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Linux kernel

Linux is an operating system kernel used by a family of Unix-like operating systems. These are popularly termed Linux operating systems and the name is also used for the various Linux distributions built on top of the operating system.

The Linux kernel is released under the GNU General Public License version 2 (GPLv2) and developed by contributors worldwide; Linux is one of the most prominent examples of Open Source software.

The Linux kernel was initially conceived and created by Finnish software engineer Linus Torvalds in 1991. Early on, the MINIX community contributed code and ideas to the Linux kernel. At the time, the GNU Project had created many of the components required for a free software operating system, but its own kernel, GNU Hurd, was incomplete and unavailable. The BSD operating system had not yet freed itself from legal encumbrances. This meant that despite the limited functionality of the early versions, Linux rapidly accumulated developers and users who adopted code from those projects for use with the new operating system. Today the Linux kernel has received contributions from thousands of programmers.


In April 1991, Linus Torvalds, then 21 years old, started working on some simple ideas for an operating system. He started with a task switcher in Intel 80386 assembly language and a terminal driver. Then, on 26 August 1991, Torvalds posted to comp.os.minix:

I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu) for 386(486) AT 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(1.08) and gcc(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 portable (uses 386 task switching etc), and it probably never will support anything other than AT-harddisks, as that's all I have :-(.''

[...] It's mostly in C, but most people wouldn't call what I write C. It uses every conceivable feature of the 386 I could find, as it was also a project to teach me about the 386. As already mentioned, it uses a MMU, for both paging (not to disk yet) and segmentation. It's the segmentation that makes it REALLY 386 dependent (every task has a 64Mb segment for code & data - max 64 tasks in 4Gb. Anybody who needs more than 64Mb/task - tough cookies). [...] Some of my "C"-files (specifically mm.c) are almost as much assembler as C. [...] Unlike minix, I also happen to LIKE interrupts, so interrupts are handled without trying to hide the reason behind them.

After that, many people contributed code to the project. By September 1991, Linux version 0.01 was released. It had 10,239 lines of code. In October 1991, Linux version 0.02 was released.

In December 1991, Linux 0.11 was released. This version was the first to be self-hosted - Linux 0.11 could be compiled by a computer running Linux 0.11. When he released version 0.12 in February 1992, Torvalds adopted the GNU General Public License (GPL) over his previous self-drafted license, which did not permit commercial redistribution.

A newsgroup alt.os.linux was started, and on January 19 1992, the first post to alt.os.linux was made. On March 31 1992, alt.os.linux became comp.os.linux.

The X Window System was soon ported to Linux. In March 1992, Linux version 0.95 was the first to be capable of running X. This large version number jump (from 0.1x to 0.9x) was due to a feeling that a version 1.0 with no major missing pieces was imminent. However, this proved to be somewhat overoptimistic, and from 1993 to early 1994, 15 development versions of version 0.99 appeared.

On March 14 1994, Linux 1.0.0 was released, with 176,250 lines of code. In March 1995, Linux 1.2.0 was released (310,950 lines of code).

Version 2 of Linux, released on June 9, 1996, was followed by additional major versions under the version 2 header:

Legal aspects

Licensing terms

Initially, Torvalds released Linux under a license which forbade any commercial exploitation. This was soon changed to the GNU General Public License (GPL), as of version 0.12. 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 the complete corresponding source code.

Torvalds has described licensing Linux under the GPL as the "best thing I ever did.

GPL version 3

Currently, Linux is licensed only under version 2 of the GPL, with (unlike much GPL software) no option to use a later version, and there is some controversy over how easily it could be changed to use later GPL versions such as the new version 3 (and whether this is desirable). Torvalds himself specifically indicated upon the release of version 2.4.0 that his own code is only under version 2. However, the terms of the GPL state that if no version is specified, then any version may be used, and Alan Cox pointed out that very few other Linux contributors have specified a particular version of the GPL. One blogger concluded that around 40% of Linux code is specifically "GPL2 or above", and another approximately 10% is "GPL" (with no version specified), making for about half of the kernel, put together. In September of 2006, a survey of 29 key kernel programmers indicated 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.

