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

On most compliers, both will give a result with the same representation. However, according to the C specification, the result of a bit shift operation on a signed argument gives implementation-defined results, so in theory 1U << i is more portable than 1 << i. In practice all C compilers you'll ever encounter treat signed left shifts the same as unsigned left shifts.

The other reason is that if nSize is unsigned, then comparing it against a signed 1 << i will generate a compiler warning. Changing the 1 to 1U gets rid of the warning message, and you don't have to worry about what happens if i is 31 or 63.

The compiler warning is most likely the reason why 1U appears in the code. I suggest compiling C with most warnings turned on, and eliminating the warning messages by changing your code.