HelloPi

This is my hobby project at writing a chat bot for the Discord multi-user chat service, written in Rust and running on my Raspberry Pi. It is also my first attempt at doing cross compilation, as I want to write and test the code on my x86 Ubuntu desktop and laptor but finally deploy it to the ARMv7 based Raspberry Pi. Details on how to set up your Ubuntu 16.04 machine for cross compiling Rust code to the Raspberry Pi, and how to build the OpenSSL library for cross-compilation are included below.

Preparing for cross compilation

Installing the GCC cross compiler

We need to install the GCC cross compiler for armv7 so that the Rust compiler will be able to link our executables, as well as build any packages that use C bindings, like the openssl crate. I own the Raspberry Pi Model B 2 which has a 32-bit ARMv7 CPU with a built in floating point unit, and is running the Raspbian Linux distribution which means that my target is the armv7-unknown-linux-gnueabihf system. The cross compilation libraries needed for this can be installed straight from the Ubuntu repos:

sudo apt-get install gcc-arm-linux-gnueabihf g++-arm-linux-gnueabihf

If you have a different Raspberry Pi that doesn't have a built in FPU, or has a more modern ARM processor then your target triple and cross compiler may be different.

Installing Rust and the cross compilation target (via rustup)

The easiset way to use multiple versions of Rust, or to have multiple compilation targets is to use the rustup tool. Instructions for installing it can be found on their website.

Once you have installed rustup we need to add the Rasperry Pi target which is called armv7-unknown-linux-gnueabihf. This can be done by running the following:

$ rustup target add armv7-unknown-linux-gnueabihf

To verify that it installed correctly you can just run the command again. You should see something like this:

info: component 'rust-std' for target 'armv7-unknown-linux-gnueabihf' is up to date

We next need to tell cargo where it can find the cross compiler for ARM that we installed earlier. To do this you need to add the following lines to your ~/.cargo/config file, creating it if it does not already exist:

[target.armv7-unknown-linux-gnueabihf]
linker = "arm-linux-gnueabihf-gcc"

You should now be able to cross compile Rust code to target the Raspberry Pi! Remember that you need to pass the --target=armv7-unknown-linux-gnueabihf option to cargo for it to actually cross compile. For example:

$ cargo new hello_world --bin
$ cd hello_world
$ cargo build --target=armv7-unknown-linux-gnueabihf
   Compiling hello_world v0.1.0 (file:///home/hello_pi/hello_world)

and the resulting executable will be in the target/armv7-unknown-linux-gnueabihf/debug folder.

Compiling OpenSSL for cross compilation

OpenSSL generates some header files at compile time that do not translate across platforms so we need to recompile OpenSSL if we want to cross compile to the Rasperry Pi.

I have only been able to get OpenSSL 1.0.2h to work with the Rust openssl crate so we will compile that verison directly.

$ wget ftp://ftp.openssl.org/source/openssl-1.0.2h.tar.gz
$ tar -xzf openssl-1.0.2h.tar.gz
$ cd openssl-1.0.2h/

Next we need to prepare our environment to build targetting ARM:

$ export CROSS=arm-linux-gnueabihf
$ export AR=$CROSS-ar
$ export AS=$CROSS-as
$ export CC=$CROSS-gcc
$ export CXX=$CROSS-g++
$ export LD=$CROSS-ld
$ ./Configure --prefix=/usr/arm-linux-gnueabihf linux-armv4

Finally we can build and install the library

$ make
$ sudo make install