Question

任何人都可以告诉我，下面显示的结构大小是24而不是20。

typedef struct
{
    double d;  // this would be 8 bytes
    char c;   // This should be 4 bytes considering 3 bytes padding
    int a;   // This would be 4 bytes
    float b; // This would be 4 bytes
} abc_t;

main()
{
    abc_t temp;
    printf("The size of struct is %d\n",sizeof(temp));
}

我的假设是当我们考虑填充时结构的大小为20但是当我运行此代码时，大小打印为24。

Answer 1

尺寸为24。这是因为最后一个成员填充了所需的字节数，因此结构的总大小应该是任何结构成员的最大对齐的倍数。

所以填充就像

typedef struct
{
    double d;  // This would be 8 bytes
    char c;    // This should be 4 bytes considering 3 bytes padding
    int a;     // This would be 4 bytes
    float b;   // Last member of structure. Largest alignment is 8.  
               // This would be 8 bytes to make the size multiple of 8 
} abc_t;

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Answer 2

也许打包属性会回答问题。

typedef struct
{
    double d;  // this would be 8 bytes
    char c;   // This should be 4 bytes considering 3 bytes padding
    int a;   // This would be 4 bytes
    float b; // This would be 4 bytes
} __attribute__((packed)) abc_t ;

假设我们有结构填充，int的大小为4，double的大小为8字节，下面这段代码中结构的大小是多少

2 个答案: