Overuse of Pragma Pack(1)

Question

What causes padding due to overuse of #pragma pack(1)? What actually happens in backstage?

#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

Your annotated code:

#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."