C File Operations: Check opened file pointer access mode

Question

A simple question:

How do I check the access mode of an already opened file pointer?

So say a function is passed an already opened FILE pointer:

Answer 1

On Linux (and possibly all UNIX systems) you could use fcntl to get the access mode of the file:

int get_file_status(FILE* f) {
    int fd = fileno(f);
    return fcntl(fd, F_GETFL);
}

Note that the returned value is an integer as combination of flags like O_RDONLY or O_RDWR, not the "r" or "w+" strings. See http://pubs.opengroup.org/onlinepubs/007908799/xsh/open.html for some of these flags.

Not sure about Windows, see On Windows/mingw, what is the equivalent of `fcntl(fd, F_GETFL) | O_ACCMODE`?.