IMS DB - Cobol Basics

We include DL/I calls inside COBOL apppcation program to communicate with IMS database. We use the following DL/I statements in COBOL program to access the database −

Entry Statement

Goback Statement

Call Statement

Entry Statement

It is used to pass the control from the DL/I to the COBOL program. Here is the syntax of the entry statement −

ENTRY  DLITCBL  USING pcb-name1
                     [pcb-name2]

The above statement is coded in the Procedure Division of a COBOL program. Let us go into the details of the entry statement in COBOL program −

The batch initiapzation module triggers the apppcation program and is executed under its control.

The DL/I loads the required control blocks and modules and the apppcation program, and control is given to the apppcation program.

DLITCBL stands for DL/I to COBOL. The entry statement is used to define the entry point in the program.

When we call a sub-program in COBOL, its address is also provided. Likewise, when the DL/I gives the control to the apppcation program, it also provides the address of each PCB defined in the program s PSB.

All the PCBs used in the apppcation program must be defined inside the Linkage Section of the COBOL program because PCB resides outside the apppcation program.

The PCB definition inside the Linkage Section is called as PCB Mask.

The relation between PCB masks and actual PCBs in storage is created by psting the PCBs in the entry statement. The sequence of psting in the entry statement should be same as they appear in the PSBGEN.

Goback Statement

It is used to pass the control back to the IMS control program. Following is the syntax of the Goback statement −

GOBACK

Listed below are the fundamental points to note about the Goback statement −

GOBACK is coded at the end of the apppcation program. It returns the control to DL/I from the program.

We should not use STOP RUN as it returns the control to the operating system. If we use STOP RUN, the DL/I never gets a chance to perform its terminating functions. That is why, in DL/I apppcation programs, Goback statement is used.

Before issuing a Goback statement, all the non-DL/I datasets used in the COBOL apppcation program must be closed, otherwise the program will terminate abnormally.

Call Statement

Call statement is used to request for DL/I services such as executing certain operations on the IMS database. Here is the syntax of the call statement −

CALL  CBLTDLI  USING DLI Function Code
                        PCB Mask
                        Segment I/O Area
                        [Segment Search Arguments]

The syntax above shows parameters which you can use with the call statement. We will discuss each of them in the following table −

Given below are the points to note about the Call statement −

CBLTDLI stands for COBOL to DL/I. It is the name of an interface module that is pnk edited with your program’s object module.

After each DL/I call, the DLI stores a status code in the PCB. The program can use this code to determine whether the call succeeded or failed.

Example

For more understanding of COBOL, you can go through our COBOL tutorial here. The following example shows the structure of a COBOL program that uses IMS database and DL/I calls. We will discuss in detail each of the parameters used in the example in the upcoming chapters.

IDENTIFICATION DIVISION.
PROGRAM-ID. TEST1.
DATA DIVISION.
WORKING-STORAGE SECTION.
01 DLI-FUNCTIONS.
   05 DLI-GU        PIC X(4)    VALUE  GU   .
   05 DLI-GHU       PIC X(4)    VALUE  GHU  .
   05 DLI-GN        PIC X(4)    VALUE  GN   .
   05 DLI-GHN       PIC X(4)    VALUE  GHN  .
   05 DLI-GNP       PIC X(4)    VALUE  GNP  .
   05 DLI-GHNP      PIC X(4)    VALUE  GHNP .
   05 DLI-ISRT      PIC X(4)    VALUE  ISRT .
   05 DLI-DLET      PIC X(4)    VALUE  DLET .
   05 DLI-REPL      PIC X(4)    VALUE  REPL .
   05 DLI-CHKP      PIC X(4)    VALUE  CHKP .
   05 DLI-XRST      PIC X(4)    VALUE  XRST .
   05 DLI-PCB       PIC X(4)    VALUE  PCB  .
01  SEGMENT-I-O-AREA        PIC X(150).
LINKAGE SECTION.
01  STUDENT-PCB-MASK.
   05 STD-DBD-NAME              PIC X(8).
   05 STD-SEGMENT-LEVEL         PIC XX.
   05 STD-STATUS-CODE           PIC XX.
   05 STD-PROC-OPTIONS          PIC X(4).
   05 FILLER                    PIC S9(5) COMP.
   05 STD-SEGMENT-NAME          PIC X(8).
   05 STD-KEY-LENGTH            PIC S9(5) COMP.
   05 STD-NUMB-SENS-SEGS        PIC S9(5) COMP.
   05 STD-KEY                   PIC X(11).
PROCEDURE DIVISION.
ENTRY  DLITCBL  USING STUDENT-PCB-MASK.
A000-READ-PARA.
110-GET-INVENTORY-SEGMENT.
   CALL ‘CBLTDLI’ USING DLI-GN
                  STUDENT-PCB-MASK
                  SEGMENT-I-O-AREA.
GOBACK.