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if posts do not occur at the ends of a line-segment-like fence (e.g., a fence between and wall-anchored to two buildings). The precise problem definition must be carefully considered, as the setup for one situation may give the wrong answer for other situations. Fencepost errors come from counting
624:
In larger numbers, being off by one is often not a major issue. In smaller numbers, however, and specific cases where accuracy is paramount, committing an off-by-one error can be disastrous. Sometimes such an issue will also be repeated and, therefore, worsened, by someone passing on an incorrect
556:
sections. This relationship is important to consider when dealing with the reverse error. The reverse error occurs when the number of posts is known and the number of sections is assumed to be the same. Depending on the design of the fence, this assumption can be correct or incorrect.
530:
The common answer of 10 posts is wrong. This response comes from dividing the length of the fence by the spacing apart from each post, with the quotient being erroneously classified as the number of posts. In actuality, the fence has 10 sections and 11 posts.
598:, consisting of 120 blocks placed at 10-year intervals between blocks, is scheduled to take 1,190 years to build (not 1,200), from the installation of the first block to the last block. One of the earliest fencepost errors involved time, where the
847:
One approach that often helps avoid such problems is to use variants of these functions that calculate how much to write based on the total length of the buffer, rather than the maximum number of characters to write. Such functions include
286:
iterates one time too many or too few, usually caused by the use of non-strict inequality (≤) as the terminating condition where strict inequality (<) should have been used, or vice versa. Off-by-one errors also stem from confusion over
706:
is that the guaranteed null termination will not write beyond the maximum length. In reality it will write a terminating null character one byte beyond the maximum length specified. The following code contains such a bug:
609:"Fencepost error" can, in rare occasions, refer to an error induced by unexpected regularities in input values, which can (for instance) completely thwart a theoretically efficient
468:
Array-related confusion may also result from differences in programming languages. Numbering from 0 is most common, but some languages start array numbering with 1.
860:, and are often considered "safer" because they make it easier to avoid accidentally writing past the end of a buffer. (In the code example above, calling
914:
813:
Off-by-one errors are common in using the C library because it is not consistent with respect to whether one needs to subtract 1 byte – functions like
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things rather than the spaces between them, or vice versa, or by neglecting to consider whether one should count one or both ends of a row.
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is false and the loop ends. However, if the comparison used were <= (less than or equal to), the loop would be carried out six times:
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if the fence is a free-standing line segment bounded by a post at each of its ends (e.g., a fence between two passageway gaps),
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The interpretation for the fence's design changes the answer to this problem. The correct number of sections for a fence is
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if the fence forms one complete, free-standing loop (e.g., enclosure accessible by surmounting, such as a boxing ring), or
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calculation, if the following person makes the same kind of mistake again (of course, the error might also be reversed).
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that involves a number that differs from its intended value by +1 or −1. An off-by-one error can sometimes appear in a
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will write past the length given them. So the programmer has to remember for which functions they need to subtract 1.
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844:. This can cause an exploitable condition where an attacker can hijack the local variables for the calling routine.
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has arrays with user-defined indices. This makes it possible to model the array indices after the problem domain.
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that iterates five times (from 0 to 4 inclusive) can be written as a half-open interval from 0 to 5:
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architectures in particular) this can result in the overwriting of the least significant byte of the
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495:) is a specific type of off-by-one error. An early description of this error appears in the works of
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implementation. This error involves the difference between expected and worst case behaviours of an
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A common off-by-one error which results in a security-related bug is caused by misuse of the
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takes the values 1, 2, 3, and 4. Both of these alternatives can cause off-by-one errors.
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316:(inclusive) are to be processed. How many items are there? An intuitive answer may be
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Fencepost errors can also occur in units other than length. For example, the
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then becomes 1, 2, 3, and finally 4 on successive iterations. At that point,
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were initialized to 1 rather than 0, there would only be four iterations:
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30 feet long with posts spaced 3 feet apart, how many posts do you need?
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A straight fence with 10 sections requires 11 posts. More generally,
465:(or vice versa.) A do-while loop is guaranteed to run at least once.
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An example of this error can occur in the computational language
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For this reason, ranges in computing are often represented by
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19:"OBOB" redirects here. For the book trivia competition, see
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The following problem demonstrates the reverse error:
367:(exclusive) to avoid fencepost errors. For example, a
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takes the values 0, 1, 2, 3, 4, and 5. Likewise, if
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Logical error that can often be found in programming
156:. Unsourced material may be challenged and removed.
1032:Humble Pi: When Math Goes Wrong in the Real World
972:An earlier version of this article was based on
568:posts, how many sections are there between them?
