Need help regarding pybind11

Question

I found some code here that I tried locally, it is all about calling a function from Python3 using pybind11. I successfully installed pybind11 however, my G++ compiler can't locate the Python.h module(since pybind11 need Python.h header), and so I searched here some solutions, some say try installing python3.8-dev, some also say try installing libpython3.8-dev, neither the two solutions worked.

I have no choice but to look for another solution, and there I found the use of -I/usr/include/python3.x and so I did:

g++ -I/usr/include/python3.8 main.cpp

I almost had it all until I found this weird looking error:

(.text._ZN8pybind116detail11type_casterIdvE4loadENS_6handleEb[_ZN8pybind116detail11type_casterIdvE4loadENS_6handleEb]+0x4f): undefined reference to `PyFloat_Type'
/usr/bin/ld: main.cpp:(.text._ZN8pybind116detail11type_casterIdvE4loadENS_6handleEb[_ZN8pybind116detail11type_casterIdvE4loadENS_6handleEb]+0x6e): undefined reference to `PyFloat_Type'
/usr/bin/ld: main.cpp:(.text._ZN8pybind116detail11type_casterIdvE4loadENS_6handleEb[_ZN8pybind116detail11type_casterIdvE4loadENS_6handleEb]+0x76): undefined reference to `PyType_IsSubtype'
/usr/bin/ld: main.cpp:(.text._ZN8pybind116detail11type_casterIdvE4loadENS_6handleEb[_ZN8pybind116detail11type_casterIdvE4loadENS_6handleEb]+0xa1): undefined reference to `PyFloat_AsDouble'
/usr/bin/ld: main.cpp:(.text._ZN8pybind116detail11type_casterIdvE4loadENS_6handleEb[_ZN8pybind116detail11type_casterIdvE4loadENS_6handleEb]+0xd9): undefined reference to `PyErr_Occurred'
/usr/bin/ld: main.cpp:(.text._ZN8pybind116detail11type_casterIdvE4loadENS_6handleEb[_ZN8pybind116detail11type_casterIdvE4loadENS_6handleEb]+0xfc): undefined reference to `PyErr_Clear'
/usr/bin/ld: main.cpp:(.text._ZN8pybind116detail11type_casterIdvE4loadENS_6handleEb[_ZN8pybind116detail11type_casterIdvE4loadENS_6handleEb]+0x11f): undefined reference to `PyNumber_Check'
/usr/bin/ld: main.cpp:(.text._ZN8pybind116detail11type_casterIdvE4loadENS_6handleEb[_ZN8pybind116detail11type_casterIdvE4loadENS_6handleEb]+0x14a): undefined reference to `PyNumber_Float'
/usr/bin/ld: main.cpp:(.text._ZN8pybind116detail11type_casterIdvE4loadENS_6handleEb[_ZN8pybind116detail11type_casterIdvE4loadENS_6handleEb]+0x174): undefined reference to `PyErr_Clear'
/usr/bin/ld: warning: creating DT_TEXTREL in a PIE
collect2: error: ld returned 1 exit status

I also tried: g++ -I/usr/include/python3.8 main.cpp -lpython3.8m

and it says:

usr/bin/ld: cannot find -lpython3.8m
collect2: error: ld returned 1 exit status

I also tried:

 g++ -o main main.py \
 -I/usr/include/python3 \
 $(python3-config --cflags) $(python3-config --ldflags)

and I got this:

/usr/bin/ld: /tmp/ccIRtkYI.o: relocation R_X86_64_32S against undefined symbol `_Py_TrueStruct' can not be used when making a PIE object; recompile with -fPIE
collect2: error: ld returned 1 exit status

I also tried:

    g++ -o main main.py \
 -I/usr/include/python3 \
 $(python3-config --cflags) $(python3-config --ldflags) fPIE

and got this:

/home/ystyphn/.local/lib/python3.8/site-packages/pybind11/include/pybind11/cast.h:1071: undefined reference to `PyErr_Occurred'
/usr/bin/ld: /home/ystyphn/.local/lib/python3.8/site-packages/pybind11/include/pybind11/cast.h:1076: undefined reference to `PyErr_Clear'
/usr/bin/ld: warning: creating DT_TEXTREL in a PIE
collect2: error: ld returned 1 exit status

Update: The code is:

#include <pybind11/embed.h>
#include <iostream>

namespace py = pybind11;

int main() {
    py::scoped_interpreter python;

    auto math = py::module::import("math");
    double root_two = math.attr("sqrt")(2.0).cast<double>();

    std::cout << "The square root of 2 is: " << root_two << "\n";
}

It is from the thread: call a Python function from c++ using pybind11

And YES after all I still like programming, hoping someone will see this

UPDATE: I created CMakeLists file instead

cmake_minimum_required(VERSION 3.4)
project(example)

set(CMAKE_LANGUAGE_COMPILER "/usr/bin/c++")
add_subdirectory(pybind11)

add_executable(example main.cpp)
target_link_libraries(example PRIVATE pybind11::embed)

And I downloaded the pybind11 from github, and I included it as a subfolder to my example

*I successfully installed pybind11* How? Did you try using one of the recommended ways to set it up from their [documentation](https://pybind11.readthedocs.io/en/latest/installing.html)? — super, Mar 13 '21 at 08:32
Also, all the undefined reference errors indicate that you are not linking against the right libs. — super, Mar 13 '21 at 08:33
https://pybind11.readthedocs.io/en/latest/compiling.html#building-manually ? — Marc Glisse, Mar 13 '21 at 08:52
I used pip install pybind11 first then I can't include it to the C++ code, its says pybind no such file in directory then I used another method which is sudo apt-get install pybind11, now pybind11 is visible however the Python.h is not visible by the compiler — Nel Carlo Bayang, Mar 13 '21 at 09:04
Okey wait, I will add, however it was just a copy pasted code it wasn't mine. — Nel Carlo Bayang, Mar 13 '21 at 09:20
Ah, before the example it wasn't clear what you wanted to do. You need `python3-config --ldflags --embed` or it thinks you are compiling a module. Don't call a C++ file main.py... — Marc Glisse, Mar 13 '21 at 09:57
There is still "/usr/bin/ld: warning: creating DT_TEXTREL in a PIE" — Nel Carlo Bayang, Mar 13 '21 at 12:49
`g++ a.cc $(python3-config --cflags --ldflags --embed) -fPIE` works for me with no warning. — Marc Glisse, Mar 13 '21 at 14:50

Need help regarding pybind11

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