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 [[ISO/IEC 9899:1999]] (C99) 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 this 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
The LDRA tool suite V 7.6.0 can detect violations of this recommendation.
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Other Languages
This rule appears in the C++ Secure Coding Standard as EXP00-CPP. Use parentheses for precedence of operation.
References
[[Dowd 06]] Chapter 6, "C Language Issues" (Precedence, pp. 287—288)
[[ISO/IEC 9899:1999]] Section 6.5, "Expressions"
[[ISO/IEC PDTR 24772]] "JCW Operator precedence/Order of Evaluation"
[[Kernighan 88]]
[[MISRA 04]] Rule 12.1
[[NASA-GB-1740.13]] Section 6.4.3, "C Language"
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