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GoTo (goto, GOTO, GO TO or other case combinations, depending on the programming language) is a
statement found in many computer
programming language
A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language.
The description of a programming ...
s. It performs a one-way transfer of control to another line of code; in contrast a
function call
In computer programming, a function or subroutine is a sequence of program instructions that performs a specific task, packaged as a unit. This unit can then be used in programs wherever that particular task should be performed.
Functions may ...
normally returns control. The jumped-to locations are usually identified using
labels
A label (as distinct from signage) is a piece of paper, plastic film, cloth, metal, or other material affixed to a container or product, on which is written or printed information or symbols about the product or item. Information printed dir ...
, though some languages use
line number
In computing, a line number is a method used to specify a particular sequence of characters in a text file. The most common method of assigning numbers to lines is to assign every line a unique number, starting at 1 for the first line, and increm ...
s. At the
machine code
In computer programming, machine code is any low-level programming language, consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). Each instruction causes the CPU to perform a very ...
level, a
goto
is a form of
branch or jump statement, in some cases combined with a stack adjustment. Many languages support the
goto
statement, and many do not (see
§ language support).
The
structured program theorem
The structured program theorem, also called 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 ...
proved that the
goto
statement is not necessary to write programs that can be expressed as
flow chart
A flowchart is a type of diagram that represents a workflow or process. A flowchart can also be defined as a diagrammatic representation of an algorithm, a step-by-step approach to solving a task.
The flowchart shows the steps as boxes of va ...
s; 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
A Turing machine is a mathematical model of computation describing an abstract machine that manipulates symbols on a strip of tape according to a table of rules. Despite the model's simplicity, it is capable of implementing any computer algori ...
, with the caveat that
code duplication In computer programming, duplicate code is a sequence of source code that occurs more than once, either within a program or across different programs owned or maintained by the same entity. Duplicate code is generally considered undesirable for a n ...
and additional variables may need to be introduced.
In the past there was considerable debate in academia and industry on the merits of the use of
goto
statements. Use of goto was formerly common, but since the advent of structured programming in the 1960s and 1970s its use has declined significantly. The primary
criticism
Criticism is the construction of a judgement about the negative qualities of someone or something. Criticism can range from impromptu comments to a written detailed response. , ''"the act of giving your opinion or judgment about the good or bad q ...
is that code that uses goto statements is harder to understand than alternative constructions. Goto remains in use in certain
common usage patterns, but
alternatives
Founded in 1994, Alternatives, Action and Communication Network for International Development, is a non-governmental, international solidarity organization based in Montreal, Quebec, Canada.
Alternatives works to promote justice and equality a ...
are generally used if available. Debates over its (more limited) uses continue in academia and software industry circles.
Usage
goto ''label''
The
goto
statement is often combined with the
if statement
In computer science, conditionals (that is, conditional statements, conditional expressions and conditional constructs,) are programming language commands for handling decisions. Specifically, conditionals perform different computations or actio ...
to cause a conditional transfer of control.
IF ''condition'' THEN goto ''label''
Programming languages impose different restrictions with respect to the destination of a
goto
statement. For example, the
C programming language
''The C Programming Language'' (sometimes termed ''K&R'', after its authors' initials) is a computer programming book written by Brian Kernighan and Dennis Ritchie, the latter of whom originally designed and implemented the language, as well as ...
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
setjmp.h is a header defined in the C standard library to provide "non-local jumps": control flow that deviates from the usual subroutine call and return sequence. The complementary functions setjmp and longjmp provide this functionality.
A ...
functions.
Criticism
At the pre-ALGOL meeting held in 1959,
Heinz Zemanek
Heinz Zemanek (actually ''Heinrich Josef Zemanek'') (1 January 1920 – 16 July 2014) was an Austrian computer pioneer who led the development, from 1954 to 1958, of one of the first complete transistorised computers on the European continent. Th ...
explicitly cast doubt on the necessity of GOTO statements; at the time no one paid attention to his remark, including
Edsger W. Dijkstra
Edsger Wybe Dijkstra ( ; ; 11 May 1930 – 6 August 2002) was a Dutch computer scientist, programmer, software engineer, systems scientist, and science essayist. He received the 1972 Turing Award for fundamental contributions to developing progra ...
