en.喂鸡.org/wiki/Integer_overflow
Since an arithmetic operation may produce a result larger than the
maximum representable value, a potential error condition may result. In
the C programming language, signed integer overflow causes undefined behavior, while unsigned integer overflow causes the number to be reduced modulo a power of two, meaning that unsigned integers "wrap around" on overflow.
擦,还有这种意思……

难怪我听说溢出越界的说法都有。。。