matlab bytesAvailableCallback with empty buffer - matlab

Question

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.

Answer 1

I have the same problem but found a workaround. I wrote a program that would read 1 byte at a time until it reached a terminator (matlab did not automatically recognize the terminators. After adding a line like this:
if u.bytesavailable == 0
fprintf(['function was called but buffer is epty' 13])
end
I noticed it would be called once for every byte I read -1. Apparently every event changing the size of the buffer (e.g. reading a byte) creates a call to your readout function again. What I found worked was to read not the entire buffer at once but the size that triggers the event. So for instance:
fread(u,BytesAvailableFcnCount)
should leave you with no more extra function calls at the cost of speed. It works for me for small amounts of data, but I am using a serial connection, not a udp one.
I tried disabling the bytesavailablefcn inside the callback function itself, as to disable any function calls shile the function is reading but I still got a load of function calls with an empty buffer (I don't know why).
Ron