Passing File Handles from TSR to Windows-Based App

The information in this article applies to:

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

SUMMARY

MS-DOS file handles are associated with a PSP (program segment prefix). For an MS-DOS terminate-and-stay-resident program (TSR) to open and operate on files on behalf of a Windows-based application, or to share file handles between MS-DOS TSRs and Windows-based applications, the TSR and application must be operating with the same PSP. This is possible because every Windows-based application has an MS-DOS PSP. Therefore, it is simply a matter of ensuring that the Windows-based application's PSP is the current PSP when the MS-DOS TSR opens or operates on a file. This article discusses some of the details involved with this process.

MORE INFORMATION

The Windows Kernel creates a unique MS-DOS PSP for every Windows-based application that is started. The only time that the application's PSP must be selected as the current PSP is when the application calls MS-DOS. Therefore, an application cannot assume that its PSP is the current PSP. Listed below are two ways to have the PSP set so that a TSR can open or operate on a file on the application's behalf:

1. The Windows-based application performs an MS-DOS call just prior to calling the TSR. This MS-DOS call must involve the file system; just retrieving the time of day is not enough. One good choice might be Get File Attributes (INT 21h, function 43h). Another possibility is to open a file. As long as the application does not yield [by calling the GetMessage, SendMessage, PeekMessage, and so forth functions], the Kernel will set the PSP for the MS-DOS call and will not change it. At this point, the application can call the TSR secure in the knowledge that the PSP is set correctly.

2. The TSR can set the current PSP using INT 21h, function 50h (the BX register is the segment of the new PSP). To do this, it is necessary that either the Windows-based application or the TSR determine the PSP for the application.

   The TSR can determine the correct PSP by following these three steps:

   a. Have the application perform an MS-DOS call just prior to the first

         call to the TSR.
   

   b. Determine the identity of the PSP by calling INT 21h, function

         51h (the segment of the current PSP is returned in the BX
         register).
   

   c. Save this information or return it to the application that will

         use the PSP as a parameter in subsequent calls to the TSR.
   

   The application can determine the PSP segment by following these three steps:

   a. Call the GetCurrentPDB function to determine its Program Data

         Base (PDB). The PDB is the Windows equivalent of the PSP.
   

   b. In protected mode, the value returned is a selector. The

         application must determine the segment address to which the
         selector maps by using the GetSelectorBase function.
   

         Note that the GetSelectorBase function is not documented in the
         Windows Software Development Kit (SDK); however, the function is
         documented in the Windows Device Development Kit (DDK). The
         function returns a LONG which is the linear address of the base.
         Dividing this value by 16 yields a segment value. Because PDBs
         are allocated using the GlobalDOSAlloc function, the linear
         address is always below 1 megabyte.
   

   c. Once the Windows-based application has determined its PSP, specify

         the PSP as a parameter when calling the TSR.