C language interview preparation I (about understanding Department)

1. How to understand c Language is a structured language ( understand c Language )

c Language is a compiled language , Its grammar is concise , Rich functions , Flexible use . Altogether 32 Keywords ,9 Control statements

Common keywords ：

Common control statements ：

1. Excellent portability ：

2. Direct access to hardware ：

1） By writing data to memory space , The hardware maps registers to memory addresses , Access the hardware directly by operating the address in memory .

2） I usually write code in protected mode , The accessible memory address is limited . Unable to access hardware mapping address .

3. With high-performance operation efficiency :

Precise control of each byte , The compiler does not have too many additional elements , Compile directly .

  Talk about c Basic unit functions of language

The concept of function ：

yes c The basic unit of a program , Returned by , Function name , Function parameter , The body of the function , There are four main parts .

1. The definition of function is relatively independent （ Can be defined anywhere outside the function ）, The program always starts with the main function .

2. Modular programming that can be invoked and executed from other parts of the program （c Language is structure oriented ）

characteristic ：
1. A function can only have one return value

2. A program can have one or more functions nested , But you can't nest definitions

3. Modularize the program , Make the program more concise , Increase readability .

  Function call

step ：

1. Find the entry address of the function through the function name , The address of the next instruction calling the function statement in the stack pressing main function .

2. Allocate space for formal parameters , Pass the value to the space corresponding to the formal parameter , Stack the formal parameters from right to left , Then there are local variables in the function

3. Execute the function body statement

4. Function return value , And free the function space .

expenses ：

1. Parameter stack

2. The program counter points to the called function （ Control moves to function ）

3. Create a new stack frame （ Open up new space on the stack ）, use ebp Value identifies the new stack frame , And save the original stack frame address .

4. Restore the original stack frame , Control moves to the caller  

Corresponding c++ Inline functions in and preprocessing #define

  When the inline function is compiled, it is expanded when the function is called , Do not create new stack frames , Save the time cost of entering and leaving the stack .

#define： Perform plain text replacement during preprocessing , There is no error detection function. Pay attention to the boundary conditions .

key word  

static

Definition ： It's a key word .

stay c Only variables can be modified in language , Its decorated variables are assigned to the static area when the program is run and defined .

stay c++ Member functions can be modified in , The class members it modifies must be initialized and outside the class . The decorated class member function does not this The pointer , Class members cannot be called , Only static data can be called .

1. Modify local variables ： Not initialized. The default is 0： Put it in .bss paragraph , Initialized to put .data paragraph
General local variables are in the stack area , Release at the end of the function call ,static Decorated local variables are allocated in the static area , Release... At the end of the program , The delay has a long life cycle , But the scope is still limited to statement blocks .

2. Modify global variable ： Not initialized. The default is 0： Put it in .bss paragraph , Initialized to put .data paragraph

The scope is limited to be visible in this source file , Not visible in other documents of the project .

extern： It extends the scope of global variables and functions, which can be seen in other files of the project .

const

  Definition ： It's a key word

c In language

const Modify the value of local variables as read-only and cannot be modified , But the memory extended by its value can be modified .

General global variables are in the static area ,const When modifying global variables , Store the global variables in the constant area .

c++ in

const Decorated variables are stored in a symbol table in the stack area , One by one correspondence with data through key value pairs .

auto Local variables allocated in stack space .

register 

stay c In language , Modifying variables , Refers to a variable that directly references a register , Operation on variable name , It is equivalent to accessing registers , Declared register variables are not necessarily stored in registers .

1. The declaration type should be the type that the register can receive , The variable length should be less than the register length

2. Cannot address a register variable .

stay c++ in , You can take the address , but regitser Keywords become invalid , The defined variables will be put into memory .

 volatile

Decorated variable , The compiler no longer optimizes , Every time I have to access variables in memory

Usually used for ：

1. When modifying other program variables in the interrupt service program, add volatile

2. Multi task shared flag bit plus volatile.

3. Memory mapped hardware registers plus volatile.

Principle and essence ：

When a thread is reading data in memory , Will first put the data and its address 64KB The data of is cached in the register .

When modifying the value of a variable in this thread, first modify the value in the register and then copy it to memory .

When other threads modify the value in memory , It will cause inconsistency between memory and register data . 

typedef And #define difference  

  The principle is different ：

typedef Is the key word , Process at compile time and only once , Can carry out type error detection .

#define It's preprocessing instructions , In pretreatment , Replace each call with plain text , No error detection , Error in compiling the developed open source program .

Different functions ：

typedef： Alias an existing type , Only type

#define： Not just types . Constant , Variable , Compile switch, etc ...

Different scopes

typedef： Only in this source file

#define： There are no scope restrictions , Available after definition .

The operation of the pointer is different （ Alias based on a type , One is plain text substitution ）

typedef：    const   typedef   p;     const Modification is typedef The type of embellishment

The difference between a structure and a consortium

struct It's a keyword , You cannot define yourself within yourself .

stay c Language is a collection of data types , Improper processing of new data types ：

1. When defining, add struct.

2. You cannot define a function in a structure .

stay c++ Is a new data type .

1. You can assign values directly with structures

2. You can define functions in the structure , The default is public space （class Default private ）

Examination site （ Byte alignment ）：

1.cpu Fixed byte addressing , Avoid meaningless multiple addressing variables , Byte alignment required .

2. Align according to the maximum of the self alignment values of all data types .（int a[10] The alignment value type is int, The space occupied is 40）

3. Align the specified alignment value with the minimum of the maximum alignment value of the data type .（#pragma（value））.

union Is a key value, all data share a piece of memory , Its size is The maximum length of each member .

Examination site （ Member coverage ）：

Cannot load multiple members at the same time , There is only one member variable at a time , The new member variable assignment will overwrite the old member . 

Operator priority and combination rate  

1. The equality operator is greater than the bitwise operator > Logical operators > Assignment operator is only greater than comma operator

2.（. Member selection ）>(++ Self increasing )>(* Take the content )

++,（=） The combination rate of assignment is from right to left