Loadable Kernel Modules and firmware

It is debated whether Loadable Kernel Modules (LKMs) should be considered derivative works under copyright law, and thereby fall under the terms of the GPL. Torvalds has stated his belief that LKMs using only a limited, "public" subset of the kernel interfaces can sometimes be non-derived works, thus allowing some binary-only drivers and other LKMs that are not licensed under the GPL. Not all Linux contributors agree with this interpretation, however, and even Torvalds agrees that many LKMs are clearly derived works, and indeed he writes that "kernel modules ARE derivative 'by default'". On the other hand Torvalds has also said that "one gray area in particular is something like a driver that was originally written for another operating system (ie. clearly not a derived work of Linux in origin). [...] THAT is a gray area, and _that_ is the area where I personally believe that some modules may be considered to not be derived works simply because they weren't designed for Linux and don't depend on any special Linux behaviour. Proprietary graphics drivers, in particular, are heavily discussed. Ultimately, it is likely that such questions can only be resolved by a court.

One point of licensing controversy is Linux's use of firmware "binary blobs" to support some hardware devices. Richard Stallman claims that these blobs make Linux partially non-free software, and that distributing Linux may even be violating the GPL (which requires "complete corresponding source code" to be available).


Linux is a registered trademark of Linus Torvalds in the United States and some other countries. This is the result of an incident in which William Della Croce, Jr., who was not involved in the Linux project, trademarked the name and subsequently demanded royalties for its use. Several Linux backers retained legal counsel and filed suit against Della Croce, who agreed in 1998 to assign the trademark to Torvalds.

SCO litigation

In March 2003, the SCO Group (SCO) filed a lawsuit against IBM claiming that IBM had violated copyrights that SCO claimed to hold over the Unix source code, by contributing portions of that code to Linux. Additionally, SCO sent letters to a number of companies warning that their use of Linux without a license from SCO may be a violation of copyright law, and claimed in the press that they would be suing individual Linux users. IBM then promised to defend its Linux customers on their behalf. This controversy has generated lawsuits by SCO against Novell, DaimlerChrysler (partially dismissed in July, 2004), and AutoZone, and retaliatory lawsuits by Red Hat and others against SCO.

In early 2007 SCO filed the specific details of the purported copyright infringement. Despite previous claims that SCO was the rightful owner of 1 million lines of code, they specified only 326 lines of code, most of which were uncopyrightable. In August 2007, the court in the Novell case ruled that SCO did not actually own the Unix copyrights to begin with.

Technical features

Linux supports true preemptive multitasking (both in user mode and kernel mode), virtual memory, shared libraries, demand loading, shared copy-on-write executables, memory management, the Internet protocol suite, and threading.


Linux is a monolithic kernel. Device drivers and kernel extensions run in kernel space (ring 0), with full access to the hardware, although some exceptions run in user space. The graphics system most people use with Linux doesn't run in the kernel, in contrast to that found in Microsoft Windows.

Kernel mode preemption allows device drivers to be preempted under certain conditions. This feature was added to handle hardware interrupts correctly and improve support for symmetric multiprocessing (SMP). Preemption also improves latency, increasing responsiveness and making Linux more suitable for real-time applications.

Build Options

The Linux kernel has configurable Linux build options that enable specific features to be added or removed from the kernel during initial compilation. Customized default parameters may also be configured during the initial compilation.

Kernel panic

In Linux, a "panic" is an unrecoverable system error detected by the kernel as opposed to similar errors detected by user space code. It is possible for kernel code to indicate such a condition by calling the panic function located in the header file sys/system.h. However, most panics are the result of unhandled processor exceptions in kernel code, such as references to invalid memory addresses. These are typically indicative of a bug somewhere in the call chain leading to the panic. They can also indicate a failure of hardware, such as a failed RAM cell or errors in arithmetic functions in the processor caused by a processor bug, overheating/damaged processor, or a soft error.

Tanenbaum-Torvalds debate

The fact that Linux is a monolithic kernel rather than a microkernel was the topic of the Tanenbaum-Torvalds debate between Andrew S. Tanenbaum and Linus Torvalds. The debate started in 1992 about Linux and kernel architecture in general on the Usenet discussion group comp.os.minix. Tanenbaum argued that microkernels are superior to monolithic kernels and that therefore Linux is obsolete. Unlike traditional monolithic kernels, device drivers are easily configured as Loadable Kernel Modules, and loaded or unloaded while running the system. This subject was revisited on 9 May 2006, and on 12 May 2006 Tanenbaum wrote a position statement.

Programming languages

Linux is written in the version of the C programming language supported by GCC (which has introduced a number of extensions and changes to standard C), together with a number of short sections of code written in the assembly language (in GCC's "AT&T-style" syntax) of the target architecture. Because of the extensions to C it supports, GCC was for a long time the only compiler capable of correctly building Linux. In 2004, Intel claimed to have modified the kernel so that its C compiler also was capable of compiling it.

