Sys_Context fails to compile ('SYS_CONTEXT' must be declared)
                                                 Last update (2008-07-31 13:38:19)
                                                                                                                   Date added (2008-07-31 13:27:55)

Summary
You use Oracle Forms Developer 2000 or Oracle Forms Developer 6.0.8 and you try to compile the Sys_Context function. For example you try to compile:

select SYS_CONTEXT('USERENV', 'OS_USER') osuser from dual;

But you get the error:
error 201: the identifier 'SYS_CONTEXT' must be declared

The cause is the SYS_CONTEXT is a new feature in Oracle 8i and is not included in Forms 6i which uses Oracle 8.0.6 required support files, but you can bypass the problem with the following workaround.

Workaround
Create a database stored function, for example with name OSUSER. This function can be referenced from a program unit or trigger in Forms.

CREATE OR REPLACE FUNCTION OSUSER 
RETURN VARCHAR2 
IS 
lv_osuser VARCHAR2(30); 
BEGIN 
SELECT SYS_CONTEXT('USERENV','OS_USER') 
INTO   lv_osuser 
FROM   DUAL; 
RETURN lv_osuser; 
END;
/

After that you can use in the Forms the statement:

select OSUSER from dual;