Callback function

[engine_motor]

How this Callback function working ,is it using pointer or function pointer? 

int CVICALLBACK LatchStatus (int panel, int control, int event,
		void *callbackData, int eventData1, int eventData2)
{
		switch (event)
		{
		case EVENT_COMMIT:
			SetCtrlVal(); 
		        break;
		}
	return 0;
}

[RobertoBozzolo]

Can you elaborate a bit more on your question? The callback paradigm makes that the associated function is called whenever an event is raised on a control or panel. Where are you wondering a pointer should be?

[engine_motor]

 The callback paradigm makes that the associated function is called whenever an event is raised on a control or panel

I agree  to your words,I Googled what is funtion pointer and where its using I got a answer

"callback function is a functionthat is called through a function pointer. If you pass the pointer (address) of a function as an argument to another, when that pointer is used to call the function it points to it is said that a call back is made."

how this related into our Labwindows CVI

[RobertoBozzolo]

The callback concept refers to functions passed as an argument to other functions: there is no such framework in the code you have posted.

I suppose, though, this is the concept that informs the run-time engine framework: the callbacks associated to controls and panels are passed (as a pointer?) to the OS which is in charge of handling machine and user events and call the associated function. Nevertheless, all this mechanism is hided to you: you must not worry about it neither you can alter it in any way.

[engine_motor]

Okay thank you, its just for my knowledge

[engine_motor]

In this code using both callbackfuntion and funtion pointer ,actually what its how its differ from normal funtion

int CVIFUNC RegisterTCPServer(unsigned portNumber, tcpFuncPtr pUserCallback, void *callbackData);
void main()
{
        if(RegisterTCPServer (iServerPortID, fnServerTCPCallback, NULL) < 0)    
	   MessagePopup("TCP Server", "Server registration failed!")
	 else
             do something
}

 /*---------------------------------------------------------------------------*/
/* TCP callback function for the server.                                     */
/*---------------------------------------------------------------------------*/
static int CVICALLBACK fnServerTCPCallback (unsigned int handle, int xType, 
                                       int errCode, void *cbData)
{
	static int i = 0;
    if (xType == TCP_CONNECT)
       else if (xType == TCP_DISCONNECT)
            else if (xType == TCP_DATAREADY)
    return 0;
}

[RobertoBozzolo]

This is the exact case of a function pointer: passing a function as an argument to a function. This can be used to tailor the behaviour of a function.

This mechanism may be a little more clear if looking at qsort function: it accepts a comparison function that you may declare and code yourself if standard ShortCompare, IntCompare... and so on cannot be used. As an example, I have used qsort to sort an array of elements defined as a structure, writing my own comparison function to sort on one structure field.

As you can see, qsort is a general tool that can be tailored by passing your own sorting algorithm coded in a function.

Another example and a brief discussion of this technique can be found here.

(Note: ShortCompare and similar functions are defined in the Programmer's Toolbox and you may look at both the definition and the actual code since the source of this library is distributed)