Oracle7 Server Application Developer's Guide

Sample User Locks

Some uses of user locks are:

providing exclusive access to a device, such as a terminal

providing application-level enforcement of read locks

detect when a lock is released and cleanup after the application

synchronizing applications and enforce sequential processing

The following Pro*COBOL precompiler example shows how locks can be used to ensure that there are no conflicts when multiple people need to access a single device.

***************************************************************** 
* Print Check                                                   * 
* Any cashier may issue a refund to a customer returning goods. * 
* Refunds under $50 are given in cash, above that by check.     * 
* This code prints the check. The one printer is opened by all  * 
* the cashiers to avoid the overhead of opening and closing it  * 
* for every check. This means that lines of output from multiple* 
* cashiers could become interleaved if we don't ensure exclusive* 
* access to the printer. The DBMS_LOCK package is used to       * 
* ensure exclusive access.                                      * 
***************************************************************** 
CHECK-PRINT 
* 
*    Get the lock "handle" for the printer lock. 
   MOVE "CHECKPRINT" TO LOCKNAME-ARR. 
   MOVE 10 TO LOCKNAME-LEN. 
   EXEC SQL EXECUTE 
      BEGIN DBMS_LOCK.ALLOCATE_UNIQUE ( :LOCKNAME, :LOCKHANDLE ); 
      END; END-EXEC. 
* 
*   Lock the printer in exclusive mode (default mode).
   EXEC SQL EXECUTE 
      BEGIN DBMS_LOCK.REQUEST ( :LOCKHANDLE ); 
      END; END-EXEC. 
*   We now have exclusive use of the printer, print the check. 

  ... 

*
*   Unlock the printer so other people can use it 
*
   EXEC SQL EXECUTE 
      BEGIN DBMS_LOCK.RELEASE ( :LOCKHANDLE ); 
      END; END-EXEC.