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Goto is astatement found in many computerprogramming languages. It performs aone-way transfer of control to another line of code; in contrast afunction call normally returns control. The jumped-to locations are usually identified usinglabels, though some languages useline numbers. At themachine code level, agoto is a form ofbranch or jump statement, in some cases combined with a stack adjustment. Many languages support thegoto statement, and many do not (see§ language support).
Thestructured program theorem proved that thegoto statement is not necessary to write programs that can be expressed asflow 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 aTuring machine, with the caveat thatcode duplication and additional variables may need to be introduced.[1]
The use of goto was formerly common, but since the advent ofstructured programming in the 1960s and 1970s, its use has declined significantly. It remains in use in certaincommon usage patterns, butalternatives 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 primarycriticism is that code that uses goto statements is harder to understand than alternative constructions. Debates over its (more limited) uses continue in academia and software industry circles.
gotolabel
Thegoto statement is often combined with theif statement to cause a conditional transfer of control.
IFconditionTHENgotolabel
Programming languages impose different restrictions with respect to the destination of agoto statement. For example, theC programming language does not permit a jump to a label contained within another function,[2] however jumps within a single call chain are possible using thesetjmp/longjmp functions.
At thepre-ALGOL meeting held in 1959,Heinz Zemanek explicitly cast doubt on the necessity of GOTO statements; at the time no one[citation needed] paid attention to his remark, includingEdsger W. Dijkstra, who later became the iconic opponent of GOTO.[3] The 1970s and 1980s saw a decline in the use of GOTO statements in favor of thestructured programmingparadigm, with GOTO criticized as leading to unmaintainablespaghetti code. Someprogramming style coding standards, for example the GNU Pascal Coding Standards, recommend against the use of GOTO statements.[4] TheBöhm–Jacopini proof (1966) 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 because its application requires the introduction of additional local variables.[5] 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 is a 1968 letter by Edsger Dijkstra called "Go-to statement considered harmful".[3] 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).[6] The letter itself sparked a debate, including a"'GOTO Considered Harmful' Considered Harmful" letter[7] sent toCommunications of theACM (CACM) in March 1987, as well as further replies by other people, including Dijkstra'sOn a Somewhat Disappointing Correspondence.[8]
An alternative viewpoint is presented inDonald Knuth'sStructured Programming with go to Statements, which analyzes many common programming tasks and finds that in some of them GOTO is the optimallanguage construct to use.[9] InThe C Programming Language,Brian Kernighan andDennis Ritchie warn thatgoto is "infinitely abusable", but also suggest that it could be used for end-of-function error handlers and for multi-level breaks from loops.[10] These two patterns can be found in numerous subsequent books on C by other authors;[11][12][13][14] 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".[11] Other programmers, includingLinux kernel designer and coderLinus Torvalds or software engineer and book authorSteve McConnell, also object to Dijkstra's point of view, stating that GOTOs can be a useful language feature, improving program speed, size and code clarity, but only when used in a sensible way by a comparably sensible programmer.[15][16] According to computer science professorJohn Regehr, in 2013, there were about 100,000 instances of goto in the Linux kernel code.[17]
Other academics took a more extreme viewpoint and argued that even instructions likebreak andreturn from the middle of loops are bad practice as they are not needed in the Böhm–Jacopini result, and thus advocated that loops should have a single exit point.[18] For instance,Bertrand Meyer wrote in his 2009 textbook that instructions likebreak andcontinue "are just the oldgoto in sheep's clothing".[19] 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.[20] Because some languages like C don't allow multi-level breaks via theirbreak keyword, some textbooks advise the programmer to usegoto in such circumstances.[14] TheMISRA C 2004 standard bansgoto,continue, as well as multiplereturn andbreak statements.[21] The 2012 edition of the MISRA C standard downgraded the prohibition ongoto from "required" to "advisory" status; the 2012 edition has an additional, mandatory rule that prohibits only backward, but not forward jumps withgoto.[22][23]
FORTRAN introduced structured programming constructs in 1978, and in successive revisions 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,[24] and by 1995 several forms of Fortran GOTO, including the Computed GOTO and the Assigned GOTO, had been deleted.[25] Some widely used modern programming languages such asJava andPython lack the GOTO statement – seelanguage support – though most provide some means of breaking out of a selection, or eitherbreaking out of ormoving 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 instanceAda[26] visually emphasizes label definitions usingangle brackets.
