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Do not make assumptions about the sign of the remainder when using the % operator.

In C89 (and historical K&R implementations), the meaning of the remainder operator for negative arguments operands was implementation-defined, but was fixed in the C99 Standard. This behavior was changed in C99, and the change remains in C11.

Because not all C - compilers are strictly C99C-conforming, you can not 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 C99The 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.

and

When integers are divided, the result of the / operator is the algebraic quotient with any fractional part discarded. If the quotient a/b is representable, the expression (a/b)*b + a%b shall equal a.

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

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

17 11 %  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).

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Noncompliant Code Example

In this non-compliant 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 non-compliant code example also violates \[[INT01-A. Use size_t for all integer values representing the size of an object]\].

Compliant Solution

To provide a true (never negative) modulo operation, use the imod() ("integer modulo") inline function:

/* modulo function giving non-negative result */
inline int imod(int i, int j) {
  return (i % j) < 0 ? (i % j) + (j < 0 ? -j : j) : i % j;
}

Risk Assessment

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

Noncompliant Code Example

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 INT01-C. Use size_t or rsize_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 INT32-C. Ensure that operations on signed integers do not result in overflow.

Compliant Solution (Unsigned Types)

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 ERR02-C. Avoid in-band error indicators, this solution fills a result argument with the mathematical result and 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.

References

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Related Guidelines

Bibliography

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Image Added Image Added Image Added operators" \[[Microsoft 07|AA. C References#Microsoft 07]\] [C Multiplicative Operators|http://msdn2.microsoft.com/en-us/library/efa0csed(VS.80).aspx] \[[Sun 05|AA. C References#Sun 05]\] C User's Guide Sun Studio 11 819-3688-10 [http://docs.sun.com/source/819-3688/]. 2005. [Appendix E, "Implementation-Defined ISO/IEC C90 Behavior"|http://docs.sun.com/source/819-3688/c90.implementation.app.html]INT09-A. Ensure enumeration constants map to unique values      04. Integers (INT)       INT11-A. Do not make assumptions about the layout of bit-field structures