, who later became the iconic opponent of GOTO. The 1970s and 1980s saw a decline in the use of GOTO statements in favor of the
structured programming
Structured programming is a programming paradigm aimed at improving the clarity, quality, and development time of a computer program by making extensive use of the structured control flow constructs of selection ( if/then/else) and repetition ( ...
paradigm
In science and philosophy, a paradigm () is a distinct set of concepts or thought patterns, including theories, research methods, postulates, and standards for what constitute legitimate contributions to a field.
Etymology
''Paradigm'' comes f ...
, with goto criticized as leading to unmaintainable
spaghetti code
Spaghetti code is a pejorative phrase for unstructured and difficult-to- maintain source code. Spaghetti code can be caused by several factors, such as volatile project requirements, lack of programming style rules, and software engineers with insu ...
. Some
programming style
Programming style, also known as code style, is a set of rules or guidelines used when writing the source code for a computer program. It is often claimed that following a particular programming style will help programmers read and understand sour ...
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 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 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 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
Donald Ervin Knuth ( ; born January 10, 1938) is an American computer scientist, mathematician, and professor emeritus at Stanford University. He is the 1974 recipient of the ACM Turing Award, informally considered the Nobel Prize of computer sc ...
'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
In computer programming, a language construct is a syntactically allowable part of a program that may be formed from one or more lexical tokens in accordance with the rules of the programming language.
The term "language construct" is often used ...
to use. In ''
The C Programming Language
''The C Programming Language'' (sometimes termed ''K&R'', after its authors' initials) is a computer programming book written by Brian Kernighan and Dennis Ritchie, the latter of whom originally designed and implemented the language, as well as ...
'',
Brian Kernighan
Brian Wilson Kernighan (; born 1942) is a Canadian computer scientist.
He worked at Bell Labs and contributed to the development of Unix alongside Unix creators Ken Thompson and Dennis Ritchie. Kernighan's name became widely known through co-au ...
and
Dennis Ritchie
Dennis MacAlistair Ritchie (September 9, 1941 – October 12, 2011) was an American computer scientist. He is most well-known for creating the C programming language and, with long-time colleague Ken Thompson, the Unix operating system and B p ...
warn that
goto
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
Linux ( or ) 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, which ...
kernel designer and coder
Linus Torvalds
Linus Benedict Torvalds ( , ; born 28 December 1969) is a Finnish software engineer who is the creator and, historically, the lead developer of the Linux kernel, used by Linux distributions and other operating systems such as Android. He also c ...
or software engineer and book author
Steve McConnell
Steven C. McConnell is an author of software engineering textbooks such as ''Code Complete'', ''Rapid Development'', and ''Software Estimation''. He is cited as an expert in software engineering and project management.
Career
McConnell graduat ...
, 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. According to computer science professor
John Regehr
John Regehr is a computer scientist specializing in compiler correctness and undefined behavior. , he is a professor at the University of Utah. He is best known for the integer overflow sanitizer which was merged into the Clang C compiler, the C ...
, 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 the Böhm–Jacopini result, and thus advocated that loops should have a single exit point. For instance,
Bertrand Meyer
Bertrand Meyer (; ; born 21 November 1950) is a French academic, author, and consultant in the field of computer languages. He created the Eiffel programming language and the idea of design by contract.
Education and academic career
Meyer recei ...
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 the programmer to use
goto
in such circumstances. The
MISRA C
MISRA C is a set of software development guidelines for the C programming language developed by The MISRA Consortium. Its aims are to facilitate code safety, security, portability and reliability in the context of embedded systems, specifically ...
