I've seen multiple questions on the site addressing unsigned integer overflow/underflow.
Most of the questions about underflow ask about assigning a negative number to an unsigned integer; what's unclear to me is what happens when an unsigned int is subtracted from another unsigned int e.g. a - b where the result is negative. The relevant part of the standard is:
A computation involving unsigned operands can never overflow, because a result that cannot be represented by the resulting unsigned integer type is reduced modulo the number that is one greater than the largest value that can be represented by the resulting type.
In this context how do you interpret "reduced"? Does it mean that UINT_MAX+1 is added to the negative result until it is >= 0?
I see that the main point is addressed by this question (which basically says that the standard chooses to talk about overflow but the main point about modulo holds for underflow too) but it's still unclear to me:
Say the result of a-b is -1; According to the standard, the operation -1%(UINT_MAX+1) will return -1 (as is explained here); so we're back to where we started.
This may be overly pedantic, but does this modulo mean a mathematical modulo as opposed to C's computational modulo?
Firstly, a result that is below the minimum value of the given integer type is not called "underflow" in C. The term "underflow" is reserved for floating-point types and means something completely different. Going out of range of an integer type is always overflow, regardless of which end of the range you cross. So the fact that you don't see the language specification talking about "underflow" doers not really mean anything in this case.
Secondly, you are absolutely right about the meaning of the word "reduced". The final value is defined by adding (or subtracting) UINT_MAX+1 from the "mathematical" result until it returns into the range of unsigned int. This is also the same thing as Euclidean "modulo" operation.