Integer overflow
In computer programming, an integer overflow occurs when an arithmetic operation attempts to create a numeric value that is outside of the range that can be represented with a given number of bits – either larger than the maximum or lower than the minimum representable value.
The most common result of an overflow is that the least significant representable bits of the result are stored; the result is said to wrap around the maximum (i.e. modulo power of two).
An overflow condition may give results leading to unintended behavior. In particular, if the possibility has not been anticipated, overflow can compromise a program's reliability and security.
For some applications, such as timers and clocks, wrapping on overflow can be desirable. The C11 standard states that for unsigned integers modulo wrapping is the defined behavior and the term overflow never applies; "a computation involving unsigned operands can never overflow."^{[1]}
On some processors like graphics processing units (GPUs) and digital signal processors (DSPs) which support saturation arithmetic, overflowed results would be "clamped", i.e. set to the minimum or the maximum value in the representable range, rather than wrapped around.
Contents
Origin
The register width of a processor determines the range of values that can be represented. Typical binary register widths for unsigned integers include:
 8 bits: maximum representable value 2^{8} − 1 = 255
 16 bits: maximum representable value 2^{16} − 1 = 65,535
 32 bits: maximum representable value 2^{32} − 1 = 4,294,967,295 (the most common width for personal computers as of 2005^{[update]}),
 64 bits: maximum representable value 2^{64} − 1 = 18,446,744,073,709,551,615 (the most common width for personal computer CPUs, as of 2017^{[update]}),
 128 bits: maximum representable value 2^{128} − 1 = 340,282,366,920,938,463,463,374,607,431,768,211,455
When an arithmetic operation produces a result larger than the maximum above for a Nbit integer, an overflow reduces the result to modulo Nth power of 2, retaining only the least significant bits of the result and effectively causing a wrap around.
In particular, multiplying or adding two integers may result in a value that is unexpectedly small, and subtracting from a small integer may cause a wrap to a large positive value (for example, 8bit integer addition 255 + 2 results in 1, which is 257 mod 2^{8}, and similarly subtraction 0 − 1 results in 255, a two's complement representation of −1).
Such wrap around may cause security problems – if an overflowed value is used as the number of bytes to allocate for a buffer, the buffer will be allocated unexpectedly small, leading to a potential buffer overflow and arbitrary code execution.
If the variable has a signed integer type, a program may make the assumption that a variable always contains a positive value. An integer overflow can cause the value to wrap and become negative, which violates the program's assumption and may lead to unexpected behavior (for example, 8bit integer addition of 127 + 1 results in −128, a two's complement of 128).
Flags
Most computers have two dedicated processor flags to check for overflow conditions.
The carry flag is set when the result of an addition or subtraction, considering the operands and result as unsigned numbers, does not fit in the given number of bits. This indicates an overflow with a carry or borrow from the most significant bit. An immediately following add with carry or subtract with borrow operation would use the contents of this flag to modify a register or a memory location that contains the higher part of a multiword value.
The overflow flag is set when the result of an operation on signed numbers does not have the sign that one would predict from the signs of the operands, e.g. a negative result when adding two positive numbers. This indicates that an overflow has occurred and the signed result represented in two's complement form would not fit in the given number of bits.
Definition variations and ambiguity
For an unsigned type, when the ideal result of an operation is outside the types representable range and the returned result is obtained by wrapping, then this event is commonly defined as an overflow. In contrast, the C11 standard defines that this event is not an overflow and states "a computation involving unsigned operands can never overflow."^{[1]}
When the ideal result of an integer operation is outside the types representable range and the returned result is obtained by clamping, then this event is commonly defined as a saturation. Usage varies as to whether a saturation is or is not an overflow. To eliminate ambiguity, the terms wrapping overflow^{[2]} and saturating overflow^{[3]} can be used.
The term underflow is most commonly used for floatingpoint math and not for integer math^{[4]}. But, many references can be found to integer underflow.^{[5]}^{[6]}^{[7]}^{[8]}^{[9]} When the term integer underflow is used, it means the ideal result was closer to minus infinity than the output type's representable value closest to minus infinity. When the term integer underflow is used, the definition of overflow may include all types of overflows or it may only include cases where the ideal result was closer to positive infinity than the output type's representable value closest to positive infinity.
When the ideal result of an operation is not an exact integer, the meaning of overflow can be ambiguous in edge cases. Consider the case where the ideal result has value 127.25 and the output type's maximum representable value is 127. If overflow is defined as the ideal value being outside the representable range of the output type, then this case would be classified as an overflow. For operations that have well defined rounding behavior, overflow classification may need to be postponed until after rounding is applied. The C11 standard ^{[1]} defines that conversions from floating point to integer must round toward zero. If C is used to convert the floating point value 127.25 to integer, then rounding should be applied first to give an ideal integer output of 127. Since the rounded integer is in the outputs range, the C standard would not classify this conversion as an overflow.
