Warning: mysqli_error() expects **Xiaobai's solution

Question

<?php

$conn=mysqli_connect("localhost","root","123456789") or die("Database server connection error".mysqli_error());

mysqli_select_db($conn,"db_library") or die("Database access error".mysqli_error( ));

mysqli_query($conn,"set names utf8");

?> ;

Warning: mysqli_error() expects exactly 1 parameter, 0 given in *********\conn\conn.php on line 3 database access error