INT16-C. Do not make assumptions about representation of signed integers

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:

• sign and magnitude
• two's complement
• ones' complement

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 ones' complement implementations.

int value;

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

Risk Assessment

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

Related Guidelines

ISO/IEC 9899:2011 Section 6.2.6.2, "Integer types"

Bibliography