The IBM Operating System Checkpoint/Restart feature is designed to be used with programs running for an extended period of time when interruptions may halt processing before the end of the job. Because the Checkpoint/Restart feature causes a lot of extra processing, use it only when you anticipate interruptions caused by machine malfunctions,input/output errors or intentional operator intervention.Checkpoint/Restart allows the interrupted program to be restarted at the job step or at a point other than the beginning of the job step.

The checkpoint routine is invoked from the COBOL load module containing your program. During execution of your program, the checkpoint routine creates checkpoint records at points you specify in your program.

The checkpoint record contains information stored in registers and main storage when the program reached the checkpoint. You specify these checkpoints by using the COBOL RERUN clause in the Environment Division.

The restart routine restarts an interrupted program. Restart occurs at the beginning of a job step or at a checkpoint, if a checkpoint record has been written. The checkpoint record contains all informationnecessary to restart the program. Restart can be initiated at any time after the program was interrupted; that is, it can be run immediately after the interrupt has; occurred, as an automatic restart, or at a later time, as adeferred restart.

The COBOL RERUN clause provides linkage to the system checkpoint routine. Any cautions and restrictions on the use of the system Checkpoint/Restart feature also apply to the use of the RERUN clause during the execution of a SORT or MERGE statement. If restart is attempted with a checkpoint taken by a COBOL program during a sort or merge operation, an error message is issued and the restart canceled. Only checkpoints taken by DFSORT are valid.

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