You can modify the JFCB in open's work area to update the secondary space quantity for DASD data sets, as shown in Example of the IFG0EX0B Exit Routine. The JFCBCTRI field contains the space request type coded in the DD statement or merged from the format-1 DSCB. The JFCBSQTY field contains the amount of secondary space requested. The JFCBPQTY field contains the amount of primary space requested. The JFCBCTRI field specifies the units in cylinders, tracks, or average blocks. If bit JFCBAVR is on, then the unit is either average blocks or average records.

Changing the value of the JFCONTIG bit has no effect on the space allocation because that bit affects only the primary space, which has already been allocated before IFG0EX0B is called.