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Linux C From entry to mastery （ The grave ）

1. C Language foundation （ Loop statement ）

2. for loop

3. Format ：
   For（ expression 1; Expression two ; expression 3）
   {
   The loop body ;
   }
   expression 1： Initial value of Fu ;
   expression 2： Termination conditions ;
   expression 3： Appreciation or impairment .
   Be careful ：
   a) No initial value expression , It can be written in for Upper edge of loop ;
   b) There is no expression 2, A cycle is an endless cycle , There is no expression 3 It's also a dead cycle , But expression 3 It can be written at the end of the loop .
   c) for（;;）： It means a dead cycle .

4. Execution order
   Execute expression first 1, Then judge the expression 2 Is it true , Set up the execution loop , Then execute the expression 3, Then judge the expression 2 Is it true , Then continue to execute the circulatory body , Then execute the expression 3…… Until the expression 2 Don't set up , The loop ends .

5. for Nesting format
   for( expression 1; expression 2; expression 3)
   {
   for( expression 1; expression 2; expression 3)
   {
   The loop body ;
   }
   }

6. while loop

7. Format 1
   While（ expression ）
   {
   The loop body ;
   }
   Execution order ： Determine if the expression holds , Set up the execution loop , Then judge whether the expression is true , Set up the execution loop ,…… Until the expression doesn't hold , The loop ends .
   Be careful ：while（1）= Dead cycle .

8. Format two ：do……while
   do{
   The loop body ;
   }while（ expression ）;
   Execution order ： First, execute the loop body , Then judge whether it is true , Set up the continuous execution loop , Not set up to end the cycle .

9. goto loop

10. Format
    loop：
    Code segment ;
    if（ expression ）
    goto loop;

11. Execution order ：
    Execute code snippets , Determine if the expression holds , Set up jump to loop Continue to execute the code snippet .

12. Loop control statement ：break;continue;return

13. break： Out of the loop （ End of cycle ）;

14. continue： End this cycle , So let's go to the next loop .

15. return： End function .Main Function , Not normally executed main Function content , It is customary to write ：return -1;

16. Array

17. Concept ：
    A collection of data consisting of one or more data types , Each data that makes up an array is called an element .

18. characteristic
    Same data type , Memory space is continuous .

19. One dimensional array

20. Concept ： An array with only one subscript .

21. Define format ：
    [ Storage type ] data type Array name [ Subscript ];
    Subscript ： Number of elements .

22. Element access ： Array name [ Subscript ].
    Value of subscript ： from 0 Start , Element number = Subscript number -1;

23. initialization
    a) Not initialized when defining
    The initial value is a random value . Next, you need to assign initial values one by one , Elements without initial values are still random values .
    b) Partial initialization during definition
    Uninitialized element value is 0.
    c) Definition time , All initialization
    Subscripts can be omitted .

24. sizeof（ Array name ）： Calculate the space occupied by the array .

25. Be careful ：

26. If the subscript is omitted during development , How many elements correspond to how many openings are initialized . If the subscript is not omitted , How many subscript values open up how many element spaces , You can only initialize the number of elements as the subscript value .

27. Once the array is defined, the memory space opened up has been determined , Subscript cannot be random value or unknown quantity when defining .

28. Cross border access to data , Don't complain , Pay attention to the problem of crossing the boundary when using .

29. Sort ： Bubbling 、 choice

30. Bubble sort ： Subscript to be 0 And the subscript is 1, Move the small number back , Then subscript 1 And the subscript is 2 Comparison , Small backward , Until you put the smallest one last . Again , Put the second smallest number in the penultimate position . Sort by analogy .

31. Two dimensional array

32. Concept ： An array with two subscripts .

33. Format ：
    [ Storage type ] data type Array name [ Line subscript ][ Column subscript ]
    Access element content ： Array name [ That's ok ][ Column ]; The subscript value is from 0 Start .

34. Traversing a two-dimensional array ： Nested loop

35. Initialization of local two-dimensional array

36. Not initialized when defining , The initial value is a random value , Next, assign values one by one .

37. Partial initialization during definition , Is initialized to 0.

38. Initialize all when defining , Line subscripts can be omitted .

39. The address of the array
    In the computer, the unit is bytes , Each byte has its own address number . If the space is defined, save the address number ,32 The address number for the operating system is 32 position , It needs to be opened up 4byte,64 Bit the address number of the operating system 64 position , It needs to be opened up 8byte.
    Array memory space is continuous , Get the first element address , You can add 1 Get the address of the second element in the form of .

