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Java, C, and C++ programmers commonly Programmers frequently make errors regarding the precedence rules of C operators due to because of the unintuitive unintuitively low - precedence levels of &, |, ^, <<, and >>. Mistakes Avoid mistakes regarding precedence rules can be avoided by through 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 , which also improves code readability. The precedence of operations by the order of the subclauses is defined in the Java Tutorials [Tutorials 2013].

Although it advises against depending on parentheses for specifying evaluation order EXP05-J. Do not follow a write by a subsequent write or read of the same object within an expression applies only to expressions that contain side effects.

Noncompliant Code Example

The intent of the expression in this noncompliant code example is to add the variable OFFSET to test the least significant bit of x.

x & 1 == 0

result of the bitwise logical 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;
}

According to the operator precedence guidelinesBecause of operator precedence rules, the expression is parsed as the following:

x & (MASK + OFFSET)

This expression is evaluated as follows, resulting in the value 0:

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

1337 - 1337)

Compliant Solution

This compliant solution uses parentheses to ensure that the expression is evaluated as intended:

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

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

Noncompliant Code Example

In this noncompliant code example, the intent is to append either "0" or "1" to the string "value=":

public class PrintValue {
  public static void main(String[] args) {
    String s = null;
    // Prints "1"
    System.out.println("value=" + s == null ? 0 : 1);
  }
}

However, the precedence rules result in the expression to be printed being parsed as ("value=" + s) == null ? 0 : 1.

Compliant Solution

This compliant solution uses parentheses to ensure that the expression evaluates as intended:

public class PrintValue {
  public static void main(String[] args) {
    String s = null;
    // Prints "value=0" as expected
    System.out.println("value=" + (s == null ? 0 : 1));
  }
}

Applicability

Mistakes regarding precedence guidelines can cause an expression to be evaluated in an unintended way, which can lead to unexpected and abnormal program behavior.

Parentheses may be omitted from mathematical expressions that follow the algebraic precedence rules. For instance, consider the following expression:

x + y * z

the By mathematical convention, multiplication is performed before the addition by mathematical convention. Consequently, parentheses to enforce this would be redundant.addition; parentheses are redundant in this case:

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.

Related Vulnerabilities

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

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

Detection of all expressions using low-precedence operators without parentheses is straightforward. Determining the correctness of such uses is infeasible in the general case, although heuristic warnings could be useful.

Automated Detection

Bibliography

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