hypervisor), 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*).[195]

- I/O Virtualization with [VFIO](/source/VFIO) and [SR-IOV](/source/SR-IOV). 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. With VFIO, however, 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).[196]

- Security mechanisms for [discretionary](/source/Discretionary_access_control) and [mandatory access control](/source/Mandatory_access_control) (SELinux, AppArmor, POSIX [ACLs](/source/Access-control_list), and others).[197][198]

- Several types of layered [communication protocols](/source/Communication_protocol) (including the [Internet protocol suite](/source/Internet_protocol_suite)).

- [Asymmetric multiprocessing](/source/Asymmetric_multiprocessing) via the [RPMsg](/source/RPMsg) subsystem.

Most [device drivers](/source/Device_driver) and kernel extensions run in [kernel space](/source/Kernel_space) ([ring 0](/source/Ring_(computer_security)) in many [CPU](/source/CPU) [architectures](/source/CPU_architecture)), with full access to the hardware. Some exceptions run in [user space](/source/User_space); notable examples are filesystems based on [FUSE](/source/Filesystem_in_Userspace)/CUSE, and parts of UIO.[199][200] Furthermore, the [X Window System](/source/X_Window_System) and [Wayland](/source/Wayland_(display_server_protocol)), 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](/source/Graphics_processing_unit) of [graphics cards](/source/Graphics_card) is an in-kernel subsystem called [Direct Rendering Manager](/source/Direct_Rendering_Manager) (DRM).

Unlike standard monolithic kernels, device drivers are easily configured as [modules](/source/Module_(Linux)), and loaded or unloaded while the system is running and can also be pre-empted under certain conditions in order to handle [hardware interrupts](/source/Hardware_interrupt) correctly and to better support [symmetric multiprocessing](/source/Symmetric_multiprocessing).[182] By choice, Linux has no stable device driver [application binary interface](/source/Application_binary_interface).[201]

Linux typically makes use of [memory protection](/source/Memory_protection) and [virtual memory](/source/Virtual_memory) and can also handle [non-uniform memory access](/source/Non-uniform_memory_access),[202] the project has absorbed [μClinux](/source/%CE%9CClinux), which also makes it possible to run Linux on [microcontrollers](/source/Microcontroller) without virtual memory.[203]

The hardware is represented in the file hierarchy. User applications interact with device drivers via entries in the [/dev](/source/Devfs) or [/sys](/source/%2Fsys) directories.[204] Process information is mapped into the [/proc](/source/Procfs) directory.[204]

Various layers within Linux, also showing separation between the userland and kernel space User mode User applications bash, LibreOffice, GIMP, Blender, 0 A.D., Mozilla Firefox, ... System components init daemon: OpenRC, runit, systemd, ... System daemons: polkitd, smbd, sshd, udevd, ... Windowing system: X11, Wayland, SurfaceFlinger (Android) Graphics: Mesa, AMD Software, ... Other libraries: GTK, Qt, EFL, SDL, SFML, FLTK, GNUstep, ... C standard library fopen, execv, malloc, memcpy, localtime, pthread_create, ... (up to 2000 subroutines) glibc aims to be fast, musl aims to be lightweight, uClibc targets embedded systems, bionic was written for Android, etc. All aim to be POSIX/SUS-compatible. Kernel mode Linux kernel stat, splice, dup, read, open, ioctl, write, mmap, close, exit, etc. (about 380 system calls) The Linux kernel System Call Interface (SCI), aims to be POSIX/SUS-compatible[205] Process scheduling subsystem IPC subsystem Memory management subsystem Virtual files subsystem Networking subsystem Other components: ALSA, DRI, evdev, klibc, LVM, device mapper, Linux Network Scheduler, Netfilter Linux Security Modules: SELinux, TOMOYO, AppArmor, Smack Hardware (CPU, main memory, data storage devices, etc.)

### Interfaces

Main pages: [Linux kernel interfaces](/source/Linux_kernel_interfaces) and [Interfaces of the Linux kernel (Category)](https://en.wikipedia.org/wiki/Category:Interfaces_of_the_Linux_kernel)

See also: [System call](/source/System_call), [POSIX](/source/POSIX), and [Single UNIX Specification](/source/Single_UNIX_Specification)

Four interfaces are distinguished: two internal to the kernel, and two between the kernel and userspace.

Linux started as a clone of UNIX, and aims toward [POSIX](/source/POSIX) and [Single UNIX Specification](/source/Single_UNIX_Specification) compliance.[206] The kernel 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.[10][15]

The Linux [application binary interface](/source/Application_binary_interface) (ABI) between the kernel and the user space has four degrees of stability (stable, testing, obsolete, removed);[207] The [system calls](/source/System_call) are expected to never change in order to preserve [compatibility](/source/Backward_compatibility) for [userspace](/source/Userspace) programs that rely on them.[208]

[Loadable kernel modules](/source/Loadable_kernel_module) (LKMs), by design, cannot rely on a stable ABI.[201] 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 ([vmlinux](/source/Vmlinux)) are statically linked by the build process.

There is no guarantee of stability of source-level in-kernel API[201] and, because of this, [device driver](/source/Device_driver) 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.[209]

#### Kernel-to-userspace API

The set of the [Linux kernel API](/source/Linux_kernel_API) that regards the interfaces exposed to user applications is fundamentally composed of UNIX and Linux-specific [system calls](/source/System_call).[210] A system call is an entry point into the Linux kernel.[211] For example, among the Linux-specific ones there is the family of the [clone(2)](/source/Clone_(Linux_system_call)) system calls.[212] Most extensions must be enabled by defining the _GNU_SOURCE [macro](/source/Macro_(computer_science)) in a [header file](/source/Header_files) or when the user-land code is being compiled.[213]

System calls can only be invoked via assembly instructions that enable the transition from unprivileged user space to privileged kernel space in [ring 0](/source/Ring_0_(computer_security)). For this reason, the [C standard library](/source/C_standard_library) (libC) acts as a wrapper to most Linux system calls, by exposing C functions that, if needed,[214] transparently enter the kernel, which will execute on behalf of the calling process.[210] For system calls not exposed by libC, such as the [fast userspace mutex](/source/Futex),[215] the library provides a function called syscall(2), which can be used to explicitly invoke them.[216]