Entry 17.10 in comp.lang.c FAQ list[27] addresses 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 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 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 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 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.[34] However, throwing and catching an exception inside a function can be extraordinarily inefficient in some languages; a prime example isObjective-C, where a goto is a much faster alternative.[37]
Another use of goto statements is to modify poorly factoredlegacy code, where avoiding a goto would require extensiverefactoring orcode 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 the relevant code, without otherwise modifying the function. This usage is consideredcode smell,[38] but finds occasional use.
The modern notion ofsubroutine was invented byDavid Wheeler when programming theEDSAC. 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 aWheeler jump.[39] This resulted in the ability to structure programs using well-nested executions of routines drawn from a library. This would not have been possible using onlygoto, since the target code, being drawn from the library, would not know where to jump back to.
Later, high-level languages such asPascal were designed around support forstructured programming, which generalized fromsubroutines (also known as procedures or functions) towardsfurthercontrol structures such as:
while,repeat until ordo, andfor statementsswitch a.k.a.case statements, a form ofmultiway branchingThese new language mechanisms replaced equivalent flows which previously would have been written usinggotos andifs. Multi-way branching replaces 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.[40]
In practice, a strict adherence to the basic three-structure template of structured programming yields highly nested code, due to inability to exit a structured unit prematurely, and acombinatorial 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 generallyexceptions – in both cases these goup the structure, returning control to enclosing blocks or functions, but do not jump to arbitrary code locations. These are analogous to 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 andcontinue allow one toterminate a loop orcontinue to the next iteration, without requiring an extrawhile orif statement. In some languages multi-level breaks are also possible. For handling exceptional situations, specializedexception handling constructs were added, such astry/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.[41]
In 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.[42] Since such "tail calls" are very common inLisp, 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 asmachine code JUMP instructions", with the machine code stack manipulation instructions "considered an optimization (rather than vice versa!)".[42] 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. InScheme, a Lisp dialect developed by Steele withGerald Jay Sussman, tail call optimization is mandatory.[43]
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, the tail call optimizations discussed by Steele turned the procedure into a credible way of implementing iteration through singletail recursion (tail recursion calling the same function). Further, tail call optimization allowsmutual recursion of unbounded depth, assuming tail calls – this allows transfer of control, as infinite-state machines, which otherwise is generally accomplished with goto statements.
Coroutines are 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 onlyresume 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 aregenerators, which are sufficient for some purposes. Even more limited areclosures – subroutines which maintain state (viastatic variables), but not execution position. A combination of state variables and structured control, notably an overall switch statement, can allow 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.
Acontinuation 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 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'scall stack in addition to a jump. Thelongjmp function of theC programming language is an example of an escape continuation that may be used to escape the current context to a surrounding one. TheCommon Lisp GO operator also has this stack unwinding property, despite the construct beinglexically scoped, as the label to be jumped to can be referenced from aclosure.
InScheme, 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.[43]
In non-procedural paradigms, goto is less relevant or completely absent. One of the main alternatives ismessage passing, which is of particular importance inconcurrent computing,interprocess communication, andobject 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 viapreemption. The influential languagesSimula andSmalltalk were among the first to introduce the concepts of messages and objects. Byencapsulating state data,object-oriented programming reduced software complexity to interactions (messages) between objects.
There are a number of different language constructs under the class ofgoto statements.
