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Loading dll using Python Ctypes

I've looked at the example given here ctypes - Beginner and followed the same steps with a different bit of C code. I've built a .dll and a .lib using C code given here: http://wolfprojects.altervista.org/articles/dll-in-c-for-python/

 //test.c
__declspec(dllexport) int sum(int a, int b) {
 return a + b;
}

In my wrapper.py I have this:

import ctypes
testlib = ctypes.CDLL("C:\\Users\\xyz\\Documents\\Python\\test.dll")

When I run the script I get this error:

self._handle = _dlopen(self._name, mode)

OSError: [WinError 193] %1 is not a valid Win32 application

If I use

testlib = ctypes.LibraryLoader("C:\\Users\\xyz\\Documents\\Python\\test.dll")

then I don't get any error on running the script. But If I try to do this:

testlib.sum(3,4)

I get the error:

dll = self._dlltype(name)

TypeError: 'str' object is not callable

The dll and the .py are in the same folder. Can anyone help me understand what's going on here. I've spent hours trying to figure this out, but have hit a wall. Thanks.

Answer*

Make sure your compiler and version of Python are both 32-bit or both 64-bit. You can't mix, which is the cause of `OSError: [WinError 193] %1 is not a valid Win32 application`.

Next, make sure to compile as a C program and not C++. That's the cause of the name mangling mention in your answer.

Example (note compiler is for **x86** not **x64**:
```none
C:\>cl /LD /W4 test.c
Microsoft (R) C/C++ Optimizing Compiler Version 17.00.61030 for x86
Copyright (C) Microsoft Corporation. All rights reserved.

test.c
Microsoft (R) Incremental Linker Version 11.00.61030.0
Copyright (C) Microsoft Corporation. All rights reserved.

/out:test.dll
/dll
/implib:test.lib
test.obj
 Creating library test.lib and object test.exp
```
Now use a **32-bit** Python:

 C:\>py -2
 Python 2.7.13 (v2.7.13:a06454b1afa1, Dec 17 2016, 20:42:59) [MSC v.1500 32 bit (Intel)] on win32
 Type "help", "copyright", "credits" or "license" for more information.
 >>> from ctypes import *
 >>> lib = CDLL('test')
 >>> lib.sum(2, 3)
 5

If you compile as C++, you can still call functions by exporting them as C, which prevents the C++ name mangling:

**test.cpp**
```py
extern "C" __declspec(dllexport) int sum(int a, int b) {
 return a + b;
}
```

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Thanks for the answers. I'm on 64-bit Windows and have installed 64-bit version of Anaconda so my Python is 64-bit. I didn't compile the dll myself as I didn't have the tools for it. So perhaps the problem was because of the compiling.

kronosjt
– kronosjt

2017年04月10日 05:50:46 +00:00
Commented Apr 10, 2017 at 5:50
1

You might could mention that it is possible to have a 32 bit and 64 bit version of python installed... since I had the 64 bit version installed and couldn't recompile the dll (as far as I know), I just installed a 32 bit of python 3.8 and it worked like a charm. Thanks!

Shmack
– Shmack

2020年05月19日 23:33:51 +00:00
Commented May 19, 2020 at 23:33
@ShanerM13 You did just mention it 🙂. If you install multiple Python versions and don't add them to the path, you can use the Python Launcher option and easily switch between any version or even call out the version for a script to run in a shebang(#!) comment.

Mark Tolonen
– Mark Tolonen

2020年05月21日 22:47:01 +00:00
Commented May 21, 2020 at 22:47

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Loading dll using Python Ctypes

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