Using cython to cross compile project from intel ubuntu to arm

Question

I have simple python + cython project (hello world example from http://docs.cython.org/src/tutorial/cython_tutorial.html) on my ubuntu 16 x86_64. I can build this project with cython for x86_64.

How can I build the project for armv7 version of ubuntu 15 without using real armv7 board/cpu?

I have arm-linux-gnueabihf-gcc (http://packages.ubuntu.com/xenial/devel/gcc-arm-linux-gnueabihf) and it can compile simple C programs for armv7. How can I change settings of cython to use cross compiler for building shared objects for arm?

Answer 1

Architecture dependent libraries and headers files are needed for cross compiling.

When testing if python3.5-dev package and others could be installed after dpkg --add-architecture armhf and apt-get update (after some modification to sources.list), the result was basically.

python3.5-dev:armhf : Depends: python3.5:armhf (= 3.5.1-10) but it is not going to be installed

apt-get install python3.5:armhf is something that doesn't work, see

The existing proposals allow for the co-installation of libraries and headers for different architectures, but not (yet) binaries.

One possible solution that does not require "full" virtual machine is provided by QEMU and chroot. A suitable directory for chroot can be created by debootstrap command. After creation, schroot can give access to that environment.

Substitute <DIRECTORY> and <USER> in the following commands:

apt-get install -y debootstrap qemu-user-static binfmt-support schroot
debootstrap --arch=armhf --foreign --include=gcc,g++,python3.5-dev xenial <DIRECTORY>
cp /usr/bin/qemu-arm-static <DIRECTORY>/usr/bin
chroot <DIRECTORY>
/debootstrap/debootstrap --second-stage
echo "deb http://ports.ubuntu.com/ubuntu-ports xenial universe" >> /etc/apt/sources.list
echo "deb http://ports.ubuntu.com/ubuntu-ports xenial multiverse" >> /etc/apt/sources.list
apt-get update
apt-get install -y cython cython3
exit
cat <<END > /etc/schroot/chroot.d/xenial-armhf
[xenial-armhf]
description=Ubuntu xenial armhf
type=directory
directory=/home/xenial-armhf
groups=sbuild,root
root-groups=sbuild,root
users=root,<USER>
END

The environment should be accessible by

schroot -c chroot:xenial-armhf

and for root user session (the user must be in a group listed in root-groups) ,

schroot -c chroot:xenial-armhf -u root

After this, it is also possible to cross compile a cython module:

hello.pyx:

print("hello world")

compiling (python3.5-config --cflags and python3.5-config --libs in chroot for options, note -fPIC):

cython hello.pyx
arm-linux-gnueabihf-gcc --sysroot <DIRECTORY> -I/usr/include/python3.5m -I/usr/include/python3.5m  -Wno-unused-result -Wsign-compare -g -fstack-protector-strong -Wformat -Werror=format-security  -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -fPIC -c hello.c
arm-linux-gnueabihf-gcc --shared --sysroot <DIRECTORY> -lpython3.5m -lpthread -ldl  -lutil -lm hello.o -o hello.so

The module can be then tested

schroot -c chroot:xenial-armhf
python3
import hello

Cross compiling cython based python modules may also work. With setup.py

from distutils.core import setup
from distutils.extension import Extension
from Cython.Distutils import build_ext

import os

os.environ['CC'] = 'arm-linux-gnueabihf-gcc'
os.environ['LDSHARED'] = 'arm-linux-gnueabihf-gcc -shared'
sysroot_args=['--sysroot', '/path/to/xenial-armhf']

setup(cmdclass = {'build_ext': build_ext},
      ext_modules= [ Extension("hello", ["hello.pyx"],
                                extra_compile_args=sysroot_args,
                                extra_link_args=sysroot_args) ])

Building a simple hello world module was possible this way. The file name for the module was wrong, in this case it was hello.cpython-35m-x86_64-linux-gnu.so. After renaming it as hello.so it was possible to import it.