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Get the C language pointer in one article

2022-07-23 09:38:00 【LuZhouShiLi】

One article C Language pointer

One 、 Memory Overview

1.1 Relevant concepts

Memory ： In the composition of the computer , Used to store programs and data , auxiliary CPU An important part of arithmetic processing
Memory ： Internal memory , Temporary program / data , Power loss
External storage ： External memory , Secondary storage , Don't lose power

Memory is communication CPU Bridge with hard disk

Temporary storage CPU Operation data in
Temporarily store data exchanged with external memory such as hard disk

1.2 Physical memory and storage address space

Physical memory ： The actual memory chip
- The memory module installed on the motherboard
- The display on the display card RAM chip
- On various adapter cards RAM Chips and ROM chip
Store address space ： The range of memory encoding , The memory we often say in software refers to this layer of meaning .
- code ： For each physical storage unit （ A byte ） Assign a number
- Addressing ： The corresponding storage unit can be found according to the assigned number , Complete data reading and writing

1.3 Memory address

Abstract the memory into a large one-dimensional character array
Coding is about memory Every byte Allocate one 32 For or 64 The number of bits （ This has something to do with the processor ）
This memory number we call memory address , Each data in memory is assigned a corresponding address
char: Allocate an address per byte
int: Occupy four bytes and allocate four addresses

Two 、 Pointers and pointer variables

2.1 Popular understanding of pointer

Start a program , The system allocates a piece of memory space to the program in memory , We programmers only need to focus on logical addresses , This memory space is composed of bytes , Every byte has its address number （ Hexadecimal representation ）, such as 32 Bit CPU, Memory addressing is 0x0000 0000 - 0xffff ffff

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that , Memory address is actually the number of memory , Let's number this （ Address ） It is called pointer

The pointer == Address == Number

2.2 Pointer to the variable

Pointer to the variable ： The variable that holds the pointer , meanwhile , Because the address number is 0x0000 0000, therefore , Pointer variables can be stored in four bytes

If it is 64 Bit compiler , The number range of memory is 0x0000 0000 0000 0000 - 0xffff ffff ffff ffff, Such a number , It takes eight bytes to store , So pointer variables also need eight bytes

2.3 summary

Each byte of the memory area has a number , This is the address
If a variable is defined in the program , When compiling or running a program , The system will allocate a memory unit to this variable , And determine the memory address
The essence of a pointer is a memory address , The pointer is the address , The address is the pointer
The pointer is the number of the memory unit , A pointer variable is a variable that holds an address
In general, we describe pointer variables as pointers , But the meaning is different .

3、 ... and 、 Definition and initialization of pointer variables

int a = 10;
int *p;
p = &a;

Three steps to define pointers
- * Combined with the symbol, it represents a pointer variable
- Whose address do you want to save , Put his definition form here
- use *p Replace the defined variable
understand ：
- p And * The combination represents that this is a pointer variable
- p It's a pointer variable ,p The type is int *（ All parts except variable names ）
- Pointer to the variable p For preservation int Type of address （ Remove variable names and recent * Outside part ）

about int **p The understanding of the ：

p It's a pointer variable ,p The type is int **
Pointer to the variable p For preservation int * Address of data type （ So it's a two-level pointer ）

Four 、 Use of the pointer

*p = 100; //* And p Combination represents ： take p The content of the space pointed to by the pointer . Direct assignment here 100

// Modify the value of the variable indirectly through the pointer variable

#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<stdlib.h>
#include<string.h>

int main()
{
    
	int a = 10;
	int* p = &a;

	*p = 100;
	printf("%d\n",a);// a The value of has changed 
	return 0;
}

Be careful ：& You can get the address of a variable in memory , But you can't go to register variables , Because register variables are not in memory , And in the CPU in , So there is no address .

