C programmers commonly make errors regarding the precedence rules of C operators due to 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.
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Noncompliant Code Example
The intent of the expression in this noncompliant code example is to test the least significant bit of x.:
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x & 1 == 0
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Because of operator precedence rules, the expression is parsed as
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x & (1 == 0)
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which evaluates to
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(x & 0)
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and then to 0.
Compliant Solution
In this compliant solution, parentheses are used to ensure the expression evaluates as expected.:
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(x & 1) == 0
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Exceptions
EXP00-C-EX1: Mathematical expressions that follow algebraic order do not require parentheses. For instance, in the expression
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x + y * z
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the multiplication is performed before the addition by mathematical convention. Consequently, parentheses to enforce this would enforce the algebraic order would be redundant.:
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x + (y * z)
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Risk Assessment
Mistakes regarding precedence rules may cause an expression to be evaluated in an unintended way. This , 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
The LDRA tool suite V 7.6.0 can detect violations of this recommendation.
Tool | Version | Checker | Description | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Axivion Bauhaus Suite |
| CertC-EXP00 | Fully implemented | ||||||
| CodeSonar |
| LANG.STRUCT.PARENS | Missing Parentheses | ||||||
| CC2.EXP00 | Fully implemented | |||||||
| Helix QAC |
| C3389, C3390, C3391, C3392, C3393, C3394, C3395, C3396, C3397, C3398, C3399, C3400 | |||||||
| Klocwork |
| CERT.EXPR.PARENS | |||||||
| LDRA tool suite |
| 361 S, 49 S | Fully implemented | ||||||
| Parasoft C/C++test |
| CERT_C-EXP00-a | Use parenthesis to clarify expression order if operators with precedence lower than arithmetic are used | ||||||
| PC-lint Plus |
| 9050 | Fully supported | ||||||
| Polyspace Bug Finder |
| Checks for possible unintended evaluation of expression because of operator precedence rules (rec. fully covered) | |||||||
| PVS-Studio |
| V502, V593, V634, V648, V1104 | |||||||
| SonarQube C/C++ Plugin |
| S864 |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Other Languages
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Related Guidelines
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| VOID EXP00-CPP. Use parentheses for precedence of operation | |
| ISO/IEC TR 24772:2013 | Operator Precedence/Order of Evaluation [JCW] |
| MISRA C:2012 | Rule 12.1 (advisory) |
References
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Bibliography
| [Dowd 2006] | Chapter 6, "C Language Issues" ("Precedence," pp. 287–288) |
| [Kernighan 1988] | |
| [NASA |
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| 6.4.3, |
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03. Expressions (EXP) 03. Expressions (EXP) EXP01-C. Do not take the size of a pointer to determine the size of the pointed-to type