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128-441: The structured program theorem proved that the goto statement is not necessary to write programs that can be expressed as flow charts ; some combination of the three programming constructs of sequence, selection/choice, and repetition/iteration are sufficient for any computation that can be performed by a Turing machine , with the caveat that code duplication and additional variables may need to be introduced. The use of goto

256-440: A goto statement. For example, the C programming language does not permit a jump to a label contained within another function, however jumps within a single call chain are possible using the setjmp/longjmp functions. At the pre-ALGOL meeting held in 1959, Heinz Zemanek explicitly cast doubt on the necessity of GOTO statements; at the time no one paid attention to his remark, including Edsger W. Dijkstra , who later became

384-472: A high-level language implementation of Unix made its porting to different computer platforms easier. Due to an earlier antitrust case forbidding it from entering the computer business, AT&T licensed the operating system's source code as a trade secret to anyone who asked. As a result, Unix grew quickly and became widely adopted by academic institutions and businesses. In 1984, AT&T divested itself of its regional operating companies , and

512-424: A minimalist functionality, while more elaborate window managers such as FVWM , Enlightenment , or Window Maker provide more features such as a built-in taskbar and themes , but are still lightweight when compared to desktop environments. Desktop environments include window managers as part of their standard installations, such as Mutter (GNOME), KWin (KDE), or Xfwm (xfce), although users may choose to use

640-516: A solution stack such as LAMP . The source code of Linux may be used, modified, and distributed commercially or non-commercially by anyone under the terms of its respective licenses, such as the GNU General Public License (GPL). The license means creating novel distributions is permitted by anyone and is easier than it would be for an operating system such as MacOS or Microsoft Windows . The Linux kernel, for example,

768-526: A GOTO statement that unwinds the stack for an out of block transfer and does not permit a transfer into a block from outside of it. Other languages may have their own separate keywords for explicit fallthroughs, which can be considered a version of goto restricted to this specific purpose. For example, Go uses the fallthrough keyword and doesn't allow implicit fallthrough at all, while Perl 5 uses next for explicit fallthrough by default, but also allows setting implicit fallthrough as default behavior for

896-474: A branch. The value of a label variable includes the address of a stack frame, and a goto out of block pops the stack. A simpler way to get an equivalent result is using a label constant array that doesn't even need an explicit declaration of a LABEL type variable: In a DOS batch file , Goto directs execution to a label that begins with a colon. The target of the Goto can be a variable. Many languages support

1024-470: A competing OS, agrees that "Linux wasn't designed, it evolved", but considers this to be a limitation, proposing that some features, especially those related to security, cannot be evolved into, "this is not a biological system at the end of the day, it's a software system." A Linux-based system is a modular Unix-like operating system, deriving much of its basic design from principles established in Unix during

1152-974: A different window manager if preferred. Wayland is a display server protocol intended as a replacement for the X11 protocol; as of 2022 , it has received relatively wide adoption. Unlike X11, Wayland does not need an external window manager and compositing manager. Therefore, a Wayland compositor takes the role of the display server, window manager, and compositing manager. Weston is the reference implementation of Wayland, while GNOME's Mutter and KDE's KWin are being ported to Wayland as standalone display servers. Enlightenment has already been successfully ported since version 19. Additionally, many window managers have been made for Wayland, such as Sway or Hyprland, as well as other graphical utilities such as Waybar or Rofi. Linux currently has two modern kernel-userspace APIs for handling video input devices: V4L2 API for video streams and radio, and DVB API for digital TV reception. Due to

1280-429: A function can be extraordinarily inefficient in some languages; a prime example is Objective-C , where a goto is a much faster alternative. Another use of goto statements is to modify poorly factored legacy code , where avoiding a goto would require extensive refactoring or code duplication . For example, given a large function where only certain code is of interest, a goto statement allows one to jump to or from only

1408-457: A label outside of the current scope, and respects object disposal and finally constructs, making it significantly less powerful and dangerous than the goto keyword in other programming languages. It also makes case and default statements labels, whose scope is the enclosing switch statement ; goto case or goto default is often used as an explicit replacement for implicit fallthrough, which C# disallows. The PL/I programing language has

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1536-567: A language where procedure calls are ubiquitous, this form of optimization considerably reduces the cost of a procedure call compared to the GOTO used in other languages. Steele argued that poorly implemented procedure calls had led to an artificial perception that the GOTO was cheap compared to the procedure call. Steele further argued that "in general procedure calls may be usefully thought of as GOTO statements which also pass parameters, and can be uniformly coded as machine code JUMP instructions", with

1664-406: A loop with two distinct exits. Reducibility was defined by Kosaraju, loosely speaking, as computing the same function and using the same "primitive actions" and predicates as the original program, but possibly using different control flow structures. (This is a narrower notion of reducibility than what Böhm–Jacopini used.) Inspired by this result, in section VI of his highly-cited paper that introduced

1792-611: A module. Most languages that have goto statements call it that, but in the early days of computing, other names were used. For example, in MAD the TRANSFER TO statement was used. APL uses a right pointing arrow, → for goto. C has goto, and it is commonly used in various idioms, as discussed above. Functional programming languages such as Scheme generally do not have goto, instead using continuations. Structured program theorem The structured program theorem , also called

1920-704: A more radical relaxation of structured programming, allowing not only multiple exit points (as in returns in non-tail position), but also multiple entry points, similar to goto statements. Coroutines are more restricted than goto, as they can only resume a currently running coroutine at specified points – continuing after a yield – rather than jumping to an arbitrary point in the code. A limited form of coroutines are generators , which are sufficient for some purposes. Even more limited are closures – subroutines which maintain state (via static variables ), but not execution position. A combination of state variables and structured control, notably an overall switch statement, can allow

