C programmers commonly make errors regarding the precedence rules of C operators due to the unintuitive low precedence levels of "&", "|", "^", "<<", and ">>". Mistakes regarding precedence rules can be avoided by the suitable use of parentheses. Using parentheses defensively reduces errors and, if not taken to excess, makes the code more readable.

Non-Compliant Code Example

The intent of the expression in this non-compliant code example is to test the least significant bit of x.

x & 1 == 0

Because of operator precedence rules, the expression is parsed as

x & (1 == 0)

which the compiler evaluates to

(x & 0)

and then to 0.

Compliant Solution

In this compliant solution, parentheses are used to ensure the expression evaluates as expected.

(x & 1) == 0

Risk Assessment

Mistakes regarding precedence rules may cause an expression to be evaluated in an unintended way. This can lead to unexpected and abnormal program behavior.

Recommendation

Severity

Likelihood

Remediation Cost

Priority

Level

EXP00-A

1 (low)

2 (probable)

2 (medium)

P4

L3

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.5, "Expressions"
\[[NASA-GB-1740.13|AA. C References#NASA-GB-1740.13]\] Section 6.4.3, "C Language"
\[[Dowd 06|AA. C References#Dowd 06]\] Chapter 6, "C Language Issues" (Precedence, pp. 287-288)

03. Expressions (EXP)      03. Expressions (EXP)       EXP01-A. Do not take the size of a pointer to determine the size of the pointed-to type