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Verifiably in-range operations are often preferable to treating out-of-range values as an error condition because the handling of these errors has been repeatedly shown to cause denial-of-service problems in actual applications. The quintessential example of this is the failure of the Ariane 5 launcher, which occurred because of an improperly handled conversion error that resulted in the processor being shut down \[[Lions 96|AA. C References#Lions 96]\]. |
A program that detects an integer overflow to be imminent may do one of two things: (1) signal some sort of error condition or (2) produce an integer result that is within the range of representable integers on that system. Some situations can be handled by an error condition, where an overflow causes a change in control flow (such as the system complaining about bad input and requesting alternative input from the user). Others are better handled by the latter option because it allows the computation to proceed and generate an integer result, thereby avoiding a denial-of-service attack. However, when continuing to produce an integer result in the face of overflow, the question of what integer result to return to the user must be considered.
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In modwrap semantics (also called _modulo_ arithmetic), integer values "wrap round." That is, adding one to {{MAX}} produces {{MIN}}. This is the defined behavior for unsigned integers in the C Standard \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] (see Section 6.2.5, "Types," paragraph 9) and is frequently the behavior of signed integers as well. However, it is more sensible in many applications to use saturation semantics instead of modwrap semantics. For example, in the computation of a size (using unsigned integers), it is often better for the size to stay at the maximum value in the event of overflow rather than suddenly becoming a very small value. |
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\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "FLC Numeric Conversion Errors" \[[Lions 96|AA. C References#Lions 96]\] |
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