2048-406: A necessary and sufficient condition for a program to be non-structured is for its CFG to have as subgraph one of any subset of three of these four graphs. He also found that if a non-structured program contains one of these four sub-graphs, it must contain another distinct one from the set of four. This latter result helps explain how the control flow of non-structured program becomes entangled in what

2176-566: A paper delivered to the ACM conference in Seattle in 1977, Guy L. Steele summarized the debate over the GOTO and structured programming, and observed that procedure calls in the tail position of a procedure can be most optimally treated as a direct transfer of control to the called procedure, typically eliminating unnecessary stack manipulation operations. Since such "tail calls" are very common in Lisp ,

2304-404: A place in server installations such as the popular LAMP application stack. The use of Linux distributions in home and enterprise desktops has been growing. Linux distributions have also become popular in the netbook market, with many devices shipping with customized Linux distributions installed, and Google releasing their own ChromeOS designed for netbooks. Linux's greatest success in

2432-502: A popular CLI shell. The graphical user interface (or GUI) used by most Linux systems is built on top of an implementation of the X Window System . More recently, the Linux community has sought to advance to Wayland as the new display server protocol, in place of X11. Many other open-source software projects contribute to Linux systems. Installed components of a Linux system include the following: The user interface , also known as

2560-481: A reversible program can also be accomplished through a reversible program using only a structured combination of control flow constructs such as sequences, selections, and iterations. Any computation achievable by a traditional, irreversible program can also be accomplished through a reversible program, but with the additional constraint that each step must be reversible and some extra output. Furthermore, any reversible unstructured program can also be accomplished through

2688-463: A selected menu option, for example). PL/I label variables achieve the effect of computed or assigned GOTO s. Up to the 1985 ANSI COBOL standard had the ALTER statement which could be used to change the destination of an existing GO TO, which had to be in a paragraph by itself. The feature, which allowed polymorphism , was frequently condemned and seldom used. In Perl , there is a variant of

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2816-481: A significant business around Linux distributions. The free software licenses , on which the various software packages of a distribution built on the Linux kernel are based, explicitly accommodate and encourage commercialization; the relationship between a Linux distribution as a whole and individual vendors may be seen as symbiotic . One common business model of commercial suppliers is charging for support, especially for business users. A number of companies also offer

2944-524: A single exit point. This purist approach is embodied in the Pascal programming language (designed in 1968–1969), which up to the mid-1990s was the preferred tool for teaching introductory programming classes in academia. Edward Yourdon notes that in the 1970s there was even philosophical opposition to transforming unstructured programs into structured ones by automated means, based on the argument that one needed to think in structured programming fashion from

3072-493: A specialized business version of their distribution, which adds proprietary support packages and tools to administer higher numbers of installations or to simplify administrative tasks. Another business model is to give away the software to sell hardware. This used to be the norm in the computer industry, with operating systems such as CP/M , Apple DOS , and versions of the classic Mac OS before 7.6 freely copyable (but not modifiable). As computer hardware standardized throughout

3200-554: A statement label (line number) which is stored in (assigned to) an integer variable. Jumping to an integer variable that had not been ASSIGNed to was unfortunately possible, and was a major source of bugs involving assigned gotos. The Fortran assign statement only allows a constant (existing) line number to be assigned to the integer variable. However, some compilers allowed accidentally treating this variable as an integer thereafter, for example increment it, resulting in unspecified behavior at goto time. The following code demonstrates

3328-523: A strict hierarchy of programs exists, nowadays called the Kosaraju hierarchy , in that for every integer n , there exists a program containing a multi-level break of depth n that cannot be rewritten as program with multi-level breaks of depth less than n (without introducing additional variables). Kosaraju cites the multi-level break construct to the BLISS programming language. The multi-level breaks, in

3456-527: A structured reversible program with only one iteration without any extra output. This theorem lays the foundational principles for constructing reversible algorithms within a structured programming framework. For the Structured Program Theorem, both local and global methods of proof are known. However, for its reversible version, while a global method of proof is recognized, a local approach similar to that undertaken by Böhm and Jacopini

3584-422: A structured unit prematurely, and a combinatorial explosion with quite complex program state data to handle all possible conditions. Two solutions have been generally adopted: a way to exit a structured unit prematurely, and more generally exceptions – in both cases these go up the structure, returning control to enclosing blocks or functions, but do not jump to arbitrary code locations. These are analogous to

3712-453: A subroutine to resume execution at an arbitrary point on subsequent calls, and is a structured alternative to goto statements in the absence of coroutines; this is a common idiom in C, for example. A continuation is similar to a GOTO in that it transfers control from an arbitrary point in the program to a previously marked point. A continuation is more flexible than GOTO in those languages that support it, because it can transfer control out of

3840-438: A switch statement, or to centralize cleanup tasks in a function with several error returns. (...) Blindly avoiding certain constructs or following rules without understanding them can lead to just as many problems as the rules were supposed to avert. Furthermore, many opinions on programming style are just that: opinions. They may be strongly argued and strongly felt, they may be backed up by solid-seeming evidence and arguments, but

3968-972: A system's software from one central location. A distribution is largely driven by its developer and user communities. Some vendors develop and fund their distributions on a volunteer basis, Debian being a well-known example. Others maintain a community version of their commercial distributions, as Red Hat does with Fedora , and SUSE does with openSUSE . In many cities and regions, local associations known as Linux User Groups (LUGs) seek to promote their preferred distribution and by extension free software. They hold meetings and provide free demonstrations, training, technical support, and operating system installation to new users. Many Internet communities also provide support to Linux users and developers. Most distributions and free software / open-source projects have IRC chatrooms or newsgroups . Online forums are another means of support, with notable examples being Unix & Linux Stack Exchange , LinuxQuestions.org and

