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

• sign Sign and magnitude
• twoTwo's complement
• onesOne's complement

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

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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 onesone's complement implementations.:

int value;

if (scanf("%d", &value) == 1) {
  if (value & 0x1 != 0) {
    /* doTake somethingaction 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 */
  }
}

Compliant Solution

Using bitwise operators is safe on unsigned integers:

unsigned int value;

if (scanf("%u", &value) == 1) {
  if (value & 0x1 != 0) {
    /* doTake somethingaction 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"

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Automated Detection

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