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In C89 (and historical K&R [implementations|BB. Definitions#implementation]), the meaning of the remainder operator for negative arguments was [implementation-defined|BB. Definitions#implementation-defined behavior]. This was fixed in the C99 standard \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\]

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 was implementation-defined, but was fixed in the C99 Standard

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

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

The C99 definition of % operator implies the following behavior:

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

Non-Compliant

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Code Example

In this non-compliant 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 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;
  }
}

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

Compliant Solution

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

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\[[Beebe 05|AA. C References#Beebe 05]\] Nelson H. F. Beebe [Re: Remainder ( % ) operator and GCC|http://gcc.gnu.org/ml/gcc-help/2005-11/msg00141.html] 2005.
\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.5.5, "Multiplicative 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]

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