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4096-549: A transformation method that goes back to enforcing single-exit. In the 1980s IBM researcher Harlan Mills oversaw the development of the COBOL Structuring Facility , which applied a structuring algorithm to COBOL code. Mills's transformation involved the following steps for each procedure. This construction can be improved by converting some cases of the selection statement into subprocedures. Material not yet covered above: Linux This

4224-461: A useful language feature, improving program speed, size and code clarity, but only when used in a sensible way by a comparably sensible programmer. According to computer science professor John Regehr , in 2013, there were about 100,000 instances of goto in the Linux kernel code. Other academics took a more extreme viewpoint and argued that even instructions like break and return from the middle of loops are bad practice as they are not needed in

4352-402: A user may interact with the application; however, certain extensions of the X Window System are not capable of working over the network. Several X display servers exist, with the reference implementation, X.Org Server , being the most popular. Server distributions might provide a command-line interface for developers and administrators, but provide a custom interface for end-users, designed for

4480-453: A vast body of work and may include both kernel modules and user applications and libraries. Linux vendors and communities combine and distribute the kernel, GNU components, and non-GNU components, with additional package management software in the form of Linux distributions. Many developers of open-source software agree that the Linux kernel was not designed but rather evolved through natural selection . Torvalds considers that although

4608-440: A while loop. Harel notes that the single loop used by the folk version of the structured programming theorem basically just provides operational semantics for the execution of a flowchart on a von Neumann computer. Another, even older source that Harel traced the folk version of the theorem is Stephen Kleene 's normal form theorem from 1936. Donald Knuth criticized this form of the proof, which results in pseudocode like

4736-468: Is goto (20,30,40) i . The equivalent construct in C is the switch statement , and in newer Fortran a SELECT CASE construct is the recommended syntactical alternative. BASIC had a 'On GoTo' statement that achieved the same goal, but in Visual Basic this construct is no longer supported. In versions prior to Fortran 95, Fortran also had an assigned goto variant that transfers control to

4864-581: Is "infinitely abusable", but also suggest that it could be used for end-of-function error handlers and for multi-level breaks from loops. These two patterns can be found in numerous subsequent books on C by other authors; a 2007 introductory textbook notes that the error handling pattern is a way to work around the "lack of built-in exception handling within the C language". Other programmers, including Linux kernel designer and coder Linus Torvalds or software engineer and book author Steve McConnell , also object to Dijkstra's point of view, stating that GOTOs can be

4992-454: Is a 1968 letter by Edsger Dijkstra called " Go-to statement considered harmful ". In that letter, Dijkstra argued that unrestricted GOTO statements should be abolished from higher-level languages because they complicated the task of analyzing and verifying the correctness of programs (particularly those involving loops). The letter itself sparked a debate, including a " 'GOTO Considered Harmful' Considered Harmful" letter sent to Communications of

5120-504: Is an accepted version of this page Linux ( / ˈ l ɪ n ʊ k s / , LIN -uuks ) is a family of open-source Unix-like operating systems based on the Linux kernel , an operating system kernel first released on September 17, 1991, by Linus Torvalds . Linux is typically packaged as a Linux distribution (distro), which includes the kernel and supporting system software and libraries — many of which are provided by

5248-458: Is called the loop and a half problem in this context. Pascal is affected by both of these problems and according to empirical studies cited by Eric S. Roberts , student programmers had difficulty formulating correct solutions in Pascal for several simple problems, including writing a function for searching an element in an array. A 1980 study by Henry Shapiro cited by Roberts found that using only

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5376-410: Is harder to understand than alternative constructions. Debates over its (more limited) uses continue in academia and software industry circles. goto label The goto statement is often combined with the if statement to cause a conditional transfer of control. IF condition THEN goto label Programming languages impose different restrictions with respect to the destination of

5504-446: Is less relevant or completely absent. One of the main alternatives is message passing , which is of particular importance in concurrent computing , interprocess communication , and object oriented programming . In these cases, the individual components do not have arbitrary transfer of control, but the overall control may be scheduled in complex ways, such as via preemption . The influential languages Simula and Smalltalk were among

5632-569: Is licensed under the GPLv2, with an exception for system calls that allows code that calls the kernel via system calls not to be licensed under the GPL. Because of the dominance of Linux-based Android on smartphones , Linux, including Android, has the largest installed base of all general-purpose operating systems as of May 2022 . Linux is, as of March 2024 , used by around 4 percent of desktop computers . The Chromebook , which runs

5760-436: Is mandatory. Although Steele's paper did not introduce much that was new to computer science, at least as it was practised at MIT, it brought to light the scope for procedure call optimization, which made the modularity-promoting qualities of procedures into a more credible alternative to the then-common coding habits of large monolithic procedures with complex internal control structures and extensive state data. In particular,

5888-410: Is not yet known. This distinction is an example that underscores the challenges and nuances in establishing the foundations of reversible computing compared to traditional computing paradigms. The Böhm–Jacopini proof did not settle the question of whether to adopt structured programming for software development, partly because the construction was more likely to obscure a program than to improve it. On

6016-461: Is popularly called " spaghetti code ". McCabe also devised a numerical measure that, given an arbitrary program, quantifies how far off it is from the ideal of being a structured program; McCabe called his measure essential complexity . McCabe's characterization of the forbidden graphs for structured programming can be considered incomplete, at least if the Dijkstra's D structures are considered

6144-484: Is possible to express the same logic without gotos, the equivalent code will be longer and often more difficult to understand). In other languages, there are structured alternatives, notably exceptions and tail calls. Situations in which goto is often useful include: These uses are relatively common in C, but much less common in C++ or other languages with higher-level features. However, throwing and catching an exception inside

