The 'Linux kernel' is a
Unix-like operating system
kernel. It is the namesake of the
Linux family of operating systems. Released under the
GNU General Public License (GPL) and developed by contributors worldwide, Linux is one of the most prominent examples of
free and open source software.
[1]
Linux was written by
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.
[2] Today the Linux kernel has received contributions from thousands of programmers.
History
Main articles: History of the Linux kernel
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 and a
terminal driver. Then, on, 25 August 1991, Torvalds posted to ''comp.os.minix'':
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.
[3]
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 the next version, Torvalds adopted the
GNU GPL over his previous self-drafted license, which did not permit commercial redistribution.
[4]
A newsgroup ''alt.os.linux'' was started, and on
January 19 1992, the first post to alt.os.linux was made.
[1] On
March 31 1992, alt.os.linux became ''comp.os.linux''.
[1]
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).
Linus decided, on
May 9 1996, to adopt
Tux the penguin as mascot for Linux.
Version 2 of Linux, released on
June 9,
1996, was a landmark. Strong development continued:
★
January 25 1999 - Linux 2.2.0 was released (1,800,847 lines of code).
★
December 18 1999 -
IBM mainframe patches for 2.2.13 were published, allowing Linux to be used on enterprise-class machines.
★
January 4 2001 - Linux 2.4.0 was released (3,377,902 lines of code).
★
December 17 2003 - Linux 2.6.0 was released (5,929,913 lines of code).
Pronunciation
In 1992, Torvalds explained how he pronounces the word ''Linux'':
Torvalds has made available an audio sample which indicates his own pronunciation, in English and Swedish.
[1][8] However, an interview from the
2001 documentary ''
Revolution OS'' indicates that his preferred pronunciation has slightly changed.
[1]
Many English speakers tend to pronounce the name as or .
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). 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."
[10]
GPL version 3
Currently, Linux 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 new version 3 (and whether this is desirable).
[11] Torvalds himself specifically indicated upon the release of version 2.4.0 that his own code is only under version 2.
[12] 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.
[13] One blogger concluded that around 40% of Linux code is specifically "GPL 2 or above", and another approximately 10% is "GPL" (with no version specified), making for about half of the kernel, put together.
[14] 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."
[15]
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."
[16] Especially
proprietary graphics drivers are heavily discussed. Ultimately, 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).
[17]
Trademark
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. 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
[1].
In August 2007, the court in
the Novell case ruled that SCO did not actually own the Unix copyrights to begin with.
[2]
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.
Architecture
Simplified Linux kernel diagram in the form of a matrix map
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. Unlike
Microsoft Windows, the
graphics system most people use with Linux doesn't run in the kernel.
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.
The fact that Linux is a monolithic kernel rather than a
microkernel was the topic of the
Tanenbaum-Torvalds debate[1] 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.
[1]
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,
[1] and on
12 May 2006 Tanenbaum authored a position statement.
[1]
Kernel panic
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.
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. Recently,
Intel claims to have modified its C compiler so that it is also capable of correctly compiling it.
[22]
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.
Portability
Main articles: Linux kernel portability and supported architectures
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
2007, Linux is the OS on more than 75% of systems on the Top 500 supercomputers list.
[23]
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 only the
a.out binary format. The next stable series (Linux 1.2) added support for
Executable and Linkable Format (ELF), which simplifies the creation of
shared libraries (used extensively by modern
desktop environments like
KDE and
GNOME[24]). ELF is the default format used by
gcc since around gcc 2.7.0
[25], 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 US$612M (467M €) in 2004 prices using the
COCOMO man-month estimation model.
[26] In 2006, a study funded by the European Union put the redevelopment cost of kernel version 2.6.8 significantly higher, to 882M € (US$1.14B).
[27]
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 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 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.
★
★ 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.
★
★ 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, 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 '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.
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 backport 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 bugfixes, in December of 2005
Adrian Bunk announced that he would keep releasing 2.6.16.y kernels when the stable team moved on to 2.6.17
[3]. 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
[4].
As of August 23, 2007, the latest stable kernel version is 2.6.22.5.
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:
Other Linux kernel programmers include
Robert Love and
Ingo Molnar.
[32]
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 included
SMP support and added support for more processors.
★ Version 2.2 of January 1999 (
The Wonderful World of Linux 2.2).
★ Version 2.4.0 of January 2001
[33]
★
★ CPU support:
Hewlett-Packard's
PA-RISC processor,
Axis Communications'
ETRAX CRIS ("Code Reduced Instruction Set") processors
★
★ added
ISA Plug-and-Play
★
★ added
USB and
PC Card support
★
★ 2.4.6: added
Bluetooth support
★
★ Filesystem and data storage
★
★
★ added
Logical Volume Manager (LVM) version 1
★
★
★ support for
RAID devices
★
★
★ 2.4.15: Support for
InterMezzo filesystem were added.
