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In a shared environment, you can access data in two ways: from exclusive files or from shared files. If you open a table for shared access, other users also have access to the file. If you open a table for exclusive access, no other user can read or write to that file. Because exclusive use defeats many of the benefits of sharing data on a network, it should be used sparingly.
Using a Table with Exclusive Access
The most restrictive way to open a file is to open it exclusively. When you open a table through the interface, it opens for exclusive use by default. You can also explicitly open a table for exclusive use by using Visual FoxPro commands.
To open a table for exclusive use
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Type the following commands in the Command window:
В 
Copy CodeSET EXCLUSIVE ON USE cMyTable
-or-
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Type the following command in the Command window:
В Copy CodeUSE cMyTable EXCLUSIVE
The following commands require you to open a table for exclusive use:
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ALTER TABLE - SQL Command
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INDEX Command when creating, adding, or deleting a compound index tag.
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INSERT Command
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MODIFY STRUCTURE Command To use this command to change a table structure, you must open the table exclusively. You can, however, use this command in read-only mode when you open the table for shared use.
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PACK Command
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REINDEX Command
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ZAP Command
Visual FoxPro returns the error, "Exclusive open of file is required," if you try to execute one of these commands on a shared table.
You can restrict access to a table by using the FLOCK( ) Function. If you use FLOCK(В ) to lock the table, other users cannot write to the table but they can read it.
Using a Table with Shared Access
When you open a table for shared use, more than one workstation can use the same table at the same time. When you open a table through the interface, you can override the default ON setting for the SET EXCLUSIVE Command. You can explicitly open a table for shared use by using Visual FoxPro commands.
To open a table for shared use
-
Type the following commands in the Command window:
В Copy CodeSET EXCLUSIVE OFF USE cMyTable
-or-
-
Type the following command in the Command window:
В Copy CodeUSE cMyTable SHARED
When you add or change data in a shared table, you must first lock the affected record or the entire table. You can lock a record or a table opened for shared use in the following ways:
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Use a command that performs an automatic record or table lock.
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Manually lock one or more records or an entire table with the record and table locking functions.
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Initiate buffering with the CURSORSETPROP( ) Function.
Associated memo and index files always open with the same share status as their table.
If your application uses a table for lookup purposes only and all users of the application access it, then you can improve performance by marking the table as read-only.