The ON statement defines the action to be taken when a specific condition is signaled during the execution of a program. Its general form is:
ON condition–name ON-unit,
or
ON condition–name SYSTEM
where:
condition-name is ANYCONDITION, AREA, ATTENTION, CONDITION(name), CONVERSION, ENDFILE(f), ENDPAGE(f), ERROR, FINISH, KEY(f), OVERFLOW, RECORD(f), SIZE, UNDEFINEDFILE(f), UNDERFLOW, USERCONDITION(expression), USERCONDITION(SS$_UNWIND), or ZERODIVIDE.
ON-unit is a BEGIN block or any statement other than DECLARE, DEFAULT, DO, END, ENTRY, FORMAT, LEAVE, OTHERWISE, PROCEDURE, RETURN, or SELECT.
The keyword SYSTEM indicates that, instead of executing user-specified code in the ON-unit, a system-generated message describing the condition is displayed and the ERROR condition is signaled. For example:
ON ENDFILE (F )
BEGIN;
.
.
.
END;
In this example, the block of statements between the BEGIN and END statement executes if the end-of-file condition is reached.
ON is a compound statement, but it cannot be nested. The ON statement's use in responding to exceptional conditions is explained more fully in the section Statement that Handles Exception Conditions.