Although many common implementations use a two's complement representation of signed integers, the C Standard declares such use as implementation-defined and allows all of the following representations:

This is a specific example of MSC14-C. Do not introduce unnecessary platform dependencies.

Noncompliant Code Example

One way to check whether a number is even or odd is to examine the least significant bit, but the results will be inconsistent. Specifically, this example gives unexpected behavior on all one's complement implementations:

int value;

if (scanf("%d", &value) == 1) {
  if (value & 0x1 != 0) {
    /* Take action if value is odd */
  }
}

Compliant Solution

The same thing can be achieved compliantly using the modulo operator:

int value;

if (scanf("%d", &value) == 1) {
  if (value % 2 != 0) {
    /* Take action if value is odd */
  }
}

Risk Assessment

Incorrect assumptions about integer representation can lead to execution of unintended code branches and other unexpected behavior.

Recommendation

Severity

Likelihood

Remediation Cost

Priority

Level

INT16-C

Medium

Unlikely

High

P2

L3

Automated Detection

Tool

Version

Checker

Description

Astrée
bitop-type
Partially checked
LDRA tool suite
50 S, 120 SPartially Implemented
Parasoft C/C++test
CERT_C-INT16-a
Bitwise operators shall only be applied to operands of unsigned underlying type
PRQA QA-C

2940, 2941, 2942, 2943, 2945, 2946, 2947, 2948


RuleChecker

bitop-type
Partially checked