Many other languages are used in some way, primarily in connection with the kernel build process (the methods whereby the bootable image is created from the sources). These include Perl, Python, and various shell scripting languages. Some drivers may also be written in C++, Fortran, or other languages, but this is strongly discouraged. Linux's build system only officially supports GCC as a kernel and driver compiler.


While not originally designed to be portable, Linux is now one of the most widely ported operating system kernels, running on a diverse range of systems from the iPAQ (a handheld computer) to the IBM System z9 (a massive mainframe server that can run hundreds or even thousands of concurrent Linux instances). Linux runs as the main operating system on IBM's Blue Gene supercomputers. As of June 2008, Linux is the OS on more than 85% of systems on the Top 500 supercomputers list. Also, Linux has been ported to various handheld devices such as TuxPhone and Apple's iPod.

Virtual machine architectures

See Comparison of virtual machines for more details on product support for Linux.

See Hypervisor for more details on terminology used with virtual machines.

The Linux kernel has extensive support for and runs on many virtual machine architectures both as the host operating system and as a client operating system. The virtual machines usually emulate Intel x86 family of processors, though in a few cases PowerPC or AMD processors are also emulated.

Supported binary formats

Linux 1.0 supported the a.out binary format and the Executable and Linkable Format (ELF), which simplifies the creation of shared libraries (used extensively by modern desktop environments like KDE and GNOME). ELF is the default format used by gcc since around gcc 2.7.0, so a.out is now rarely if ever used, and ELF is now the primary binary format.

Linux supports many other binary formats, including binfmt misc for associating files to a program (such as an interpreter) to run or display that file.

Estimated cost to redevelop

The cost to redevelop the Linux kernel version 2.6.0 in a traditional proprietary development setting has been estimated to be $612 million USD (€467 million) in 2004 prices using the COCOMO man-month estimation model. In 2006, a study funded by the European Union put the redevelopment cost of kernel version 2.6.8 higher, at €882 million ($1.14 billion).


Further developing his own code and integrating changes made by other programmers, Linus Torvalds keeps releasing new versions of the Linux kernel. These are called "vanilla" kernels, meaning they have not been further modified by anyone. Many Linux operating system vendors modify the kernels of their product, mainly in order to add support for drivers or features which have not officially been released as stable, while some distributions rely on vanilla kernels.

Version numbering

The version number of the Linux kernel currently consists of four numbers, following a recent change in the long-standing policy of a three-number versioning scheme. For illustration, let it be assumed that the version number is composed thus: A.B.C[.D] (e.g. 2.2.1, 2.4.13 or

  • The A number denotes the kernel version. It is rarely changed, and only when major changes in the code and the concept of the kernel occur. It has been changed twice in the history of the kernel: In 1994 (version 1.0) and in 1996 (version 2.0).
  • The B number denotes the major revision of the kernel.
    • Prior to the Linux 2.6.x series, even numbers indicate a stable release, i.e. one that is deemed fit for production use, such as 1.2, 2.4 or 2.6. Odd numbers have historically been development releases, such as 1.1 or 2.5. They were for testing new features and drivers until they became sufficiently stable to be included in a stable release. It was an even/odd version number scheme.
    • Starting with the Linux 2.6.x series, there is no significance to even or odd numbers, with new feature development going on in the same kernel series. Linus Torvalds has stated that this will be the model for the foreseeable future.
  • The C number indicates the minor revision of the kernel. In the old three-number versioning scheme, this was changed when security patches, bug fixes, new features or drivers were implemented in the kernel. With the new policy, however, it is only changed when new drivers or features are introduced; minor fixes are handled by the D number.
  • A D number first occurred when a grave error, which required immediate fixing, was encountered in 2.6.8's NFS code. However, there were not enough other changes to legitimize the release of a new minor revision (which would have been 2.6.9). So, was released, with the only change being the fix of that error. With 2.6.11, this was adopted as the new official versioning policy. Bug-fixes and security patches are now managed by the fourth number, whereas bigger changes are only implemented in minor revision changes (the C number).

Also, sometimes after the version there will be some more letters such as 'rc1' or 'mm2'. The 'rc' refers to release candidate and indicates a non-official release. Other letters are usually (but not always) the initials of a person. This indicates a development branch of the kernel by that person. e.g. ck stands for Con Kolivas, ac stands for Alan Cox, whereas mm stood for Andrew Morton. Sometimes, the letters are related to the primary development area of the branch the kernel is built from, for example, wl indicates a wireless networking test build.

