Creating a 16-bit Thunk DLL

After you compile the thunk script to create the 16-bit code for the thunk, you must create the 16-bit thunk DLL. To implement the 16-bit thunk DLL, use the following steps.

To implement the 16-bit thunk DLL

1. Add a function named DllMain to your 16-bit DLL. This function should have the following structure:

   BOOL FAR PASCAL __export DllMain(DWORD dwReason, WORD hInst, 
           WORD wDS, WORD wHeapSize, DWORD dwReserved1, 
           WORD wReserved2) 
   { 
       if( !(XXX_ThunkConnect16( "DLL16.DLL",  // name of 16-bit DLL 
           "DLL32.DLL",                        // name of 32-bit DLL 
           hInst, dwReason)) ) 
       { 
           return FALSE; 
       } 
       return TRUE; 
   } 

   Note that DllMain calls the following function, which was generated by the thunk compiler:

   BOOL FAR PASCAL __export XXX_ThunkConnect16(LPSTR pszDll16, 
       LPSTR pszDll32, WORD hInst, DWORD dwReason); 

   In this example, XXX is the base name — that is, the name of the thunk script file, not including the path and filename extension. If you used the /t option with the thunk compiler to specify a different base name when you compiled the thunk script, use that base name here.

2. Add the following statements to your 16-bit DLL's module definition (.DEF) file, choosing ordinals appropriate for your DLL.

   EXPORTS
   DllMain             @1 RESIDENTNAME
   XXX_ThunkData16     @2 RESIDENTNAME
   
   IMPORTS
   C16ThkSL01     = KERNEL.631
   ThunkConnect16 = KERNEL.651

3. Build the DLL. Be sure to link with the 16-bit .OBJ file created from the thunk script. This file contains the code for XXX_ThunkData16, which is a table of function calls the thunk compiler uses to implement the thunks.
4. Mark the DLL with version 4.0, using the 16-bit resource compiler included with the Platform SDK.

   rc -40 DLL_Name

To ensure that your application works correctly, it is important that you use the following rules when implementing your thunks:

• The 16-bit DllMain function is called each time the module's usage count is incremented or decremented. The dwReason parameter is 1 when the usage count is incremented and zero when it is decremented.
• Because the system calls the DllMain function while loading is under way, the value returned by the GetModuleUsage function is not reliable if you call it inside your DllMain function.
• Do not call thunks inside the DllMain routines. Do not perform operations that reenter the 16-bit loader, yield the 16-bit scheduler, or release the 16-bit subsystem. These actions could cause unpredictable results and are not guaranteed to work in future versions of the thunk compiler.
• You can have multiple ThunkConnect calls, as long as they connect to the same DLL. In fact, this is the only way to get both 32- to 16-bit and 16-bit to 32-bit thunks from one set of thunk DLLs. The way to do this is to have a thunk script for each direction and link both into each DLL. Then each entry-point function will have two ThunkConnect calls.