confirm valid file identifier before flcose?

I am using GUI callbacks to close a file. Sometimes the file is aready closed, and I get an invalid file identifier error. Is there a robust way to check if the file is still open before calling fclose? I have tried using fopen('all') but that does not always catch it.

Answer 1

[Les Beckham]

If you call fopen on the file identifier when the file is closed it will return empty (instead of returning the filename). So, you can do this:

if ~isempty(fopen(fid))
    fclose(fid);
end

Answer 2

[Voss]

try
    fclose(fid);
catch
    % file already closed, do nothing
end

Answer 3

[CM]

Thanks Les and Voss. I have actually combined both techniques, with the try inside the if loop. I am not using multithreading, so I am not sure if it is possible for the fid to become invalid within the space of the fopen and the if statements, but I have been going round in circles on this for a while and want to make sure it is robust.