40. One dimensional array
    The array name of one-dimensional array is one-dimensional array d Forehead address , Is the address of the first element , It's an address constant , Once an array is defined , The first address is determined , Can't change .
    a) Address +1： What you get is the address of the next element .
    b) Add... Before the address *, Indicates the content of the address .
    c) Direct access to element content ：st[i] *(st+i);
    d) Directly access the element address ：&st[i] st+i.

41. Two dimensional array
    The array name of the two-dimensional array represents the address of the first row of the two-dimensional array , It's a line address , Is the address constant . Array name plus 1, Indicates the address of the next line .
    a) The second use of ： Add , Reduce row address to column address .
    b) Two dimensional arrays can be regarded as multiple one-dimensional arrays , The array name of each one-dimensional array is ：st[0]、st[1]、st[2]……st[n-1].
    c) Array name [ Line subscript ]: Represents the address of the first element of each line .
    d) Direct access to element content ：st[i][j]; ((st+i)+j); *(st[i]+j)
    e) Directly access the element address ：&st[i][j]; *(st+i)+j; st[i]+j.

42. String operation function

43. gets Get the string from the terminal
    a） function ： Get the string from the terminal , And will automatically add ‘\0’;
    b） Parameters ： Get the first address of string storage .
    c） Return value ： Successful return to get the address where the string is stored
    Failure to return NULL.
    Be careful ： How much does the terminal input ,gets Take how much , There is no boundary crossing problem of half detection array , To use less .

44. puts Output string to terminal
    a） function ： Output the string to the terminal display .
    b） Parameters ： The first address of the output string .
    c） Return value ： success ： Number of output characters ; Failure ：EOF（-1）.

45. Zero clearing function ：memset; bzero

46. memset（s,c,n）：
    a） function ： Zero clearing .
    b） Parameters ：s： Clear the first address of the content ;c：0;n： Empty bytes .
    c） Return value ： success ： Clear the first address of the content s; Failure ：NULL.

47. bzero（s,n）
    a) function ： Zero clearing .
    b) Parameters ：s: Clear the first address of the content ;n: Empty bytes .
    c) Return value ： nothing .

48. The pointer

49. First level pointer
    a) The pointer ： It's the address .
    b) Define format ：[ Storage type ] data type * Pointer variable name
    Be careful ： What kind of variable , You need to define the pointer of the corresponding type to save the address of the variable , Can access variables indirectly through pointers .

50. Three uses of
    a) Add before address ： Fetch address content .
    b) Add... Before the line address *： Demote to column address （ Element address ）.
    c) Data type followed by *： Represents the definition of a pointer variable of the corresponding type .

51. Pointers are used in combination with arrays

52. Pointers and one-dimensional arrays ：
    Save the first address of one-dimensional array through pointer , Let pointer variables instead of one-dimensional array names indirectly access one-dimensional arrays .
    The array name is the address constant . Pointer variables are variables , Changes can occur during program operation

53. Indirect access to one-dimensional arrays
    Address ：p+i; &p[i];
    Content ：*(p+i); p[i].

54. The operation of the pointer

55. Arithmetic operation
    a) + ：p+n The pointer moves in the direction of the larger address n Data
    b) –：p-n The pointer moves in the direction of small address n Data
    c) ++ ：p++ The pointer moves a number in the direction of the larger address
    d) – ：p— The pointer moves in the direction of small address 1 Data
    e) - ：p-q The number of data elements separated between two pointers

56. Relationship between operation

< >= <= != ==
The large address is larger than the small address .

