Do not make assumptions about the sign of the resulting value from the remainder % operator.
The meaning of the remainder operator for negative arguments was defined to be implementation defined in 1989 ISO C (and was so in historical K&R implementations), but that was fixed in the 1999 Standard.
Because not all C-compilers are strictly C99-conforming, you can not rely on the behavior of the % operator if you need to run on a wide range of platforms with many different compilers.
The div() function provides the same result as the remainder % operator.
According to C99:
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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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 definition of % operator implies the following behavior:
| Code Block |
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17 % 3 -> 2
17 % -3 -> 2
-17 % 3 -> -2
-17 % -3 -> -2
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The result has the same sign as the dividend (the first operand in the expression)The sign of the remainder % operator is implementation-defined when the operands are of different sign. This can result in a portability problem, when the programmer has assumed that i % j is always positive.
Non-Compliant Coding Example
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Search for vulnerabilities resulting from the violation of this rule on the CERT website.
References
| Wiki Markup |
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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] |