Important additional information to note about Read and Write

In addition to setting the current output record data, the I/O exit should also set the current output record length during a read or write operation. If the current output record length is not set, or is set explicitly to zero, any changes in the current output record data buffer will be ignored, and the current input record will be used as the current output record.
When processing a variable length sequential file, the data portion does not include the record descriptor word (RDW) containing the record length. The length is stored in a separate field in the control block.
The I/O exit code should not assume that it is operating on records in sequential order. For example, the File Manager editor might read or write individual records out of order.
The I/O exit could receive read and write calls in any sequence. For example, the File Manager Find/Change utility might read a number of records and then write a number of records, and then go back to reading.