C中奇怪的结构声明方法

Question

This is from the header file for the library functions of CSR8670 Bluetooth chip

typedef struct TaskData { void (*handler)(Task, MessageId, Message); } TaskData;  

What kind of structure declaration is this? What are the member data for this structure?
Here is the full header file for context:

/* This file was automatically generated from syscalls.in 17.2 */

#ifndef __MESSAGE__H

#define __MESSAGE__H

#include <csrtypes.h>
/*! @file message_.h @brief Message types */
/*!
Message identifier type.
*/
typedef uint16 MessageId;
/*!
Message delay type.
*/
typedef uint32 Delay;
/*!
Message type.
*/
typedef const void *Message;
/*!
Task type.
*/
typedef struct TaskData *Task;
/*!
TaskData type.
*/
typedef struct TaskData { void (*handler)(Task, MessageId, Message); } TaskData;

#endif  

I am still not sure what *handler means. I have not been able to find any other references to handler in the other header file. If it's relevant, Task represents a sort of routine running on the firmware that accepts and processes the message that the firmware may receive from external sources (for example, a bluetooth device trying to connect to the CSR board).

Answer 1

struct TaskData只有一个成员，它是一个函数指针

void (*handler)(Task, MessageId, Message);

Answer 2

handler is a pointer to a function that returns void and has parameters with types Task , MessageId , and Message in that order.

TaskData is a structure containing that one member.

It's probably used by some library function to call a function that the user of that library has to define. (These are known as callback functions and are idiomatic in C.)

Answer 3

void (*handler)(Task, MessageId, Message);

This is a function pointer, which is the only member of the structure struct TaskData .

The function pointer is a pointer to a function which should have

• Return type as void .
• Three input parameter of type Task , MessageId , Message , respectively, which are again some typedef s.

---

EDIT:

Usage

as Mentioned in the comment below, for a variable TaskData task; the access should be [in pseudocode]

// void somefunc(Task t, MessageId mid, Message m) is the function
task.handler = somefunc;  

and

Task p;
MessageId q;
Message r;

task.handler(p,q,r);   
//function somefunc() will be called with argument p, q,and r

Answer 4

typedef struct TaskData { void (*handler)(Task, MessageId, Message); } TaskData;

The only member of this structure is:

void (*handler)(Task, MessageId, Message);

ie a function pointer named handler which can point to a function which returns void & takes arguments of type Task , MessageId , and Message

This can be accessed for example like this:

typedef struct
{
  void (*hand)(int a);
} str;

void func(int a)
{
  printf("Value of a = %d\n", a);
}

int main ()
{
  str var;

  var.hand = func;
  var.hand(25);

  return 0;
}

Answer 5

Any thing inside a structure is its member. handler is a function pointer to a function which accepts three arguments of types Task, MessageId, Message and returns void . This is a member of structure.

The main use of function pointers in structure is used to get the object oriented feature of Polymorphism (virtual function) in C.