Error using fprintf Invalid file identifier. Use fopen to generate a valid file identifier.

Hello, I have a problem, every program I try to run gives me this error. Even with the matlab examples.

This is the code from the mathworks page:

A = magic(4);

fileID = fopen('myfile.txt','w');
nbytes = fprintf(fileID,'%5d %5d %5d %5d\n',A);

fclose(fileID);

type('myfile.txt')

[Benjamin Kraus]

Can you clarify your question?

• When you say "every program", do you mean every command you try to run in MATLAB?
• If you run those commands one at a time, do you get any other error messages?
• Are you running those commands in a directory in which you have write access?
• What is the value of fileID after calling fopen?

Answer 1

[Benjamin Kraus]

When fopen fails, it does not generate an error message. Instead, it sets the fileID to -1.

Check the value of fileID after calling fopen.

If it is -1, check whether you have write permissions in the directory you are trying to write to. Most likely you are trying to write a file to a directory that is read-only, and you either need to change directories or specify an absolute path instead of a relative path.

[Alex Javier]

Hi, Benjamin

The value of FID is -1 when I run the program. How do I know if I have write permissions in the directory? or How do I specify an absolute path?

I need to create a program that prints a table in an external file, but when I created and empty file called my "file.txt" and I run the program it says:

"Error using fprintf Invalid file identifier. Use fopen to generate a valid file identifier.

Error in CP5_test (line 6) nbytes = fprintf(fileID,'%5d %5d %5d %5d\n',A);

in the command window.

[Walter Roberson]

"How do I know if I have write permissions in the directory"

The "Permission denied" message tells you that you do not have permission.

You can check some aspects of permissions on your current folder by executing

fileattr .

where the '.' is part of the command.

If you want the details, then you can look at the directory security settings https://msdn.microsoft.com/en-us/library/bb727008.aspx

To specify an absolute pathname, give it in the fopen. For example,

[fileID, message] = fopen('C:\Users\AlJa\Documents and Settings\MATLAB\myfile.txt', 'w');

[Alex Javier]

It worked, thanks!

Answer 2

[Walter Roberson]

You do not have write access to the directory you are in. Your current directory is probably a directory that MATLAB is installed in.

Change

fileID = fopen('myfile.txt','w');

to

[fileID, message] = fopen('myfile.txt','w');
if fileID < 0
   error('Failed to open myfile because: %s', message);
end

[Alex Javier]

Error using CP5_test (line 7) Failed to open myfile because: Permission denied

Answer 3

[Akash kumar]

if you open the fopen file in his format and you run the again matlab file then it gives the error.

First:- You close the fopen file in your system . In your case it is 'myfile.txt'

second:- Now you run your code.

Note:- YOUR code will be definately run.

Thanks!

[Walter Roberson]

No, this is not correct for this situation.

In MATLAB, if you fopen() the same disk file multiple times within MATLAB, you are permitted to do that. How the various instances interact is not documented. It will not give you "permission denied".

In MATLAB on Mac or Linux, if you fopen() a file that a different program has open, then in most cases you are permitted ot do that. There is a "hardened" Linux that the open might fail on, though.

In MATLAB on Windows, if you fopen() a file that a different program has open, then the results depend on how the other program opened the file. By default, if simple methods were used to open the file in the other process, then the file will be locked for opening inside MATLAB. However, the fopen() will not give back "permission denied": instead the file is "busy"

"permission denied" as an error message occurs in a few situations in MS Windows:

• the file already exists, but the protections for the file are set in a way that do not permit you to open the file for the mode you are requesting (e.g., you might have read access to the file but not write access.)
• the file does not already exist, but the protections for the containing directory are set in a way that do not permit you to create a new file in the directory
• regardless of the permission and ownership of the file, if the file is underneath one of MS Windows two main directories for installed executables, then write access to the file will be denied unless you are running as Administrator.

The last of those is the most common situation that people encounter: MATLAB on Windows tends to start up the first time with its initial directory set to the main directory that MATLAB was installed into, which is under one of the places that MS WIndows denies user access to.

[Akash kumar]

But, if you open the matlab on windows then if you open the excel data sheet which is generated by the current running code. And you try to again run the same code then it will give the "Error using fprintf Invalid file identifier. Use fopen to generate a valid file identifier." Error.

[Walter Roberson]

But it will not be a "permission denied" situation. The message will be different. See https://www.mathworks.com/matlabcentral/answers/378848-error-using-fprintf-invalid-file-identifier-use-fopen-to-generate-a-valid-file-identifier#answer_301617 for how to get the message.

And when you cannot open a file because it is open for writing in Excel, then fclose() of the file in your MATLAB session does not help: you would need to convince Excel to close the file.