SAMPLE: Maintaining a Hidden Application

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.1 and 3.0

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.

Download HIDAPP.EXE from the Microsoft Software Library (MSL) on the following services:

• Microsoft Download Service (MSDL)

        Dial (206) 936-6735 to connect to MSDL
        Download HIDAPP.EXE (size: 41742 bytes) 
  

• Internet (anonymous FTP)

        ftp ftp.microsoft.com
        Change to the \SOFTLIB\MSLFILES directory
        Get HIDAPP.EXE (size: 41742 bytes) 
  

MORE INFORMATION

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.