C语言中的Shift运算符以1而不是0开头 [英] Shift operator in C prepends ones instead of zeros

问题描述

这是代码:

#define u8 char
#define u32 unsigned int

typedef struct {
    //decoded instruction fields
    u8 cond; // Condition (f.ex. 1110 for always true)
    u8 instruction_code; // Is a constant since we only use branch
    u32 offset; // Offset from current PC
} dcdinst;

u8 mem[1024];

mem[0x0] = 0b11101010;

u8* instruction_addr = &mem[pc];

if (instruction_addr == NULL) {
    return false;
}

unsigned int first_part = instruction_addr[0];

// Here is the code that presents a problem:
// I try to get the upper part of the first byte
inst.cond = first_part >> 4;

first_part是以下字节:11101010. inst.cond变为11111110，但我需要将其设置为00001110.

first_part is the following byte: 11101010. inst.cond becomes 11111110, but I need it to be 00001110.

所以，我的实际问题是我想获取指令的前4位，该位的起始地址为instruction_addr.我试图通过使用右移位运算符>>来做到这一点，但问题是它不是在字节的左边加0，而是在前面加1.

So, my actual problem is that I want to get the first 4 bits of my instruction which starts at the address instruction_addr. I tried to do so by using the right shift operator >> but the problem is that instead of prepending 0s to the left of the byte, it prepends 1s.

我在stackoverflow上发现，我首先必须将值转换为无符号值，这就是我使用变量first_part所做的事情，但是我仍然遇到相同的问题.我不明白为什么这种转变似乎将我的变量视为"负变量，而其类型却是无符号的".

I found on stackoverflow that I first had to cast the value to a unsigned one, and that's what I did by using the variable first_part, but I still have the same problem. I don't understand why the shift seems to "see" my variable as a negative one while its type is specifically "unsigned".

有人有主意吗?

推荐答案

您的u8类型使用的是char，没有指定签名，这意味着它具有未定义的签名.默认情况下，您的编译器可能正在使用signed char.因此，在操作过程中和升级过程中，您都需要进行符号扩展.

Your u8 type is using char without specifying signedness, which means it has undefined signedness. It's likely your compiler is using a signed char by default. Thus, you're subject to sign-extension during operations and during promotion.

更改:

#define u8 char
#define u32 unsigned int

收件人:

typedef unsigned char u8;
typedef unsigned int u32;

(或正确使用stdint.h类型)，并且您的存储空间实际上应未签名.

(or use stdint.h types properly), and your storage should be actually unsigned.

使用typedef也意味着编译器会涉及此别名，这不仅是预处理器文本替换，而且消除了一类细微的错误.

Using typedefs also means the compiler is involved with this aliasing, it's not just a preprocessor text replacement, eliminating a class of subtle errors.

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