3. The assignment operation ：
   = += -=
   It is meaningless to operate between pointers of different data types .
4. The secondary pointer
5. Concept ： Save the address of the meter and pointer .
6. Format ：[ Storage type ] data type ** Pointer variable name ;
   Be careful ： As long as it's a pointer , What you save is the address number ,32 Bit operating system pointer is opened 4byte.
7. Const Constant quantity
8. const Modifying variables
   const Modifying variables , The value of a variable cannot be modified by its name , But you can define a pointer to a variable , Access the address content through the pointer to modify the value .
9. const Modify a pointer
   a) const int *p
   The pointer can be modified , What the pointer points to cannot be modified by the pointer .
   b) int * const p
   The pointer to cannot be modified , The content pointed to by the pointer can be modified by the pointer .
   Be careful ： Because the pointer cannot be modified , Only one point can be assigned at the time of definition .
   c) const int * const p
   The pointer to cannot be modified , The content pointed to by the pointer cannot be modified by the pointer .
10. void Empty type
    No, void Variable of type , It can be used void Modify a pointer , Decorate a function or as a function parameter .
11. void Modify a pointer
    void* A pointer to a type can match an address of any type , However, it needs to be forcibly converted to the corresponding address type .
12. Strong go
13. Strong conversion of data type
    Format ： Variable =（ data type ） value ;
    Generally, it is implicit strong rotation , When calculating, turn low precision into high precision .
14. Strong rotation of pointer type
    Format ： Pointer to the variable =（ data type *） Address values .
15. Large and small end
    a) Big end storage : Low address stores high byte content , High address stores low byte content .
    b) Small end storage ： Low address stores low byte content , High address stores high byte content .
16. function
    Three elements of the function ： function 、 Parameters 、 Return value .
17. Definition ： Encapsulate the code module of a specific function into a function （ Interface ）.
18. Define format ：
    [ Storage type ] data type Function name （ Shape parameter ）
    {
    The body of the function ;
    Return Constants or variables or expressions ;
    }
    Be careful ：
    a) Returned constant or variable or expression , The data type must be consistent with the function data type . If there is no return , The function type is defined as void type , There is no return sentence .
    b) The defined function needs to be directly or indirectly main Function call can realize function .
    c) If defined function does not need to pass parameters , The formal parameters of the function prototype are written as void. If not void, Pass the actual argument value , No mistake. , But this argument value will take up resource space , If you use this space later, there will be errors , It's easy to find the cause of the error .
19. Function call ： Function name （ Actual parameters ）
20. Function declaration ：[ Storage type ] data type Function name （ Shape parameter ）;
    a) The function is declared in main Upper edge of function , Declaration can omit formal parameter variable name .
    b) If the function prototype is defined in main Upper edge of function , The statement may not be written .
21. The difference between formal parameter and argument
    a) Shape parameter ： Formal parameters are formal parameter variables defined when defining functions , Is a formally existing value , Function calls will open up memory space .
    b) Actual parameters ： The argument is the value passed when calling the function , Is the actual value , There is memory space for development and storage .
22. Arguments to functions
23. Value passed
    Copy a copy of the value and pass it to other functions , Change the copied content , The original content has not changed .
24. Address delivery
    Pass the address of the variable to other functions , The meta content can be modified through the address .
25. Array passing
    The essence is also address passing , Passing is the first address of the array .
26. Command line arguments
    int main(int argc,const char *argv[])
27. argc： Number of command line arguments
28. argv: Save command line parameter content , The first address of each parameter is saved .
29. string Family of functions
30. strlen（char *s）
    a） function ： Calculate the length of the string , barring ‘\0’;
    b） Parameters ：s： The first address of the string to be calculated ;
    c） Return value ： Length of string
31. strlrn and sizeof The difference between
    a） strlen Is the function ,sizeof Is the key word .
    b） strlen Is to calculate the actual number and length of characters in the string , barring ‘\0’;
    c） sizeof It is used to calculate the memory space occupied , Include ‘\0’;
32. strcpy（char *dest,char *src）
    a） function ： Copy the string
    b） Parameters ：dest： The storage location of the copied string ;src： The string to copy .
    c） Return value ： The first address of the copied string .
33. strcat（char *dest,char *src）
    a） function ： Realize string splicing , take dest After ‘\0’ Remove and splice .
    b） Parameters ：dest： The storage location of the string to be spliced ;src： The string to be spliced .
    c） Return value ： The first address of the concatenated string .
34. strcmp（char *s1,char *s2）
    a） function ： Compare the size of two strings , Compare by character , Compare characters ASCII code .
    b） Parameters ： The first address of two strings .
    c） Return value ：s1>s2 Return positive number ;s1<s2 Return negative ;s1=s2 return 0.