The JFCB address provided in the OIEXJFCB field of the parameter list points to a copy of the JFCB in the combined work and control block area. If you are accessing concatenated partitioned data sets, other JFCBs might be associated with the OPEN macro.

If you modify the JFCB through IFG0EX0B, you must provide return code 4 to open. This causes open to update the system copy of the JFCB. Do not modify the JFCB through IFG0EX0B if the user program has set JFCNWRIT (bit 4) in byte JFCBTSDM, because this indicates that the system copy of the JFCB should not be updated. A sample IFG0EX0B routine that modifies the JFCB is shown in Example of the IFG0EX0B Exit Routine.

If the OPEN macro specifies EXTEND or OUTINX, and the disposition in the JFCB is not MOD, open temporarily changes its JFCB copy to specify DISP=MOD.