C中的算术下溢和溢出是什么？

从http://en.wikipedia.org/wiki/Arithmetic_overflow：

the condition that occurs when a
calculation produces a result that is
greater in magnitude than that which a
given register or storage location can
store or represent.

所以,例如：

uint32_t x = 1UL << 31;
x *= 2;  // Overflow!

请注意,由于@R在下面的评论中提到,C标准表明：

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.

当然,这是一个比较特殊的“溢出”定义.大多数人会将模数减少(即环绕)称为“溢出”.

潜流

从http://en.wikipedia.org/wiki/Arithmetic_underflow：

the condition in a computer program that
can occur when the true result of a
floating point operation is smaller in
magnitude (that is,closer to zero)
than the smallest value representable
as a normal floating point number in
the target datatype.

所以,例如：

float x = 1e-30;
x /= 1e20; // Underflow!