You can specify in an exit list the address of a routine that completes or modifies a DCB and does any additional processing required before the data set is completely open. The routine is entered during the opening process after the JFCB has been used to supply information for the DCB. Filling in the DCB describes other functions performed by OPEN before and after the DCB OPEN exit. The routine can determine data set characteristics by examining fields completed from the data set labels. When your DCB exit routine receives control, the 3 low-order bytes of register 1 will contain the address of the DCB currently being processed. See Changing and Testing the DCB and DCBE.

When opening a data set for output and the record format is fixed or variable, you can force the system to calculate an optimal block size by setting the block size in the DCB or DCBE to zero before returning from this exit. The system uses DCB block size if it is not using the large block interface (LBI). See Large Block Interface (LBI). If the zero value you supply is not changed by the DCB OPEN installation exit, OPEN determines a block size when OPEN takes control after return from the DCB OPEN installation exit. See System-Determined Block Size.

As with label processing routines, the contents of register 14 must be preserved and restored if any macros are used in the routine. Control is returned to the operating system by a RETURN macro; no return code is required.

This exit is mutually exclusive with the JFCBE exit. If you need both the JFCBE and DCB OPEN exits, you must use the JFCBE exit to pass control to your routines.

The DCB OPEN exit is intended for modifying or updating the DCB. System functions should not be attempted in this exit before returning to OPEN processing. In particular, dynamic allocation, OPEN, CLOSE, EOV, and DADSM functions should not be invoked because of an existing OPEN enqueue on the SYSZTIOT resources.