You locate a buffer that contains the RBA you are interested in by issuing a SCHBFR macro, a read-only GET, or a GET for update. When you issue GET for update, you get exclusive control of the buffer. Whether you have exclusive control or not, you can mark the buffer for output by issuing the MRKBFR macro with MARK=OUT, then change the buffer's contents. Without exclusive control, you should not change the control information in the CIDF or RDFs (do not change the record lengths).

MRKBFR MARK=OUT, indicates that the buffer's contents are modified. You must modify the contents of the buffer itself, not a copy. Therefore, when you issue SCHBFR or GET to locate the buffer, you must specify RPL OPTCD=LOC. (If you use OPTCD=MVE, you get a copy of the buffer but do not learn its location.) The buffer is written when a WRTBFR is issued or when VSAM is forced to write a buffer to satisfy a GET request.

If you are sharing a buffer or have exclusive control of it, you can release it from shared status or exclusive control with MRKBFR MARK=RLS. If the buffer was marked for output, MRKBFR with MARK=RLS does not nullify it; the buffer is eventually written. Sequential positioning is lost. MRKBFR with MARK=RLS is similar to the ENDREQ macro.