Integer overflow is undefined behavior. This means that implementations have a great deal of latitude in how they deal with signed integer overflow. An implementation that defines signed integer types as being modulo, for example, need not detect integer overflow. Implementations may also trap on signed arithmetic overflows, or simply assume that overflows will never happen and generate object code accordingly. (see See guideline MSC15-C. Do not depend on undefined behavior.) . For these reasons, it is important to ensure that operations on signed integers do no result in signed overflow. Of particular importance, however, are operations on signed integer values that originate from untrusted sources and are used in any of the following ways:

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The following sections examine specific operations that are susceptible to integer overflow. When operating on small integer types (smaller than int), integer promotions are applied. The usual arithmetic conversions may also be applied to (implicitly) convert operands to equivalent types before arithmetic operations are performed. Make sure you understand integer conversion rules before trying to implement secure arithmetic operations. (see See guideline INT02-C. Understand integer conversion rules.).

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