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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

ECLAIR

1.2

exprprns

Fully implemented

LDRA tool suite

9.7.1

361 S

Fully implemented

PRQA QA-C
Unable to render {include} The included page could not be found.

3389
3390
3391
3392
3393
3394
3395
3396
3397
3398
3399
3400
3401

Fully implemented

Related Vulnerabilities

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

Related Guidelines

Bibliography

[Dowd 2006]Chapter 6, "C Language Issues" ("Precedence," pp. 287–288)
[ISO/IEC 9899:2011]Section 6.5, "Expressions"
[Kernighan 1988] 
[NASA-GB-1740.13]Section 6.4.3, "C Language"

 

 


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