Alert log

The alert log file (also referred to as the ALERT.LOG) is a chronological log of messages and errors written out by an Oracle Database.
Typical messages found in this file is: database startup, shutdown, log switches, space errors, etc.
This file should constantly be monitored to detect unexpected messages and corruptions.
Oracle will automatically create a new alert log file whenever the old one is deleted.

#######Location of the ALERT.LOG file
Oracle will write the alert.log file to the directory as specified by the BACKGROUND_DUMP_DEST parameter.
If this parameter is not set, the alert.log will be created in the ORACLE_HOME/rdbms/trace directory.

SQL> show parameter BACKGROUND_DUMP_DEST

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
background_dump_dest string /app/oracle/diag/rdbms/o11gr1/
o11gr1/trace
##### Writing to the ALERT.LOG file
Users can write messages to the alter.log file. Example:

-- Write message to alert.log
exec dbms_system.ksdwrt(2, 'Look Ma, I can write to the alert.log file!');
PL/SQL procedure successfully completed.

-- Flush the buffer
exec dbms_system.ksdfls;
PL/SQL procedure successfully completed.





http://www.orafaq.com/wiki/Alert.log