[Pseudo filesystems](/source/Synthetic_file_system) (e.g., the [sysfs](/source/Sysfs) and [procfs](/source/Procfs) filesystems) and [special files](/source/Special_file) (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.[217][218]

#### Kernel-to-userspace ABI

Main article: [Linux Standard Base](/source/Linux_Standard_Base)

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](/source/Instruction_set_architecture) 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](/source/Linux_Standard_Base) (LSB).[219][220] 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.[221] The LSB version 5 is built upon several standards and drafts (POSIX, SUS, X/Open, [File System Hierarchy](/source/Filesystem_Hierarchy_Standard) (FHS), and others).[222]

The parts of the LSB more relevant to the kernel are the *General ABI* (gABI),[223] especially the [System V ABI](/source/System_V_Interface_Definition)[224][225] and the [Executable and Linking Format](/source/Executable_and_Linkable_Format) (ELF),[226][227] and the *Processor Specific ABI* (psABI), for example the *Core Specification for X86-64.*[228][229]

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.[230][231][232]

#### In-kernel API

At XDC2014, Alex Deucher from AMD announced the unified kernel-mode driver.[233] The proprietary Linux graphic driver, [libGL-fglrx-glx](/source/AMD_Catalyst), will share the same [DRM](/source/Direct_Rendering_Manager) infrastructure with [Mesa 3D](/source/Mesa_3D). As there is no stable in-kernel [ABI](/source/Application_binary_interface), AMD had to constantly adapt the former [binary blob](/source/Binary_blob) used by Catalyst.

There are several internal kernel APIs between kernel 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 to dynamically loadable modules (e.g., device drivers loaded on demand) whether they're exported with the EXPORT_SYMBOL() and EXPORT_SYMBOL_GPL() macros[234][235] (the latter reserved to modules released under a GPL-compatible license).[236]

Linux provides in-kernel APIs that manipulate data structures (e.g., [linked lists](/source/Linked_list), [radix trees](/source/Radix_tree),[237] [red-black trees](/source/Red%E2%80%93black_tree),[238] [queues](/source/Queue_(abstract_data_type))) 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.[239][240][241]

In-kernel APIs include libraries of low-level common services used by device drivers:

- [SCSI](/source/SCSI) Interfaces and [libATA](/source/LibATA) – respectively, a peer-to-peer packet based communication protocol for storage devices attached to USB, SATA, SAS, Fibre Channel, FireWire, ATAPI device,[242] and an in-kernel library to support [S]ATA host controllers and devices.[243]

- [Direct Rendering Manager](/source/Direct_Rendering_Manager) (DRM) and [Kernel Mode Setting](/source/Kernel_Mode_Setting) (KMS) – for interfacing with GPUs and supporting the needs of modern 3D-accelerated video hardware,[244] and for setting screen resolution, color depth and refresh rate[245]

- [DMA](/source/Direct_memory_access) buffers ([DMA-BUF](/source/DMA-BUF)) – for sharing buffers for hardware direct memory access across multiple device drivers and subsystems[246][247][248]

- [Video4Linux](/source/Video4Linux) – for video capture hardware

- [Advanced Linux Sound Architecture](/source/Advanced_Linux_Sound_Architecture) (ALSA) – for sound cards

- [New API](/source/New_API) – for [network interface controllers](/source/Network_interface_controller)

- [mac80211](/source/Mac80211) and cfg80211 – for wireless network interface controllers[249][250]

#### 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.[201]

### Process management

See also: [Process (computing)](/source/Process_(computing)), [Thread (computing)](/source/Thread_(computing)), and [Process management (computing)](/source/Process_management_(computing))

Linux, as other[kernels](/source/Kernel_(operating_system)), has the ability to manage processes including creating, suspending, resuming and terminating. Unlike other operating systems, the Linux kernel implements processes as a group of threads called tasks. If two tasks share the same TGID, then they are called in the kernel terminology a task group. Each task is represented by a *task_struct* [data structure](/source/Data_structure). When a process is created it is assigned a globally unique identifier called *PID* and cannot be shared[251][252]

A new process can be created by calling clone[253] family of system calls or fork [system call](/source/System_call). Processes can be suspended and resumed by the kernel by sending signals like SIGSTOP and SIGCONT. A process can terminate itself by calling exit system call, or terminated by another process by sending signals like SIGKILL, SIGABRT or SIGINT.

If the executable is dynamically linked to shared libraries, a [dynamic linker](/source/Dynamic_linker) is used to find and load the needed objects, prepare the program to run and then run it.[254]

The [Native POSIX Thread Library](/source/Native_POSIX_Thread_Library) (NPTL)[255] provides the POSIX standard thread interface (*pthreads*) to userspace. The kernel isn't aware of processes nor threads but it is aware of *tasks*, thus threads are implemented in userspace. Threads in Linux are implemented as *tasks* sharing resources, while if they aren't sharing called to be independent processes.

The kernel provides the futex(7) (fast user-space mutex) mechanisms for user-space locking and synchronization.[256] The majority of the operations are performed in userspace but it may be necessary to communicate with the kernel using the futex(2) system call.[215]

As opposed to userspace threads described above, *kernel threads* run in kernel space.[257] They are threads created by the kernel itself for specialized tasks; they are privileged like the kernel and aren't bound to any process or application.

### Scheduling

Main articles: [Completely Fair Scheduler](/source/Completely_Fair_Scheduler) and [Earliest eligible virtual deadline first scheduling](/source/Earliest_eligible_virtual_deadline_first_scheduling)

See also: [Scheduling (computing) § Linux](/source/Scheduling_(computing)#Linux)

The Linux [process scheduler](/source/Process_scheduler) is modular, in the sense that it enables different scheduling classes and policies.[258][259] Scheduler classes are pluggable 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.[16]: 46–47 Entities may be threads, group of threads, and even all the processes of a specific user.

