[C language] advanced pointer --- do you really understand pointer?

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Preface

     What we are learning today is Advanced pointer .

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One 、 Classification of knowledge points

Today's advanced pointer includes character pointer , Array pointer , Pointer array , Array parameter passing and pointer parameter passing , A function pointer , Function pointer array , A pointer to an array of function pointers , Callback functions, etc .

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Two 、 The ginseng

1. One dimensional array parameters

// Formal parameters can be written in the form of arrays and pointers 
// Real arrays will not be created here , So there is no need to write any size here 
// Because only the address of the first element of the array is passed 
void test1(int arr[10]);
void test1(int arr[]);
void test1(int *arr);
void test2(int**arr);
void test2(int*arr[]);
int main()
{
    
	int arr1[10]={
    0};
	int*arr2[20]={
    0};
	test1(arr1);
	test2(arr2);
	return 0;
}

2. Pass parameters of two-dimensional array

The first element of a two-dimensional array actually represents the address of the first row

1. Formal parameters are written in the form of arrays
test(int arr[3][5]); Because it's the address in the first line , So here 3 It doesn't make sense , Just columns （ Columns cannot be omitted ）
2. Formal parameters are written as pointers
void test(int (*p)[5]);
This is an array pointer ,*p Means it is a pointer ,[5] Represents that this array has five elements ,int The type representing each element is int
It must not be written as void test(int(*p)[3][5]); This represents the entire two-dimensional array , And the address of the first line is only needed for parameter transmission

3、 ... and 、 Various pointers and arrays

1. Special examples of array names

The array name usually represents the address of the first element , But there are two exceptions
1.sizeof( Array name )：sizeof The size of the whole array is calculated with a single array name .
2.& Array name ：& Followed by a separate array name, the address of the entire array is taken

2. A function pointer

A function pointer –> Pointer to function
The form of function pointer ：
Like functions
int Add(int x,int y)
Then the function pointer is
int (*pf)(int,int);

int Add(int x,int y)
{
    
	return x+y;
}
int test(char*str)
{
    

}
int main()
{
    
	int arr[10];
	int(*p)[10]=&arr;
	printf("%p\n",&Add);
	printf("%p\n",Add);// It can be explained that Add and &Add The address of is the same 
	// The writing is different , But the meaning is the same , All we get is the address of the function 
	int(*pf)(int,int)=Add;
	// Equate to 
    //int(*pf)(int,int)=&Add;
    
    int ret=(*pf)(2,3);
    //int ret=(********pf)(2,3);
    //int ret=Add(2,3);
	
	// there pf That's the function pointer 
	int(*pf2)(char*)=test;//&test
	printf("%d\n",(*pf)(2,3));
	printf("%d\n",ret);
	return 0;
}

Explanation of function pointer examples

int main()
{
    
//1.
	(*(void(*)())0)();
	//void(*)() Is the type of a function pointer 
	//(void(*)())0 That is the 0 Cast the type to such a function pointer 
	// signify 0 There is such a function at the address , Which function has the address 0（ Here knowledge assumes access 0 This address ）
	// The application is not accessible 0 At this address 
	// and int (*pf)(int,int)=Add; analogy , That is to say pf A function is placed at 
	// Then dereference this address *(void(*)())0 Find this function , Call again *(void(*)())0()
	
//2.
	void (*signal(int, void(*)(int)))(int);
	//void(*)(int) yes signal Second parameter of , Is a function pointer 
	//signal The return type of a function is a function pointer 
}

3. Function pointer array

int Add(int x,int y)
{
    
	return x+y;	
}
int Sub(int x,int y)
{
    
	return x-y;
}
int Mul(int x,int y)
{
    
	return x*y;
}
int Div(int x,int y)
{
    
	return x/y;
}
int main()
{
    
	// Pointer array 
	// Character pointer array 
	char*arr[5];
	// Array of integer pointers 
	int*arr2[4];
	int(*pf1)(int,int)=Add;
	int(*pf2)(int,int)=Sub;
	int(*pf3)(int,int)=Mul;
	int(*pf4)(int,int)=Div;
	// Function pointer array 
	int (*pf[4])(int,int)={
    Add,Sub,Mul,Div};
	printf("%d",pf[0](1,2));
	return 0;
	// First, pf[4] Represents an array , The size is 4
	// The type of each element is int (*)(int,int);
}

