Sharing Files with Windows for Workgroups Clients

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Opening a file with READ_WRITE access and using the OF_SHARE_DENY_NONE file sharing flag is possible from more than one application each running from a different workstation. Applications sharing a file from a server must use the same file sharing flags. Keeping the file open may be a necessary specification for networked database management systems because it is more efficient to keep the file open than to open and close the file for each file-access operation. However, not all applications should be designed to keep the file open during the use of a file.

To better understand file sharing, consider an application running on Workstation A opening a file that resides on the disk of Server S. An application running on Workstation B can also open the same file so that each application is aware of the updates made to the file on Server S if the file is opened with READ_WRITE access and the OF_SHARE_DENY_NONE sharing flag.

To ensure that two or more applications do not access the shared file in the same region at the same time, locking is used to create exclusive access to a region of the file. File region locking by one application causes failure of file I/O operations performed by other applications sharing the file. The sharing component of Windows for Workgroups, VSHARE.386, keeps track of the region that is locked and fails the file I/O operation attempting to access the shared file, including any portion of the locked region. This is very critical to file sharing; if one application is writing information into a location of a shared file at the same instance another application is reading from the same location of the shared file, there is a loss of data integrity.

In Windows for Workgroups, file I/O operations are redirected to the server drive via the network redirector component, VREDIR.386 (in enhanced mode) or WORKGRP.SYS (in standard mode). The network redirector of Windows for Workgroups caches file I/O operations. The redirector will disable the cache if file locking is enabled, and therefore even if two applications are sharing files and are being careful so that there is no file access concurrency, Windows for Workgroups will still fail to perform the file I/O unless file locking is used. One application reading from the file will not be reading the latest updates from the application writing to the file or data will not seem to be written to disk.

The following summarizes key points when implementing applications to share files residing on a server for full read-write access.

• Open the file with READ_WRITE access and OF_SHARE_DENY_NONE sharing flag.
• Before reading or writing, lock the region of the file about to be accessed. Use _locking() from the Microsoft C Run-time library or MS-DOS Interrupt 21h function 5Ch to lock the file region.
• Immediately following a read or a write, unlock the region. Use _locking() or Interrupt 21h function 5Ch to unlock the file region. For courtesy to other applications using the file, do not exclude the use of a file region for a long period of time. Lock the file region only during accessing (reading or writing).
• If the lock fails, this means that some portion of the region of the file is locked by another application. The application is not allowed access to this region until the other application releases it by unlocking the region.