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Wiki MarkupIn C89 (and historical K&R [implementations|BB. Definitions#implementation]), the meaning of the remainder operator for negative operands was [implementation -defined|BB. Definitions#implementation defined behavior]. This was changed in the C99 standard \[[ISO/IEC 9899-1999|AA. Bibliography#ISO/IEC 9899-1999]\].. This behavior was changed in C99, and the change remains in C11.

Because not all C compilers are strictly C99 C-conforming, you programmers cannot rely on the behavior of the % operator if you they need to run on a wide range of platforms with many different compilers.According to C99

The C Standard, subclause 6.5.5 [ISO/IEC 9899:2011], states:

The result of the / operator is the quotient from the division of the first operand by the second; the result of the % operator is the remainder. In both operations, if the value of the second operand is zero, the behavior is undefined.

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Discarding the fractional part of the remainder is often called truncation toward zero.

The C99 C definition of the % operator implies the following behavior:

 1711 %  35  ->  21
 1711 % -35  ->  21
-1711 %  35  -> -21
-1711 % -35  -> -21

The result has the same sign as the dividend (the first operand in the expression).

Noncompliant Code Example

In this noncompliant code example, the insert() function adds values to a buffer in a modulo fashion, that is, by inserting values at the beginning of the buffer once the end is reached. However, both size and index are declared as int and , consequently , not are not guaranteed to be positive. Depending on the implementation and on the sign of size and index, the result of (index + 1) % size may be negative, resulting in a write outside the bounds of the list array.

int insert(int index, int *list, int size, int value) {
  if (size != 0) {
    index = (index + 1) % size;
    list[index] = value;
    return index;
  }
  else {
    return -1;
  }
}

This code also violates recommendation ERR02-C. Avoid in-band error indicators.

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Taking the absolute value of the modulo operation returns a positive value:

int insert(int index, int *list, int size, int value) {
  if (size != 0) {
    index = abs((index + 1) % size);
    list[index] = value;
    return index;
  }
  else {
    return -1;
  }
}

However, this noncompliant code example violates recommendation INT01-C. Use rsizesize_t or sizersize_t for all integer values representing the size of an object. There is also a possibility that (index + 1) could result in a signed integer overflow in violation of rule INT32-C. Ensure that operations on signed integers do not result in overflow.

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The most appropriate solution in this case is to use unsigned types to eliminate any possible implementation-defined behavior, as in this compliant solution. For compliance with recommendation ERR02-C, we fill a 'result' . Avoid in-band error indicators, this solution fills a result argument with the mathematical result , and we return returns nonzero only if the operation succeeds.

int insert(size_t* result, size_t index, int *list, size_t size, int value) {
  if (size != 0 && size != SIZE_MAX) {
    index = (index + 1) % size;
    list[index] = value;
    *result = index;
    return 1;
  }
  else {
    return 0;
  }
}

Risk Assessment

Incorrectly assuming that the result of the remainder operator for signed operands will always be positive can lead to an out-of-bounds memory accessor other flawed logic.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

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Bibliography

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Beebe

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2005]
[ISO/IEC 9899:2011]	Subclause 6.5.5, "Multiplicative Operators"
[Microsoft 2007]	C Multiplicative Operators
[Sun 2005]	Appendix E, "Implementation-Defined ISO/IEC C90 Behavior"

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Image Added Image Added Image Added|AA. Bibliography#Beebe 05]\] \[[Microsoft 2007|AA. Bibliography#Microsoft 07]\] [C Multiplicative Operators|http://msdn2.microsoft.com/en-us/library/efa0csed(VS.80).aspx] \[[Sun 2005|AA. Bibliography#Sun 05]\] [Appendix E, "Implementation-Defined ISO/IEC C90 Behavior"|http://docs.sun.com/source/819-3688/c90.implementation.app.html]Image Removed      04. Integers (INT)       INT11-C. Take care when converting from pointer to integer or integer to pointer