Microsoft KB Archive/154093

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HOWTO: Call 32-bit Code from 16-bit Code Under Windows 95

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Q154093

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The information in this article applies to:


 * Microsoft Win32 Software Development Kit (SDK), used with:
 * Microsoft Windows 95

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SUMMARY
It is often desirable to port Win16 applications and DLLs to Win32 a little at a time rather than all at once. For example, you may want to port 16-bit DLLs to Win32 but still be able to call them from 16-bit code. This article discusses the mechanism by which 16-bit DLLs can call 32-bit DLLs. The mechanism is called a thunk and the method implemented under Windows 95 is called a flat thunk.

The three major steps in writing the thunk code are:


 * 1) Creating the thunk Script.
 * 2) Building the 16-bit DLL.
 * 3) Building the 32-bit DLL.

MORE INFORMATION
A flat thunk consists of a 16-bit and a 32-bit DLL that work together. A Win16 application calls the 16-bit DLL and the 16-bit DLL calls an exported function in the 32-bit DLL. When the function in the 32-bit DLL returns, it returns back to the 16-bit DLL, which in turn returns back to the Win16 application. The 16-bit and 32-bit DLLs work by calling the Windows 95 16- bit and 32-bit kernels to handle all of the low-level details necessary to make the transition from 16-bit to 32-bit code and back.

Designing a new flat thunk involves creating a thunk script (.THK file). This script is compiled with the thunk compiler into an assembly-language file which is assembled twice; one time with each of two flags: -DIS_16 and -DIS_32. The result is a 16-bit and a 32-bit object module. These object modules are linked into the 16-bit and 32-bit DLLs respectively. The following diagram summarizes the files involved in building the DLLs:

                      ++ | 16to32.THK | ++                            |                       ++                       | 16to32.ASM | ++                       /          \                -DIS_16 /            \ -DIS_32 /             \                  +---+ +---+                 | 16THK.OBJ | | 32THK.OBJ | +---+ +---+                      /                  \       +---+    +---+             +---+      | APP16 | -&gt; | DLL16 | -- THUNK -- | DLL32 | +---+   +---+             +---+

Tools Needed to Build Flat Thunks

 * 1) Microsoft Visual C++ 1.5x (16-bit compiler) for the 16- bit side of the thunk. The 16-bit side of the thunk is a 16-bit DLL.
 * 2) Microsoft Visual C++ 2.x or higher (32-bit compiler) for the 32-bit side of the thunk. The 32-bit side of the thunk is a 32-bit DLL.
 * 3) Thunk compiler (thunk.exe) from the Microsoft Win32 SDK for compiling thunk scripts.
 * 4) Microsoft Macro Assembler (MASM) version 6.1 or higher for assembling the assembly-language output of the thunk compiler.
 * 5) The 16-bit RC.EXE from the BINW16 directory of the Microsoft Win32 SDK for marking the 16-bit thunk DLL as version 4.0.

Creating the Thunk Script
You need to create a script that will be used by the thunk compiler to create a thunk. A thunk script is a text file that contains type definitions, the function prototypes of the functions you wish to call via thunks and a specification of the direction of the parameters for each function. For example, some functions require both input and output parameters while others may only require input parameters. Thunk scripts use special syntax to describe whether parameters are input, output, or both input and output.

A thunk script for 16->32 thunks begins with the following statement:

"enablemapdirect1632 = true;" By default, the 32-bit DLL is loaded only when a thunk to it is executed for the first time. Because this late binding is used, 16-bit code must not depend on any action taken by the initialization of the 32-bit DLL. Because the 32-bit DLL will load only when the first thunk to it executes, problems in loading the 32-bit DLL will not be detected when the 16-bit DLL first loads. To disable late binding of the 32-bit DLL, add the following line in your thunk script:

"preload32=true;" The thunk compiler expects that the 16-bit side of the thunk is declared as __far __pascal, and that the 32-bit side is __stdcall. (The WINAPI declaration takes care of this on both sides.) The __cdecl and __fastcall calling conventions are not supported by the thunk compiler. Note, however, that the thunk compiler does not actually accept the __far, __pascal, or __stdcall keywords; they are assumed.

The following thunk script describes a function with no parameters:

  enablemapdirect1632 = true;

void MyThunk32 {  } The equivalent declaration would be:

  C   language:    void WINAPI MyThunk32(void); C++ Language:   extern "C" void WINAPI MyThunk32; The function in the following example script takes two parameters and returns a value. The second parameter is an output parameter and contains a pointer that is passed back to the 16-bit DLL:

  enablemapdirect1632 = true;

typedef int  BOOL; typedef char *LPSTR;

BOOL MyThunk32(LPSTR lpstrInput, LPSTR lpstrOutput) {     lpstrInput  = input;   // optional; input is default lpstrOutput = output; } The statement "lpstrOutput = output" tells the thunk compiler that the 32- bit function will return an address that needs to be converted from a 32- bit linear address to a selector:offset pointer.

The following thunk script uses more complex parameter types such as structures. This example also shows how to specify input and output parameters:

  enablemapdirect1632 = true;

typedef unsigned int UINT; typedef char *LPSTR;

typedef struct _POINT { UINT x;     UINT y;   }POINT, *LPPOINT;

typedef struct _CIRCLE { POINT center; UINT radius; }CIRCLE, *LPCIRCLE;

void MyThunk32( LPCIRCLE lpCircleInOut) {     lpCircleInOut = inout; } The statement "lpCircleInOut = inout" tells the thunk compiler that this pointer is going to be used for input and output. This causes the thunk compiler to convert lpCircleInOut from a 16-bit far (selector:offset) pointer to a 32-bit linear address when the function is called and then back to a 16-bit far pointer when the function returns. The conversion is handled by the thunk created by the thunk compiler.

