Linux kernel

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{{Infobox_Software2 | name = Linux kernel | logo = Image:Tux.png | caption = | developer = Linus Torvalds | operating system = | genre = Kernel | license = GPL | website = www.kernel.org }}

The Linux kernel is a Unix-like operating system kernel that was begun by Linus Torvalds in 1991 and subsequently developed with the assistance of developers worldwide.

Developed under the GNU General Public License, Linux is free software.

The kernel is best known for its use in the GNU/Linux operating system.

Contents

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History

The project was launched in 1991 with a famous post to the Usenet newsgroup comp.os.minix that includes this sentence:

"I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu) for 386(486) AT clones..." [1]

At the time, the GNU project had created many of the components required for a free operating system, but its own kernel project, the GNU Hurd, was incomplete and unavailable. The BSD operating system had not yet freed itself from legal encumbrances. This left a space for the Linux kernel to fill, and despite the limited functionality of the early versions it rapidly accumulated developers and users. Early on, Minix hackers contributed code and ideas to the Linux kernel, and today it has received contributions from thousands of programmers.

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Timeline

  • Apr 1991 - Linus Torvalds, then 21, starts working on some simple ideas for an operating system. He starts with a task switcher in 386 assembly and a terminal driver.
  • 25 August 1991 - Torvalds posts to comp.os.minix: [2]
"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"
  • September 1991 - Linux version 0.01 is released. (10,239 lines of code.)
  • October 1991 - Linux version 0.02 is released. [3]
  • December 1991 - Linux 0.11 is released. This version is the first that is self-hosted (Linux 0.11 can be compiled under Linux 0.11)
  • 19 January 1992 - First post to alt.os.linux newsgroup. [4]
  • 31 March 1992 - The newsgroup comp.os.linux is created. [5]
  • March 1992 - Linux version 0.95 is the first to be capable of running the X Window System.
  • During the whole of 1993, and early 1994 - 15 development versions 0.99.*, with 0.99.11 (July 1993) introducing BogoMips into the kernel
  • 14 March 1994 - Linux 1.0.0 is released. (176,250 lines of code.)
  • March 1995 - Linux 1.2.0 is released (310,950 lines of code.)
  • 9 May 1996 - Tux the penguin is suggested as mascot for Linux
  • 9 June 1996 - Linux 2.0.0 is released. (777,956 lines of code.)
  • 25 January 1999 - Linux 2.2.0 is released, very buggy at first. (1,800,847 lines of code.)
  • 18 December 1999 - IBM mainframe patches for 2.2.13 published, bringing Linux into the biggest enterprises.
  • 4 January 2001 - Linux 2.4.0 is released. (3,377,902 lines of code.)
  • 17 December 2003 - Linux 2.6.0 is released. (5,929,913 lines of code.)
  • 20 March 2006 - Linux 2.6.16 is released. [6].
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Mascot

The Linux mascot is a penguin named Tux, created by Larry Ewing in May 1996 [7] after an initial suggestion made by Alan Cox [8] and further refined by Linus Torvalds on the Linux Kernel mailing list [9]. The mascot is a penguin because as Torvalds put it, "Linus likes penguins. That's it." [10] The name Tux was suggested to represent (T)orvalds (U)ni(X) [11], and it stuck.

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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). This license allows distribution and even sale of possibly modified versions of Linux but requires that all those copies be released under the same license and be accompanied by source code.

Torvalds has described licensing Linux under the GPL as the "best thing I ever did." [12]

One general question about the application of the GPL to Linux involves whether loadable kernel modules are considered "derived works" under copyright law, and thereby fall under the terms of the GPL. Torvalds has stated his belief that modules using only a limited, "public" subset of the kernel interfaces can sometimes be non-derived works, thus allowing some binary-only drivers and other modules not obeying the GPL. Not all kernel contributors agree with this interpretation, however, and even Torvalds agrees that many kernel modules are clearly derived works, and indeed he states that kernel modules ARE derivative "by default". Ultimately, such questions can only be resolved by a court.

Currently, the Linux kernel is licensed under version 2 of the GPL, and there is some controversy over how easily it could be changed to use later GPL versions such as the upcoming version 3 (and whether this is desirable) [13]. Torvalds himself indicated in version 2.4.0 that his own code is only under version 2 [14], but Alan Cox pointed out that very few other kernel contributors have specified a particular version of the GPL [15]. (The terms of the GPL state that if no version is specified, then any version may be used.)

Linux is a registered trademark of Linus Torvalds. 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.

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Litigation

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In March 2003, the SCO Group (SCO) filed a lawsuit against IBM claiming that IBM had contributed some portions of SCO's copyrighted Unix source code to the Linux kernel project in violation of IBM's license to use that Unix source code. 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. 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.

