Warning: mysqli_error() expects **小白求解

Question

<?php

     $conn=mysqli_connect("localhost","root","123456789") or die("数据库服务器连接错误".mysqli_error());

     mysqli_select_db($conn,"db_library") or die("数据库访问错误".mysqli_error( ));

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

?>

Warning: mysqli_error() expects exactly 1 parameter, 0 given in *********\conn\conn.php on line 3数据库访问错误