In depth anatomy of C language - key words & supplementary contents

Hello, I'm Mu Xixi

---

Keyword classification

In ordinary books ,C Language keywords are 32 individual , But this is all C90(C89) Standards for . Actually C99 And then I added 5 Key words . however , Current mainstream compilers , Yes C99 The support is not good , according to C90 standard , The idea that 32 individual .

keyword	explain
auto	Declare automatic variables
short	Declare short integer variables or functions
int	Declare an integer variable or function
long	Declare long integer variables or functions
float	Declare floating-point variables or functions
double	Declare a double precision variable or function
char	Declare character type variables or functions
struct	Declare a structural variable or function
union	Declare shared data type
enum	Declare enumeration type
typedef	Used to alias data types
const	Declare read-only variable
unsigned	Declare an unsigned type variable or function
signed	Declare a symbolic type variable or function
extern	Declare variables are being declared in other files
register	Declare register variables
static	Declare static variables
volatile	Explain that variables can be implicitly changed during program execution
void	Declares that the function returns no value or takes no arguments , Declare no type pointer
if	Conditional statements
else	Conditional statement negates Branch （ And if Continuous use ）
switch	For switch statements
case	Switch statement Branch
for	A circular statement
do	The body of a loop statement
while	The loop condition of a loop statement
goto	Jump statements without conditions
continue	End the current cycle , Start next cycle
break	Jump out of current loop
default	In the switch statement “ other ” Branch
sizeof	Calculate the data type length
return	Subroutine return statement （ With parameters , Or without parameters ） The loop condition

Supplementary content

first C Program

#include<stdio.h>
#include<windows.h>//windows.h System header file , Just to stop the screen 
int main()
{
    
	printf("hello world!\n");
	system("pause");//pause Stop screen 
	return 0;
}

The file code is converted into an executable program after generating the solution （ Binary .exe）

You can empty the executable by emptying the solution

• stay windows in , Double click the essence of the program to run it , Load the program into memory .
• Any program must be loaded into memory before it can be run .
• Load into memory , Fast .

Von Neumann

Definitions and statements

Variable

stay Memory Open up a specific size of space in , To hold data

Definition of variables

 type    Variable name  =  The default value is 
int a = 10;
char c = 'c';

Define the reason for the variable ： Because there is data that needs to be saved temporarily , Waiting for further processing .
The nature of variables

• The essence of all variables is to open up space somewhere in memory .
• The program runs , It needs to be loaded into memory
• Program calculation , You need to use variables

Define the nature of variables ： Open up a space in memory , To hold data .( Why must it be memory ： Because defining variables , It's also part of program logic , The program has been loaded into memory )

Definition ： Open up space , There can only be one time .
Statement ： inform , Many times .

int a = 10;//a The definition of 
a = 20;// assignment 
a = 100;// assignment 

Classification of variables

Variables are divided into ： Local and global variables

local variable ： The variables contained in the code block are called local variables . Local variables are temporary . Enter code block , Automatically form local variables , The exit code block is automatically released .[ Many on the Internet say that the variables in the function are local variables , You can't say wrong , But the statement is inaccurate ]（ Defined in a code block ）

Global variables ： Variables defined outside all functions , It's called a global variable . Global variables are global .（ Defined outside the code block ）
Code block ： use {} The enclosed area , It's called a code block

#include<stdio.h>
int g_val = 100;// Global variables 
int main()
{
    
	int a = 10;// local variable ,main Functions are also functions , There are also code blocks {}
	if(a == 10)
	{
    
		int b = 10;// local variable 
    }
	printf("a=%d\n", a);
	return 0;
}

Scope of variable

Scope ： It refers to the code area of the variable that can be accessed normally .
local variable ： Valid only within this code block .
Global variables ： During the whole program running , Are effective .

Where global variables ：

1. Can be accessed in any code block
2. Can be accessed in any code block , Even modified .
3. When a global variable has the same name as a local variable , Local variables take precedence .

