Pointer - function pointer

Catalog

1 What is a function pointer ？

2 Function pointer standard

        2.1 Meaning of function name

        2.2 Function （） The meaning of

        2.3 A function pointer

        2.4 Assignment and use of function pointer

3 Two interesting function pointers

        No.1

        No.2

        No.2 Simplification of ——typedef

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1 What is a function pointer ？

Let's make an analogy

	int* pa;   // Integer pointer 
	char* pb;  // Character pointer 
	int(*pc)[3];  // Array pointer , Point to storage 3 individual int Array of 
	char(*pd)[3]; // Array pointer , Point to storage 3 individual char Array of 

seeing the name of a thing one thinks of its function , A function pointer is a pointer to a function

2 Function pointer standard

        2.1 Meaning of function name

When we study arrays, we know that the array name indicates the address of the first element of the array ,& The array name represents the address of the entire array , Because arrays occupy a continuous space in space , So is the function . The function name represents the address of the function , But it is a little different from the array name ：& Function name is equivalent to function name .

#include<stdio.h>
void aa(void) {    // Definition aa function 
	printf("123");
}
int main(void) {

	void(*p)(void);  // Define a function pointer , Don't worry about this 
	p = aa;
	printf("%p\n", p);    
	printf("%p\n", *p);
	printf("%p\n", aa);
	printf("%p\n", &aa);    // The four results are exactly the same 

}

        2.2 Function （） The meaning of

We usually call functions , Is a function name plus a （）,（） According to the situation, put the corresponding parameters . Remember the array 【】 Do you ？ It means to dereference the pointer on the left after operation .（） It means to access the address on the left after passing in parameters （ It doesn't work here * Visit it ,（） The function is defined in the design ）.

        2.3 A function pointer

int Add(int a,int b){}  // A function definition 

int (*p)(int, int);    // A function pointer variable definition ,p Precede * Union knowing is a pointer 
					// And again （） combination , Indicates a pointing function .
					// With the int,int The function pointed to by the combination representation has two parameters 
					// Finally and int combination , Indicates that the pointed function returns a int

// About why brackets should be added , because p It will precede the one on the right （） The combination results in a return value of int* Function of type 
// priority *<[]<()

        2.4 Assignment and use of function pointer

When assigning an array pointer , We assign the address of the array to it , It can be considered as a type one level higher than the first level pointer （& Array name ）. The same is true of function pointers , We assign the address of the function to the function pointer variable , Because it has no first element address , So it can be regarded as a first level pointer .

#include<stdio.h>
int Add(int a, int b) {
	printf("%d\n", a + b);
	
	return a + b;
}

int main(void) {
	int (*p)(int, int);  
	p = Add;   // Assign a value to it 

	Add(2, 3);
	(*p)(2, 3);		//p=Add,（） Represents a dereference , Then why do we have * Well ？
    (p)(2,3);
	(**p)(2, 3);	
	(***p)(2, 3);	// there * It's just a decoration      
	printf("%d", (*p)(2, 3)); // The program will print twice in this step 5
	
}

3 Two interesting function pointers

        No.1

	(*(void(*)())0)()

    // First look at void(*)() This is a function pointer type
    // Yes 0 Cast (void(*)())0
    // You may wonder ,0 Can I also cast types ？ As I said before, data is in memory , It's space + The way （ The rules ）
    // That is, the type , So type conversion is in the computer unless otherwise specified , Otherwise, it would be “ mandatory ”.
    // This time is equivalent to defining a point 0 Array pointer variable of    (void(*)())0 Here we use p Show me
    // Replace it ,(*p)(), Call this function .

It can also be written like this

	void(*p)();    
	p = 0;
	(*p)();  // take p as  (void(*)())0  that will do 

        No.2

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

    // First we see signal,（） Priority is greater than *, This is a function
    // Parameters are integer and function pointer types , No variable is known as a function declaration
    // What about the return value ？ Let's look at the function of normal point

int Add(int , int );
int            // Add the function name and （） What is left is the type of return value 

    // take signal（int,void（*）（int）） Get rid of
    // be left over void(*)(int), Is the type of return value ： Function pointer type

, describe ：

//signal Is a function description
//signal There are two arguments to the function , The first is int. The second is function pointers , The function pointer points to a function whose argument is int, The return type is void
//signal The return type of a function is a function pointer , The function pointer points to a function whose argument is int, The return type is void

void(*)(int)    signal   (int, void(*)(int));
// It's understandable , But it can't be written like this 

        No.2 Simplification of ——typedef

We know typedef You can add names to types with complex names , Here you can also use the function pointer type name

	typedef void(*)(int) fun_p; // Normally it is written like this , But function pointers are a little special 
	typedef void(*fun_p)(int);   // It should be written like this 
	fun_p signal(int, fun_p);   // Function declarations can be simplified