Undefined reference to `main` error when embedding Python in C++

I’m trying to embed Python in C++. This is my Python file (with the name EmbedTest.py):

from __future__ import division

class model:  
    def init(self,a,b):  
        self.a = a  
        self.b = b  
         def test_method(a,b):  
    m = model(a,b)  
    m.add(1)  
    print("a: ",a,"b: ",b)  
    return (a+b,a-b,a*b)

This is my C++ file (with the name EmbedTest.cpp and located in the same folder as EmbedTest.py)

#include <Python.h>  

int main(int argc, char *argv[]) {  
    PyObject *pName, *pModule, *pFunc;  
    PyObject *pArgs, *pValue, *pValue_1, *pValue_2;  
    double sum,diff,prod;  
    double a = atof(argv[1]);  
    double b = atof(argv[2]);   
    Py_Initialize();  
    pName = PyUnicode_DecodeFSDefault("EmbedTest.py");  
    pModule = PyImport_Import(pName);  
    Py_DECREF(pName);  

    if(pModule != NULL) {
      pFunc = PyObject_GetAttrString(pModule,"test_method");    
      if(pFunc && PyCallable_Check(pFunc)) {  
          pArgs = PyTuple_New(2);  
          pValue_1 = PyFloat_FromDouble(a);  
          pValue_2 = PyFloat_FromDouble(b);  
          if (!pValue_1) {
          Py_DECREF(pArgs);
            Py_DECREF(pModule);
        fprintf(stderr, "Cannot convert argumentn");
        return 1;  
        }  
        if (!pValue_2) {
        Py_DECREF(pArgs);
        Py_DECREF(pModule);
        fprintf(stderr, "Cannot convert argumentn");
        return 1;  
        }  
        PyTuple_SetItem(pArgs, 0, pValue_1);  
        PyTuple_SetItem(pArgs, 1, pValue_2);  
        
        pValue = PyObject_CallObject(pFunc, pArgs);  
            Py_DECREF(pArgs);  
            if (pValue != NULL) {
            sum = PyFloat_AsDouble(PyTuple_GetItem(pValue,0));
            diff = PyFloat_AsDouble(PyTuple_GetItem(pValue,1));
            prod = PyFloat_AsDouble(PyTuple_GetItem(pValue,2));  
              printf("a: %f b: %f sum: %f diff: %f prod: %f",a,b,sum,diff,prod);  
              Py_DECREF(pValue);  
            }  
            else {  
                Py_DECREF(pFunc);  
                Py_DECREF(pModule);  
                PyErr_Print();  
                fprintf(stderr,"Call failedn");  
                return 1;  
            }       
          } else {  
              if (PyErr_Occurred())  
                PyErr_Print();  
              fprintf(stderr, "Cannot find function "%s"n", argv[2]);  
          }  
          Py_XDECREF(pFunc);  
          Py_DECREF(pModule);  
        }  
        else {  
          PyErr_Print();  
          fprintf(stderr, "Failed to load "%s"n", argv[1]);  
          return 1;  
    }  
    if (Py_FinalizeEx() < 0) {  
        return 120;  
    }  
    return 0;  }

Compiling is fine. I use the flags suggested by python3.6-config --cflags. Hence

gcc -c -I/home/MyFolder/anaconda3/include/python3.6m -Wno-unused-result -Wsign-compare -march=nocona -mtune=haswell -ftree-vectorize -fPIC -fstack-protector-strong -O3 -pipe -fdebug-prefix-map==/usr/local/src/conda/- -fdebug-prefix-map==/usr/local/src/conda-prefix -fuse-linker-plugin -ffat-lto-objects -flto-partition=none -flto -DNDEBUG -fwrapv -O3 -Wall -Wstrict-prototypes EmbedTest.cpp

works fine.

However, when I try to link, I get a problem. I use the flags suggested by python3.6-config --ldflags. Hence I try

gcc -o EmbedTest.o -L/home/MyFolder/anaconda3/lib/python3.6/config-3.6m-x86_64-linux-gnu -L/home/MyFolder/anaconda3/lib -lpython3.6m -lpthread -ldl -lutil -lrt -lm -Xlinker -export-dynamic.

However, I get the following error message:

/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crt1.o: In function `_start':
/build/glibc-6V9RKT/glibc-2.19/csu/../sysdeps/x86_64/start.S:118: undefined reference to `main'
collect2: error: ld returned 1 exit status

Any idea what goes wrong here?

Answer

You are not linking the .o file, you are outputing to it.

Change

gcc -o EmbedTest.o -L/home/MyFolder/anaconda3/lib/python3.6/config-3.6m-x86_64-linux-gnu -L/home/MyFolder/anaconda3/lib -lpython3.6m -lpthread -ldl  -lutil -lrt -lm  -Xlinker -export-dynamic

gcc -o EmbedTest EmbedTest.o -L/home/MyFolder/anaconda3/lib/python3.6/config-3.6m-x86_64-linux-gnu -L/home/MyFolder/anaconda3/lib -lpython3.6m -lpthread -ldl  -lutil -lrt -lm  -Xlinker -export-dynamic

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Answer