DOS:Driver Strings

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There are switches or "driver strings" you can set to control the way your application creates, reads, and writes files with a specific driver. Driver strings are simply messages or parameters that are sent to the file driver at run-time to control its behavior. See Common Driver Features--Driver Strings for an overview of these runtime Database Driver switches and parameters.

 

Some driver strings have no effect after the file is open, so no SEND function syntax is listed for those strings. However, the SEND function syntax to return the value of the switch is listed for all driver strings.

The DOS Driver supports the following Driver Strings:

FILEBUFFERS

 

DRIVER('DOS', '/FILEBUFFERS = n' )

[ Buffers" = ] SEND(file, 'FILEBUFFERS [ = n ]' )

Sets the size of the buffer used to read and write to the file, where the buffer size is (n * 512 bytes). Use the /FILEBUFFERS driver string to increase the buffer size if access is slow. Maximum buffer size is 4,294,967,264. SEND returns the size of the buffer in bytes.

 

The default buffer size for files opened denying write access to other users is the larger of 1024 or (2 * record size), and the larger of 512 or record size for all other open modes.

QUICKSCAN

 

DRIVER('DOS', '/QUICKSCAN = on | off' )

[ QScan" = ] SEND(file, 'QUICKSCAN [ = on | off ]' )

Specifies buffered access behavior. The DOS driver reads a buffer at a time (not a record), allowing faster access. In a multi-user environment these buffers are not 100% trustworthy for subsequent access, because another user may change the file between accesses. As a safeguard, the driver rereads the buffers before each record access. To disable the reread, set QUICKSCAN to ON. The default is ON for files opened denying write access to other users, and OFF for all other open modes. SEND returns the Quickscan setting (ON or OFF) in the form of a STRING(3).