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 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 enter and leave 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
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
and
Python
Python may refer to:
Snakes
* Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia
** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia
* Python (mythology), a mythical serpent
Computing
* Python (pro ...
lack 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
Ada may refer to:
Places
Africa
* Ada Foah, a town in Ghana
* Ada (Ghana parliament constituency)
* Ada, Osun, a town in Nigeria
Asia
* Ada, Urmia, a village in West Azerbaijan Province, Iran
* Ada, Karaman, a village in Karaman Province, Tur ...
visually emphasizes label definitions using
angle brackets
A bracket is either of two tall fore- or back-facing punctuation marks commonly used to isolate a segment of text or data from its surroundings. Typically deployed in symmetric pairs, an individual bracket may be identified as a 'left' or 'r ...
.
Entry 17.10 in comp.lang.c FAQ list addresses the issue of GOTO use directly, stating
Common usage patterns
While overall usage of gotos 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:
* To make the code more readable and easier to follow
* To make smaller programs, and get rid of code duplication
* Implement a
finite-state machine
A finite-state machine (FSM) or finite-state automaton (FSA, plural: ''automata''), finite automaton, or simply a state machine, is a mathematical model of computation. It is an abstract machine that can be in exactly one of a finite number o ...
, using a
state transition table
State may refer to:
Arts, entertainment, and media Literature
* ''State Magazine'', a monthly magazine published by the U.S. Department of State
* ''The State'' (newspaper), a daily newspaper in Columbia, South Carolina, United States
* ''Our S ...
and goto to switch between states (in absence of
tail call elimination
In computer science, a tail call is a subroutine call performed as the final action of a procedure. If the target of a tail is the same subroutine, the subroutine is said to be tail recursive, which is a special case of direct recursion (computer s ...
), particularly in automatically generated C code. For example,
goto in the canonical LR parser.
* Implementing multi-level break and continue if not directly supported in the language; this is a common idiom in C. Although Java reserves the goto keyword, it doesn't actually implement it. Instead, Java implements labelled break and labelled continue statements. According to the Java documentation, the use of gotos for multi-level breaks was the most common (90%) use of gotos in C. Java was not the first language to take this approach—forbidding goto, but providing multi-level breaks— the
BLISS
BLISS is a system programming language developed at Carnegie Mellon University (CMU) by W. A. Wulf, D. B. Russell, and A. N. Habermann around 1970. It was perhaps the best known system language until C debuted a few years later. Since then, C b ...
programming language (more precisely the BLISS-11 version thereof) preceded it in this respect.
* Surrogates for single-level break or continue (retry) statements when the potential introduction of additional loops could incorrectly affect the control flow. This practice has been observed in
Netbsd
NetBSD is a free and open-source Unix operating system based on the Berkeley Software Distribution (BSD). It was the first open-source BSD descendant officially released after 386BSD was forked. It continues to be actively developed and is a ...
code.
* Error handling (in absence of exceptions), particularly cleanup code such as resource deallocation. C++ offers an alternative to goto statement for this use case, which is :
Resource Acquisition Is Initialization (RAII) through using destructors or using try and catch exceptions used in
Exception handling
In computing and computer programming, exception handling is the process of responding to the occurrence of ''exceptions'' – anomalous or exceptional conditions requiring special processing – during the execution of a program. In general, an ...
.
setjmp
setjmp.h is a header defined in the C standard library to provide "non-local jumps": control flow that deviates from the usual subroutine call and return sequence. The complementary functions setjmp and longjmp provide this functionality.
A t ...
and longjmp are another alternative, and have the advantage of being able to unwind part of the
call stack
In computer science, a call stack is a stack data structure that stores information about the active subroutines of a computer program. This kind of stack is also known as an execution stack, program stack, control stack, run-time stack, or ma ...
.
* Popping the stack in, e.g., Algol, PL/I.
* Specialized scripting languages that operate in a linear manner, such as a dialogue system for video games.
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 a function can be extraordinarily inefficient in some languages; a prime example is
Objective-C
Objective-C is a general-purpose, object-oriented programming language that adds Smalltalk-style messaging to the C programming language. Originally developed by Brad Cox and Tom Love in the early 1980s, it was selected by NeXT for its NeXTS ...
, where a goto is a much faster alternative.
Another use of goto statements is to modify poorly factored
legacy code
In computing, a legacy system is an old method, technology, computer system, or application program, "of, relating to, or being a previous or outdated computer system", yet still in use. Often referencing a system as "legacy" means that it paved ...
