Calling C++ from python using boost python: introductory example not working

Question

I am interested in using Boost.Python to call C++ functions from my Python scripts.

This example here is an introductory example on Boost python's home-pagewhich I am unable to run. Can someone help me out with this?

This is what I tried

I created a file named hello_ext.cpp as follows

#include <boost/python.hpp>
char const* greet()
{
   return "hello, world";
}


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

I then compiled it to a shared library as follows

g++ -c -Wall -Werror -fpic hello_ext.cpp -I/usr/include/python2.7
g++ -shared -o libhello_ext.so hello_ext.o

Finally firing up the ipython interpreter I tried to import hello_ext but got the following error message. Where did I go wrong?

In [1]: import hello_ext
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-1-18c4d6548768> in <module>()
----> 1 import hello_ext

ImportError: ./hello_ext.so: undefined symbol: _ZNK5boost6python7objects21py_function_impl_base9max_arityEv

@ n.m. Thank you! Can you tell me how I should do that? Will I need to modify the first or the second line? — smilingbuddha
– smilingbuddha, Commented Jan 14, 2016 at 23:49

J.J. Hakala · Accepted Answer · 2016-01-15 01:44:27Z

3

You should include some libraries in your link command,

 g++ -shared -Wl,--no-undefined hello_ext.o -lboost_python -lpython2.7 -o hello_ext.so

With -Wl,--no-undefined linker option it will be an error if some symbols are missing.

edited Jan 15, 2016 at 1:44

answered Jan 15, 2016 at 0:00

J.J. Hakala

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4 Comments

smilingbuddha Over a year ago

Thank you! It looks like it cannot find the libboost_python library. It seems not to be installed. Anyway I'll accept this answer for now! Thanks.

Jochen Müller Over a year ago

Afaik object file and libraries need to be stated in the proper order in the linker call, with the dependent module first: g++ -shared -Wl,--no-undefined -o hello_ext.so hello_ext.o -lboost_python -lpython2.7

smilingbuddha Over a year ago

@jomuel Oh thanks so much! The code was still not compiling. It works fine now!

J.J. Hakala Over a year ago

@jomuel I guess g++ / ld may allow one to get away with that wrong link order. If I remember correctly, link order has usually mattered more in case of static libraries. I edited my answer.

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