What causes an invalid file identifier in MATLAB?

Question

I have a MATLAB script that I could have sworn worked fine the last time I used it (a year ago). Now, I get this error:

Invalid file identifier.  Use fopen to generate a valid file identifier.

If I understand correctly, it is failing to find or open(?) a file specified elsewhere in the script. Is this right? If so, what could cause it?

Answer 1

fid (file identifier) is the output of fopen. It's an integer, but not related to the file permanently. You need to use fopen to get the fid. It seems to me that you are using incorrect fid (file identifier) in some file-related I/O command, such as fread, fscanf or fclose. Unsuccessful fopen gives fid of -1. For any valid normal file successful fopen will give fid that is 3 or greater integer.

However, without any code it's impossible to say where or what the bug or error is. You could use MATLAB debugger to single-step the code from relevant fopen (set breakpoint there and run your program) until the relevant fclose and see if fid (or whatever variable name you use for file identifier) or any data structure for your file identifiers (if have more than one file identifier in your code) changes in any point between relevant fopen and fclose.

Answer 2

I solved this problem for my self by adding permission option to fopen. As you see in http://www.mathworks.se/help/matlab/ref/fopen.html , fopen syntax is:

fileID = fopen(filename,permission)

Possible permissions, for example are: 'r' (default) | 'w' | 'a' | 'r+' | 'w+' | 'a+' | ...

'r' – Open file for reading.

'w' – Open or create new file for writing. Discard existing contents, if any.

'a' – Open or create new file for writing. Append data to the end of the file.

'r+' – Open file for reading and writing.

'w+' – Open or create new file for reading and writing. Discard existing contents, if any.

'a+' – Open or create new file for reading and writing. Append data to the end of the file.

...

If I use fopen without permission option, or if I use 'r' (default) option, fopen will return -1, which is error. I success with this:

fid=fopen('tmp.txt', 'w');
fid=fopen('tmp.txt', 'a');