, where avoiding a goto would require extensive
refactoring
In computer programming and software design, code refactoring is the process of restructuring existing computer code—changing the '' factoring''—without changing its external behavior. Refactoring is intended to improve the design, structure ...
or
code duplication In computer programming, duplicate code is a sequence of source code that occurs more than once, either within a program or across different programs owned or maintained by the same entity. Duplicate code is generally considered undesirable for a n ...
. 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 considered
code smell
In computer programming, a code smell is any characteristic in the source code of a program that possibly indicates a deeper problem. Determining what is and is not a code smell is subjective, and varies by language, developer, and development meth ...
, but finds occasional use.
Alternatives
Structured programming
The modern notion of
subroutine
In computer programming, a function or subroutine is a sequence of program instructions that performs a specific task, packaged as a unit. This unit can then be used in programs wherever that particular task should be performed.
Functions may ...
was invented by
David Wheeler when programming the
EDSAC
The Electronic Delay Storage Automatic Calculator (EDSAC) was an early British computer. Inspired by John von Neumann's seminal ''First Draft of a Report on the EDVAC'', the machine was constructed by Maurice Wilkes and his team at the Universi ...
. 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
The Wheeler Jump is a type of subroutine call methodology that was used on some early computers that lacked hardware support for saving the return address. The concept was developed by David Wheeler while working on the pioneering EDSAC machine i ...
. 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 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
Pascal, Pascal's or PASCAL may refer to:
People and fictional characters
* Pascal (given name), including a list of people with the name
* Pascal (surname), including a list of people and fictional characters with the name
** Blaise Pascal, Fren ...
were designed around support for
structured programming
Structured programming is a programming paradigm aimed at improving the clarity, quality, and development time of a computer program by making extensive use of the structured control flow constructs of selection ( if/then/else) and repetition ( ...
, which generalized from
subroutine
In computer programming, a function or subroutine is a sequence of program instructions that performs a specific task, packaged as a unit. This unit can then be used in programs wherever that particular task should be performed.
Functions may ...
s (also known as procedures or functions) towards
further
control structures
In computer science, control flow (or flow of control) is the order in which individual statements, instructions or function calls of an imperative program are executed or evaluated. The emphasis on explicit control flow distinguishes an ''imper ...
such as:
* Loops using
while
,
repeat until
or do
, and
for
statements
*
switch
a.k.a.
case
statements, a form of
multiway branch Multiway branch is the change to a program's control flow based upon a value matching a selected criteria. It is a form of conditional statement. A multiway branch is often the most efficient method of passing control to one of a set of program l ...
ing
These new language mechanisms replaced equivalent flows which previously would have been written using
goto
s and
if
s. 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 go to statements of legacy programs by replacing them with multilevel loop exit statements.
Exceptions
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 a
combinatorial explosion
In mathematics, a combinatorial explosion is the rapid growth of the complexity of a problem due to how the combinatorics of the problem is affected by the input, constraints, and bounds of the problem. Combinatorial explosion is sometimes used ...
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 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
In computing and computer programming, exception handling is the process of responding to the occurrence of ''exceptions'' – anomalous or exceptional conditions requiring special processing – during the execution of a program. In general, an ...
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.
Tail calls
In a paper delivered to the ACM conference in Seattle in 1977,
Guy L. Steele
Guy Lewis Steele Jr. (; born October 2, 1954) is an American computer scientist who has played an important role in designing and documenting several computer programming languages and technical standards.
Biography
Steele was born in Missouri ...
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
A lisp is a speech impairment in which a person misarticulates sibilants (, , , , , , , ). These misarticulations often result in unclear speech.
Types
* A frontal lisp occurs when the tongue is placed anterior to the target. Interdental lisping ...
, 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
In computer programming, machine code is any low-level programming language, consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). Each instruction causes the CPU to perform a very ...
JUMP instructions", with 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 scheme is a systematic plan for the implementation of a certain idea.
