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.
Section 6.5 of the C standard [ISO/IEC 9899:2011] defines the precedence of operation by the order of the subclauses.
Noncompliant Code Example
The intent of the expression in this noncompliant 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 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
Exceptions
EXP00-EX1: Mathematical expressions that follow algebraic order do not require parentheses. For instance, in the expression
x + y * z
the multiplication is performed before the addition by mathematical convention. Consequently, parentheses to enforce the algebraic order would be redundant.
x + (y * z)
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-C | low | probable | medium | P4 | L3 |
Automated Detection
Tool | Version | Checker | Description |
|---|---|---|---|
| 9.7.1 | 361 S | Fully implemented | |
| 1.2 | exprprns | Fully implemented | |
| PRQA QA-C | Unable to render {include} The included page could not be found. | Fully implemented |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Related Guidelines
CERT C++ Secure Coding Standard: EXP00-CPP. Use parentheses for precedence of operation
ISO/IEC 9899:2011 Section 6.5, "Expressions"
ISO/IEC TR 24772 "JCW Operator precedence/order of evaluation"
MISRA Rule 12.1
Bibliography
[Dowd 2006] Chapter 6, "C Language Issues" (Precedence, pp. 287–288)
[Kernighan 1988]
[NASA-GB-1740.13] Section 6.4.3, "C language"