Embedded Python in C++

[Xeno]

ah i see, this works in release mode, but not in debug mode (i do have the python3_d.lib installed, don't know why the return will not work in debug build)

[Xeno]

another side question, is there a sample link to add a callback function pointer from c++ to python?

[Gribouillis]

is there anything linker wise i need to add in vc++?

Sorry I can't tell you: I don't know vs2019 nor the Windows OS. I got my compiler options by running these commands in the Linux terminal

Output:
$ python3.8-config --embed --cflags
-I/usr/include/python3.8 -I/usr/include/python3.8  -Wno-unused-result -Wsign-compare -g -fdebug-prefix-map=/build/python3.8-uvizni/python3.8-3.8.10=. -specs=/usr/share/dpkg/no-pie-compile.specs -fstack-protector -Wformat -Werror=format-security  -DNDEBUG -g -fwrapv -O3 -Wall
$ python3.8-config --embed --ldflags
-L/usr/lib/python3.8/config-3.8-x86_64-linux-gnu -L/usr/lib -lpython3.8 -lcrypt -lpthread -ldl  -lutil -lm -lm 

I added -fpie because the linker complained and suggested to add -fpie.

[Gribouillis]

another side question, is there a sample link to add a callback function pointer from c++ to python?

I don't understand what you want. Can you explain it with more details?

[Xeno]

try to make a callback to the c++ from python code

added this c++ function

extern "C" {
	PyObject* CPython::spam_system(PyObject* self, PyObject* args)
	{
		const char* command;
		int sts;

		if (!PyArg_ParseTuple(args, "s", &command))
			return NULL;
		sts = system(command);
		return PyLong_FromLong(sts);
	}
}

trying to call back to c++ using ctypes, but not sure how to setup the args and returns when defining the c++ function in python

import ctypes

class TestClass:
    def __init__(self, count):
        self.count = count
        self.lib = ctypes.CDLL(
            r'C:\Users\WINSIM\PycharmProjects\testProject\cpp\cppTester\x64\Release\cppPythonWrapper.dll')
        # c++ function
        self.spam_system = getattr(self.lib, "?spam_system@CPython@@QEAAPEAU_object@@PEAU2@0@Z")
        # how to setup the c++ function callback?
        self.spam_system.argtypes = [c_string]
        self.spam_system.restype = c_long

    # the callback function
    def print_cb(self):
        self.spam_system("ls -s")

    def print_hi(self, name):
        if __name__ == '__main__':
            print(f'PYTHON: function is being run directly {name} {self.count}')
            self.print_cb()
            return "Completed"
        else:
            print(f'PYTHON: function is being imported {name} {self.count}')
            return "Skipped"

[Gribouillis]

I would try

        self.spam_system.argtypes = (ctypes.py_object,)
        self.spam_system.restype = ctypes.py_object

[Xeno]

that does seem to fire the callback, and I can see the c++ getting the function calls

but I am getting an Access violation on both PyArg_ParseTuple or PyUnicode_AsEncodedString when trying to decode the PyObject* args (which is a tuple type)

    PyObject* CPython::spam_system(PyObject* self, PyObject* args)
    {
        const char* command; 
        if (!PyArg_ParseTuple(args, "s", &command))

PyObject* args don't seem to be populated?

tuple(iterable=(), /)
--
Built-in immutable sequence.
If no argument is given, the constructor returns an empty tuple.
If iterable is specified the tuple is initialized from iterable's items.
If the argument is a tuple, the return value is the same object.

my Python call to the c++

    # the callback function
    def print_cb(self):
        self.spam_system("ls -s")

[Gribouillis]

but I am getting an Access violation on both PyArg_ParseTuple

You may need to include the 'self' argument as well, try with

self.spam_system.argtypes = (ctypes.py_object, ctypes.py_object,)

[Xeno]

just want to close the issue, and thank you for all the help

I reverted back to 3.8.10, the Access violation just stopped, and i can decode the PyObjects from python now. no code change is need.
don't know if this is a bug or not in 3.10

[Gribouillis]

I reverted back to 3.8.10, the Access violation just stopped, and i can decode the PyObjects from python now.

I don't use the CPython api very often, so I can't interprete the access violation. In Python 3.10 you could probably use the new PyObject_CallOneArg() function here, and also PyObject_CallMethod()...