Q92828: Maintaining a Hidden Application

Article: Q92828
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.0,3.1
Operating System(s): 
Keyword(s): kbfile kbsample kb16bitonly kbOSWin310
Last Modified: 06-DEC-1999

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

- Microsoft Windows Software Development Kit (SDK) versions 3.0, 3.1 
-------------------------------------------------------------------------------

SUMMARY
=======

HIDAPP is a sample DLL (dynamic-link library) that uses a hidden application to
do file I/O for the DLL. Using this architecture, it is possible for multiple
applications to link to the DLL and access the file through the DLL.

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

The following files are available for download from the Microsoft Download
Center:

Hidapp.exe

For additional information about how to download Microsoft Support files, click
the article number below to view the article in the Microsoft Knowledge Base:

  Q119591 How to Obtain Microsoft Support Files from Online Services

Microsoft used the most current virus detection software available on the date of
posting to scan this file for viruses. Once posted, the file is housed on secure
servers that prevent any unauthorized changes to the file.


It is often desirable to have a DLL open and manage a data file. This way, many
applications can link to the DLL and access the file. Because all access is done
through the DLL, there are no problems associated with having more than one
application opening the file at the same time.

Under Windows, DLLs cannot own files; only applications can. File handles are
indices into the file table, which is stored in the task database. When a DLL
opens a file, the application that called the DLL owns that file handle. If a
DLL saves the file handle in a global variable and then later tries to use it
when it is called by a different application, then that file handle will be
invalid.

This HIDAPP sample application discusses one way to work around this limitation.
It consists of a DLL that uses a hidden application to access files for the DLL
that act as a server. When the first instance of client application links to the
DLL, the hidden application is executed and a data file is opened. The handle
for this data file is owned by the hidden application. After this, other client
applications may link to the DLL and call functions to add and retrieve records
from the data file. When the last client application shuts down, the DLL closes
the data file and shuts down the hidden application.

Additional query words: 3.00 3.10 softlib HIDAPP.EXE kbFile

======================================================================
Keywords          : kbfile kbsample kb16bitonly kbOSWin310 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK300 kbWinSDK310
Version           : WINDOWS:3.0,3.1

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

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.

Copyright Microsoft Corporation 1986-2002.