cython_tutorial | A porject for learning Cython | Machine Learning library

Here is what I finally used that now seems to work:

You have written a cdef function and are trying to call it from python. Unfortunately, cdef functions are not visible from python (see http://docs.cython.org/en/latest/src/userguide/language_basics.html#python-functions-vs-c-functions) so you need to do a little bit more work to expose your function to python. In particular you would need to write either a def function or a cpdef function that calls your cdef function and then call that function from python.

This was solved in a comment but posted as an answer for completeness:

Python is describing the functions as "built-in function" purely to mean that it's compiled in C, rather than meaning that it's a standard library function, and so there was issue with this - the correct function was being found in the correct module.

On my PC it appears as which is a more helpful message. I think this might be a Python 3 improvement (but I'm not 100% sure).

Normally, the linker searches for a library with name libXXX.so and not libXXX.so.1.0 (and if not successful, then for library with name libXXX.a). Usually libXXX.so is a symlink to libXXX.so.Y.0 which is the real library.

You can watch the linker working by calling the link-command with -Wl,--verbose, e.g.:

Could you rename primes.cpp to e.g. primesc.cpp? I think primes.pyx is compiled to primes.cpp by Cython as part of the build, so there's a filename clash.