atomically creating a file lock in MATLAB (file mutex)

Question

I am looking for a simple already implemented solution for atomically creating a file lock in MATLAB.

Something like: file_lock('create', 'mylockfile'); %this will block until it creates the lock file. file_lock('remove', 'mylockfile'); %this will remove the lock file:

This question has already been asked several times, with some proposed solution ideas (such as using Java FileLock), but I didn't find a simple already implemented solution.

Are you aware of such an implemented solution?

Notes: http://groups.google.com/group/comp.soft-sys.matlab/browse_thread/thread/eb4ebeb8700ea440/13bd6710e8429a70?lnk=raot

http://www.mathworks.com/matlabcentral/newsreader/view_thread/279510

Answer 1

Write to a new file, then rename it. Renaming is an atomic operation, and all the new content will become visible at once.

Answer 2

At the end I did one implementation based on two consecutive tests (movefile, and verify the contents of the moved file).

not very well written, but it works for now for me.

+++++ file_lock.m ++++++++++++++++++++++++

function file_lock(op, filename)
%this will block until it creates the lock file:
%file_lock('create', 'mylockfile') 
%
%this will remove the lock file:
%file_lock('remove', 'mylockfile')


% todo: verify that there are no bugs

filename = [filename '.mat'];

if isequal(op, 'create')
  id = [tempname() '.mat'] 
  while true
    save(id, 'id');
    success = fileattrib(id, '-w');
    if success == 0; error('fileattrib'); end

    while true
      if exist(filename, 'file');        %first test
        fprintf('file lock exists(1). waiting...\n');
        pause(1); 
        continue;
      end
      status = movefile(id, filename);   %second test
      if status == 1; break; end
      fprintf('file lock exists(2). waiting...\n');
      pause(1);
    end

    temp = load(filename, 'id');         % third test.
    if isequal(id, temp.id); break; end

    fprintf('file lock exists(3). waiting...\n');
    pause(1)
  end

elseif isequal(op, 'remove')
  %delete(filename);
  execute_rs(@() delete(filename));

else
  error('invalid op');
end



function execute_rs(f)
while true
  try 
    lastwarn('');
    f();
    if ~isequal(lastwarn, ''); error(lastwarn); end  %such as: Warning: File not found or permission denied 
    break;
  catch exception
    fprintf('Error: %s\n.Retrying...\n', exception.message);
    pause(.5);
  end
end

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