1. 分析结构体大小

   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;
   };

2. 分析结构体大小

   #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;
   };

3. 分析结构体大小

   #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;
   };

4. 分析结构体大小

   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()

Q.E.D.