KSAM LOGICAL RECORD POINTER

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 key file that identifies the current record in the data file. 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. Pointerent 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. (Refer to Table 3-3 “Positioning the Logical Record Pointer” for a summary of those procedures that either position the pointer or are dependent on the pointer position.)

Table 3-3 Positioning the Logical Record Pointer

Procedure Name	Pointer- Dependent	Position of Pointer After Execution of Procedure
CKSTART	NO	Points to key whose value was specified in call.
CKREADBYKEY	NO	Points to key whose value was specified in call.
CKWRITE	NO	Points to key whose value is next in key sequence to key value in record just written.
CKREAD	YES	Pointer 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.
CKDELETE	YES	Points to next key value in ascending sequence following key value in record just deleted.
CKREWRITE	YES (sequential mode)	Pointer remains positioned to key value for record jus modified; unless any key value in record was changed, in which case, it points to next key in ascending seuence after the key in the modified record.
CKREWRITE	NO (random or dynamic mode)

SHARED ACCESS

Particular care must be taken when using the logical record pointer during shared access (the file was opened with CKOPENSHR). Since the record pointer is maintained in a separate control block for each open file, if more than one user opens the same file, one user may modify the key file causing the record pointers of other users to point to the wrong key.

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 3-2 Representation of KSAMFILE Used in COBOL Examples is used in all subsequent examples associated with the COBOL procedure calls.

Figure 3-2 Representation of KSAMFILE Used in COBOL Examples

KSAM LOGICAL RECORD POINTER

Technical documentation

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SHARED ACCESS