Development model

The development model for Linux 2.6 was a significant change from the development model for Linux 2.5. Previously there was a stable branch (2.4) where only relatively minor and safe changes were merged, and an unstable branch (2.5), where bigger changes and cleanups were allowed. This meant that users would always have a well-tested 2.4 version with the latest security and bug fixes to use, though they would have to wait for the features which went into the 2.5 branch. The downside of this was that the "stable" kernel ended up so far behind that it no longer supported recent hardware and lacked needed features. In the late 2.5.x series kernel some maintainers elected to try and back port their changes to the stable series kernel which resulted in bugs being introduced into the 2.4.x series kernel. The 2.5 branch was then eventually declared stable and renamed to 2.6. But instead of opening an unstable 2.7 branch, the kernel developers elected to continue putting major changes into the 2.6 "stable" branch. This had the desirable effect of breaking changes into smaller and easier to test batches, making new features quickly available, and getting more testing of the latest code.

However, the new 2.6 development model also meant that there was no stable branch for people just wanting security and bug fixes, and not needing the latest features. Fixes were only put into the latest version, so if a user wanted a version with all known bugs fixed they would also get all the latest features, which had not been well tested, and risked breaking things which had previously worked. A partial fix for this was the previously mentioned fourth version number digit (y in 2.6.x.y), which are series of point releases created by the stable team (Greg Kroah-Hartman, Chris Wright, maybe others). The stable team only released updates for the most recent kernel however, so this did not solve the problem of the missing stable kernel series. Linux distribution vendors, such as Red Hat and Debian, maintain the kernels which ship with their releases, so a solution for some people is to just follow a vendor kernel.

As a response to the lack of a stable kernel tree where people could coordinate the collection of bug fixes, in December 2005 Adrian Bunk announced that he would keep releasing 2.6.16.y kernels when the stable team moved on to 2.6.17 He also plans to include driver updates, making the maintenance of the 2.6.16 series very similar to the old rules for maintenance of a stable series such as 2.4

As of October 10, 2008, the latest stable kernel version is 2.6.27

In February 2008, the developers also responded to the lack of what one might call an unstable kernel tree - the linux-next tree was created to serve as a place where patches aimed to be merged during the next development cycle are gathered

Historical version numbering

The first version of the kernel was 0.01. This was followed by 0.02, 0.03, 0.10, 0.11, 0.12 (the first GPL version), 0.95, 0.96, 0.97, 0.98, 0.99 and then 1.0. From 0.95 on there were many patch releases between versions.


While Linus Torvalds supervises code changes and releases to the latest kernel versions, he has delegated the maintenance of older versions to other programmers. Major releases as old as 2.0 (officially obsoleted with the kernel 2.2.0 release in January 1999) are maintained as needed, although at a very slow pace.

Kernel series Current version Maintainer
2.0 2.0.40 David Weinehall
2.2 2.2.27-rc2 Marc-Christian Petersen (former maintainer Alan Cox)
2.4 2.4.37-rc1 Willy Tarreau (former maintainer Marcelo Tosatti)
2.6.16 Adrian Bunk
2.6.17+ (changes rapidly) Linus Torvalds
2.6.x-mm (changes rapidly) Andrew Morton

Other Linux kernel programmers include Robert Love and Ingo Molnár.

Revision control

The Linux kernel source code used to be maintained without the help of an automated source code management system, mostly because of Linus Torvalds' dislike of centralized SCM systems.

In 2002, Linux kernel development switched to BitKeeper, a SCM system which satisfied Linus Torvalds' technical requirements. BitKeeper was made available to Linus and several others free of charge, but was not free software, which was source of controversy. The system did provide some interoperability with free SCM systems such as CVS and Subversion.

In April 2005, however, efforts to reverse-engineer the BitKeeper system by Andrew Tridgell led BitMover, the company which maintained BitKeeper, to stop supporting the Linux development community. In response, Linus Torvalds and others wrote a new source code control system for the purpose, called Git. The new system was written within weeks, and in two months the first official kernel release was made using git. Git soon developed into a separate project in its own right and gained wider adoption in the free software community.

Stable version history

Kernel items

Kernel items are functions, variables, header files, and macros.

Kernel headers

Kernel headers are C header files which provide some definitions of the low-level kernel ABI, to be shared between the kernel and userspace programs. Most userspace programs should not include kernel headers; they are only intended to be used directly by system libraries and low-level tools and daemons.

The "make headers_install" command, when executed in the kernel's source tree, exports the kernel's header files in a form suitable for use by userspace programs.

When exported, most kernel headers reside in the /usr/include/asm and /usr/include/linux directories.

See also


External links

  • - The Linux Kernel Archives, the official kernel repository

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