When use , Take... For an expression , Will subtract one level from the expression , If you take... For the expression &, Will increase one level **

int *p;
int **q;

q = &p;//  The secondary pointer   Deposit is p The address of 

// q  type  int **
// p  type  int *
//  take &  Add one level *

*q = p;
//  take *  Minus one level

Four 、 The size of the pointer variable

No matter what type of pointer variable , The size is related to the compiler type ,32 Bit four bytes ,64 Bit eight bytes

5、 ... and 、 The width and step size of the pointer

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adopt * When taking the content of the space pointed to by the pointer variable , Getting the width of memory is related to the type of pointer variable itself

#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<stdlib.h>
#include<string.h>

int main()
{
    
	int num = 0x01020304;
	char* p1 = (char*)&num;//int *  Coercive transformation 
	short* p2 = (short*)&num;
	int* p3 = &num;

	//  adopt * When taking the content of the space pointed to by the pointer variable , The width of the memory taken is related to the type of the pointer variable itself 
	printf("%x\n",*p1);//  Only one byte can be taken   Print 04
	printf("%x\n",*p2);// Take two bytes  0403
	printf("%x\n",*p3);//  Take four bytes  01020304

	return 0;
}

The width of the pointer = Match the pointer variable to the nearest pointer variable * Drag black , The rest of the types .

char *p  1
short *p  2
int *p  4
int **p  sizeof(int *)   4

#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<stdlib.h>
#include<string.h>

int main()
{
    
	int num = 0x01020304;
	char* p1 = (char*)&num;//int *  Coercive transformation 
	short* p2 = (short*)&num;
	int* p3 = &num;

	//  adopt * When taking the content of the space pointed to by the pointer variable , The width of the memory taken is related to the type of the pointer variable itself 
	printf("%x\n",*p1);//  Only one byte can be taken   Print 04
	printf("%x\n",*p2);// Take two bytes  0403
	printf("%x\n",*p3);//  Take four bytes  01020304


	printf("%u\n",p1);
	printf("%u\n", p2);
	printf("%u\n", p3);


	printf("%u\n", p1 + 1);//  Add one 
	printf("%u\n", p2 + 1);//  Add 2
	printf("%u\n", p3 + 1);//  Add  4

	return 0;
}

6、 ... and 、 Wild pointer

int *p;

*p = 200;

A wild pointer is a pointer that is not initialized , The direction of the pointer is random , You cannot manipulate the wild pointer . The pointer p The saved address must be defined （ Applied to the system ）

7、 ... and 、 Null pointer

The function of null pointer ： If you finish using the pointer, assign the pointer to NULL, Judge whether the pointer is NULL, Know whether the pointer has been used .

#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<stdlib.h>
#include<string.h>

int main()
{
    
	int a;
	//  Assign the value of the pointer to 0,0x000000 = NULL
	int* p = NULL;//  Give the pointer p The content of is assigned to 0
	*p = 200;
	printf("%d\n",*p);
	return 0;
}

8、 ... and 、 Universal pointer

void b; You can't define void Variable of type Because the compiler doesn't know how many bytes to allocate to variables
But it can be defined as void * type Because the pointer must be four bytes

#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<stdlib.h>
#include<string.h>

int main()
{
    
	int a = 10;
	void* p = (void*)&a;//  Universal pointer can save   Any address   But pointer type conversion is required  
	// printf("%d\n",*p);//  Can't print   because sizeof(void)  I don't know how many bytes   There is no way to get the content in the address 
	printf("%d\n",*(int *)p);//  First convert the pointer type to the original type   Getting content 
	return 0;
}

Nine 、const Decorated pointer variable

const Modifier constant You cannot directly modify the content , It needs to be modified by pointer


#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define _CRT_SECURE_NO_WARNINGS

int main()
{
    
	const int a = 10;//  Modifying variables a  No more passing a modify a What's in memory 
	int *p = &a;
	*p = 100;//  You can use the pointer p Modify the content 
	printf("%d\n",*p);
	printf("%d\n",a);
	return 0;
}