To date, no proof of SCO's claims of copied code in Linux has been provided and SCO's claims have varied widely.

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Technical features

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Architecture

The Linux kernel includes true multitasking, virtual memory, shared libraries, demand loading, shared copy-on-write executables, proper memory management, and TCP/IP networking.

Today Linux is a module-loading monolithic kernel. Device drivers and kernel extensions typically run in ring 0, with full access to the hardware, although some run in user space. Unlike standard monolithic kernels, device drivers are easily configured as modules, and loaded or unloaded while running the system. Also unlike standard monolithic kernels, device drivers can be pre-empted under certain conditions. This latter feature was added to handle hardware interrupts correctly, and to improve support for symmetric multiprocessing. Preemption also improves latency, increasing responsiveness and making Linux more suitable for real-time applications.

The fact that Linux is not a microkernel was the topic of a famous flame war between Linus Torvalds and Andy Tanenbaum on comp.os.minix in 1992. [16] [17]

The complete source code of various versions of the Linux kernel can be browsed at http://lxr.linux.no .

The complete source code of the latest versions of the Linux kernel can be downloaded from http://www.kernel.org .

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Kernel panic

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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 or overheating/damaged processor.

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Programming languages

The Linux kernel is written in that version of the C programming language which is supported by the GNU C compiler (which has introduced a number of extensions and changes to standard C), together with a number of relatively 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 a Linux kernel. Recently, Intel claims to have modified its C compiler so that it also is capable of correctly compiling the kernel.

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 behavior is strongly discouraged. The kernel's build system only officially supports the GCC as a kernel and driver compiler.

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Portability

While not originally intended as a portable operating system, 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 is intended to run as the main operating system on IBM's new Blue Gene supercomputer architecture when it is finished. Linus included, perhaps humorously, BogoMips into the kernel as a performance pseudo-comparison tool.

It is important to note that Linus' efforts were also directed successfully at a different sort of portability. Portability, according to Linus, was the ability to easily compile applications from a variety of sources on his system; thus Linux originally became popular in part because it required the least effort to get popular GPLed and other open source applications running.

Linux currently runs on the following machine architectures:

A complete list of ports is at Linux Ports.

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Versions

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 modified by anyone. Many providers of GNU/Linux operating systems 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, such as Slackware, rely on vanilla kernels.

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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 2.6.12.3).

  • The A number denotes the kernel version. It is changed least frequently, 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. 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. This has changed with the Linux 2.6.x series, 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, bugfixes, 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, 2.6.8.1 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 'am2'. The 'rc' refers to release candidate and indicates a non-official release. Other letters are usually the initials of a person. This indicates a slight fork of the kernel by that person. e.g. am would stand for Andrew Morton.

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Maintenance

While Linus Torvalds supervises code changes and releases to the latest kernel versions, he has delegated the maintenance of older versions to other programmers:

Kernel series Maintainer
2.0 David Weinehall
2.2 Marc-Christian Petersen (formerly Alan Cox)
2.4 Marcelo Tosatti
2.6 Andrew Morton / Linus Torvalds

Other Linux kernel programmers include Robert Love and Ingo Molnar. (See the Linux MAINTAINERS file).

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Stable version history

  • Version 1.0 of March 1994 supported only single-processor i386 machines.
  • Version 1.2 of March 1995 added support for Alpha, Sparc and Mips.
  • Version 2.0 of June 1996 added support for more processors and included SMP support.
  • Version 2.2 of January 1999 (The Wonderful World of Linux 2.2).
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References

  • Torvalds, Linus; Diamond, David (2001). Just For Fun: The Story of an Accidental Revolutionary. HarperBusiness. ISBN 0066620724 (hardcover); HarperAudio ISBN 0694525391 (audio tape, abridged ed., read by David Diamond) - on the beginnings of the Linux kernel
  • [18], Nikolai Bezroukov. Portraits of Open Source Pioneers. Ch 4: A benevolent dictator (Softpanorama e-book).
  • LinkSys and binary modules, LWN.net Weekly Edition, October 16 2003.
  • Revolution OS - a documentary on the history of Linux featuring several interviews with prominent hackers, including Torvalds

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External links

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The following are to be merged

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Linux 2.6

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Pages with links to kernel resources

cs:Linuxové jádro de:Linux-Kernel es:Linux (núcleo) fa:هسته لینوکس fr:Noyau Linux ilo:Linux it:Kernel Linux ja:Linuxカーネル lv:Linux nl:Linux-kernel no:Linux-kjernen nn:Linux-kjernen pl:Linux (jądro) pt:Núcleo do Linux ru:Linux (ядро) sh:Linux sv:Linuxkärnan th:ลินุกซ์ เคอร์เนล tl:Linux vi:Hạt nhân Linux zh:Linux 内核

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