6272-453: Is referred to as a computed goto in documentation of the C compilers that support it; its semantics are a superset of Fortran's assigned goto, because it allows arbitrary pointer expressions as the goto target, while Fortran's assigned goto doesn't allow arbitrary expressions as jump target. As with the standard goto in C, the GNU C extension allows the target of the computed goto to reside only in

6400-462: Is still possible to use GOTO in these programming languages, albeit with some restrictions. One can emulate GOTO in Java, JavaScript, and Python. PL/I has the data type LABEL , which can be used to implement both the "assigned goto" and the "computed goto." PL/I allows branches out of the current block. A calling procedure can pass a label as an argument to a called procedure which can then exit with

6528-403: Is that the Linux kernel and other components are free and open-source software. Linux is not the only such operating system, although it is by far the most widely used. Some free and open-source software licenses are based on the principle of copyleft , a kind of reciprocity: any work derived from a copyleft piece of software must also be copyleft itself. The most common free software license,

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6656-594: Is the Bourne-Again Shell (bash), originally developed for the GNU Project, other shells such as Zsh are also used. Most low-level Linux components, including various parts of the userland , use the CLI exclusively. The CLI is particularly suited for automation of repetitive or delayed tasks and provides very simple inter-process communication . On desktop systems, the most popular user interfaces are

6784-475: Is typically built into the firmware and is highly tailored to the system. This includes routers , automation controls, smart home devices , video game consoles , televisions (Samsung and LG smart TVs ), automobiles (Tesla, Audi, Mercedes-Benz, Hyundai, and Toyota), and spacecraft ( Falcon 9 rocket, Dragon crew capsule, and the Ingenuity Mars helicopter). The Unix operating system

6912-514: Is typically credited to a 1966 paper by Corrado Böhm and Giuseppe Jacopini . David Harel wrote in 1980 that the Böhm–Jacopini paper enjoyed "universal popularity", particularly with proponents of structured programming. Harel also noted that "due to its rather technical style [the 1966 Böhm–Jacopini paper] is apparently more often cited than read in detail" and, after reviewing a large number of papers published up to 1980, Harel argued that

7040-415: Is used on a wide variety of devices including PCs, workstations , mainframes and embedded systems . Linux is the predominant operating system for servers and is also used on all of the world's 500 fastest supercomputers . When combined with Android , which is Linux-based and designed for smartphones , they have the largest installed base of all general-purpose operating systems . The Linux kernel

7168-416: The goto statement that is not a traditional GOTO statement at all. It takes a function name and transfers control by effectively substituting one function call for another (a tail call ): the new function will not return to the GOTO, but instead to the place from which the original function was called. There are several programming languages that do not support GOTO by default. By using GOTO emulation, it

7296-634: The goto statement, and many do not. In Java , goto is a reserved word , but is unusable, although compiled .class files generate GOTOs and LABELs. Python does not have support for goto, although there are several joke modules that provide it. There is no goto statement in Seed7 and hidden gotos like break- and continue-statements are also omitted. In PHP there was no native support for goto until version 5.3 (libraries were available to emulate its functionality). C# and Visual Basic .NET both support goto . However, it does not allow jumping to

7424-628: The ACM (CACM) in March 1987, as well as further replies by other people, including Dijkstra's On a Somewhat Disappointing Correspondence . An alternative viewpoint is presented in Donald Knuth 's Structured Programming with go to Statements , which analyzes many common programming tasks and finds that in some of them GOTO is the optimal language construct to use. In The C Programming Language , Brian Kernighan and Dennis Ritchie warn that goto

7552-544: The Böhm–Jacopini theorem , is a result in programming language theory . It states that a class of control-flow graphs (historically called flowcharts in this context) can compute any computable function if it combines subprograms in only three specific ways ( control structures ). These are The structured chart subject to these constraints, particularly the loop constraint implying a single exit (as described later in this article), may however use additional variables in

7680-497: The Free Software Foundation and wrote the GNU General Public License (GNU GPL) in 1989. By the early 1990s, many of the programs required in an operating system (such as libraries, compilers , text editors , a command-line shell , and a windowing system ) were completed, although low-level elements such as device drivers , daemons , and the kernel , called GNU Hurd , were stalled and incomplete. Minix

7808-660: The GNU Project — to create a complete operating system. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses and recommends the name " GNU/Linux " to emphasize the use and importance of GNU software in many distributions, causing some controversy . Thousands of distributions exist, many based directly or indirectly on other distributions; popular Linux distributions include Debian , Fedora Linux , Arch Linux , and Ubuntu , while commercial distributions include Red Hat Enterprise Linux and SUSE Linux Enterprise . Other than

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7936-761: The GNU toolchain , which includes the GNU Compiler Collection (GCC) and the GNU Build System . Amongst others, GCC provides compilers for Ada , C , C++ , Go and Fortran . Many programming languages have a cross-platform reference implementation that supports Linux, for example PHP , Perl , Ruby , Python , Java , Go , Rust and Haskell . First released in 2003, the LLVM project provides an alternative cross-platform open-source compiler for many languages. Proprietary compilers for Linux include

8064-473: The GUI shells , packaged together with extensive Desktop environments , such as KDE Plasma , GNOME , MATE , Cinnamon , LXDE , Pantheon , and Xfce , though a variety of additional user interfaces exist. Most popular user interfaces are based on the X Window System, often simply called "X". It provides network transparency and permits a graphical application running on one system to be displayed on another where

8192-476: The shell , is either a command-line interface (CLI), a graphical user interface (GUI), or controls attached to the associated hardware, which is common for embedded systems. For desktop systems, the default user interface is usually graphical, although the CLI is commonly available through terminal emulator windows or on a separate virtual console . CLI shells are text-based user interfaces, which use text for both input and output. The dominant shell used in Linux

