LOCK...UNLOCK Statement
 ----------------------------------------------------------------------------


 Action

 Controls access by other processes to all or part of an opened file.


 Syntax



  LOCK #  filenumber% ,{ record& |  start&  TO  end&}
 .

 .

 .



  UNLOCK #  filenumber% ,{ record& |  start&  TO  end&}


 Remarks


 These statements are used in networked environments where several processes
 might need access to the same file. The  LOCK and  UNLOCK statements use the
 following arguments.

-----------------------------------------------------------------------------
 Argument                                Description
 ----------------------------------------------------------------------------
  filenumber%                            The number used in the  OPEN
                                         statement to open the file.

  record&                                The number of the record or byte
                                         to be locked. The argument
                                         record& can be any number from 1
 Argument                                Description
 ----------------------------------------------------------------------------
                                         record& can be any number from 1
                                         to 2,147,483,647 (equivalent to
                                         231 -1). A record can be up to
                                         32,767 bytes in length.

  start&                                 The number of the first record or
                                         byte to be locked.

  end&                                   The number of the last record or
                                         byte to be locked..



 For binary-mode files,  record&,  start&, and  end& represent the number of
 a byte relative to the beginning of the file. The first byte in a file is
 byte 1.

 For random-access files,  record&,  start&, and  end& represent the number
 of a record relative to the beginning of the file. The first record is
 record 1.

 If the file has been opened for sequential input or output,  LOCK and
 UNLOCK affect the entire file, regardless of the range specified by  start&
 and  end&.

 The  LOCK and  UNLOCK statements are always used in pairs. The arguments to
 LOCK and  UNLOCK must match exactly.

 If you specify just one record, then only that record is locked or unlocked.
 If you specify a range of records and omit a starting record ( start&), then
 all records from the first record to the end of the range ( end&) are locked
 or unlocked.  LOCK with no  record& locks the entire file, while  UNLOCK
 with no  record& unlocks the entire file.

  LOCK and  UNLOCK execute only at run time if you are using OS-2 or versions
 of DOS that support networking (version 3.1 or later). In DOS, you must run
 the SHARE.EXE program to enable locking operations. Using earlier versions
 of DOS causes BASIC to generate the error message Feature unavailable if
 LOCK and  UNLOCK are executed.


 Warning

 Be sure to remove all locks with an  UNLOCK statement before closing a file
 or terminating your program. Failing to remove locks produces unpredictable
 results.
 The arguments to  LOCK and  UNLOCK must match exactly.
 Do not use  LOCK and  UNLOCK on devices or ISAM tables.


 If you attempt to access a file that is locked, BASIC may generate the
 following error messages.

 Bad record number

 Permission denied


 Example

 The following example illustrates the use of the  LOCK and  UNLOCK
 statements. The program creates a sample data record in a random-access
 file. Once the sample record is created and closed, the program again opens
 the file to allow customer information to be updated. As a record is opened,
 it is locked to prevent use by another terminal that has access to the same
 file. After the update is complete, the record is unlocked, again allowing
 modifications by others with access.

 ' 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 section 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.


     UNLOCK #1, Number%


     LOCATE 17, 10. INPUT "Update another? ", Continue$


     Update$ = UCASE$(LEFT$(Continue$, 1))


 LOOP WHILE Update$ = "Y"


 CLOSE #1


 KILL "MONITOR"' Remove file from disk.


 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