* const int *p = &a; Cannot pass *p  To modify the P What it points to 

*p = 100;// error


* int* const p = &a; p The saved address cannot be modified 

p = &b;// error p The value of itself cannot be modified 

* const int *const p = &a;// p The address pointed to by itself cannot be changed    It can't pass *p modify p The address pointed to saves the content

Ten 、 Multi level pointer

The secondary pointer stores the address of the primary pointer
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int *p = &a;

int **q = &p;//  The secondary pointer 

//  If * and & meet , Counteract 
// **q == *(*q) == *(p) == a

11、 ... and 、 Pointers and arrays

11.1 Manipulate array elements through pointers

Pointer plus one , Across an element


#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define _CRT_SECURE_NO_WARNINGS

int main()
{
    
	int a[10] = {
    1,2,3,4,5,6,7,8,9,10};
	// a  Array name , The address of the first element 

	int* p = a;//  The pointer p Save the address of the first element 
	for (int i = 0; i < sizeof(a) / sizeof(a[0]); i++)
	{
    
		printf("%d ",*(p + i));
	}

	return 0;
}

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define _CRT_SECURE_NO_WARNINGS

int main()
{
    
	int a[10] = {
    1,2,3,4,5,6,7,8,9,10};
	// a  Array name , The address of the first element 

	int* p = a;//  The pointer p Save the address of the first element 
	for (int i = 0; i < sizeof(a) / sizeof(a[0]); i++)
	{
    
		*(p + i) = i;
		printf("%d ",*(p + i));
	}

	return 0;
}

11.2 Pointer arithmetic

Subtraction of two pointers , Get how many elements are crossed in the middle , There is no point in adding two pointers


#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define _CRT_SECURE_NO_WARNINGS

int main()
{
    
	int a[10] = {
    1,2,3,4,5,6,7,8,9,10};
	// a  Array name , The address of the first element 
	int* p = a;
	// int* q = &a[9];
	//  We must change here int *  Because add one or subtract one   Will span the entire array of elements 
	//  After the transformation   Just across an element size address 
	int* q = (int*)(&a + 1) - 1;//  Point to last element 
	printf("%d\n",q - p);// 9
	printf("%d\n", *(p + 3));
	return 0;
}

11.3 [] Not exclusive to arrays

a[i] = p[i] = *(p + i) = *(a + i)


#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define _CRT_SECURE_NO_WARNINGS

int main()
{
    
	int a[10] = {
    1,2,3,4,5,6,7,8,9,10};
	int* p = a;

	for (int i = 0; i < sizeof(a) / sizeof(a[0]); i++)
	{
    
		//printf("%d ",*(a + i));
		//printf("%d ",*(p + i));
		printf("%d ",p[i]);

	}

	return 0;
}

Twelve 、 Pointer array

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#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define _CRT_SECURE_NO_WARNINGS

int main()
{
    
	int a = 10;
	int b = 20;
	int c = 30;

	//  Pointer array ： Every element in the array is a pointer （ Address ）
	int* num[3] = {
    &a,&b,&c};
	printf("%d \n",sizeof(num));

	//  Print a b c
	for (int i = 0; i < sizeof(num) / sizeof(num[0]); i++)
	{
    
		printf("%d ",*num[i]);
	}

	//  Define a pointer to hold the array num The address of the first element 
	//  First num The first element itself is a pointer （ Address ）
	// Num = &num[0] = &(int *) = int **
	int** k = num;//  The second level pointer manipulates the pointer array 

