有没有定义INT_MAX一个可移植的方法？ [英] Is there a portable way to define INT_MAX?

问题描述

我发现在/usr/include/limits.h以下定义：

I found the following definitions in /usr/include/limits.h:

＃定义INT_MIN（-INT_MAX - 1）

＃定义INT_MAX 2147483647

另外，似乎在此头文件中的所有XXX_MAX的显式从数字常数定义

Also, it seems that all XXX_MAX's in this header file are explicitly defined from a numerical constant.

我（跨平台针对不同的字大小）不知道是否有一个可移植的方式来定义INT_MAX？

I wonder if there is a portable way (against different word sizes across platforms) to define a INT_MAX ?

我试过如下：

〜（（INT）-1）

但这似乎不正确。

一个短的解释也高度重视。

A short explanation is also highly regarded.

推荐答案

对于 INT_MAX 在标准头 limits.h中，实现者的手被它的要求是在preprocessor ​​可用＃如果指令的事实追平。这排除了任何涉及的sizeof 或转换。

For the INT_MAX in the standard header limits.h, the implementor's hands are tied by the fact that it's required to be usable in preprocessor #if directives. This rules out anything involving sizeof or casts.

如果你只是想在实际的C EX pressions使用的版本，也许这会工作：

If you just want a version that works in actual C expressions, perhaps this would work:

(int)-1U/2 == (int)(-1U/2) ? (int)-1U : (int)(-1U/2)

这里的概念是， INT 可能具有相同的数值位为无符号，或者少一个值位; C标准允许使用。为了测试它是，检查转换结果（INT）-1U 。如果 -1U 在 INT 适合，其值必须是演员不变，所以平等是真实的。如果 -1U 不适合在 INT ，然后投结果类型的实现定义的结果 INT 。不管该值是什么，虽然，平等将假仅通过可能值的范围内。

The concept here is that int may have the same number of value bits as unsigned, or one fewer value bit; the C standard allows either. In order to test which it is, check the result of the conversion (int)-1U. If -1U fits in int, its value must be unchanged by the cast, so the equality will be true. If -1U does not fit in int, then the cast results in an implementation-defined result of type int. No matter what the value is, though, the equality will be false merely by the range of possible values.

请注意，从技术上，转换到 INT 可能会导致一个实现定义的信号被提出，而不是获得一个实现定义的值，但这不是会当你处理一个恒定的前pression这将在编译时进行评估的情况发生。

Note that, technically, the conversion to int could result in an implementation-defined signal being raised, rather than an implementation-defined value being obtained, but this is not going to happen when you're dealing with a constant expression which will be evaluated at compile-time.

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