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

this is likely a name mangling issue, try making add use C calling convention as in the first comment — Sam Mason
– Sam Mason, Commented Oct 22, 2019 at 13:50
In other words: The functions you want to call should be made extern "C". — Max Langhof
– Max Langhof, Commented Oct 22, 2019 at 14:31
Well with the code you show it can't possibly work (unless you give ctypes the mangled name). — Max Langhof
– Max Langhof, Commented Oct 22, 2019 at 14:44

Mark Tolonen · Accepted Answer · 2019-10-22 16:42:41Z

3

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

answered Oct 22, 2019 at 16:42

Mark Tolonen

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1 Comment

IT World Over a year ago

Thank you thank you so much you're the best It's working

Paul Roub · Accepted Answer · 2019-10-22 20:32:32Z

-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, 2019 at 20:32

Paul Roub

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answered Oct 22, 2019 at 13:57

triplekimchi

1

1 Comment

IT World Over a year ago

Can you give me an example please!

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