1, Array parameter, pointer parameter

When writing code, it is inevitable to pass [array] or [pointer] to the function. How to design the parameters of the function?

1. One dimensional array parameter transfer

void test(int arr[])//ok
{}
void test(int arr[10])//ok
{}
void test(int* arr)//ok
{}
void test2(int* arr[20])//ok
{}
void test2(int** arr)//ok
{}
int main()
{
	int arr[10] = { 0 };
	int* arr2[20] = { 0 };
	test(arr);
	test2(arr2);
	return 0;
}

2. Two dimensional array parameter transfer

void test(int arr[3][5])//ok
{}
void test(int arr[][])//no
{}
void test(int arr[][5])//ok
{}
//Summary: for the design of two-dimensional array parameters and function parameters, only the first [] number can be omitted.
//Because for a two-dimensional array, you can't know how many rows there are, but you must know how many elements there are in a row.
//This is convenient for calculation.
void test(int *arr)//no
{}
void test(int* arr[5])//no
{}
void test(int (*arr)[5])//ok
{}
void test(int **arr)//no
{}
int main()
{
 int arr[3][5] = {0};
 test(arr);
}

3. Primary pointer transfer parameter

#include <stdio.h>
void print(int *p, int sz) 
{
    int i = 0;
    for(i=0; i<sz; i++)
    {
        printf("%d\n", *(p+i));
    }
}
int main()
{
    int arr[10] = {1,2,3,4,5,6,7,8,9};
    int *p = arr;
    int sz = sizeof(arr)/sizeof(arr[0]);
    //First level pointer p, passed to function
    print(p, sz);
    return 0; 
}

4. Secondary pointer transfer parameter

#include <stdio.h>
void test(int** ptr) 
{
	printf("num = %d\n", **ptr);
}
int main()
{
	int n = 10;
	int* p = &n;
	int** pp = &p;
	test(pp);
	test(&p);
	return 0;
}

2, Function pointer

Let's start with a piece of code:

void test()
{
	printf("hehe\n");
}
int main()
{
	printf("%p\n", test);
	printf("%p\n", &test);
	return 0;
}

The output is two addresses, which are the addresses of the test function.
Save function address: void (*pfun1)();
Pfun1 is first combined with * to explain that pfun1 is a pointer, the pointer points to a function, the pointed function has no parameters, and the return value type is void.

Read two interesting pieces of code:

//Code 1 
(*(void (*)())0)();
//Code 2
void (*signal(int , void(*)(int)))(int);

Code 1: is a function call. The address of a function in the code that converts the 0 cast type to void(*) (). Then dereference the address of 0, that is, go to the function at the address of 0. The called function has no parameters and the return type is void
Code 2: is a function declaration. The declared function name is signal. Signal has two parameters. The first is the type of int, the second is the function pointer type of void (*) (int), and the return type of signal function is still the function pointer type of void (*) (int).
Simplified code 2:

typedef void(*pfun_t)(int);
pfun_t signal(int, pfun_t);

3, Function pointer array

1. Definitions

The address of the function is stored in an array, which is called the function pointer array

int (*parr1[10])();

Parr1 is first combined with [], indicating that parr1 is an array, and the contents of the array are function pointers of type int (*) ().

2. Purpose

Purpose of function pointer array: transfer table
Example: (calculator)

int add(int a, int b) {
	return a + b;
}
int sub(int a, int b) {
	return a - b;
}
int mul(int a, int b) {
	return a * b;
}
int div(int a, int b) {
	return a / b;
}
int main()
{
	int x, y;
	int input = 1;
	int ret = 0;
	int(*p[5])(int x, int y) = { 0, add, sub, mul, div }; //Transfer table
	while (input)
	{
		printf("*************************\n");
		printf(" 1:add           2:sub \n");
		printf(" 3:mul           4:div \n");
		printf("*************************\n");
		printf("Please select:");
		scanf("%d", &input);
		if ((input <= 4 && input >= 1))
		{
			printf("Input operand:");
			scanf("%d %d", &x, &y);
			ret = (*p[input])(x, y);
		}
		else
			printf("Incorrect input\n");
		printf("ret = %d\n", ret);
	}
	return 0;
}

4, Pointer to array of function pointers

The pointer to the array of function pointers is a pointer, the pointer points to an array, and the elements of the array are function pointers.
How to define?

