matlab bytesAvailableCallback with empty buffer

When opening any communication instrument in matlab, you can set the object with a BytesAvailableFcn. i.e.

u.BytesAvailableFcn = @bytesAvailable_callback;

Every time the callback is called, it clears the buffer completely. For example, with a udp client:

function bytesAvailable_callback(udp_obj, event)

bytesAvailable = get(udp_obj, 'BytesAvailable');
fprintf('callback - bytesAvailable = %d\n',bytesAvailable);
while bytesAvailable
    data = fread(udp_obj, bytesAvailable, 'int32');
    fprintf('rec = %d\n',data(1));
    bytesAvailable = get(udp_obj, 'BytesAvailable');
end
end

what happens next is the problem: the buffer has been cleared but the BytesAvailableFcn is still in the call stack and will be called many time, only to return nothing.

an example running the above callback with udp:

clc
echoudp('on', 8000)
%
u = udp('127.0.0.1', 8000);
u.OutputBufferSize = 10000;
u.InputBufferSize = 10000;

u.BytesAvailableFcn = @bytesAvailable_callback;
u.BytesAvailableFcnCount = 10;
u.BytesAvailableFcnMode = 'byte';

fopen(u);
%

for i = 1:2
    fprintf('send = %d\n',i );
    fwrite(u, [i ones(1,10)], 'int32');
end
pause(1)
fclose(u);
delete(u);
clear u
echoudp('off');

the results are:

send = 1
send = 2
callback - bytesAvailable = 88
rec = 1
rec = 2
callback - bytesAvailable = 0
callback - bytesAvailable = 0
callback - bytesAvailable = 0
callback - bytesAvailable = 0
callback - bytesAvailable = 0
callback - bytesAvailable = 0
callback - bytesAvailable = 0

We can easlly see the unwanted behaviour - callback called when the InputBuffer is empty. The above can be very demanding in a busy program where every callback counts. Is there a way to "flush" the above unwanted callback calls?

Using drawnow did not help - it executed the callbacks, so still accesses the function.