The ON statement gives your Open PL/I programs the ability to respond to exception conditions that occur during the execution of a program, such as end-of-file, or computational errors such as division by zero. For example:
ON ERROR
BEGIN;
.
.
.
END;
ON statement execution establishes the ON-unit as a block of statements that are executed if the specified condition occurs. If the specified condition occurs after the ON statement is executed, processing is interrupted and the specified ON–unit is executed. In the above example, the block of statements between the BEGIN and END statements are established as the ON–unit, which executes if the ERROR condition occurs. Execution of the ON statement does not cause immediate execution of the ON-unit.
An ON-unit is established within its containing block's current activation and remains established until that block returns to its caller, or until another ON-unit is established for the same condition within the same block activation, or until the ON-unit is reverted using the REVERT statement.
If one of the possible conditions occurs during the execution of a block, and that block does not have an established ON-unit for that condition, the calling block's ON-unit is used to respond to the condition. If the calling block has no established ON-unit for the condition, its caller's ON-unit is used, and so on. If no ancestor has an ON-unit for the condition, a default action is taken. Except for the ENDPAGE, FINISH, and UNDERFLOW conditions, this default action ends program execution and issues a run-time error message.
Notifications can be set up whenever a condition is detected that triggers an ON-unit. This allows you to investigate program state at ON-units to aid debugging and set up optional breakpoints. See Checking Program State with ON-unit Notification for more information.
ON-units for the ERROR condition cannot resume execution of the statement in which the error was detected.