结构体 字节对齐的练习

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;
   };        
   

   image

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;
   };
   

   image

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;
   };
   

   image

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()
   
   

![image](http://upload-images.jianshu.io/upload_images/5396392-b1feba86ffd81779.jpg?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)