Page History

Although many common implementations use a two's complement representation of signed integers, the C99 standard declares this as C Standard declares such use as implementation-defined, and allows all of the following representations:

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

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

...

One way to check whether a number is even or odd is to examine the least significant bit. This will give inconsistent results, but the results will be inconsistent. Specifically, this example will give 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 * do something/
  }
}

Compliant Solution

Using bitwise operators is safe on unsigned integers:

unsigned int value;

if (scanf("%u", &value) == 1) {
  if (value & 0x1 != 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.

Related Guidelines

ISO/IEC 9899:1999 Section 6.2.6.2

Bibliography

Automated Detection

...

Image Added Image Added Image Removed      04. Integers (INT)      Image Modified