8320-400: The "computed goto" in which the instruction to jump to is determined dynamically (conditionally). Under certain conditions it is possible to eliminate local goto statements of legacy programs by replacing them with multilevel loop exit statements. In practice, a strict adherence to the basic three-structure template of structured programming yields highly nested code, due to inability to exit

8448-399: The 1970s and 1980s. Such a system uses a monolithic kernel , the Linux kernel, which handles process control, networking, access to the peripherals , and file systems . Device drivers are either integrated directly with the kernel or added as modules that are loaded while the system is running. The GNU userland is a key part of most systems based on the Linux kernel, with Android being

8576-454: The 1980s, it became more difficult for hardware manufacturers to profit from this tactic, as the OS would run on any manufacturer's computer that shared the same architecture. Most programming languages support Linux either directly or through third-party community based ports . The original development tools used for building both Linux applications and operating system programs are found within

8704-546: The Böhm–Jacopini result, and thus advocated that loops should have a single exit point. For instance, Bertrand Meyer wrote in his 2009 textbook that instructions like break and continue "are just the old goto in sheep's clothing". A slightly modified form of the Böhm–Jacopini result, however, allows the avoidance of additional variables in structured programming, as long as multi-level breaks from loops are allowed. Because some languages like C don't allow multi-level breaks via their break keyword, some textbooks advise

8832-674: The GNU General Public License (GPL), is a form of copyleft and is used for the Linux kernel and many of the components from the GNU Project. Linux-based distributions are intended by developers for interoperability with other operating systems and established computing standards. Linux systems adhere to POSIX, SUS , LSB , ISO , and ANSI standards where possible, although to date only one Linux distribution has been POSIX.1 certified, Linux-FT. Free software projects, although developed through collaboration , are often produced independently of each other. The fact that

8960-441: The GOTO statement – see language support – though most provide some means of breaking out of a selection, or either breaking out of or moving on to the next step of an iteration. The viewpoint that disturbing the control flow in code is undesirable may be seen in the design of some programming languages, for instance Ada visually emphasizes label definitions using angle brackets . Entry 17.10 in comp.lang.c FAQ list addresses

9088-464: The Linux kernel, key components that make up a distribution may include a display server (windowing system) , a package manager , a bootloader and the Bash shell . Linux is one of the most prominent examples of free and open-source software collaboration. While originally developed for x86 based personal computers , it has since been ported to more platforms than any other operating system, and

9216-650: The Linux kernel-based ChromeOS , dominates the US K–12 education market and represents nearly 20 percent of sub-$ 300 notebook sales in the US. Linux is the leading operating system on servers (over 96.4% of the top one million web servers' operating systems are Linux), leads other big iron systems such as mainframe computers , and is used on all of the world's 500 fastest supercomputers (as of November 2017 , having gradually displaced all competitors). Linux also runs on embedded systems , i.e., devices whose operating system

9344-613: The Linux kernel. On July 3, 1991, to implement Unix system calls , Linus Torvalds attempted unsuccessfully to obtain a digital copy of the POSIX standards documentation with a request to the comp.os.minix newsgroup . After not finding the POSIX documentation, Torvalds initially resorted to determining system calls from SunOS documentation owned by the university for use in operating its Sun Microsystems server. He also learned some system calls from Tanenbaum's Minix text. Torvalds began

9472-502: The Pascal-provided control structures, the correct solution was given by only 20% of the subjects, while no subject wrote incorrect code for this problem if allowed to write a return from the middle of a loop. In 1973, S. Rao Kosaraju proved that it's possible to avoid adding additional variables in structured programming, as long as arbitrary-depth, multi-level breaks from loops are allowed. Furthermore, Kosaraju proved that

9600-629: The ability to structure programs using well-nested executions of routines drawn from a library. This would not have been possible using only goto , since the target code, being drawn from the library, would not know where to jump back to. Later, high-level languages such as Pascal were designed around support for structured programming , which generalized from subroutines (also known as procedures or functions) towards further control structures such as: These new language mechanisms replaced equivalent flows which previously would have been written using goto s and if s. Multi-way branching replaces

9728-471: The beginning, it was rather casually hacked on by huge numbers of volunteers coordinating only through the Internet. Quality was maintained not by rigid standards or autocracy but by the naively simple strategy of releasing every week and getting feedback from hundreds of users within days, creating a sort of rapid Darwinian selection on the mutations introduced by developers." Bryan Cantrill , an engineer of

9856-457: The behavior of the goto i when line i is unspecified: Several C compilers implement two non-standard C/C++ extensions relating to gotos originally introduced by gcc . The GNU extension allows the address of a label inside the current function to be obtained as a void* using the unary, prefix label value operator && . The goto instruction is also extended to allow jumping to an arbitrary void* expression. This C extension

9984-424: The building blocks. Up to 1990 there were quite a few proposed methods for eliminating gotos from existing programs, while preserving most of their structure. The various approaches to this problem also proposed several notions of equivalence, which are stricter than simply Turing equivalence, in order to avoid output like the folk theorem discussed above. The strictness of the chosen notion of equivalence dictates

10112-447: The complexity and diversity of different devices, and due to the large number of formats and standards handled by those APIs, this infrastructure needs to evolve to better fit other devices. Also, a good userspace device library is the key to the success of having userspace applications to be able to work with all formats supported by those devices. The primary difference between Linux and many other popular contemporary operating systems

10240-407: The construction was more likely to obscure a program than to improve it because its application requires the introduction of additional local variables. It did, however, spark a prominent debate among computer scientists, educators, language designers and application programmers that saw a slow but steady shift away from the formerly ubiquitous use of the GOTO. Probably the most famous criticism of GOTO

