Python ctypes DLL is "not a valid Win32 application"

Question

I am using ctypes to load a 64-bit Windows DLL into a 64-bit Python 3.6.2 environment, but Python complains that the DLL is not a 32-bit DLL. This is all 64-bit, so I don't know why it expects a 32-bit DLL. This same error message appears in Visual Studio 2017 and in PyCharm 2017.1.5.

Here is the problem code:

ThisDLL = ctypes.WinDLL(r"C:\Temp2\Test_Project\Std_Math_Formulas.dll")

Here are the error messages:

File "C:\Project Backups\Python Projects - PyCharm\Test_DLL\Main_Entry.py", line 73, in CTypes_Test_02 ThisDLL = ctypes.WinDLL(r"C:\Temp2\Test_Project\Std_Math_Formulas.dll") File "C:\Program Files\Python36\lib\ctypes\__init__.py", line 348, in __init__ self._handle = _dlopen(self._name, mode) OSError: [WinError 193] %1 is not a valid Win32 application

I have researched this at length, and the answers all apply to a mixed 32-bit to 64-bit situation -- 32-bit DLL in 64-bit environment or 64-bit DLL in 32-bit environment. Here they both environment and DLL are 64-bit.

I know that Windows 32-bit DLLs use the stdcall calling convention and 64-bit uses the fastcall convention, but ctypes should be able to load a 64-bit DLL.

How do you know that your Python environment is 64-bit? Can you print the output of import platform; print(platform.architecture())? — Dietrich Epp
– Dietrich Epp, Commented Dec 28, 2017 at 0:33
I'm not completely sure, but I wouldn't be surprised that the Windows error 193 is not "bit-localized" - i.e., it's a generic loader error, and will spit "it is not a valid Win32 application" in reply to an invalid file, regardless of it expecting a 32 or 64 bit executable. If you open that dll in e.g. Dependency Walker does it see it as a valid dll? — Matteo Italia
– Matteo Italia, Commented Dec 28, 2017 at 0:36

Mike Müller · Accepted Answer · 2017-12-28 00:41:41Z

1

You are mixing up win32 and 32-bit. 64-bit applications are also win32 applications. So your problem has nothing to do with 32 and 64-bit systems.

answered Dec 28, 2017 at 0:41

Mike Müller

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RTC222 Over a year ago

Dietrich: import platform; print(platform.architecture()) and it confirms that it is a 64-bit environment. Matteo: Dependency Walker will open the file, so it is a valid DLL, but x64dbg disassembler says that the DLL is 32-bit, although it is compiled for 64-bit (NASM), and all the registers in the disassembly are 32-bit although they're written for 64-bit registers. Mike: I assume that you are right -- the error message is more generic than specific to 32-bit, so I will investigate the DLL and post when I have an answer. Thanks to all three of you for your input.

RTC222 · Accepted Answer · 2017-12-29 19:42:21Z

I solved the problem. I assembled this DLL with the NASM assembler before linking. Originally, I used -f COFF for the output format. However, when you assemble with NASM for 64-bit, you must use -f Win64 as the output format, not -f COFF. So the proper output format (with no other switches) is:

nasm -f Win64 FileName.asm -o FileName.obj

And that solved this problem.

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