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COBOL UNLOCK Statement

The UNLOCK statement is used for releasing file locks in multi-user environments.

Overview

The UNLOCK statement provides a mechanism for releasing file locks that were previously established on file records. This is essential in multi-user environments where multiple programs or users need concurrent access to the same files.

When a file record is locked, other users or processes are prevented from accessing that record until the lock is released. The UNLOCK statement allows you to explicitly release these locks when they are no longer needed.

Syntax

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UNLOCK file-name RECORD

Basic syntax for the UNLOCK statement. The file-name specifies which file to unlock, and the RECORD keyword indicates that the current record should be unlocked.

Parameters

  • file-name: The name of the file from which to release locks
  • RECORD: Optional keyword indicating that the current record should be unlocked

Practical Examples

Example 1: Basic Record Unlocking

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       IDENTIFICATION DIVISION.
       PROGRAM-ID. UNLOCK-EXAMPLE.
       
       ENVIRONMENT DIVISION.
       INPUT-OUTPUT SECTION.
       FILE-CONTROL.
           SELECT CUSTOMER-FILE ASSIGN TO "CUSTOMER.DAT"
               ORGANIZATION IS INDEXED
               ACCESS MODE IS RANDOM
               RECORD KEY IS CUSTOMER-ID.
       
       DATA DIVISION.
       FILE SECTION.
       FD  CUSTOMER-FILE.
       01  CUSTOMER-RECORD.
           05  CUSTOMER-ID       PIC 9(6).
           05  CUSTOMER-NAME     PIC X(30).
           05  CUSTOMER-BALANCE  PIC 9(8)V99.
       
       WORKING-STORAGE SECTION.
       01  WS-CUSTOMER-ID        PIC 9(6) VALUE 123456.
       
       PROCEDURE DIVISION.
       MAIN-PROCESS.
           OPEN I-O CUSTOMER-FILE
           
           * Read and lock a record
           READ CUSTOMER-FILE
               INVALID KEY DISPLAY "Customer not found"
               NOT INVALID KEY
                   * Process the record
                   DISPLAY "Processing customer: " CUSTOMER-NAME
                   
                   * Update the record
                   ADD 100 TO CUSTOMER-BALANCE
                   REWRITE CUSTOMER-RECORD
                   
                   * Unlock the record
                   UNLOCK CUSTOMER-FILE RECORD
           END-READ
           
           CLOSE CUSTOMER-FILE
           STOP RUN.

This example demonstrates basic record unlocking. The program reads a customer record, updates the balance, rewrites the record, and then unlocks it to allow other users access. The UNLOCK statement releases the lock that was established when the record was read.

Example 2: Multi-User File Access

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       IDENTIFICATION DIVISION.
       PROGRAM-ID. MULTI-USER-ACCESS.
       
       ENVIRONMENT DIVISION.
       INPUT-OUTPUT SECTION.
       FILE-CONTROL.
           SELECT INVENTORY-FILE ASSIGN TO "INVENTORY.DAT"
               ORGANIZATION IS INDEXED
               ACCESS MODE IS RANDOM
               RECORD KEY IS PRODUCT-ID.
       
       DATA DIVISION.
       FILE SECTION.
       FD  INVENTORY-FILE.
       01  INVENTORY-RECORD.
           05  PRODUCT-ID        PIC 9(6).
           05  PRODUCT-NAME      PIC X(30).
           05  QUANTITY-ON-HAND  PIC 9(5).
       
       WORKING-STORAGE SECTION.
       01  WS-PRODUCT-ID         PIC 9(6).
       01  WS-QUANTITY-TO-ADD    PIC 9(5).
       
       PROCEDURE DIVISION.
       UPDATE-INVENTORY.
           DISPLAY "Enter product ID: "
           ACCEPT WS-PRODUCT-ID
           
           DISPLAY "Enter quantity to add: "
           ACCEPT WS-QUANTITY-TO-ADD
           
           OPEN I-O INVENTORY-FILE
           
           * Read the product record
           MOVE WS-PRODUCT-ID TO PRODUCT-ID
           READ INVENTORY-FILE
               INVALID KEY 
                   DISPLAY "Product not found"
                   CLOSE INVENTORY-FILE
                   STOP RUN
               NOT INVALID KEY
                   * Update quantity
                   ADD WS-QUANTITY-TO-ADD TO QUANTITY-ON-HAND
                   
                   * Rewrite the record
                   REWRITE INVENTORY-RECORD
                   
                   * Unlock to allow other users access
                   UNLOCK INVENTORY-FILE RECORD
                   
                   DISPLAY "Inventory updated successfully"
           END-READ
           
           CLOSE INVENTORY-FILE
           STOP RUN.

