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The Java Language Specification (JLS), Section 15 §15.17.3, "Remainder Operator %" [JLS 2013], states,

The remainder operation for operands that are integers after binary numeric promotion (§5.6.2) produces a result value such that (a/b)*b+(a%b) is equal to a. This identity holds even in the special case that the dividend is the negative integer of largest possible magnitude for its type and the divisor is -1 (the remainder is 0). It follows from this rule that the result of the remainder operation can be negative only if the dividend is negative, and can be positive only if the dividend is positive; moreover, the magnitude of the result is always less than the magnitude of the divisor.

The result of the remainder operator has the same sign as the dividend (the first operand in the expression).:

5 % 3 produces 2
5 % (-3) produces 2
(-5) % 3 produces -2
(-5) % (-3) produces  -2

As a result, code that depends upon on the remainder operation to always returns return a positive result is erroneous.

Noncompliant Code Example

This noncompliant code example uses the integer hashKey as an index into the hash array. The hash key input could be negative, producing a negative result from the remainder operator. Consequently, the lookup function will throw a java.lang.ArrayIndexOutOfBoundsException. 

private int SIZE = 16;	
public int[] hash = new int[SIZE];
	
public int lookup(int hashKey) {
  return hash[hashKey % SIZE];
}

A negative hash key produces a negative result from the remainder operator, causing the lookup() method to throw java.lang.ArrayIndexOutOfBoundsException.

Compliant Solution

This compliant solution calls a method that the imod() method, which always returns a positive remainder that is always positive.:

// methodMethod imod() gives non-negativenonnegative result
private int SIZE = 16;
public int[] hash = new int[SIZE];

private int imod(int i, int j) {
  int temp = i % j;
  return (temp < 0) ? -temp : temp; 
  // unaryUnary -minus will succeed without overflow  
                                    // because temp cannot be Integer.MIN_VALUE
}
	
public int lookup(int hashKey) {
  return hash[imod(hashKey, sizeSIZE)];
}

...

Applicability

Incorrectly assuming a positive remainder from a remainder operation can result in erroneous code.

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Automated Detection

Automated Detection

Automated detection of uses of the % operator is straightforward. Sound determination of whether those uses correctly reflect the intent of the programmer is infeasible in the general case. Heuristic warnings could be useful.

Related Vulnerabilities

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

Related Guidelines

C Secure Coding Standard: INT10-C. Do not assume a positive remainder when using the % operator

C++ Secure Coding Standard: INT10-CPP. Do not assume a positive remainder when using the % operator

Bibliography

\[[JLS 2005|AA. Bibliography#JLS 05]\] [Section 15.17.3|http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.17.3] "Remainder Operators"

Bibliography

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Image Added Image Added Image AddedImage Removed      03. Integers (INT)      NUM10-J. Beware of precision loss when converting primitive integers to floating-point