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1、 Yes const and volatile The understanding of the

const：

• const Indicates that the decorated variable cannot be modified , If modified , The compiler will report an error .
• If you have a const Modify the modified variable after taking the address or reference , Then there will be undefined behavior , The compiler is not guaranteed to get the desired results .

volatile：

• volatile Be sure to pay attention to , It has nothing to do with multithreading ;
• Its function is to Memory visibility , Prevent the compiler from volatile Modify the variables to do some related optimization ;

Do not add variables volatile, The compiler will optimize variables ：

And added volatile modification , The generated assembly is like this ：

C++ Of volatile Generally, it is only used to communicate with hardware , We can hardly program with .

For details, see ：https://en.cppreference.com/w/cpp/language/cv

2、Linux The process of generating executable programs

Four processes ！

1、 Preprocessing ; Mainly did the following work ：

• Expand all #define Macro definition , Replace text ;
• Delete all comments in the program ;
• Handle all conditional compilation ,#if、#ifdef、#elif etc. ;
• Deal with all #include Instructions , Copy the contents of these header files to the referenced source file ;
• Add line number and filename ID , It is convenient for the compiler to generate warnings and debugging information ;
• Keep all #pragma Compiler Directives , Because the compiler will use them ;

2、 compile ; Perform a series of operations on the preprocessed file to generate the corresponding assembly file

• Lexical analysis ： Also known as lexical scanning , Through the scanner , Using the algorithm of finite state machine, the string in the source code is divided into a series of marks , Such as addition, subtraction, multiplication, division, number brackets, etc .
• Syntax analysis ： Use the parser to analyze the marks generated by lexical analysis by means of context free grammar , Generate parsing tree . During this period, if the expression is illegal （ Brackets don't match, etc ）, You're going to report a mistake .
• Semantic analysis ： Syntax analysis checks whether the expression is legal , Check whether the expression makes sense , For example, a floating-point integer is assigned to a pointer , The compiler will report an error .
• Interlanguage generation ： Do some syntax tree optimization , Such as 6+2=8.
• Object code generation and optimization ： Generate object assembly code from intermediate code .

3、 assembly ;

• Use The assembler converts the assembly code into instructions that the machine can execute , In fact, it is to translate the assembly instructions and machine instructions one by one according to the comparison table .

4、 link ;

• The basic units processed above are source files , After the above processing, multiple source files become multiple target files ;
• link Namely The process of packaging and assembling multiple object files into a whole , This The whole is an executable program .

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3、Linux How does the program work

This involves many knowledge points ：

• What is a process , How the process is established ？
• What is virtual memory ？ Why have virtual memory ？
• How the program works ？

1、 adopt fork System call creates a new process ;
2、 adopt execve The system call executes the specified ELF file , With environment variables and parameters ;
3、 Check ELF Validity of executable , Like magic number （ The file format can be determined by magic number ）、Segment The quantity of ;
4、 Looking for dynamically linked segments , Set dynamic linker path ;
5、 according to ELF Program header table description of executable file , Yes ELF File mapping , For example code 、 data 、 Read only data ;
6、 initialization ELF Process environment ;
7、 Change the return address of the system call to ELF The entry address of the executable

4、 The process of missing pages in memory

It involves many pre knowledge points ：

• Virtual memory management
• Mapping of virtual memory to physical memory
• The concept of page table and page

Memory page missing The page to be accessed is not in main memory , The operating system needs to load the page into main memory and then access it , The execution of the command will be temporarily stopped , Generate an exception that the page does not exist .

The process of page break processing is as follows ：
1、 If there are free physical pages in memory , Assign a physical page frame r, And then turn to 4 Step , Or turn to 2 Step ;
2、 Choose a page replacement algorithm , Select a physical page frame to be replaced r, Its corresponding logical page is q, If the page has been modified during memory , Write it back to external storage ;
3、 take q The corresponding page table entry is modified , Put the dwell position 0;
4、 Will need to visit the page p Load to physical page r in ;
5、 modify p The content of the corresponding page table entry , Put the dwell position 1, Set the physical page frame number to x;
6、 Rerun the interrupted instruction .

5、 The concepts of left and right values

What can take the address is the lvalue , What can't get the address is the right value .

6、 Mobile semantics

The official definition of ：

• Mobile semantics mainly provides the ability to move expensive objects from one address to another in memory , At the same time, steal the source resources in order to build the target with the least cost .

Main uses ：

• Be able to use mobile constructor and mobile assignment operator , When we use temporary objects , Reduce unnecessary copy operations .

7、 Advantages and disadvantages of template use ？

advantage ：

• More flexible , reusable ;
• Automatically generate code at compile time , Greatly reduce development time ;

shortcoming ：

• It's difficult to debug ;
• Generally, the implementation of functions in the template will be placed in the header file , The implementation details will be exposed ;
• Extensive use of templates , It will take a long time to compile .

8、 Program output

for (char i = 0; i < 256; ++i)
	{
    
		printf("%d\n", i);
	}

Output is ：
0 1 2 … 127 -128 -127 … -1 0 1… Infinite loop ！

127 The binary of is 0111 1111（ Complement code ）
-128 The binary of is 1000 0000（ Complement code ）

reason ：

As can be seen from the results above ,ch stay 127 after , The values are determined by 127 Turned into -128, Continue to perform when ch by -1 Back to the original 0 value . Just like the closed loop drawn above , 0->127,-128->0->127…

Let's analyze it from the perspective of binary ：

In the computer , Signed numbers are represented by complements .

about char Type of 1 byte 8 Digit number , The first bit is the sign bit , The rest of the bits are numeric .

2^7-1 = 127

• 127 The binary of is 0111 1111（ Complement code ）
• -128 The binary of is 1000 0000（ Complement code ）

because 127 + 1 Backward position , Make the high bit overflow to the sign bit .

The sign bit marks the positive and negative of the array , Then the value changes from a positive number to a negative number .

and 1000 0000（ Complement code ） yes -128 Complement , Then he means signed number -128 .

The relationship between the source code and complement of negative numbers is ： Original code -> Take the opposite +1 => Complement code .

For positive numbers , Do not distinguish the original code / Inverse code / Complement code , Are expressed in their own original code , Only negative numbers are distinguished .

For signed numbers , We take one as the sign bit in the first place , Indicates the positive and negative of the value .

And for 0, We have two representations ： namely +0 ⇒ 0000 0000 、 -0 ⇒ 1000 0000 , This is clearly unreasonable .

+0 And -0 Should represent the same number , And we can find ：

• 1000 0000( primary ) As the original code , Its complement is still 1000 0000( repair )（ High overflow , discarded ）
• -1271111 1111( primary ) Minus one ( Plus minus one ） The binary value of is also 1000 0000 . and -127 - 1 The value of is -128 .( Negative numbers are complemented ,1111 1111 [phone] =》1000 0000)

Then we can use Complement code 1000 0000 Express -128 . And because we cancel “ negative 0” The concept , A binary bit combination is left to represent other values （ Used to represent -128）.

It can be seen from the previous analysis that , Complement code 1000 0000 The original code value of can represent negative 0, However, we don't need “ negative 0”, So for Complement code 1000 0000 We do not study its original code , It only has complement meaning , Express -128 . therefore , stay 127 After adding one, it becomes -128 .