Program Communication Statements

Program communication statements are used to manage interactions between parts of the program, different programs or between the program and external entities. They enable structured programming by sharing code and data among different parts of a program or between separate programs. The program communication statements are –

CALL Statement
Cancel Statement
STOP RUN Statement
EXIT PROGRAM Statement
GOBACK Statement
GO TO Statement

CALL -

CALL statement is used to invoke the subprogram to complete a task. The CALL statement is always coded in the calling or main program and the program name in the CALL statement is called or subprogram.

Syntax -

CALL literal-1 [USING variable1, ..., variablen]

literal-1 - The name of the subprogram (or the data item holding it) to be invoked.
variable1 to variablen - These are the data items or literals that are passed from the calling program to the called program. They can be used to send data and receive data from the subprogram.

CALL Types -

CALLs are two types based on how the subprogram is called from main program -

Static call - CALL statement with SUBPROG name in quotes makes the call as STATIC CALL. When we compile the MAINPROG, the SUBPROG load module is attached to the MAINPROG load module and stored together.
```
CALL "subprogram-name" [USING variable1, ..., variablen]
```
MAINPROG should be compiled with NODYNAM compiler option. Example - Static call
```
CALL "SUBPROG" USING WS-INPUT1, WS-INPUT2.
```
Dynamic call - A CALL statement with a variable with a SUBPROG name makes the call as DYNAMIC CALL. When we compile MAINPROG and SUBPROG, their load modules are stored separately, and the load modules are not linked to each other.
```
CALL ws-variable [USING variable1, ..., variablen]
```
MAINPROG should be compiled with DYNAM compiler option.
Example - Dynamic call
```
MOVE "SUBPROG"  TO WS-SUBPROG.
CALL WS-SUBPROG USING WS-INPUT1, WS-INPUT2.
```

Passing Parameters -

There are three ways of passing parameters from MAINPROG to SUBPROG, and those are -

BY REFERENCE - Default option. It is used to pass the memory location of the actual data, not a copy of the data. If the SUBPROG modifies the value of the passing input variable, the new value is reflected in the MAINPROG.
```
CALL subprog-name USING BY REFERENCE PARM1, PARM2, ...
```
Example - Dynamic call with BY REFERENCE
```
CALL WS-SUBPROG USING BY REFERENCE WS-INP1, WS-PARM2, ...
```
BY CONTENT - It is used to pass the copy of the data to the subprogram. If the SUBPROG modifies the value of the passing input variable, the new value is not reflected in the MAINPROG.
```
CALL subprog-name USING BY CONTENT PARM1, PARM2, ...
```
Example - Dynamic call with BY CONTENT
```
CALL WS-SUBPROG USING BY CONTENT WS-INP1, WS-PARM2, ...
```

Example -

Scenario - Static Call from MAINPROG to SUBPROG and receiving the result back from SUBPROG.

Main Program -

----+----1----+----2----+----3----+----4----+----5----+
       IDENTIFICATION DIVISION.
       PROGRAM-ID. MAINPROG.
       DATA DIVISION.
       WORKING-STORAGE SECTION.
       01 WS-VAR.
          05 WS-INP1      PIC 9(02) VALUE 47. *> Input1
          05 WS-INP2      PIC 9(02) VALUE 25. *> Input2
          05 WS-RESULT    PIC 9(04).          *> Result Variable

	   PROCEDURE DIVISION. 
	  * Calling subprogram staically with two inputs 
      * and receiving the result from SUBPROG
           CALL "SUBPROG" USING WS-INP1, WS-INP2, WS-RESULT.

           DISPLAY "INPUTS:  " WS-INP1 ", " WS-INP2.
           DISPLAY "RESULTS: " WS-RESULT.
           STOP RUN.

Sub Program -

----+----1----+----2----+----3----+----4----+----5----+
       IDENTIFICATION DIVISION. 
       PROGRAM-ID. SUBPROG.
       AUTHOR. MTH.

       DATA DIVISION.
       LINKAGE SECTION.
       01 LK-INP1      PIC 9(02).  *> To receive input1 from MAINPROG
       01 LK-INP2      PIC 9(02).  *> To receive input2 from MAINPROG
       01 LK-RESULT    PIC 9(04).  *> To send result to MAINPROG

	  *Receiving data from main program CALL statement
       PROCEDURE DIVISION USING LK-INP1, LK-INP2, LK-RESULT. 
           COMPUTE LK-RESULT = LK-INP1 * LK-INP2.
           GOBACK.

