firefox/third_party/rust/libdbus-sys/cross_compile.md

Cross compiling dbus

There are (at least) three different approaches to cross compiling dbus. Choose which one fits you best:

• Using rust-embedded/cross, see below
• There is an alternative guide at issue 184, which also contains a powershell script that set things up for you.
• Setting things up manually, see below

The examples below all assume you're trying to compile for Raspberry Pi 2 or 3 running Raspbian. Adjust target triples accordingly if your target is something else.

Cross compiling using rust-embedded/cross

Thanks to jobale for providing these instructions (taken from issue 292).

Tested on Ubuntu 20.04 | rustc 1.47.0

• Install rust-embedded/cross
• In your project directory, create a Cross.toml file: touch Cross.toml Add this code in it:

[target.armv7-unknown-linux-gnueabihf]
image = "rustcross:dbus-armhf"

[build.env]
passthrough = [
	"RUSTFLAGS",
]

• In your project directory create a Dockerfile: touch Dockerfile Put this code in it:

# Base image for rapsberrypi 3 target
FROM rustembedded/cross:armv7-unknown-linux-gnueabihf

# Install libdbus libraries and pkg-config
RUN dpkg --add-architecture armhf && \
	    apt-get update && \
	    apt-get install --assume-yes libdbus-1-dev libdbus-1-dev:armhf pkg-config

For whatever reason, in the docker image, armhf libraries are installed in at least 2 locations. That's the reason of all my troubles:

• /usr/arm-linux-gnueabihf/lib/

• /usr/lib/arm-linux-gnueabihf/

• Cross needs to know those locations. We pass them to the compiler through a command flag. For convenience I put it in a bash script:

#!/bin/bash
set -o errexit
set -o nounset
set -o pipefail
set -o xtrace

readonly TARGET_ARCH=armv7-unknown-linux-gnueabihf
readonly LINK_FLAGS='-L /usr/arm-linux-gnueabihf/lib/ -L /usr/lib/arm-linux-gnueabihf/'

RUSTFLAGS=${LINK_FLAGS} cross build --release --target=${TARGET_ARCH}

Some more explanations for newcomers like me:

• Cross command act as cargo command (e.g: cross build is the same as cargo build but for cross-compiling).
• In Cross.toml, passthrough = [ "RUSTFLAGS",] is what enable us to pass flags/parameters to the compiler in the docker image.

Setting up cross compiling manually

A cross linker

Apparently, rustc in itself can generate code for many archs but not assemble the generated code into the final executable. Hence you need a cross linker.

Install it - here follow whatever guide you have for the target arch. Distributions may also ship with cross toolchains. Example for Ubuntu 18.04:

sudo apt install gcc-8-multilib-arm-linux-gnueabihf

Tell rustc where to find it - in .cargo/config add the following:

[target.armv7-unknown-linux-gnueabihf]
linker = "arm-linux-gnueabihf-gcc-8"

Target rust std

This one's easy, just run rustup:

rustup target add armv7-unknown-linux-gnueabihf

Target dbus libraries

Installing the libraries

Now to the more challenging part. Since we link to a C library libdbus-1.so, we also need the target version of that library. However, libdbus-1.so in itself links to a systemd library (at least it does so here) which in turn links to other libraries etc, so we need target versions of those libraries too.

Getting an entire rootfs/image is probably the easiest option. The rootfs needs to have libdbus-1-dev installed. I e:

• Boot your target (i e, a raspberry), install libdbus-1-dev on it, turn it off and put the SD card into your computer's SD card reader. Mount the partition.
• If the above is not an option, you could download an image, mount it at the right offset, like this (ugly hack!) sudo mount -o loop,offset=50331648 2019-04-08-raspbian-stretch-lite.img /tmp/mnt and then, to manually make the symlink that libdbus-1-dev does for you: cd /tmp/mnt/usr/lib/arm-linux-gnueabihf && ln -s ../../../lib/arm-linux-gnueabihf/libdbus-1.so.3 libdbus-1.so.
• Or you can use the alternative guide's approach to download, extract and post-process the relevant .deb files manually. This might be a preferrable option if an entire image/rootfs would be too large.

Finding the libraries

When not cross compiling, finding the right library is done by a build.rs script which calls pkg-config. This will not work when cross compiling because it will point to the libdbus-1.so on the host, not the libdbus-1.so of the target. Maybe it is possible to teach pkg-config how to return the target library instead, but I have not tried this approach. Instead we can override build script altogether and provide the same info manually. This is possible because libdbus-sys has a links = dbus line.

For the example below we assume that we have mounted a Raspbian rootfs on /tmp/mnt, and that the cross linker came with some basic libraries (libc, libpthread etc) that are installed on /usr/arm-linux-gnueabihf/lib. Unfortunately, we can't use the basic libraries that are present on the image, because they might contain absolute paths.

And so we add the following to .cargo/config:

[target.armv7-unknown-linux-gnueabihf.dbus]
rustc-link-search = ["/usr/arm-linux-gnueabihf/lib", "/tmp/mnt/usr/lib/arm-linux-gnueabihf"]
rustc-link-lib = ["dbus-1"]

Finally

If we are all set up, you should be able to successfully compile with:

cargo build --target=armv7-unknown-linux-gnueabihf