Scheme or schemer may refer to:
Arts and entertainment
* ''The Scheme'' (TV series), a BBC Scotland documentary series
* The Scheme (band), an English pop band
* ''The Schem ...
, a Lisp dialect developed by Steele with
Gerald Jay Sussman
Gerald Jay Sussman (born February 8, 1947) is the Panasonic Professor of Electrical Engineering at the Massachusetts Institute of Technology (MIT). He received his S.B. and Ph.D. degrees in mathematics from MIT in 1968 and 1973 respectively. ...
, tail call optimization 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, the tail call optimizations discussed by Steele turned the procedure into a credible way of implementing iteration through single
tail recursion
In computer science, a tail call is a subroutine call performed as the final action of a procedure. If the target of a tail is the same subroutine, the subroutine is said to be tail recursive, which is a special case of direct recursion. Tail recur ...
(tail recursion calling the same function). Further, tail call optimization allows
mutual recursion
In mathematics and computer science, mutual recursion is a form of recursion where two mathematical or computational objects, such as functions or datatypes, are defined in terms of each other. Mutual recursion is very common in functional progra ...
of unbounded depth, assuming tail calls – this allows transfer of control, as in
finite state machine
A finite-state machine (FSM) or finite-state automaton (FSA, plural: ''automata''), finite automaton, or simply a state machine, is a mathematical model of computation. It is an abstract machine that can be in exactly one of a finite number o ...
s, which otherwise is generally accomplished with goto statements.
Coroutines
Coroutine
Coroutines are computer program components that generalize subroutines for non-preemptive multitasking, by allowing execution to be suspended and resumed. Coroutines are well-suited for implementing familiar program components such as cooperative ...
s 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 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 variable
In computer programming, a static variable is a variable that has been allocated "statically", meaning that its lifetime (or "extent") is the entire run of the program. This is in contrast to shorter-lived automatic variables, whose storage is ...
s), 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.
Continuations
A
continuation
In computer science, a continuation is an abstract representation of the control state of a computer program. A continuation implements ( reifies) the program control state, i.e. the continuation is a data structure that represents the computati ...
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's
call stack
In computer science, a call stack is a stack data structure that stores information about the active subroutines of a computer program. This kind of stack is also known as an execution stack, program stack, control stack, run-time stack, or ma ...
in addition to a jump. The
longjmp
setjmp.h is a header defined in the C standard library to provide "non-local jumps": control flow that deviates from the usual subroutine call and return sequence. The complementary functions setjmp and longjmp provide this functionality.
A t ...
function of the
C programming language
''The C Programming Language'' (sometimes termed ''K&R'', after its authors' initials) is a computer programming book written by Brian Kernighan and Dennis Ritchie, the latter of whom originally designed and implemented the language, as well as ...
is an example of an escape continuation that may be used to escape the current context to a surrounding one. The
Common Lisp
Common Lisp (CL) is a dialect of the Lisp programming language, published in ANSI standard document ''ANSI INCITS 226-1994 (S20018)'' (formerly ''X3.226-1994 (R1999)''). The Common Lisp HyperSpec, a hyperlinked HTML version, has been derived fro ...
GO operator also has this stack unwinding property, despite the construct being
lexically scoped
In computer programming, the scope of a name binding (an association of a name to an entity, such as a variable) is the part of a program where the name binding is valid; that is, where the name can be used to refer to the entity. In other parts o ...
, as the label to be jumped to can be referenced from a
closure.
In
Scheme A scheme is a systematic plan for the implementation of a certain idea.
Scheme or schemer may refer to:
Arts and entertainment
* ''The Scheme'' (TV series), a BBC Scotland documentary series
* The Scheme (band), an English pop band
* ''The Schem ...
, 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.
Message passing
In non-procedural paradigms, goto is less relevant or completely absent. One of the main alternatives is
message passing
In computer science, message passing is a technique for invoking behavior (i.e., running a program) on a computer. The invoking program sends a message to a process (which may be an actor or object) and relies on that process and its supporting i ...
, which is of particular importance in
concurrent computing
Concurrent computing is a form of computing in which several computations are executed '' concurrently''—during overlapping time periods—instead of ''sequentially—''with one completing before the next starts.
