工会内的位域对齐 [英] Alignment of bitfields inside unions

问题描述

  struct Thing 
我有点困惑于下面的代码如何在内存中放置： {
 union 
 {
无符号值：24; 
 uint8_t bytes [3]; 
}; 
 Thing（int v）
：value（v）
 {} 
 $ b void foo（）
 {
 printf（Thing％ p值=％d！\ n，this，value）; 
} 
 
} __attribute __（（__ packed__））; 
  

在Linux上的gcc 3.3,4.3或4.6上（没有我能想到的特殊选项 - 只有 -W在4.6），结构的大小总是4：

  $ pahole ./union 
 struct Thing {
 union {
 unsigned int value; / * 4 * / 
 unsigned char bytes [3]; / * 3 * / 
}; 
 [...] 
  

我们在这里有一些类似的代码， ：24在一个结构中，并且有人加入了联合并且无意中将结构的大小从3增加到了4字节。
如果我试图将联合定义为packed - 大小仍然是4，那么会发生同样的事情。这种行为是否符合C ++规范？会有什么解释？

稍后编辑：用C ++规范替换C规范。

解决方案

您错过了匿名联盟的打包属性。考虑这个例子：

pre $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $＃ } PACKED;
struct S2 {union {char a [3];无符号值：24; }; } PACKED;
struct S3 {union {char a [3];无符号值：24; } PACKED; };


int main（）{
std :: cout<< sizeof（S1）<<<的std :: ENDL;
std :: cout<< sizeof（S2）<<<的std :: ENDL;
std :: cout<< sizeof（S3）<的std :: ENDL;
}

输出：

  3 
 4 
 3 
  

打包的属性有点奇怪，我总是尝试和测试每种可能的组合以获得正确的结果。

I'm a bit puzzled by how the following code gets layed out in memory:

struct Thing
{
    union
    {
        unsigned value:24;
        uint8_t bytes[3];
    };
    Thing(int v)
            :value(v)
    {}

    void foo()
    {
        printf("Thing %p value=%d !\n", this, value);
    }

} __attribute__((__packed__));

On gcc 3.3, 4.3 or 4.6 on Linux (without any special options I can think of - only "-Wall -g" on 4.6), the size of the structure is always 4:

$ pahole ./union
struct Thing {
        union {
                unsigned int               value;                /*           4 */
                unsigned char              bytes[3];             /*           3 */
        };
[...]

We had some similar code here where we had the unsigned value:24 in a structure, and somebody added the union and inadvertently increased the size of the struct from 3 to 4 bytes. The same thing happens if I try to define the union as "packed" - size is still 4. Is this behavior as per the C++ spec? What would be the explanation?

later edit: replaced "C spec" with "C++ spec".

解决方案

You missed the packed attributes to your anonymous union. Consider this example:

#define PACKED __attribute__((__packed__))
struct S1 { unsigned value:24; } PACKED ;
struct S2 { union { char a[3]; unsigned value:24; };  } PACKED ;
struct S3 { union { char a[3]; unsigned value:24; } PACKED ;  };


int main() {
   std::cout << sizeof(S1) << std::endl;
   std::cout << sizeof(S2) << std::endl;
   std::cout << sizeof(S3) << std::endl;
}

Output:

3
4
3

The packed attribute is a little weird, I always try and test every possible combination to get proper result.

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