While loop stuck in callback function

Question

Maheen Siddiqui 2016-4-1

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/276600-while-loop-stuck-in-callback-function

评论： Image Analyst 2016-4-5

I have written a callback function in Matlab. My laptop is communicating with another laptop that is sending it bytes every few seconds that are recorded in a text file. For e.g. the laptop sends "66" and my laptop writes to the file Event_Markers.txt "66" continuously until the other laptop sends something else. The code is below.

The problem that I am currently facing is that in my callback function (below) I use a while loop to continuously write the same "information" (e.g. "66") to the text file until the other laptop sends something else. But this while loop gets stuck. This part is of a larger script that is acquiring data from a spectrometer and adding it to my script and causes everything to become stuck and the rest of the script is not executed. I tried to use an if loop instead of while and it only writes "66" twice instead of writing it continuously. It is, however, writing to the text file as I want it to.

Does anybody know if I need to add some other line of code to stop it becoming stuck? I have tried to add in a break but that results in the same problem as when I have an if loop, it only records "66" twice instead of continuously.

Thanks!

appenderFile=fopen('Event_Markers.txt','a+t');
s=serial('COM3');
set(s,'BytesAvailable',{@myCallback,appenderFile});
set(s,'BytesAvailableFcnCount',1);
set(s,'BytesAvailableFcnMode','byte');
fopen(s);
 function myCallback(s,~,appenderFile)
    bytes=(s,'BytesAvailable')
    if(bytes==1)
         [data count msg] = fread(s,bytes);
    end 
    fprintf(appenderFile,'%d \n',data);
    bytes=(s,'BytesAvailable');
    while bytes==0
         fprintf(appenderFile,'%d \n',data);
         bytes=get(s,'BytesAvailable');
      %if bytes~=0
        %break 
      %end
    end
end

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Answer 1

Image Analyst 2016-4-1

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在 MATLAB Online 中打开

Never use a while loop without a failsafe or else you get the problem you did. So you can put a loop counter on there

loopCounter = 1;
maxNumLoops = 1000; % Whatever you'd expect the max to ever get to.
while condition && loopCounter < maxNumLoops
    % code
    % Increment loop counter
    maxNumLoops = maxNumLoops + 1;
end

Or else use tic and toc to bail out after so many seconds

maxSecondsToWait = 5; % Whatever
startTime = tic;
elapsedSeconds  = 0;
while condition && elaspedSeconds < maxSecondsToWait 
    % code
    % Update elapsed time.
    elapsedSeconds = toc(startTime); 
end

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Maheen Siddiqui 2016-4-4

I call fclose at the end of the bigger script.

After debugging I found that it was the fread and fprintf which are very slow. But I'm unsure what I could do instead? Do you think I should save "data" in an array instead of writing to a file?

Image Analyst 2016-4-5

You could use SSDs instead of HDs.

Or, if you have your data originate from the same session of the same program, then sure, keep it as an array in memory and do not write to a temporary file on the drive. It will be faster.

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