# Cross Compile with OSX as a host

## compile crosstool-ng

One debian systems, there are pre-compiled toolchains, for OSX however they have to be built from scratch. 
A decent tool for creating such toolchains is [crosstool-ng](http://crosstool-ng.github.io/docs/).

*Compilation Note* there's a problem with having ncurses installed via homebrew. Add the libs and include paths to LDFLAGS/CXXFLAGS before the configure step
For python problems on gdb, see https://stackoverflow.com/a/11600411/4583130

crosstool-ng works similar to linux kernel configurations. A complete example can be found below 01_aarch64-unknown-linux-gnu.config
.
*Execution Note* the tool requires a case-sensitive filesystem. OSX is not case-sensitive by default. So we have to create a new Volume which is:
```
#!/bin/bash
ImageName=CrossToolNG
ImageNameExt=${ImageName}.sparseimage
diskutil umount force /Volumes/${ImageName} && true
rm -f ${ImageNameExt} && true
hdiutil create ${ImageName} -volname ${ImageName} -type SPARSE -size 8g -fs HFSX
hdiutil mount ${ImageNameExt}
cd /Volumes/$ImageName
```
Found here: https://docs.zephyrproject.org/1.9.0/getting_started/installation_mac.html#creating-a-case-sensitive-file-system

Once the configuration is built correctly (use samples and modify them for the correct version), the folder layout looks as follows:
```
 ➭ ll ~/workspace/toolchains/x86_64/gcc-7.5/x86_64-unknown-linux-gnu
total 4096
dr-xr-xr-x   9 karsten  staff      288 Oct 16 00:54 ./
drwxr-xr-x   5 karsten  staff      160 Oct 19 11:49 ../
dr-xr-xr-x  37 karsten  staff     1184 Oct 16 00:53 bin/
-r--r--r--   1 karsten  staff  2095985 Oct 16 00:53 build.log.bz2
dr-xr-xr-x   2 karsten  staff       64 Oct 16 00:49 include/
dr-xr-xr-x   7 karsten  staff      224 Oct 16 00:49 lib/
dr-xr-xr-x   3 karsten  staff       96 Oct 16 00:49 libexec/
dr-xr-xr-x   5 karsten  staff      160 Oct 16 00:53 share/
dr-xr-xr-x   8 karsten  staff      256 Oct 16 00:49 x86_64-unknown-linux-gnu/
```

## cross compile with clang
one does not have to cross-compile with a special created gcc compiler. Clang itself (the one on the host platoform) is by defualt a cross compiler.
once the toolchain is created with clang, it requires two arguments set, i.e. `sysroot` and `gcc-toolchain`.
```
[ 50%] Building CXX object CMakeFiles/karsten_hello_world.dir/main.cpp.o
/usr/local/opt/ccache/libexec/clang++ --target=x86_64-unknown-linux-gnu --sysroot=/Users/karsten/workspace/toolchains/x86_64/gcc-7.5/x86_64-unknown-linux-gnu/x86_64-unknown-linux-gnu/sysroot   --gcc-toolchain=/Users/karsten/workspace/toolchains/x86_64/gcc-7.5/x86_64-unknown-linux-gnu/x86_64-unknown-linux-gnu/sysroot/../.. -o CMakeFiles/karsten_hello_world.dir/main.cpp.o -c /Users/karsten/workspace/ctrlx/snap_ws/ctrlx_hello_world/main.cpp
[100%] Linking CXX executable karsten_hello_world
/usr/local/Cellar/cmake/3.18.4/bin/cmake -E cmake_link_script CMakeFiles/karsten_hello_world.dir/link.txt --verbose=1
/usr/local/opt/ccache/libexec/clang++ --target=x86_64-unknown-linux-gnu --sysroot=/Users/karsten/workspace/toolchains/x86_64/gcc-7.5/x86_64-unknown-linux-gnu/x86_64-unknown-linux-gnu/sysroot --gcc-toolchain=/Users/karsten/workspace/toolchains/x86_64/gcc-7.5/x86_64-unknown-linux-gnu/x86_64-unknown-linux-gnu/sysroot/../.. -L/usr/local/opt/llvm/lib -Wl,-rpath,/usr/local/opt/llvm/lib -fuse-ld=lld CMakeFiles/karsten_hello_world.dir/main.cpp.o -o karsten_hello_world
[100%] Built target karsten_hello_world
/usr/local/Cellar/cmake/3.18.4/bin/cmake -E cmake_progress_start /Users/karsten/workspace/ctrlx/snap_ws/ctrlx_hello_world/build/CMakeFiles 0
```

