Difficulties in Debugging Flat Thunks

Debugging flat thunks is difficult not only because the mechanism itself is complex, but also because the necessary debugging tools are difficult to master. The flat-thunk mechanism is complex because it must transform function calls in 32-bit compiled code into calls compatible with 16-bit code and vice versa. Because 32-bit code uses data types and CPU register sets that are different than those used with 16-bit code, the flat-thunk mechanism must translate function parameters, switch stacks, and translate return values. The flat-thunk mechanism is optimized for speed, yet must allow preemptive 32-bit code to call nonpreemptive 16-bit code. The thunk compiler makes creating flat thunks much easier than manually creating them, but it is not foolproof.

To debug flat thunks, you must use a system-level debugger, rather than the application-level debugger you would generally use. Application-level debuggers such as the Microsoft Visual C++ debugger and WinDBG cannot trace through thunks because they consist of both 32-bit and 16-bit code and cause the system to claim or release the Win16Mutex. To trace through a thunk, you need to use a system-level debugger such as WDEB386.EXE. Using WDEB386.EXE is more complicated than using an application-level debugger, because you must know Intel x86 assembly language, know how Intel x86 microprocessors work, and remember many debugger commands.