Structure size and memory alignment rules

memory alignment rules

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1. #include "stdafx.h"  
2.   
3. //#pragma pack(8) is related to the environment. The default value of the window system is 8, and the default value of linux is 4.  
4.   
5. #if 0  
6.   
7. The essence of memory alignment is to sacrifice space for time  
8.   
9. ---- Memory alignment rules:  
10. (1): Take pack(n), take the size m of the largest variable type in the structure ( char short int )    
11. n=8 m=4; take the smaller value of the two numbers Y=4 outer alignment (Y is outer alignment)  
12.   
13.   
14. (2): 1( char ) 2( short ) 4( int ) (actual type size) Compare a series of internal alignment rules X 1 2 4      
15. (Compare the actual type size with the Y value, and take the smaller one to get x)  
16.   
17.   
18. (3): The so-called internal alignment (starting address is 0), that is, the place where the address value is divisible by x starts to store data   
19. (The value of the address /X is stored from here if it is divisible)  
20.    
21.   
22. (4): The so-called outer alignment is the end address, which is the smallest integer multiple of the outer alignment  
23. (The end can be determined according to the actual situation)  
24.   
25. -------------------------------------------------------------  
26. //The body of the structure is char double float short  
27. n 8      m 8      Y 8  
28. 1 8 4 2     X 1 8 4 2  
29.   
30. If #pragma pack(1) takes 1  
31.   
32. n 1      m 8      Y 1  
33. 1. 1 8 4 2 X 1 1 1 1 The actual size can be divided by X   
    2. Then according to the rules, it is stored in order  
    3. So the size is 1+8+4+2=15  
    4.   
    5.   
    6. #endif  
    7.   
    8.   
    9. struct type  
    10. {  
    11.     char a;   //1  
    12.     double b; //8  
    13.     float c;  //4  
    14.     short d;  //2  
    15. };  
    16.   
    17. int _tmain(int argc, _TCHAR* argv[])  
    18. {  
    19.     printf("size=%d\n", sizeof(struct type));  
    20.     return 0;  
    21. }  

结构体的大小

[cpp]  view plain  copy

1. #include "stdafx.h"  
2.   
3.   
4. struct type  
5. {  
6.     char a;  //1   //空了3个字节 内存对齐的需要   
7.     //short c;  short在这个位置type大小为8  
8.     int b;   //4  
9.     //short c; short在这个位置type大小为12  
10. }/*var*/;  //结构体的类型 type不占空间 但var占空间  
11.   
12.   
13. int _tmain(int argc, _TCHAR* argv[])  
14. {  
15.     struct type var;  
16.   
17.     printf("sizeof(struct type)=%d  sizeof(var)=%d\n",  
18.         sizeof(struct type), sizeof(var));  
19.   
20.     printf("&var.a=%p  &var.b=%p\n", &var.a, &var.b);  
21.   
22.     return 0;  
23. }  

Original blog address: https://blog.csdn.net/swordarcher/article/details/78546429