4. The function pointer array implements the arithmetic unit

int Add(int x,int y)
{
    
	return x+y;	
}
int Sub(int x,int y)
{
    
	return x-y;
}
int Mul(int x,int y)
{
    
	return x*y;
}
int Div(int x,int y)
{
    
	return x/y;
}

void menu()
{
    
	printf("********************\n");
	printf("*** 1.Add ***\n");
	printf("*** 2.Sub ***\n");
	printf("*** 3.Mul ***\n");
	printf("*** 4.Div ***\n");
	printf("*** 0.Exit ***\n");
}
// Implement calculator 
int main()
{
    
	int input;
	int x,y;
	int ret;
	int (*pf[5])(int,int) = (0,Add,Sub,Mul,Div);
	// This initialization is because the array is from 0 At the beginning 
	do
	{
    
		menu();
		printf(" Please select :>");
		scanf("%d\n",&input);
		if(input==0)
		{
    
			printf(" Exit calculator .....\n");
			break;
		}
		else if(input>=1&&input<=4)
		{
    
			printf(" Please enter two operands :>");
			scanf("%d %d",&x,&y);
			ret=pf[input](x,y);
			printf("ret=%d\n",ret);
		}
		else 
		{
    
			printf(" Wrong choice \n");
		}
	}while(input);
	return 0;
}

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3、 ... and 、 Callback function and qsort function

1. Callback function

Concept ：
A callback function is a function called through a function pointer . If you pass the pointer of a function, that is, the address, as a parameter to another function , When this pointer is used to call the function it points to , Let's just say this is a callback function
The callback function is not called directly by the function's implementer , It's called by another party when a particular event or condition occurs , Used to respond to time or conditions .
A The address of the function is passed to B function , stay B Functions are called callback functions

void test()
{
    
	printf("zhupi\n");
}
void print_zhupi(void(*pf)())
{
    
	if( A condition )// If a certain condition or time occurs , The response 
	pf();
}
int main()
{
    
	print_zhupi(test);
	return 0;
}

Use the callback function to complete the calculator

int Add(int x,int y)
{
    
	return x+y;	
}
int Sub(int x,int y)
{
    
	return x-y;
}
int Mul(int x,int y)
{
    
	return x*y;
}
int Div(int x,int y)
{
    
	return x/y;
}

void calc(int (*pf)(int,int))
{
    
	int x,y;
	int ret=0;
	printf(" Please enter two operands :>");
	scanf("%d %d",&x,&y);
	ret=pf(x,y);
	printf("%d\n",ret);
}
void menu()
{
    
	printf("********************\n");
	printf("*** 1.Add ***\n");
	printf("*** 2.Sub ***\n");
	printf("*** 3.Mul ***\n");
	printf("*** 4.Div ***\n");
	printf("*** 0.Exit ***\n");
}
// Implement calculator 
int main()
{
    
	int input;
	int x,y;
	int ret;
	int (*pf[5])(int,int) = (0,Add,Sub,Mul,Div);
	// This initialization is because the array is from 0 At the beginning 
	do
	{
    
		menu();
		printf(" Please select :>");
		scanf("%d\n",&input);
		switch(input)
		{
    
		case 1:
			calc(Add);
			break;
		case 2:
			calc(Sub);
			break;
		case 3:
			calc(Mul);
			break;
		case 1:
			calc(Div);
			break;
		case 0:
			exit(-1);
			
		}
		
	return 0;
}

2.qsort function

qsort Function is a library function , It is a sorting function based on quick sorting algorithm
qsort The header file to be quoted is #include <stdlib.h>
qsort This function can sort any type of data
Functions written by ourselves may not be able to
For example, our bubble sort

void bubble_sort(int arr[],int len)
{
    
	for(int i=0;i<len-1;i++)
	{
    
		for(int j=0;j<len-1-i;j++)
		{
    
			if(arr[j]>arr[j+1])
			{
    
				int temp=arr[j];
				arr[j]=arr[j+1];
				arr[j+1]=temp;
			}
		}
	}
}
int main()
{
    
	int arr[]={
    9,8,7,6,5,4,3,2,1,0};
	int len=sizeof(arr)/sizeof(arr[0]);
	bubble_sort(arr,len);
	for(int i=0;i<len;i++)
	{
    
		printf("%d ",arr[i]);
	}
	return 0;
}
// It can be seen that , Not universal 

qsort：

qsort The sorting function defined in the needs to return >0 <0 =0 The number of , After getting the result qsort You can sort according to the returned number
Right now qsort Parameters of

void qsort(void*base,
			size_t num,
			size_t width,
			int(*cmmp)(const void*e1,const void*e2));
//base Is the actual location of the data to be sorted 
//num Is the number of elements of the array 
//width Is the size of each element 
//elem1 elem2 Is the address of the two elements to be compared 
// This comparison function requires qsort The user of the function customizes a comparison function 
// Because the element types are different , Users according to the actual situation , Provide a comparison function 

give an example =>

int cmp_int(const void*e1,const void*e2)
{
    
	//void* Is a pointer without an exact type , Cannot dereference directly 
	// Because the type of pointer represents the permission to access several bytes when dereferencing , and void* It's not clear ;

