To update existing records, you must use the READ KU and WRITE K combination. Because READ KU implies that the record will be rewritten in the data set, the system retains the DECB and the buffer used in the READ KU and uses them when the record is written. If you decide not to write the record, you should use the same DECB in another READ or WRITE macro, or if dynamic buffering was used, issue a FREEDBUF macro. If you issue several READ KU or WRITE K macros before checking the first one, you could destroy some of your updated records unless the records are from different blocks.

When you are using scan mode with QISAM and you want to issue PUTX, issue an ENQ on the data set before processing it and a DEQ after processing is complete. ENQ must be issued before the SETL macro, and DEQ must be issued after the ESETL macro. When you are using BISAM to update the data set, do not modify any DCB fields or issue a DEQ until you have issued CHECK or WAIT.