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](https://github.com/diwic/dbus-rs/issues/184#issuecomment-520228758), which also contains a [powershell script](https://github.com/diwic/dbus-rs/issues/184#issuecomment-520791888) 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](https://github.com/jobale) for providing these instructions (taken from [issue 292](https://github.com/diwic/dbus-rs/issues/292)). Tested on Ubuntu 20.04 | rustc 1.47.0 - Install [rust-embedded/cross](https://github.com/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](https://doc.rust-lang.org/cargo/reference/config.html) 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](https://doc.rust-lang.org/cargo/reference/config.html): ``` [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`