如何为带有位字段的结构分配值？ [英] How to assign value to a struct with bit-fields?

问题描述

我有一个位域（总共32位宽）的结构，我有一个32位的变量。当我尝试分配变量值到我的结构，我得到一个错误：

I have a struct with bit-fields (totally 32 bit width) and I have a 32-bit variable. When I try to assign the variable value to my struct, I got an error:

错误：从'uint32_t {aka unsigned int} '到非标量类型'main（）:: CPUID'请求。

error: conversion from ‘uint32_t {aka unsigned int}’ to non-scalar type ‘main()::CPUID’ requested.

struct CPUIDregs
    {
       uint32_t EAXBuf;
    };
CPUIDregs CPUIDregsoutput;   


int main () {

 struct CPUID          
    {
          uint32_t   Stepping         : 4;         
          uint32_t   Model            : 4;        
          uint32_t   FamilyID         : 4;        
          uint32_t   Type             : 2;        
          uint32_t   Reserved1        : 2;         
          uint32_t   ExtendedModel    : 4;         
          uint32_t   ExtendedFamilyID : 8;          
          uint32_t   Reserved2        : 4;          
    };

    CPUID CPUIDoutput = CPUIDregsoutput.EAXBuf;

你有什么想法如何以最短的方式做？感谢

Do you have any idea how to do it in the shortest way? Thanks

当然，我在实际代码中有更适当的EAX值，但我想这不会影响这里。

P.S. Of course I have more appropriate value of EAX in real code, but I guess it doesn't affect here.

推荐答案

从不依赖于编译器如何在内存中布局您的结构。

You should never rely on how the compiler lays out your structure in memory. There are ways to do what you want with a single assignment, but I will neither recommend nor tell you.

完成作业的最佳方式如下：

The best way to do the assignment would be the following:

static inline void to_id(struct CPUid *id, uint32_t value)
{
    id->Stepping         = value & 0xf;
    id->Model            = (value & (0xf << 4)) >> 4;
    id->FamilyID         = (value & (0xf << 8)) >> 8;
    id->Type             = (value & (0x3 << 12)) >> 12;
    id->Reserved1        = (value & (0x3 << 14)) >> 14;
    id->ExtendedModel    = (value & (0xf << 16)) >> 16;
    id->ExtendedFamilyID = (value & (0xff << 20)) >> 20;
    id->Reserved2        = (value & (0xf << 28)) >> 28;
}

相反

static inline uint32_t from_id(struct CPUid *id)
{
    return id->Stepping
         + ((uint32_t)id->Model << 4)
         + ((uint32_t)id->FamilyID << 8)
         + ((uint32_t)id->Type << 12)
         + ((uint32_t)id->Reserved1 << 14)
         + ((uint32_t)id->ExtendedModel << 16)
         + ((uint32_t)id->ExtendedFamilyID << 20)
         + ((uint32_t)id->Reserved2 << 28);
}

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