Infinite loop: Difference between revisions

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{{shortShort description|Programming idiom}}

{{About-distinguish|the programming term|Infinite Loop (street)|Infinite Loop (book)}}

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{{Loop constructs}}<!-- NOTE:DO Please don'tNOT remove. Discuss navigation concept at [[Talk:Do_while_loopDo while loop#Helpbox_experimentHelpbox experiment]] -->

In [[computer programming]], an '''infinite loop''' (or '''endless loop''')<ref>{{cite web

|url=https://www.yourdictionary.com/endless-loop

|title=Endless loop dictionary definition

|access-date=2020-01-22

|archive-date=2020-08-01

|archive-url=https://web.archive.org/web/20200801213748/https://www.yourdictionary.com/endless-loop

|url-status=live

}}</ref><ref>{{cite web

|url=https://whatis.techtarget.com/definition/infinite-loop-endless-loop

|title=What is infinite loop (endless loop)

|access-date=2020-01-22

|archive-date=2019-07-15

|archive-url=https://web.archive.org/web/20190715101446/https://whatis.techtarget.com/definition/infinite-loop-endless-loop

|url-status=live

}}</ref> is a sequence of instructions that, as written, will continue endlessly, unless an external intervention occurs, ("pullsuch theas turning off power via a switch or pulling a plug"). It may be intentional.

==Overview==

This differs from "a type of computer program that runs the same instructions continuously until it is either stopped or interrupted".<ref>{{cite news

|newspaperwork=[[The New York Times]]

|url=https://archive.nytimes.com/www.nytimes.com/library/tech/99/08/biztech/articles/16digi.html

|title=Overload of Hangers-On Creates Bumpy Ride for Internet Stocks

|last=Caruso |authorfirst=Denise Caruso

|date=August 16, 1999

|access-date=December 27, 2019

|archive-date=December 27, 2019

|archive-url=https://web.archive.org/web/20191227145519/https://archive.nytimes.com/www.nytimes.com/library/tech/99/08/biztech/articles/16digi.html

|url-status=live

}}</ref> Consider the following [[pseudocode]]:

<syntaxhighlight lang="lua">

how_many = 0

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==Details==

An ''infinite loop'' is a sequence of instructions in a [[computer program]] which loops endlessly, either due to the [[control flow#Loops|loop]] having no terminating condition,<ref>{{cite magazine

|magazine=Flow Journal

|url=http://www.flowjournal.org/tag/loop-media/?print=print-search

|title=Codes and Modes: The Character of Documentary Culture

|quote=an infinite loop is one that lacks .. an exit condition

|date=November 2014

|access-date=2020-01-23

|archive-date=2020-08-01

|archive-url=https://web.archive.org/web/20200801213624/http://www.flowjournal.org/tag/loop-media/?print=print-search

|url-status=live

}}</ref> having one that can never be met, or one that causes the loop to start over. In older [[operating system]]s with [[cooperative multitasking]],<ref>also known as non-preemptive-multitasking: {{cite magazine

|author=<!-- Unstated -->

|magazine=[[PC Magazine]]

|url=https://www.pcmag.com/encyclopedia/term/48051/non-preemptive-multitasking

|title=Non-preemptive Multitasking

|access-date=August 15, 2015

|archiveaccess-date=JulyFebruary 267, 20192024

|archive-date=July 26, 2019

|archive-url=https://web.archive.org/web/20190726232111/https://www.pcmag.com/encyclopedia/term/48051/non-preemptive-multitasking

|url-status=live

}}</ref> infinite loops normally caused the entire system to become unresponsive. With the now-prevalent preemptive multitasking model, infinite loops usually cause the program to consume all available processor time, but can usually be terminated by thea user. [[Busy waiting|Busy wait]] loops are also sometimes called "infinite loops". Infinite loops are one possible cause for a computer "[[hangHang (computing)|hanging or freezing]]"; others include [[thrashingThrashing (computer science)|thrashing]], [[deadlock]], and [[segmentation fault|access violation]]s.

==Intended vs unintended looping==

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Modern interactive computers require that the computer constantly be monitoring for user input or device activity, so at some fundamental level there is an infinite processing [[idle loop]] that must continue until the device is turned off or reset. In the [[Apollo Guidance Computer]], for example, this outer loop was contained in the Exec program,<ref>{{cite web

|url=http://klabs.org/history/history_docs/mit_docs/1711.pdf

|title=The History of Apollo On-board Guidance, Navigation, and Control

|author=David Hoag

|date=September 1976

|publisher=Charles Stark Draper Laboratory

|access-date=2020-01-23

|archive-date=2016-11-05

|archive-url=https://web.archive.org/web/20161105060425/http://klabs.org/history/history_docs/mit_docs/1711.pdf

|url-status=live

}}</ref> and if the computer had absolutely no other work to do, it would loop run a dummy job that would simply turn off the "computer activity" indicator light.

Modern computers also typically do not halt the processor or motherboard circuit-driving clocks when they crash. Instead they fall back to an error condition displaying messages to the operator (such as the [[blue screen of death]]), and enter an infinite loop waiting for the user to either respond to a prompt to continue, or reset the device.

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[[File:Infinite loop BSOD.jpg|thumb|A [[blue screen of death]] on [[Windows XP]]. "The [[device driver]] got stuck in an infinite loop."]]

Most often, the term is used for those situations when this is not the intended result; that is, when this is a [[software bug|bug]].<ref>{{cite web

|url=https://nyxcrossword.com/2013/10/1013-13-new-york-times-crossword.html

|title=New York Times Crossword Answers

|quote=computing .. a defect .. which .. to loop

|date=October 13, 2013

|access-date=January 22, 2020

|archive-date=August 2, 2020

|archive-url=https://web.archive.org/web/20200802040416/https://nyxcrossword.com/2013/10/1013-13-new-york-times-crossword.html

|url-status=live

}}</ref> Such errors are most common amongby novice programmers, but can be made by experienced programmers as wellalso, because their causes can be quite subtle.

