':param fd:  A file descriptor.
:return: the file for the corresponding fd or None
If the fd does not exist, a new fd is created with a warning
If concrete_fs is set then accessing a non_existing fd will return None'

def get_file(self, fd):

    fd = self.state.make_concrete_int(fd)
    if (fd not in self.files):
        if self.concrete_fs:
            return None
        l.warning('Accessing    non-existing    file    with    fd    %d.    Creating    a    new    file.', fd)
        self.open(('tmp_%d' % fd), Flags.O_RDWR, preferred_fd=fd)
    return self.files[fd]