	//  Print the elements in the array , Like printing b  Then first pass k Go to the first element address  k + 1 obtain b The address of , after *(k + 1)  obtain num[1] The content of , Last **（k+ 1） obtain b
	for (int i = 0; i < sizeof(num) / sizeof(num[0]); i++)
	{
    
		printf("%d ", **(k + i));//  Manipulate the pointer array through the secondary pointer 
	}

	return 0;
}

13、 ... and 、 Pointer as parameter

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define _CRT_SECURE_NO_WARNINGS

void swap(int* x, int* y)
{
    
	//  The incoming address is   Formal parameters can be written in the form of pointers 
	int k = *x;
	*x = *y;
	*y = k;
}

int main()
{
    
	int a = 10;
	int b = 20;
	int c = 30;

	swap(&a,&b);//  The incoming address is 

	printf("%d %d\n",a,b);

	return 0;
}

fourteen 、 Arrays as arguments to functions

Arrays as arguments to functions , Will degenerate directly into pointer type

void print_arr(int* b, int len)
{
    
	int n = sizeof(b) / sizeof(b[0]);// Print  1
	printf("%d\n",n);
	for (int i = 0; i < len; i++)
	{
    
		printf("%d ",b[i]);
	}
	printf("\n");
}

int main()
{
    
	int b[10] = {
    1,2,3,4,5,6,7,8,9,10};

	print_arr(b,10);

	return 0;
}

15、 ... and 、 Pointer as the return value of the function

The return is an address

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<time.h>
#define _CRT_SECURE_NO_WARNINGS
int num = 0;

int* getnum()
{
    
	//{} Variables defined in are called local variables , The space of local variables will be released after the end of the function 
	srand(time(NULL));
	num = rand();
	return &num;
}


int main()
{
    
	int* p = getnum();

	printf("%d", *p);

	return 0;
}

sixteen 、 Pointers and strings

16.1 Pointer and character array

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<time.h>
#define _CRT_SECURE_NO_WARNINGS

int main()
{
    
	char a[] = "helloworld";//  Defines an array of characters   The length is eleven , The contents of the character array are helloworld\0  It's a string 
	//  Define a pointer to hold the address of the first element of the array 
	char* p = a;
	printf("%s\n",p);// %s  Print a string   What you want is the address of the first character  
	//  The pointer p What you save is the address of the first character 
	return 0;
}

You can change a single character through a character array pointer

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<time.h>
#define _CRT_SECURE_NO_WARNINGS

int main()
{
    
	char a[] = "helloworld";//  Defines an array of characters   The length is eleven , The contents of the character array are helloworld\0  It's a string 
	//  Define a pointer to hold the address of the first element of the array 
	char* p = a;
	printf("%s\n",p);// %s  Print a string   What you want is the address of the first character  
	//  The pointer p What you save is the address of the first character 

	printf("%s\n",p + 2);

	*p = 'x';//  Change the character directly through the character pointer 
	p++;
	*p = 'o';
	printf("%s\n", a);

	return 0;
}

16.2 String constant

Assign a string constant to a pointer , The pointer saves the address of the string constant , You can print strings through pointers , But you can't modify the character of the string through the pointer

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<time.h>
#define _CRT_SECURE_NO_WARNINGS

int main()
{
    
	//  String exists in array   The stack area   Can be changed 
	char a[] = "helloworld";//  Defines an array of characters   The length is eleven , The contents of the character array are helloworld\0  It's a string 
	//  Define a pointer to hold the address of the first element of the array 
	char* p = a;
	p = "abcdef";//  String constants are stored in the text constant area , When use , Get the address of the first element of the string 
	//  The content of the text constant area cannot be changed 

	printf("%s\n",p);// %s  Print a string   What you want is the address of the first character  
	//  The pointer p What you save is the address of the first character 

	printf("%d\n",sizeof(p));// 4
	printf("%d\n",sizeof("abcdef"));// 7

	printf("%d\n",strlen(p));// 6
	printf("%d\n",strlen("abcdef"));// 6

	return 0;
}

16.3 Character pointer as function parameter

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<time.h>
#define _CRT_SECURE_NO_WARNINGS

char* my_strcat(char* src, char* dst)
{
    
	//  Concatenate two strings 
	int n = strlen(src);//  Calculate the number of valid characters 
	int i = 0;

