C language callback function [C language]

What is a callback function

Let's take a look at how Baidu Encyclopedia defines callback functions ：

A callback function is a function called through a function pointer . If you put a pointer to a function （ Address ） Pass as argument 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 the event or condition .

This passage is quite long , Also more tongue twister . Next, I will illustrate what callback is through a figure ：

Suppose we want to use a sort function to sort the array , Then in the main program (Main program) in , Let's go through the library first , Select a library sort function (Library function). But there are many sorting algorithms , There's bubble sort , Selection sort , Quick sort , Merge sort . meanwhile , We may also need to sort special objects , Such as specific structures . The library function will choose a sort algorithm according to our needs , Then call the function that implements the algorithm to complete the sorting . The called sorting function is the callback function (Callback function).

Combine this picture with the above explanation of callback function , We can find out , To implement a callback function , The key point is to pass the pointer of a function to a function ( The above figure shows the library function ), Then the function can call the callback function through this pointer . Be careful , The callback function is not C Language specific , Almost any language has callback functions . stay C In language , We implement the callback function by using the function pointer . What is the function pointer ？ take it easy , Now let's take a look at what is a function pointer .

What is a function pointer

Function pointers are also pointers , It just doesn't point to an integer , Character type, but function . stay C in , Each function is stored in memory after compilation , And each function has an entry address , According to this address , We can access and use this function . The function pointer points to the entry of this function , To call this function .

The use of function pointers

Definition of function pointer

Although the function pointer is also a pointer , But it is defined in a way that looks very different from other pointers , Let's see how it is defined ：

/*  Method 1 */
void (*p_func)(int, int, float) = NULL;

/*  Method 2 */
typedef void (*tp_func)(int, int, float);
tp_func p_func = NULL;

Both methods define a point whose return value is  void  type , Parameter is  (int, int, float)  Function pointer of . The second method is to make the function pointer easier to understand , Especially in complex environments ; For general function pointers , Just use the first method .
If you haven't seen function pointers before , You may find the definition of function pointer strange , Why not  void ()(int, int, float) *p_func  It is  void (*p_func)(int, int, float)  This form ？ I don't know this question , There's no need to tangle , Take a moment to understand the difference between it and ordinary pointers , If you can't, just remember its form first .

Assignment of function pointer

After defining the function pointer , We need to assign a value to it. We have two ways to assign a value to a function pointer ：

void (*p_func)(int, int, float) = NULL;
p_func = &func1;
p_func = func2;

Both of the above methods are legal , For the second method , The compiler implicitly converts  func_2  from  void ()(int, int, float)  Type conversion to  void (*)(int, int, float)  type , therefore , Both methods work . For more detailed instructions , Take a look at the following stackoverflow Of link .

Call a function... Using a function pointer

Because function pointers are also pointers , Therefore, a conventional belt can be used  *  Method to call the function . The same as the assignment of function pointer , We can also use two methods ：

/*  Method 1 */
int val1 = p_func(1,2,3.0);

/*  Method 2 */
int val2 = (*p_func)(1,2,3.0);

Method 1 It's the same as we usually call functions directly , Method 2 It's used  *  Value the function pointer , So as to realize the call to the function .

Pass the function pointer as an argument to the function

Function pointers are the same as ordinary pointers , We can pass it to the function as a parameter of the function , Now let's see how to realize the parameter passing of function pointer ：

/* func3  Put the function pointer  p_func  As its formal parameter  */
void func3(int a, int b, float c, void (*p_func)(int, int, float))
{
    (*p_func)(a, b, c);
}

/* func4  Call function func3 */
void func4()
{
    func3(1, 2, 3.0, func_1);
    /*  perhaps  func3(1, 2, 3.0, &func_1); */
}

Function pointer as function return type

With the above foundation , It should not be difficult to write a function whose return type is a function pointer , The following example is a function whose return type is a function pointer ：

void (* func5(int, int, float ))(int, int)
{
    ...
}

ad locum , func5  With  (int, int, float)  Is the parameter , Its return type is  void (*)(int, int) . stay C In language , The declaration of variables or functions is also a big learning , Want to know more about the topic of declaration , Please refer to my previous articles - C Experts programming 》 Reading notes (1-3 Chapter ). The third chapter of this book spends a whole chapter explaining how to read C Declaration of language .

Function pointer array

Before we start talking about callback functions , Finally, let's introduce the function pointer array . Since the function pointer is also a pointer , Then we can use arrays to store function pointers . Let's take a look at an example of a function pointer array ：

/*  Method 1 */
void (*func_array_1[5])(int, int, float);

/*  Method 2 */
typedef void (*p_func_array)(int, int, float);
p_func_array func_array_2[5];

Both of the above methods can be used to define the function pointer array , They define a number of elements as 5, The type is  void (*)(int, int, float)  An array of function pointers for .

