How to read data from a txt-file that is written by another program

I'm creating a design optimization loop between Matlab and a simulation software. My Matlab code writes up a simulation and instructs the other program to solve it and write its results in a textfile (this part works well). To finish the loop i need a function to wait for the simulation to finish and then read the results that are in the text file. I have tried using a while-loop with with fopen, to wait for a file identifier that is not -1. But this loop continues forever, (unless i pause the process manually and restart it, then it suddenly finds it) even though the file is finished and closed by the other program in the meantime. I also have a regular expression that i know will match only with the last line of the result file so i tried a while loop that waited for a match. But since i need to use fileread to get the text in the file, and fileread uses fopen i have the same problem with it always returning a -1 file identifier. What is happening here, and does anyone have a solution?

function result=readresult(gen,n)
genstr = num2str(gen);
nstr   = num2str(n,'%04d');
filename = ['OUTPUT_generation' genstr '.txt']; % Results filename e.g. OUTPUT_generation5.txt
fid= -1;
while fid==-1
    pause(0.5);
    fid = fopen(filename);
end

matchexp = ['\s+' genstr '\.' nstr '\s+']; % This expression matches with the last line in the results-file only

exp='(\d+\.\d+)\s*(\d+\.\d+)\s*(\d+\.\d+)\s*'; % This expression matches a set of results with tokens

result = zeros(n,3);

checkmatch = {};
while isempty(checkmatch) % wait for results file to be complete
    text = fileread(filename);
    checkmatch = regexp(text,matchexp,'once'); % check if results file is complete
    
end
    result = regexp(text,exp,'tokens');

[darova]

Didn't you forget about fclose on your main code?

[Jóhann Egholm Vørmadal]

My main code has other files opened, but not this specific one.

if by main code you are referring to the function above you are right, i am missing fclose in this function. But it does not solve the problem unfortunately.

[Walter Roberson]

Is this MS Windows?

[Jóhann Egholm Vørmadal]

Yes! Windows 10 Home version 1903

[Walter Roberson]

https://www.mathworks.com/matlabcentral/answers/9957-using-net-filesystemwatcher-to-listen-for-new-files

[Jóhann Egholm Vørmadal]

Below is what managed to put together using a borrowed file system watcher code from: https://stackoverflow.com/questions/20110702/matlab-system-io-filewatcher?fbclid=IwAR3viFWHTXF7RnJAadDwmsE8feZEbkBn889RWadt6bl10rOaYRqALh9lU_c

Now i am initially calling the 'FileWatch' function instead of 'readresults'. But how can i now pass the 'results' variable back to the main function?

function listeners = FileWatch(pathToWatch, callback)
    persistent Listeners;

    % to check/clear the existing ones:
    listeners = [];
    switch pathToWatch
        case 'clear'
           Listeners = [];
           return
        case 'list'
           listeners = Listeners;
           return;
    end

    fileObj = System.IO.FileSystemWatcher(pathToWatch);
    fileObj.EnableRaisingEvents = true;

    changeListener = addlistener(fileObj, 'Changed', callback);


    if isempty(Listeners)
        Listeners = changeListener;
    else
        Listeners(end+1) = changeListener;
    end
end
    
function handlefileevent(event,n,gen)
     result = readresult(gen,n);                
end

[Walter Roberson]

http://matlab.wikia.com/wiki/FAQ#How_can_I_share_data_between_callback_functions_in_my_GUI.28s.29.3F