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Because not all C compilers are strictly 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.

The C Standard, section 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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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 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.

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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 ERR02-C. Avoid in-band error indicators, we fill this solution fills a result argument with the mathematical result , and we return returns nonzero only if the operation succeeds.

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Bibliography

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