10368-498: The consumer market is perhaps the mobile device market, with Android being the dominant operating system on smartphones and very popular on tablets and, more recently, on wearables . Linux gaming is also on the rise with Valve showing its support for Linux and rolling out SteamOS , its own gaming-oriented Linux distribution, which was later implemented in their Steam Deck platform. Linux distributions have also gained popularity with various local and national governments, such as

10496-402: The contents of the Böhm–Jacopini proof were usually misrepresented as a folk theorem that essentially contains a simpler result, a result which itself can be traced to the inception of modern computing theory in the papers of von Neumann and Kleene . Harel also writes that the more generic name was proposed by H.D. Mills as "The Structure Theorem" in the early 1970s. This version of

10624-427: The contrary, it signalled the beginning of the debate. Edsger Dijkstra 's famous letter, " Go To Statement Considered Harmful ," followed in 1968. Some academics took a purist approach to the Böhm–Jacopini result and argued that even instructions like break and return from the middle of loops are bad practice as they are not needed in the Böhm–Jacopini proof, and thus they advocated that all loops should have

10752-531: The current context to a surrounding one. The Common Lisp GO operator also has this stack unwinding property, despite the construct being lexically scoped , as the label to be jumped to can be referenced from a closure . In Scheme , continuations can even move control from an outer context to an inner one if desired. This almost limitless control over what code is executed next makes complex control structures such as coroutines and cooperative multitasking relatively easy to write. In non-procedural paradigms, goto

10880-428: The current function, something that a GOTO cannot do in most structured programming languages. In those language implementations that maintain stack frames for storage of local variables and function arguments, executing a continuation involves adjusting the program's call stack in addition to a jump. The longjmp function of the C programming language is an example of an escape continuation that may be used to escape

11008-468: The current function. Attempting to jump outside the current function results in unspecified behavior. Some variants of BASIC also support a computed GOTO in the sense used in GNU C, i.e. in which the target can be any line number, not just one from a list. For example, in MTS BASIC one could write GOTO i*1000 to jump to the line numbered 1000 times the value of a variable i (which might represent

11136-467: The design of Unix served as a scaffolding, "Linux grew with a lot of mutations – and because the mutations were less than random, they were faster and more directed than alpha-particles in DNA ." Eric S. Raymond considers Linux's revolutionary aspects to be social, not technical: before Linux, complex software was designed carefully by small groups, but "Linux evolved in a completely different way. From nearly

11264-508: The development of 386BSD , from which NetBSD , OpenBSD and FreeBSD descended, predated that of Linux. Linus Torvalds has stated on separate occasions that if the GNU kernel or 386BSD had been available at the time (1991), he probably would not have created Linux. While attending the University of Helsinki in the fall of 1990, Torvalds enrolled in a Unix course. The course used a MicroVAX minicomputer running Ultrix , and one of

11392-532: The development of the Linux kernel on Minix and applications written for Minix were also used on Linux. Later, Linux matured and further Linux kernel development took place on Linux systems. GNU applications also replaced all Minix components, because it was advantageous to use the freely available code from the GNU Project with the fledgling operating system; code licensed under the GNU GPL can be reused in other computer programs as long as they also are released under

11520-476: The development of the components of the system and free software. An analysis of the Linux kernel in 2017 showed that well over 85% of the code was developed by programmers who are being paid for their work, leaving about 8.2% to unpaid developers and 4.1% unclassified. Some of the major corporations that provide contributions include Intel , Samsung , Google , AMD , Oracle , and Facebook . Several corporations, notably Red Hat, Canonical , and SUSE have built

11648-510: The federal government of Brazil . Linus Torvalds is the lead maintainer for the Linux kernel and guides its development, while Greg Kroah-Hartman is the lead maintainer for the stable branch. Zoë Kooyman is the executive director of the Free Software Foundation, which in turn supports the GNU components. Finally, individuals and corporations develop third-party non-GNU components. These third-party components comprise

11776-414: The first to introduce the concepts of messages and objects. By encapsulating state data, object-oriented programming reduced software complexity to interactions (messages) between objects. There are a number of different language constructs under the class of goto statements. In Fortran , a computed GOTO jumps to one of several labels in a list, based on the value of an expression. An example

11904-471: The first version of the Linux kernel on the Internet . Like GNU and 386BSD, Linux did not have any Unix code and therefore avoided any current legal issues . Desktop Linux distributions include a windowing system such as X11 or Wayland and a desktop environment such as GNOME , KDE Plasma or Xfce . Distributions intended for servers may not have a graphical user interface at all or include

12032-509: The form a leave label keyword were actually introduced in the BLISS-11 version of that language; the original BLISS only had single-level breaks. The BLISS family of languages didn't provide an unrestricted goto. The Java programming language would later follow this approach as well. A simpler result from Kosaraju's paper is that a program is reducible to a structured program (without adding variables) if and only if it does not contain

12160-439: The form of bits (stored in an extra integer variable in the original proof) in order to keep track of information that the original program represents by the program location. The construction was based on Böhm's programming language P′′ . The theorem forms the basis of structured programming , a programming paradigm which eschews goto commands and exclusively uses subroutines, sequences, selection and iteration. The theorem

12288-431: The get go. The pragmatic counterpoint was that such transformations benefited a large body of existing programs. Among the first proposals for an automated transformation was a 1971 paper by Edward Ashcroft and Zohar Manna . The direct application of the Böhm–Jacopini theorem may result in additional local variables being introduced in the structured chart, and may also result in some code duplication . The latter issue

12416-484: The goto statement does not necessarily lead immediately to beautiful programming: an unstructured programmer is just as capable of constructing a Byzantine tangle without using any goto's (perhaps substituting oddly-nested loops and Boolean control variables, instead). Many programmers adopt a moderate stance: goto's are usually to be avoided, but are acceptable in a few well-constrained situations, if necessary: as multi-level break statements, to coalesce common actions inside

