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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.

Wiki MarkupSection Subclause 6.5 of \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] (C99) defines the precedence of operation by the order of the subclauses.

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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 non-compliant 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 the compiler 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

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It is common for C programmers to assign the return value of a function while concurrently checking for and in-band error condition.

if(ret = foo() != err) {
  /* use ret */
}

However, since comparison binds tighter than assignment, the value of the comparison is stored in ret. So if foo() succeeds, ret will always be set to 0, and the if-statement will execute if and only if foo() fails, exactly opposite of what the programmer expects.

Compliant Solution

if((ret = foo()) != err) {
  /* use ret */
}

Exceptions

Exceptions

EXP00-C-EX1: Mathematical EXP00-EX1: Mathemtical 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. Therefore parenthesis to enforce this would 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 , which can lead to unexpected and abnormal program behavior.

Automated Detection

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Tool	Version	Checker	Description
Axivion Bauhaus Suite	Include Page Axivion Bauhaus Suite_V Axivion Bauhaus Suite_V	CertC-EXP00	Fully implemented
CodeSonar	Include Page CodeSonar_V CodeSonar_V	LANG.STRUCT.PARENS	Missing Parentheses
ECLAIR	Include Page ECLAIR_V ECLAIR_V	CC2.EXP00	Fully implemented
Helix QAC	Include Page Helix QAC_V Helix QAC_V	C3389, C3390, C3391, C3392, C3393, C3394, C3395, C3396, C3397, C3398, C3399, C3400
Klocwork	Include Page Klocwork_V Klocwork_V	CERT.EXPR.PARENS
LDRA tool suite	Include Page LDRA_V LDRA_V	361 S, 49 S	Fully implemented
Parasoft C/C++test	Include Page Parasoft_V Parasoft_V	CERT_C-EXP00-a	Use parenthesis to clarify expression order if operators with precedence lower than arithmetic are used
PC-lint Plus	Include Page PC-lint Plus_V PC-lint Plus_V	9050	Fully supported
Polyspace Bug Finder	Include Page Polyspace Bug Finder_V Polyspace Bug Finder_V	CERT C: Rec. EXP00-C	Checks for possible unintended evaluation of expression because of operator precedence rules (rec. fully covered)
PVS-Studio	Include Page PVS-Studio_V PVS-Studio_V	V502, V593, V634, V648, V1104
SonarQube C/C++ Plugin	Include Page SonarQube C/C++ Plugin_V SonarQube C/C++ Plugin_V	S864

The LDRA tool suite V 7.6.0 is able to detect violations of this recommendation.

Related Vulnerabilities

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

References

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Related Guidelines

Bibliography

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Image Added Image Added Image Added03. Expressions (EXP)      03. Expressions (EXP)       EXP01-A. Do not take the size of a pointer to determine the size of the pointed-to type