Embedded development: embedded foundation callback function

　　 Callback function is a basic and often critical concept for developers to create drivers or custom libraries . A callback function is a reference to executable code , It is passed as a parameter to other code , Allow lower level software layers to call functions defined in higher level layers (10). Callbacks allow driver or library embedded developers to specify behavior at a lower level , But leave the implementation definition to the application layer .

　　 The simplest callback function is just a function pointer , It's passed as an argument to another function . in the majority of cases , The callback will consist of three parts ：

　　• Callback function

　　• Callback registration

　　• Callback execution

　　 The following figure shows how these three parts work together in a typical callback implementation .

　　

 

　　 First , Developers create libraries or modules that will have implementation elements determined by the application developer . for example , The developer created a GPIO The driver , The driver has an interrupt service routine , The code is specified by the application developer . Interrupts can handle button presses or other functions . The driver doesn't care about functionality , It is only concerned that at runtime it knows what function should be called when an interrupt is triggered . The code that calls a callback function in a module is often called a signal handler .

　　 Next , There needs to be some way to tell the underlying code what functions to execute . There are many ways to do this , But for driver modules , The recommended practice is to create a function in the module that is specifically used to register the function as a callback . Having a separate function to register a callback function makes it clear to embedded developers that the callback function is being registered to a specific signal handler . When a register function is called , Pass the called required function as a parameter to the module and store the function address .

　　 Last , Application developers write their applications , This includes the creation of callbacks and the implementation of initialization code , This code registers the function in the library or module . When executing an application , Low level code stores the callback function address , When you need to perform a function , It dereferences the callback function and executes it .

　　 There are two main examples that developers can consider using callbacks . First , In the driver , The developer will not know how the final application might need to use any interrupt service program . If the developer is creating libraries for some microcontroller peripherals , You can use callbacks to specify all interrupt behaviors . Using callbacks will allow developers to ensure that if the application developer does not register a custom callback function , Each interrupt has a default service routine . When callbacks are used with interrupts , Developers need to remember , Need to follow best practices for interruptions .

　　

 

　　 secondly , As long as there are common behaviors in the application that may have implementation specific behaviors , You can use callbacks . for example , Initializing arrays is a very common task , Need to execute in the application . If for some applications , Embedded developers want to initialize array elements to all zeros , In another application, they want to initialize array elements to random numbers ? under these circumstances , They can use callbacks to initialize arrays .

　　ArrayInit Function accepts a pointer to an array with element size , Then it takes a pointer to the function that returns an integer . The function at this time has not been defined , But it can be defined by the application code . When calling ArrayInit when , Developers will pass any function they choose to initialize array elements .

　　 Function with callback ：

　　void ArrayInit(int * Array, size_t size, int (*Function)(void))

　　{

　　for(size_t i = 0; i < size; i++)

　　{

　　Array[i] = Function();

　　}

　　}

　　 Initialize element to 0：

　　int Zeros(void)

　　{

　　return 0;

　　}

　　 Initialize the element to a random number ：

　　int Random(void)

　　{

　　return rand();

　　}

　　 function Zeros or Random Be passed on to ArrayInit, It depends on how the application embedded developer wants to initialize the array .