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 FLOCK()
 Lock an open and shared database file
------------------------------------------------------------------------------
 Syntax

     FLOCK() --> lSuccess

 Returns

     FLOCK() returns true (.T.) if an attempt to lock a database file in USE
     in the current work area succeeds; otherwise, it returns false (.F.).
     For more information on file locking, refer to the Network Programming
     chapter in the Programming and Utilities guide.

 Description

     FLOCK() is a database function used in network environments to lock an
     open and shared database file, preventing other users from updating the
     file until the lock is released.  Records in the locked file are
     accessible for read-only operations.

     FLOCK() is related to USE...EXCLUSIVE and RLOCK().  USE...EXCLUSIVE
     opens a database file so that no other user can open the same file at
     the same time and is the most restrictive locking mechanism in
     CA-Clipper.  RLOCK() is the least restrictive and attempts to place an
     update lock on a shared record, precluding other users from updating the
     current record.  FLOCK() falls in the middle.

     FLOCK() is used for operations that access the entire database file.
     Typically, these are commands that update the file with a scope or a
     condition such as DELETE or REPLACE ALL.  The following is a list of
     such commands:

     Commands that require an FLOCK()
     ------------------------------------------------------------------------
     Command                       Mode
     ------------------------------------------------------------------------
     APPEND FROM                   FLOCK() or USE...EXCLUSIVE
     DELETE (multiple records)     FLOCK() or USE...EXCLUSIVE
     RECALL (multiple records)     FLOCK() or USE...EXCLUSIVE
     REPLACE (multiple records)    FLOCK() or USE...EXCLUSIVE
     UPDATE ON                     FLOCK() or USE...EXCLUSIVE
     ------------------------------------------------------------------------

     For each invocation of FLOCK(), there is one attempt to lock the
     database file, and the result is returned as a logical value.  A file
     lock fails if another user currently has a file or record lock for the
     same database file or EXCLUSIVE USE of the database file.  If FLOCK() is
     successful, the file lock remains in place until you UNLOCK, CLOSE the
     DATABASE, or RLOCK().

     By default, FLOCK() operates on the currently selected work area as
     shown in the example below.

 Notes

     .  SET RELATION: CA-Clipper does not automatically lock all work
        areas in the relation chain when you lock the current work area, and
        an UNLOCK has no effect on related work areas.

 Examples

     .  This example uses FLOCK() for a batch update of prices in
        Inventory.dbf:

        USE Inventory NEW
        IF FLOCK()
           REPLACE ALL Inventory->Price WITH ;
                 Inventory->Price * 1.1
        ELSE
           ? "File not available"
        ENDIF

     .  This example uses an aliased expression to attempt a file lock
        in an unselected work area:

        USE Sales NEW
        USE Customer NEW
        //
        IF !Sales->(FLOCK())
           ? "Sales is in use by another"
        ENDIF

 Files:  Library is CLIPPER.LIB.