How can I call a c++ function using python

Question

I want to know if there's a way to use a c++ function inside a python code

For example, after doing my research I did find this solution using .dll file. But It can't find the function My code:

fun.cpp:

#include <iostream>
extern int add(int a, int b) {
return a+b;
}
int main()
{
    std::cout << "Hello World! from C++" << std::endl;
    return 0;
}

compiling it using cmd:

g++ fun.cpp -o fun.dll

Calling the function using Python, ctypes:

from ctypes import *
import os
mydll = cdll.LoadLibrary("C:/Users/User/Desktop/ctypes/fun.dll")

result= mydll.add(10,1)
print("Addition value:-"+result)

But I had this error:

Traceback (most recent call last): File "c:\Users\User.vscode\extensions\ms-python.python-2019.10.41019\pythonFiles\ptvsd_launcher.py", line 43, in main(ptvsdArgs) File "c:\Users\User.vscode\extensions\ms-python.python-2019.10.41019\pythonFiles\lib\python\old_ptvsd\ptvsd__main__.py", line 432, in main run() File "c:\Users\User.vscode\extensions\ms-python.python-2019.10.41019\pythonFiles\lib\python\old_ptvsd\ptvsd__main__.py", line 316, in run_file runpy.run_path(target, run_name='main') File "C:\Python36\lib\runpy.py", line 263, in run_path pkg_name=pkg_name, script_name=fname) File "C:\Python36\lib\runpy.py", line 96, in _run_module_code mod_name, mod_spec, pkg_name, script_name) File "C:\Python36\lib\runpy.py", line 85, in _run_code exec(code, run_globals) File "c:\Users\User\Desktop\ctypes\test.py", line 5, in result= mydll.add(10,1) File "C:\Python36\lib\ctypes__init__.py", line 361, in getattr func = self.getitem(name) File "C:\Python36\lib\ctypes__init__.py", line 366, in getitem func = self._FuncPtr((name_or_ordinal, self)) AttributeError: function 'add' not found

https://stackoverflow.com/questions/145270/calling-c-c-from-python — agent_bean, Oct 22 '19 at 13:45
this is likely a name mangling issue, try making `add` use C calling convention as in the first comment — Sam Mason, Oct 22 '19 at 13:50
In other words: The functions you want to call should be made `extern "C"`. — Max Langhof, Oct 22 '19 at 14:31
Well with the code you show it can't possibly work (unless you give ctypes the mangled name). — Max Langhof, Oct 22 '19 at 14:44
@ITWorld you need to use `extern "C"` not just `extern`, see https://stackoverflow.com/q/1041866/1358308 for example. this was the first hit from google! — Sam Mason, Oct 22 '19 at 15:14
@SamMason I already did it, if you see my code, you'll see it — IT World, Oct 22 '19 at 15:25
it's not in the code you posted, hence my comment! you only have `extern`, check the many linked questions for the correct syntax — Sam Mason, Oct 22 '19 at 15:27

score 3 · Accepted Answer · answered Oct 22 '19 at 16:42

C++ mangles exported names. Here's an example that should compile on Windows and Linux. __declspec(dllexport) is required to export a function on Windows, but not Linux. extern "C" is required to export the function name using C conventions, instead of the name-mangled C++ conventions. The name-mangling is required in C++ to indicate the function parameters and return type, since C++ can have multiple functions with the same name but take different parameters. The C convention does not support multiple functions with the same name.

fun.cpp:

#ifdef _WIN32
#   define API __declspec(dllexport)
#else
#   define API
#endif

extern "C" API int add(int a, int b) {
    return a+b;
}

Python:

from ctypes import *

dll = CDLL('fun')
result = dll.add(10,1)
print('Addition value:',result)

Output:

Addition value: 11

Thank you thank you so much you're the best It's working – IT World Oct 23 '19 at 21:25 — IT World, Oct 23 '19 at 21:25

score -1 · Answer 2 · edited Oct 22 '19 at 20:32

-1

I think you should check Python's LoadLibrary is calling "fun.dll"'s main() function correctly. It should look for the DllMain() function when loading, then you need to get the function address to call the function.

edited Oct 22 '19 at 20:32

Paul Roub

36,322
27
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answered Oct 22 '19 at 13:57

triplekimchi

1

Can you give me an example please! – IT World Oct 22 '19 at 13:59

How can I call a c++ function using python

2 Answers2