	//void* It's a garbage can , Any type can be put in , Then it's OK to force type conversion ;

//1.
	if(*(int*)e1>*(int*)e2)
	{
    
		return 1;
	}
	else if(*(int*)e1<*(int*)e2)
	{
    
		return -1;
	}
	else
	{
    
		return 0;
	}
//2.
	return (*(int*)e1-*(int*)e2;
}
void test()
{
    
	int arr[]={
    9,8,7,6,5,4,3,2,1,0};
	int len=sizeof(arr)/sizeof(arr[0]);
	qsort(arr,len,sizeof(arr[0]),cmp_int);
	for(int i=0;i<10;i++)
	{
    
		printf("%d ",arr[i]);
	}
}
int main()
{
    
	test();
	return 0;
}

Use qsort Sort structure

struct Stu
{
    
	char mame[20];
	int age;
	double score;

};
int cmp_stu_by_name(const void*e1,const void*e2)
{
    
	return strcmp(((struct Stu*)e1)->mame, ((struct Stu*)e2)->mame);
}
void test()
{
    
	struct Stu arr[3] = {
     {
    "zhangsan",20,55.5},{
    "lisi",30,88.0},{
    "wangwu",10,90.0} };
	int sz = sizeof(arr) / sizeof(arr[0]);

	qsort(arr,sz,sizeof(arr[0]),cmp_stu_by_name);

}

int main()
{
    
	test();
	return 0;
}

3. General bubble sort

int cmp_int(const void* e1, const void* e2)// Compare what you decide 
{
    
	return *(int*)e1 - *(int*)e2;
}
void Swap(char*buf1,char*buf2,int width)
{
    
	for (int i = 0; i < width; i++)
	{
    
		char temp = *buf1;
		*buf1 = *buf2;
		*buf2 = temp;
		buf1++;
		buf2++;
	}
	// Byte by byte exchange 
}

void bubble_sort(void* base, size_t num, size_t width, int (*cmp)(const void* e1, const void* e2))
{
    
	for (int i = 0; i < num - 1; i++)
	{
    
		for (int j = 0; j < num - 1 - i; j++)
		{
    
			// Compare , Because I know the width of each element , So cast the type to char*
			if (cmp((char*)base+j*width,(char*)base+(j+1)*width)>0)
			{
    
				// In exchange for 
				Swap((char*)base + j * width, (char*)base + (j + 1) * width,width);
			}
		}
	}
}
void print_arr(int * base, int sz)
{
    
	for (int i = 0; i < sz; i++)
	{
    
		printf("%d ",*(base+i));
	}
}
void test2()
{
    
	int arr[] = {
     9,8,7,6,5,4,3,2,1,0 };
	int sz = sizeof(arr) / sizeof(arr[0]);
	bubble_sort(arr, sz, sizeof(arr[0]), cmp_int);
	print_arr(arr,sz);

}
int main()
{
    
	test2();
	return;
}

Four 、 Various sizeof and strlen The calculation of ！！！

1. One dimensional array

int main()
{
    
	int a[] = {
     1,2,3,4 };
	printf("%d\n",sizeof(a));//16
	printf("%d\n",sizeof(a+0));//4/8, What is in parentheses is not a separate array name , So it represents the address of the first element , An address is a pointer 
	printf("%d\n",sizeof(*a));//4  Not a separate array name , therefore a Represents the address of the first element , Quoting , It's the first element 
	printf("%d\n",sizeof(a+1));//4/8  It calculates the size of the address of the second element 
	printf("%d\n",sizeof(a[1]));//4  It calculates the size of the second element 
	printf("%d\n",sizeof(&a));//4/8 &a Take out   Is the address of the array , The address of the array is also the address , It's the address （ The pointer ） Size is 4/8 Bytes 
	printf("%d\n",sizeof(*&a));//16 &a It takes out the address of the entire array , Then dereference to get the whole array , So we calculate the size of the whole array 
	printf("%d\n",sizeof(&a+1));//4/8 &a What you get is the address of the whole array ,+1 Is to skip the entire array , But it's also the address  
	printf("%d\n",sizeof(&a[0]));//4/8  Represents the address of the first element 
	printf("%d\n",sizeof(&a[0]+1));//4/8  Represents the address of the second element 
	return 0;
}