	//  Loop through the second string 
	while (*(dst + i) != 0)
	{
    
		//  The cycle condition is   The current character is not \0

		// src  At the beginning, it points to the first character of the string   add n Then point to the character at the end of the string \0  And then directly cover \0
		*(src + n + i) = *(dst + i);
		i++;
	}
	*(src + n + i) = 0;//  add to \0 character 
	return src;
}

int main()
{
    
	char a[] = "zxcvb";
	char b[] = "wqeqwrrt";

	printf("%s\n",my_strcat(a,b));

	return 0;
}

16.4 Character pointer array

Character pointer array , Every element is a character pointer （ String address ）

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<time.h>
#define _CRT_SECURE_NO_WARNINGS

int main()
{
    
	char* p1 = "heihei";
	char* p2 = "haha";
	char* p3 = "xixi";
	char *num[3] = {
    p1,p2,p3};//  Character pointer array   Every element is a character pointer （ String address ）

	for (int i = 0; i < 3; i++)
	{
    
		printf("%s\n",num[i]);// num The address of the string is stored in 
	}

	printf("%c\n",*num[0]);//  Print  h
	printf("%c\n",*(num[1] + 1));//  Print a
	printf("%c\n",*(num[2] + 2));

	return 0;
}

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16.5 Character pointer array as main Function parameter

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<time.h>
#define _CRT_SECURE_NO_WARNINGS

//char* argv[] = { ".*.exe","hello","123456" };

int main(int argc,char *argv[])
{
    
	
	for (int i = 0; i < argc; i++)
	{
    
		// argc Represents the number of parameters 
		// argv Represents an array of pointers   The address of each parameter string is stored   Default argv[0] Is the address string of the program 
		for (int i = 0; i < argc; i++)
		{
    
			printf("%s\n",argv[i]);//  Pass in the string address   Print each string 
		}
	}

	return 0;
}

seventeen 、 String commonly used processing functions

String copy function stcpy

char str1[128] = ".";
char str2[128] = "world";
strcpy(str1,str2);
printf("%s\n",str1);

str2 It will cover directly from the beginning str1, And copy \0 character , So the following code will print world

char str1[128] = "hhhhhhhiiiiiii";
char str2[128] = "world";
strcpy(str1,str2);
printf("%s\n",str1);

strncpy String copy function
take str2 Middle front n Copy characters to str1 in , If the copy is insufficient N individual , encounter \0 Copy end
strcat(str1,str2) String concatenation function
take str2 String connected to dest Tail of ,\0 It will be added to the past , When you link , encounter \0 end
strncat(str1,str2,n)
take str2 In character array N Copy characters to str1 Character array , encounter \0 end
strcmp(str1,str2) Compare strings
str1 Array neutralization str2 The array takes out an element for comparison , Continue to think about the following comparison , The comparison is of characters ascii value , If str1 > str2 The return value is equal to 1,str1 == str2 The return value is equal to 0, If str1 < str2 The return value is equal to -1
sprintf()： Package function

int len = sprintf(buf,“ Format ”,“ data ”);// Package the data according to the format , Store in array buf in ,sprintf The return value of the function is the effective length of the finished package

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<time.h>
#define _CRT_SECURE_NO_WARNINGS
#pragma warning(disable:4996)

int main()
{
    
	int year = 2018;
	int month = 10;
	int day = 20;
	char buf[1024] = "";

	int len = sprintf(buf,"year = %d %c month = %d day = %d",year,0,month,day);
	printf("buf = [%s]\n",buf);//  encounter \0 Don't print 
	printf("%d\n",len);//  Return string length 
	
	return 0;
}

sscanf(): Unpack function

sscanf(buf,“ Format ”, data );// take buf Format the content in and output it to data

