Learning notes on February 18, 2022 (C language)

22/2/5 Learning notes （Part 3）

Preliminary common keywords

• auto - Automatically - Each local variable consists of auto modification （ Basically, they have been omitted , There are other uses in the new grammar .）

• break

• case

• char

• const

• continue

• default - Default -

• do

• double

• else

• enum - enumeration -

• extern - Declare external symbols -

• float

• for

• goto

• if

• int

• long

• register - Register key - It is recommended to store in the register （ A lot of frequently used data , But today compilers automatically recognize ）

• return

• short

• signed - The signed , Example positive and negative -

• sizeof

• static - Static -
  1、static Modify local variables （ Changing the life cycle of local variables , Essentially changes the storage type of variables .）
  
  2、static Modify global variable （ Global variables can be used throughout the project , Because global variables have external link properties .）
  static After modifying global variables , Make the whole global variable Only in their own source file （.c） It can be used internally , Other source files cannot be used ！ because static After modification , Global variables become internal link properties , Other source files cannot link this static global variable .
  3、static Modify function （ Be similar to static Modify global variable .）
  static After decorating the function , The function You can only use it inside your own source file , Essentially ,static Changed link properties .

• struct

• switch

• typedef - Type as defined -

 typedef unsigned int u_int;// Alias 
 int main()
 {
    
      unsigned int num = 100;
      u_int num2 = 100;// At this point, the two code definitions are the same 
      
      return 0;
 }

• union - Consortium -
• unsigned
• void - nothing 、 empty -
• volatile
• while

define Define constants and macros

define It's a preprocessing instruction .

• define Defining macro ：

 #define ADD(X,Y) X+Y
 int main()
 {
    
       printf("%d\n",ADD(2, 3));// The result is 5
       return 0;
  }

 #define ADD(X,Y) X+Y
 int main()
 {
    
       printf("%d\n", 4*ADD(2, 3));// The result is 11,4*2+3 = 8+3 =11
       return 0;
  }

 #define ADD(X,Y) ((X)+(Y))// Defined as a macro , Instead of a simple number 
 int main()
 {
    
       printf("%d\n",4*ADD(2, 3));// The result is 20
       return 0;
  }

The pointer

• Memory
  Memory is a very important memory on a computer , All programs in the computer run in memory .
  Memory is divided into small memory units , The size of each memory unit is 1 Bytes .
  For effective access to each unit of memory , Memory units have numbers , These numbers are called the address of the memory unit .

• How memory is numbered ？
  32 position —32 Root address line — Physical line — Electrify —1/0 2^32 Root address line （ Memory units ）
  64 position —64 Root address line — Physical line — Electrify —1/0 2^64 Root address line （ Memory units ）

• How big is a memory unit ？
  One byte per memory unit 8 individual bit.

• Query address code ：
  
  

• Pointer size
  

Structure

• Structure can make C Language Create a new type come out .
• Structural variable . Member variables
  

little tips

• About Storage type
  
• "%p\n" Specifically for printing addresses .
• pa It's used to store the address , stay c In language, it is called pointer variable .