callback function (Nice to understand) __ function

callback function

When looking at LWIP, see Using callback function, look at a foreigner company OPC source code, see Use callback function. Look at some of my domestic code (my company software, etc.) is useless. So, I have a lot of curiosity about the callback function. I used to use the callback function when I wrote the VC program, but I didn't use it in C language. Recently, see a lot of classical code abroad widely used in the callback function (LWIP, the OPC program of two companies, etc.), are the C language to achieve, rather than VC Windows program for others to achieve their own use of the kind.

To understand the mystery of this function, first ask three questions:

1. What the callback function is.

2. callback function How to develop, how to use.

3. The function of the callback function, should be used under what circumstances.

To learn with questions, with a purpose. Oh, personal experience.

Open baidu.com, google.cn search for a lot of information, as follows:

By the way, one of the gentleman's signature I admire: 1 Live Well, because we will die for a long time. 2 5,000 years of civilization 200 years of helplessness

First question:

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In fact, the callback is a function of the function of the pointer to call the process.

Why do you use a callback? For example, I want to write a sub module for you to receive remote socket Send command. When I receive the command, I need to call the function of your main module to handle it accordingly. But I don't know which function you're going to use to handle this command, and I don't know what your main module is. cp   P or. h, or, I don't care what you do with it in the main module, or what functions you use to handle it ... What to do?

Use callback!

--lone Wolf

Using a callback function is actually a function (usually an API function) that passes the address of one of its functions (which is the callback function) as an argument to that function. And that function, when needed, invokes the callback function with the address passed, at which point you can use this opportunity to process the message in the callback function or to complete a certain operation.

--An expert

The callback function is written by yourself. You need to call another function, and one of the arguments to this function is the name of your callback function. This way, when necessary, the system invokes the callback function that you write, so you can do what you want to do in the callback function.

--Green leaves

Http://hi.baidu.com/zhuyipeng/blog/item/863fefdb7c736c63d1164eec.html is a relatively good article.

What is a callback function?
A callback function is a function that an application provides to a Windows system DLL or other DLL, typically used to intercept messages, get system information, or handle asynchronous events. The application tells the DLL the address pointer of the callback function, and the DLL calls the function when it is appropriate. The callback function must conform to the predetermined parameter format and delivery method, or the DLL's invocation of it can cause the program or system to crash. In general, the callback function takes the standard WINDOWSAPI invocation, that is, the __stdcall, of course, the DLL can define the invocation method on its own, but the client program must follow the same rules. In the __stdcall mode, the function's arguments are pressed from right to left in the stack, except explicitly indicating that the pointer or reference, the parameters are passed by value, the function returns before returning the parameter from the stack.
Understand the callback function!

--jufengfeng

Function pointers provide the concept of callback functions.

--newty.de

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After reading so much information, I only summed up the definition of each one in one sentence: The callback function is a use of the function pointer.

In part of the data, a lot of discussion about how the callback function is called, who is called, there are many graphics, I do not see the nature of the problem.

Second question:

*********************************************************************

I implemented a very simple callback function.

#include <stdio.h>

void printwelcome (int len)

{

printf ("Welcome welcome--%d/n", Len);

}

void Printgoodbye (int len)

{

printf ("A visitor to the visitor-%d/n", Len);

}

void callback (int times, void (* print) (int))

{

int i;

for (i = 0; I < times; ++i)

{

print (i);

}

printf ("N/a) I do not know whether you are welcome or see a visitor!/n/n");

}

void Main (void)

{

Callback (Printwelcome);

Callback (Printgoodbye);

Printwelcome (5);

}

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The above code is not called by any system function, which means that those things are just the eyes of the lost man in the scattered soil. Also have the object programming, with class to encapsulation is also to deceive, don't be puzzled by appearance.

Third question:

*********************************************************************

STL people know that in the STL many algorithms and procedures are used to callback functions, which implements a strategy. As long as any functions and calculations that meet my criteria can be used in my formula. You can implement a variety of callback functions, as long as they fit in my format.

As far as the above program is concerned, you can work as long as the function format conforms to the cllback of the second parameter, whether you cook for someone or make a bed. This is the role of the callback, leaving the callback implementation to others.

This is a usage.

A friend explained the callback mechanism with a layered concept: The callback function is B, the main function and the print* function are layer A, the layer a calls the B-level callback function Callmeback, and the B-level callback function calls the implementation function print* of layer a. Plainly, the B-layer is an interface.

This is my understanding. Over.

Turn from: http://blog.csdn.net/callmeback/article/details/4242260