Too many output arguments while launching workers with parfeval

I try to write and read in parallel to/from a hardware device (under Linux) for the purpose of testing the device and it must happen infinitely until instructed to stop.

script:

write_file='testdata';
read_file='result';
pattern=int16([ 0x0102, 0x0304, 0x0506, 0x0708; %test pattern is chosen so individual bytes could be identified
                0x090a, 0x0b0c, 0x0d0e, 0x0f10;
                0x1112, 0x1314, 0x1516, 0x1718;
                0x191a, 0x1b1c, 0x1d1e, 0x1f20]);
repetition=128;
testdata=repmat(pattern, 1, 8192*repetition); %replicate for 32768* repetition test data points
read_dim=[4, 32768*repetition];
write_thread=parfeval(@writeXillybus, 2, write_file, testdata);
read_thread=parfeval(@readXillybus, 2, read_file, read_dim);
pause %let the workers run indefinitely

cancel(write_thread);
cancel(read_thread);

functions:

function out=writeXillybus(device, testdata)

test_fid=fopen(['/dev/xillybus_', device], 'w');%open device file

cleanup = onCleanup(@() fclose(test_fid));
while(true)
    out=fwrite(test_fid, testdata,'int16');

end

end


function out=readXillybus(device, format)

data_fid=fopen(['/dev/xillybus_', device], 'r'); 
cleanup = onCleanup(@() fclose(data_fid));
    while(true)
        out=fread(data_fid, format, '*int16');
 
    end

    
    
end

However the workers crash with future reporting. 'Too many output arguments.' I have trouble making sense out of it, because it rises deep from Matlab internals: remote cause shows line 39 of toolbox/parallel/cluster/+parallel/+internal/+queue/evaluateRequest.m as origin.

Anyway, isn't parfeval supposed to be able to handle arbitrary number of outputs?

Answer 1

[Yongjian Feng]

According to the help of parfeval, the second input argument is NUMOUT. You gave 2?

[Quant]

I read docs a few times, but every time I somehow understood it refers to the input arguments. what an embarassing lapse in attention