One common cause, for example, is that thea programmer intends to iterate over sequence of nodes in a [[data structure]] such as a [[linked list]] or [[treeTree (data structure)|tree]], executing the loop code once for each node. Improperly formed links can create a ''reference loop'' in the data structure, where one node links to another that occurs earlier in the sequence. This makes part of the data structure into a [[ringRing (data structure)|ring]], causing naive code to loop forever.

While most infinite loops can be found by close inspection of the code, there is no general method to determine whether a given program will ever halt or will run forever; this is the [[undecidable problem|undecidability]] of the [[halting problem]].<ref>{{cite web|url=https://www.geeksforgeeks.org/halting-problem-in-theory-of-computation|title=Halting Problem in Theory of Computation|date=3 October 2018|access-date=22 January 2020|archive-date=9 August 2020|archive-url=https://web.archive.org/web/20200809100104/https://www.geeksforgeeks.org/halting-problem-in-theory-of-computation/|url-status=live}}</ref>

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==Interruption==

As long as the system is responsive, infinite loops can often be interrupted by sending a signal to the process (such as [[SIGINT (POSIX)|SIGINT]] in Unix), or an [[interrupt]] to the processor, causing the current process to be aborted. This can be done in a [[task manager]], in a terminal with the [[Control-C]] command,<ref>{{cite web

|url=https://pen-testing.sans.org/resources/papers/gcih/buffer-overflow-exploit-dameware-remote-control-software-104168

|title=A Buffer Overflow Exploit Against the DameWare Remote Control software

|quote=As soon as the command shell is closed with a control-c combination ...

|date=December 19, 2003

|access-date=January 22, 2020

|archive-date=July 24, 2020

|archive-url=https://web.archive.org/web/20200724200739/https://pen-testing.sans.org/resources/papers/gcih/buffer-overflow-exploit-dameware-remote-control-software-104168

|url-status=live

}}</ref> or by using the [[killKill (command)|kill]] command or [[system call]]. However, this does not always work, as the process may not be responding to signals or the processor may be in an uninterruptible state, such as in the [[Cyrix coma bug]] (caused by overlapping uninterruptible instructions in an [[instruction pipeline]]). In some cases other signals such as [[SIGKILL]] can work, as they do not require the process to be responsive, while in other cases the loop cannot be terminated short of system shutdown.

==Language support==

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This creates a situation where <code>x</code> will never be greater than 5, since at the start of the loop code, <code>x</code> is assigned the value of 1 (regardless of any previous value) before it is changed to <code>x</code> + 1. Thus the loop will always result in <code>x</code> = 2 and will never break. This could be fixed by moving the <code>x = 1</code> instruction outside the loop so that its initial value is set only once.

In some languages, programmer confusion about the mathematical symbols may lead to an unintentional infinite loop. For example, here is a snippet in [[C (programming language)|C]]:

<syntaxhighlight lang=c>

#include <stdio.h>

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</syntaxhighlight>

The expected output is the numbers 0 through 9, with an interjected "a equals 5!" between 5 and 6. However, in the line "<code>if (a = 5)</code>" above, the programmer has confused the = (assignment) operator was confused with the == (equality test) operator. Instead, this will assign the value of 5 to <code>a</code> at this point in the program. Thus, <code>a</code> will never be able to advance to 10, and this loop cannot terminate.

===Rounding errors===

{| style="float:right; border: 1px solid grey;"

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Infinite recursion is a special case of an infinite loop that is caused by [[recursion (computer science)|recursion]].

The following example in [[Visual Basic for Applications|VBA]] (VBA) returns a [[stack overflow]] error:

<syntaxhighlight lang="vbscript">

Sub Test1()

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===Alderson loop===

''Alderson loop'' is a rare slang or [[The Jargon File|jargon]] term for an infinite loop where there is an exit condition available, but inaccessible in the currentan implementation of the code, typically due to a programmer's error. These are most common and visible while [[debugging]] [[user interface]] code.

A C-like pseudocode example of an Alderson loop, where the program is supposed to sum numbers given by the user until zero is given, but where the programmerwrong hasoperator is used the wrong operator:

<syntaxhighlight lang="C">

int sum = 0;

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The term allegedly received its name from a programmer (last name Alderson) who in 1996<ref>{{cite web

|url=https://www.lee-dohm.com/2013/05/24/alderson-loop

|title=Alderson loop

|author=Lee Dohm

|date=May 24, 2013

|access-date=January 22, 2020

|archive-date=June 19, 2020

|archive-url=https://web.archive.org/web/20200619200434/https://www.lee-dohm.com/2013/05/24/alderson-loop/

|url-status=live

}}</ref> had coded a [[modal window|modal]] [[dialog box]] in [[Microsoft Access]] without either an OK or Cancel button, thereby disabling the entire program whenever the box came up.<ref>{{Cite web |url=http://www.catb.org/~esr/jargon/html/A/Alderson-loop.html |title=Alderson Loop |website=[[The Jargon File]], Version 4.4.7 |url-status=live |archive-url=https://web.archive.org/web/20060515053043/http://www.catb.org/~esr/jargon/html/A/Alderson-loop.html |archive-date=2006-05-15 |access-date=2006-05-21}}</ref>

==See also==

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==References==

{{Reflist}}

<references/>

==External links==

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{{DEFAULTSORT:Infinite Loop}}

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