...
The following portable compliant solution can be used for on any conforming implementation, including those where this assertion failsthat do not have an integer type that is at least twice as big as int.:
| Code Block | ||||
|---|---|---|---|---|
| ||||
#include <limits.h>
void func(signed int si_a, signed int si_b) {
signed int result;
if (si_a > 0) { /* si_a is positive */
if (si_b > 0) { /* si_a and si_b are positive */
if (si_a > (INT_MAX / si_b)) {
/* Handle error */
}
} else { /* si_a positive, si_b nonpositive */
if (si_b < (INT_MIN / si_a)) {
/* Handle error */
}
} /* si_a positive, si_b nonpositive */
} else { /* si_a is nonpositive */
if (si_b > 0) { /* si_a is nonpositive, si_b is positive */
if (si_a < (INT_MIN / si_b)) {
/* Handle error */
}
} else { /* si_a and si_b are nonpositive */
if ( (si_a != 0) && (si_b < (INT_MAX / si_a))) {
/* Handle error */
}
} /* End if si_a and si_b are nonpositive */
} /* End if si_a is nonpositive */
result = si_a * si_b;
} |
...
The C Standard, 6.5.7 paragraph 4 [ISO/IEC 9899:2011], states
...
In almost every case, an attempt to shift by a negative number of bits or by more bits than exist in the operand indicates a bug (logic error). These issues are covered by INT34-C. Do not shift a negative number of bits or more bits than exist in the operand.
Noncompliant Code Example
This noncompliant code example can result in an unrepresentable value.
...
Compliant Solution
This compliant solution eliminates the possibility of overflow resulting from a left-shift operation:
...