12544-573: The iconic opponent of GOTO. The 1970s and 1980s saw a decline in the use of GOTO statements in favor of the structured programming paradigm , with GOTO criticized as leading to unmaintainable spaghetti code . Some programming style coding standards, for example the GNU Pascal Coding Standards, recommend against the use of GOTO statements. The Böhm–Jacopini proof (1966) did not settle the question of whether to adopt structured programming for software development, partly because

12672-408: The issue of GOTO use directly, stating Programming style, like writing style, is somewhat of an art and cannot be codified by inflexible rules, although discussions about style often seem to center exclusively around such rules. In the case of the goto statement, it has long been observed that unfettered use of goto's quickly leads to unmaintainable spaghetti code. However, a simple, unthinking ban on

12800-473: The machine code stack manipulation instructions "considered an optimization (rather than vice versa!)". Steele cited evidence that well optimized numerical algorithms in Lisp could execute faster than code produced by then-available commercial Fortran compilers because the cost of a procedure call in Lisp was much lower. In Scheme , a Lisp dialect developed by Steele with Gerald Jay Sussman , tail call optimization

12928-516: The mid-1990s in the supercomputing community, where organizations such as NASA started to replace their increasingly expensive machines with clusters of inexpensive commodity computers running Linux. Commercial use began when Dell and IBM , followed by Hewlett-Packard , started offering Linux support to escape Microsoft 's monopoly in the desktop operating system market. Today, Linux systems are used throughout computing, from embedded systems to virtually all supercomputers , and have secured

13056-501: The minimal set of control flow structures needed. The 1988 JACM paper by Lyle Ramshaw surveys the field up to that point, as well proposing its own method. Ramshaw's algorithm was used for example in some Java decompilers because the Java virtual machine code has branch instructions with targets expressed as offsets, but the high-level Java language only has multi-level break and continue statements. Ammarguellat (1992) proposed

13184-441: The notable exception. The GNU C library , an implementation of the C standard library , works as a wrapper for the system calls of the Linux kernel necessary to the kernel-userspace interface, the toolchain is a broad collection of programming tools vital to Linux development (including the compilers used to build the Linux kernel itself), and the coreutils implement many basic Unix tools . The GNU Project also develops Bash ,

13312-489: The notion of cyclomatic complexity , Thomas J. McCabe described an analogue of Kuratowski's theorem for the control-flow graphs (CFG) of non-structured programs, which is to say, the minimal subgraphs that make the CFG of a program non-structured. These subgraphs have a very good description in natural language. They are: McCabe actually found that these four graphs are not independent when appearing as subgraphs, meaning that

13440-541: The one below, by pointing out that the structure of the original program is completely lost in this transformation. Similarly, Bruce Ian Mills wrote about this approach that "The spirit of block structure is a style, not a language. By simulating a Von Neumann machine, we can produce the behavior of any spaghetti code within the confines of a block-structured language. This does not prevent it from being spaghetti." The proof in Böhm and Jacopini's paper proceeds by induction on

13568-501: The operating system to their specific needs. Distributions are maintained by individuals, loose-knit teams, volunteer organizations, and commercial entities. A distribution is responsible for the default configuration of the installed Linux kernel, general system security, and more generally integration of the different software packages into a coherent whole. Distributions typically use a package manager such as apt , yum , zypper , pacman or portage to install, remove, and update all of

13696-426: The opposing opinions may be just as strongly felt, supported, and argued. It's usually futile to get dragged into "style wars", because on certain issues, opponents can never seem to agree, or agree to disagree, or stop arguing. While overall usage of goto has been declining, there are still situations in some languages where a goto provides the shortest and most straightforward way to express a program's logic (while it

13824-549: The programmer to use goto in such circumstances. The MISRA C 2004 standard bans goto , continue , as well as multiple return and break statements. The 2012 edition of the MISRA C standard downgraded the prohibition on goto from "required" to "advisory" status; the 2012 edition has an additional, mandatory rule that prohibits only backward, but not forward jumps with goto . FORTRAN introduced structured programming constructs in 1978, and in successive revisions

13952-552: The relatively loose semantic rules governing the allowable use of goto were tightened; the "extended range" in which a programmer could use a GOTO to leave and re-enter a still-executing DO loop was removed from the language in 1978, and by 1995 several forms of Fortran GOTO, including the Computed GOTO and the Assigned GOTO, had been deleted. Some widely used modern programming languages such as Java and Python lack

14080-451: The relevant code, without otherwise modifying the function. This usage is considered code smell , but finds occasional use. The modern notion of subroutine was invented by David Wheeler when programming the EDSAC . To implement a call and return on a machine without a subroutine call instruction, he used a special pattern of self-modifying code, known as a Wheeler jump . This resulted in

14208-455: The required texts was Operating Systems: Design and Implementation by Andrew S. Tanenbaum . This textbook included a copy of Tanenbaum's Minix operating system. It was with this course that Torvalds first became exposed to Unix. In 1991, he became curious about operating systems. Frustrated by the licensing of Minix, which at the time limited it to educational use only, he began to work on his operating system kernel, which eventually became

14336-412: The same or a compatible license. Torvalds initiated a switch from his original license, which prohibited commercial redistribution, to the GNU GPL. Developers worked to integrate GNU components with the Linux kernel, creating a fully functional and free operating system. Linus Torvalds had wanted to call his invention " Freax ", a portmanteau of "free", "freak", and "x" (as an allusion to Unix). During

