用设定最显著位 [英] set most significant bit in C

问题描述

我试图创下最显著位在长隆符号，X。
要做到这一点，我使用这行code的：

I am trying to set the most significant bit in a long long unsigned, x. To do that I am using this line of code:

x |= 1<<((sizeof(x)*8)-1);

我想这应该工作，因为sizeof的字节大小给人，所以我再乘以8，并减去一个设置最后一位。每当我这样做，编译器有这样的警告：警告：类型的左移计数> =宽度

I thought this should work, because sizeof gives size in bytes, so I multiplied by 8 and subtract one to set the final bit. Whenever I do that, the compiler has this warning: "warning: left shift count >= width of type"

我不明白为什么这个错误发生。

I don't understand why this error is occurring.

推荐答案

的 1 您移行类型的常量 INT ，这意味着你被的sizeof（无符号长长）* 8）移位的 INT 值 - 1 位。这种转变可以很容易地超过 INT 的宽度，这显然是在您的情况发生了什么事了。

The 1 that you are shifting is a constant of type int, which means that you are shifting an int value by sizeof(unsigned long long) * 8) - 1 bits. This shift can easily be more than the width of int, which is apparently what happened in your case.

如果你想获得无符号长长类型的某些位掩码面具，你应该的初始位掩码无符号长启动长类型，而不是 INT 键入

If you want to obtain some bit-mask mask of unsigned long long type, you should start with an initial bit-mask of unsigned long long type, not of int type.

1ull << (sizeof(x) * CHAR_BIT) - 1

这是可以说是更好的方式来构建相同的面具将

An arguably better way to build the same mask would be

~(-1ull >> 1)

或

~(~0ull >> 1)

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