#include<stdio.h>
int g_vax = 10;// Global variables 
void test()
{
    
	int g_val = 100;// Of a local variable g_val Can only be accessed in this code 
	printf("%d\n", g_val);// In global variables g_val Any code block can be accessed , Even modified 
	// The output is local , That is, when part and all have the same name , Preferential local .
}
int main()
{
    
	test();
	printf("%d\n", g_vax);// Can be accessed in any code block 
	return 0;
}

Life cycle of variable

Life cycle concept ： It refers to the time range from the definition of the variable to its release , The so-called release , It refers to the space once opened up ” Be released “.
local variable ： Enter code block , Form local variables [ Open up space ], Exit code block ," Release " local variable .
Global variables : After definition , Throughout the life cycle of the program , This variable is always valid .

Scope vs Life cycle

Scope ： Measure the extent of the variable's impact , The valid scope of this variable .
Life cycle ： Describes the length of a variable's lifetime . The concept of time ： When was it opened up , When is it released .

The most generous keyword - auto

How to use ： Variables typically defined in code blocks , Local variable , The default is auto Embellished , But generally omit .
By default, all variables are auto Do you ？ No , Generally used to modify local variables
Local variables include automatic variables , Temporary and local variables .

#include <stdio.h>
int main()
{
    
	for (int i = 0; i < 10; i++) // local variable i Can also be auto modification 
	{
    
		printf("i=%d\n", i);
		if (1)
		{
    
			auto int j = 0;// Automatic variable 
			printf("before: j=%d\n", j);
			j += 1;
			printf("after : j=%d\n", j);
		}
	}
	return 0;
}

auto Already very old , Basically not used .

The fastest keyword - register

CPU It is mainly the hardware unit responsible for calculation , But for ease of calculation , Generally, the first step is to read the data from memory to CPU Inside , Then you need CPU It has certain temporary data storage capacity . Be careful ：CPU It's not the time to calculate , To read specific data to CPU Inside , That's too slow .
So modern CPU Inside , Are integrated with a set of hardware called registers , Used to save temporary data .

Storage pyramid

distance CPU The closer the storage hardware , The faster the speed. .

Understanding of register

CPU A set of storage hardware is integrated into the , This set of hardware is called registers .

The essence of register existence

On the hardware level , Improve the computing efficiency of the computer . Because you don't need to read data from memory .

register Modifying variables

As far as possible The modified variable , Put in CPU In the deposit area , So as to achieve the purpose of improving efficiency
So what kind of variables , May adopt register Well ？
1. Local ( The overall situation will lead to CPU The register is occupied for a long time )
2. Will not be written ( Writing requires writing back to memory , If you need to read the test later , It doesn't make sense ）
3. High frequency read ( Improving efficiency lies in )
4. If you want to use , Please don't use a lot of , Because the number of registers is limited
5.register Decorated variable , Address not available ( Because it's already in the deposit area , Address is a memory related concept )

register –cache– Memory –SSD/flash/ Hard disk – Compact disc – disk
Hard disk is converted into memory through cache technology , Memory is converted into registers through cache technology .
distance CPU The nearer the storage unit , The more efficient , The higher the unit price cost .
distance CPU The farther away the storage unit , The less efficient , The cheaper the unit price .
For any kind of hardware , Act as a cache for upstream hardware .
example ： Memory can be regarded as a large cache of hard disk ;cache And registers are equivalent to some kind of cache in memory .

CPU When accessing data , Achieve the highest efficiency at the lowest cost .

#include<stdio.h>
int main()
{
    
	register int a = 10;
	printf("%p\n", &a);
	return 0;
}

At the end

Friends who think it's good can pay attention to , Like or collect ！ Thank you for your support .

Yours ️ Praise is the source of my creative power
Your collection is the direction of my struggle
Your attention is my greatest support
Yours ️ Comments are a bright light for me to move forward
It's not easy to create , I hope you can support Xiaomu