You are viewing an old version of this page. View the current version.

Compare with Current View Page History

« Previous Version 18 Next »

Java, C, and 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.

This defines the precedence of operation by the order of the subclauses.

This recommendation is similar to EXP01-J, however it applies to more generally than expressions containing side effects.

Noncompliant Code Example

The intent of the expression in this noncompliant code example is to add the variable OFFSET with the result of the bitwise and between x and MASK.

public static final int MASK = 1337;
public static final int OFFSET = -1337;

public static int computeCode(int x) {
  return x & MASK + OFFSET;
}

Because of operator precedence rules, the expression is parsed as

x & (MASK + OFFSET)

Which gets evaluated as

x & (1337 - 1337)

Resulting in 0.

Compliant Solution

In this compliant solution, parentheses are used to ensure the expression evaluates as expected.

public static final int MASK = 1337;
public static final int OFFSET = -1337;

public static int computeCode(int x) {
  return (x & MASK) + OFFSET;
}

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

EXP09-J

low

probable

medium

P4

L3

Related Vulnerabilities

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

EXP01-J. Do not depend on operator precedence while using expressions containing side-effects.

Other Languages

This rule appears in the C++ and C Secure Coding Standard as EXP00-CPP. Use parentheses for precedence of operation. and EXP00-C. Use parentheses for precedence of operation..

References

[Tutorials 08] Expressions, Statements, and Blocks

  • No labels