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Linux kernel

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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 (GPL), Linux is free software.

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

Contents

History

The project was launched in 1991. 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.

Timeline

Mascot

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

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." [link]

One point of licensing controversy is the kernel's use of firmware "binary blobs" to support some hardware devices. Richard Stallman, author of the GPL, has suggested that kernel versions including such firmware blobs are partially non-free, and may even be violating the GPL (which requires "complete corresponding source code" to be available).[link]

More generally, it is debated 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 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." [link] Especially proprietary graphics drivers are heavily discussed. 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) [link]. Torvalds himself indicated in version 2.4.0 that his own code is only under version 2 [link], but Alan Cox pointed out that very few other kernel contributors have specified a particular version of the GPL [link]. (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.

Litigation

For more details on this topic, see SCO-Linux controversies.
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.

Technical features

Architecture

The Linux kernel supports true preemptive multitasking, virtual memory, shared libraries, demand loading, shared copy-on-write executables, 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. [link]

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

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 is also capable of correctly compiling the kernel.[link]

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.

Portability

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), to the iPod (a portable mp3 player). Linux is intended to run as the main operating system on IBM's new Blue Gene supercomputer architecture when it is finished. Torvalds included, perhaps humorously, BogoMips into the kernel as a performance pseudo-comparison tool.

It is important to note that Torvalds' efforts were also directed successfully at a different sort of portability. Portability, according to Torvalds, 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 free software and other open source applications running.

Linux currently runs on the following machine architectures:

A complete list of ports is at [Linux Ports].

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.

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).

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 stands for Andrew Morton.

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]).

Stable version history

References


Linux
Linux distributions - Comparison of Linux distributions
Open source - Linus's law - Linus Torvalds
Linux kernel - GNU/Linux naming controversy

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External links

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Linux 2.6

 

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