Linux provides both [user preemption](/source/Preemption_(computing)) as well as (selectable, up to)[260][261] full [kernel preemption](/source/Kernel_preemption).[16]: 62–63 Preemption reduces [latency](/source/Latency_(engineering)), increases responsiveness,[262] and makes Linux more suitable for desktop and [real-time](/source/Real-time_computing) applications. This comes at a cost of throughput, as work being interrupted worsens cache behavior.[261]

For normal tasks, by default, the kernel uses the [Completely Fair Scheduler](/source/Completely_Fair_Scheduler) (CFS) class[*[needs update](https://en.wikipedia.org/wiki/Wikipedia:Manual_of_Style/Dates_and_numbers#Chronological_items)*], introduced in version 2.6.23.[184] The scheduler is defined as a macro in 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).[16]: 46–50

With user preemption, the kernel scheduler can replace the current process with the execution of a [context switch](/source/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 vruntime 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.[263] 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[264] real-time scheduling classes named [SCHED_FIFO](/source/SCHED_FIFO) (realtime [first-in-first-out](/source/FIFO_(computing_and_electronics))) and [SCHED_RR](/source/SCHED_RR) (realtime [round-robin](/source/Round-robin_scheduling)), both of which take precedence over the default class.[258] An additional scheduling policy known as [SCHED DEADLINE](/source/SCHED_DEADLINE), implementing the [earliest deadline first algorithm](/source/Earliest_deadline_first_scheduling) (EDF), was added in kernel version 3.14, released on 30 March 2014.[265][266] SCHED_DEADLINE takes precedence over all the other scheduling classes.

Real-time [PREEMPT_RT](/source/PREEMPT_RT) patches, included into the mainline Linux since version 2.6, provide a [deterministic scheduler](/source/Real-time_operating_system#Scheduling), the removal of preemption and interrupt disabling (where possible), PI Mutexes (i.e., locking primitives that avoid priority inversion),[267][268] support for [High Precision Event Timers](/source/High_Precision_Event_Timers) (HPET), preemptive [read-copy-update](/source/Read-copy-update) (RCU), (forced) IRQ threads, and other minor features.[269][270][271]

In 2023, Peter Zijlstra proposed replacing CFS with an [earliest eligible virtual deadline first scheduling](/source/Earliest_eligible_virtual_deadline_first_scheduling) (EEVDF) scheduler,[272][273] to prevent the need for CFS "latency nice" patches.[274] The EEVDF scheduler replaced CFS in version 6.6 of the Linux kernel.[183]

### Synchronization

The kernel has different causes of concurrency (e.g., interrupts, bottom halves, preemption of kernel and users tasks, symmetrical multiprocessing).[16]: 167

For protecting critical regions (sections of code that must be executed atomically), shared memory locations (like [global variables](/source/Global_variable) and other data structures with global scope), and regions of memory that are asynchronously modifiable by hardware (e.g., having the C [volatile](/source/Volatile_(computer_programming)) [type qualifier](/source/Type_qualifier)), Linux provides a large set of tools. They consist of [atomic types](/source/Linearizability) (which can only be manipulated by a set of specific operators), [spinlocks](/source/Spinlock), [semaphores](/source/Semaphore_(programming)), [mutexes](/source/Mutual_exclusion),[275][16]: 176–198[276] and [lockless algorithms](/source/Non-blocking_algorithm) (e.g., [RCUs](/source/Read-copy-update)).[277][278][279] Most lock-less algorithms are built on top of [memory barriers](/source/Memory_barrier) for the purpose of enforcing [memory ordering](/source/Memory_ordering) and prevent undesired side effects due to [compiler optimization](/source/Optimizing_compiler).[280][281][282][283]

[PREEMPT_RT](/source/PREEMPT_RT) code included in mainline Linux provide *RT-mutexes*, a special kind of Mutex that do not disable preemption and have support for priority inheritance.[284][285] Almost all locks are changed into sleeping locks when using configuration for realtime operation.[286][271][285] [Priority inheritance](/source/Priority_inheritance) avoids priority inversion by granting a low-priority task that holds a contended lock the priority of a higher-priority waiter until that lock is released.[287][288]

Linux includes a kernel lock validator called *Lockdep*.[289][290]

### Interrupts

Although the management of [interrupts](/source/Interrupt) could be seen as a single job, it is divided into two. 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](/source/Interrupt_handler) (ISR) 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*).[16]: 133–137

Linux interrupt service routines can be nested. A new IRQ can trap into a high priority ISR that preempts any other lower priority ISR.

### Memory

See also: [Memory management](/source/Memory_management) and [Virtual memory](/source/Virtual_memory)

The Linux kernel manages both physical and virtual memory. It divides physical memory into zones,[291] each of which has a specific purpose.

- ZONE_DMA: this zone is suitable for[DMA](/source/Direct_memory_access).

- ZONE_NORMAL: for normal memory operations.

- ZONE_HIGHMEM: part of physical memory that is only accessible to the kernel using temporary mapping.

Those zones are the most common, but others exist.[291]

Linux implements [virtual memory](/source/Virtual_memory) with 4 or 5-level [page tables](/source/Page_table).[292] The kernel is not [pageable](/source/Memory_paging) (meaning it is always resident in physical memory and cannot be swapped to the disk) and there is no memory protection (no SIGSEGV signals, unlike in user space), therefore memory violations lead to instability and system crashes.[16]: 20 User memory is pageable by default, although paging for specific memory areas can be disabled with the mlock() [system call](/source/System_call) family.

[Page frame](/source/Page_frame) information is maintained in apposite data structures (of type struct page) that are populated immediately after boot and kept until shutdown, regardless of whether they are associated with virtual pages. The physical address space is divided into different zones, according to architectural constraints and intended use. NUMA systems with multiple memory banks are also supported.[293]

Small chunks of memory can be dynamically allocated in kernel space via the family of kmalloc() APIs 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[294]

The kernel used to include the SLAB, SLUB and SLOB allocators as configurable alternatives.[295][296] The SLOB allocator was removed in Linux 6.4[297] and the SLAB allocator was removed in Linux 6.8.[298] The sole remaining allocator is SLUB, which aims for simplicity and efficiency,[296] is [PREEMPT_RT](/source/PREEMPT_RT) compatible[299] and was introduced in Linux 2.6.

