C5_USE

USE
 Open an existing database (.dbf) and its associated files
------------------------------------------------------------------------------
 Syntax

     USE [<xcDatabase>
        [INDEX <xcIndex list>]
        [ALIAS <xcAlias>] [EXCLUSIVE | SHARED]
        [NEW] [READONLY]
        [VIA <cDriver>]]

 Arguments

     <xcDatabase> is the name of the database file to be opened and may
     be specified either as a literal file name or as a character expression
     enclosed in parentheses.

     INDEX <xcIndex list> specifies the names of 1 to 15 index files to
     be opened in the current work area.  Specify each index as a literal
     file name or as a character expression enclosed in parentheses.  The
     first index in the list becomes the controlling index.  If you specify
     an <xcIndex> as an expression and the value returned is spaces or NIL,
     it is ignored.

     ALIAS <xcAlias> specifies the name to associate with the work area
     when the database file is opened.  You may specify the alias name as a
     literal name or as a character expression enclosed in parentheses.  A
     valid <xcAlias> may be any legal identifier (i.e., it must begin with an
     alphabetic character and may contain numeric or alphabetic characters
     and the underscore).  Within a single application, Clipper will not
     accept duplicate aliases.  If this clause is omitted, the alias defaults
     to the database file name.

     EXCLUSIVE opens the database file for nonshared use in a network
     environment.  All other users are denied access until the database file
     is CLOSEd.

     SHARED opens the database file for shared use in a network
     environment.  Specifying this clause overrides the current EXCLUSIVE
     setting.

     NEW opens <xcDatabase> in the next available work area making it the
     current work area.  If this clause is not specified, <xcDatabase> is
     opened in the current work area.

     READONLY opens <xcDatabase> with a read-only attribute.  This lets
     you open database files marked read-only.  If you cannot open the
     <xcDatabase> this way, a runtime error is generated.  If this clause is
     not specified, <xcDatabase> is opened as read-write.

     VIA <cDriver> specifies the replaceable database driver (RDD) with
     which to process the current work area.  <cDriver> is the name of the
     RDD specified as a character expression.  If <cDriver> is specified as a
     literal value, it must be enclosed in quotes.

     If the VIA clause is omitted, the DBFNTX driver is used by default.
     Note that if the specified driver is not linked, an unrecoverable error
     occurs.

     In no arguments are specified, the database file open in the current
     work area is closed.

 Description

     USE opens an existing database (.dbf) file, its associated memo (.dbt)
     file, and optionally associated index (.ntx or .ndx) file(s) in the
     current or the next available work area.  In Clipper, there are 250
     work areas with a maximum of 255 total files open in DOS 3.3 and above.
     Before USE opens a database file and its associated files, it closes any
     active files already open in the work area.  When a database file is
     first opened, the record pointer is positioned at the first logical
     record in the file (record one, if there is no index file specified).

     In a network environment, you may open database files as EXCLUSIVE or
     SHARED.  EXCLUSIVE precludes the USE of the database file by other users
     until the file is closed.  SHARED allows other users to USE the database
     file for concurrent access.  If the database file is SHARED, responsibility
     for data integrity falls upon the application program.  In Clipper,
     FLOCK() and RLOCK() are the two basic means of denying other users
     access to a particular work area or record.  If a USE is specified and
     neither EXCLUSIVE nor SHARED is specified, the database file is opened
     according to the current EXCLUSIVE setting.  In Clipper, all USE
     commands should explicitly specify how the database file is to be
     opened, EXCLUSIVE or SHARED.  The implicit open mode specified by SET
     EXCLUSIVE is supplied for compatibility purposes only and not
     recommended.

     Opening a database file in a network environment requires some special
     handling to be successful.  First, attempt to USE the database file
     without specifying the INDEX list.  Then, test for the success of the
     operation using NETERR().  If NETERR() returns false (.F.), the open
     operation succeeded and you can SET INDEX TO the index list.  A USE will
     fail in a network environment if another user has EXCLUSIVE USE of the
     database file.  Refer to the "Network Programming" chapter in the
     Programming and Utilities Guide for more information on opening files in
     a network environment.

     You can open index files with USE or SET INDEX.  The first index in the
     list of indexes defines the current ordering of records when they are
     accessed.  This index is referred to as the controlling index.  You can
     change the current controlling index without closing any files by using
     the SET ORDER command.

     To close a database and its associated files in the current work area,
     specify USE or CLOSE with no arguments.  To close database files in all
     work areas, use CLOSE DATABASEs.  To close index files in the current
     work area without closing the database file, use CLOSE INDEX or SET
     INDEX TO with no arguments.

     Refer to the "Basic Concepts" chapter in the Programming and Utilities
     Guide for more information about the Clipper database paradigm.

 Notes

     .  Setting the maximum open files: Control of the number of file
        handles available to a Clipper application is controlled by a
        combination of the CONFIG.SYS FILES command, and the F parameter of
        the CLIPPER environment variable.  The F parameter specifies the
        maximum number of files that can be opened at any one time within the
        current Clipper program.  Clipper determines the number of
        files that can be opened using the smaller of the two parameters.
        For example, if the FILES command is set to 120 and the F parameter
        is set to 50, the maximum number of files that can be opened is 50.
        In a network environment, file handles also need to be set in the
        network configuration file.

        The file limit is controlled by the operating system.  Under DOS
        versions less than 3.3, the maximum number of files that can be
        opened at one time is 20 files.  In DOS versions 3.3 and greater, the
        maximum limit is 255 files.

     .  Opening the same database file in more than one work area:
        Although opening a database file in more than one work area is
        possible in a network environment, this practice is strongly
        discouraged.  If done, each file must be opened with a different
        alias, otherwise a runtime error will occur.

     .  Opening two database files with the same names, in different
        directories: Although opening two database files with the same
        names in different directories is possible, the database files MUST
        have unique alias names; otherwise, a runtime error will occur.

 Examples

     .  This example opens a shared database file with associated
        index files in a network environment.  If NETERR() returns false
        (.F.), indicating the USE was successful, the indexes are opened:

        USE Accounts SHARED NEW
        IF !NETERR()
           SET INDEX TO AcctNames, AcctZip
        ELSE
           ? "File open failed"
           BREAK
        ENDIF

     .  This example opens a database file with several indexes
        specified as extended expressions.  Note how the array of index names
        is created as a constant array:

        xcDatabase = "MyDbf"
        xcIndex = {"MyIndex1", "MyIndex2", "MyIndex3"}
        USE (xcDatabase) INDEX (xcIndex[1]), ;
              (xcIndex[2]), (xcIndex[3])

 Files   Library is CLIPPER.LIB.

See Also: CLOSE DBRSELECT() DBSETINDEX() DBUSEAREA() NETERR()



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3 responses to “C5_USE

  1. Pingback: DB Commands | Viva Clipper !

  2. Pingback: C5 Commands | Viva Clipper !

  3. Pingback: C5_ZAP | Viva Clipper !

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