Compile C++ class with a header file to use it in python?

Question

I have a my_class.h:

// my_class.h

namespace N
{
    class my_class
    {
    public:
        void hello_world();
    };
}

with a my_class.cpp file.

// my_class.cpp

extern "C" {
    #include "my_class.h"
}
#include <iostream>

using namespace N;
using namespace std;

void my_class::hello_world()
{
    cout << "Hello, World!" << endl;
}

I want to create a shared object my_class.so file to use it in python. Therefore, I am using the g++ compiler.

g++ -fPIC -shared -o my_class.so my_class.cpp

Using the shared object in python my_class.so

import ctypes

my_class = ctypes.cdll.LoadLibrary('./my_class.so')
my_class.hello_world

I get the following error message:

Exception has occurred: AttributeError
./my_class.so: undefined symbol: hello_world

I do not know how to interpret this.
Note:

my_class.hello_world() results the same error

Why are you including a c++ header inside extern C? You are using C linkage with C++ code, so it wont work — Ruiqi Li
– Ruiqi Li, Commented Oct 7, 2021 at 15:31
Do you mean that I need to remove the extern "C" wrapper in my_class.cpp? It is still not working even without it. — ajawa
– ajawa, Commented Oct 8, 2021 at 12:56
I do not know how python handles mangled names. However, extern C definitely shouldn’t be there — Ruiqi Li
– Ruiqi Li, Commented Oct 8, 2021 at 20:22

RMiller · Accepted Answer · 2021-10-09 19:06:59Z

2

Much to talk about, but I'll try to be brief:

extern "C" is going to make those symbols available in the C-syntax. (You do want this)
C-syntax does not support classes or namespaces.
the python dynamic library loader is grabbing ALL the symbols - not just your class. (It seems there is some confusion there with the naming scheme)

To Fix: Simply eliminate your class and namespaces (or add a C-API pass-through).

my_class.h

extern "C" {
   void hello_world();
}

my_class.cpp

#include "my_class.h"
#include <iostream>

void hello_world()
{
        std::cout<<"Hello, World" << std::endl;
}

After you build, use nm or objdump to verify that the symbol hello_world is defined and not mangled. Example:

>> nm my_class.so | grep hello
0000000000000935 T hello_world

And your python code will need an open close parenthesis to tell it to execute the function:

import ctypes

my_class = ctypes.cdll.LoadLibrary('./my_class.so')
my_class.hello_world()

answered Oct 9, 2021 at 19:06

RMiller

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2 Comments

ajawa Over a year ago

Thank you! But I did not get the usage of extern "C" {...} in this context. Why do I need it in the header file and why do I need it at all?

RMiller Over a year ago

If it worked without the extern "C", then the python package ctypes might be smart enough to search for the mangled version as well. C++ mangles it's symbols, C does not. For example, run nm on a binary with a hello_world() function compiled as cpp. You should see something like "_Z11hello_worldv". This is the actual name written to the binary file for future lookups. to ensure the mangling doesn't happen, you can use 'extern "C"'.

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