Callback function

What we talked about earlier is function pointers , Let's get back to the point , Let's see how the callback function is implemented . The following is a simple callback function example of four operations ：

#include <stdio.h>
#include <stdlib.h>

/****************************************
 *  Function pointer structure 
 ***************************************/
typedef struct _OP {
    float (*p_add)(float, float); 
    float (*p_sub)(float, float); 
    float (*p_mul)(float, float); 
    float (*p_div)(float, float); 
} OP; 

/****************************************
 *  Addition, subtraction, multiplication and division function 
 ***************************************/
float ADD(float a, float b) 
{
    return a + b;
}

float SUB(float a, float b) 
{
    return a - b;
}

float MUL(float a, float b) 
{
    return a * b;
}

float DIV(float a, float b) 
{
    return a / b;
}

/****************************************
 *  Initialize function pointer 
 ***************************************/
void init_op(OP *op)
{
    op->p_add = ADD;
    op->p_sub = SUB;
    op->p_mul = &MUL;
    op->p_div = &DIV;
}

/****************************************
 *  Library function 
 ***************************************/
float add_sub_mul_div(float a, float b, float (*op_func)(float, float))
{
    return (*op_func)(a, b);
}

int main(int argc, char *argv[]) 
{
    OP *op = (OP *)malloc(sizeof(OP)); 
    init_op(op);
    
    /*  Call a function directly using a function pointer  */ 
    printf("ADD = %f, SUB = %f, MUL = %f, DIV = %f\n", (op->p_add)(1.3, 2.2), (*op->p_sub)(1.3, 2.2), 
            (op->p_mul)(1.3, 2.2), (*op->p_div)(1.3, 2.2));
     
    /*  Call callback function  */ 
    printf("ADD = %f, SUB = %f, MUL = %f, DIV = %f\n", 
            add_sub_mul_div(1.3, 2.2, ADD), 
            add_sub_mul_div(1.3, 2.2, SUB), 
            add_sub_mul_div(1.3, 2.2, MUL), 
            add_sub_mul_div(1.3, 2.2, DIV));

    return 0; 
}

This example is a little long , I explain step by step how to use callback functions .

First step

To complete addition, subtraction, multiplication and division , We need to define four functions to realize the operation of addition, subtraction, multiplication and division , These functions are :

/****************************************
 *  Addition, subtraction, multiplication and division function 
 ***************************************/
float ADD(float a, float b) 
{
    return a + b;
}

float SUB(float a, float b) 
{
    return a - b;
}

float MUL(float a, float b) 
{
    return a * b;
}

float DIV(float a, float b) 
{
    return a / b;
}

The second step

We need to define four function pointers to these four functions ：

/****************************************
 *  Function pointer structure 
 ***************************************/
typedef struct _OP {
    float (*p_add)(float, float); 
    float (*p_sub)(float, float); 
    float (*p_mul)(float, float); 
    float (*p_div)(float, float); 
} OP; 

/****************************************
 *  Initialize function pointer 
 ***************************************/
void init_op(OP *op)
{
    op->p_add = ADD;
    op->p_sub = SUB;
    op->p_mul = &MUL;
    op->p_div = ÷
}

The third step

We need to create one “ Library function ”, This function takes the function pointer as an argument , Use it to call different functions ：

/****************************************
 *  Library function 
 ***************************************/
float add_sub_mul_div(float a, float b, float (*op_func)(float, float))
{
    return (*op_func)(a, b);
}

Step four

When these films are finished , We can call the callback function ：

/*  Call callback function  */ 
printf("ADD = %f, SUB = %f, MUL = %f, DIV = %f\n", 
        add_sub_mul_div(1.3, 2.2, op->p_add), 
        add_sub_mul_div(1.3, 2.2, op->p_sub), 
        add_sub_mul_div(1.3, 2.2, MUL), 
        add_sub_mul_div(1.3, 2.2, DIV));

The callback function can be implemented in four simple parts . In these four steps , We can even omit the second step , Pass the function name directly into “ Library function ”, For example, the above multiplication and division operations . The core of the callback function is the function pointer , As long as you understand the function pointer, you can learn the callback function , That's easy to catch .

summary

This article mainly talks about how to use function pointers and callback functions . The core of the callback function is the function pointer , So I spent a lot of time explaining function pointers . For the implementation of callback function , I gave an example , I hope this example can help you . Callback function is very important , If you can't even do it ,C Language is really not a beginner . Yes, of course , Even if I can do it , Don't be proud , because C There are still too many things we need to learn about language 、 practice .

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