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Besides the possible issues that Lundin told you, there is also the possibility that the current directory of the program is not what you think it is (note that the current directory is not necessarily the directory the executable is in, nor is it necessarily your user directory). This problem is unlikely if you started it directly from the command line, but if you started it through another program (for example from a file manager, or from your development environment), you might want to check that program's documentation on what directory it puts as current when it starts programs.

If the program is started from a directory you don't have a write permissions to, and the file does not yet exist there (which is likely if you are in the “wrong” directory), then the code will try to create it, which fails if you don't have write permission on the directory.