How to fix a simple callback function with a tcpip object?

Hi Everyone,

I have a very simple problem: I am trying to get a callback function to execute after receiving data through a tcp/ip connection. I have been reading the asynchronuous writing and reading section of the MATLAB documentation here, and I am essentially reproducing what they suggest without any success.

Here is a simple script which illustrates my issue, when I execute it, I get a series of error and warning messages:

echotcpip('on',4000);
t = tcpip('127.0.0.1',4000);
t.BytesAvailableFcn = 'dispcallback';
fopen(t);

% Send some data:
fprintf(t, 'Test 123');

When I run the above script, I get these messages:

Error using dispcallback (line 13)
Not enough input arguments.

Error in instrcb (line 20)
        evalin('base', val);

Warning: The BytesAvailableFcn is being disabled. To enable the callback property
either connect to the hardware with FOPEN or set the BytesAvailableFcn property.

Why is this? How can I properly invoke the callback function? I have tried with MATLAB 2014b and 2015b with the same result.

Thanks!

Answer 1

[Dave Behera]

Try replacing the line

 t.BytesAvailableFcn = 'dispcallback';

with

 set(t, 'BytesAvailableFcn', @dispcallback)

In the former, the event arguments were not being passed to the callback function, for some reason. Using the 'set' command should fix the issue.

[Andy]

is there a way to open a useful udp server in Matlab (no echoudp...)?

Answer 2

[Andy]

Should we specify a function:

function dispcallback(obj,event)
    callbackTime = datestr(datenum(event.Data.AbsTime));
    fprintf(['A ' event.Type ' event occurred for ' obj.Name ' at '  callbackTime '. Obj is the UDP object. Event is a struct w/ two fields: Type and Data. Data received over UDP: ' fscanf(obj) '\n']);        
end

is required in the execution folder ... dispcallback doesn't seem to be available by default.

Main code:

% this opens a UDP server listening on ALL local IPs and port 8000
echoudp('on', 8000);
% this enacts a client conneting into 127.0.0.1:8000
u = udp('127.0.0.1', 8000);
fopen(u);
u.Status

% the below makes sure anything sent back to the client, gets echoed through the dispcallback
u.ReadAsyncMode = 'continuous';
set(u, 'BytesAvailableFcn', @dispcallback);

Run:

% this writes / sends an udp packet towards 127.0.0.1:8000
fprintf(u, 'Hello net.')

Output:

% Since echoudp will echo back anything you throw at it ... the below is the result of us running the fprintf above command.
"A BytesAvailable event occurred for UDP-127.0.0.1 at 02-Apr-2020 16:00:29. Obj is the UDP object. Data: Hello net."