Using the Thunk Compiler
The thunk compiler usage is as follows:

"thunk.exe /options -o" The following line shows how to compile a 16->32 thunk script. This line takes a script named 16to32.thk and produces an assembly-language file named 16to32.asm:

"thunk -t thk 16to32.thk -o 16to32.asm" The "-t thk" option tells the thunk compiler to prefix the thunk functions in the assembly-language file with "thk_". This prefix is used when linking multiple thunk scripts into a pair of DLLs and is useful for creating a pair of DLLs that contain both 16->32 and 32->16 thunks.

Building the 16-bit DLL
  The 16-bit DLL must export a function named "DllEntryPoint". This function must make a call to a function created by the thunk compiler named thk_ThunkConnect16 ("thk" is the prefix from the thunk compiler -t switch) every time DllEntryPoint is called:   // prototype for function in .OBJ from the thunk script BOOL WINAPI __export thk_ThunkConnect16(LPSTR lpDll16,                                          LPSTR lpDll32,                                           WORD  hInst,                                           DWORD dwReason);

BOOL WINAPI thk_thunkConnect16(LPSTR, LPSTR, WORD, DWORD);

BOOL WINAPI __export DllEntryPoint(DWORD dwReason,                                      WORD  hInst,                                       WORD  wDS,                                       WORD  wHeapSize,                                       DWORD dwReserved1,                                       WORD  wReserved 2) {      if (!thk_ThunkConnect16("DLL16.DLL", "DLL32.DLL", hInst, dwReason)) return FALSE;

return TRUE; }   Include the following lines in the IMPORTS section of the module definition (DEF) file for the 16-bit DLL:   C16ThkSL01      = KERNEL.631 ThunkConnect16 = KERNEL.651   Include the following lines in the EXPORTS section of the module definition (.DEF) file for the 16-bit DLL. THK_THUNKDATA16 is defined in the object file that is assembled from the output of the thunk compiler. Both of these symbols must have the RESIDENTNAME keyword, but can have any ordinal number:   THK_THUNKDATA16 @1  RESIDENTNAME DllEntryPoint  @2  RESIDENTNAME  Add the thunk functions that the Win16 application will call to the EXPORTS statement of the 16-bit DLL's module definition (DEF) file. Be sure they are declared and defined as __far __pascal __export (or WINAPI __export). If the DLL is written in C++, be sure to declare them as extern "C" as well.</li>  Compile the thunk as follows (if not already compiled): <pre class="CODESAMP">  thunk -t thk 16to32.thk -o 16to32.asm </li>  Next, assemble the assembly-language file produced by the thunk compiler as a 16-bit object module. The following line shows an example: <pre class="CODESAMP">  ml /DIS_16 /c /W3 /nologo /Fo thk16.obj 16to32.asm </li> Link this object module as part of the 16-bit DLL.</li>  Mark the 16-bit DLL as version 4.0. To do this, use the resource compiler (RC.EXE). The following line shows the syntax: "rc -40 <DLL file>" This -40 option is available in the resource compiler that is provided with the Win32 SDK.

NOTE: Be sure to use the RC.EXE in the BINW16 directory so that the DLL is marked with version 4.0. The RC.EXE that ships with 16-bit versions of Microsoft Visual C++ will not mark the DLL as version 4.0. </li></ol>

Building the 32-bit DLL
<ol>  In the DllMain of your 32-bit DLL, you must make a call to a function created by the thunk compiler named thk_ThunkConnect32 every time DllMain is called, as shown here ("thk" is the prefix from the thunk compiler - t switch): <pre class="CODESAMP">  // prototype for function in .OBJ from the thunk script BOOL WINAPI thk_ThunkConnect32 (LPSTR    lpDll16,                                   LPTSR     lpDll32,                                   HINSTANCE hDllInst,                                   DWORD     dwReason);

BOOL WINAPI DllMain (HINSTANCE hDLLInst,                       DWORD     dwReason,                        LPVOID    lpvReserved) {     if (!thk_ThunkConnect32("DLL16.DLL", "DLL32.DLL", hDLLInst, dwReason)) {        return FALSE; }     switch (dwReason) {        case DLL_PROCESS_ATTACH: break;

case DLL_PROCESS_DETACH: break;

case DLL_THREAD_ATTACH: break;

case DLL_THREAD_DETACH: break; }     return TRUE; } </li>  Include the following lines in the EXPORTS section of the module definition (.DEF) file for the 32-bit DLL: <pre class="CODESAMP">  thk_ThunkData32 </li> Export the functions to which you are thunking. You can use either the 32-bit DLL's module definition (DEF) file or the __declspec(dllexport) keyword. Be sure they are declared and defined as __stdcall. If the 32-bit DLL is written in C++, be sure to declare them as extern "C" as well. These functions are called by the 16-bit side of the thunk.</li>  Compile the thunk script as follows (if not already compiled): <pre class="CODESAMP">  thunk -t thk 16to32.thk -o 16to32.asm </li>  Assemble the assembly-language file produced by the thunk compiler as a 32-bit object module. The following line shows an example: <pre class="CODESAMP">  ml /DIS_32 /c /W3 /nologo /coff /Fo thk32.obj 16to32.asm </li> Link this object module as part of the 32-bit DLL.</li> Link thunk32.lib as part of the 32-bit DLL. This is the 32-bit import library provided in the Win32 SDK that contains references to the 32-bit thunking APIs that the code created by the thunk compiler uses.</li></ol>