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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.
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// method imod() gives non-negative 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; // unary - will succeed without overflow
// because temp cannot be Integer.MIN_VALUE
}
public int lookup(int hashKey) {
return hash[imod(hashKey, SIZE)];
}
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Analysis
Incorrectly assuming a positive remainder from a remainder operation can result in erroneous code.
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Guideline
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Severity
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Likelihood
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Remediation Cost
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Priority
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Level
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NUM51-JG
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low
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unlikely
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P1
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L3
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 Guidelines
The CERT C Secure Coding Standard | INT10-C. Do not assume a positive remainder when using the % operator |
The CERT C++ Secure Coding Standard | INT10-CPP. Do not assume a positive remainder when using the % operator |
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03. Numeric Types and Operations (NUM) 03. Numeric Types and Operations (NUM)