Interpretation of C basic syntax: summarize some commonly used but easily confused functions (i++ and ++i) in the program (bit field)

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1> i++ And ++i

i++ and ++i yes C Language operators ,i++ It means increasing suffix ,++i Prefix increment means .

• i++ It means increasing suffix ,i++ Is to do the expression operation first , And then we're going to do the self-increment . hold i++ Split the operation process of , Equivalent to i=i+1 You can see that the calculation results are consistent .

  x = i ++;    // First, let x become i Value 1, let i Add 1
  

• ++i Prefix increment means ,++i Is to perform self increasing or self decreasing operation first , And then we do the expression operation . The result of the operation shows that , Only i From the value of ,++i and i++ The final i The value is the same , All are i Since then 1.

  x = ++i;     // First, let i Add 1,  let x become i Value 1
  

2> Understanding of bit field (int a:4);

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The following procedure is in linux The system has been compiled

  1 /*--------------------------------------------------------------------- 2 * Function: bit area 3 * Author: CDamogu 4 * Date: 2022/7/12 5 * Description: How to unstandard bit area,for example: int a:4; 6 * Opeartion System: Linux 7 ----------------------------------------------------------------------*/
  8 #include <stdio.h>
  9 
 10 
 11 struct bs
 12 {
    
 13         unsigned        int a:4;
 14         unsigned        int b:4;
 15         unsigned        int c:2;
 16         unsigned        int d:6;
 17 }structa;
 18 
 19 
 20 void main() 
 21 {
           
 22         structa.a=15;
 23         structa.b=8;      
 24         structa.c=2; 
 25         structa.d=23;
 26           
 27         printf("a is %d\n",structa.a);
 28         printf("b is %d\n",structa.b);
 29         printf("c is %d\n",structa.c);
 30         printf("d is %d\n",structa.d);
 31         printf("structa is %X(H)\n",structa);
 32         printf("5E8F To Bin is: 0101 1110 1000 1111\n");
 33         printf("5E8F To Bin is: 010111 10 1000 1111\n");
 34         printf("5E8F To Bin is: 23 2 8 15 \n");
 35 
 36 } 

The results are as follows ：

[[email protected] tes1]# ./a.out 
a is 15
b is 8
c is 2
d is 23
structa is 5E8F(H)
5E8F To Bin is: 0101 1110 1000 1111
5E8F To Bin is: 010111 10 1000 1111
5E8F To Bin is:  23    2   8    15 
[[email protected] tes1]# 

1. 5E8F This value is after I execute , Then the following program adds three lines of parsing , In order to facilitate the understanding of the defined order and the high and low positions stored in the space after execution

2. Prototype of bit field function

   struct name
   {
         
       type var_name : n;
   };
   

   • In structure name Summary , Member variables var_name The occupied space is n position .
   • n As a positive integer , Its value must be less than type Number of digits occupied by type .
     • Such as type If it is int, Occupy 4 byte 32 position , that n Must be 1~31 Integer between .
   • For members of bit field type , If the actual value exceeds n The range that bits can express , Then the excess will be cut off , Save only the low value .
     • Such as int var:4, Itself only 4 Bit space , If the assignment is var = 20, because 20 The binary value of is 10100, Actually five , At this time var What is actually assigned is the lower four digits ,0100, namely 4.
   • because C The address in the language is calculated for bytes , Therefore, member variables of bit field type do not support address fetching
     • That is, for variables a, If there is a bit field member variable var, that &a.var It's illegal. , Compilation error .

3. A bit field must be stored in the same byte , Can't span two bytes .

   • If there is not enough space left for one byte to store another bit field , The bit field should be stored from the next unit . You can also intentionally start a bit field from the next unit .

   struct bs 
   {
          
   unsigned a:4 
   unsigned :0 /* airspace */ 
   unsigned b:4 /* Store from next unit */ 
   unsigned c:4 
   }
   
   // In this bit field definition ,a The first byte 4 position , after 4 Place filling 0 No use ,b Start with the second byte , Occupy 4 position ,c Occupy 4 position .
   

4. A bit domain can be a bit free domain name , At this time, it is only used for filling or adjusting the position . Nameless bit fields cannot be used . for example ：

   struct k 
   {
          
   int a:1 
   int :2 /* The 2 Bit cannot be used */ 
   int b:3 
   int c:2 
   };
   
   // As can be seen from the above analysis , Bit fields are essentially structural types ,  But its members are allocated in binary .