2500 words, hand-in-hand to take you to a preliminary understanding of static keywords and pointers, structures

Typedef As the name implies, it is a type definition, which should be understood as a type renaming.

PS: Just like a person's nickname, typedef is to put a nickname for the variable type

Here, the int type is re-named a, that is, at this time a is equal to the int type, we can use a to create a variable and print it

#include<stdio.h>
int main()
{
	typedef int a;        //类型重命名
	a i = 10;             //新名字来定义变量
	printf("%d", i);
	return 0;
}

keyword static

In C language: static is used to modify variables and functions 1. Modified local variables - called static local variables

2. Decorate global variables - called static global variables

3. Decorated functions - called static functions

decorate local variables

Before static is not used

After using static

We found that after using static, the printed result is different from before. This is because before the static function is not used, every time i is printed, the space of i will be released, and it will be re-entered when it is entered again. Create a space for i and assign the initial value of i to 1, so 10 1s will be printed

Why can I print 1-10 after using static?

This is because: when static is used to modify local variables, the local variables will be placed in the static area. At this time, whenever i is printed, that is, after the function is executed, the i in the static area will not be released, but will continue here. Save it. When i is used again next time, i will still use the value of the last function as the initial value for the next operation.

The i before static is not used will be released immediately after each function execution ends, because this temporary variable i is created in the stack area, and is released as soon as the function exits, and after the static modification, i is placed in the static area, extending The life cycle of i is made so that the life cycle of i is almost as long as the life cycle of the main function

static modifies global variables

When we create a variable in a .c file and want to use it in another .c file, we need to add extern to declare it in the .c file to be used before it can be used

As follows:

Here we create variable i in test1.c, use variable i in test.c, and use extern in test.c to declare and use

After modifying external variables with static

We found that the program will report an error

At this point, the program cannot resolve the variable i from the outside, but it can be used normally when we use i in the test1.c file

This is because: after modifying the global variable with static, it will change the external link attribute of the local variable to the internal link attribute, which means that after modifying the global variable with static, when you want to use this variable in another .c file, will not be used.

Decorate a function with static

Before a .c file references a function of another .c file and static is not used

prints fine

After using static

Cannot print normally at this time

And after we use static in the .c file

You can print normally at this time

This is because: when static modifies an external function, it will cause the external function to lose external connectivity and maintain internal connectivity, so an error will be reported

Summarize:

When static modifies a local variable, the variable will be placed in the static area, and the memory used by the temporary variable will not be released after each use, and the life cycle will be delayed until the end of the program.

When static modifies global variables and functions, global variables and functions will lose external linkage, but maintain internal linkage, so when static modifies global variables and functions, they cannot be referenced in another .c file

Introduction to the register keyword

register keyword when registering

The register keyword can only act on local variables, not global variables.

The register keyword specifies to store local variables in registers.

register is only a request to store this variable in the register, but it may not be able to request successfully

#define define constants and macros

Define identifier constants with define

Define macros with define

Note: macro parameters are untyped

pointer

RAM

The memory is a particularly important memory on the computer, and the operation of the programs in the computer is carried out in the memory. Therefore, in order to use the memory efficiently, the memory is divided into small memory units, and the size of each memory unit is 1 byte . In order to effectively access each unit of memory, the memory unit is numbered, and these numbers are called the address of the memory unit.

print the address of a

#include<stdio.h>
int main()
{
 int a=10;     //定义变量a，开辟四个字节的空间
 int *c=&a;    //定义指针变量c，并将a的地址放入其中
printf("%d",*c);      // 打印c地址中所1存放的数值
printf("%p",c)        //打印c的地址
return 0;
}

&a meaning: take out the address of a, & is the address symbol

Define a variable of type int * to assign the number stored in the address of a to the variable c of type int *,

The meaning of int * c=&a, int indicates that the object pointed to by c is of type int , * indicates that p is a pointer variable ,

No matter what type of pointer it is, it is creating a pointer variable, such as char *a, which is a pointer variable

Pointer variables are used to store addresses

The size of the pointer variable depends on how much space is needed when an address is stored

size of pointer variable

The size of the pointer variable is related to the number of operands in the system.

Note: *c refers to the variable stored in the address where the variable c of type int * is located. c refers to the address pointed to by a pointer variable of type int*.

structure

Structure is a particularly important knowledge point in C language. Structure makes C language capable of describing complex types. For example, when describing a student, the student includes: name + age + gender + student number. Only structures can be used to describe them here.

define structure

Here the phone uses 12 spaces because the \0 in the string also occupies a space

#include <stdio.h>
struct Stu
{
    char name[100]; //姓名
    int age;         //年龄
    char sex[10];    //性别
    char tele[12]; //电话
};

struct Stu is a structure type, which is a structure variable

assign value to structure

Assignment method: assign corresponding values in order according to the variable types in the defined structure.

struct Stu a is to define a variable aa is a structure type struct Stu

reference structure variable

Direct reference: add one more structure variable a. Then select the corresponding structure variable

Reference as a pointer: use the structure name -> apply