How to run parallel optimization when fitness function uses input/output files for [X] and [F].

Hi, I am "gamultiobj" for multiobjective optimization. my fitness function is as below. It calls a Fortran compiled file (CNEW.exe) to calculate the objective functions. Input variables [X] from the optimiser are written to a file "xinput" which is read by CNEW.exe and it write the function values in "fout.txt". All works fine when fitness function evaluation setting is on "in serial". I tried to use paralell computation in MATLAB to reduce the time. It seems that parallel processing works fine as I can see in the task manager each core runs an individual CNEW.exe task. But since all of them communicate with the same in/out files on the hard drive, the oucome is terribly mixed and optimization goes wrong. Can someone please tell me what should I do?

function y = mymgt(x)
%write the x data
xh = fopen('xinput.txt', 'w');
fprintf(xh, '%f %f %f\n', x);
fclose(xh);

%run the program
system('CNEW.exe');
%read the f data

fh = fopen('fout.txt');
y = fscanf(fh, '%f  %f');
fclose(fh);

end