### Virtual filesystem

See also: [Virtual file system](/source/Virtual_file_system)

Since Linux supports numerous filesystems with different features and functionality, it is necessary to implement a generic filesystem that is independent from underlying filesystems. The [virtual file system](/source/Virtual_file_system) interfaces with other Linux subsystems, userspace, or[APIs](/source/Api) and abstracts away the different implementations of underlying filesystems. VFS implements system calls like create, open, read, write and close. VFS implements a generic [superblock](/source/Superblock_(file_system))[300] and [inode block](/source/Inode) that is independent from the one that the underlying filesystem has.

In this subsystem directories and files are represented by a struct file [data structure](/source/Data_structure). When [userspace](/source/Userspace) requests access to a file it is returned a [file descriptor](/source/File_descriptor) (non negative integer value) but in [kernel space](/source/Kernel_space) it is a struct file structure. This structure stores all the information the kernel knows about a file or directory.

[sysfs](/source/Sysfs) and [procfs](/source/Procfs) are virtual filesystems that expose hardware information and [userspace](/source/Userspace) programs' runtime information. These filesystems aren't present on disk and instead the kernel implements them as a [callback](/source/Callback_(computer_programming)) or routine that gets called when they are accessed by userspace.

### Supported architectures

See also: [List of Linux-supported computer architectures](/source/List_of_Linux-supported_computer_architectures) and [Linux-powered device](/source/Linux-powered_device)

[TiVo](/source/TiVo) [DVR](/source/Digital_video_recorder), a consumer device running Linux

While not originally designed to be [portable](/source/Porting),[19][301] Linux is now one of the most widely ported operating system kernels, running on a diverse range of systems from the [ARM architecture](/source/ARM_architecture) to IBM [z/Architecture](/source/Z%2FArchitecture) [mainframe computers](/source/Mainframe_computer). The first port was performed on the [Motorola 68000](/source/Motorola_68000) platform. The modifications to the kernel were so fundamental that Torvalds viewed the Motorola version as a [fork](/source/Fork_(software)) and a "Linux-like operating system".[301] 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](/source/Digital_Equipment_Corporation) [Alpha AXP](/source/Alpha_AXP) 64-bit platform.[302][303][301]

Linux is used as the primary operating system on [IBM’s Summit](/source/Summit_(supercomputer)) supercomputer. In October 2019, all systems on the TOP500 list of the world’s fastest supercomputers ran operating systems based on the Linux kernel,[73] whereas the first Linux-based supercomputer appeared on the list in 1998.[304]

Linux has also been ported to various handheld devices such as [iPhone 3G](/source/IPhone_3G) and [iPod](/source/IPod).[305]

### Supported devices

In 2007, the LKDDb project has been started to build a comprehensive database of hardware and protocols known by Linux kernels.[306] 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.[307]

### Live patching

Kernel updates can be applied without rebooting by using [live patching](/source/Live_patching) technologies such as [Ksplice](/source/Ksplice), [kpatch](/source/Kpatch) and [kGraft](/source/KGraft). Initial support for live kernel patching was merged into the Linux kernel mainline in version 4.0, released on 12 April 2015. The feature, known as *livepatch*, uses the kernel's [ftrace](/source/Ftrace) functionality and provides a common infrastructure for hot patches supplied through kernel modules, together with interfaces for userspace management tools. In its initial Linux 4.0 implementation, livepatch support was limited to the [x86](/source/X86) architecture and provided basic infrastructure rather than a complete live-patching consistency model.

### Security

Kernel bugs present potential security issues. For example, they may allow for [privilege escalation](/source/Privilege_escalation) or create [denial-of-service attack](/source/Denial-of-service_attack) vectors. Over the years, numerous bugs affecting system security were found and fixed.[308] New features are frequently implemented to improve the kernel's security.[309][310]

Capabilities(7) have already been introduced in the section about the processes and threads. Android makes use of them and [systemd](/source/Systemd) gives administrators detailed control over the capabilities of processes.[311]

Linux offers a wealth of mechanisms to reduce kernel attack surface and improve security that are collectively known as the [Linux Security Modules](/source/Linux_Security_Modules) (LSM).[312] They comprise the [Security-Enhanced Linux](/source/Security-Enhanced_Linux) (SELinux) module, whose code has been originally developed and then released to the public by the [NSA](/source/National_Security_Agency),[313] and [AppArmor](/source/AppArmor)[198] among others. SELinux is now actively developed and maintained on [GitHub](/source/GitHub).[197] SELinux and AppArmor provide support to access control security policies, including [mandatory access control](/source/Mandatory_access_control) (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.[314]

Critics have accused kernel developers of covering up security flaws, or at least not announcing them; in 2008, Torvalds responded to this with the following:[315][316]

I personally consider security bugs to be just "normal bugs". I don't cover them up, but I also don't have any reason what-so-ever to think it's a good idea to track them and announce them as something special...one reason I refuse to bother with the whole security circus is that I think it glorifies—and thus encourages—the wrong behavior. It makes "heroes" out of security people, as if the people who don't just fix normal bugs aren't as important. In fact, all the boring normal bugs are *way* more important, just because there's[sic] a lot more of them. I don't think some spectacular security hole should be glorified or cared about as being any more "special" than a random spectacular crash due to bad locking.

Linux distributions typically release security updates to fix vulnerabilities in the Linux kernel. Many offer [long-term support](/source/Long-term_support) releases that receive security updates for a certain Linux kernel version for an extended period of time.

