COBOL COMMON

The COMMON clause in COBOL is used for defining shared data areas and common storage accessible across multiple programs. This feature enables efficient inter-program communication and data sharing in modular COBOL applications.

Overview and Purpose

COMMON data areas in COBOL provide a mechanism for sharing data between multiple programs or subprograms without the overhead of copying data. These shared areas exist in a common memory space and can be accessed by different programs simultaneously. This capability is essential for building modular applications where programs need to share configuration data, maintain shared counters, or communicate status information efficiently.

Basic COMMON Area Definition

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LINKAGE SECTION.
01  COMMON-DATA-AREA.
    05  SYSTEM-STATUS       PIC X(10).
    05  RECORD-COUNTER      PIC 9(8) COMP.
    05  ERROR-FLAG          PIC X.
    05  LAST-UPDATE-TIME    PIC X(14).

This example shows a basic COMMON data area defined in the LINKAGE SECTION. The structure includes system status information, a record counter, an error flag, and timestamp data. This common area can be shared between multiple programs that need to coordinate their activities or share operational data. The LINKAGE SECTION placement indicates that this data is external to the program and shared with other programs.

Shared Configuration Data

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01  COMMON-CONFIG.
    05  APPLICATION-MODE    PIC X(10) VALUE "PRODUCTION".
    05  DEBUG-LEVEL         PIC 9 VALUE 0.
    05  MAX-RECORDS         PIC 9(7) VALUE 1000000.
    05  OUTPUT-DIRECTORY    PIC X(50).
    05  LOG-FILE-NAME       PIC X(30).

This configuration structure demonstrates how COMMON areas can be used to share application settings across multiple programs. All programs in the application suite can access the same configuration values, ensuring consistency in behavior. When configuration changes are needed, they can be made in one place and immediately affect all programs that reference the common area.

Inter-Program Communication Buffer

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01  COMMUNICATION-BUFFER.
    05  MESSAGE-TYPE        PIC X(8).
    05  SENDER-PROGRAM      PIC X(8).
    05  MESSAGE-LENGTH      PIC 9(4) COMP.
    05  MESSAGE-DATA        PIC X(200).
    05  RESPONSE-CODE       PIC X(4).
    05  TIMESTAMP           PIC X(26).

This communication buffer structure enables programs to exchange messages through a shared common area. One program can write a message with metadata about the sender and message type, while another program can read and process the message. This pattern is useful for implementing simple inter-program communication without complex messaging systems.

Tutorial: Implementing Shared Data Management

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01  SHARED-WORKSPACE.
    05  CONTROL-FLAGS.
        10  INITIALIZATION-DONE PIC X VALUE 'N'.
        10  SHUTDOWN-REQUESTED  PIC X VALUE 'N'.
        10  ERROR-OCCURRED      PIC X VALUE 'N'.
    05  COUNTERS.
        10  TOTAL-RECORDS       PIC 9(8) COMP VALUE ZERO.
        10  ERROR-COUNT         PIC 9(6) COMP VALUE ZERO.

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UPDATE-SHARED-COUNTER.
    ADD 1 TO TOTAL-RECORDS OF SHARED-WORKSPACE
    IF TOTAL-RECORDS > 999999
        MOVE 'Y' TO ERROR-OCCURRED
        MOVE 'COUNTER_OVERFLOW' TO LAST-ERROR-CODE
    END-IF.

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INITIALIZE-SHARED-AREA.
    IF INITIALIZATION-DONE = 'N'
        MOVE 'Y' TO INITIALIZATION-DONE
        MOVE ZERO TO TOTAL-RECORDS
        MOVE ZERO TO ERROR-COUNT
        MOVE SPACES TO LAST-ERROR-CODE
    END-IF.

Practical Exercises

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01  BATCH-STATISTICS.
    05  PROCESSING-STATS.
        10  RECORDS-PROCESSED   PIC 9(8) COMP.
        10  RECORDS-REJECTED    PIC 9(6) COMP.
        10  START-TIME          PIC X(8).
        10  END-TIME            PIC X(8).
    05  ERROR-SUMMARY.
        10  VALIDATION-ERRORS   PIC 9(6) COMP.
        10  SYSTEM-ERRORS       PIC 9(4) COMP.

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01  APPLICATION-CONFIG.
    05  ENVIRONMENT-SETTINGS.
        10  ENVIRONMENT-TYPE    PIC X(10).
        10  LOG-LEVEL           PIC 9.
        10  TRACE-ENABLED       PIC X.
    05  FILE-PATHS.
        10  INPUT-PATH          PIC X(100).
        10  OUTPUT-PATH         PIC X(100).
        10  BACKUP-PATH         PIC X(100).

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01  WORK-QUEUE-CONTROL.
    05  QUEUE-STATUS.
        10  ACTIVE-WORKERS      PIC 9(3) COMP.
        10  PENDING-TASKS       PIC 9(6) COMP.
        10  COMPLETED-TASKS     PIC 9(8) COMP.
    05  COORDINATION-FLAGS.
        10  QUEUE-SHUTDOWN      PIC X.
        10  EMERGENCY-STOP      PIC X.

Advanced COMMON Area Techniques

Version Control and Compatibility

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01  COMMON-HEADER.
    05  STRUCTURE-VERSION   PIC X(8) VALUE "V2.1.0".
    05  CREATED-DATE        PIC X(8).
    05  LAST-MODIFIED       PIC X(8).
    05  CHECKSUM            PIC 9(8) COMP.

Include version information in your common areas to ensure compatibility between different versions of programs. This helps detect when programs are using incompatible data structures and allows for graceful handling of version mismatches.

Synchronization and Locking

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01  SYNCHRONIZATION-CONTROL.
    05  LOCK-FLAG           PIC X VALUE 'N'.
    05  LOCK-OWNER          PIC X(8).
    05  LOCK-TIMESTAMP      PIC X(26).
    05  WAITING-PROGRAMS    PIC 9(3) COMP.

Implement simple locking mechanisms to coordinate access to shared data. Use flags and timestamps to manage exclusive access when programs need to modify shared data safely. This prevents data corruption from simultaneous updates.

Error Handling and Recovery

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01  ERROR-MANAGEMENT.
    05  LAST-ERROR-CODE     PIC X(8).
    05  ERROR-PROGRAM       PIC X(8).
    05  ERROR-TIMESTAMP     PIC X(26).
    05  RECOVERY-ACTION     PIC X(20).

Include error tracking and recovery information in common areas to help programs coordinate error handling and recovery procedures. This enables better system resilience and debugging capabilities when problems occur in multi-program environments.