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C programmers commonly make errors regarding the precedence rules of C operators because of 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.

Subclause 6.5 of the C Standard 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-C-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, which 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

CodeSonar
5.0p0

LANG.STRUCT.PARENS

Missing Parentheses

ECLAIR

1.2

CC2.EXP00

Fully implemented

Klocwork
2018
MISRA.EXPR.PARENS.2012
LDRA tool suite
9.7.1

361 S, 49 S

Fully implemented

Parasoft C/C++test
10.4

CERT_C-EXP00-a

Use parenthesis to clarify expression order if operators with precedence lower than arithmetic are used

Polyspace Bug Finder

R2018a

Possibly unintended evaluation of expression because of operator precedence rules

MISRA C:2012 Rule 12.1

Operator precedence rules cause unexpected evaluation order in arithmetic expression

The precedence of operators within expressions should be made explicit

PRQA QA-C
9.3

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

Fully implemented
PVS-Studio

6.23

V502, V593, V634, V648
SonarQube C/C++ Plugin
3.11
S864

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)
[Kernighan 1988]
[NASA-GB-1740.13]Section 6.4.3, "C Language"


 


4 Comments

  1. Good rule of thumb: if you have to look at an operator precedence chart to *write* your code, then so will your reader; add parens if there is any doubt.

    1. Sound advice indeed.

      Is the assignment operator (especially in conditions) addressed separately?

  2. added exception for standard algebraic precedence.