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dbms_system

kdswrt

This procedure allows to write user defined messages into the alert.log. The first parameter needs to be 2, the second is the string that should be written.

read_ev

Which events are set? 
declare 
  lvl number; 
begin 
  for n in 10000..10999 loop 
    sys.dbms_system.read_ev(n,lvl); 
    if (lvl > 0) then 
      dbms_output.put_line('Event: ' || to_char(n) || ', Level: ' || to_char(lvl )); 
    end if; 
  end loop; 
end; 
/

set_bool_param_in_session

Sets a boolean parameter in another session.

set_ev

dbms_support.set_ev(
  sid,
  serial#,
  event_number,
  event_level,
  event_name
);
This procedure allows to set a diagnost event in a session.
In the special case of the diagnostic event 10046, use dbms_support.start_trace.

set_int_param_in_session

Sets a integer parameter in another session.

set_sql_trace_in_session

Using dbms_system.set_sql_trace_in_session, it is possible to enable tracing of SQL statements in other sessions the the one issuing it. For the own session, one might consider using 
alter session set sql_trace=true
If sql tracing is to be enabled for the entire instance, either set sql_trace in the init.ora file or issue 
alter system set sql_trace=true
Enable sql tracing is a prerequisite for tk prof