Open maintains a protected copy of the user DCB (in the user's program) in the combined work and control block area. IFG0EX0B can use open's copy of the DCB to test DCB field values. If you use IFG0EX0B to modify either the user DCB or OPEN's copy, you must also modify the other. The protection key of the user DCB is supplied in the parameter list passed to IFG0EX0B. IFG0EX0B must use this key to get information from, or modify the user DCB. Open does not copy the DCBE. OPEN requires that the DCBE be writable in the same key as the program that issued the OPEN macro. To maintain system integrity, your exit routine must switch to the key of the issuer of OPEN whenever accessing the user's DCB or DCBE.

If you use IFG0EX0B to change values in the DCB for a data set that has been allocated to a system-managed volume, you must not specify values that would change the data set to a type that cannot be SMS managed. Doing so causes abnormal termination of the task. For example, you cannot specify an unmovable data set organization.

Be sure you determine the type of DCB and device passed to the exit before testing access-method or device-dependent fields in the DCB. The sample exit shown in Example of the IFG0EX0B Exit Routine provides an example of excluding all DCBs from processing except for QSAM DCBs being opened to a DASD or tape device.