2. A character array

int main()
{
    
	char arr[] = {
    'a','b','c','d','e','f'};
	printf("%d\n",sizeof(arr));//6
	printf("%d\n",sizeof(arr+0));//4/8
	printf("%d\n",sizeof(*arr));//1
	printf("%d\n",sizeof(arr[1]));//1
	printf("%d\n",sizeof(&arr));//4/8
	printf("%d\n",sizeof(&arr+1));//4/8
	printf("%d\n",sizeof(&arr[0]+1));//4/8
	return 0;
}

int main()
{
    
	char arr[] = {
     'a','b','c','d','e','f' };
	//strlen Look for the end expression \0
	printf("%d\n",strlen(arr));// Random value ,arr There is no \0, therefore strlen The function will continue to look back \0, Statistics \0 The number of characters that appear before 
	printf("%d\n",strlen(arr+0));// Random value 
	printf("%d\n",strlen(*arr));//error arr Is the address of the first element of the array ,*arr Is the first element of the array ,'a'-97,strlen hold 97 As an address , From this address 
								// Look back \0, There may be wild pointer problems , Error accessing memory , So the code itself is wrong , Did not get a valid address 
								// In the eyes of pointer variables , Everything is an address , Because the meaning of his natural existence is to store addresses 
	printf("%d\n",strlen(arr[1]));//error
	printf("%d\n",strlen(&arr));// Random value , look for \0
	printf("%d\n",strlen(&arr+1));// Random value 
	printf("%d\n",strlen(&arr[0]+1));// Random value 
	return 0;
}

int main()
{
    
	char arr[] = "abcdef";
	printf("%d\n",sizeof(arr));//7
	printf("%d\n",sizeof(arr+0));//4/8
	printf("%d\n",sizeof(*arr));//1
	printf("%d\n",sizeof(arr[1]));//1
	printf("%d\n",sizeof(&arr));//4/8
	printf("%d\n",sizeof(&arr+1));//4/8
	printf("%d\n",sizeof(&arr[0]+1));//4/8


	printf("%d\n", strlen(arr));//6
	printf("%d\n", strlen(arr + 0));//6
	printf("%d\n", strlen(*arr));//error
	printf("%d\n", strlen(arr[1]));//error
	printf("%d\n", strlen(&arr));//6
	printf("%d\n", strlen(&arr + 1));// Random value 
	printf("%d\n", strlen(&arr[0] + 1));//5
	return 0;
}

int main()
{
    
	char* p = "abcdef";// Constant string 
	//p yes 'a' The address of 
	printf("%d\n",sizeof(p));//4/8
	printf("%d\n",sizeof(p+1));//4/8 p+1 yes 'b' The address of 
	printf("%d\n",sizeof(*p));//18
	printf("%d\n",sizeof(p[0]));//1
	printf("%d\n",sizeof(&p));//4/8 &p Is the address of the pointer , Double pointer 
	printf("%d\n",sizeof(&p+1));//4/8
	printf("%d\n",sizeof(&p[0]+1));//4/8

	printf("%d\n",strlen(p));//6
	printf("%d\n",strlen(p+1));//5
	printf("%d\n",strlen(*p));//error
	printf("%d\n",strlen(p[0]));//error
	printf("%d\n",strlen(&p));// Random value 
	printf("%d\n",strlen(&p+1));// Random value 
	printf("%d\n",strlen(&p[0]+1));//5
	return 0;
}

3. Two dimensional array

int main()
{
    
	int a[3][4] = {
     0 };
	printf("%d\n",sizeof(a));//48 sizeof( Array name ) It calculates the size of the entire array 
	printf("%d\n",sizeof(a[0][0]));//4
	printf("%d\n",sizeof(a[0]));//16 a[0] yes *(a+0) Is the address of the first element in the first line , That is, the array name in the first line , therefore sizeof(a[0]) In fact, it calculates the size of the first row 
	printf("%d\n",sizeof(a[0]+1));//4/8 a[0] Is the array name in the first row ,a[0]+1 That is, the address of the second element in the first line 
	printf("%d\n",sizeof(*(a[0]+1)));//4  That is to say a[0][1]
	printf("%d\n",sizeof(a+1));//4/8  The array name of a two-dimensional array represents the address of the first line , therefore a+1 It represents the address in the second line 
	printf("%d\n",sizeof(*(a+1)));//16 a+1 Is the address on the second line , After dereference, the address of the first element in the second line , So we're calculating the size of the second line 
	printf("%d\n",sizeof(&a[0]+1));//4/8  It represents the address in the second line 
	printf("%d\n",sizeof(*(&a[0]+1)));//16  The parentheses indicate the address of the first element in the second line   That is, the array name in the second line , It calculates the size of the second row of the array 
	printf("%d\n",sizeof(*a));//16  The address of the first row array is calculated 
	printf("%d\n",sizeof(a[3]));//16 a[3] Is the address of the first element in the fourth line 
	return 0;
}

5、 ... and 、 5、 ... and The color spot Beautiful Of One some Scrap word

        When you see here , I believe the above content has been memorized ‍️‍️‍️. If you find it helpful , Use your little hand to point . Is there a problem with one button three times ？ No problem , What are these ？ traditional code of conduct . It's not easy to create , Support a lot .