Why does 'fopen' return a NULL pointer?

There are many reasons fopen can return NULL including (but certainly not limited to):

• The file doesn't exist
• The file is opened in a mode that doesn't allow other accesses
• The network is down
• The file exists, but you don't have permissions
• A file exists with the name you gave, but the current directory of the process is not what you expected so the relative pathname fails to find and open the file.

The way to find out which is responsible is to dig into the errno code.

However just because you resolve this particular error doesn't mean you can assume fopen will never return NULL. When dealing with I/O operations your code simply has to expect failure. It's not possible to predict the success of I/O operations, and they can always fail.

The proper thing to do is check errno when fopen returns NULL.

I'm going to guess that your problem is that you're trying to write to a filesystem that doesn't allow \n in filenames, but it could be a permissions issue as well.

It means that the file might not exist or some permission error occurred while accessing a file such as "Read-Only" or "Write-Protected", so in those cases fopen will return 0 (a NULL pointer). On success it will return a file pointer as a handler.

fp=fopen("c:\\ABC.txt", "r"); cannot be the same as fp=fopen("c:\\abc.txt", "r");.

Use // instead of \\ in a Linux environment.

P.S.: In Linux and Unix-like operating systems file names are case-sensitive.