splice() syscall

I found that splice() syscall does not transfer data, when in and out file descriptors are the same or refer to the same file. Instead destination ‘space’ is filled with zeroes while supposed to contain input buffer content.

Here is my code for the reference (with some debug added):

static int eblob_splice_data_one(int *fds, int fd_in, loff_t *off_in,
		int fd_out, loff_t *off_out, size_t len)
{
	int err;
	size_t to_write = len;

	while (to_write > 0) {
		err = splice(fd_in, off_in, fds[1], NULL, to_write, 0);
		printf("splice  in: %zu bytes from fd: %d, off: %llu: %d\n",
				to_write, fd_in, *off_in, err);
		if (err == 0) {
			err = -ENOSPC;
			goto err_out_exit;
		}
		if (err < 0) {
			err = -errno;
			perror("splice1");
			goto err_out_exit;
		}
		to_write -= err;
	}

	to_write = len;
	while (to_write > 0) {
		err = splice(fds[0], NULL, fd_out, off_out, to_write, 0);
		printf("splice out: %zu bytes into fd: %d, off: %llu: %d\n",
				to_write, fd_out, *off_out, err);
		if (err == 0) {
			err = -ENOSPC;
			goto err_out_exit;
		}
		if (err < 0) {
			err = -errno;
			perror("splice2");
			goto err_out_exit;
		}
		to_write -= err;
	}

	err = 0;

err_out_exit:
	return err;
}

Unfortunately it does not even return error, but silently corrupts data.

I would be happy to be wrong of course.