Bad check for oddness¶
ID: cpp/incomplete-parity-check
Kind: problem
Security severity:
Severity: warning
Precision: medium
Tags:
- reliability
- correctness
- types
Query suites:
- cpp-security-and-quality.qls
Click to see the query in the CodeQL repository
This rule finds code that uses x % 2 == 1 to check whether a number x is odd, which does not work for negative numbers. Applying % to negative numbers produces negative results. For example, (-5) % 2 equals -1, not 1. As a result, this check incorrectly considers all negative numbers as even.
Recommendation¶
Consider using x % 2 != 0 or (x & 1) == 1 instead.
References¶
MSDN Library: Multiplicative Operators and the Modulus Operator.
Wikipedia: Modulo Operation - Common pitfalls.