★ Version 2.6 - current (
December 17 2003 to the present)
[34]
★
★ integrated
µClinux (for
microcontrollers)
★
★ CPU support: with support for
Hitachi's
H8/300 series, the
NEC v850, and
Motorola's embedded
m68k processors,
NUMA support, support for
NCR's
Voyager architecture, support for
Intel's
hyperthreading and
Physical Address Extension (PAE)
★
★ integrated the
ALSA sound driver
★
★ OS support:
★
★
★ Improved
APIC support.
★
★
★ Increased the maximum number of users and groups each from 65,536 (= 2
16) to 4,294,967,296 (2
32).
★
★
★ Increased the maximum number of process ids from 32,768 (= 2
15) to 1,073,741,824 (2
30).
★
★
★ Increased the maximum number of device types (major device) from 255 to 4095 and the maximum number of devices of each type (minor device) from 255 to more than a million.
★
★
★ Improved 64-bit support and
filesystems of up to 16
terabytes on common hardware.
★
★
★ Improvements to the "overall responsiveness" for interactive processes (the kernel became fully
pre-emptible and the
I/O scheduler was rewritten).
★
★
★ Support for
futexes, a rewrite of threading infrastructure to allow the
Native POSIX Thread Library (NPTL) to be used.
★
★
★ An improved module loader.
★
★
★
User-mode Linux integration.
★
★
★ 2.6.11
Infiniband support
★
★ Storage Support:
★
★
★ LVM version 2
★
★
★ support for
SGI's
XFS filesystem.
★
★
★ A new "system filesystem" called
sysfs, destined to relieve
procfs of its system related information.
★
★
★ 2.6.2
ATA over Ethernet support
★
★
★ 2.6.12 (
17 June 2005)
iSCSI support
★
★
★ 2.6.13
inotify support
★
★
★ 2.6.14
9P support
★
★
★ 2.6.14
FUSE support
★
★
★ 2.6.17 Online reshaping of software raid5/6
See also
★
menuconfig
References
★ (audio tape, abridged ed., read by David Diamond) - on the beginnings of the Linux kernel
★
[5], ''Nikolai Bezroukov''. Portraits of Open Source Pioneers. Ch 4: A benevolent dictator (Softpanorama e-book).
★
LinkSys and binary modules
★
FreeBSD/Linux kernel source code cross-reference. Browsable Linux (and other operating system) kernel source cross-reference.
★
LXR: The Linux Cross-Reference project. Browsable Linux kernel source.
★
KernelHQ - a browsable kernel source tree - with ''all'' versions present, and with browsable
diffs
★
For the love of Hacking
★
The LWN 2001 Linux Timeline
★
Everyone's Favorite Linux Mascot
★
Linux Timeline
★
History of Linux
★
Upgrade to 2.6 kernel
★
The Wonderful World of Linux 2.6; Joseph Pranevich; December 2003
★
Understanding the Linux 2.6.8.1 CPU Scheduler; Josh Aas, 17 February 2005
★
LinuxChanges
★
Seminar Paper on Linux Kernel 2.6
★
Linux Device Drivers, 3rd Edition
★
Understanding the Linux Kernel, 3rd Edition (Book)
★
Linux: The GPL And Binary Modules
★
Anatomy of the Linux kernel
1.
2. Free as in Freedom by Sam Williams. O'Reilly books, 2002
3.
4. Release Notes for Linux v0.12
5.
6.
7.
8. Linus pronouncing Linux in English and Swedish
9.
10. The Pragmatist of Free Software
11. GPLv3 and the kernel
12. Linux-2.4.0-test8
13. Re: GPL V3 and Linux
14. How much Linux Kernel code is GPL 2 only?
15. Top Linux programmers pan GPL 3
16. http://www.ussg.iu.edu/hypermail/linux/kernel/0312.0/0670.html
17. Linux, GNU, and freedom
18.
19.
20.
21.
22. http://www.pyrillion.org/index.html?showframe=linuxkernelpatch.html
23. Operating system Family
24. ldd /usr/bin/kwin lists 40 shared libraries (kwin 3.5.5a); ldd /usr/bin/gnome-panel lists 68 (gnome-panel 2.14.3).
25. The Linux ELF HOWTO
26. http://www.dwheeler.com/essays/linux-kernel-cost.html
27. Economic impact of FLOSS on innovation and competitiveness of the EU ICT sector, Table 3 on page 50.
28. [ANNOUNCE] Linux-kernel 2.0.40 aka ``The Moss-covered Tortoise'' David Weinehall
29. Linux 2.2.27-rc2 Marc-Christian Petersen
30.
31.
32. See the Linux MAINTAINERS file.
33. Wonderful World of Linux 2.4
34. The Wonderful World of Linux 2.6
External links
★
kernel.org - The Linux Kernel Archives, the official kernel repository
★
kernelnewbies.org - information for (beginning) Linux kernel developers
★
'Linux Kernel in a Nutshell' - free (Creative Commons Attribution-ShareAlike 2.5 license) downloadable book on the Linux kernel
★
Kernel Traffic, a summary of the high-traffic Linux kernel mailing list up to 10-Nov-2005
★
KernelTrap
★
LWN.net kernel page
★
Linux Kernel Janitor
★
Anatomy of the Linux kernel M. Tim Jones
★
Interactive Linux kernel map
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