结构体 字节对齐的练习

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        struct S1
        {
              char c1;
              double d;
              char c2;
              char c3;
        };
        struct S2
        {
              char c1;
              char c2;
              char c3;
              double d;
        };
        struct S3
        {
              char c1;
              S1 s;
              char c2;
              char c3;
        };        

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        #pragma pack(1)
        struct S1
        {
              char c1;
              double d;
              char c2;
              char c3;
        };
        #pragma pack()
        struct S3
        {
              char c1;
              S1 s;
              char c2;
              char c3;
        };

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        #pragma pack(4)
        struct S1
        {
              char c1;
              double d;
              char c2;
              char c3;
        };
        #pragma pack()
        struct S3
        {
              char c1;
              S1 s;
              char c2;
              char c3;
        };

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        struct S1
        {
              char c1;
              double d;
              char c2;
              char c3;
        };
        #pragma pack(1)
        struct S3
        {
              char c1;
              S1 s;
              char c2;
              char c3;
        };
        #pragma pack()