How can call C++ code in Python, that uses

Question

I am trying to call some of my C++ machine learning code in Python. I have tried multiple other methods, such as this question and this explanation, both of which are very similar. I followed these to the letter, with the same filenames and code.

For example, I have the following C++ code...

file.cpp

#include <iostream>

void some_code()
{
    std::cout << "Hello world!";
}

extern "C"
{
    void hello_there() { some_code(); }
}

I then compile it with...

g++ -c -fPIC file.cpp -o foo.o

g++ -shared -Wl,-soname,lib.so -o lib.so foo.o

code.py

from ctypes import cdll

lib = cdll.LoadLibrary('./lib.so')

lib.hello_there()

Traceback (most recent call last):
  File "c:\some-path-here\1 - calling-cpp-with-python.py", line 5, in <module>
    lib = cdll.LoadLibrary('./lib.so')
  File "C:\Users\USERNAME\AppData\Local\Programs\Python\Python310\lib\ctypes\__init__.py", line 452, in LoadLibrary
    return self._dlltype(name)
  File "C:\Users\USERNAME\AppData\Local\Programs\Python\Python310\lib\ctypes\__init__.py", line 374, in __init__
    self._handle = _dlopen(self._name, mode)
FileNotFoundError: Could not find module 'C:\some-path-here\lib.so' (or one of its dependencies). Try using the full path with constructor syntax.

What am I doing wrong? I have also tried with compiling it to a DLL file, and adding __declspec(dllexport) before the "void" in the extern bit.

Additional Notes

If it is helpful...

I am on Windows 10
I am using the MinGW G++
The name of the directory with the files in has spaces in it.
This is running on Python 3.10, but the error also occurs in 3.9

You probably need to tell Python how to find the C++ standard library, because it is unlikely to be in a place where Python knows to look at. Can you compile a C++ *program* and the run it from a cmd window (not from a mingw shell)? If not, then it's the same problem you are seeing here. — n. 1.8e9-where's-my-share m., Feb 14 '22 at 17:11
You are using `.so` files in windows. I have doubt whether it will work or not, but probably WSL can help you. It looks like you have some path issue. — akm, Feb 14 '22 at 17:11
You can try removing the `./` part and write it like `lib.so`. — akm, Feb 14 '22 at 17:15
However, I would still like to use Windows, to make my code cross-platform — Password-Classified, Feb 14 '22 at 19:24
@n.1.8e9-where's-my-sharem. I have not used it in the mingw shell, but rather use the g++ that I installed alongside mingw — Password-Classified, Feb 14 '22 at 19:46
@akm I tried this on WSL, it works. Do you know of any windows solutions, perhaps with dlls? — Password-Classified, Feb 14 '22 at 20:20
See [this](https://stackoverflow.com/questions/18583061/c-dll-loads-in-c-program-not-in-python-ctypes/18650202#18650202) for debugging which file can't be found. The code works with the Microsoft compiler using `__declspec(dllexport` and `.dll` files. Use a DLL and make sure the C++ runtime is also present in the same directory or DLL search path or it is statically linked — Mark Tolonen, Feb 15 '22 at 01:34
@Password-Classified Related, [this](https://stackoverflow.com/questions/59330863/cant-import-dll-module-in-python?noredirect=1&lq=1) can help you... — akm, Feb 15 '22 at 07:50
OK try to examine (1) your library and (2) your executable in the dependency walker. Do you see a dependency in one that is not in the other? Also try building with `-static-libstdc++ -static-libgcc` flags. — n. 1.8e9-where's-my-share m., Feb 15 '22 at 09:41

anirudh · Answer 1 · 2022-02-14T16:59:24.123

1

Looks like a path issue based on the error. I was able to quickly try out your example, and it worked for me. This error probably has nothing to do with iostream, but you could confirm that by changing the function quickly to use printf.

I do not have experience working on c++ in windows 10, but could you check if you're able to find the library file in python, before accessing using something like the below code

import os    
print(os.path.exists('./lib.so'))

edited Feb 14 '22 at 16:59

answered Feb 14 '22 at 16:54

anirudh

3,920
2
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I have tried with printf, and the error does not pop up – Password-Classified Feb 14 '22 at 19:30
It works with printf, but I would like to use iostream – Password-Classified Feb 14 '22 at 19:41

score 1 · Answer 2 · edited Mar 23 '22 at 01:45

1

I encountered exactly the same problem on Win 10, MinGW & Python 3.7-3.10.

I finally used dependencywalker and found out that the importing relies on the compiler environment. With exaction of dependent dlls I solved this problem. Using static compile mode also helps.

edited Mar 23 '22 at 01:45

eyllanesc

221,139
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answered Mar 21 '22 at 03:19

Serena

19
3

Thanks for making the modifications; if you happen to remember, can you include the command line option (or other brief instruction) on how to enable static linking on the MinGW environment? – nanofarad Mar 23 '22 at 01:48

How can call C++ code in Python, that uses

file.cpp

code.py

Additional Notes

2 Answers2