Pack alignment exception issue

[Lyu.1]

Hi,Master:

platform: STM32G070CB

keil ide: 

 

arm Compiler:version 5 (c99 mode)

Question:

#pragma pack(8)
typedef struct{
    uint32_t a;
    uint8_t  b;
    uint32_t c;
    uint32_t d;
    uint32_t  e;
} x_t;
#pragma pack()

x_t a[10] = {0};
int sz = sizeof(a[0]);

sz is 20, Why not 24？

Thank you very much!

[STOne-32]

Dear @Lyu.1 ,

 Can you please show on the debugger how are packed on memory sz ?  

Here is how it works for ARM/Keil for adding padding or not for each variable inside the structure? By default it should be 8 packed . 
May be also to know the optimization level used as well , so we can discuss with Keil colleagues .

https://developer.arm.com/documentation/dui0491/i/Compiler-specific-Features/-pragma-pack-n-

Ciao,

STOne-32

[Pavel A. · Accepted solution]

The fine manual has the answer.

https://developer.arm.com/documentation/dui0375/g/Compiler-specific-Features/-pragma-pack-n-

"This pragma aligns members of a structure to the minimum of n and their natural alignment."

[Lyu.1]

Thank you for your answer, I understand.

[Lyu.1]

Thank you for your answer, I understand.