C programmers commonly make errors regarding the precedence rules of C operators due to the nonintuitively 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 following C expression, intended to test the least significant bit of x

x & 1 == 0

However, it is parsed as

x & (1 == 0)

which the compiler would probably evaluate at compile time to

(x & 0)

and then to 0.

Compliant Solution

Adding parentheses to indicate precedence will cause the expression to evaluate as expected.

(x & 1) == 0

Risk Assessment

Mistakes regarding precedence rules may result in improper computations. This can lead to unintended program behavior

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

EXP00-A

1 (low)

2 (probable)

2 (medium)

P4

L3

References

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