unsigned char型C = 255＆QUOT; 11111111＆QUOT;或不？ [英] Unsigned char c = 255 is "11111111" or not?

问题描述

下面是我的问题code：

Here is my problem code:

#include "stdio.h"

int main()
{
        char a = -1;
        unsigned char b = 255;
        unsigned char c = 0;
        if((~a) ==  c)
                printf("OK 1");
        else
                printf("bad 1");
        printf("\n");
        if((~b) ==  c)
                printf("OK 2");
        else
                printf("bad 2");
        printf("\n");
}

我预计打印：

OK 1

OK 2

不过，我得到OK 1和2不好！

But, I get OK 1 and bad 2!

如果 unsigned char型b 为255（11111111），那么〜乙方应00000000。为什么它不等于C吗？

If unsigned char b is 255 (11111111), then ~b should be 00000000. Why does it not equal c?

我在SUSE Linux的工作，用gcc。

I work on Linux SUSE, using gcc.

推荐答案

你要通过整数促销咬伤。当你这样做：

You're getting bitten by integer promotions. When you do:

~b == c

B 和 C 都得到提升到 INT 。这意味着你真的这样做的：

b and c both get promoted to int. That means you're really doing:

~0x000000ff == 0

这结束了​​比较：

0xffffff00 == 0

不匹配。它可以在你的第一个情况，因为你的字符类型签名，并得到在促进符号扩展：

Which don't match. It works in your first case because your char type is signed, and gets sign extended in the promotion:

~a == c
~(-1) == 0
~0xffffffff == 0
0 == 0

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