Calling Functions from within class using CTypes

Question

I am trying to call a function from within a class which from what I understand, you just call as expected "object.class.function" is this correct, or am I doing something else wrong?

cLibrary.c

#include <stdio.h>
#include "clibrary.h"
 
void Clibrary::prompt()
{
 printf("hello world\n");
}

cLibrary.h

class Clibrary
{
public:
 Clibrary(){};
 ~Clibrary(){};
 static void prompt();
};

Then build it

g++ -fPIC -shared -o clibrary.so cLibrary.c

Then test if it has the correct symbol

$ nm -gDC clibrary.so 
 w __cxa_finalize
 w __gmon_start__
 w _ITM_deregisterTMCloneTable
 w _ITM_registerTMCloneTable
 U puts
000000000000111a T Clibrary::prompt()

Then call it using CTypes in test.py.

#!/usr/bin/python3
import ctypes
libObject = ctypes.CDLL('./clibrary.so')
libObject.Clibrary.prompt()

Then try to run it

./test.py 
Traceback (most recent call last):
 File "./test.py", line 5, in <module>
 libObject.Clibrary.prompt()
 File "/usr/lib/python3.8/ctypes/__init__.py", line 386, in __getattr__
 func = self.__getitem__(name)
 File "/usr/lib/python3.8/ctypes/__init__.py", line 391, in __getitem__
 func = self._FuncPtr((name_or_ordinal, self))
AttributeError: ./clibrary.so: undefined symbol: Clibrary

NO STACK OVERFLOW MODERATORS: it is not like Calling C function from python using ctypes because it is within a class.

• Mark Tolonen: It's "C" types. It can't call functions within a C++ class. It understands C functions and types.

Answer 1

ctypes understands C linkage and a C++ library normally needs to create extern "C" wrapper functions. In this special case you can force ctypes to load the name-decorated C++ symbol for the static method, but it isn't recommended.

Here's both demonstrated:

test.cpp

#include <stdio.h>
#ifdef _WIN32
# define API __declspec(dllexport) // Windows requires explicit export of functions.
#else
# define API
#endif
class API Clibrary // exported to see name decoration, but not needed
{
public:
 Clibrary() {};
 ~Clibrary() {};
 static void prompt() { printf("hello world\n"); }
};
// wrapper function with C linkage
extern "C" API void prompt() { Clibrary::prompt(); }

List of symbols exported from the DLL:

ordinal hint RVA name
 1 0 00001020 ??0Clibrary@@QEAA@XZ
 2 1 00001030 ??1Clibrary@@QEAA@XZ
 3 2 00001040 ??4Clibrary@@QEAAAEAV0@AEBV0@@Z
 4 3 00001050 ?prompt@Clibrary@@SAXXZ
 5 4 00001000 prompt

test.py

import ctypes
libObject = ctypes.CDLL('./test')
# Hack but works. Decorated name is not standard and could
# be different using a different compiler.
other = getattr(libObject,'?prompt@Clibrary@@SAXXZ')
other()
libObject.prompt() # best and portable

Output:

hello world
hello world