The MODCB macro lets you customize the control blocks generated with the GENCB macro. The MODCB macro can be used to modify the contents of an access method control block, an exit list, or a request parameter list. Typical reasons to modify a request parameter list are to change the length of a record (RECLEN) when you are processing a data set whose records are not all the same length, and to change the type of request (OPTCD), such as from direct to sequential access or from full-key search argument to generic key search argument.

When modifying any field in a RPL, you must first make sure there is no active or inactive process that is still connected to the RPL. Inactive processes that are still connected to their RPLs include sequential, skip-sequential and POINT processes. You can disconnect the RPL from any process using the ENDREQ macro.