How to assign needed file descriptor to a file

Let's imagine we have opened a file and want it to have some exact descriptor number, not the one kernel gave to us.

The information we have is

	struct fd {
		struct file *file;
		int tgt_fd;
	} *fd;

and we've just done the

	int fd;

	fd = open_a_file(fd->file);

what's next?