14464-441: The software licenses explicitly permit redistribution, however, provides a basis for larger-scale projects that collect the software produced by stand-alone projects and make it available all at once in the form of a Linux distribution. Many Linux distributions manage a remote collection of system software and application software packages available for download and installation through a network connection. This allows users to adapt

14592-586: The start of his work on the system, some of the project's makefiles included the name "Freax" for about half a year. Initially, Torvalds considered the name "Linux" but dismissed it as too egotistical. To facilitate development, the files were uploaded to the FTP server ( ftp.funet.fi ) of FUNET in September 1991. Ari Lemmke, Torvalds' coworker at the Helsinki University of Technology (HUT) who

14720-407: The structure of the flow chart. Because it employed pattern matching in graphs , the proof of Böhm and Jacopini's was not really practical as a program transformation algorithm, and thus opened the door for additional research in this direction. The Reversible Structured Program Theorem is an important concept in the field of reversible computing . It posits that any computation achievable by

14848-437: The tail call optimizations discussed by Steele turned the procedure into a credible way of implementing iteration through single tail recursion (tail recursion calling the same function). Further, tail call optimization allows mutual recursion of unbounded depth, assuming tail calls – this allows transfer of control, as in finite state machines , which otherwise is generally accomplished with goto statements. Coroutines are

14976-438: The theorem replaces all the original program's control flow with a single global while loop that simulates a program counter going over all possible labels (flowchart boxes) in the original non-structured program. Harel traced the origin of this folk theorem to two papers marking the beginning of computing. One is the 1946 description of the von Neumann architecture , which explains how a program counter operates in terms of

15104-471: The use case of the system. This custom interface is accessed through a client that resides on another system, not necessarily Linux-based. Several types of window managers exist for X11, including tiling , dynamic , stacking , and compositing . Window managers provide means to control the placement and appearance of individual application windows, and interact with the X Window System. Simpler X window managers such as dwm , ratpoison , or i3wm provide

15232-680: The use of a return statement in non-terminal position – not strictly structured, due to early exit, but a mild relaxation of the strictures of structured programming. In C, break and continue allow one to terminate a loop or continue to the next iteration , without requiring an extra while or if statement. In some languages multi-level breaks are also possible. For handling exceptional situations, specialized exception handling constructs were added, such as try / catch / finally in Java. The throw-catch exception handling mechanisms can also be easily abused to create non-transparent control structures, just like goto can be abused. In

15360-559: The various distribution-specific support and community forums, such as ones for Ubuntu , Fedora, Arch Linux , Gentoo , etc. Linux distributions host mailing lists ; commonly there will be a specific topic such as usage or development for a given list. There are several technology websites with a Linux focus. Print magazines on Linux often bundle cover disks that carry software or even complete Linux distributions. Although Linux distributions are generally available without charge, several large corporations sell, support, and contribute to

15488-429: The word "Linux" should be pronounced, he included an audio guide with the kernel source code. However, in this recording, he pronounces Linux as /ˈlinʊks/ ( LEEN -uuks ) with a short but close front unrounded vowel , instead of a near-close near-front unrounded vowel as in his newsgroup post. The adoption of Linux in production environments, rather than being used only by hobbyists, started to take off first in

15616-508: Was conceived and implemented in 1969, at AT&T 's Bell Labs , in the United States by Ken Thompson , Dennis Ritchie , Douglas McIlroy , and Joe Ossanna . First released in 1971, Unix was written entirely in assembly language , as was common practice at the time. In 1973, in a key pioneering approach, it was rewritten in the C programming language by Dennis Ritchie (except for some hardware and I/O routines). The availability of

15744-498: Was created by Andrew S. Tanenbaum , a computer science professor, and released in 1987 as a minimal Unix-like operating system targeted at students and others who wanted to learn operating system principles. Although the complete source code of Minix was freely available, the licensing terms prevented it from being free software until the licensing changed in April 2000. Although not released until 1992, due to legal complications ,

15872-510: Was designed by Linus Torvalds , following the lack of a working kernel for GNU , a Unix -compatible operating system made entirely of free software that had been undergoing development since 1983 by Richard Stallman . While a separate working Unix-compatible system called Minix was later released, its license was not entirely free at the time. The first entirely free Unix for personal computers, 386BSD , did not appear until 1992, by which time Torvalds had already built and publicly released

16000-411: Was formerly common, but since the advent of structured programming in the 1960s and 1970s, its use has declined significantly. It remains in use in certain common usage patterns , but alternatives are generally used if available. In the past, there was considerable debate in academia and industry on the merits of the use of goto statements. The primary criticism is that code that uses goto statements

16128-459: Was later originally developed, it represented the first successful commercial attempt at distributing a primarily single-user microcomputer that ran a Unix operating system. With Unix increasingly "locked in" as a proprietary product, the GNU Project , started in 1983 by Richard Stallman , had the goal of creating a "complete Unix-compatible software system" composed entirely of free software . Work began in 1984. Later, in 1985, Stallman started

16256-513: Was one of the volunteer administrators for the FTP server at the time, did not think that "Freax" was a good name, so he named the project "Linux" on the server without consulting Torvalds. Later, however, Torvalds consented to "Linux". According to a newsgroup post by Torvalds, the word "Linux" should be pronounced ( / ˈ l ɪ n ʊ k s / LIN -uuks ) with a short 'i' as in 'print' and 'u' as in 'put'. To further demonstrate how

16384-518: Was released from its obligation not to enter the computer business; freed of that obligation, Bell Labs began selling Unix as a proprietary product, where users were not legally allowed to modify it. Onyx Systems began selling early microcomputer-based Unix workstations in 1980. Later, Sun Microsystems , founded as a spin-off of a student project at Stanford University , also began selling Unix-based desktop workstations in 1982. While Sun workstations did not use commodity PC hardware, for which Linux

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