Ç整数溢出 [英] C integer overflow

问题描述

我在C整数工作，努力探索更多的溢出是如何发生的时候再打开。

I was working with integers in C, trying to explore more on when and how overflow happens.

我注意到，当我加了两个正数，其中溢出的总和，我总是得到一个负数。

I noticed that when I added two positive numbers, the sum of which overflows, I always got a negative number.

在另一方面，如果我添加了两个负数，其中溢出的总和，我总是有一个正数（包括0）。

On the other hand, if I added two negative numbers, the sum of which overflows, I always got a positive number (including 0).

我做了几个实验，但我想知道这是不是真的对每一个案件。

I made few experiments, but I would like to know if this is true for every case.

推荐答案

整数溢出在C未定义的行为。

Integer overflows are undefined behavior in C.

ç说，一名前pression涉及整数溢的，如果通常的算术转换后的结果是结果的类型psented签署的类型，不能再$ P $的。转让和投前pressions是个例外，因为它们是由整数转换裁定。

C says an expression involving integers overflows, if its result after the usual arithmetic conversions is of a signed typed and cannot be represented in the type of the result. Assignment and cast expressions are an exception as they are ruled by the integer conversions.

防爆pressions不能溢出，它们包住，即。克， 0U - 1 是 UINT_MAX

Expressions of unsigned type cannot overflow, they wrap, e. g., 0U - 1 is UINT_MAX.

例如：

INT_MAX + 1    // integer overflow
UINT_MAX + 1   // no overflow, the resulting type is unsigned
(unsigned char) INT_MAX // no overflow, integer conversion occurs 

不要让任何整数EX pression overlflows，现代的编译器（如 GCC ）充分利用整数溢出是执行各种类型的优化未定义的行为。

Never let any integer expression overlflows, modern compilers (like gcc) take advantage of integer overflows being undefined behavior to perform various types of optimizations.

例如：

a - 10 < 20

在 A 的类型为 INT 升级后，前pression在<$ C降低$ C> GCC （优化在启用时）为：

when a is of type int after promotion, the expression is reduced in gcc (when optimization are enabled) to:

a < 30

它采用前pression的优势是不确定的行为，当 A 的范围为 INT_MIN + 10 - 1 到 INT_MIN 。

该优化无法完成时 A 是 unsigned int类型，因为如果 A 是 0 ，那么 A - 10 已被评估为 UINT_MAX - 9 （无未定义行为）。优化 A - 10 LT; 20 到 A＆LT; 30 然后会导致不同的结果比要求之一，当 A 是 0 来 9 。

This optimization could not be done when a is unsigned int because if a is 0, then a - 10 has to be evaluated as UINT_MAX - 9 (no undefined behavior). Optimizing a - 10 < 20 to a < 30 would then lead to a different result than the required one when a is 0 to 9.

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