weird structure declaration method in C

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

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.)