Methods to mitigate integer overflow problems
Language  Unsigned integer  Signed integer 

Ada  modulo the type's modulus  raise Constraint_Error 
C/C++  modulo power of two  undefined behavior 
C#  modulo power of 2 in unchecked context; System.OverflowException is raised in checked context^{[10]}


Java  N/A  modulo power of two 
JavaScript  all numbers are doubleprecision floatingpoint  
MATLAB  Builtin integers saturate. Fixedpoint integers configurable to wrap or saturate  
Python 2  N/A  convert to long type (bigint) 
Seed7  N/A  raise OVERFLOW_ERROR^{[11]} 
Scheme  N/A  convert to bigNum 
Simulink  configurable to wrap or saturate  
Smalltalk  N/A  convert to LargeInteger 
Swift  Causes error unless using special overflow operators.^{[12]} 
There are several methods of handling overflow:
 Avoidance: by carefully ordering operations, checking operands in advance and selecting the correct data type, it is possible to ensure that the result will never be larger than can be stored. Static analysis tools, formal verification and design by contract techniques can be used to more confidently and robustly ensure that an overflow cannot accidentally result.
 Handling: If it is anticipated that overflow may occur and when it happens detected and other processing done. Example: it is possible to add two numbers each two bytes wide using just a byte addition in steps: first add the low bytes then add the high bytes, but if it is necessary to carry out of the low bytes this is arithmetic overflow of the byte addition and it becomes necessary to detect and increment the sum of the high bytes. CPUs generally have a way of detecting this to support addition of numbers larger than their register size, typically using a status bit.
 Propagation: if a value is too large to be stored it can be assigned a special value indicating that overflow has occurred and then have all successive operation return this flag value. This is useful so that the problem can be checked for once at the end of a long calculation rather than after each step. This is often supported in Floating Point Hardware called FPUs.
Programming languages implement various mitigation methods against an accidental overflow: Ada, Seed7 (and certain variants of functional languages), trigger an exception condition on overflow, while Python (since 2.4) seamlessly converts internal representation of the number to match its growth, eventually representing it as long
– whose ability is only limited by the available memory.^{[13]}
Runtime overflow detection implementation AddressSanitizer
is also available for C compilers.
In languages with native support for Arbitraryprecision arithmetic and type safety (such as Python or Common Lisp), numbers are promoted to a larger size automatically when overflows occur, or exceptions thrown (conditions signaled) when a range constraint exists. Using such languages may thus be helpful to mitigate this issue. However, in some such languages, situations are still possible where an integer overflow can occur. An example is explicit optimization of a code path which is considered a bottleneck by the profiler. In the case of Common Lisp, this is possible by using an explicit declaration to typeannotate a variable to a machinesize word (fixnum)^{[14]} and lower the type safety level to zero^{[15]} for a particular code block.^{[16]}^{[17]}^{[18]}^{[19]}
In Java 8, there are overloaded methods, for example like Math.addExact(int, int)
, which will throw ArithmeticException
in case of overflow.
Computer emergency response team (CERT) developed the Asif Infinitely Ranged (AIR) integer model, a largely automated mechanism to eliminate integer overflow and truncation in C/C++ using runtime error handling.^{[20]}
In computer graphics or signal processing, it is typical to work on data that ranges from 0 to 1 or from −1 to 1. An example of this is a grayscale image where 0 represents black, 1 represents white, and values inbetween represent varying shades of gray. One operation that one may want to support is brightening the image by multiplying every pixel by a constant. Saturated arithmetic allows one to just blindly multiply every pixel by that constant without worrying about overflow by just sticking to a reasonable outcome that all these pixels larger than 1 (i.e. "brighter than white") just become white and all values "darker than black" just become black.
Examples
Unanticipated arithmetic overflow is a fairly common cause of program errors. Such overflow bugs may be hard to discover and diagnose because they may manifest themselves only for very large input data sets, which are less likely to be used in validation tests.
Taking the arithmetic mean of two numbers by adding them and dividing by two, as done in many search algorithms, causes error if the sum (although not the resulting mean) is too large to be represented, and hence overflows.^{[21]}
An unhandled arithmetic overflow in the engine steering software was the primary cause of the crash of the 1996 maiden flight of the Ariane 5 rocket.^{[22]} The software had been considered bugfree since it had been used in many previous flights, but those used smaller rockets which generated lower acceleration than Ariane 5.
On 30 April 2015, the Federal Aviation Authority announced it will order Boeing 787 operators to reset its electrical system periodically, to avoid an integer overflow which could lead to loss of electrical power and ram air turbine deployment, and Boeing deployed a software update in the fourth quarter.^{[23]} The European Aviation Safety Agency followed on 4 May 2015.^{[24]} The error happens after 2³¹ centiseconds (248.55134814815 days), indicating a 32bit signed integer.
