Using FREEDBUF

Note the use of the FREEDBUF macro in Figure 1. Usually, the FREEDBUF macro has two functions:

To indicate to the ISAM routines that a record that has been read for update will not be written back
To free a dynamically obtained buffer.

In Figure 1, because the read operation was unsuccessful, the FREEDBUF macro frees only the dynamically obtained buffer.

The first function of FREEDBUF lets you read a record for update, then decide not to update it without performing a WRITE for update. You can use this function even when your READ macro does not specify dynamic buffering, if you have included S (for dynamic buffering) in the MACRF field of your READ DCB.

You can cause an automatic FREEDBUF merely by reusing the DECB; that is, by issuing another READ or a WRITE KN to the same DECB. You should use this feature whenever possible, because it is more efficient than FREEDBUF. For example, in Figure 1, the FREEDBUF macro could be eliminated, because the WRITE KN addressed the same DECB as the READ KU.

Figure 1. Directly Updating an Indexed Sequential Data Set

//INDEXDD   DD       DSNAME=SLATE.DICT,DCB=(DSORG=IS,BUFNO=1,...),---
//TAPEDD    DD       ---
            ...
ISUPDATE    START    0
            ...
NEXTREC     GET      TPDATA,TPRECORD
            ENQ      (RESOURCE,ELEMENT,E,,SYSTEM)
            READ     DECBRW,KU,,'S',MF=E    Read into dynamically
*                                           obtained buffer
            WAIT     ECB=DECBRW
            TM       DECBRW+24,X'FD'        Test for any condition
            BM       RDCHECK                but overflow
            L        3,DECBRW+16            Pick up pointer to
*                                           record
            MVC      ISUPDATE-ISRECORD      Update record
            WRITE    DECBRW,K,MF=E
            WAIT     ECB=DECBRW
            TM       DECBRW+24,X'FD'        Any errors?
            BM       WRCHECK
            DEQ      (RESOURCE,ELEMENT,,SYSTEM)
            B        NEXTREC
RDCHECK     TM       DECBRW+24,X'80'        No record found
            BZ       ERROR                  If not, go to error
*                                           routine
            FREEDBUF DECBRW,K,ISDATA        Otherwise, free buffer
            MVC      ISKEY,KEY              Key placed in ISRECORD
            MVC      ISUPDATE,UPDATE        Updated information
*                                           placed in ISRECORD
            WRITE    DECBRW,KN,,WKNAREA,'S',MF=E Add record to data set
            WAIT     ECB=DECBRW
            TM       DECBRW+24,X'FD'        Test for errors
            BM       ERROR
            DEQ      (RESOURCE,ELEMENT,,SYSTEM)  Release exclusive
*                                          control
            B        NEXTREC
WKNAREA     DS       4F                     BISAM WRITE KN work field
ISRECORD    DS       0CL50                  50-byte record from ISDATA
            DS       CL19                   DCB First part of ISRECORD
ISKEY       DS       CL10                   Key field of ISRECORD
            DS       CL1                    Part of ISRECORD
ISUPDATE    DS       CL20                   Update area of ISRECORD
            ORG      ISUPDATE               Overlay ISUPDATE with
TPRECORD    DS       0CL30                  TPRECORD 30-byte record
KEY         DS       CL10                   from TPDATA DCB Key
*                                            for locating
UPDATE      DS       CL20                   ISDATA record update
RESOURCE    DC       CL8'SLATE'             information or new data
ELEMENT     DC       C'DICT'
            READ     DECBRW,KU,ISDATA,'S','S',KEY,MF=L
ISDATA      DCB      DDNAME=INDEXDD,DSORG=IS,MACRF=(RUS,WUA),       C
                     MSHI=INDEX,SMSI=2000
TPDATA      DCB      ---
INDEX       DS       2000C
            ...