Utlfile package usage

I want to use UTL file package. I have created a sample procedure to test. Procedure is created without any errors but when executing it is giving the erroes like given below.

Could any one help I have to set any parameters to use utl file package.

Please throw some light on this and thanks in advance.

Regards
Eswar

get D:\ESWAR\PROCE\utl_test.txt
  1 create or replace procedure utl_test as   2 begin
  3 declare
  4 vfile_handle utl_file.file_type;
  5 vdir varchar2(50):='/c/temp';   6 vfilename varchar2(25):='a1.txt';   7 begin
  8 vfile_handle := utl_file.fopen(vdir,vfilename,'w');   9 utl_file.put_line(vfile_handle,'Hai file created');  10 utl_file.fclose(vfile_handle);
 11 end;
 12* end;
SQL> / Procedure created.

SQL> exec utl_test
begin utl_test; end;

*
ERROR at line 1:

ORA-06510: PL/SQL: unhandled user-defined exception
ORA-06512: at "SYS.UTL_FILE", line 82
ORA-06512: at "SYS.UTL_FILE", line 120
ORA-06512: at "ESWAR.UTL_TEST", line 8
ORA-06512: at line 1