Because not all C compilers are strictly C-conforming, programmers cannot rely on the behavior of the
% operator if they need to run on a wide range of platforms with many different compilers.
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.
When integers are divided, the result of the
/operator is the algebraic quotient with any fractional part discarded. If the quotient
a/bis representable, the expression
(a/b)*b + a%bshall equal
Discarding the fractional part of the remainder is often called truncation toward zero.
The C definition of the
% operator implies the following behavior:
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
index are declared as
int and consequently are not guaranteed to be positive. Depending on the implementation and on the sign of
index, the result of
(index + 1) % size may be negative, resulting in a write outside the bounds of the
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:
However, this noncompliant code example violates INT01-C. Use rsize_t or size_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.
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.
Could detect the specific noncompliant code example. It could identify when the result of a % operation might be negative and flag usage of that result in an array index. It could conceivably flag usage of any such result without first checking that the result is positive, but it would likely introduce many false positives
Can detect violations of this recommendation with the CERT C Rule Pack
|Parasoft C/C++test||9.5||BD-PB-ARRAY||Partially implemented|
|Polyspace Bug Finder||R2016a||Tainted modulo operand|
|PRQA QA-C||8.2||3103||Fully implemented|
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
|[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"|