In 2024, researchers disclosed that the Linux kernel contained a serious vulnerability, CVE-2024-50264, located in the AF_VSOCK subsystem. This bug is a use-after-free flaw, a class of memory corruption issue that occurs when a program continues to use memory after it has been freed.[317][318] Such flaws are particularly dangerous in the kernel, as they can allow attackers to escalate privileges. The bug was resolved in May 2025.[319]

## Legal

### Licensing terms

Initially, Torvalds released Linux under a license that forbade any commercial use.[320] This was changed in version 0.12 by a switch to the [GNU General Public License](/source/GNU_General_Public_License) version 2 (GPLv2).[24] 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.[321] Torvalds has described licensing Linux under the GPLv2 as the "best thing I ever did".[320]

The Linux kernel is licensed explicitly under [GNU General Public License](/source/GNU_General_Public_License) version 2 only (GPL-2.0-only) with an explicit syscall exception (Linux-syscall-note),[10][13][14] without offering the licensee the option to choose any later version, which is a common GPL extension. Contributed code must be available under [GPL-compatible license](/source/GPL_compatibility).[15][209]

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.[322] Torvalds himself specifically indicated upon the release of version 2.4.0 that his own code is released only under version 2.[323] The terms of the GPL state that if no version is specified, then any version may be used,[324] and [Alan Cox](/source/Alan_Cox_(computer_programmer)) pointed out that very few other Linux contributors had specified a particular version of the GPL.[325]

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."[326] This group of high-profile kernel developers, including Torvalds, [Greg Kroah-Hartman](/source/Greg_Kroah-Hartman) and [Andrew Morton](/source/Andrew_Morton_(computer_programmer)), commented on mass media about their objections to the GPLv3.[327] They referred to clauses regarding [DRM](/source/Digital_rights_management)/[tivoization](/source/Tivoization), patents, "additional restrictions" and warned a [Balkanisation](/source/Balkanization) of the "Open Source Universe" by the GPLv3.[327][328] Torvalds, who decided not to adopt the GPLv3 for the Linux kernel, reiterated his criticism even years later.[329]

### Loadable kernel modules

It is debated whether some [loadable kernel modules](/source/Loadable_kernel_module) (LKMs) are to be considered [derivative works](/source/Derivative_work) 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[234][235] are non-derived works, thus Linux gives system administrators the mechanisms to load out-of-tree binary objects into the kernel address space.[15]

There are some out-of-tree loadable modules that make legitimate use of the *dma_buf* kernel feature.[330] GPL compliant code can certainly use it. A different possible use case would be [Nvidia Optimus](/source/Nvidia_Optimus) that pairs a fast GPU with an Intel integrated GPU, where the Nvidia GPU writes into the [Intel](/source/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.[236] [Alan Cox](/source/Alan_Cox_(computer_programmer)) replied on [LKML](/source/Linux_kernel_mailing_list), rejecting a request from one of Nvidia's engineers to remove this technical enforcement from the API.[331] Torvalds clearly stated on the LKML that "[I] claim that binary-only kernel modules ARE derivative "by default"'".[332]

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".[333] [Proprietary](/source/Proprietary_software) graphics drivers, in particular, are heavily discussed.

Whenever proprietary modules are loaded into Linux, the kernel marks itself as being "tainted",[334] and therefore bug reports from tainted kernels will often be ignored by developers.

### Firmware binary blobs

The official kernel, that is, Torvalds's git branch at the kernel.org repository, contains [binary blobs](/source/Binary_blob) released under the terms of the GNU GPLv2 license.[10][15] Linux can also load binary blobs, proprietary firmware, drivers, or other executable modules from the filesystem, and link them into kernel space.[335]

When necessary (e.g., for accessing boot devices or for speed), firmware can be built-in to the kernel, meaning building the firmware into [vmlinux](/source/Vmlinux); nonetheless, 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 not GPL compatible, although this is quite common nonetheless).[336]

### Trademark

Linux is a registered [trademark](/source/Trademark) of Linus Torvalds in the United States, the European Union, and some other countries.[337][338] 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.[339][340][341]

### Removal of Russian maintainers

In October 2024, during the [Russian invasion of Ukraine](/source/Russian_invasion_of_Ukraine), kernel developer [Greg Kroah-Hartman](/source/Greg_Kroah-Hartman) removed some kernel developers whose email addresses suggested a connection to Russia from their roles as maintainers.[342][343] [Linus Torvalds](/source/Linus_Torvalds) responded that he did not support Russian aggression and would not revert the patch, insinuating that opponents of the patch were [Russian trolls](/source/Russian_web_brigades).[344] James Bottomley, a kernel developer, issued an apology for the handling of the situation and clarified that the action was a consequence of [U.S. sanctions against Russia](/source/International_sanctions_during_the_Russian_invasion_of_Ukraine).[345]

## See also

- [Linux portal](https://en.wikipedia.org/wiki/Portal:Linux)
- [Free and open-source software portal](https://en.wikipedia.org/wiki/Portal:Free_and_open-source_software)

- [Linux](/source/Linux) – Family of Unix-like operating systems

- [Linux kernel version history](/source/Linux_kernel_version_history) – Version history of the Linux kernel

- [Comparison of operating systems](/source/Comparison_of_operating_systems)

- [Comparison of operating system kernels](/source/Comparison_of_operating_system_kernels)

- [Microkernel](/source/Microkernel) – Kernel that provides fewer services than a traditional kernel

- [Minix 3](/source/Minix_3) – Unix-like operating system

- [macOS](/source/MacOS) – Operating system for Apple computers

- [Microsoft Windows](/source/Microsoft_Windows) – Computer operating system

## Notes

1. **[^](#cite_ref-8)** with [GNU extensions](/source/GNU_Compiler_Collection);[5] [C11](/source/C11_(C_standard_revision)) (gnu11) since 5.18,[6] [C89](/source/C89_(C_version)) (gnu89) before[7]

1. **[^](#cite_ref-10)** see [Rust for Linux](/source/Rust_for_Linux)[8]

1. **[^](#cite_ref-18)** As a whole, Linux source is provided under the terms of the [GPL-2.0-only](/source/GNU_General_Public_License) license with an explicit syscall exception.[13][14] Aside from that, individual files can be provided under a different license that is required to be compatible with the GPL-2.0-only license (i.e., the GNU General Public License version 2) or a dual license, with one of the choices being the GPL version 2 or a GPLv2 compatible license.[15]

## References

1. **[^](#cite_ref-HvDj4_1-0)** ["Linux Logos and Mascots"](https://web.archive.org/web/20100815085106/http://www.linux.org/info/logos.html). Linux Online. 2008. Archived from [the original](https://www.linux.org/info/logos.html) on 15 August 2010. Retrieved 19 September 2024.

