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This compliant solution guarantees there is no possibility of signed overflow on systems where long long is at least twice the size of int.

signed int si1, si2, result;
static_assert(sizeof(long long) >= sizeof(int), "Unable to detect overflow after multiplication");

signed long long tmp = (signed long long)si1 * (signed long long)si2;

/*
 * If the product cannot be represented as a 32-bit integer, handle as an error condition
 */
if ( (tmp > INT_MAX) || (tmp < INT_MIN) ) {
  /* handle error condition */
}
result = (int)tmp;

The compliant solution uses a static assertion to ensure that the overflow detection will succeed. See DCL03-A. Use a static assertion to test the value of a constant expression for a discussion of static assertions.

preceding code is compliant only on systems where long long is at least twice the size of int. On systems where this relationship does not exist, the following compliant solution may be used to ensure signed overflow does not occur.

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