562:
501:
499:. The following problem illustrates the error:
991:"Why numbering should start at zero (EWD 831)"
821:will never write past the length given them (
356:(inclusive) is represented by the range from
8:
426:The loop body is executed first of all with
94:. There might be a discussion about this on
916:Who first described the "fence-post error?"
805:// Final parameter should be: sizeof(buf)-1
60:Learn how and when to remove these messages
545:posts. Conversely, if the fence contains
234:Learn how and when to remove this message
216:Learn how and when to remove this message
114:Learn how and when to remove this message
905:
825:subtracts 1 itself, and only retrieves
326:, but that is off by one, exhibiting a
702:routine. A common misconception with
989:Dijkstra, Edsger Wybe (May 2, 2008).
7:
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154:adding citations to reliable sources
457:Another such error can occur if a
14:
604:calculated leap years incorrectly
41:This article has multiple issues.
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71:
30:
1015:system this issue is listed as
864:instead would remove the bug.)
639:function, whose parameters are
534:In this scenario, a fence with
141:needs additional citations for
49:or discuss these issues on the
1:
999:University of Texas at Austin
829:bytes), whereas others, like
862:strlcat(buf, s, sizeof(buf))
278:context. It often occurs in
1013:Common Weakness Enumeration
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1064:Computer security exploits
675:would achieve a sequence
21:Oregon Battle of the Books
18:
1017:CWE-193: Off-by-one Error
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503:If you build a straight
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330:; the correct answer is
874:Boundary-value analysis
549:posts, it will contain
519:sections would require
484:(occasionally called a
995:E. W. Dijkstra Archive
680:but instead would get
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527:
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461:is used in place of a
418:/* Body of the loop */
1030:Parker, Matt (2021).
689:Security implications
514:
889:Zero-based numbering
879:Pigeonhole principle
667:number of increments
637:linear interpolation
308:of items, and items
289:zero-based numbering
280:computer programming
150:improve this article
84:confusing or unclear
1034:. Riverhead Books.
913:Moniot, Robert K.,
538:sections will have
346:half-open intervals
300:Looping over arrays
254:(known by acronyms
92:clarify the article
921:Fordham University
695:C standard library
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493:picket fence error
165:"Off-by-one error"
947:"The Jargon File"
836:On some systems (
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1024:Further reading
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1059:Software bugs
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927:on 2016-03-05
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167: –
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161:Find sources:
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139:This article
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1002:. Retrieved
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950:. Retrieved
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929:, retrieved
925:the original
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827:(length − 1)
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596:Time Pyramid
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564:If you have
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440:index < 5
430:equal to 0;
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304:Consider an
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276:mathematical
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148:Please help
143:verification
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90:Please help
81:
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43:Please help
40:
663:upper value
659:lower value
647:upper value
643:lower value
611:binary tree
602:originally
272:logic error
1053:Categories
1004:2011-03-16
982:permission
931:2016-07-07
895:References
634:linspace()
489:lamp-post,
463:while loop
176:newspapers
86:to readers
46:improve it
900:Citations
632:with the
619:algorithm
497:Vitruvius
206:July 2020
52:talk page
868:See also
655:and not
312:through
104:May 2022
1011:In the
965:Sources
857:strlcpy
851:strlcat
831:strncat
823:fgets()
819:strncpy
815:fgets()
775:strncat
704:strncat
699:strncat
282:when a
270:) is a
190:scholar
82:may be
1038:
978:FOLDOC
952:17 May
793:sizeof
763:sizeof
745:memset
630:MATLAB
526:posts.
470:Pascal
192:
185:
178:
171:
163:
505:fence
452:index
448:index
444:index
436:index
432:index
428:index
406:index
394:index
382:index
306:array
295:Cases
197:JSTOR
183:books
1036:ISBN
954:2021
854:and
817:and
736:char
721:char
712:void
397:<
369:loop
284:loop
268:OBOB
266:and
256:OBOE
169:news
976:at
802:));
799:buf
781:buf
772:));
769:buf
751:buf
739:buf
715:foo
613:or
588:+ 1
577:− 1
554:− 1
543:+ 1
524:+ 1
491:or
376:for
365:+ 1
352:to
339:+ 1
264:OB1
260:OBO
250:or
246:An
152:by
1055::
997:.
993:.
919:,
685:.
665:,
661:,
649:,
645:,
621:.
480:A
409:++
341:.
335:−
321:−
291:.
262:,
258:,
55:.
1044:.
1007:.
984:.
956:.
935:.
808:}
796:(
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778:(
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757:0
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718:(
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657:(
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586:n
581:n
575:n
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547:n
541:n
536:n
522:n
517:n
421:}
415:{
412:)
403:;
400:5
391:;
388:0
385:=
379:(
363:n
358:m
354:n
350:m
337:m
333:n
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194:·
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58:(
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