InFortran, acomputedGOTO jumps to one of several labels in a list, based on the value of an expression. An example isgoto (20,30,40) i.[44] The equivalent construct in C is theswitch statement, and in newer Fortran aSELECT CASE construct is the recommended syntactical alternative.[45]BASIC had a'On GoTo' statement that achieved the same goal, but inVisual Basic this construct is no longer supported.[46]
In versions prior to Fortran 95, Fortran also had anassigned goto variant that transfers control to 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.[47] The Fortranassign 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 atgoto time. The following code demonstrates the behavior of thegoto i when linei is unspecified:
assign200toii=i+1gotoi! unspecified behavior200write(*,*)"this is valid line number"
Several C compilers implement two non-standard C/C++ extensions relating to gotos originally introduced bygcc.[48] The GNU extension allows the address of a label inside the current function to be obtained as avoid* using the unary, prefixlabel value operator&&. The goto instruction is also extended to allow jumping to an arbitraryvoid* expression. This C extension is referred to as acomputed 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.[49] As with the standard goto in C, the GNU C extension allows the target of the computed goto to reside only in the current function. Attempting to jump outside the current function results in unspecified behavior.[49]
Some variants of BASIC also support a computed GOTO in the sense used in GNU C, i.e. in which the target can beany line number, not just one from a list. For example, inMTS BASIC one could writeGOTO i*1000 to jump to the line numbered 1000 times the value of a variablei (which might represent a selected menu option, for example).[50]
PL/Ilabel variables achieve the effect of computed or assignedGOTOs.
Up to the 1985 ANSICOBOL 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.[51] The feature, which allowedpolymorphism, was frequently condemned and seldom used.[52]
InPerl, there is a variant of thegoto 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 (atail call): the new function will not return to the GOTO, but instead to the place from which the original function was called.[53]
There are several programming languages that do not support GOTO by default. By using GOTO emulation, it is still possible to use GOTO in these programming languages, albeit with some restrictions. One can emulate GOTO in Java,[54] JavaScript,[55] and Python.[56][57]
PL/I has the data typeLABEL, 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 a branch. The value of a label variable includes the address of a stack frame, and a goto out of block pops the stack.
/* This implements the equivalent of */ /* the assigned goto */ declare where label; where = somewhere; goto where; ... somewhere: /* statement */ ; ...
/* This implements the equivalent of */ /* the computed goto */ declare where (5) label; declare inx fixed; where(1) = abc; where(2) = xyz; ... goto where(inx); ... abc: /* statement */ ; ... xyz: /* statement */ ; ...
A simpler way to get an equivalent result is using alabel constant array that doesn't even need an explicit declaration of aLABEL type variable:
/* This implements the equivalent of */ /* the computed goto */ declare inx fixed; ... goto where(inx); ... where(1): /* statement */ ; ... where(2): /* statement */ ; ...
In a DOSbatch file, Goto directs execution to a label that begins with a colon.The target of the Goto can be a variable.
@echo offSETD8str=%date%SETD8dow=%D8str:~0,3%FOR%%Din(Mon Wed Fri)doif"%%D"=="%D8dow%"gotoSHOP%%Decho Today,%D8dow%, is not a shopping day.gotoend:SHOPMonecho buy pizza for lunch - Monday is Pizza day.gotoend:SHOPWedecho buy Calzone to take home - today is Wednesday.gotoend:SHOPFriecho buy Seltzer in case somebody wants a zero calorie drink.:end
Many languages support thegoto statement, and many do not. InJava,goto is areserved word, but is unusable, although compiled.class files generate GOTOs and LABELs.[58] Python does not have support for goto, although there are several joke modules that provide it.[56][57] There is no goto statement inSeed7 and hidden gotos like break- and continue-statements are also omitted.[59] InPHP there was no native support forgoto until version 5.3 (libraries were available to emulate its functionality).[60]
C# andVisual Basic .NET both supportgoto.[61][62] However, it does not allow jumping to a label outside of the current scope, and respects object disposal and finally constructs, making it significantly less powerful and dangerous than thegoto keyword in other programming languages. It also makescase anddefault statements labels, whose scope is the enclosingswitch statement;goto case orgoto default is often used as an explicit replacement for implicit fallthrough, which C# disallows.
ThePL/I programing language has 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 ofgoto restricted to this specific purpose. For example, Go uses thefallthrough keyword and doesn't allow implicit fallthrough at all,[63] while Perl 5 usesnext for explicit fallthrough by default, but also allows setting implicit fallthrough as default behavior for a module.
Most languages that have goto statements call it that, but in the early days of computing, other names were used. For example, inMAD the TRANSFER TO statement was used.[64]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.
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