sizeof求一个结构体的大小为什么会多了两个字节呢？？？？？？？？

Adrian

如题。我定义了个结构体：

typedef        struct
{
        int16u        ChaPa_Z;
        int16u        ChaPa_K;
        int16u        JingPa_Z;
        int16u        JingPa_K;
        int16u        Temp_Z;
        int16u        Temp_K;
        int16u        Kv;
        int16u        Kp;
        int32u        Ba;
        int16u        CRCFM;
}Glo_T;
sizeof(Glo_T);是24，仿真和下载程序后测试都多了两个字节！！！！！！
我用的编译器是keil uv3。

数据类型定义为：
typedef unsigned long     int32u;   // Unsigned 32 bit quantity
typedef unsigned long     * pint32u;
typedef signed   short    int16s;   // Signed   16 bit quantity
typedef signed   short    * pint16s;
typedef unsigned short    int16u;   // Unsigned 16 bit quantity
typedef unsigned short    * pint16u;

Adrian

先顶。等高人解答。

Adrian

我这样测试
typedef        struct
{
int16u        CRCFM;
}Glo_T;

sizeof(Glo_T);的大小是4！！！！！！！！！！！！！！！！！！！

yunhuisong

看来是32位处理的啊，不够就补齐

yunhuisong

看来是32位处理的啊，不够就补齐

stm32w

typedef struct
{
   …………
  int32u Ba;     //32位变量，结构体整体对齐方式是4字节（也就是说结构体整体占用空间将是4的整数倍）
  int16u CRCFM;
}Glo_T;

stm32w

typedef struct
{
   …………
  int32u Ba;     //32位变量，结构体整体对齐方式是4字节（也就是说结构体整体占用空间将是4的整数倍）
  int16u CRCFM;
}Glo_T;

stm32w

typedef struct
{
   …………
  int32u Ba;     //32位变量，结构体整体对齐方式是4字节（也就是说结构体整体占用空间将是4的整数倍）
  int16u CRCFM;
}Glo_T;

stm32w

32位，结构体默认32位整体对齐。
（结构体整体占用存储空间是4的整数倍）。

（8为机移植到16位，32位都要考虑的问题）

一般可以用编译器扩展关键字，或者预处理，改变结构体对齐方式，可以按单字节对齐。

Adrian

回复【11楼】stm32w  

一般可以用编译器扩展关键字，或者预处理，改变结构体对齐方式，可以按单字节对齐。
-----------------------------------------------------------------------

请问这个具体如何实现？不大清楚。谢谢。

AWEN2000

结构定义时加__packed关键字，让结构按字节对齐

否则按4字节对齐的

Adrian

typedef        struct       
{
        int16u        ChaPa_Z;
        int16u        ChaPa_K;
        int16u        JingPa_Z;
        int16u        JingPa_K;
        int16u        Temp_Z;
        int16u        Temp_K;
        int16u        Kv;
        int16u        Kp;
        int32u        Ba;
        int16u        CRCFM;
}__packed        Glo_T ;//这样好了。

dachun

Adrian 发表于 2010-10-28 10:46
typedef        struct        
{
        int16u        ChaPa_Z;

我也遇到这样的问题，解决办法应该是这个样子的：


#pragma pack (1) /*指定按1字节对齐*/  
struct D
{      
char b;     
int a;     
short c;  
};  
#pragma pack () /*取消指定对齐，恢复缺省对齐*/

zhugean

对于32位的单片机千万不要用pack压缩，严重影响效率
一般都是浪费几个字节

zhugean

请参考http://www.amobbs.com/thread-5547367-1-1.html

xf331785508

Comparisons of an unpacked struct, a __packed struct, and a struct with individually __packed fields, and of a __packed struct and a #pragma packed struct
ARM Compiler toolchain v5.02 for µVision Using the Compiler   

Home > Compiler Coding Practices > Comparisons of an unpacked struct, a __packed struct, and a struct with individually __packed fields, and of a __packed struct and a #pragma packed struct

Comparisons of an unpacked struct, a __packed struct, and a struct with individually __packed fields, and of a __packed struct and a #pragma packed struct
These comparisons illustrate the differences between the methods of packing structures.

Comparison of an unpacked struct, a __packed struct, and a struct with individually __packed fields
The differences between not packing a struct, packing an entire struct, and packing individual fields of a struct are illustrated by the three implementations of a struct shown in Table 12.

Table 12. C code for an unpacked struct, a packed struct, and a struct with individually packed fields

1. //Unpacked struct  
   
2. struct foo
   
3. {
   
4.     char one;
   
5.     short two;
   
6.     char three;
   
7.     int four;
   
8. } c;
   
9. //__packed struct
   
10. __packed struct foo
    
11. {
    
12.     char one;
    
13.     short two;
    
14.     char three;
    
15.     int four;
    
16. } c;
    
17. 
    
18. //__packed fields
    
19. struct foo
    
20. {
    
21.     char one;
    
22.     __packed short two;
    
23.     char three;
    
24.     int four;
    
25. } c;
    

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In the first implementation, the struct is not packed. In the second implementation, the entire structure is qualified as __packed. In the third implementation, the __packed attribute is removed from the structure and the individual field that is not naturally aligned is declared as __packed.

Table 13 shows the corresponding disassembly of the machine code produced by the compiler for each of the sample implementations of Table 12, where the C code for each implementation has been compiled using the option -O2.

Table 13. Disassembly for an unpacked struct, a packed struct, and a struct with individually packed fields

1. Unpacked struct  
   
2. ; r0 contains address of c
   
3. ; char one
   
4. LDRB    r1, [r0, #0]
   
5. ; short two
   
6. LDRSH   r2, [r0, #2]
   
7. ; char three
   
8. LDRB    r3, [r0, #4]
   
9. ; int four
   
10. LDR     r12, [r0, #8]
    
11. 
    
12. __packed struct
    
13. ; r0 contains address of c
    
14. ; char one
    
15. LDRB  r1, [r0, #0]
    
16. ; short two
    
17. LDRB  r2, [r0, #1]
    
18. LDRSB r12, [r0, #2]
    
19. ORR   r2, r12, r2, LSL #8
    
20. ; char three
    
21. LDRB  r3, [r0, #3]
    
22. ; int four
    
23. ADD   r0, r0, #4
    
24. BL    __aeabi_uread4
    
25. 
    
26. __packed fields
    
27. ; r0 contains address of c
    
28. ; char one
    
29. LDRB  r1, [r0, #0]
    
30. ; short two
    
31. LDRB  r2, [r0, #1]
    
32. LDRSB r12, [r0, #2]
    
33. ORR   r2, r12, r2, LSL #8
    
34. ; char three
    
35. LDRB  r3, [r0, #3]
    
36. ; int four
    
37. LDR   r12, [r0, #4]

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zhuser

好贴，我之前也遇到了此类似问题。

FUK_In_Love

4字节对齐，编译器位数有关

mmiker

把int32u  那行和下面那行换个位试试。