"Zero setup" cross compilation and "cross testing" of Rust crates
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Latest commit ebee639 Jan 21, 2017 @homunkulus homunkulus Auto merge of #62 - japaric:xargo-up, r=japaric
move to Xargo v0.3.5



crates.io crates.io


"Zero setup" cross compilation and "cross testing" of Rust crates

`cross test`ing a crate for the aarch64-unknown-linux-gnu target
`cross test`ing a crate for the aarch64-unknown-linux-gnu target

Disclaimer: Only works on a x86_64 Linux host (e.g. Travis CI is supported)


  • cross will provide all the ingredients needed for cross compilation without touching your system installation.

  • cross provides an environment, cross toolchain and cross compiled libraries (e.g. OpenSSL), that produces the most portable binaries.

  • "cross testing", cross can test crates for architectures other than i686 and x86_64.

  • The stable, beta and nightly channels are supported.



$ cargo install cross


cross has the exact same CLI as Cargo but as it relies on Docker you'll have to start the daemon before you can use it.

# Start the Docker daemon, if it's not already running
$ sudo systemctl start docker

# MAGIC! This Just Works
$ cross build --target aarch64-unknown-linux-gnu

# EVEN MORE MAGICAL! This also Just Works
$ cross test --target mips64-unknown-linux-gnuabi64

# Obviously, this also Just Works
$ cross rustc --target powerpc-unknown-linux-gnu --release -- -C lto


You can place a Cross.toml file in the root of your Cargo project to tweak cross's behavior:

Custom Docker images

The default Docker image that cross uses provides a C environment that tries to cover the most common cross compilation cases. However, it can't cover every single use case out there. When the default image is not enough, you can use the target.$TARGET.image field in Cross.toml to use custom Docker image for a specific target:

image = "my/image:tag"

In the example above, cross will use a image named my/image:tag instead of the default one. Normal Docker behavior applies, so:

  • Docker will first look for a local image named my/image:tag

  • If it doesn't find a local image, then it will look in Docker Hub.

  • If only image:tag is specified, then Docker won't look in Docker Hub.

  • If only tag is omitted, then Docker will use the latest tag.

It's recommended to base your custom image on the default Docker image that cross uses: japaric/$TARGET:$VERSION (where $VERSION is cross's version). This way you won't have to figure out how to install a cross C toolchain in your custom image. Example below:

FROM japaric/aarch64-unknown-linux-gnu:v0.1.4

RUN dpkg --add-architecture arm64 && \
    apt-get update && \
    apt-get install libfoo:arm64
$ docker build -t my/image:tag path/to/where/the/Dockerfile/resides

Use Xargo instead of Cargo

By default, cross uses cargo to build your Cargo project unless you are building for one of the thumbv*-none-eabi* targets; in that case, it uses xargo. However, you can use the build.xargo or target.$TARGET.xargo field in Cross.toml to force the use of xargo:

# all the targets will use `xargo`
xargo = true


# only this target will use `xargo`
xargo = true

Note that xargo = false has no effect as you can't use cargo with targets that only support xargo.

Supported targets

A target is considered as "supported" if cross can cross compile a "non-trivial" (binary) crate, usually Cargo, for that target.

Testing support is more complicated. It relies on QEMU user emulation, so testing may sometimes fail due to QEMU bug sand not because there's a bug in the crate. That being said, cross test is assumed to "work" (test column in the table below) if it can successfully run compiler-builtins test suite.

Also, testing is very slow. cross will actually run units tests sequentially because QEMU gets upset when you spawn several threads. This also means that, if one of your unit tests spawns several threads then it's more likely to fail or, worst, "hang" (never terminate).

Target libc GCC OpenSSL C++ QEMU test
aarch64-unknown-linux-gnu 2.19 4.8.2 1.0.2j 2.8.0
arm-unknown-linux-gnueabi 2.19 4.8.2 1.0.2j 2.8.0
armv7-unknown-linux-gnueabihf 2.15 4.6.2 1.0.2j 2.8.0
i686-unknown-freebsd [1] 10.2 5.3.0 1.0.2j N/A
i686-unknown-linux-gnu 2.15 4.6.2 1.0.2j N/A
i686-unknown-linux-musl 1.1.15 5.3.1 N/A N/A
mips-unknown-linux-gnu 2.23 5.3.1 1.0.2j 2.8.0
mips64-unknown-linux-gnuabi64 2.23 5.3.1 1.0.2j 2.8.0
mips64el-unknown-linux-gnuabi64 2.23 5.3.1 1.0.2j 2.8.0
mipsel-unknown-linux-gnu 2.23 5.3.1 1.0.2j 2.8.0
powerpc-unknown-linux-gnu 2.19 4.8.2 1.0.2j 2.7.1
powerpc64-unknown-linux-gnu 2.19 4.8.2 1.0.2j 2.7.1
powerpc64le-unknown-linux-gnu 2.19 4.8.2 1.0.2j 2.7.1
s390x-unknown-linux-gnu 2.23 5.3.1 1.0.2j 2.8.0
sparc64-unknown-linux-gnu [2] 2.23 5.3.1 1.0.2j 2.8.0
thumbv6m-none-eabi N/A 5.3.1 N/A N/A
thumbv7em-none-eabi N/A 5.3.1 N/A N/A
thumbv7em-none-eabihf N/A 5.3.1 N/A N/A
thumbv7m-none-eabi N/A 5.3.1 N/A N/A
x86_64-unknown-dragonfly [1] [2] 4.6.0 5.3.0 1.0.2j N/A
x86_64-unknown-freebsd [1] 10.2 5.3.0 1.0.2j N/A
x86_64-unknown-linux-gnu 2.15 4.6.2 1.0.2j N/A
x86_64-unknown-linux-musl 1.1.15 5.3.1 1.0.2j N/A
x86_64-unknown-netbsd[1] 7.0 5.3.0 1.0.2j N/A

[1] For *BSD targets, the libc column indicates the OS release version from where libc was extracted.

[2] No std component available as of 2017-01-10


Caveats / gotchas

  • cross will mount the Cargo project as READ ONLY. Thus, if any crate attempts to modify its "source", the build will fail. Well behaved crates should only ever write to $OUT_DIR and never modify $CARGO_MANIFEST_DIR though.


Licensed under either of

at your option.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.