Somewhat on impulse I ordered a Raspberry Pi Zero “W”. Not sure how I hadn’t heard about this until now (Zero first came out in 2015, “Zero W” with Wifi/Bluetooth in 2017), but as soon as I saw it I had to have one:

Dawww… look at that lil’ guy.

Way back in early 2000’s I did a project with wireless “Motes” running TinyOS:

I can’t remember which exact model we had but they all seem to have around 4 KB of SRAM and 512 KB flash memory. Here we are nigh 20 years later and the Zero is an absolute beefcake with 512 MB RAM, micro SD slot (here 16 GB), and paired with a single-core 32-bit ARM CPU.

Equipment:

• Raspberry Pi Zero- preferably a “W” or “WH”
• Micro SD card (at least 8GB)
• Micro SD card reader, or SD card reader with adapter
• “Regular” USB-A (male) to micro-B (male) cable
  • Come with many things (e.g. all but the latest Android devices using USB-C)
  • From reading forum posts, some charging cables might not have data wires and won’t work. If no device is found, try another cable.

Optional, but occasionally useful:

• Power adapter with USB micro-B connector or another USB-A to micro-B cable like above
  • You can power the Zero via a cable from your PC to the micro USB port labeled “USB” on the Zero, but it’s convenient to have separate power so you can unplug the first
• mini HDMI to standard/type-A HDMI adapter or cable (to connect Zero to external display)
• USB-A (female) to micro-B (male) OTG adapter (to connect/keyboard mouse to Zero)

Setup

Doing a basic install of Raspbian is similar to with the Pi 3:

1. Download Raspbian
2. Install

Prior to the Zero W/WH with wifi, there was an approach to configure the system completely headless and get it on the network that’s still pretty useful:

• Raspberry Pi OTG Mode
• Setting Up Pi OTG - The Quick Way
• Adafruit guide

For macOS:

# Write raspbian image to SD in /dev/disk2
diskutil unmountDisk /dev/disk2
sudo dd bs=1m if=~/Downloads/2018-11-13-raspbian-stretch.img of=/dev/rdisk2 conv=sync
cd /Volumes/boot
# Enable SSH
touch ssh
vim config.txt # Add to the bottom: dtoverlay=dwc2
vim cmdline.txt # After `rootwait` add: modules-load=dwc2,g_ether
sudo diskutil eject /dev/rdisk2

Insert the SD card into the Pi Zero and connect your PC to the micro USB port labeled “USB” (it’s the one closest to the middle of the unit) to power it on.

After ~30 seconds, in System Preferences > Network you should see:

Connect with:

# Setup ssh key authentication. Default password is `raspberry`
ssh-copy-id pi@raspberrypi.local
ssh -X pi@raspberrypi.local

If you’ve got multiple devices, or you re-flash at some point you’ll see the very scary:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: POSSIBLE DNS SPOOFING DETECTED! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
The ECDSA host key for raspberrypi.local has changed,
and the key for the corresponding IP address 169.254.28.197
is unknown. This could either mean that
DNS SPOOFING is happening or the IP address for the host
and its host key have changed at the same time.
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that a host key has just been changed.
The fingerprint for the ECDSA key sent by the remote host is
SHA256:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX.
Please contact your system administrator.
Add correct host key in /Users/XXX/.ssh/known_hosts to get rid of this message.
Offending ECDSA key in /Users/XXX/.ssh/known_hosts:37
ECDSA host key for raspberrypi.local has changed and you have requested strict checking.
Host key verification failed.

Find the line like Offending ECDSA key in /Users/XXX/.ssh/known_hosts:37. It means you should edit ~/.ssh/known_hosts and remove line 37 to fix the nasty.

Next, configure Internet sharing:

To get it online, open System Preferences > Sharing.For Share your connection from select your computer’s internet connection (probably either LAN or Wifi) and To computers using select the RNDIS/Ethernet Gadget.

Note that if the wifi uses 802.1x authentication (like our office) connection sharing won’t work.

Software

One important difference is the Zero is armv6 (with VFPv2 but no NEON- like the Raspberry Pi 1) instead of armv7 like the Raspberry Pi 3. This means the available .NET Core, PowerShell, VS Code and other binaries won’t “just” work. We may look into building 32-bit .NET Core from source, but with a single core puttering along at 1 GHz and only 512 MB of RAM I doubt VS Code would run well.

Remember when we advocated learning vim or emacs? Guess what works beautifully on low-end devices like the Zero.

Thankfully, Rust also works! All we really need is Rust anyway.

Cross Compiling

It’s easy to install Rust on the Zero via rustup:

curl https://sh.rustup.rs -sSf | sh

Let’s build runng.

On a 2017 MacBook Pro cargo clean; cargo build takes about 20 seconds.

On the Pi Zero, it takes 66 minutes. Ouch!

This looks like a job for cross compiling! This is perhaps the most complete document on cross-compiling with Rust, but it is a bit dated. We’ll look at using musl to create stand-alone binaries. Afterall, who wants to deal with shared libraries?

The best approach depends on your situation:

• Which machines/devices/OSes are available
• Complexity/requirements of the build environment
• Familiarity with docker/cargo (especially when it comes to cacheing crates)

Here’s a summary of the options I’ll cover:

These aren’t meant to be directly comparible since I didn’t have identical hardware to run everything on. But, it should give you some idea of the convenience/flexibility vs. build time trade-offs you can make.

