Callback function for UDP set with DatagramReceivedFcn is invoking only for one time?

After the first execution of UDP callback function system gives a warning message of "Warning: The DatagramReceivedFcn is being disabled. To enable the callback property either connect to the hardware with FOPEN or set the DatagramReceivedFcn property." in command window. And the callback function does not invoke again. Code for setting the UDP object is as follows :

global objudp1   %%%%global variable for udp object
strClientIP=get(handles.edClientIPAddr,'string');
objudp1=udp(strClientIP,2000,'LocalPort',2500);
objudp1.ReadAsyncMode = 'continuous';
objudp1.DatagramReceivedFcn  ={@OnDataReceivedOnUdp1};%%%,handles};
objudp1.DatagramTerminateMode='On';
flushinput(objudp1);
fopen(objudp1);

[Walter Roberson]

I suspect the flushinput() should go after the fopen()

[Dominic]

Thanks.

Now I changed the code as follows :

/*******************************************/
global objudp1   %%%%global variable for udp object
strClientIP=get(handles.edClientIPAddr,'string');
objudp1=udp(strClientIP,2000,'LocalPort',2500);
objudp1.ReadAsyncMode = 'continuous';
objudp1.DatagramReceivedFcn  ={@OnDataReceivedOnUdp1};%%%,handles};
objudp1.DatagramTerminateMode='On';
fopen(objudp1);
flushinput(objudp1);
/*******************************************/

But still the problem exists.

Answer 1

[Walter Roberson]

Looking at http://www.mathworks.com/matlabcentral/newsreader/view_thread/294903 I see a hint that the bit about the callback being disabled might be due to the connection breaking from the remote end (which would be an unusual thing for udp), or due to the udp object being destroyed or going out of scope.

Ah, and suppose the LocalPort is already in use, then the fopen() would not do what you want and the callback would probably disable itself.

Answer 2

[Greg]

That warning is almost always due to an error in your DatagramReceivedFcn code. This thread is a year old, so I doubt the original poster will come back to it, but for anyone still having this issue: debug your DatagramReceivedFcn for code errors.

[Duijnhouwer]

This was my problem, thanks!

Specifically, I was using DatagramReceivedFcn in a object oriented context, so my handle was

O.udp_connection.DatagramReceivedFcn = @O.parse_message;

The parse_message method needs 3 arguments then, like

classdef test < handle
    properties
        u;
    end
    methods
        function O=test
            O.u=udp('127.0.0.1','RemotePort',5050,'LocalPort',5051);
            O.u.DatagramReceivedFcn = @O.parse_message;
            fopen(O.u);
        end
        function parse_message(O,~,~)
            msg=fscanf(O.udp_connection)
            % parsing msg here....
        end
    end
end

If it doens't have three methods the OP's error message is thrown before even entering parse_message, which precluded waiting there with a breakpoint to debug the body...