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<time.h>
#define _CRT_SECURE_NO_WARNINGS
#pragma warning(disable:4996)

int main()
{
    
	int year = 2018;
	int month = 10;
	int day = 20;
	char buf[1024] = "2018:10:20";

	sscanf(buf,"%d:%d:%d",&year,&month,&day);//  from Buf Get data in format 
	printf("%d %d %d\n",year,month,day);
	
	return 0;
}

strchr(str,‘c’) Intercept the string starting from a certain character

char str[] = "xixihellogworld";
char *p = strchr(str,'g);
printf("%s\n",p);//  Print gworld

Do it yourself

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<time.h>
#define _CRT_SECURE_NO_WARNINGS
#pragma warning(disable:4996)

char* my_strchr(char *p,char ch)
{
    
	int i = 0;
	while (*(p + i) != 0)
	{
    
		if (*(p + i) == ch)
		{
    
			// return (p + i);//  Address fetch 
			//  Or written 
			return &p[i];
		}
		i++;
	}

	if (*(p + i) == 0)
	{
    
		return NULL;
	}
}

int main()
{
    
	char str[] = "xixixihellogworld";
	char* p = my_strchr(str,'g');
	printf("%s\n",p);
	return 0;
}

strstr(str1,str2)

Intercept the remaining strings from a certain string

int main()
{
    
	char str[] = "xixiabcxihellogworld";
	char str1[] = "abc";
	char* p = strstr(str,str1);
	printf("%s\n",p);
	return 0;
}

strtok(str,“c”) Cut string

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<time.h>
#define _CRT_SECURE_NO_WARNINGS
#pragma warning(disable:4996)


int main()
{
    
	char str[] = "xixiab#cxihello#gworld";
	char* p1 = strtok(str,"#");//  stay str1 Lieutenant general # cutting , Return the string before cutting 
	printf("%s\n",p1);

	char* p2 = strtok(NULL,"#");
	printf("%s\n",p2);

	char* p3 = strtok(NULL,"#");
	printf("%s\n",p3);
	
	return 0;
}

atoi() function Convert string to integer

atoi() Will scan the string , Skip the preceding space character , Until you encounter a number or a plus or minus sign and start converting , Encountered non numeric or string terminator \0 End the transition , And return the result to the return value integer .

char num[] = "123";
int sum = atoi(num);

Allied atof() Convert to decimal

eighteen 、 String case

18.1 Find the number of times a string appears in the string

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<time.h>
#define _CRT_SECURE_NO_WARNINGS
#pragma warning(disable:4996)

int strstr_num(char* src, char* dst)
{
    
	int i = 0;
	char* p = src;
	int n = 0;

	do
	{
    
		p = strstr(p,dst);

		if (p != NULL)
		{
    
			//  If the address returned from the search is not NULL  It means that 
			n++;
			p = p + strlen(dst);
		}

	} while (p != NULL);

	return n;
}

int main()
{
    
	char src[] = "hdabchabchjfsjabcjdsjabc";
	char dst[] = "abc";
	int n = strstr_num(src,dst);
	printf("%d\n",n);
	return 0;
}

18.2 Two plug model


int main()
{
    
	char buf[] = " hello world ";
	char dst[128] = "";
	
	char* start = buf;
	char* end = &buf[strlen(buf) - 1];// end Point to the last character 

	//  Find the first position that is not a space 
	while (*start != ' ' && *start != 0)
	{
    
		start++;
	}

	while (*end != ' ' && end != start)
	{
    
		end--;
	}

	printf("%d\n",end - start + 1);//  Print length 
	strncpy(dst,start,end - start + 1);
	printf("%s\n",dst);

	return 0;
}

18.3 String flip

int main()
{
    
	char buf[] = "abcdef";
	char* start = buf;
	char* end = &buf[strlen(buf) - 1];//  Go to the last character 

	while (end > start)
	{
    
		char ch = *end;
		*end = *start;
		*start = ch;
		end--;
		start++;
	}
	printf("%s",buf);
	return 0;
}