MacOS

musl-cross

On Macs, we can use the same approach we use to target AWS Lambda or our Pi 3 pm2.5 monitor.Let’s start with something simple, a “hello world” binary. On a Mac (not on the Zero):

cargo new --bin hello_muscles
cd hello_muscles/

In hello_muscles/.cargo/config (or ~/.cargo/config) add:

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

# Install arm/x86_64 linux-musl cross-compilers
brew install FiloSottile/musl-cross/musl-cross --with-arm-hf
# Install rust target
rustup target add arm-unknown-linux-musleabihf
# Build
CROSS_COMPILE=arm-linux-musleabihf- cargo build --release --target arm-unknown-linux-musleabihf

Check target/arm-unknown-linux-musleabihf/release/ for output:

$ file target/arm-unknown-linux-musleabihf/release/hello_muscles

target/arm-unknown-linux-musleabihf/release/hello_muscles: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV), statically linked, with debug_info, not stripped

$ ls -l target/arm-unknown-linux-musleabihf/release/hello_muscles

-rwxr-xr-x 2 jake staff 2674552 Mar 18 16:28 target/arm-unknown-linux-musleabihf/release/hello_muscles

A ~2.7MB self-contained ARM32 executable. Not bad, but we can do even better by stripping the executable:

$ arm-linux-musleabihf-strip target/arm-unknown-linux-musleabihf/release/hello_muscles

$ ls -l target/arm-unknown-linux-musleabihf/release/hello_muscles

-rwxr-xr-x 2 jake staff 201760 Mar 28 14:51 target/arm-unknown-linux-musleabihf/release/hello_muscles

A sliver over 200 KB! Let’s give it a whirl on the Zero:

# No passwords needed, thanks key-auth!
scp target/arm-unknown-linux-musleabihf/release/hello_muscles pi@raspberrypi.local:~/
ssh pi@raspberrypi.local

pi$ ./hello_muscles 
Hello, world!

crosstool-NG

A lot of older posts mention crosstool-NG, but MacOS is no longer officially supported. We didn’t look into it further, but it might be useful for Windows/Linux environments.

Linux

There’s numerous different approaches on Linux and we’re not going to cover all of them. Feel free to leave a comment if you have a favorite not mentioned here. Regardless, there’s a few other commadns that come in handy:

# Get general information about executable
file <path_to_binary>
# More detailed information than `file`, such as whether soft or hard-float, etc.
readelf -A <path to binary>

rust-musl-cross

If you’re comfortable with docker and building your project can be nicely containerized, rust-musl-cross might be a good option:

alias rust-musl-builder='docker run --rm -it -v "$(pwd)":/home/rust/src messense/rust-musl-cross:arm-musleabihf'
rust-musl-builder cargo build --release

There’s tags for other exotic architectures like armv7-musleabihf, etc.

There’s also rust-musl-builder: primary targetted at x86_64 although it has some limited form of support for armv7 (i.e. Raspberry Pi 2 and 3, but not Zero).

Cross

The Rust embedded team has a tool cross that might make everything easier:

# Try this first. It's working if `cross build` pulls docker images.
cargo install cross
cross build --target arm-unknown-linux-musleabi

# At the time of this writing, most recent release is from Nov. 2017 and didn't work for me
$ cross -V
cross 0.1.14

# Use HEAD
cd
git clone https://github.com/rust-embedded/cross.git
cd cross
cargo build --release
cd ../some_project_source
~/cross/target/release/cross build --target arm-unknown-linux-musleabihf

Make sure you have completed the Post-installation steps for Linux because it assumes you don’t need sudo docker.

Even if you can get cross working, the default images may not work for whatever you’re trying to build. In fact, not all target triples supported by Rust have docker images:

$ cross build --target arm-unknown-linux-musleabihf

Unable to find image 'japaric/arm-unknown-linux-musleabihf:latest' locally
docker: Error response from daemon: pull access denied for japaric/arm-unknown-linux-musleabihf, repository does not exist or may require 'docker login'.

To work around this, cross enables you to specify custom docker images.

Docker and Qemu

We’re using Docker and Qemu to simplify building and testing for multiple platforms.

If none of the above solutions work for you, it’s pretty easy to work with docker and qemu:

# Do this once after starting docker
docker run --rm --privileged multiarch/qemu-user-static:register --reset
# Create interactive console and mount PWD as /usr/src
docker run -it --rm -v $(pwd):/usr/src multiarch/debian-debootstrap:armhf-stretch

docker$ apt-get install -y # Pre-requisites...
# Note that Debian "armhf" is armv7
docker$ cargo build --target arm-unknown-linux-musleabi
# When you're done leave the interactive shell
docker$ exit

# Check target/arm-unknown-linux-muslXXX/YYY/ for binaries

Performance will vary depending on docker/qemu/host resources, but it should be slower than cross-compiling and much faster than the Zero.

Pi 3

A final option is to do the build on another ARM device like a Raspberry Pi 3 where you won’t have to cross-compile. The probably only makes sense if you’ve got a low-end laptop or are using the Pi as your dev PC.

The End?

Hardly. We’ve got Linux installed and running on the Zero, and we’ve got a dev environment so we can start building something. Now for the next step.