Example: release positioning for another request

In this example, the ENDREQ macro is used to cause VSAM to release exclusive control of a control interval containing a record. There are two request parameter lists, both of which require VSAM to be able to remember its position until VSAM is explicitly requested to forget its position.

BLOCK   ACB    MACRF=(SEQ,                                                       x
               DIR),STRNO=2
SEQ     RPL    ACB=BLOCK,          VSAM must remember its position.              x
               OPTCD=SEQ
DIRUPD  RPL    ACB=BLOCK,          VSAM must remember its position and maintain  x
               OPTCD=(DIR,UPD)     exclusive control until explicitly requested  x
        .                          to forget it by PUT or ENDREQ.
        .
LOOP    GET    RPL=SEQ             VSAM now remembers its position for           x
                                   this request only while it is
        LTR    15,15               processing the request.
        BNZ    ERROR
        GET    RPL=DIRUPD          VSAM can remember its position for this       x
                                   request.
        LTR    15,15               The control interval will be placed in        x
                                   exclusive control until either
        BNZ    ERROR               ENDREQ or PUT UPD is issued.

Decide whether to update the record:

        B      FORGET              No; do not update the record.
        PUT    RPL=DIRUPD          Yes; update the record, causing VSAM          x
                                   to forget its position for DIRUP.
        LTR    15,15
        BNZ    ERROR
        B      LOOP
FORGET  ENDREQ RPL=DIRUPD          Cause VSAM to forget its position for DIRUPD.
        LTR    15,15               Release exclusive control.
        BNZ    ERROR
        B      LOOP
ERROR   xxx                        Request wasn't accepted or failed.

The use of ENDREQ shown here causes VSAM to release exclusive control of the control interval for a record. When PUT is issued after a DIRUPD GET request, ENDREQ need not be issued, because PUT causes VSAM to release exclusive control (the next DIRUPD GET does not depend on VSAM's remembering its position). Another result of ENDREQ is that current buffers are written if they have been modified.

To cause VSAM to give up its position associated with a chain of request parameter lists, specify the first request parameter list in the chain in your ENDREQ macro.

ENDREQ can also be used to cancel an asynchronous request, rather than suspending processing with CHECK.

Because VSAM remembers its position after a direct GET with OPTCD=UPD, LOC or (NUP, NSP), if no PUT or ENDREQ follows, you can switch to sequential access and use the positioning for a GET.

Requirement: If you are sharing subtasks or if you have issued an asynchronous request for access to a data set, you must issue a CHECK or an ENDREQ on all RPLs before you issue a CLOSE or CLOSE TYPE=T. Otherwise, concurrent data set I/O activity causes unpredictable results during a close.