Page History

...

• Sign and magnitude
• Two's complement
• Ones' complement

Noncompliant Code Example

One way to check whether a number is even or odd is to examine the least significant bit. This will give inconsistent results. Specifically, this example will give unexpected behavior on all ones' complement implementations.

int value;

if (scanf("%d", &value) == 1) {
  if (value & 0x1 == 1) {
    /* 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 == 1) {
    /* 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.

References

\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.2.6.2