Review of C language (byte alignment)

Byte alignment

Alignment rules ：

1. The first member is offset from the structure variable by 0 The address of .

2. Other member variables should be aligned to the address of an integer multiple of the alignment number . Align numbers = Compiler default alignment number And The smaller value of the member size .

3. The total size of the structure is the maximum number of alignments （ Each member variable has an alignment number ） Integer multiple .

4. If the structure is nested , The nested structure is aligned to an integer multiple of its own alignment number , Alignment number of nested structure = The maximum number of alignments in a nested structure And The smaller value of the default alignment number of the current structure . The overall size of the total structure is the maximum number of alignments （ The number of alignments with nested structures ） Integer multiple .

   
   //4 Byte alignment 
   struct S1
   {
    char d; 
    double c; // 4+8  The length is 8  Default 4 Byte alignment , So the alignment number is 4
    double i; //4+8+8
   };
   printf("%d\n", sizeof(struct S1)); //20 
   
   struct S2
   {
    char c1;
    struct S1 s1;//4+20   Align the nested structure to its own alignment number  4  At an integral multiple of  
    int d;//4+20+4
   };
   printf("%d\n",sizeof(struct S2)); // 28  Because the maximum parity of the largest whole is 4, So there is no need to make up 
   

   //8 Byte alignment 
   struct S1
   {
    char d; 
    double c;// 8+8  The length is 8  Default 8 Byte alignment , So the alignment number is 8
    double i;//8+8+8
   };
   printf("%d\n", sizeof(struct S1)); //24
   
   struct S2
   {
    char c1;
    struct S1 s1; //8+24  Align the nested structure to its own alignment number  8  At an integral multiple of 
    int d;//8+24+4
   };
   printf("%d\n",sizeof(struct S2)); // 40  After alignment is 36, Because the maximum number of alignments in a nested structure is 8, So the maximum number of alignments in the total structure is 8, Finally, we need to 8 Byte completion , So for 40
   
   

   // Default 8 Byte alignment 
   struct S1
   {
    char c;
    double d; // 8+8  The length is 8  Default 8 Byte alignment , So the alignment number is 8
    double i;
   };
   printf("%d\n", sizeof(struct S1)); //24
   
   #pragma pack (4) // It's all in the back 4 Byte alignment , Until the end of this document 
   struct S2
   {
    char c1;
    struct S1 s1; //4+24  Although the alignment number of the previously nested structure to its own is  8 , But the current structure S2 The default alignment number is  4 , therefore S1 The alignment number of is 4
    int d;//4+24+4
   };
   printf("%d\n",sizeof(struct S2)); // 32  Because the maximum number of alignments is 4, So there is no need to make up 
   

   // Default 8 Byte alignment 
   struct S1
   {
    char c;
    char d; 
    char i;
   };
   printf("%d\n", sizeof(struct S1)); //3
   
   //#pragma pack (4)
   struct S2
   {
    char c1;
    struct S1 s1;
    int d;
   };
   printf("%d\n",sizeof(struct S2)); // 8 
   

   struct S1
   {
    char c;
    char d; 
    char i;
    char a;
    char b;
   }; 
   printf("%d\n", sizeof(struct S1)); //5
   
   //#pragma pack (4)
   struct S2
   {
    char c1;
    struct S1 s1;
    int d;
   };
   printf("%d\n", sizeof(struct S2)); //12