DL/I and CA IDMS/DB › DL/I Commands › Program Communication

Program Communication

The Program Communication Block (PCB)

When an application performs operations against a database, it always does so through a program communication block (PCB). The PCB restricts the application's access to specific segments selected from the definition of an underlying database.

Regardless of whether the definition is for a physical or a logical database, the database always appears to the application as hierarchical. This is an important point because the flow of the application's processing must always conform to a specific hierarchical path. In other words, application access to a database always starts at a root segment occurrence and proceeds downward through the hierarchy, moving from left to right among segment occurrences on the same level.

PCB Provides for Transfer and Control of Information

At program run time, DL/I maintains an I/O area for each PCB defined in the PSB. The PCB area provides for the transfer of data and control information between the application and DL/I. The PCB I/O area contains a control block with a number of fields. DL/I updates the control fields after each DL/I call. An application's access to these fields is established by declaring the fields as program variables. It is the application's responsibility to check the control fields, as appropriate, after each DL/I call.

Basic DL/I Control Fields

The basic DL/I control fields (with sample names) are:

• DBD-NAME ── Name of the DBD referenced by the PCB. This DBD determines the access path available to the application.
• SEG-LEVEL ── The current segment level in the hierarchy.
• STATUS-CODE ── DL/I result status code.
• PROCOPTS ── Processing options in effect, as specified in the PCB definition.
• SEG-NAME ── Segment name for the segment occurrence last accessed.
• KEY-LENGTH ── Length of the concatenated key for the segment occurrence last accessed.
• SEN-SEGS ── Number of segments available to the application, as specified in SENSEG statements in the PCB.
• KEY-AREA ── Key feedback area for the concatenated key of the segment occurrence last accessed.