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'This example illustrates the use of the LOCK and UNLOCK statements. A sample
'data record is created in a random-access file containing a customer record.
'The program allows you to update the file, locking the file while you use it
'to prevent access from another terminal.
 
'Note: In order to use this program, you must have DOS version 3.1 or
'later, and you must run the DOS SHARE.EXE to enable locking operations
'before entering the BASIC programming environment.
 
'Define the record.
TYPE AccountRec
   Payer AS STRING * 20
   Address AS STRING * 20
   Place AS STRING * 20
   Owe AS SINGLE
END TYPE
 
DIM CustRec AS AccountRec
 
'This section creates a sample record to use.
ON ERROR GOTO ErrHandler
 
OPEN "MONITOR" FOR RANDOM SHARED AS #1 LEN = LEN(CustRec)
 
CustRec.Payer = "George Washington"
CustRec.Address = "1 Cherry Tree Lane"
CustRec.Place = "Mt. Vernon, VA"
CustRec.Owe = 12!
PUT #1, 1, CustRec          'Put one record in the file.
CLOSE #1
 
'This sections opens the sample record for updating.
OPEN "MONITOR" FOR RANDOM SHARED AS #1 LEN = LEN(CustRec)
DO
    Number% = 0             'Reset to zero.
    DO UNTIL Number% = 1    'Force user to input 1.
        CLS : LOCATE 10, 10
        INPUT "Customer Number?   #"; Number%
    LOOP
    'Lock the current record so another process
    'doesn't change it while you're using it.
    LOCK #1, Number%
    GET #1, Number%, CustRec
    LOCATE 11, 10: PRINT "Customer: "; CustRec.Payer
    LOCATE 12, 10: PRINT "Address:  "; CustRec.Address
    LOCATE 13, 10: PRINT "Currently owes: $"; CustRec.Owe
    LOCATE 15, 10: INPUT "Change (+ or -)", Change!
    CustRec.Owe = CustRec.Owe + Change!
    PUT #1, Number%, CustRec
    'Unlock the record so others can use it.
    UNLOCK #1, Number%
    LOCATE 17, 10: INPUT "Update another? ", Continue$
    Update$ = UCASE$(LEFT$(Continue$, 1))
LOOP WHILE Update$ = "Y"
CLOSE #1
 
'Remove file from disk.
KILL "MONITOR"
END
 
ErrHandler:
IF ERR = 70 THEN        'Permission denied error.
    CLS
    PRINT "You must run SHARE.EXE before running this example."
    PRINT "Exit the programming environment, run SHARE.EXE, and"
    PRINT "reenter the programming environment to run this"
    PRINT "example. Do not shell to DOS to run SHARE.EXE or you"
    PRINT "may not be able to run other programs until you reboot."
END IF
END