This is a property of a syst ...
,
interprocess communication
In computer science, inter-process communication or interprocess communication (IPC) refers specifically to the mechanisms an operating system provides to allow the processes to manage shared data. Typically, applications can use IPC, categori ...
, and
object oriented programming
Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of pro ...
. 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
Simula is the name of two simulation programming languages, Simula I and Simula 67, developed in the 1960s at the Norwegian Computing Center in Oslo, by Ole-Johan Dahl and Kristen Nygaard. Syntactically, it is an approximate superset of ALGOL 6 ...
and
Smalltalk
Smalltalk is an object-oriented, dynamically typed reflective programming language. It was designed and created in part for educational use, specifically for constructionist learning, at the Learning Research Group (LRG) of Xerox PARC by Alan Ka ...
were among the first to introduce the concepts of messages and objects. By
encapsulating state data,
object-oriented programming
Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of pr ...
reduced software complexity to interactions (messages) between objects.
Variations
There are a number of different language constructs under the class of ''goto'' statements.
and
In
Fortran, a computed
GOTO
jumps to one of several labels in a list, based on the value of an expression. An example is
goto (20,30,40) i
. The equivalent construct in C is the
switch statement
In computer programming languages, a switch statement is a type of selection control mechanism used to allow the value of a variable or expression to change the control flow of program execution via search and map.
Switch statements function some ...
and in newer Fortran a
CASE
statement is the recommend syntactical alternative.
BASIC
BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of general-purpose, high-level programming languages designed for ease of use. The original version was created by John G. Kemeny and Thomas E. Kurtz at Dartmouth College ...
had a
'On GoTo'
statement that achieved the same goal, but in
Visual Basic Visual Basic is a name for a family of programming languages from Microsoft. It may refer to:
* Visual Basic .NET (now simply referred to as "Visual Basic"), the current version of Visual Basic launched in 2002 which runs on .NET
* Visual Basic (cl ...
this construct is no longer supported.
In versions prior to Fortran 95, Fortran also had an assigned 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. The Fortran
assign
statement only allows a constant (existing) line number to be assigned to the integer variable. However, it was possible to accidentally treat this variable as an integer thereafter, for example increment it, resulting in unspecified behavior at
goto
time. The following code demonstrates the behavior of the
goto i
when line
i is unspecified:
assign 200 to i
i = i+1
goto i ! unspecified behavior
200 write(*,*) "this is valid line number"
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 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 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 a selected menu option, for example).
PL/I
PL/I (Programming Language One, pronounced and sometimes written PL/1) is a procedural, imperative computer programming language developed and published by IBM. It is designed for scientific, engineering, business and system programming. I ...
''label variables'' achieve the effect of computed or assigned
GOTO
s.
ALTER
Up to the 1985 ANSI
COBOL
COBOL (; an acronym for "common business-oriented language") is a compiled English-like computer programming language designed for business use. It is an imperative, procedural and, since 2002, object-oriented language. COBOL is primarily us ...
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.
Perl GOTO
In
Perl
Perl is a family of two high-level, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offici ...
, there is a variant of 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
In computer science, a tail call is a subroutine call performed as the final action of a procedure. If the target of a tail is the same subroutine, the subroutine is said to be tail recursive, which is a special case of direct recursion. Tail recur ...
): the new function will not return to the GOTO, but instead to the place from which the original function was called.
Emulated GOTO
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, JavaScript, and Python.
PL/I label variables
PL/I
PL/I (Programming Language One, pronounced and sometimes written PL/1) is a procedural, imperative computer programming language developed and published by IBM. It is designed for scientific, engineering, business and system programming. 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 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 a ''label constant array'' that doesn't even need an explicit declaration of a LABEL type variable:
/* This implements the equivalent of */
/* the computed goto */
declare inx fixed;
...
goto where(inx);
...
where(1): /* statement */ ;
...
where(2): /* statement */ ;
...
