是什么在C位域使用的正确方法？ [英] What's the correct way of using bitfields in C?

问题描述

我使用的位域工作再上一个浮点数库我试图做出不带FPU微控制器方便地访问。

I am using bitfields to get easy access on a float library I am trying to make for a microcontroller with no FPU.

问题是，我似乎无法使其与位域工作。请看下图：

The problem is that I can't seem to make it work with bitfields. Take a look:

typedef struct
{
   union{
    unsigned long mantissa: 23;
    unsigned long exponent: 8;
    unsigned long sign: 1;
	float all;

      };

}_float __attribute__((__packed__));

问题是，当我试图访问或更改任何它认为分别从末端位域为1,8,23位。而应该是从端23位，则8位，然后最后一个比特。除非我已经完全误解了使用位字段。我想到用包装将解决这个问题，但你可以看到它没有。

The problem is that when I try to access or change anything it considers the bitfields as 1,8,23 bits from the end respectively. While it should be 23 bits from the end, then 8 bits and then the last bit. Unless I have totally misunderstood the use of bitfields. I thought that using packed would solve the problem but as you can see it didn't.

任何帮助将是非常美联社preciated。而谷歌搜索不止一次让我有很高的期望，我一直带领这个网站。

Any help would be really appreciated. I have been lead to this site while googling more than once so I have high hopes.

推荐答案

您可能忽略了结构的联合中。

You might be missing a struct inside your union.

typedef struct
{
    union{
       struct {
           unsigned long mantissa: 23;
           unsigned long exponent: 8;
           unsigned long sign: 1;
       } float_parts;
       float all;
    };
}_float __attribute__((__packed__));

注意尾数/指数和符号的顺序取决于CPU的一个字节顺序。

Note that the order of mantissa/exponent and sign depends one the endianess of the cpu.

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