#include<stdio.h>
void test(const char* str) {
	printf("%s\n", str);
}
int main()
{
	//Function pointer pfun
	void (*pfun)(const char*) = test;
	//Array of function pointers pfunArr
	void (*pfunArr[5])(const char* str);
	pfunArr[0] = test;
	//Pointer to function pointer array pfunArr ppfunArr
	void (*(*ppfunArr)[10])(const char*) = &pfunArr;
	return 0;
}

5, Callback function

A callback function is a function called through a function pointer. If you pass the pointer (address) of a function as a parameter to another function, when the pointer is used to call the function it points to, we say it is a callback function. The callback function is not called directly by the implementer of the function, but by another party when a specific event or condition occurs, which is used to respond to the event or condition.

Note: void * - there is no specific type of pointer and can receive any type of address. Disadvantages: no operation, no ± integer, no dereference

First, demonstrate the use of the qsort function:
qsort:

strcmp:

//The user of the qosrt function has to implement a comparison function
//Compare two integer functions
int cmp_int(const void* e1, const void* e2) 
{
	return (*(int*)e1 - *(int*)e2);
}
//Compare structure data
struct Stu
{
	char name[20];
	int age;
};
int cmp_by_name(const void* e1, const void* e2)
{
	return strcmp(((struct Stu*)e1)->name, ((struct Stu*)e2)->name);
}
void print_arr(int arr[], int sz)
{
    int i = 0;
    for(i = 0; i < sz; i++)
    {
        printf("%d ", arr[i]);
    }
}
int main()
{
    struct Stu s[3] = {{"Zhang San",15}, {"Li Si",30},{"Wang Wu",10}};
	int arr[] = { 9,8,7,6,5,4,3,2,1,0};
	
	int sz = sizeof(s)/sizeof(s[0]);
	int sz = sizeof(arr) / sizeof(arr[0]);
	
	//By name
	qsort(s,sz,sizeof(s[0],cmp_by_name);
	//in years
	qsort(s,sz,sizeof(s[0],cmp_by_age);
	
	qsort(arr, sz, sizeof(arr[0]), cmp_int);
	return 0;
}

Use the callback function to simulate the implementation of qsort (bubbling)

#include <stdio.h>
#include<stdlib.h>
#include <string.h>
void print_arr(int arr[], int sz)
{
	int i = 0;
	for (i = 0; i < sz; i++)
	{
		printf("%d ", arr[i]);
	}
}
int cmp_int(const void* p1, const void* p2) 
{
	return (*(int*)p1 - *(int*)p2);
}
void Swap(char* buf1, char* buf2, int width) 
{
	int i = 0;
	for (i = 0; i < width; i++)
	{
		char tmp = *buf1;
		*buf1 = *buf2;
		*buf2 = tmp;
		buf1++;
		buf2++;
	}
}
void BubbleSort(void* base, size_t num, size_t width, int (*cmp)(const void*e1, const void*e2))
{
	int i = 0;
	//Number of trips
	for (i = 0; i < num - 1; i++)
	{
		//Logarithm of comparison
		int j = 0;
		for (j = 0; j < num - i - 1; j++)
		{
			//base[j] ==> *(base+j)
			if (cmp((char*)base + j * width, (char*)base + (j + 1) * width) > 0)
			{
				//exchange
				Swap((char*)base + j * width, (char*)base + (j + 1) * width, width);
			}
		}
	}
}
void test()
{
	int arr[] = { 9,8,7,6,5,4,3,2,1,0 };
	int sz = sizeof(arr) / sizeof(arr[0]);
	BubbleSort(arr, sz, sizeof(arr[0]), cmp_int);
	print_arr(arr, sz);
}
int main()
{
	test();
	return 0;
}