The equivalent cmake toolchain file:
```
set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR x86_64)

set(CMAKE_SYSROOT /Users/karsten/workspace/toolchains/x86_64/gcc-7.5/x86_64-unknown-linux-gnu/x86_64-unknown-linux-gnu/sysroot)
set(CMAKE_CXX_FLAGS_INIT "--gcc-toolchain=${CMAKE_SYSROOT}/../..")
set(CMAKE_C_FLAGS_INIT "--gcc-toolchain=${CMAKE_SYSROOT}/../..")

set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)

set(triple x86_64-unknown-linux-gnu)

set(CMAKE_C_COMPILER clang)
set(CMAKE_C_COMPILER_TARGET ${triple})
set(CMAKE_CXX_COMPILER clang++)
set(CMAKE_CXX_COMPILER_TARGET ${triple})
```

## Cross Compile ROS2

Used toolchain file being used is in the file below: 02_aarch64_toolchain.cmake in comibination with a aarch64-unknown-linux-gnu toolchain generated by crosstool-ng.

### Compile Python
We need to cross-compile Python and enhance our toolchain with this:

```
wget -c https://www.python.org/ftp/python/3.8.6/Python-3.8.6.tar.xz
```

```
PATH=~/workspace/toolchains/aarch64/aarch64-unknown-linux-gnu/bin:$PATH ./configure CC=aarch64-unknown-linux-gnu-gcc CXX=aarch64-unknown-linux-gnu-g++ AR=aarch64-unknown-linux-gnu-ar LD=aarch64-unknown-linux-gnu-ld RANLIB=aarch64-unknown-linux-gnu-ranlib --host=aarch64-unknown-linux-gnu --build=x86_64-apple-darwin18 --target=aarch64 -prefix=`pwd`/python3_install --disable-ipv6 ac_cv_have_long_long_format=yes ac_cv_file__dev_ptmx=no ac_cv_file__dev_ptc=no --enable-shared --enable-optimizations && make && make install
```
We then have to integrate these files into our crosstool-ng toolchain sysroot folder. I would assume that's the same as specifying the toolchain sysroot as a `--prefix` in the `configure` step above.
```
cp -r ~/workspace/ros2/ros2_cross_compile/third_party/python3.8.6/Python-3.8.6/python3_install/bin/* bin/
cp -r ~/workspace/ros2/ros2_cross_compile/third_party/python3.8.6/Python-3.8.6/python3_install/lib/* lib/
cp -r ~/workspace/ros2/ros2_cross_compile/third_party/python3.8.6/Python-3.8.6/python3_install/include/* include/
cp -r ~/workspace/ros2/ros2_cross_compile/third_party/python3.8.6/Python-3.8.6/python3_install/share/* share/
```

## Compile TinyXml2, libexpat, logging-log4cxx

The three packages listed above are all cmake-based packages and can thus conveniently be installed via colcon.

## Compile a ROS2 workspace

Foonathan memory can't really be cross-compiled, cyclone_dds is easier. So we skip fastrtps and their dependencies.
There is some code generation for cyclonedds needed as well. For this, the `ddsconf` tool has to be executed from the host.
Luckily, cyclonedds lets you easily pass in that tool via `-DDDSCONF_EXECUTABLE`. 
```
colcon build --merge-install --build-base build_aarch64 --install-base install_aarch64 --event-handler=console_cohesion+  --cmake-args -DBUILD_TESTING=OFF -DCMAKE_TOOLCHAIN_FILE=~/workspace/toolchains/aarch64/aarch64_toolchain.cmake -DDDSCONF_EXECUTABLE=/Users/karsten/workspace/ros2/ros2_cross_compile/tmp/cyclonedds/build/bin/ddsconf --packages-ignore fastrtps rmw_fastrtps_shared_cpp rmw_fastrtps_cpp rmw_fastrtps_dynamic_cpp foonathan_memory_vendor rosidl_typesupport_fastrtps_cpp rosidl_typesupport_fastrtps_c --cmake-clean-ca --packages-up-to examples_rclcpp_minimal_publisher
```