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The following sections examine specific operations that are susceptible to integer overflow. The specific tests that are required to guarantee that the operation does not result in an integer overflow depend on the signedness of the integer types. When operating on small types (smaller than int integer conversion rules apply. The usual arithmetic conversions may also be applied to (implicitly) convert operands to equivalent types before arithmetic operations are performed. Make sure you understand implicit conversion rules before trying to implement secure arithmetic operations.
Addition
Addition in C is between two operands of arithmetic type, or between a pointer to an object type and an integer type. (Incrementing is equivalent to adding one.)
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unsigned int ui1, ui2, sum;
if (~ui1 < ui2) {
/* handle error condition */
}
sum = ui1 + ui2;
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Subtraction
Subtraction in C is between two operands of arithmetic type, two pointers to qualified or unqualified versions of compatible object types, or between a pointer to an object type and an integer type. (Decrementing is equivalent to subtracting one.)
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signed int si1, si2, result;
if (si1 > 0){ /* si1 is positive */
if (si2 > 0) { /* si1 and si2 are positive */
if (si1 > (INT_MAX / si2)) {
/* handle error condition */
}
} /* end if si1 and si2 are positive */
else { /* si1 positive, si2 non-positive */
if (si2 < (INT_MIN / si1)) {
/* handle error condition */
}
} /* si1 positive, si2 non-positive */
} /* end if si1 is positive */
else { /* si1 is non-positive */
if (si2 > 0) { /* si1 is non-positive, si2 is positive */
if (si1 < (INT_MIN / si2)) {
/* handle error condition */
}
} /* end if si1 is non-positive, si2 is positive */
else { /* si1 and si2 are non-positive */
if( (si1 != 0) && (si2 < (INT_MAX / si1))) {
/* handle error condition */
}
} /* end if si1 and si2 are non-positive */
} /* end if si1 is non-positive */
result = si1 * si2;
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Division
Division in C is between two operands of arithmetic type. Overflow can occur during twos-complement signed integer division when the dividend is equal to the minimum (negative) value for the signed integer type and the divisor is equal to -1. Both signed and unsigned division operations are also susceptible to divide-by-zero errors.
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The unary negation operator in C takes an operand of arithmetic type. Overflow can occur during twos-complement unary negation when the operand is equal to the minimum (negative) value for the signed integer type.
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Left Shift Operator
The shift operator in C is between two operands of integer type.
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Right Shift Operator
The shift operator in C is between two operands of integer type.
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