ID: Q154093
The information in this article applies to:
- Microsoft Windows 95
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.
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 | -> | DLL16 | -- THUNK -- | DLL32 |
+-------+ +-------+ +-------+
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.
the 32-bit side of the thunk. The 32-bit side of the
thunk is a 32-bit DLL.
for compiling thunk scripts.
for assembling the assembly-language output of the thunk
compiler.
Microsoft Win32 SDK for marking the 16-bit thunk DLL as
version 4.0.
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;
preload32=true;
The following thunk script describes a function with no parameters:
enablemapdirect1632 = true;
void MyThunk32()
{
}
C language: void WINAPI MyThunk32(void);
C++ Language: extern "C" void WINAPI MyThunk32();
enablemapdirect1632 = true;
typedef int BOOL;
typedef char *LPSTR;
BOOL MyThunk32(LPSTR lpstrInput, LPSTR lpstrOutput)
{
lpstrInput = input; // optional; input is default
lpstrOutput = output;
}
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 thunk compiler usage is as follows:
thunk.exe /options <inputfile> -o <outputfile>
thunk -t thk 16to32.thk -o 16to32.asm
1. 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;
}
module definition (DEF) file for the 16-bit DLL:
C16ThkSL01 = KERNEL.631
ThunkConnect16 = KERNEL.651
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
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.
thunk -t thk 16to32.thk -o 16to32.asm
thunk compiler as a 16-bit object module. The following
line shows an example:
ml /DIS_16 /c /W3 /nologo /Fo thk16.obj 16to32.asm
8. 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.
1. 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):
// 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;
}
module definition (.DEF) file for the 32-bit DLL:
thk_ThunkData32
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.
compiled):
thunk -t thk 16to32.thk -o 16to32.asm
compiler as a 32-bit object module. The following line
shows an example:
ml /DIS_32 /c /W3 /nologo /coff /Fo thk32.obj 16to32.asm
7. 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.
For information about how to debug flat thunks, please refer to the following article in the Microsoft Knowledge Base:
ARTICLE-ID: Q133722
TITLE : How to Debug Flat Thunks
Last Reviewed: September 10, 1998