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KSAM Logical Record Pointer

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E0300 Edition 4 ♥
E0394 Edition 3

Many of the KSAM procedures use a logical record pointer to indicate the current record in the file. This pointer points to a key value in the index area that identifies the current record in the data area. The particular key used, if the file has more than one key, is the key specified in the current procedure or the last procedure that referenced a key.

Procedures that use pointers are either pointer-dependent or pointer-independent. Pointer-dependent procedures expect the pointer to be positioned at a particular record in order to execute correctly. Pointer-independent procedures, on the other hand, execute regardless of where the pointer is positioned and, in most cases, they position the pointer.

Table A-1 Positioning the Logical Record Pointer

Procedure Name Pointer-
Dependent		 Position of Pointer After Execution of Procedure
CKSTARTNOPoints to key whose value was specified in call.
CKREADBYKEYNOPoints to key whose value was specified in call.
CKWRITENOPoints to key whose value is next in key sequence to key value in record just written.
CKREADYESPointer remains positioned to key value for record just read; unless next call is to CKREAD, or to CKREWRITE followed by CKREAD, in which case, next CKREAD moves pointer to next key in key sequence before reading the record.
CKDELETEYESPoints to next key value in ascending sequence following key value in record just deleted.
CKREWRITEYES (sequential mode)

NO (random or dynamic mode)		Pointer remains positioned to key value for record just modified, unless any key value in record was changed; in this case, it points to next key in ascending sequence after the key in the modified record.

Shared Access

Particular care must be taken when using the logical record pointer during shared access (the file was opened with CKOPENSHR). If more than one user opens the same file, one user may modify the record pointer. This causes other users to access the data record.

To avoid this problem, you should always lock the file in a shared environment before calling a procedure that sets the pointer and leave the file locked until all procedures that depend on the pointer have been executed. Thus, if you want to read the file sequentially, delete a record, or modify a record, you should lock the file, call a procedure that sets the pointer (such as CKSTART), and then call CKREAD, CKDELETE, or CKREWRITE. When the operation is complete, you can then unlock the file to give other users access to it.

Sample KSAM File

The file KSAMFILE illustrated in Figure A-2 "Representation of KSAMFILE Used in COBOL Examples" is used in all subsequent examples associated with the COBOL procedure calls.

Figure A-2 Representation of KSAMFILE Used in COBOL Examples 

 :BUILD KSAMFILE;REC=-74,,F,ASCII;KSAMXL;&
 : KEY=(B,3,20;B,23,8,DUP)
 
 :BUILD KSAMFILE;REC=-74,,F,ASCII;KSAM64;&
 : KEY=(B,3,20;B,23,8,DUP)


[Representation of KSAMFILE
Used in COBOL Examples]
A File Description in Working Storage for Figure A-2 appears below. 

  File Description in Working Storage (Figure A-2).
  
  WORKING-STORAGE SECTION
  77  RECSIZE           PIC S9(4)   COMP VALUE 74.
  77  RESULT            PIC 9(4)    VALUE 0.
  01  REC.
      03  FILLER        PIC XX      VALUE SPACES.
      03  NAME          PIC X(20).
      03  PHONE         PIC X(8).
      03  OTHERDATA     PIC X(44).
  01  DAT.
      03  NAME          PIC X(20).
      03  PHONE         PIC X(8).
      03  OTHERDATA     PIC X(44).
  01  FILETABLE.
      03  FILETABLE     PIC S9(4)   COMP VALUE 0.
      03  FILENAME      PIC X(8)    VALUE "KSAMFILE".
      03  I-O-TYPE      PIC S9(4)   COMP VALUE 0.
      03  A-MODE        PIC S9(4)   COMP VALUE 0.
      03  PREV-OP       PIC S9(4)   COMP VALUE 0.
  01  STAT.
      03  STATUS-KEY-1  PIC X.
      03  STATUS-KEY-2  PIC X.



Status Parameter

CKCLOSE