1. **[^](#cite_ref-wikidata-af77cbd4d7c117510b0afaacf307cfe730fa2272-v20_2-0)** Greg Kroah-Hartman (19 June 2026). ["Linux 7.1.1"](https://lore.kernel.org/lkml/2026061931-reversal-stopped-29e3@gregkh/). Retrieved 24 June 2026.

1. **[^](#cite_ref-wikidata-3616d9c75de8eab329ad30ed63b43de1278bbd03-v20_3-0)** Greg Kroah-Hartman (2 April 2026). ["Linux 6.18.21"](https://lore.kernel.org/lkml/2026040214-emit-obstinate-46ae@gregkh/). Retrieved 8 April 2026.

1. **[^](#cite_ref-wikidata-7f3f827bd2930ede44497e2e531bcf730c0e56b4-v20_4-0)** Linus Torvalds (7 June 2026). ["Linux 7.1-rc7"](https://lore.kernel.org/lkml/CAHk-=whAmWe+G5_jtrraOx0JAu4ZcpWvLAsFqH3N5wWkVrCbww@mail.gmail.com/). Retrieved 9 June 2026.

1. **[^](#cite_ref-5)** [Russell, Paul Rusty](/source/Rusty_Russell). ["GNU Extensions"](https://people.netfilter.org/rusty/unreliable-guides/kernel-hacking/conventions-gnu-extns.html). [*Unreliable Guide To Hacking The Linux Kernel*](https://people.netfilter.org/rusty/unreliable-guides/kernel-hacking/lk-hacking-guide.html). Retrieved 4 August 2025.

1. **[^](#cite_ref-6)** MaskRay (12 May 2024). ["Exploring GNU extensions in the Linux kernel"](https://maskray.me/blog/2024-05-12-exploring-gnu-extensions-in-linux-kernel). *MaskRay*. [Archived](https://web.archive.org/web/20250729112739/https://maskray.me/blog/2024-05-12-exploring-gnu-extensions-in-linux-kernel) from the original on 29 July 2025. Retrieved 4 August 2025.

1. **[^](#cite_ref-7)** Bergmann, Arnd (3 March 2022). ["Kbuild: move to -std=gnu11"](https://web.archive.org/web/20230716094848/https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e8c07082a810f). *Linux kernel mailing list* (Mailing list). Archived from [the original](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e8c07082a810f) on 16 July 2023. Retrieved 19 October 2025.

1. **[^](#cite_ref-heise-stable_9-0)** Leemhuis, Thorsten (10 December 2025). ["Linux Kernel: Rust Support Officially Approved"](https://www.heise.de/en/news/Linux-Kernel-Rust-Support-Officially-Approved-11109808.html). *[heise.de](/source/Heise.de)*. Retrieved 12 December 2025.

1. **[^](#cite_ref-11)** ["torvalds/linux: Linux kernel source tree"](https://github.com/torvalds/linux). *GitHub*. [Archived](https://web.archive.org/web/20180604121742/https://github.com/torvalds/linux) from the original on 4 June 2018. Retrieved 6 May 2026.

1. ^ [***a***](#cite_ref-COPYING_File_12-0) [***b***](#cite_ref-COPYING_File_12-1) [***c***](#cite_ref-COPYING_File_12-2) [***d***](#cite_ref-COPYING_File_12-3) ["COPYING"](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/COPYING). *git.kernel.org*. [Archived](https://web.archive.org/web/20210902152553/https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/COPYING) from the original on 2 September 2021. Retrieved 2 September 2021.

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## Further reading

- Torvalds, Linus; Diamond, David (2001). [*Just for Fun: The Story of an Accidental Revolutionary*](/source/Just_for_Fun_(book)). [HarperBusiness](/source/HarperBusiness). [ISBN](/source/ISBN_(identifier)) [978-0066620732](https://en.wikipedia.org/wiki/Special:BookSources/978-0066620732).

- Bezroukov, Nikolai. ["Ch 4: A benevolent dictator"](http://www.softpanorama.org/People/Torvalds/index.shtml). *Portraits of Open Source Pioneers* (e-book). Softpanorama. [Archived](https://web.archive.org/web/20051013082354/http://www.softpanorama.org/People/Torvalds/index.shtml) from the original on 13 October 2005. Retrieved 3 October 2005.

- Corbet, Jonathan (16 October 2003). ["LinkSys and binary modules"](https://lwn.net/Articles/53780/). LWN.net Weekly Edition. [Archived](https://web.archive.org/web/20160801080742/http://lwn.net/Articles/53780/) from the original on 1 August 2016. Retrieved 21 July 2016.

- ["Everyone's Favorite Linux Mascot"](http://www.nd.edu/~ljordan/linux/tuxhistory.html). [Archived](https://web.archive.org/web/20050816235544/http://www.nd.edu/~ljordan/linux/tuxhistory.html) from the original on 16 August 2005. Retrieved 16 June 2005.

- Pranevich, Joseph (December 2003). ["The Wonderful World of Linux 2.6"](https://web.archive.org/web/20030716054145/http://www.kniggit.net/wwol26.html). Archived from [the original](http://kniggit.net/wwol26.html) on 16 July 2003.

- ["LinuxChanges"](http://wiki.kernelnewbies.org/LinuxChanges). [Archived](https://web.archive.org/web/20051031211753/http://wiki.kernelnewbies.org/LinuxChanges) from the original on 31 October 2005. Retrieved 31 October 2005.

