Modifying a memory-mapped data file

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Ken
Ken on 6 Jan 2018
Commented: Walter Roberson on 1 Mar 2024 at 19:30
I am using memmapfile to managed and update structured data records. After I modify a record in the file, the "modified date" of the file is not updated. I do confirm that the record has been modified. Unfortunately, it seems that the memory-mapped file is not updated by by cloud service, presumably because the file "modified date" is not changed.
Does anyone know how this might be fixed/managed?

Answers (2)

Ben Drebing
Ben Drebing on 12 Jan 2018
Unfortunately, this is a current limitation of MATLAB. But a quick way to get around this is to open and close it again after you've modify the file so that the modified time stamp gets updated. You can just do something like:
fileID = fopen('MyFile.dat','w');
fclose(fileID);
This will update the date modified without actually changing the file.
  8 Comments
Eivind Hennestad
Eivind Hennestad on 1 Mar 2024 at 10:05
Also, for the record and to repeat what is noted in the discussion here, the first solution proposed by Ben is not correct. The following statement will wipe out the contents of the file:
fileID = fopen('MyFile.dat','w');
Walter Roberson
Walter Roberson on 1 Mar 2024 at 19:30
Ah, if the process is to open a file, read the first byte, rewind, and write back the first byte, then you should open with 'r+' permissions.
Opening with 'a+' would not be desirable because for 'a' and 'a+' there is a hidden fseek() to eof before every write.

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Jan
Jan on 12 Jan 2018
Under Windows you can use https://www.mathworks.com/matlabcentral/fileexchange/24671-filetime to set the modification date manually.

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