Overflow bugs are evident in computer games. In the arcade game Donkey Kong, it is impossible to advance past level 22 due to an integer overflow in its time/bonus. The game takes the level number a user is on, multiplies it by 10 and adds 40. When they reach level 22, the time/bonus number is 260, which is too large for its 8bit 256 value register, so it resets itself to 0 and gives the remaining 4 as the time/bonus – too short to finish the level. In Donkey Kong Jr. Math, when trying to calculate a number over 10000, it shows only the first 4 digits. Overflow is the cause of the famous Split Screen in PacMan^{[25]} and the Nuclear Gandhi in Civilization series. It also caused the Far Lands in Minecraft which existed from the Infdev development period to Beta 1.7.3, however it was later fixed in Beta 1.8 but still exist in the Pocket Edition and Windows 10 Edition versions of Minecraft.^{[26]}
Microsoft / IBM MACRO Assembler (MASM) Version 1.00, and likely all other programs built by the same Pascal compiler, had an integer overflow and signedness error in the stack setup code, which prevented them from running on newer DOS machines or emulators under some common configurations with more than 512 KB of memory. The program either hangs or displays an error message and exits to DOS.^{[27]}
In August 2016, a Casino machine at Resorts World Casino printed a prize ticket of $42,949,672.76 as a result of an overflow bug. The Casino refused to pay this amount calling it a malfunction, using in their defense that the machine clearly stated that the maximum payout was $10,000, so any prize higher than that had to be the result of a programming bug. The Iowa Supreme Court ruled in favor of the Casino.^{[28]}
See also
 Buffer overflow
 Heap overflow
 Pointer swizzling
 Software testing
 Stack buffer overflow
 Static program analysis
 Unix signal
References
 ^ ^{a} ^{b} ^{c} ISO. "ISO/IEC 9899:2011 Information technology  Programming languages  C". webstore.ansi.org.
 ^ "Wrap on overflow  MATLAB & Simulink". www.mathworks.com.
 ^ "Saturate on overflow  MATLAB & Simulink". www.mathworks.com.
 ^ Arithmetic underflow
 ^ "CWE  CWE191: Integer Underflow (Wrap or Wraparound) (3.1)". cwe.mitre.org.
 ^ "Overflow And Underflow of Data Types in Java  DZone Java". dzone.com.
 ^ Mir, Tabish (4 April 2017). "Integer Overflow/Underflow and Floating Point Imprecision". medium.com.
 ^ "Integer underflow and buffer overflow processing MP4 metadata in libstagefright". Mozilla.
 ^ "Avoiding Buffer Overflows and Underflows". developer.apple.com.
 ^ BillWagner. "Checked and Unchecked (C# Reference)". msdn.microsoft.com.
 ^ Seed7 manual, section 15.2.3 OVERFLOW_ERROR.
 ^ The Swift Programming Language. Swift 2.1 Edition. October 21, 2015.
 ^ Python documentation, section 5.1 Arithmetic conversions.
 ^ "Declaration TYPE". Common Lisp HyperSpec.
 ^ "Declaration OPTIMIZE". Common Lisp HyperSpec.
 ^ Reddy, Abhishek (20080822). "Features of Common Lisp".
 ^ Pierce, Benjamin C. (2002). Types and Programming Languages. MIT Press. ISBN 0262162091.
 ^ Wright, Andrew K.; Matthias Felleisen (1994). "A Syntactic Approach to Type Soundness". Information and Computation. 115 (1): 38–94. doi:10.1006/inco.1994.1093.
 ^ Macrakis, Stavros (April 1982). "Safety and power" (requires subscription). ACM SIGSOFT Software Engineering Notes. 7 (2): 25–26. doi:10.1145/1005937.1005941.
 ^ Asif Infinitely Ranged Integer Model
 ^ "Extra, Extra  Read All About It: Nearly All Binary Searches and Mergesorts are Broken". googleresearch.blogspot.co.uk.
 ^ Gleick, James (1 December 1996). "A Bug and A Crash". New York Times Magazine. Retrieved 9 December 2013.
 ^ Mouawad, Jad (30 April 2015). "F.A.A. Orders Fix for Possible Power Loss in Boeing 787". New York Times.
 ^ "US20150907 : Electrical Power – Deactivation". Airworthiness Directives. European Aviation Safety Agency. 4 May 2015.
 ^ Pittman, Jamey. "The PacMan Dossier".
 ^ Minecraft Gamepedia. "Minecraft Gamepedia Page".
 ^ Lenclud, Christophe. "Debugging IBM MACRO Assembler Version 1.00".
 ^ Kravets, David (June 15, 2017). "Sorry ma'am you didn't win $43M – there was a slot machine 'malfunction'". Ars Technica.
External links
 Phrack #60, Basic Integer Overflows
 Phrack #60, Big Loop Integer Protection
 Efficient and Accurate Detection of Integerbased Attacks
 WASC Threat Classification – Integer Overflows
 Understanding Integer Overflow in C/C++
 Binary Overflow – Binary Arithmetic
 ISO C11 Standard