What is an integer overflow error?

Question

What is an integer overflow error? Why do i care about such an error? What are some methods of avoiding or preventing it?

Solution 1

Integer overflow occurs when you try to express a number that is larger than the largest number the integer type can handle.

If you try to express the number 300 in one byte, you have an integer overflow (maximum is 255). 100,000 in two bytes is also an integer overflow (65,535 is the maximum).

You need to care about it because mathematical operations won't behave as you expect. A + B doesn't actually equal the sum of A and B if you have an integer overflow.

You avoid it by not creating the condition in the first place (usually either by choosing your integer type to be large enough that you won't overflow, or by limiting user input so that an overflow doesn't occur).

Solution 2

The easiest way to explain it is with a trivial example. Imagine we have a 4 bit unsigned integer. 0 would be 0000 and 1111 would be 15. So if you increment 15 instead of getting 16 you'll circle back around to 0000 as 16 is actually 10000 and we can not represent that with less than 5 bits. Ergo overflow...

In practice the numbers are much bigger and it circles to a large negative number on overflow if the int is signed but the above is basically what happens.

Another way of looking at it is to consider it as largely the same thing that happens when the odometer in your car rolls over to zero again after hitting 999999 km/mi.

Solution 3

When you store an integer in memory, the computer stores it as a series of bytes. These can be represented as a series of ones and zeros.

For example, zero will be represented as 00000000 (8 bit integers), and often, 127 will be represented as 01111111. If you add one to 127, this would "flip" the bits, and swap it to 10000000, but in a standard two's compliment representation, this is actually used to represent -128. This "overflows" the value.

With unsigned numbers, the same thing happens: 255 (11111111) plus 1 would become 100000000, but since there are only 8 "bits", this ends up as 00000000, which is 0.

You can avoid this by doing proper range checking for your correct integer size, or using a language that does proper exception handling for you.

Solution 4

An integer overflow error occurs when an operation makes an integer value greater than its maximum.

For example, if the maximum value you can have is 100000, and your current value is 99999, then adding 2 will make it 'overflow'.

You should care about integer overflows because data can be changed or lost inadvertantly, and can avoid them with either a larger integer type (see long int in most languages) or with a scheme that converts long strings of digits to very large integers.

Solution 5

Overflow is when the result of an arithmetic operation doesn't fit in the data type of the operation. You can have overflow with a byte-sized unsigned integer if you add 255 + 1, because the result (256) does not fit in the 8 bits of a byte.

You can have overflow with a floating point number if the result of a floating point operation is too large to represent in the floating point data type's exponent or mantissa.

You can also have underflow with floating point types when the result of a floating point operation is too small to represent in the given floating point data type. For example, if the floating point data type can handle exponents in the range of -100 to +100, and you square a value with an exponent of -80, the result will have an exponent around -160, which won't fit in the given floating point data type.

You need to be concerned about overflows and underflows in your code because it can be a silent killer: your code produces incorrect results but might not signal an error.

Whether you can safely ignore overflows depends a great deal on the nature of your program - rendering screen pixels from 3D data has a much greater tolerance for numerical errors than say, financial calculations.

Overflow checking is often turned off in default compiler settings. Why? Because the additional code to check for overflow after every operation takes time and space, which can degrade the runtime performance of your code.

Do yourself a favor and at least develop and test your code with overflow checking turned on.

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Comments

[David]

this answers the what but not the why and the how

[dthorpe]

Overflow is an artifact of math operations, not on assignment / storage.

[indiv]

"less than 5 bits", not "less than 5 bytes".

[indiv]

10000000 is INT_MIN (e.g., -128 for 1 signed byte) in 2's compliment. -1 is FFFFFFFF.