Linking error with meson and boost_python

Question

i am trying to use boost to expose a c++ class to python, but i cannot figure out the linking error i am getting.

My demo program looks like this:

#include <boost/python.hpp>
#include <Python.h>

char const* greet()
{
   return "hello, world";
}

BOOST_PYTHON_MODULE(hello_ext)
{
    using namespace boost::python;
    def("greet", greet);
}

and my meson file looks like this:

project('test', 'cpp', version : '0.0.1',default_options : ['c_args= -g'])
add_project_arguments(language : 'cpp')
cxx = meson.get_compiler('cpp')
python_dep = dependency('python3')
boost_dep = dependency('boost',modules: ['python3'],)

src = ['./test.cpp']

executable('test', src,
                     dependencies : [ python_dep,boost_dep])

When i run it i get a bunch of undefined references to python methods:

/usr/include/boost/python/object_core.hpp:400: undefined reference to `_Py_NoneStruct'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyExc_ValueError'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyLong_AsLong'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyNumber_InPlaceFloorDivide'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyBool_Type'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyObject_GetAttr'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyDict_GetItemString'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyObject_Call'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyNumber_And'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyExc_IndexError'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyEval_GetGlobals'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyTuple_Type'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyNumber_InPlaceSubtract'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyList_Type'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyObject_GetAttrString'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyModule_Type'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyMem_Free'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyType_IsSubtype'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyStaticMethod_Type'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyErr_WarnEx'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyUnicode_InternFromString'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyTuple_New'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyObject_SetAttr'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyObject_IsInstance'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyErr_NoMemory'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyExc_AttributeError'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyNumber_Add'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyUnicode_FromFormat'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyList_Append'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyType_Type'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyExc_ReferenceError'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyArg_ParseTupleAndKeywords'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyStaticMethod_New'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyNumber_InPlaceMultiply'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyTuple_Size'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyErr_SetObject'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `_PyObject_New'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyDict_Keys'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyType_GenericAlloc'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyErr_Format'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyObject_CallFunction'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyList_Reverse'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyCFunction_Type'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyExc_OverflowError'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyMem_Malloc'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyErr_ExceptionMatches'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyComplex_ImagAsDouble'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyList_New'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyImport_ImportModule'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyProperty_Type'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyUnicode_FromString'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyNumber_InPlaceOr'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyBytes_Size'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyType_GetFlags'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyObject_DelItem'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyNumber_FloorDivide'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyErr_Clear'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyRun_StringFlags'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyDict_GetItem'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyWeakref_NewRef'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyNumber_Multiply'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `_Py_NewReference'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyNumber_Lshift'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyDict_New'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyErr_SetString'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyDict_Update'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PySlice_New'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyRun_FileExFlags'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyObject_SetItem'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyUnicode_AsUTF8'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyObject_SetAttrString'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyUnicode_FromEncodedObject'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyLong_AsUnsignedLongLong'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyExc_RuntimeError'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyNumber_Xor'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyBytes_AsString'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `_Py_NotImplementedStruct'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyNumber_Rshift'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyBaseObject_Type'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyExc_StopIteration'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyObject_GetItem'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyExc_TypeError'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyDict_Values'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `_Py_fopen'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyType_Ready'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyObject_Size'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyMethod_Type'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyMethod_New'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyObject_ClearWeakRefs'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyLong_AsSsize_t'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyLong_AsLongLong'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyEval_CallFunction'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyNumber_Remainder'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyLong_FromLong'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyErr_NewException'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyObject_CallMethod'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyNumber_Or'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyObject_IsTrue'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyList_Sort'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyModule_Create2'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyUnicode_AsWideChar'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyLong_AsUnsignedLong'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyNumber_InPlaceRemainder'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyDict_Type'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyErr_Occurred'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyDict_Copy'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyLong_Type'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyNumber_InPlaceAnd'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyNumber_InPlaceXor'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `_PyType_Lookup'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyDict_Size'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyNumber_InPlaceRshift'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyFloat_Type'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyIter_Next'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyCallable_Check'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyComplex_RealAsDouble'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyLong_FromUnsignedLong'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyList_Insert'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyObject_RichCompare'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyDict_Items'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyCMethod_New'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyTuple_GetItem'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyNumber_Subtract'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyComplex_Type'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyUnicode_AsUTF8String'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyUnicode_FromStringAndSize'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyDict_Clear'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyNumber_InPlaceLshift'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyBool_FromLong'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libboost_python39.so.1.74.0: undefined reference to `PyNumber_InPlaceAdd'

I seen some suggestions online about it being the order of the linking, i have tried changing the order of the dependency array without luck.

Answer 1

That looks like you also need to link with python, because for some reason it's not being pulled in as a dependency of boost python adding dependency('python3') to your dependency list should solve that.

Answer 2

Starting from python3.8, the python3.pc file does not provide the library flags necessary to link programs against an embedded python interpreter. Instead, packages need to use python3-embed, so you should use

python_dep = dependency('python3-embed')

and two more things:

• you are building python "module" (since you will use your exposed method greet() in python, right?), so you should use shared_library() as target (not executable())
• according to the way how python is looking for modules - the library should be named exactly as your named it BOOST_PYTHON_MODULE (in the simplest case, alternatives you can see here ), thus the name suffix is overridden (default is 'lib')

 lib = shared_library('hello_ext',
            src,
            dependencies : [ python_dep, boost_dep],
            name_prefix : '')

All steps, with complications with importing module from another path (with eg import importlib.util):

% meson build
% ninja -C build
% cd build
$ python3
Python 3.8.10 (default, Nov 26 2021, 20:14:08) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import hello_ext
>>> hello_ext.greet()
'hello, world'
>>>