如何检查是否发生溢出? [英] How to check if overflow occured?

问题描述

可能重复:
检测C/C ++中整数溢出的最佳方法

Possible Duplicate:
Best way to detect integer overflow in C/C++

这可能是一个菜鸟问题，但是如何检查某些溢出影响了C中我的数字的值.例如，在将整数相乘并等待整数结果时，如果实际结果大于max-integer值，实际结果已更改(对吗?).那么，如何判断是否发生了这种情况?

This is probably a rookie question, but how can I check some overflow affected the value of my numbers in C. For example, when multiplying integers, and waiting for an integer result, if actual result was bigger than max-integer value, actual result is altered(right?). So how can I tell if something like this occured?

推荐答案

带符号的整数溢出就像被零除-导致未定义的行为，因此您必须在执行前检查它们是否会出现.潜在溢出的操作.一旦溢出，所有赌注都将消失-您的代码可以执行任何操作.

Signed integer overflow is like division by zero - it leads to undefined behaviour, so you have to check if it would occur before executing the potentially-overflowing operation. Once you've overflowed, all bets are off - your code could do anything.

在<limits.h>中定义的*_MAX和_MIN宏可以派上用场，但是您需要注意不要在测试本身中调用未定义的行为.例如，要检查在给定的int a, b;下a * b是否会溢出，可以使用:

The *_MAX and _MIN macros defined in <limits.h> come in handy for this, but you need to be careful not to invoke undefined behaviour in the tests themselves. For example, to check if a * b will overflow given int a, b;, you can use:

if ((b > 0 && a <= INT_MAX / b && a >= INT_MIN / b) ||
    (b == 0) ||
    (b == -1 && a >= -INT_MAX) ||
    (b < -1 && a >= INT_MAX / b && a <= INT_MIN / b))
{
    result = a * b;
}
else
{
    /* calculation would overflow */
}

(请注意，这避免了一个细微的陷阱，即您无法计算INT_MIN / -1-这样的数字不能保证可表示，并且确实会导致常见平台上的致命陷阱.)

(Note that one subtle pitfall this avoids is that you can't calculate INT_MIN / -1 - such a number isn't guaranteed to be representable and indeed causes a fatal trap on common platforms).

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