MS/DOS GOTO
In a DOS
batch file
Batch may refer to:
Food and drink
* Batch (alcohol), an alcoholic fruit beverage
* Batch loaf, a type of bread popular in Ireland
* A dialect term for a bread roll used in North Warwickshire, Nuneaton and Coventry, as well as on the Wirral ...
, Goto directs execution to a label that begins with a colon.
The target of the Goto can be a variable.
@echo off
SET D8str=%date%
SET D8dow=%D8str:~0,3%
FOR %%D in (Mon Wed Fri) do if "%%D" "%D8dow%" goto SHOP%%D
echo Today, %D8dow%, is not a shopping day.
goto end
:SHOPMon
echo buy pizza for lunch - Monday is Pizza day.
goto end
:SHOPWed
echo buy Calzone to take home - today is Wednesday.
goto end
:SHOPFri
echo buy Seltzer in case somebody wants a zero calorie drink.
:end
Language support
Many languages support the
goto
statement, and many do not. In
Java
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
,
goto
is a
reserved word
In a computer language, a reserved word (also known as a reserved identifier) is a word that cannot be used as an identifier, such as the name of a variable, function, or label – it is "reserved from use". This is a syntactic definition, and a re ...
, but is unusable, although the compiled file.class generates GOTOs and LABELs.
[ Unlike C and C++, the Java programming language has no goto statement; identifier statement labels are used with break (§14.15) or continue (§14.16) statements appearing anywhere within the labeled statement. The keywords const and goto are reserved, even though they are not currently used. This may allow a Java compiler to produce better error messages if these C++ keywords incorrectly appear in programs.] Python does not have support for goto, although there are several joke modules that provide it. There is no goto statement in
Seed7
Seed7 is an extensible general-purpose programming language designed by Thomas Mertes. It is syntactically similar to Pascal and Ada. Along with many other features, it provides an extension mechanism. Daniel Zingaro"Modern Extensible Languages" ...
and hidden gotos like break- and continue-statements are also omitted. In
PHP
PHP is a general-purpose scripting language geared toward web development. It was originally created by Danish-Canadian programmer Rasmus Lerdorf in 1993 and released in 1995. The PHP reference implementation is now produced by The PHP Group ...
there was no native support for
goto
until version 5.3 (libraries were available to emulate its functionality).
The
C# programming language has
goto
. However, it does not allow jumping to a label outside of the current scope, 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
In computer programming languages, a switch statement is a type of selection control mechanism used to allow the value of a variable or expression to change the control flow of program execution via search and map.
Switch statements function some ...
; ''goto case'' or ''goto default'' is often used as an explicit replacement for implicit fallthrough, which C# disallows.
The
PL/I
PL/I (Programming Language One, pronounced and sometimes written PL/1) is a procedural, imperative computer programming language developed and published by IBM. It is designed for scientific, engineering, business and system programming. 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 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 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.
See also
*
COMEFROM
In computer programming, COMEFROM (or COME FROM) is an obscure control flow structure used in some programming languages, originally as a joke. COMEFROM is the inverse of GOTO in that it can take the execution state from any arbitrary point in code ...
*
Control flow
In computer science, control flow (or flow of control) is the order in which individual statements, instructions or function calls of an imperative program are executed or evaluated. The emphasis on explicit control flow distinguishes an ''imper ...
*
GOSUB
BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of general-purpose, high-level programming languages designed for ease of use. The original version was created by John G. Kemeny and Thomas E. Kurtz at Dartmouth College in ...
*
Switch statement
In computer programming languages, a switch statement is a type of selection control mechanism used to allow the value of a variable or expression to change the control flow of program execution via search and map.
Switch statements function some ...
– a
multiway branch Multiway branch is the change to a program's control flow based upon a value matching a selected criteria. It is a form of conditional statement. A multiway branch is often the most efficient method of passing control to one of a set of program l ...
(or conditional goto)
*
Unstructured programming
Non-structured programming is the historically earliest programming paradigm capable of creating Turing-complete algorithms. It is often contrasted with the structured programming paradigm, in particular with the use of unstructured control flow u ...
Notes
References
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{{refend
BASIC commands
Control flow
Edsger W. Dijkstra