- ["Seminar Paper on Linux Kernel 2.6"](https://web.archive.org/web/20070202002917/http://www.engineeringproject.net/seminars/linux.htm). Archived from [the original](https://engineeringproject.net/seminars/linux.htm) on 2 February 2007.

- ["Linux Device Drivers"](https://lwn.net/Kernel/LDD3/) (3rd ed.). [Archived](https://web.archive.org/web/20160727085953/http://lwn.net/Kernel/LDD3/) from the original on 27 July 2016. Retrieved 21 July 2016.

- [*Understanding the Linux Kernel*](https://www.oreilly.com/catalog/understandlk/) (Book) (3rd ed.). [ISBN](/source/ISBN_(identifier)) [978-0-596-00565-8](https://en.wikipedia.org/wiki/Special:BookSources/978-0-596-00565-8). [Archived](https://web.archive.org/web/20051217094234/http://www.oreilly.com/catalog/understandlk/) from the original on 17 December 2005. Retrieved 22 December 2005.

- Rosen, Rami (2014). [*Linux Kernel Networking*](https://link.springer.com/content/pdf/bfm%3A978-1-4302-6197-1%2F1.pdf) (PDF). Berkeley, CA: Apress. [doi](/source/Doi_(identifier)):[10.1007/978-1-4302-6197-1](https://doi.org/10.1007%2F978-1-4302-6197-1). [ISBN](/source/ISBN_(identifier)) [978-1-4302-6196-4](https://en.wikipedia.org/wiki/Special:BookSources/978-1-4302-6196-4). Retrieved 10 August 2025.

- ["Linux: The GPL And Binary Modules"](https://web.archive.org/web/20050723031159/http://kerneltrap.org/node/1735). Archived from [the original](https://kerneltrap.org/node/1735) on 23 July 2005.

- ["Anatomy of the Linux kernel"](https://www.ibm.com/developerworks/linux/library/l-linux-kernel/). *[IBM](/source/IBM)*. [Archived](https://web.archive.org/web/20070627093507/http://www.ibm.com/developerworks/linux/library/l-linux-kernel/) from the original on 27 June 2007. Retrieved 9 June 2007.

- Tanenbaum, Andrew; Bos, Herbert (2015). *Modern Operating Systems*. United States of America: Pearson. p. 722. [ISBN](/source/ISBN_(identifier)) [9781292061429](https://en.wikipedia.org/wiki/Special:BookSources/9781292061429). [OCLC](/source/OCLC_(identifier)) [892574803](https://search.worldcat.org/oclc/892574803).

## External links

Wikimedia Commons has media related to [Linux kernel](https://commons.wikimedia.org/wiki/Linux_kernel).

Wikibooks has more on the topic of: ***[Linux kernel](https://en.wikibooks.org/wiki/Special:Search/Linux_kernel)***

- [Official website](https://kernel.org) - [Linux kernel documentation index](https://kernel.org/doc/) - [Linux kernel man pages](https://kernel.org/doc/man-pages/) - [Kernel bugzilla](https://bugzilla.kernel.org/), and [regressions](https://bugzilla.kernel.org/show_bug.cgi?id=15790) for each recent kernel version

- [Kernel Newbies](https://kernelnewbies.org/), a source of various kernel-related information

- [Kernel coverage at LWN.net](https://lwn.net/Kernel/), an authoritative source of kernel-related information

- [Bootlin's Elixir Cross Referencer](https://elixir.bootlin.com/linux/latest/source), a Linux kernel source code cross-reference

- [Greg Kroah Hartman on the Linux kernel](https://www.youtube.com/watch?v=L2SED6sewRw) on [YouTube](/source/YouTube_video_(identifier))

- [Virtual filesystem](https://www.geeksforgeeks.org/virtual-file-system), linux generic filesystem implementation

v t e Linux kernel Organization Kernel Linux Foundation Linux Mark Institute Linus's law Tanenbaum–Torvalds debate Tux SCO disputes Linaro GNU GPL v2 menuconfig Supported computer architectures Version history Criticism Support Developers The Linux Programming Interface kernel.org LKML Linux conferences Users Linux User Group (LUG) People Werner Almesberger H. Peter Anvin Jens Axboe Moshe Bar Suparna Bhattacharya Andries Brouwer Rémy Card Alan Cox Matthew Garrett Avi Kivity Con Kolivas Greg Kroah-Hartman Robert Love David S. Miller Ingo Molnár Andrew Morton Hans Reiser Rusty Russell Shuah Khan Linus Torvalds Theodore Ts'o Stephen Tweedie Harald Welte Chris Wright Technical Debugging CRIU ftrace kdump Linux kernel oops SystemTap BPF eBPF Startup vmlinux System.map dracut initrd initramfs ABIs Linux Standard Base x32 ABI APIs Kernel System Call Interface POSIX ioctl select open read close sync … Linux-only futex epoll splice dnotify inotify readahead … In-kernel ALSA Crypto API io_uring DRM kernfs Memory barrier New API RCU Video4Linux IIO Userspace Daemons, File systems bpffs configfs devfs devpts debugfs FUSE hugetlbfs pipefs procfs securityfs sockfs sysfs tmpfs systemd udev Kmscon binfmt_misc Wrapper libraries C standard library glibc uClibc Bionic libhybris dietlibc EGLIBC klibc musl Newlib libcgroup libdrm libalsa libevdev libusb liburing Components Kernel modules BlueZ cgroups Console bcache Device mapper dm-cache dm-crypt DRM EDAC evdev Kernel same-page merging (KSM) LIO Framebuffer LVM KMS driver Netfilter Netlink nftables Network scheduler perf SLUB zram zswap Process and I/O schedulers: Brain Fuck Scheduler Completely Fair Scheduler (CFS) Earliest eligible virtual deadline first (EEVDF) Noop scheduler O(n) scheduler O(1) scheduler SCHED_DEADLINE SCHED_FIFO SCHED_RR Security Modules: AppArmor Exec Shield seccomp SELinux Smack Tomoyo Linux Linux PAM Device drivers 802.11 graphics Raw device initramfs KernelCare kexec kGraft kpatch Ksplice Variants Mainline Linux kernel Linux-libre High-performance computing INK Compute Node Linux SLURM Real-time computing RTLinux RTAI Xenomai PREEMPT_RT MMU-less μClinux PSXLinux Virtualization Hypervisor KVM Xen OS-level virtualization Linux-VServer Lguest LXC OpenVZ Other L4Linux User-mode Linux MkLinux coLinux Adoption Range of use Desktop Embedded Gaming Thin client: LTSP Server: LAMP LYME-LYCE Devices Adopters List of Linux adopters Linux portal Free and open-source software portal Category

