过度使用 Pragma Pack(1)

Question

过度使用 #pragma pack(1) 导致填充的原因是什么？后台到底发生了什么？

#pragma pack(1)
typedef struct
{
    union
    {
        uint8_t SAM;
        #pragma pack(1)
        struct {
                uint8_t sm:1;  
                uint8_t sm1:3 ;
                uint8_t sm2 :1 ; 
                uint8_t sm3 :1 ; 
                uint8_t sm4 :1 ;
                uint8_t sm5:1 ; 
            };
          #pragma pack()
        };
}SAM1;
#pragma pack()

Answer 1

您的注释代码：

#pragma pack(1)
// from here on the compiler will align fields on 1 byte
typedef struct
{
    union
    {
        uint8_t SAM;
        #pragma pack(1)
        // there is no use for this pragma on a bit field
        // packing was already at 1 byte
        struct {
                uint8_t sm:1;  
                uint8_t sm1:3 ;
                uint8_t sm2 :1 ; 
                uint8_t sm3 :1 ; 
                uint8_t sm4 :1 ;
                uint8_t sm5:1 ; 
            };
          #pragma pack()
          // from the next struct or union declaration the compiler will use default packing
        };
}SAM1;
#pragma pack()
// from here on the compiler uses default packing

"Pack takes effect at the first struct, union, or class declaration after the pragma is seen."

过度使用 Pragma Pack(1)

Overuse of Pragma Pack(1)

c

c++

gnu