This easy writing to the alert log is accomplished by using Oracle’s utl_file package. The Oracle utl_file package allows Oracle SQL and PL/SQL to read and write directly from flat files on the server.

Writing custom messages to the Oracle alert log requires the following steps:

1 - Locate the background dump directory (the location of the alert log).
2 - Set the utl_file_dir initialization parameter.
3 - Execute utl_file.fopen to open the file for write access.
4 - Use dbms_output.put_line to write the custom message to the alert log.
5 - Execute utl_file.fclose to close the file

-- ******************************************************
-- Gather the location of the alert log directory
-- ******************************************************

select
name into :alert_loc
from
v$parameter
where
name = ‘background_dump_destination’;

-- ******************************************************
-- Set the utl_file_dir
-- (prior to Oracle9i, you must bounce the database)
-- ******************************************************
alter system set utl_file_dir = ‘:alert_log’);


-- ******************************************************
-- Open the alert log file for write access
-- ******************************************************
utl_file.fopen(':alert_loc',’alertprod.log’,'W');

-- ******************************************************
-- Write the custom message to the alert log file
-- ******************************************************
dbms_output.put_line('invalid_application_error');

-- ******************************************************
-- Close the alert log file
-- ******************************************************
utl_file.fclose(':alert_loc');

Oracle lets a database program write to flat files using the utl_file utility

FILE SYSTEM 3

DESCRIPTION
(Directory of files created using the utl_file package)

The user interaction directory is placed here. The files created using the package utl_file are generated in this directory. Since users will frequently read and write files in this directory, this should be separated. The directory should be owned by Oracle with permissions set for read, write and execute for everyone.

Please note then in Oracle 9i, the utl_file package does not need this parameter to be set in init.ora to manipulate a file. The DIRECTORY object can be dynamically created and used as the directory. Therefore, the days of this filesystem could be numbered.
 

Also see these related notes on Oracle utl_file:

Oracle utl_file server directory vulnerability discovered

Oracle Table Design utl_file External Tables

Incorrect utl_file permission umask tips