Non Blocking Channel Operations in COBOL

Our first example demonstrates non-blocking operations using COBOL. While COBOL doesn’t have built-in channel operations like some modern languages, we can simulate similar behavior using file handling and conditional statements.

       IDENTIFICATION DIVISION.
       PROGRAM-ID. NON-BLOCKING-OPERATIONS.
       ENVIRONMENT DIVISION.
       INPUT-OUTPUT SECTION.
       FILE-CONTROL.
           SELECT MESSAGE-FILE ASSIGN TO "messages.txt"
               ORGANIZATION IS LINE SEQUENTIAL.
           SELECT SIGNAL-FILE ASSIGN TO "signals.txt"
               ORGANIZATION IS LINE SEQUENTIAL.
       DATA DIVISION.
       FILE SECTION.
       FD MESSAGE-FILE.
       01 MESSAGE-RECORD.
           05 MESSAGE-TEXT PIC X(50).
       FD SIGNAL-FILE.
       01 SIGNAL-RECORD.
           05 SIGNAL-VALUE PIC X.
       WORKING-STORAGE SECTION.
       01 WS-EOF PIC X VALUE 'N'.
       01 WS-MESSAGE PIC X(50).
       01 WS-SIGNAL PIC X.
       
       PROCEDURE DIVISION.
       MAIN-PROCEDURE.
           PERFORM NON-BLOCKING-RECEIVE
           PERFORM NON-BLOCKING-SEND
           PERFORM MULTI-WAY-SELECT
           STOP RUN.

       NON-BLOCKING-RECEIVE.
           OPEN INPUT MESSAGE-FILE
           READ MESSAGE-FILE
               AT END
                   DISPLAY "No message received"
               NOT AT END
                   DISPLAY "Received message: " MESSAGE-TEXT
           END-READ
           CLOSE MESSAGE-FILE.

       NON-BLOCKING-SEND.
           MOVE "hi" TO WS-MESSAGE
           OPEN EXTEND MESSAGE-FILE
           IF WS-EOF = 'N'
               WRITE MESSAGE-RECORD FROM WS-MESSAGE
               DISPLAY "Sent message: " WS-MESSAGE
           ELSE
               DISPLAY "No message sent"
           END-IF
           CLOSE MESSAGE-FILE.

       MULTI-WAY-SELECT.
           OPEN INPUT MESSAGE-FILE
           OPEN INPUT SIGNAL-FILE
           READ MESSAGE-FILE
               AT END
                   READ SIGNAL-FILE
                       AT END
                           DISPLAY "No activity"
                       NOT AT END
                           DISPLAY "Received signal: " SIGNAL-VALUE
                   END-READ
               NOT AT END
                   DISPLAY "Received message: " MESSAGE-TEXT
           END-READ
           CLOSE MESSAGE-FILE
           CLOSE SIGNAL-FILE.

In this COBOL program, we simulate non-blocking operations using file handling. Here’s a breakdown of the main sections:

We define two files: MESSAGE-FILE and SIGNAL-FILE to simulate channels.
In the NON-BLOCKING-RECEIVE procedure, we attempt to read from the MESSAGE-FILE. If successful, we display the message; otherwise, we indicate that no message was received.
The NON-BLOCKING-SEND procedure attempts to write a message to the MESSAGE-FILE. If successful, it displays the sent message; otherwise, it indicates that no message was sent.
The MULTI-WAY-SELECT procedure simulates a multi-way non-blocking select. It first tries to read from MESSAGE-FILE, then from SIGNAL-FILE, and if both are empty, it indicates no activity.

This COBOL implementation provides a similar concept to non-blocking channel operations, adapted to COBOL’s file-handling capabilities. The program will check for messages and signals without waiting indefinitely, mimicking the non-blocking behavior of the original example.

To run the program, compile it using a COBOL compiler and execute the resulting binary. The output will depend on the contents of the messages.txt and signals.txt files at runtime.

Timeouts in COBOL

Closing Channels in COBOL

Learn X By Example

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Non Blocking Channel Operations in COBOL