KnowledgeBase Archive

An Archive of Early Microsoft KnowledgeBase Articles

View on GitHub

Article: Q101421
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.1
Operating System(s): 
Keyword(s): kbprogramming kb16bitonly
Last Modified: 06-NOV-1999

-------------------------------------------------------------------------------
The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) 3.1 
- Microsoft Windows for Workgroups version 3.1 
-------------------------------------------------------------------------------

SUMMARY
=======

Multiple applications, each running in Windows for Workgroups version 3.1 on
different workstations, can share a file residing on a server with full
read-write access while simultaneously keeping the file open. File region
locking ensures that data integrity of the file is not lost. The Windows for
Workgroups network redirector caches file I/O operations unless file locking is
used. It will seem that Windows for Workgroups clients are failing to write data
unless the applications lock file regions.

MORE INFORMATION
================

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.

Additional query words: remote locking

======================================================================
Keywords          : kbprogramming kb16bitonly 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWFWSearch kbWFW310 kbWinSDKSearch kbWinSDK310
Version           : WINDOWS:3.1
Issue type        : kbinfo

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Q101421: Sharing Files with Windows for Workgroups Clients