JCL -

//MATEPKRJ JOB MSGLEVEL=(1,1),NOTIFY=&SYSUID     
//*
//STEP01  EXEC PGM=MAINPROG                      
//STEPLIB  DD  DSN=MATEPK.COBOL.LOADLIB,DISP=SHR 
//SYSOUT   DD  SYSOUT=*

Output -

INPUTS:  47, 25
RESULTS:  1175

Explaining Example -

In the above example:

MAINPROG is the main program, and SUBPROG is the subprogram.
WS-INP1 and WS-INP2 are the inputs passed from MAINPROG to the SUBPROG. SUBPROG receives the data into LK-INP1, LK-INP2 from MAINPROG, multiplies those values, and places the result into LK-RESULT.
SUBPROG returns the output in LK-RESULT, and MAINPROG displays the result received from SUBPROG.

CANCEL -

The CANCEL statement is used to release the resources associated with a previously called subprogram. If the subprogram is no longer needed, the CANCEL statement can be used to free up the resources used by subprogram.

CANCEL subprogram-name

Example - Let us assume SUBPROG is the subprogram name and WS-SUBPROG is working-storage variable that holds the subprogram name.

MOVE "SUBPROG" TO WS-SUBPROG.
CANCEL WS-SUBPROG

STOP RUN -

The STOP RUN statement is used to terminate the execution of a program. Once STOP RUN is executed, control returns to the operating system. STOP RUN always uses with main or calling program.

STOP RUN.

Example - MAINPROG with STOP RUN.

       IDENTIFICATION DIVISION.
       PROGRAM-ID. MAINROG.
	   AUTHOR. MTH.
	   ...
       PROCEDURE DIVISION.
           ...
           STOP RUN.

EXIT PROGRAM -

It is used to mark the end of a called program's (subprogram) processing and to return control to the calling program (main program). It's mainly used in subprograms, or called programs within COBOL.

EXIT PROGRAM.

Example - MAINPROG calls the SUBPROG and it is coded with EXIT PROGRAM.

       IDENTIFICATION DIVISION.
       PROGRAM-ID. SUBPROG.
	   AUTHOR. MTH.
	   ...
       PROCEDURE DIVISION.
           ...
           EXIT PROGRAM.

Control returns to the MAINPROG when the EXIT PROGRAM is executed.

GOBACK -

The GOBACK statement specifies the logical end of a program and gives the control back from where it was received. It can code in both the main program and subprogram.

GOBACK.

Example - MAINPROG coded with GOBACK.

       IDENTIFICATION DIVISION.
       PROGRAM-ID. MAINPROG.
	   AUTHOR. MTH.
	   ...
       PROCEDURE DIVISION.
           ...
           GOBACK.

Control returns to the operating system when the GOBACK is executed.

GO TO -

It is used to transfer control to another part of the program, allowing the program to "jump" to a different paragraph or section. Different ways of using GO TO statement are -

Unconditional GO TO - It is used to transfer control unconditionally to another part of the program.

GO TO paragraph-1.

Example - Passing control to a END-PARA.

PROCEDURE DIVISION.
BEGIN-PARA.
    DISPLAY 'Start'.
    GO TO END-PARA.
    DISPLAY 'This will be skipped'.
	STOP RUN.
END-PARA.
    DISPLAY 'End'.

Output -

Start
End

Conditional GO TO - It is not advised to use individually. But, it is advised to use with conditional statements like IF and EVALUATE..

GO TO paragraph-1.

Example - GO TO with IF.

PROCEDURE DIVISION.
PARA0.
    DISPLAY 'Para0'.
	MOVE 3      TO WS-P
    PERFORM PARA1
	   THRU PARA1-EXIT.
    DISPLAY 'Return to para0'.
	STOP RUN.
PARA1.
    DISPLAY 'Para1'.
	IF WS-P > 1
	   GO TO PARA1-EXIT
	ELSE
	   COMPUTE WS-P = WS-P + 1
	END-IF.
	DISPLAY 'WS-P: ' WS-P.
PARA1-EXIT.
    EXIT.

Output -

Para0
Return to para0