This example shows how UNLOCK is used in a multi-user inventory system. After updating the inventory quantity, the record is unlocked to allow other users to access the same product record. This prevents conflicts when multiple users are updating inventory simultaneously.

Example 3: Error Handling with UNLOCK

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       IDENTIFICATION DIVISION.
       PROGRAM-ID. UNLOCK-ERROR-HANDLING.
       
       ENVIRONMENT DIVISION.
       INPUT-OUTPUT SECTION.
       FILE-CONTROL.
           SELECT ACCOUNT-FILE ASSIGN TO "ACCOUNT.DAT"
               ORGANIZATION IS INDEXED
               ACCESS MODE IS RANDOM
               RECORD KEY IS ACCOUNT-NUMBER.
       
       DATA DIVISION.
       FILE SECTION.
       FD  ACCOUNT-FILE.
       01  ACCOUNT-RECORD.
           05  ACCOUNT-NUMBER    PIC 9(10).
           05  ACCOUNT-BALANCE   PIC 9(10)V99.
           05  ACCOUNT-STATUS    PIC X(1).
       
       WORKING-STORAGE SECTION.
       01  WS-ACCOUNT-NUMBER     PIC 9(10).
       01  WS-WITHDRAWAL-AMOUNT  PIC 9(8)V99.
       
       PROCEDURE DIVISION.
       PROCESS-WITHDRAWAL.
           DISPLAY "Enter account number: "
           ACCEPT WS-ACCOUNT-NUMBER
           
           DISPLAY "Enter withdrawal amount: "
           ACCEPT WS-WITHDRAWAL-AMOUNT
           
           OPEN I-O ACCOUNT-FILE
           
           * Read the account record
           MOVE WS-ACCOUNT-NUMBER TO ACCOUNT-NUMBER
           READ ACCOUNT-FILE
               INVALID KEY 
                   DISPLAY "Account not found"
                   CLOSE ACCOUNT-FILE
                   STOP RUN
               NOT INVALID KEY
                   * Check if withdrawal is possible
                   IF ACCOUNT-BALANCE >= WS-WITHDRAWAL-AMOUNT
                       * Process withdrawal
                       SUBTRACT WS-WITHDRAWAL-AMOUNT FROM ACCOUNT-BALANCE
                       
                       * Update the record
                       REWRITE ACCOUNT-RECORD
                       
                       * Unlock the record
                       UNLOCK ACCOUNT-FILE RECORD
                       
                       DISPLAY "Withdrawal processed successfully"
                   ELSE
                       * Insufficient funds - unlock without updating
                       UNLOCK ACCOUNT-FILE RECORD
                       DISPLAY "Insufficient funds"
                   END-IF
           END-READ
           
           CLOSE ACCOUNT-FILE
           STOP RUN.

This example demonstrates error handling with UNLOCK. Even when there are insufficient funds and no update is made, the record is still unlocked to release the lock. This ensures that other users can access the account record even when the current operation fails.

Best Practices and Considerations

Best Practices

  • Always unlock records when you are finished processing them
  • Unlock records even when errors occur to prevent deadlocks
  • Use UNLOCK in conjunction with proper error handling
  • Consider the timing of UNLOCK operations in your application logic
  • Test UNLOCK behavior in multi-user scenarios

Considerations

  • File organization and access mode compatibility
  • Multi-user environment requirements
  • Performance implications of locking and unlocking
  • Error handling and recovery procedures
  • Deadlock prevention strategies

Test Your Knowledge

1. What is the primary purpose of the UNLOCK statement in COBOL?

  • To unlock program execution
  • To release file locks in multi-user environments
  • To unlock memory resources
  • To unlock database connections

2. In which COBOL division is the UNLOCK statement typically used?

  • IDENTIFICATION DIVISION
  • ENVIRONMENT DIVISION
  • DATA DIVISION
  • PROCEDURE DIVISION

3. What is the relationship between UNLOCK and file sharing?

  • They are unrelated
  • UNLOCK enables file sharing by releasing locks
  • File sharing enables UNLOCK
  • They are the same thing

4. When should you use the UNLOCK statement?

  • Always after every file operation
  • When you want to release file locks to allow other users access
  • Only when closing files
  • Only for error conditions

5. What happens when an UNLOCK statement is executed?

  • The file is closed
  • File locks are released, allowing other users access
  • The program stops
  • Memory is freed

Frequently Asked Questions

Related Concepts

READ Statement

Reading records from files

REWRITE Statement

Updating existing records

File Organization

Understanding different file organizations

File Sharing

File sharing and access modes

Error Handling

Handling file operation errors

TRANSFORMUNSTRING and END-UNSTRING