v t e Linux Linux kernel History Linus's law Linux-libre Booting process Kernel oops Tux more… Controversies Criticism of Linux Criticism of desktop Linux GNU/Linux naming controversy Tanenbaum–Torvalds debate SCO and Linux Distributions General comparison Distributions list Netbook-specific comparison Distributions that run from RAM Lightweight Security-focused operating system Package manager Package format List of software package managers Organizations LinuxChix Linux Counter Linux Documentation Project Linux Foundation Linux Mark Institute Linux User Group (LUG) Adoption Adopters Desktop Embedded Gaming Mobile Range of use State/government-sponsored Linux malware Media DistroWatch Free Software Magazine Full Circle Linux.com Linux Format Linux Gazette Linux Journal Linux Magazine LinuxUser Ubuntu User Linux Outlaws Linux Voice LugRadio LWN.net Phoronix Revolution OS The Code Security certifications CompTIA Linux+ Linux Foundation Red Hat Linux portal Free and open-source software portal Category

v t e Mobile operating systems Comparison Open-source mobile phones, OSes Open Handset Alliance Open Mobile Alliance Mobile app distributions Mobile app development Linux Kernel Active: Aurora OS HarmonyOS (Linux kernel subsystem) ChromeOS ChromeOS Flex KaiOS Smart Feature OS Kindle firmware LuneOS Maemo Leste Tizen Defunct: Bada Firefox OS B2G OS MeeGo MontaVista MotoMagx Mobilinux OpenEmbedded Ångström Openmoko Linux QtMoko SHR OpenEZX WebOS Android Active: Android Go ColorOS OxygenOS Realme UI EMUI Fire OS GrapheneOS HyperOS LineageOS /e/ MagicOS Nothing OS One UI Wear OS Defunct: CyanogenMod GNU/ Linux Active: Fedora Mobility Manjaro ARM postmarketOS PureOS Sailfish OS SteamOS Ubuntu Touch UBports Mobian Defunct: Maemo Mer Moblin XNU iOS iPadOS watchOS bridgeOS QNX BlackBerry 10 BlackBerry Tablet OS Microsoft Windows Windows XP Tablet PC Edition Windows Mobile Windows Phone 7 8 8.1 Windows RT Windows 10 Mobile Windows 8 Windows 8.1 Windows 10 Windows 11 Zircon Fuchsia FreeBSD, NetBSD PlayStation Vita system software HongMeng Kernel Active: OpenHarmony HarmonyOS HarmonyOS NEXT Defunct: EMUI (non GMS-based) LiteOS Huawei Band OS FreeRTOS Amazfit OS Zepp OS MIUI Band OS Nintendo system software Nintendo 3DS system software Nintendo DSi system software Nintendo Switch system software Minix 3 Intel Management Engine Symbian S60 S80 S90 UIQ Others Binary Runtime Environment for Wireless (BREW) BlackBerry OS EPOC GEOS Magic Cap Nokia Asha platform PalmDOS Palm OS PlayStation Portable system software S30 S30+ S40 Sony Ericsson Java Platform ThreadX Software Mobile operating systems Mobile applications Telecommunication

v t e Routing software Operating systems and network operating systems Linux- based Entirely free Endian fli4l Floppyfw IPFire LEDE libreCMC OpenWrt VyOS Zeroshell Partly proprietary AirOS and EdgeOS Alliedware Plus DD-WRT ExtremeXOS FRITZ!Box RouterOS Smoothwall Tomato Vyatta FreeBSD- based Entirely free m0n0wall OPNsense pfSense Partly proprietary Junos OS Proprietary Cisco IOS ExtremeWare NX-OS TiMOS VRP Routing daemons Babel B.A.T.M.A.N. BIRD FRRouting GNU Zebra OpenBGPD OpenOSPFD Quagga XORP Other software Captive portal Neighbornode Nym Tor Related articles List of router and firewall distributions List of router firmware projects National Security Agency#Software backdoors

v t e Contributors to the Linux operating system Kernel H. Peter Anvin Jens Axboe Andries Brouwer Rémy Card Alan Cox Matthew Garrett Michael Kerrisk Shuah Khan Con Kolivas Greg Kroah-Hartman Robert Love David S. Miller Ingo Molnár Andrew Morton Keith Packard Hans Reiser Rusty Russell Sage Sharp Linus Torvalds Theodore Ts'o Stephen Tweedie Sage Weil Harald Welte Chris Wright GNU Richard Stallman Michael Tiemann Distributions Solar Designer Gaël Duval Marc Ewing Benjamin Mako Hill Peter MacDonald Ian Murdock Daniel Robbins Jane Silber Mark Shuttleworth Patrick Volkerding Bob Young Other developers Timothee Besset Matthias Ettrich Olivier Fourdan Mark Galassi Jim Gettys Ryan C. Gordon Harald Hoyer Miguel de Icaza Havoc Pennington Lennart Poettering Kay Sievers Dave Taylor Jeff Waugh Advocates Valerie Aurora Donna Benjamin Ken Coar Mary Gardiner Jon Hall Federico Heinz Bradley M. Kuhn Bruce Perens Eric S. Raymond Others Pia Andrews Jono Bacon Keith Bergelt Pamela Jones Allison Randal Karen Sandler Dave Sifry Luis Villa Linux portal

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Adapted from the Wikipedia article [Linux kernel](https://en.wikipedia.org/wiki/Linux_kernel) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Linux_kernel?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.