The surprise would be if you wanted to, say, add 100 to a uint8 and have it wrap, but were assigning the result to a uint64. Then your arithmetic wouldn't wrap,

The surprise would be if you wanted to, say, add 100 to a uint8 and have it wrap, but were assigning the result to a uint64. Then your arithmetic wouldn't wrap, unless you explicitly told it to, i.e. "x = (y + 100) & 0xff;" Either way leads to surprises if your intuition tells you the other thing should happen. Strictly, in C the behaviour is undefined when a signed integer overflows. Although it wouldn't help in the author's case, since he's using unsigned integers, it is valid for a compiler to internally promote the operands of signed arithmetic.