Struct Align

基本介紹

C 以變數類型配置記憶體空間，常見類型及大小如下

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char 	        1 byte 	        -128 to 127 or 0 to 255
unsigned char 	1 byte 	        0 to 255
signed char 	1 byte 	        -128 to 127
int 	        2 or 4 bytes 	-32,768 to 32,767 or -2,147,483,648 to 2,147,483,647
unsigned int 	2 or 4 bytes 	0 to 65,535 or 0 to 4,294,967,295
short 	        2 bytes 	    -32,768 to 32,767
unsigned short 	2 bytes 	    0 to 65,535
long 	        4 bytes 	    -2,147,483,648 to 2,147,483,647
unsigned long 	4 bytes 	    0 to 4,294,967,295

32 bits 的架構上，一次的資料存取也就是32 bits (4bytes)

4 bytes 不是隨便從哪個點抓都可以，而是以4 bytes 為單位

比如說第 0，4，8 ，12….等，而不會是從3，7，9 開始

這表示了對齊 (alignment)的重要，因為會影響到幾次才抓的完。

如果要抓一個4 bytes 的int，而這個int是從 6 擺到 10，那就表示要抓兩次

因為會先 (4),5,6,7 再來 (8),9,10,11，範例

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int main(int argc, char* argv[])
{
        struct align_test
        {
            char        x;  // 1 byte
            int         y;  // 4 byte
            short int   z;  // 2 byte
        };
        struct align_test test;
        
        
        printf("test size   is %d\n", sizeof(test));
        printf("x size      is %d\n", sizeof(test.x));
        printf("y size      is %d\n", sizeof(test.y));
        printf("z size      is %d\n", sizeof(test.z));
                
        printf("address of test is  %p\n", &test);
        printf("address of x is     %p\n", &test.x);
        printf("address of y is     %p\n", &test.y);
        printf("address of z is     %p\n", &test.z);
        return 0;
}

test size   is 12
x size      is 1
y size      is 4
z size      is 2

address of test is  0x7fffb6237f00
address of x is     0x7fffb6237f00
address of y is     0x7fffb6237f04
address of z is     0x7fffb6237f08


所以放的位址如下
+---+---+---+---+---+---+---+---+---+---+---+---+
| x | p | p | p | y | y | y | y | z | p | p | p |
+---+---+---+---+---+---+---+---+---+---+---+---+
  0   1   2   3   4   5   6   7   8   9   10  11  
  

照理說應該只要 1+4+2 = 7 byte 空間
但是因為 4 byte align 造成浪費，其中的 p 為 padding
所以實際上會變為

struct align_test
{
    char        x;          // 1 byte
    char        padding0[3];
    int         y;          // 4 bytes
    short int   z;          // 2 bytes
    char        padding1[2];
};

如果不想浪費空間可以改為

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#pragma pack(push)      /* push current alignment to stack */
#pragma pack(1)         /* set alignment to 1 byte boundary */
struct align_test
{
    char x;         // 1 byte
    int y;          // 4 bytes
    short int z;    // 2 bytes
};
#pragma pack(pop)       /* restore original alignment from stack */

test size   is 7
x size      is 1
y size      is 4
z size      is 2
address of test is  0x7fff7230a290
address of x is     0x7fff7230a290
address of y is     0x7fff7230a291
address of z is     0x7fff7230a295


所以放的位址如下
+---+---+---+---+---+---+---+
| x | y | y | y | y | z | z |
+---+---+---+---+---+---+---+
  0   1   2   3   4   5   6

參考

參考

參考