Is there any difference between 1U and 1 in C?

Question

    while ((1U << i) < nSize) {
        i++;
    }

Any particular reason to use 1U instead of 1?

Answer 1

1U is unsigned.

The reason why they used an unsigned value in that is expression is (I guess) because nSize is unsigned too, and compilers (when invoked with certain parameters) give warnings when comparing a signed and an unsigned values.

Another reason (less likely, in my opinion, but we cannot know without knowing wath value nSize is supposed to assume) is that unsigned values can be twice as big as signed ones, so nSize could be up to ~4*10^9 instead of ~2*10^9.